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A model study of the relative influences of scavenging and circulation on 230Th and 231Pa in the western North Atlantic
Journal Pre-proof A Model Study of the Relative Influences of Scavenging and Circulation on 231 Pa in the western North Atlantic
230 Th and
Paul Lerner, Olivier Marchal, Phoebe J. Lam, Wilford Gardner, Mary Jo Richardson, Alexey Mishonov PII:
S0967-0637(19)30305-X
DOI:
https://doi.org/10.1016/j.dsr.2019.103159
Reference:
DSRI 103159
To appear in:
Deep-Sea Research Part I
Received Date: 18 October 2019 Accepted Date: 6 November 2019
Please cite this article as: Lerner, P., Marchal, O., Lam, P.J., Gardner, W., Richardson, M.J., Mishonov, 230 231 A., A Model Study of the Relative Influences of Scavenging and Circulation on Th and Pa in the western North Atlantic, Deep-Sea Research Part I, https://doi.org/10.1016/j.dsr.2019.103159. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
1
2
A Model Study of the Relative Influences of Scavenging and Circulation on
230
Th and
231
Pa in the western North Atlantic
3
Paul Lerner∗a , Olivier Marchalb , Phoebe J. Lamc , Wilford Gardnerd , Mary Jo Richardsond ,
4
and Alexey Mishonove
5
6
Goddard Institute for Space Studies, New York City, NY 10026, USA
b
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c
7
University of California Santa Cruz, Santa Cruz, CA 95064, USA d
8
9
a
e
Texas A&M University, College Station, TX 77843, USA
Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA
November 19, 2019
10
∗ Corresponding
Author. Address: Goddard Institute for Space Studies, 2880 Broadway, NYC, NY (Tel:1-212-678-5548)
email address: [email protected]
1
Generated using version 3.2 of the official AMS LATEX template
ABSTRACT
11
12
The oceanic cycles of thorium-230 and protactinium-231 are affected by a number of processes, such as
13
removal by adsorption to settling particles and transport by ocean currents. Measurements obtained as part
14
of GEOTRACES and earlier programs have shown that, in the North Atlantic, the activities of dissolved
15
230
Th (230 Thd ) and
231
Pa (231 Pad ) at abyssal depths are lower near the western margin than in the basin 230
interior. At least two factors could explain the lower
17
scavenging in benthic nepheloid layers (BNLs) extending a thousand meters or more above the seafloor; and
18
(ii) ventilation by relatively
19
Current (DWBC).
230
Thd - and
231
Thd and
231
16
Pad near the margin: (i) intensified
Pad -poor waters emanating from the Deep Western Boundary
20
Here a regional ocean circulation model with ‘eddy-permitting’ resolution (1/4◦ ) that incorporates 230 Th
21
and 231 Pa is used in an effort to reproduce the observed distributions of 230 Th and 231 Pa in the western North
22
Atlantic. In this model,
23
particulate matter that is derived from a recent synthesis of nephelometer and transmissometer data. The
24
model simulates a meandering Gulf Stream and a DWBC along the continental slope and rise, although
25
noticeable differences with physical observations also exist. A model solution is found that explains most
26
of the variance of
27
cruises. On the other hand, measurements of particulate
28
difficult to reproduce, with the same solution accounting for only 49% (11%) of the 230 Thp (231 Pap ) variance.
29
Sensitivity experiments suggest that the low
30
are due to enhanced removal rates of both nuclides in BNLs rather than to deep water ventilation from
31
the western boundary. The radionuclide activities present in the DWBC at its inflow location are also
32
found to strongly influence the basin-scale distributions of
33
BNLs as important sites of
34
simultaneously radionuclide measurements in dissolved and particulate forms in the studied area.
35
36
Keywords:
230
230
230
Th and
231
Pa removal from solution is governed by a prescribed distribution of
Thd measurements (85%) and
230
Th and
231
230
231
Pad measurements (81%) from (pre-)GEOTRACES
Thd and
231
230
Th (230 Thp ) and
231
Pa (231 Pap ) are more
Pad activities observed near the western margin
230
Th and
231
Pa. Overall, our study points to
Pa scavenging in the ocean and illustrates the difficulty to explain
Th, 231 Pa, regional ocean circulation model, Deep Western Boundary Current, reversible
exchange, particle concentration effect, nepheloid layer
1
37
1. Introduction
38
Thorium-230 (half-life of 75.6 kyr; Cheng et al. (2013)) and protactinium-231 (32.8 kyr; Robert et al.
39
(1969)) are two naturally-occurring radioisotopes that are produced from the radioactive decay of uranium-
40
234 and uranium-235, respectively. Uranium isotopes display quasi-conservative behaviour in seawater and
41
concentrations proportional to salinity (e.g., Owens et al. (2011)), so that the production rates of
42
and
43
thought to be removed from the water column through a reversible exchange (adsorption and desorption)
44
with settling particles (Nozaki et al. 1981; Bacon and Anderson 1982), a process often referred to as ‘particle
45
scavenging’. A convenient measure of the intensity of particle scavenging is provided by the distribution
46
coefficient, KD = Ap /(Ad P ), where Ad (Ap ) is the activity (in units of dpm) of the radionuclide in the
47
dissolved (particulate) phase per kg of seawater and P is the bulk particle concentration in units of grams
48
of particles per gram of seawater. In the open ocean, KD for
49
indicating a strong tendency for Th to adsorb onto marine particles, whereas KD for 231 Pa is generally lower
50
by one order of magnitude (Moran et al. 2002; Hayes et al. 2015a). As a result of their different sensitivities
51
to particle scavenging, Th and Pa represent two model types for the fate of trace metals, Th being removed
52
from the open ocean primarily by settling particles and Pa largely by lateral transport to other environments
53
(Anderson et al. 1983b,a; Henderson and Anderson 2003).
54
231
230
Pa are expected to be approximately uniform in the world’s oceans. Both
230
Th and
231
230
Th
Pa are
Th is typically of the order of 107 g g−1 ,
The distributions of 230 Th and 231 Pa in the North Atlantic have been documented in a number of studies. 230
Th (230 Thd ) or dissolved
231
Pa (231 Pad ) from distant
55
Early work showed that the activities of dissolved
56
locations exhibit a similar increase with depth in the upper ∼ 1000 m of the water column but present
57
large horizontal variations in the abyssal region below (for syntheses see Marchal et al. (2007); Luo et al.
58
(2010)). More recently, the distributions of
59
been determined with unprecedented horizontal and vertical resolutions along the U.S. GEOTRACES North
60
Atlantic section GA03 (Hayes et al. 2015a). Consistent with pre-GEOTRACES data,
61
distant locations showed a similar increase with depth in the upper 1000 m but large lateral variations at
62
greater depths. Below about 1000 m, stations west of Bermuda displayed lower
63
stations east of Bermuda (Fig. 1).
230
Th and
2
231
Pa in both dissolved and particulate forms have
230
230
Thd or 231 Pad from
Thd and
231
Pad than
230
0
0.2
Thd (dpm/m 3)
0.4
0.6
0.8
1
1.2
0 1000
depth (m)
2000 3000 4000 5000 6000
64
231
-0.2 0
Pad (dpm/m 3)
0
0.2
0.4
0.6
1000
depth (m)
2000 3000 4000 5000
65
66
6000 Fig. 1. Vertical profiles of dissolved
230
Th and
231
Pa activities in the North Atlantic. Data from stations
67
west (east) of Bermuda are shown with black (blue) circles and data from station GT11-16, near the TAG
68
hydrothermal vent, are shown with red circles. The horizontal bars show the measurement uncertainties
69
(see Table 1 for data sources). 230
Thd and
231
70
At stations west of Bermuda,
Pad increased with depth down to about 2000–4000 m but
71
decreased with depth below. Whereas the activity increase with depth observed in the upper part of the
72
water column is consistent with a reversible exchange of Th and Pa with settling particles (Bacon and
73
Anderson 1982), the reversal in the vertical activity gradient at mid-depth and the large horizontal activity
74
gradients in the deeper part await further investigation. 3
75
At least three factors could explain the departures of
230
Thd and
231
Pad measurements from the profiles
76
expected from reversible exchange: (i) spatial variations in particle scavenging due to variations in bulk
77
particle concentration, (ii) spatial variations in particle scavenging due to variations in particle composition,
78
and (iii) the ventilation of deep layers by components of the North Atlantic Deep Water (NADW). Each of
79
these factors is introduced in turn below.
80
Particle scavenging in oceanic waters appears to vary with bulk particle concentration (P ). Two observed
81
characteristics of metal sorption in natural waters are in apparent contradiction with physico-chemical ad-
82
sorption theory: (i) the time scales of sorption reactions are long, reaching days to weeks and more, and (ii)
83
KD is negatively correlated with particle concentration – the so-called “particle concentration effect” (e.g.,
84
Honeyman and Santschi (1989)). Colloids have for a long time been invoked to explain the slow Th sorption
85
kinetics (e.g., Tsunogai and Minagawa (1978)). In the model developed by these authors, dissolved Th
86
species rapidly and irreversibly adsorb onto colloidal particles, which then reversibly aggregate with larger
87
filterable particles. Santschi et al. (1986) proposed that if the sorption of a metal ion has particle aggregation
88
as a rate-limiting step, then the rate of sorption should vary with particle number or particle concentration.
89
In the Brownian-pumping model, both characteristics (i-ii) result from the same underlying processes: the
90
rapid formation of metal-colloid surface site complexes (adsorption) and the slow coagulation of colloids with
91
filterable particles (Honeyman and Santschi 1989). More generally, the negative correlation between KD and
92
P could arise from at least two factors: (i) higher concentrations of Th-binding colloids, small enough to
93
pass through typical filters, may keep apparent Th concentrations high in the ‘dissolved’ phase, and (ii) the
94
surface area per mass of particles may decrease at greater particle mass, which would reduce the number of
95
available binding sites (Henderson et al. 1999; Pavia et al. 2018). Owing to their high specific surface area
96
and complexation capacity, colloidal particles may be of great importance in the cycling of particle-reactive
97
elements (e.g., Guo et al. (1997)), and laboratory studies suggested that they could significantly influence
98
Th scavenging in oceanic waters (e.g., Guo et al. (1997); Quigley et al. (2001, 2002)).
99
The 230 Th, 231 Pa, and particle data collected along GA03 indicated that particle concentration influences
100
solid-solution partitioning and sorption rates in North Atlantic waters (Hayes et al. 2015b; Lerner et al. 2017).
101
Hayes et al. (2015b) found that KD for 230 Th and 231 Pa generally decreased with particle concentration along
102
GA03. Lerner et al. (2017) derived estimates of the apparent first-order rate constant for Th adsorption onto
103
particles (k1 ) from an inversion of size-fractionated 228,230,234 Th and particle data gathered at eleven stations
104
east of Bermuda. They found that k1 generally decreases with depth, with the time scale 1/k1 averaging
105
1.0 yr in the upper 1000 m and (1.4 – 1.5) yr below. A positive relationship between k1 and P was found,
106
consistent with the notion that k1 increases with the number of surface sites available for adsorption (e.g.,
107
Honeyman et al. (1988)). In contrast to the influence of colloids as envisioned by the Brownian pumping
4
108
hypothesis, Lerner et al. (2017) provided evidence that the particle concentration effect may arise from the
109
joint effect of P on the rate constants for Th attachment to, and detachment from, particles.
110
Particle scavenging could be intensified at abyssal depths in the western North Atlantic due to the
111
presence of benthic nepheloid layers (BNLs) (Fig. 2). Vertical traces from nephelometers have long revealed
112
that BNLs are common near the western margin in depths greater than 3000 m where a clear water minimum
113
is typically found; in contrast to the western basins, the eastern basins generally show low turbidity (Eittreim
114
et al. 1969, 1976; Biscaye and Eittreim 1977). A recent synthesis of light scattering and attenuation data
115
illustrated that the particle load in excess of the concentration at the clear water minimum decreases eastward
116
in the western North Atlantic by up to one order of magnitude (Gardner et al. 2017). In line with this result,
117
particulate matter concentrations estimated from size-fractionated particle composition data featured intense
118
BNLs along GA03 in the western North American margin with P > 1000 mg m−3 , two to three orders of
119
magnitude higher than in surrounding waters (Lam et al. 2015). These observations suggest that intensified
120
scavenging due to high particle concentrations could contribute to the relatively low
121
the deep waters west of Bermuda.
5
230
Thd and
231
Pad in
PM (mg m-3) 0
0
50
100
150
200
1000
depth (m)
2000 3000 4000 5000 6000 122
123
Fig. 2. Regionally averaged vertical profile of particulate matter (PM) concentration in the western North
124
Atlantic. Each circle is an average based on (i) optical measurements converted empirically to PM
125
concentration and (ii) a linear interpolation at the same vertical levels of the concentration estimates
126
obtained from optical profiles at different stations. The horizontal bars show the standard errors of the
127
averages (data compilation from Gardner et al. (2017)).
128
Particle scavenging in the ocean seems to vary also with the chemical composition of particles (e.g.,
129
Anderson et al. (1983a); Chase et al. (2002); Geibert and Usbeck (2004); Scholten et al. (2005); Roberts
130
et al. (2009); Chuang et al. (2013); Lin et al. (2014)). A number of studies provided evidence for a role
131
of particle composition on the sorption of
132
(2005) reported that
133
is positively correlated with Mn, Ba, and lithogenic fractions and does not covary with CaCO3 or biogenic
134
silica (bSi). Radioisotope and particle data from section GA03 provided further insight into the effects
135
of particle type on Th and Pa scavenging in the North Atlantic (Hayes et al. 2015b; Lerner et al. 2018).
230
230
Th and
231
Pa in North Atlantic waters. Roy-Barman et al.
Th of particles collected by sediment traps deployed in the eastern North Atlantic
6
136
Hayes et al. (2015b) inferred enhanced scavenging by authigenic Fe and Mn (hydr)oxides in the form of
137
hydrothermal particles emanating from the Middle Atlantic Ridge (MAR) and particles resuspended from
138
reducing environments near the seabed off the West African coast. Biogenic silica was not found to be a
139
significant scavenging phase for Th and Pa, presumably because of its low abundance and small variance
140
at the sampled stations. Lerner et al. (2018) examined the dependence of their k1 estimates (below about
141
100 m) on particle composition (particulate organic C, CaCO3 , bSi, lithogenic material, and Fe and Mn
142
(hydr)oxides) at stations east of Bermuda. Particle composition was found to explain a larger fraction
143
of k1 variance for stations within the Mauritanian upwelling region (‘eastern stations’) than for stations
144
west of that region (‘western stations’). The k1 variance explained by particle composition was mainly due
145
to biogenic particles at the ‘eastern stations’ and to Mn oxides at the ‘western stations’. Interestingly, the
146
proportions of k1 variance explained by particle composition and particle concentration were not significantly
147
different (Lerner et al. 2018).
148
In addition to variations in particle scavenging intensity, ventilation of the deep basins by components 230
Thd and
231
149
of NADW has been postulated to impact the distributions of
Pad in the North Atlantic
150
(e.g., Moran et al. (1997); Vogler et al. (1998); Moran et al. (2002)). The Deep Western Boundary Current
151
(DWBC) in the western North Atlantic transports cold waters from the northern North Atlantic toward the
152
equatorial region (e.g., Schmitz and McCartney (1993)) and appears to act as a major conduit from which
153
deep layers in the North Atlantic are ventilated. The dynamical processes responsible for the ventilation of
154
deep basins from the western boundary are varied and complex, involving, e.g., recirculation gyres (e.g., Hogg
155
et al. (1986); Pickart and Hogg (1989)), mesoscale eddies (e.g., Pickart et al. (1997)), offshore entrainment in
156
the region where the DWBC and the Gulf Stream cross over (e.g., Bower and Hunt (2000a,b)), deep cyclones
157
accompanying meander troughs in the Gulf Stream path (e.g., Andres et al. (2016)), and stirring between
158
the boundary and the interior (e.g., Le Bras et al. (2017, 2018)).
159
Tracer measurements have shed considerable light on deep ventilation in the North Atlantic. Tritium
160
and excess helium-3 data indicated that the deep water in the subpolar gyre is ventilated by NADW on
161
time scales of about 10–15 yr (Doney and Jenkins 1994). Chlorofluorocarbon (CFC) data showed that deep
162
layers in the North Atlantic are renewed on decadal time scales by Labrador Sea Water (LSW) and Denmark
163
Strait Overflow Water (DSOW) originating from the western boundary (e.g., Rhein et al. (2015)). From
164
multiple tracer data collected along GA03, Holzer et al. (2018) estimated that abyssal waters have a mean
165
age ranging from 200 to 400 yr and are younger in the western basins. Overflow waters last ventilated in
166
the Arctic Ocean and in the Norwegian and Greenland Seas would contribute about 40% of waters present
167
in the western basins where ∼60% of these overflow waters are younger than 160 yr (Holzer et al. 2018).
168
Early measurements of
230
Thd in the Labrador Sea and the Denmark Strait presented relatively small 7
169
values compared to those observed at more southern locations in the Atlantic Ocean (Moran et al. 1995,
170
1997, 2002), suggesting that ventilation by LSW and DSOW would tend to decrease
171
North Atlantic. Similarly, recent measurements obtained on filtered seawater collected in the Labrador Sea,
172
Irminger Sea, Iceland Basin, and West European Basin along GEOTRACES section GA01 (Deng et al.
173
2018) showed
174
Ocean. The low
175
related to the presence of young (e.g., CFC-rich) waters, whilst enhanced scavenging of both nuclides near
176
the seafloor would also occur in overflow waters (Deng et al. 2018).
230
Thd and 230
231
Thd and
Thd in the deep
Pad activities that are lower than those measured further south in the Atlantic 231
Pad near the bottom in the Labrador Sea and Irminger Sea appeared to be
In summary, at least two processes could contribute to the relatively low
177
230
230
Thd and
231
Pad observed at
178
abyssal depths in the western North Atlantic: (i) intensified particle scavenging in thick or rapidly cycling
179
BNLs and (ii) ventilation by 230 Thd - and 231 Pad -poor components of NADW from the western boundary. In
180
this paper, we apply a regional ocean circulation model in order to study the effects of these two processes on
181
the distributions of both radionuclides. The model is configured to represent the western North Atlantic with
182
‘eddy-permitting’ (1/4◦ ) resolution and includes a description of 230 Th and 231 Pa cycling. In this description,
183
the rates of 230 Th and 231 Pa removal from solution are determined by a prescribed distribution of particulate
184
matter concentration based on a recent compilation of nephelometer and transmissometer data. Results from
185
the circulation-scavenging model are then compared to (i) physical oceanographic observations provided from
186
satellite altimetry and repeat sections between the New England continental shelf and Bermuda; and (ii)
187
radionuclide measurements obtained from (pre-)GEOTRACES cruises. Finally, numerical experiments are
188
performed with the model in order to evaluate the sensitivity of the simulated distributions of
189
190
231
230
Th and
Pa in both dissolved and particulate forms to (i) the intensity of particle scavenging, (ii) the strength of
the DWBC, and (iii) the radionuclide activities that are present in the DWBC.
191
Thorium-230 and protactinium-231 have been incorporated in ocean models in previous studies. Both
192
tracers have been included in three-dimensional (3D) models of global ocean circulation (Henderson et al.
193
1999; Siddall et al. 2005; Dutay et al. 2009; Rempfer et al. 2017; Gu and Liu 2017; van Hulten et al. 2018),
194
in a zonally-averaged circulation model (Marchal et al. 2000), in a 2D (latitude-depth) box model (Luo et al.
195
2010), and in 3D regional inverse models (Marchal et al. 2007; Burke et al. 2011). Most of these studies were
196
aimed at testing paleoceanographic applications of
197
230
230
Th and
231
Pa, more specifically the use of sediment
Th activity data to correct accumulation rates for the effects of sediment redistribution (Bacon 1984) and 231
Pa/230 Th ratio from Atlantic sediments as a paleocirculation indicator (Yu et al. 1996).
198
the use of the
199
The present work complements these earlier studies by isolating the relative influences of particle scavenging
200
and ocean circulation on radionuclide distributions in the western North Atlantic using a model with higher
201
horizontal resolution and a treatment of sorption kinetics that relates specific rates of adsorption to a particle
8
202
concentration field derived from optical data. On the other hand, our model does not account for fluxes
203
of radionuclides from the seafloor such as due to sediment resuspension (e.g., Rutgers van der Loeff and
204
Boudreau (1997)). Emphasis is placed on understanding water column measurements, although our results
205
may also have implications for the interpretation of sediment records of
230
Th and
231
Pa.
206
The remainder of this paper is organized as follows. In section 2, the physical, radionuclide, and optical
207
data sets used in this study are briefly described. Nephelometer and transmissometer data are converted
208
into estimates of particulate matter (PM) concentration using published calibrations, and an estimate of the
209
3D distribution of PM concentration in the study area is derived. The ocean model is presented in section 3.
210
The physical and geochemical components of the model are detailed, together with the conditions imposed
211
at the surface and at the open boundaries of the regional domain. In section 4, a particular model solution
212
is presented that shows relatively good agreement with physical oceanographic and radionuclide data. The
213
influences of particle scavenging intensity and of DWBC properties (strength and radionuclide contents) are
214
then illustrated in section 5 through a number of sensitivity experiments. Our results are discussed in section
215
6, with emphasis on the role of BNLs in particle scavenging. A summary and perspectives follow in section
216
7.
217
2. Data
218
a. Physical Data
219
This study relies on a number of physical datasets. Bathymetric data used to establish the model
220
bottom topography are obtained from ETOPO2 version 2 (ETOPO2v2 2006), a gridded product with 2-min
221
resolution and derived from satellite altimeter and shipboard sounding data. Temperature and salinity data
222
employed to constrain the initial and boundary conditions of the model (section 4 and appendix C) are
223
obtained from the 1/4◦ resolution climatology (decadal averages, objectively analyzed) of the World Ocean
224
Atlas (Locarnini et al. 2013; Zweng et al. 2013). Climatologic values of potential temperature are first derived
225
from the values of in situ temperature, salinity, and depth of the Atlas using the algorithm developed by
226
Jackett et al. (2004) and assuming 1 dbar = 1 m. Potential temperature and salinity data at model levels
227
are then obtained from their respective climatologic values by linear interpolation (and extrapolation, where
228
needed). Note that the longitudes and latitudes of model grid points are set to coincide with the longitudes
229
and latitudes of the climatologic grid, so no horizontal interpolation is necessary.
230
Two types of satellite data sets are used in this study. First, surface wind stresses used to provide the
231
mechanical forcing to the model come from the Scatterometer Climatology of Ocean Winds, SCOW (Risien
9
232
and Chelton 2008). The monthly means of zonal and meridional wind stresses, available at 1/4◦ resolution,
233
are averaged to produce an annual mean field. The annual mean wind stresses at a given model grid point are
234
then obtained from the closest values of the climatologic fields. Second, observations of dynamic topography
235
are used in order to test the modeled circulation. Monthly observations of sea surface height (SSH) between
236
1993–2012 are derived from satellite altimetry maps prepared by Ssalto/Duacs and distributed by Aviso+,
237
with support from CNES (https://www.aviso.altimetry.fr). These observations are averaged to produce
238
a multi-year annual mean field which is compared to model results.
239
Finally, horizontal velocity data from line W, between the New England continental shelf and Bermuda,
240
and coinciding with the western segment of GA03 (Fig. 3), are used to provide another test of the modeled
241
circulation. Velocity measurements along line W were obtained from shipboard deployments of Lowered
242
Accoustic Doppler Current Profilers and from moored profiler and current meters between 2004 and 2014
243
(Toole et al. 2017). The velocity data used in this study are 10-yr averages derived by averaging daily profiles
244
of subinertial-filtered velocity (note that the eastern-most mooring only contains data between 2008 and
245
2014, so only 6-yr averages are used for that mooring). The daily profiles were constructed by combining
246
moored profiler and current meter data from six moorings deployed along line W (John Toole, personal
247
communication).
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
248
249
Fig. 3. Map of the study area showing the location of GEOTRACES stations (red triangles with
250
numerals), pre-GEOTRACES stations (red triangles with letters), and nephelometer and transmissmeter
251
stations (circles). Black lines are isobaths of 200, 1000, and 3000 m, and blue arrows show schematic
10
252
pathways of the Gulf Stream (GS), Deep Western Boundary Current (DWBC), Northern Recirculation
253
Gyre (NRG), and Subtropical Gyre (SG). Also shown are the approximate locations of Bermuda (BER),
254
the New England Seamounts (NES), and the Sohm Abyssal Plain (SAP). The thin black line protruding
255
from the continental shelf and slope south of New England is line W.
256
257
258
b. Radionuclide Data The
230
Th and
231
Pa activity data considered in this study originate from both pre-GEOTRACES
programs and the GA03 section (Table 1).
11
Table 1. Thorium-230 and Protactinium-231 Data Used in this Study
259
#a
230
errorb
reference
0
1σ
Cochran et al. (1987)
19
0
2σ
Guo et al. (1995)
0
0
0
2σ
Luo et al. (2010)
19
0
0
0
2σ
Luo et al. (2010)
69.75◦ W
19
0
0
0
2σ
R. Fran¸cois (pers. com.)
32◦ N
64◦ W
19
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-2
37◦ N
69◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-3
33◦ N
69◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-4
36◦ N
68◦ W
10
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-5
38◦ N
70◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
GT11-01
39.69◦ N
69.81◦ W
25
10
25
10
1σ
Hayes et al. (2015a)
GT11-02
39.35◦ N
69.54◦ W
17
12
17
12
1σ
Hayes et al. (2015a)
GT11-03
38.67◦ N
69.10◦ W
20
12
20
12
1σ
Hayes et al. (2015a)
GT11-04
38.09◦ N
68.70◦ W
16
12
16
12
1σ
Hayes et al. (2015a)
GT11-06
37.61◦ N
68.39◦ W
20
12
21
12
1σ
Hayes et al. (2015a)
GT11-08
35.42◦ N
66.52◦ W
17
12
17
12
1σ
Hayes et al. (2015a)
GT11-10
31.75◦ N
64.17◦ W
28
12
28
11
1σ
Hayes et al. (2015a)
GT11-12
29.70◦ N
56.82◦ W
18
12
18
12
1σ
Hayes et al. (2015a)
GT11-14
27.58◦ N
49.63◦ W
21
12
21
11
1σ
Hayes et al. (2015a)
Thd
#
230
Thp
#
231
station
latitude
longitude
Pad
CMME-13
32.76◦ N
70.78◦ W
8
8
11
S1
36.05◦ N
74.43◦ W
11
10
EN407-3
39.47◦ N
68.37◦ W
11
EN407-4
38.6◦ N
68.89◦ W
EN407-6
39.73◦ N
BATS
#
231
Pap
260
261
262
263
264
a
number of observations.
b
σ is the standard error for data from R. Fran¸cois (pers. com.) and the standard
deviation for all other data.
Radionuclide data in both dissolved and particulate forms are used and displayed in Appendix D (Fig. A1–A7).
12
265
1) Thorium-230
266
Measurements of
230
Thd from pre-GEOTRACES cruises generally show an increase with depth in the
267
upper 2500–3000 m of the water column (Fig. A1). Below 2500–3000 m, stations sampled during these cruises
268
feature either uniform
269
only two studies reported
270
published
271
measurements from a station off Cape Hatteras in the Middle Atlantic Bight. The 230 Thp activities measured
272
at both stations generally increase with depth. The
273
230
230
230
Thd or a 230
230
Thd decrease with depth. To our knowledge, prior to GEOTRACES,
Thp measurements in the investigated region (Fig. A1). Cochran et al. (1987)
Thp data for a station in the Hatteras Abyssal Plain, and Guo et al. (1995) reported
230
Thp
Thd,p data collected along GA03 and used in this study have been described by Hayes et al. 230
274
(2015a), so only a brief overview is provided below. Broadly,
275
of the water column at all stations (Fig. A2). At the deepest stations, the
276
increase with depth down to 3000–4000 m, where vertical gradients of
277
are particularly noticeable at stations GT11-04 to GT11-10. At station GT11-12 (east of Bermuda),
278
below ∼ 4000 m tend to be more uniform compared to the activities measured at stations west of Bermuda.
279
Similarly to 230 Thd data, 230 Thp data from GA03 generally show an increase with depth (Fig. A3). Stations
280
GT11-06, GT11-10, and GT11-12 show indications of a reversal in the vertical
281
3000 m, with measurements presenting a local maximum in the deepest sample(s), although 230 Thp data from
282
GT11-10 and GT11-12 also display relatively large dispersion and (or) uncertainties. Particularly spectacular
283
are the extremely large
284
GT11-08. The
285
two orders of magnitude than
286
230
230
230
Thd increased with depth in the upper part
230
230
Thd data show a continued
Thd change sign. These inversions
230
230
Thd
Thp gradient near 2000–
Thp activities measured on four near-bottom samples at stations GT11-04 and
Thp activities in these samples approach or exceed 1 dpm m−3 , which is larger by one to 230
Thp activities measured at similar depths at other stations and larger than
Thd activities measured on the same or nearby samples at the same stations.
287
2) Protactinium-231
288
Only a few measurements of
231
Pa in the western North Atlantic existed prior to GEOTRACES (Fig.
231
Pad profiles at two locations at 38.5◦ N and 39.5◦ N near the western
289
A4). Luo et al. (2010) reported
290
boundary, which generally show an increase with depth down to 2000–2500 m and a decrease with depth
291
below. Similarly to 230 Thd,p data, the 231 Pad,p data obtained along GA03 and used in this study have already
292
been discussed (Hayes et al. 2015a). As for
293
upper part of the water column at all stations (Fig. A5). At the deepest stations, measurements of
294
present maxima between 2000–3000 m and a decrease with depth below. Measurements of 231 Pap display an
295
increase with depth down to 2000–3000 m and a decrease with depth below except for the deepest samples
230
Thd data,
13
231
Pad data show an increase with depth in the 231
Pad
230
296
(Fig. A6). As for
Thp , stations GT11-04 and GT11-08 show extreme
297
samples which largely exceed
298
c. Optical Data
231
Pap and
231
231
Pap values in near-bottom
Pad measured on the same or nearby samples.
299
A relatively large number of optical measurements from nephelometers and beam transmissometers are
300
available in the western North Atlantic (for a recent synthesis, see Gardner et al. (2017)). These instruments
301
can be used to measure the distribution of, respectively, light attenuation and light scattering with vertical
302
resolutions of approximately 2 m and 100-250 m, respectively (note that the range for the light scattering
303
resolution reflects the dependence of the resolution on the rate at which the nephelometer is lowered in
304
the water column;Gardner et al. (1985b)). From measurements of these optical properties, the vertical
305
distribution of PM concentration (P , mg m−3 ) can be estimated using empirical calibrations.
306
The following approach is applied to derive a field of PM concentration for use in the geochemical
307
component of the model. First, the light scattering and light attenuation data compiled by Gardner et al.
308
(2017, 2018b) are converted into P estimates using calibration formulae reported by Gardner et al. (2017)
309
for light scattering and Gardner et al. (2018b) for light attenuation. In addition, the light attenuation data
310
from GA03 and available from the GEOTRACES Intermediate Data Product (Schlitzer et al. 2018) are used
311
and converted to P estimates following Gardner et al. (2018b). Second, the PM concentrations at locations
312
where optical data are available are interpolated linearly at the model grid levels. The errors produced
313
by vertical interpolation are expected to be generally small given the high vertical resolution of the optical
314
profiles and are neglected in this work. Note that the model is based on terrain-following (s) coordinates, not
315
depth coordinates (section 3), so that the levels at which P values are interpolated occur along s-surfaces,
316
not depth surfaces. Finally, the interpolated P values are mapped along each s-surface using a minimum
317
variance (Gauss-Markov) procedure in order to obtain an estimate of PM concentration at each model grid
318
point along that surface (for details see Appendix A). Note that in this study the PM concentrations at
319
model grid points are fixed and not allowed to be transported by advection or mixing.
320
3. Ocean Model
321
a. Domain
322
The model domain considered in this study is the western North Atlantic north of 28◦ N and west of
323
55◦ W (Fig. 3). It is bounded in the west by the 200-m isobath, which is taken as a closed boundary. The
324
other two boundaries – the latitude of 28◦ N and the longitude of 55◦ W – are open. The model grid has a
14
325
horizontal resolution of 1/4◦ and includes 31 vertical levels. It is determined by a trade-off between a desire
326
to simulate the circulation with greatest detail possible and the large computational cost associated with
327
the simulation of steady-state distributions of
328
are s-coordinates defined from
230
Th and
s=
231
Pa (section 4). The vertical levels of the model
z−η . h+η
(1)
329
Here z is the local depth, η is the free surface elevation, and h = h(x, y) is the water depth where x and y
330
denote, respectively, the longitude and latitude coordinates in the domain. The model topography h(x, y)
331
is derived by averaging the bathymetric data from ETOPO2v2 (2006) in model grid cells.
332
b. Physical Component
333
The physical component of the model is the Princeton Ocean Model (POM) – a primitive-equation model
334
based on s-coordinates and a free surface (Blumberg and Mellor 1987; Mellor 2002). The computer code
335
applied in this study is pom2k.f. Note that in the present application, the variation of the Coriolis parameter
336
with latitude is taken into account according to f = fo + β(y − yo ), where fo = 2Ω sin φo , β = (2Ω/r) cos φo ,
337
Ω = 7.29 × 10−5 s−1 is Earth angular velocity, r = 6371 km is the Earth radius, φo = 36.375◦ N, and yo is the
338
value of y corresponding to the latitude φo . The basic equations of POM are the governing equations for the
339
zonal velocity component (u) and the meridional velocity component (v), the hydrostatic approximation, the
340
governing equations for (potential) temperature (T ) and salinity (S), and the condition of incompressibility.
341
These equations are complemented with a nonlinear equation of state (Mellor 1991).
342
c. Geochemical Component
343
The geochemical component of the model includes governing equations for
230
Th and
231
Pa in both the
344
dissolved phase and the particulate phase. The dissolved and particulate activities in the model are intended
345
to represent the dissolved and particulate activities as measured on water samples and defined operationally
346
by filtration. The basic equations of the geochemical component are ∂Ad + u · ∇Ad ∂t ∂Ap ∂Ap + u · ∇Ap + wp ∂t ∂z 230
Th or
= = 231
∂ ∂Ad λAπ + k−1 Ap − k1 Ad + κT + Fd , ∂z ∂z ∂ ∂Ap k1 Ad − k−1 Ap + κT + Fp . ∂z ∂z
(2a) (2b)
Pa in dissolved (particulate) form expressed in dpm m−3 , u is
347
Here Ad (Ap ) is the activity of
348
the fluid velocity, wp is the particle settling speed, λ is the radioactive decay constant for
349
Aπ is the activity of the radioactive parent (234 U for
15
230
Th and
235
U for
231
230
Th or
231
Pa,
Pa), k1 (k−1 ) is a first-order
350
apparent rate constant for Th or Pa adsorption onto particles (Th or Pa desorption from particles), κT
351
is a vertical turbulent diffusivity, and the terms (Fd , Fp ) denote the effects of horizontal mixing. Clearly,
352
t is time and ∇ = x ˆ(∂/∂x) + yˆ(∂/∂y) + zˆ(∂/∂z) is the gradient operator where (ˆ x, yˆ, zˆ) are unit vectors.
353
Vertical and horizontal mixing processes are represented as for temperature and salinity: the diffusivity
354
κT is obtained from a turbulent closure scheme (Mellor and Yamada 1982) which involves the solution of
355
governing equations for turbulent kinetic energy (q 2 /2) and for q 2 l, where l is a turbulence length scale, and
356
the terms (Fd , Fp ) are parameterized according to Smagorinsky (1963).
357
Note that the radioactive decay terms are omitted in (2a-2b). The half-lives of
230
Th and
231
Pa exceed
358
by several orders of magnitude the time scales of solid-solution equilibration of Th and Pa in laboratory
359
experiments (e.g., Nyffeler et al. (1984); Geibert and Usbeck (2004)). Likewise, the value of λ = 9.17 × 10−6
360
yr−1 for
361
in oceanic waters (for a compilation see Marchal and Lam (2012); Lerner et al. (2017)).
230
Th is very small compared to observational estimates of k1 and k−1 of ≥ O(10−1 yr
−1
) for Th
362
The influence of particle concentration on particle scavenging is accounted for in the model as follows.
363
Both theoretical considerations (e.g., Honeyman et al. (1988)) and empirical evidence (e.g., Lerner et al.
364
(2017)) suggest that the apparent rate constant for metal adsorption onto particles, k1 , increases with the
365
(bulk) concentration of particles. In our model, k1 is set to vary with particle concentration according to k1 (x, y, z) = k1,b + k10 P (x, y, z).
(3)
366
Here P (x, y, z) is the particle concentration estimated from the optical measurements (Appendix A), k10
367
describes the variation of the adsorption rate constant with particle concentration, and k1,b is a background
368
value intended to account for the effect of particles that are not detected by the optical instruments (for a
369
discussion of instrumental sensitivities, see, e.g., McCave (1986) and Boss et al. (2009)).
370
Three comments are in order regarding the geochemical component of the model. First, in a more
371
general treatment, k1 would be a function of P b where the exponent b could be different from one. From
372
a compilation of k1 estimates reported in the literature, Honeyman et al. (1988) showed that k1 appears
373
to be proportional to P b with b = 0.51 (their Table 2). These authors argued that such a variation of
374
the adsorption rate constant with particle concentration could explain the particle concentration effect. In
375
contrast, in their analysis of Th isotope and particle data collected east of Bermuda along GA03, Lerner
376
et al. (2017) estimated that b ranges from 0.95 to 1.62 (their Table 4), depending on the technique used to
377
regress lnk1 against lnP and on the application of a smoothing condition on the vertical distribution of the
378
rate parameters. Consistent with the latter result, a value of b = 1 is assumed in this study.
379
Second, in contrast to k1 , the desorption rate constant (k−1 ) and the particle settling speed (wp ) are
380
assumed to be independent of particle concentration or any other particle (or water) property. The k−1 16
381
values estimated from Th isotope and particle data at GA03 stations east of Bermuda (Lerner et al. 2017)
382
do not exhibit a consistent vertical trend, except at four stations where k−1 appears to decrease with depth.
383
Near-surface values of k−1 estimated at these stations are also high relative to the other stations. The wp
384
values estimated by Lerner et al. (2017) tend to be larger and display enhanced vertical variability below
385
2000 m, except at the TAG hydrothermal vent of the MAR, where their wp estimates are low relative to
386
the other stations. In the present study, k−1 and wp are taken as uniform throughout the model domain for
387
simplicity. Finally, the rate parameters (k1,b , k10 , k−1 ) are allowed to be different for
388
230
Th and
231
Pa in the model
389
in order to account for the differences in the sorption kinetics of Th and Pa in oceanic waters (e.g., Moran
390
et al. (2002); Hayes et al. (2015a)). The particle settling speed, wp , is also allowed to be different for the two
391
nuclides, a treatment suggested by the fact that (i) Th and Pa may be carried by different particulate phases
392
in the ocean (e.g., Chase et al. (2002); Geibert and Usbeck (2004); Roberts et al. (2009); Chuang et al.
393
(2013)), and (ii) different particles types may have different sinking speeds (e.g., biogenic silica, lithogenic
394
material, and calcium carbonte may sink much more rapdily than particulate organic carbon (Armstrong
395
et al. 2001; Klaas and Archer 2002)). The value of wp for
396
least-squares) the equation
397
230
230
Thp (z) =
230
230
Th is determined by fitting (using ordinary
Thp (0) + λ234 Th-230 Uz/wp to a composite profile compiled from all
Thp data in the upper 3000 m for stations GT11-01 to GT11-12 (not shown). This popular approach to 230
398
constrain wp (Th) from
399
Rutgers van der Loeff and Berger (1993); Scholten et al. (1995); Venchiarutti et al. (2008)) yields an estimate
400
of wp (Th) of 1800 m yr−1 , the value used in our simulations. In contrast, the value of wp for
401
generally less sensitive to particles and thus more strongly influenced by circulation than 230 Th, is determined
402
by a trial-and-error approach until good agreement with
403
for
404
231
230
Thp data (e.g., Bacon and Anderson (1982); Krishnaswami et al. (1976, 1981);
231
231
Pa, which is
Pad,p data is achieved. Thus, the value of wp
Thp is determined by disregarding the effects of advection and mixing, whereas the value of wp for
Pap is determined by taking such effects into account.
405
A trial-and-error approach is also used to determine the value of the other geochemical parameters with
406
two exceptions: k1,b for Th is fixed to 0.4 yr−1 , a value consistent with k1 estimates for GA03 stations east
407
of Bermuda in waters with low particle concentration (Lerner et al. 2017), and k1,b for Pa is fixed to a lower
408
value, 0.04 yr−1 , consistent with the different particle sensitivities of Pa and Th in seawater (e.g., Moran
409
et al. (2002); Hayes et al. (2015a)). Values for each parameter in the geochemical component of the model
410
are listed in Table 3.
17
411
d. Boundary Conditions
412
The conditions imposed at the horizontal and lateral boundaries of the model domain are summarized
413
below (for details see appendices B-C). Consider first the horizontal boundaries. At the sea surface, the
414
model is forced with an annual mean wind field derived from satellite scatterometry (Risien and Chelton
415
2008), and the simulated values of temperature and salinity in the surface layer are restored to annual mean
416
fields from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013) (section 2a). The restoring
417
approach implies that the surface (T, S) fields simulated by the model will not depart too markedly from the
418
climatologic fields – a desirable result – with the caveat that the modeled variability due to eddy activity
419
will be muted to some degree. At the bottom, a shear stress condition is applied and a condition of no flux
420
is specified for
230
Thd,p and
231
Pad,p .
421
Consider then the lateral boundaries. The western boundary (aligned with the 200-m isobath) is a
422
closed boundary. This treatment implies that the model simulations presented in this paper will not rep-
423
resent the exchanges of momentum, energy, and material between the continental shelf and the continental
424
slope (e.g., Garvine et al. (1988); Lozier and Gawarkiewicz (2001); Gawarkiewicz et al. (2004); Churchill
425
and Gawarkiewicz (2009)). At the other two lateral boundaries (along 28◦ N and along 55◦ W), radiation
426
conditions are specified in order to minimize the reflection of perturbations generated within the domain.
427
Along specific segments of these boundaries, inflows and outflows representing the Gulf Stream, the DWBC,
428
and Sargasso Sea water entering or exiting the domain are also applied, following previous regional model
429
studies of the western North Atlantic (e.g., Thompson and Schmitz (1984); Ezer (2016a,b)). Note that, at
430
the DWBC inflow, the boundary values of
431
the Labrador Sea, except for
432
reference solution that more closely agreed with the available radionuclide measurements. At all other lo-
433
cations along the open boundaries,
434
which is situated to the east of the model domain (Appendix C).
435
e. Method of Solution
231
230
Th and
231
Pa are based on measurements in deep waters in
Pad whose value exceeds these measurements, which we found produced a
230
Th and
231
Pa values are based on measurements at station GT11-14,
436
The differential equations of the model are solved as follows (for details see Blumberg and Mellor (1987);
437
Mellor (2002)). First, the differential equations are expressed in terrain-following (s) coordinates. The
438
particle settling speed wp , which appears in equation (2b), is converted into an equivalent speed in s-
439
coordinates using the relationship, ωp = wp cos θ,
18
(4)
440
where θ is the angle between the vertical axis and the normal to the s-surface. Here cos θ is approximated
441
from cos θ = p
1 (∂z/∂x)2s + (∂z/∂y)2s + 1
,
(5)
442
where the partial derivatives are evaluated using central differences. Second, the dynamical equations of
443
POM are separated into equations for slow motions (internal mode) and fast motions (external mode). This
444
‘mode splitting’ technique allows the calculation of the free surface elevation at relatively small computational
445
cost. Finally, the differential equations represented in s-coordinates are solved using finite differences on a
446
staggered C grid. The advection terms in the governing equations for (T, S, Ad , Ap , q 2 /2, q 2 l) are solved using
447
a central difference scheme, and these equations are integrated forward in time using a leap frog scheme with
448
an Asselin filter. The equations for the external mode are integrated with a time step of 15 s and the
449
equations for the internal mode, including the activity equations (2a–2b), are integrated with a time step of
450
450 s. Unless stipulated otherwise, the model parameters take the values listed in Tables 2–3.
19
Table 2. Parameters of the physical model component
451
Physical Parameters
value
units
ρo
reference density
1025
kg m−3
g
acceleration due to gravity
9.806
m s−2
C
Smagorinsky coefficient
0.2
1
κ
von K´arm´an constant
0.4
1
κu,o
background vertical viscosity
0
m2 s−1
κT,o
background vertical diffusivity
0
m2 s−1
Pr
turbulent Prandtl number
5
1
zr
bottom roughness parameter
0.01
m
τ
restoring time scale for SST and SSS
14.4
d
value
units
452
Numerical Parameters
453
454
∆tE
time step for external mode
15
s
∆tI
time step for internal mode
450
s
∆s
step interval for advective termsa
5
1
hmax
maximum depth in radiation condition
200
m
umax
maximum velocity for CFL violation
100
m s−1
c
constant of Asselin filter
0.1
1
α
weight for sea surface slope termb
0
1
a
Step interval during which the advective terms of the external mode are not updated
b
Weight used in the external mode equations 20
Table 3. Parameters of the geochemical model component
455
λTh-230
radioactive decay constant of
230
λPa-231
radioactive decay constant of
231
Th Pa
value
units
9.17 × 10−6
yr−1
2.12 × 10−5
yr−1
234
U
activity of
234
U
2750
dpm m−3
235
U
activity of
235
U
108
dpm m−3
k1 (Th)
adsorption rate constant for Th
variable
yr−1
k1 (Pa)
adsorption rate constant for Pa
variable
yr−1
k1,b (Th)
background value of k1 (Th)
0.4
yr−1
k1,b (Pa)
background value of k1 (Pa)
0.04
yr−1
k10 (Th)
sensitivity of k1 (Th) to particle concentration
0.04
yr−1 mg−1 m3
k10 (Pa)
sensitivity of k1 (Pa) to particle concentration
0.02
yr−1 mg−1 m3
k−1 (Th)
desorption rate constant for Th
3.69
yr−1
k−1 (Pa)
desorption rate constant for Pa
18.45
yr−1
1800
m yr−1
2400
m yr−1
456
457
wp (Th)
settling speed of
230
wp (Pa)
settling speed of
231
Thp Pap
4. Reference Solution
458
A large number of experiments have been conducted with the model in an effort to reproduce the
459
circulation and the radionuclide distributions observed in the western North Atlantic. In this section, we
460
describe a particular model solution, obtained with parameter values listed in Tables 2–3, which shows
461
relatively good agreement with the observations. This solution is called the ‘reference’ solution below. The
462
word ‘reference’ is meant to imply that this solution is used as a pivot against which results from other
463
simulations are compared, not that it is the most accurate simulation that could be obtained from the
464
model.
465
The model is integrated forward in time until the circulation and the radionuclide distributions reach
466
quasi-steady states. The initial conditions of the model are the following: (i) the ocean is at rest (u = 0,
467
q 2 = 0, and q 2 l = 0) with a flat surface (η = 0), (ii) the (T, S) distributions are set to the annual mean
468
distributions from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013), and (iii) the
21
230
Thd,p
231
469
and
Pad,p distributions are specified from idealized vertical profiles which broadly reproduce data from
470
station GT11-14 (located east of the domain along GA03) and which are also used as lateral boundary
471
conditions (Appendix C; Fig. A7). The model is first integrated diagnostically for a period of 10 days, with
472
(T, S, 230 Thd,p , 231 Pad,p ) fixed to their initial values, and then prognostically for a period of 2545 days (∼
473
7 yr), with (T, S, 230 Thd,p , 231 Pad,p ) allowed to vary according to their respective governing equations. At
474
this time, the domain averages of kinetic energy ρo |u2 |/2,
475
values (Fig. 4).
Thd,p , and
231
Pad,p have reached quasi-steady
Mean kinetic energy (m2/s2)
230
Mean
Mean
230
230
Thp (dpm m-3)
Thd (dpm m-3)
days
days
476
477
478
Pap (dpm m-3) Mean
Mean
231
231Pa
d
(dpm m-3)
days
days
days Fig. 4. Time series of the domain-averaged kinetic energy,
230
reference solution
22
Thd ,
230
Thp ,
231
Pad , and
231
Pap in the
230
Notice that the integration time needed to attain quasi-steady state is longer for
480
than it is for the mean kinetic energy (∼ 1 yr). Hence the model results reported in this paper are averages
481
for the last 2190 days (6 yr) of the integration for sea surface elevation and velocities, and for the last 365
482
d (1 yr) of the integration for
483
a. Circulation
230
Thd,p and
231
Th and
231
479
Pa (∼ 6 yr)
Pad,p .
484
1) Sea Surface Elevation
485
The distribution of the sea surface elevation simulated by the model is compared to the distribution of
486
the multi-year annual mean SSH derived from satellite altimetry (Fig. 5).
23
mean elevation (1993-2012)
CH
model elevation
CH
487
488
Fig. 5. Averages of sea surface height (m) as observed from satellite altimeter data during the period
489
1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf
490
Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
491
The SSH distribution from satellite altimeters reveals the average pathway of the Gulf Stream during the
24
492
observation period (1993–2012). Maxima in SSH occur just south of the Gulf Stream, downstream of the
493
region where this separates from the margin near Cape Hatteras, and minima in SSH occur in the region
494
between the Gulf Stream and the continental slope east of Cape Hatteras. The northward depression in SSH
495
across the Gulf Stream is of the order of 1 m and occurs on an horizontal scale of O(100 km). The pattern
496
of sea surface elevation simulated by the model is broadly consistent with the observed pattern, although
497
the magnitude of the SSHs and the gradients of SSH across the Gulf Stream as observed by altimeters are
498
generally underestimated by the model. The relatively small SSH gradients in the simulation implies that the
499
strength of the Gulf Stream as predicted by the model may be lower than observed, an inference consistent
500
with a comparison with velocity data along line W (see next section).
501
502
The standard deviations of SSH values derived by satellite altimetry are compared with the standard deviations of sea surface elevation calculated by the model (Fig. 6).
25
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
503
504
Fig. 6. Standard deviation of sea surface height (m) as observed from satellite altimeter data during the
505
period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the
506
Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
507
As expected, the standard deviations observed by altimeters show the largest values along the average
26
508
pathway of the Gulf Stream, which reflects the meandering and eddy activity of the Gulf Stream. Maxima
509
in standard deviations are also predicted in the vicinity of the Gulf Stream by the model, although the
510
standard deviations from the model are in general strongly underestimated compared to those observed.
511
Eddy variability is thus underpredicted by the model. Model-data differences in SSH (mean and standard
512
deviation) could be ascribed to various factors, such as insufficient spatial resolution of the model, the
513
restoring of surface (T, S) to climatologic fields, and the fact that model averages (6 yr) and data averages
514
(20 yrs) are not calculated over the same time span.
515
2) Horizontal Velocity
516
The horizontal velocities simulated by the model in the surface layer and at a depth of 3500 m are
517
displayed in Figure 7.
27
518
519
520
Fig. 7. Field of horizontal velocity in the surface layer (top) and at a depth of 3500 m (bottom) simulated
521
in the reference solution. The horizontal arrow at the lower right outside each panel is the maximum speed
28
522
in units of m s−1 in the corresponding field.
523
In the surface layer, the model simulates a strong current – the Gulf Stream – which flows northward along
524
the margin. The simulated current detaches from the margin north of Cape Hatteras, i.e., too far to the north
525
compared to the observations (Fig. 5), and then flows to the northeast as a meandering structure. Inaccurate
526
Gulf Stream separation has been a long-standing problem with many models, although simulations generally
527
improve when high resolution is used (e.g., Ezer (2016b); and references therein). The largest speeds in the
528
modeled Gulf Stream reach O(1 m s−1 ), which is consistent with observations from extensive field programs
529
(e.g., Meinen and Luther (2016)). At a depth of 3500 m, the model predicts a relatively strong current
530
flowing to the southwest along the boundary – the Deep Western Boundary Current. The modeled DWBC
531
presents distinct cores at some locations but remains generally noticeable all along the western boundary.
532
533
The simulated horizontal velocities between the New England continental shelf and Bermuda are compared to the velocities observed during the line W program (Fig. 8).
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
a
b
c
d
depth (m)
Model
534
535
Fig. 8. Distribution of horizontal velocity components between the New England continental shelf and
536
Bermuda as measured during the line W program (top) and as simulated in the reference solution
537
(bottom). At the top of each panel, red vertical lines show the position of GA03 stations GT11-01 to
538
GT11-06, and grey vertical lines show the position of mooring locations. Coordinates along the horizontal
29
539
axis are distances from 40.125◦ N,70.125◦ W (line W data from Toole et al. (2017)).
540
The observed velocity components both parallel and perpendicular to line W show the largest magnitudes in
541
the vicinity of the Gulf Stream, i.e., at a distance of ≥ 200 km from the shelf break. The Gulf Stream is the
542
most conspicuous feature along the section, showing flow to the northeast with velocity maxima in the upper
543
∼ 1000 m. Below the Gulf Stream and along the continental rise is the Deep Western Boundary Current,
544
which flows to the southwest and is characterized by much lower vertical shears and speeds compared to the
545
Gulf Stream. In accordance with these observations, the model simulates strong currents in the upper ∼
546
1000 m and along the western boundary. Whereas the simulated speeds in the Gulf Stream and DWBC have
547
the observed orders of magnitude, the horizontal velocities in the Gulf Stream (DWBC) are underestimated
548
(overestimated) by the model compared to the observations. Moreover, both currents in the model simulation
549
occur north of their respective observed locations. Again, model errors such as due to too coarse spatial
550
resolution and surface thermodynamical forcing may contribute to the differences between the simulation
551
and the observations.
552
b. Radionuclide Activities
553
We compare in this section the distributions of 230 Thd,p and 231 Pad,p calculated in the reference solution to
554
those observed from pre-GEOTRACES campaigns and along the western segment of GA03. Model results are
555
compared with both (i) station-averaged profiles of 230 Thd,p and 231 Pad,p computed by linearly interpolating
556
all radionuclide data at the same vertical levels and averaging the interpolated data along each vertical
557
level, and (ii) measured radionuclide profiles at individual stations. The rationale for considering both (i)
558
and (ii) is that a model driven by climatologic forcing may better reproduce station-averaged profiles than
559
profiles measured at specific locations and specific times. Notice that in the comparison between measured
560
and simulated radionuclide activities, the extreme
561
stations GT11-04 and GT11-08 (Figs. A3 and A6) are excluded.
562
The station-averaged profiles of
230
Thd and
231
230
Thp and
231
Pap values measured on two samples at
Pad display a reversal in activity gradients at mid-depth
563
(Figs. 9 and 10), as expected from the inspection of the profiles measured at the individual stations (Figs.
564
A1-A2 and Figs. A4-A5). Note also the relatively high
565
levels, below 4500 m. These averages are based on data from a relatively small number of stations and are
566
strongly influenced by data from station GT11-12, situated at > 500 km to the southeast of Bermuda (Fig.
567
3). Notably, the 230 Thd activities measured below ∼ 4500 m at station GT11-12 exceed 0.8 dpm m−3 , which
568
is larger by a least a factor of two than the
569
stations of GA03 (Fig. A2). The deepest value of 230 Thd shown in figure 9 comes from station GT11-12 only,
230
230
Thd and
231
Pad averages at the three deepest
Thd activities measured below ∼ 4500 m at the other deep
30
570
whilst the two values directly above are averages of data from GT11-12 as well as a few other stations. As a
571
result, the deepest value
572
by large standard errors (Fig. 9). Speculatively, the much higher 230 Thd and 231 Pad activities measured near
573
the bottom at GT11-12 compared to other stations of GA03 stem from the much weaker BNL at GT11-12,
574
as revealed by the sections of the beam attenuation coefficient for particles (Fig. 3 of Hayes et al. (2015a))
575
and of the bulk particle concentration (Fig. 5 of Lam et al. (2015)).
230
Thd is high, and the other two values are also relatively high and characterized
31
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th 576
230
Fig. 9. Profile of station-averaged
578
and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles
579
580
Thd (top) and
230
577
Thp (bottom) as calculated from pre-GEOTRACES
show averages of measurements from several stations with the following exceptions: for shallowest circle is a measurement from a single station (OC278-5), and for
32
230
230
Thd the
Thd,p the deepest circle is a
581
582
583
measurement from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230
measurements; Table 1). The extreme values of
230
Thd measurement and the deepest
230
Thd,p
Thp near the bottom of stations GT11-04 and GT11-08
are excluded from the station-averaged profile of the measurements.
depth (m)
584
d
(dpm m-3)
depth (m)
231Pa
585
586
587
231Pa Fig. 10. Profile of station-averaged
231
p
(dpm m-3)
Pad (top) and
231
Pap (bottom) as calculated from
pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution 33
588
589
590
591
592
(black line). The circles show averages of measurements from several stations with the following exceptions: for
231
Pad (231 Pap ), the three (four) deepest circles show measurements from a single station
(GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
231
Pap near the bottom
of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
593
1) Distribution of
230
594
The vertical profile of
230
Th
Thd obtained by averaging the available data in the study area is reproduced 230
595
rather closely in the reference solution (Fig. 9). The solution shows both the
596
in the upper ∼ 3000 m and the
597
holds over most of the water column, except for the three deepest observations. In contrast to
598
and consistent with the observed averages, the station-averaged
599
generally downwards over the entire ocean depth, although the observed
600
by the model (Fig. 9).
230
Thd increase with depth
Thd decrease below. The good agreement of the model with the data
230
230
Thd ,
Thp simulated by the model increases 230
Thp are generally overestimated
601
Similarly to the station averages, the simulated 230 Thd and 230 Thp profiles compare in general favourably
602
with the observed profiles at individual stations (Figs. A8 and A10). Noticeable differences with the
603
observations occur at pre-GEOTRACES station S1 (Guo et al. 1995) where
604
m of the water column are underestimated (Fig. A8), and at station GT11-12 where
605
3000 m are also underpredicted (Fig. A9). The reference solution generally overestimates
606
stations, particularly (i) below ∼ 2000 m at stations GT11-03, GT11-04, GT11-06, and GT11-08 and (ii)
607
over most of the water column at the deeper stations GT11-10 and GT11-12 (Fig. A10). Thus, whereas the
608
reference solution captures reasonably well the station-averaged profiles of
609
Atlantic, it is less successful at explaining the observations made at individual stations.
610
611
230
230
Thp data in the upper 1000 230
Thd data below ∼ 230
Thp at GA03
Thd,p in the western North
For future reference, we provide measures of the (dis)agreement of the reference solution with activity measurements from pre-GEOTRACES campaigns and the western segment of GA03 (Fig. 11).
34
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3 230
0.4
0.6
0.7
0.04
-0.02 -0.02
0.8
10
(dpm p data
0.05
3.5
Thp data (dpm/m 3)
0.1
0.04
0.06
0.1
m-3)
0.12
0.14
-3
3 2.5 2
230
m-3)
0.15
231Pa
Thd data (dpm/m 3)
0.2
0.08
Thp model (dpm/m 3) 230Th230model (dpm p
r2 = 0.11 p<0.01 n = 83
4
230
(dpm m-3) d data 231Pa
4.5
r2 = 0.81 p <0.01 n = 167
0.25
0.02
(dpm m-3) 5
0.3
0
Thd model (dpm/m 3)
230Th model d
0.35
0.5
0.06
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
0.12
(dpm m-3)
0.8
1.5 1 0.5 0 0
0 0
612
0.05
0.1
231Pa
0.15
0.2
-3) Thd model (dpm/m 3m ) (dpm d model
0.25
0.5
0.3
1
1.5
2
231Pa
230
230
2.5
3
3.5
Thp model (dpm/m 3)
-3 p model (dpm m )
4
4.5
5 10
-3
613
Fig. 11. Scatter plots of measured radionuclide activities versus simulated radionuclide activities in the
614
reference solution. Shown in each panel are the squared Pearson correlation coefficient (r2 ), the p value of
615
the correlation, and the number of measurements (n). In each panel, the black line is the line of perfect
616
agreement. For panel (b) and (d), the extreme measured values of
230
Thp and
231
Pap near the bottom of
617
stations GT11-04 and GT11-08, along with the corresponding model values, are excluded from the scatter
618
plot. 230
619
We find that the reference solution accounts for 85% of the variance in the
620
and for 49% of the variance in the
621
simulated activities is very significant for both the dissolved phase and the particulate phase (p < 0.01). The
622
root mean square difference between the measured and simulated activities amounts to 0.078 dpm m−3 for
623
624
230
Thd and to 0.028 dpm m−3 for
230
230
Thd measurements (n = 254)
Thp measurements (n = 98). The correlation between measured and
Thp (Table 4).
Table 4. Root Mean Square Difference Between Observed & Simulated Activitiesa
35
230
n
Thd
230
Thp
231
Pad
231
Pap
238
100
161
84
reference solution
0.078
0.028
0.039
0.001
k10 / 2
0.086
0.025
0.041
0.001
k10 × 2
0.086
0.037
0.037
0.003
DWBC inflow = 10 Sv
0.081
0.030
0.040
0.002
DWBC inflow = 40 Sv
0.076
0.029
0.037
0.002
Ad,p (DWBC inflow) / 2
0.078
0.025
0.033
0.001
Ad,p (DWBC inflow) × 2
0.165
0.041
0.110
0.002
uniform k1 (T h) & k1 (P a)
0.075
0.043
0.041
0.002
Ad,p (DWBC inflow) / 2, DWBC inflow = 10 Sv
0.093
0.027
0.041
0.002
625
626
a
All values in dpm m−3
627
2) Distribution of
628
The station-averaged
231
231
Pa
Pad for (pre-)GEOTRACES data are broadly reproduced in the reference solu-
629
tion in the upper 3000 m of the water column, but they are overpredicted at greater depths except near 5000
630
m (Fig. 10). The reversal in the vertical
631
m) than in the observations (∼ 3000 m). As for
632
data tend to be overestimated at most levels in the reference solution, except between about 2000–3000 m
633
where the simulation agrees closely with the observational averages. Note that the station-averaged profiles
634
of 231 Pad,p are largely unaltered if wp for 231 Pap is decreased from its reference value of 2400 m yr−1 to 1800
635
m yr−1 , the value used for
636
agreement with the measured
230
231
Pad gradient occurs deeper in the model simulation (at ∼ 4000 231
Pad , the station-averaged
Thp (not shown). The simulated 231
231
231
Pap computed from GA03
Pad profiles show in general the closest
Pad profiles at individual stations in the upper ∼ 3000 m of the water 36
637
column (Figs. A11 and A12). Below this depth, the model overestimates the 231 Pad measurements, with the
638
notable exception of station GT11-12, where the observed 231 Pad are rather closely reproduced below 3000 m
639
and underestimated between about 2000–3000 m. Inspection of
640
that the largest differences between the simulated and observed
641
GT11-03 to GT11-08 and in the upper ∼ 2000 m at station GT11-10 (Fig. A13). 230
642
As for
643
surements of
644
231
231
Pap profiles at individual stations shows
231
Pap occur near the seafloor at stations
Thd,p , we quantify the (dis)agreement of the reference solution with (pre-)GEOTRACES mea231
Pad,p activities (Fig. 11). The reference solution “explains” 81% of the variance in the
Pad measurements (n = 167) and only 11% of the variance in the
231
Pap measurements (n = 83). The
645
correlation between measured and simulated activities is very significant, even for the particulate phase
646
(p < 0.01). The root mean square difference between the simulated and observed activities amounts to 0.039
647
dpm m−3 for
231
Pad and to 0.001 dpm m−3 for
231
231
Pap (Table 4).
Pa/230 Th
648
3) Comparison to Sediment
649
The enhanced deposition of particle-reactive substances in ocean-margin sediments is often referred to as
650
‘boundary scavenging’ (Spencer et al. 1981). Anderson et al. (1994) found that deposition rates of 230 Th and
651
231
Pa measured during the SEEP-I and SEEP-II programs in the Middle Atlantic Bight exceed their local
652
rates of supply, consistent with boundary scavenging. However, they also found that the 231 Pa/230 Th activity
653
ratios of surface sediments are consistently less than the
654
odds with the notion that
655
is generally less prone to particle scavenging than
656
its production site). The authors postulated that the anomalous boundary scavenging of 230 Th and 231 Pa in
657
the Middle Atlantic Bight could be explained by the export from the region of fine-grained Mn-rich particles
658
that would scavenge greater portions of
231
231
Pa/230 Th production ratio of 0.093, which is at
Pa should be preferentially deposited over
231
230
230
Th in marginal sediments (231 Pa
Th and thus more likely to be transported away from
Pad than of
230
Thd .
659
In this section, the distribution of 231 Pap /230 Thp near the bottom (deepest grid point) which is simulated
660
in the reference solution is compared to surface sediment 231 Pa/230 Th data which are available in the western
661
North Atlantic (Table 5).
37
Table 5. Surface sediment
662
231
Pa/230 Th data in the western North Atlantic.
231
Pa/230 Th
core
latitude
longitude
depth (m)
reference
OCE152-BC1
39.49◦ N
70.57◦ W
1126
0.082
Anderson et al. (1994)
OCE152-BC8
32.47◦ N
70.58◦ W
1596
0.071
Anderson et al. (1994)
OCE152-BC9
39.42◦ N
70.55◦ W
1981
0.091
Anderson et al. (1994)
OCE152-BC5
39.08◦ N
70.56◦ W
2691
0.063
Anderson et al. (1994)
EN123-BC4
39.48◦ N
70.56◦ W
1280
0.076
Anderson et al. (1994)
EN123-BC6
39.49◦ N
70.55◦ W
1643
0.066
Anderson et al. (1994)
EN123-BC3
39.35◦ N
70.55◦ W
2344
0.061
Anderson et al. (1994)
EN123-BC1
39.08◦ N
70.55◦ W
2736
0.053
Anderson et al. (1994)
EN179-BC5
37.38◦ N
74.13◦ W
384
0.127
Anderson et al. (1994)
EN179-BC2
37.37◦ N
74.10◦ W
892
0.050
Anderson et al. (1994)
EN179-BC3
37.38◦ N
74.09◦ W
1031
0.075
Anderson et al. (1994)
EN179-BC4
37.32◦ N
74.02◦ W
1318
0.071
Anderson et al. (1994)
EN179-BC7
37.25◦ N
73.49◦ W
1989
0.051
Anderson et al. (1994)
EN187-BC4
37.37◦ N
74.13◦ W
512
0.063
Anderson et al. (1994)
EN187-BC10
36.52◦ N
74.37◦ W
580
0.089
Anderson et al. (1994)
EN187-BC8
36.52◦ N
74.34◦ W
1020
0.053
Anderson et al. (1994)
EN187-BC5
37.37◦ N
74.10◦ W
1045
0.069
Anderson et al. (1994)
EN187-BC11
37.02◦ N
74.34◦ W
1125
0.062
Anderson et al. (1994)
EN187-BC9
36.52◦ N
74.34◦ W
1165
0.075
Anderson et al. (1994)
EN187-BC6
37.24◦ N
73.5◦ W
2000
0.055
Anderson et al. (1994)
OCE325-GGC5
33.7◦ N
57.6◦ W
4550
0.054
McManus et al. (2004)
VM26-176
32.76◦ N
70.78◦ W
1126
0.065
Yu (1994)
663
664
The sediment
231
Pa/230 Th data used for this comparison originate from the compilation reported by
665
Henry et al. (2016) and come for the most part from the SEEP-I and SEEP-II programs (Anderson et al.
666
1994).
667
It is seen that the near-bottom 231 Pap /230 Thp ratio in our reference solution show relative minima between 231
Pap /230 Thp present
668
1000–3000 m in the Middle Atlantic Bight; east of the 3000-m isobath, the simulated
669
maxima in multiple regions predominately located in the northern part of the domain (Fig. 12). 38
a a
c b
670
231
Pap /230 Thp in the reference experiment. The filled circles are
671
Fig. 12. Distribution of near-bottom
672
surface sediment data (Table 5), and the solid black lines are the 200 m, 1000 m, and 3000 m isobath,
673
respectively. 231
Pap /230 Thp ratios below the production ratio of 0.093 along the margin is consistent
674
The simulation of
675
with measurements of surface sediment
676
port of fine-grained Mn-rich particles (Anderson et al. 1994) may not be necessary to explain the anomalous
677
boundary scavenging observed in the Bight. In other words, sediment
678
duction ratio may also arise from the joint effects of ocean circulation and particle scavenging as represented
679
in our model. The dissolved and particulate
680
tively, 0.22/0.40 = 0.55 and 0.003/0.03 = 0.10 (Appendix C), i.e., to values that exceed the
681
production ratio. Consequently, the near-bottom 231 Pap /230 Thp ratios below the production ratio which are
682
simulated in the Middle Atlantic Bight do not reflect the
683
ary but should rather result from processes operating within the model domain. On the other hand, the
684
sediment
685
13), indicating that the ability of the model to explain the variability of sediment
686
Bight is very limited. Various factors could explain the small correlation between the simulated near-bottom
687
231
231
231
Pa/230 Th in the Middle Atlantic Bight, suggesting that the ex-
231
231
Pap /230 Thp ratios below the pro-
Pa/230 Th ratios at the DWBC inflow are set to, respec-
230
Thd,p and
231
231
Pa/230 Th
Pad,p values set at the bound-
Pa/230 Th show a positive but insignificant relationship with the simulated 231
231
Pap /230 Thp (Fig.
Pa/230 Th within the
Pap /230 Thp and the measured sediment 231 Pa/230 Th, such as (i) model errors, including errors due to the
688
omission of the process postulated by Anderson et al. (1994), (ii) bioturbation within the sediment column,
689
and (iii) sediment lateral redistribution (e.g., Kretschmer et al. (2010, 2011)).
39
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.08
0.1
0.12
0.14
0.16
0.18
PaTh sed model modeled 231Pa/230Th
690
691
0.06
Fig. 13. Scatter plot of surface sediment
231
Pa/230 Th data versus the near-bottom
231
Pap /230 Thp
692
simulated near the corresponding data location in the reference experiment. The regression coefficient
693
(slope) is 0.35 ± 0.36 (one standard error) and the Pearson correlation coefficient is 0.21 (n = 22). The
694
black line is the line of perfect agreement.
695
696
5. Sensitivity Experiments In this section, we explore the sensitivity of the
230
Thd,p and
231
Pad,p distributions simulated by the
697
circulation-geochemical model to the intensity of particle scavenging and to properties of the DWBC. Specif-
698
ically, numerical experiments are conducted with varying values of (i) the sensitivity of the adsorption rate
699
constant k1 to particle concentration (k10 ), (ii) the volume transport of the Deep Western Boundary Current
700
at its inflow location, or (iii) the 230 Thd,p and 231 Pad,p activities of the DWBC at its inflow location. For each
701
experiment, the other model parameters and boundary conditions are the same as those for the reference
702
solution (section 4).
40
703
a. Particle Scavenging Intensity
704
We consider two numerical experiments where the sensitivity of k1 to P is either halved or doubled
705
relatively to the values of k10 assumed for Th and Pa in the reference solution (Table 3). Consider first the
706
effect of changing k10 for Th, k10 (Th), from its reference value of 0.04 yr−1 mg−1 m3 to either 0.02 or 0.08
707
yr−1 mg−1 m3 . A larger value of k10 means that a larger influence of PM concentration and hence of BNLs
708
on chemical scavenging is incorporated into the model. As expected, 230 Thd is found to decrease and 230 Thp
709
is found to increase as k10 (Th) is enhanced (top panels of Fig. 14). d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
710
Fig. 14. Profile of station-averaged
711
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
712
(pre-)GEOTRACES measurements (black circles) and as simulated for k10 (Th) = 0.02 yr−1 mg−1 m3 and
713
k10 (Pa) = 0.01 yr−1 mg−1 m3 (blue lines), k10 (Th) = 0.04 yr−1 mg−1 m3 and k10 (Pa) = 0.02 yr−1 mg−1 m3
714
(black, reference solution), and k10 (Th) = 0.08 yr−1 mg−1 m3 and k10 (Pa) = 0.04 yr−1 mg−1 m3 (green).
715
The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for
716
717
718
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages
(measurement error for the shallowest three (four) deepest measurements of
230
231
Thd measurement, the deepest
230
Thd,p measurements, and the
Pad (231 Pap ); Table 1). The extreme values of
41
230
Thp (Fig. 3) and
719
231
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
720
230
721
The agreement with
722
compared to the reference solution: the root mean square difference between the observed and simulated
723
230
Thd measured at (pre-)GEOTRACES stations, however, is only weakly modified
Thd , rmsd(230 Thd ), amounts to 0.086 dpm m−3 for both k10 (Th) = 0.02 and 0.08 yr−1 mg−1 m3 (Table
724
4). The solution with k10 (Th) = 0.02 yr−1 better describes the observed
725
solution, although the agreement with the observed 230 Thd averages deteriorates (Fig. 14) and rmsd(230 Thp )
726
is only slightly reduced compared to the reference solution (Table 4). The solution with k10 (Th) = 0.08 yr−1
727
mg−1 m3 strongly overestimates the observed
728
the surface and the bottom of the profile (Fig. 14), with rmsd(230 Thp ) reaching 0.037 dpm m−3 (Table 4).
729
230
230
Thp averages than the reference
Thp averages over most of the water column, except near
Consider then the effect of altering k10 (Pa) from its reference value of 0.02 yr−1 mg−1 m3 to either 0.01 or
730
0.04 yr−1 mg−1 m3 (bottom panels of Fig. 14). Similarly to
230
731
decrease (increase) as k10 (Pa) is enhanced. Interestingly, the
231
732
a small sensitivity to k10 (Pa) in the upper ∼ 3000 m, which suggests that particle scavenging is in general a
733
small term in the budget of
734
the low
735
particles is assumed to be more sensitive to particle concentration, although rmsd(231 Pad ) is reduced only
736
slightly (Table 4). However, in this case, the model strongly overestimates the 231 Pap averages observed over
737
the entire column (Fig. 14), which illustrates a difficulty to explain simultaneously the observed
738
dissolved and particulate forms. As shown above, a similar difficulty arises for 230 Th, although the reference
739
solution appears to better reproduce the observed
740
(Fig. 14).
741
b. Strength of the DWBC Inflow
742
231
231
Thd,p , the station-averaged
231
Pad (231 Pap )
Pad averages simulated by the model show
Pad in this part of the water column. Compared to the reference solution,
Pad averages observed below 3000 m are better reproduced by the model if Pa adsorption onto
230
Thd,p averages than the observed
231
231
Pa in
Pad,p averages
We now examine how the strength of the Deep Western Boundary Current at its inflow location (between
743
42.5–44.875◦ N along 55◦ W; Appendix C) affects the distributions of
744
particulate forms in the western North Atlantic domain. Observations along the continental slope and rise in
745
the western North Atlantic suggest considerable temporal variations in the volume transport of the DWBC.
746
Four repeat hydrographic sections across the DWBC at 55◦ W were occupied in order to investigate the inter-
747
annual variability of the deep flow (Pickart and Smethie 1998). The volume transport was estimated from
748
geostrophic velocities in four density layers corresponding to Upper Labrador Sea Water (ULSW), Classical
749
Labrador Sea Water (CLSW), Iceland-Scotland Overflow Water (ISOW), and Denmark Strait Overflow
42
230
Th and
231
Pa in both dissolved and
750
Water (DSOW). The 4-layer summed transport during occupations in 1991, 1994, and 1995 was estimated
751
to range from 12.6 to 25.2 Sv, with an average of 18.8±6.3 Sv. More recently, current meter observations
752
collected along line W from May 2004 to April 2008 showed that the 5-d averaged transport summed in the
753
ULSW, CLSW, ISOW, and DSOW layers ranged from 3.5 Sv to 79.9 Sv, with a record mean of 25.1 Sv and
754
standard deviation of 12.5 Sv (Toole et al. 2011). Bias adjustment to account for the finite width of the
755
mooring array increased the mean transport estimate to 28.7 Sv (Toole et al. 2011).
756
Here two numerical experiments are considered, where the volume transport of the DWBC inflow is
757
changed from its value of 20 Sv in the reference solution to either 10 Sv or 40 Sv. These experiments are not
758
intended to be realistic; their sole purpose is to document the sensitivity of the radionuclide distributions
759
to sizeable variations in the strength of the DWBC inflow in the model. It is seen that the station-averaged
760
profiles of
761
10 to 40 Sv (Fig. 15).
230
Thd,p and
Pad,p experience only modest changes if the DWBC inflow is strengthened from
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231
231Pa
p
(dpm m-3)
depth (m)
231Pa
762
763
Fig. 15. Profile of station-averaged
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
764
(pre-)GEOTRACES measurements (black circles) and as simulated when the strength of the DWBC at its
765
inflow location is set to 10 Sv (green lines), 20 Sv (black, reference solution), and 40 Sv (blue). The circles
766
show averages of measurements from several stations, with the exceptions listed in Figure 9 for
43
230
Thd,p
767
and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages (measurement 230
768
error for the shallowest
769
deepest measurements of
Thd measurement, the deepest
231
230
Thd,p measurements, and the three (four)
Pad (231 Pap ); Table 1). The extreme values of
230
Thp (Fig. 3) and
231
Pap
770
(Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile
771
of the measurements.
772
The differences between the simulated radionuclide activities and the measured radionuclide activities from
773
(pre-)GEOTRACES cruises are not greatly altered by changes in the DWBC inflow: as the DWBC inflow is
774
increased from 10 to 40 Sv, rmsd(230 Thd ) changes from 0.081 to 0.076 dpm m−3 , rmsd(230 Thp ) from 0.030
775
to 0.029 dpm m−3 , and rmsd(231 Pad ) from 0.040 to 0.037 dpm m−3 , whereas rmsd(231 Pap ) amounts to the
776
same value of 0.002 dpm m−3 (Table 4).
777
Notice that the small variations of the station-averaged profiles with DWBC inflow do not imply that the
778
DWBC inflow has also a modest influence on local radionuclide profiles. A change in DWBC inflow from 10
779
to 40 Sv does produce appreciable changes at some locations, particularly in the vicinity of the inflow and
780
near the depth of the current core (not shown).
781
c. Radionuclide Activities in the DWBC Inflow
782
Finally, we explore the sensitivity of the 230 Thd,p and 231 Pad,p distributions to the radionuclide activities 230
which are assumed at the location of the DWBC inflow. Although
784
available to constrain the variability of the two radionuclides near this location, hydrographic observations
785
indicate that water properties of the DWBC can exhibit significant inter-annual variations. In their analysis
786
of repeat sections along 55◦ W, Pickart and Smethie (1998) reported that the largest property variability
787
between 1983–1995 occurred in the CLSW, which in the 1990s became markeldy colder, fresher, and richer
788
in dissolved oxygen and CFCs, all suggestive of ‘new ventilation’. Observations along line W from 1995 to
789
2014 revealed water mass changes that are consistent with changes in source water properties upstream in
790
the Labrador Sea (Le Bras et al. 2017). Particularly evident was the cold, dense, and deep class of Labrador
791
Sea Water (dLSW) that was presumably created by recurring convection events during severe winters in
792
1987–1994. From 2010 to 2014, the density of DSOW within the DWBC along line W was found to decrease
793
as a result of warming overcompensating a slight salinity increase (Andres et al. 2016).
794
Here we consider two numerical experiments where the
230
Th and
231
783
Thd,p and
231
Pa measurements are not
Pad,p activities at the DWBC
795
inflow location are halved or doubled, compared to their respective values in the reference solution (Appendix
796
C). It is seen that the station-averaged
797
variations in radionuclide activities at the DWBC inflow (Fig. 16).
230
Thd,p and
231
44
Pad,p vary markedly at most depths in response to
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
798
799
Fig. 16. Profile of station-averaged
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
800
(pre-)GEOTRACES measurements (black circles) and as simulated when the radionuclide activities at the
801
DWBC inflow are halved (green lines) or doubled (blue) compared to their values in the reference solution
802
(black). The circles show averages of measurements from several stations, with the exceptions listed in
803
Figure 9 for
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the
804
averages (measurement error for the shallowest
230
Thd measurement, the deepest
805
and the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
806
(Fig. 3) and
231
230
Thd,p measurements, 230
Thp
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
807
230
As a result, the agreement with observational averages is noticeably modified: as
809
the DWBC inflow are changed from half to twice their reference values, rmsd(230 Thd ) (rmsd(230 Thp )) varies
810
from 0.078 and 0.165 dpm m−3 (0.025 to 0.041 dpm m−3 ), whereas rmsd(231 Pad ) (rmsd(231 Pap )) varies
811
from 0.033 and 0.110 dpm m−3 (0.001 to 0.002 dpm m−3 ) (Table 4). Thus, in the model, the radionuclide
812
contents of the DWBC inflow more strongly influence the station-averaged profiles than the strength of the
813
DWBC inflow.
45
Thd,p and
231
808
Pad,p at
814
815
6. Discussion The results presented in section 4 show that the circulation-geochemical model considered in this study 230
can capture most (> 80%) of the variance in the measurements of
817
obtained from (pre-)GEOTRACES cruises in the western North Atlantic. Notably, the reversal in the 230 Thd
818
and
819
m on average for
820
and 231 Pa in the particulate phase could also be broadly reproduced, although a model simulation could not
821
be found that closely replicates radionuclide measurements in the two phases simultaneously. Conceivably, a
822
combination of model parameters and (or) boundary conditions different than the ones that have been tested
823
could lead to a significantly better fit of model results with radionuclide measurements in the two phases.
824
Such a combination, however, is probably best sought by using an inverse procedure, which is beyond the
825
scope of this study.
231
Th and
231
816
Pa in the dissolved phase
Pad gradients observed at mid-depth can be reproduced by the model, though too deeply by ∼ 1000 231
Pad . Results from sensitivity experiments (section 5) show that measurements of
230
Th
826
Despite the difficulty experienced to closely replicate 230 Th and 231 Pa data in both the dissolved phase and
827
the particulate phase, the present model does seem to be appropriate for investigating important questions
828
about the observed distributions of 230 Th and 231 Pa in the western North Atlantic. Of particular significance
829
are the observed reversals in the vertical 230 Thd and 231 Pad gradients at mid-depth, leading to radionuclides
830
activities in deep waters that are markedly lower near the western margin than far from the margin (Fig. 1).
831
The model experiments illustrated in section 5 suggest that the elevated particle concentrations in benthic
832
nepheloid layers could enhance chemical scavenging and hence produce the low
833
which have been measured in these waters (Fig. 14).
230
Thd and
231
Pad activities
834
In the remainder of this section, we first examine in more detail the importance of particle scavenging
835
relatively to DWBC inflow in setting the radionuclide distributions in the western North Atlantic. Our
836
results are then compared to those obtained in previous model studies. The potential of BNLs as sites of
837
intensified scavenging and the possibility of significant temporal variations in chemical scavenging in the
838
deep western North Atlantic are subsequently discussed. Finally, we clarify the implications of our results
839
for the use of
840
(2002)).
841
a. Importance of Particle Scavenging
842
230
Th as a ventilation tracer (e.g., Moran et al. (1997); Vogler et al. (1998); Moran et al.
We consider another experiment that further illustrates the potential role of spatial variations in particle 230
scavenging, and of BNLs in particular, in setting the distributions of
844
Atlantic. In this experiment, the apparent rate constants for Th and Pa adsorption are uniform and equal
46
Th and
231
843
Pa in the western North
845
to the domain-averaged values of k1 (Th) and k1 (Pa) in the reference solution. This treatment ensures that
846
the differences in the radionuclide distributions between this experiment and the reference experiment are
847
due to different assumptions about spatial variations in k1 (uniform versus non-uniform), and not due to
848
different domain-averaged k1 values. We find that, with uniform k1 (Th) and k1 (Pa),
849
850
851
230
Thd and
231
Pad averages increase monotonically with
depth, in contrast to the reference solution (and the observations), which shows inversions in 231
230
Thd and
Pad profiles at mid-depth (Fig. 17). d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
852
Fig. 17. Profile of station-averaged
853
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
854
(pre-)GEOTRACES measurements (black circles) and as simulated for uniform k1 (Th) and k1 (Pa) when
855
the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (red), and 40 Sv (blue).
856
The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230
857
858
859
860
861
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages
(measurement error for the shallowest three (four) deepest measurements of 231
230
231
Thd measurement, the deepest
230
Thd,p measurements, and the
Pad (231 Pap ); Table 1). The extreme values of
230
Thp (Fig. 3) and
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements. 47
230
As a result, the low
863
The root mean square differences for
864
or higher than those for variable k1 (Th) and k1 (Pa) (Table 4). Experiments with a DWBC inflow of 10, 20,
865
or 40 Sv all show a monotonic increase of
866
(Fig. 17). Overall, these results suggest that the low 230 Thd and 231 Pad activities observed near the western
867
margin are more likely due to enhanced scavenging in the deep water column than to ventilation by
868
and
231
Thd and
231
862
Pad activities observed in deep waters are not reproduced by the model. 230
Thd,p and
230
231
Thd and
Pad,p for uniform k1 (Th) and k1 (Pa) are comparable
231
Pad with depth if k1 (Th) and k1 (Pa) are uniform
230
Thd -
Pad -poor waters from the western boundary.
869
The small sensitivity to changes in DWBC inflow (section 5b) may be a consequence of our choice of
870
lateral boundary conditions. To test this possibility, we consider an experiment where two modifications to
871
the reference experiment are brought simultaneously: (i) the DWBC inflow is increased from 20 Sv to 40
872
Sv and (ii) the radionuclide activities at the DWBC inflow location are halved compared to the reference
873
values. The resulting values of the rmsd between observed and simulated activities are listed in Table 4. It
874
is seen that these values are generally larger than those for the reference experiment and for the experiment
875
where the activities at the DWBC inflow are halved but the strength of the DWBC inflow is kept equal to 20
876
Sv. Compared to these two experiments, strengthening the DWBC inflow and lowering the activities at the
877
DWBC inflow noticeably lowers the average
878
m (Fig. 18). However, the dissolved and particulate activities below ∼ 3500 m remain largely unaltered in
879
the experiment where (i) and (ii) are both implemented, suggesting that the small sensitivity of radionuclide
880
activities in deep waters to changes in DWBC inflow is a robust result of the model.
d
(dpm m-3)
d
(dpm m-3)
Thd,p and
231
Pad,p in the upper water column above ∼ 3500
230Th
p
(dpm m-3)
depth (m)
230Th
230
231Pa
depth (m)
231Pa
881
48
p
(dpm m-3)
Fig. 18. Profile of station-averaged
882
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
883
(pre-)GEOTRACES measurements (black circles) and as simulated for the reference experiment (black),
884
when the radionuclide activities at the DWBC inflow locations are halved (green), and when the
885
radionuclide activities at the DWBC inflow locations are halved, and the DWBC at its inflow is 40 Sv
886
(blue). The circles show averages of measurements from several stations, with the exceptions listed in
887
Figure 9 for
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the
888
averages (measurement error for the shallowest
230
Thd measurement, the deepest
889
and the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
890
(Fig. 3) and
231
230
Thp
station-averaged profile of the measurements.
b. Comparison to Previous Model Studies In this section, we briefly put our results in the context of three recent model studies on the distribution
893
894
Thd,p measurements,
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the
891
892
230
231
Pa and
230
Th in the global ocean (Rempfer et al. 2017; Gu and Liu 2017; van Hulten et al. 2018).
895
Some of the key differences between these previous models and the one considered in this study are worth
896
mentioning. In these previous models, particle scavenging is determined from the distribution of different
897
particle types simulated by the models, in contrast to our model which includes a single particle field directly
898
based on observations. Moreover, the models of Gu and Liu (2017) and van Hulten et al. (2018) simulate
899
total activities, not dissolved and particulate activities separately. This approach has the benefits that the
900
radionuclide distributions can be calculated by solving a single equation and that sorption reactions need
901
not be treated explictly, although it does require assumptions about solid-solution partitioning. Perhaps
902
more important, none of these previous models represent the large particle concentrations that characterize
903
benthic (and intermediate) nepheloid layers.
904
Interestingly, the difficulty to produce simulations that agree simultaneously with
230
Th and
231
Pa mea-
905
surements in dissolved and particulate forms is a common result of all three models (Rempfer et al. 2017;
906
Gu and Liu 2017; van Hulten et al. 2018). Rempfer et al. (2017) acknowledged the difficulty to produce a
907
simulation that simultaneously accords with 230 Th and 231 Pa data in the two phases, and found that a better
908
agreement is obtained by representing in their model additional sinks at the sea floor (‘bottom scavenging’)
909
and at continental boundaries (‘boundary scavenging’). van Hulten et al. (2018) reported that the
910
and
911
of the ocean, but that
912
nepheloid layers. Finally, Gu and Liu (2017) stated that their model can simulate
231
230
Thd
Pad distributions simulated by their model compare ‘well’ with GEOTRACES data in many parts 230
Thp and
231
Pap activities are underpredicted because of missing particles from
49
230
Thd and
231
Pad dis-
230
Thp and
231
913
tributions that are in ‘good agreement’ with observations but that
Pap are generally smaller
914
than observed in the deep ocean (and generally larger than observed in the surface ocean).
915
The difficulty to reproduce in the same simulation the observed radionuclide distributions in the two
916
phases is also encountered in this work, although it is perhaps less severe owing to the consideration of BNLs
917
and of the associated enhancement in particle scavenging in deep water. Rempfer et al. (2017) included
918
simplified representations of ‘bottom scavenging’ and ‘boundary scavenging’ in their computationally efficient
919
model with coarse horizontal resolution (36×36 grid cells). ‘Bottom scavenging’ was represented by applying
920
a globally uniform concentration of resuspended particles to grid cells adjacent to the bottom, and ‘boundary
921
scavenging’ was represented by increasing by a uniform factor the adsorption rate constant for
922
cells adjacent to continents. The simulation representing both forms of scavenging and illustrated in their
923
paper (Rempfer et al. (2017); their Fig. 2) shows that the deep water 230 Thd and 231 Pad activities are reduced
924
compared to a simulation where ’bottom scavenging’ and ‘boundary scavenging’ are absent. However, in this
925
simulation, both
926
to present the
927
apparent lack of
928
be due to the fact that ‘bottom scavenging’ was represented only in the deepest grid cells of their model,
929
whereas we allow for enhanced k1 over the entire thickness of BNLs as estimated from the optically-derived
930
particle field.
931
c. BNLs as Sites of Intensified Scavenging
230
230
Thd and
230
231
Thd and 231
Thd and
231
Pa in grid
Pad appear to increase monotonically with depth, i.e., they did not seem
Pad inversions at mid-depth as shown in observations and in our model. The
231
Pad inversions in the simulation illustrated by Rempfer et al. (2017) could
932
Whereas boundary scavenging is expected to affect all particle-reactive species with long residence times,
933
the processes leading to enhanced deposition of these species in marginal sediments are not completely
934
understood. One process, however, could operate at many margins: the intensified scavenging in particle-
935
rich nepheloid layers (benthic and intermediate) that may be found near continental slopes (for reviews
936
see McCave (1986); Gardner et al. (2018a)). Benthic nepheloid layers near the western North American
937
margin are characterized by particle concentrations that are much higher than, and particle compositions
938
that are distinct from, those in surrounding waters (e.g., Lam et al. (2015); Gardner et al. (2017)). Particles
939
sampled from BNLs near the western margin along GA03 presented concentration levels of up to 1648 mg
940
m−3 and were dominated by lithogenic material (Lam et al. 2015). If the adsorption rates of particle-reactive
941
metals increase with particle abundance, as postulated on theoretical grounds (e.g., Honeyman et al. (1988))
942
and supported by observational evidence (e.g., Honeyman et al. (1988); Lerner et al. (2017)), then these
943
layers would be characterized by relatively large removal rates of Th and Pa from solution. Less obvious,
50
944
however, is the effect of the chemical composition of resuspended sediment on Th and Pa scavenging in the
945
water column. Lithogenic material was not found to strongly contribute to k1 variance at GA03 stations
946
east of Bermuda (Lerner et al. 2017), although this analysis focused on data collected outside BNLs. More
947
generally, particles resuspended from the seabed would lead to scavenging rates that are different from those
948
in surrounding waters if their chemical composition differs from that of the primary settling material from
949
surface waters. Various factors could produce such differences: (i) temporal variations in the composition of
950
the primary flux, (ii) diagenetic changes in the sediment (later resuspended), (iii) differential resuspension of
951
particles by type and size, and (iv) different original sources for the resuspended material and the primary
952
material (Gardner et al. 1985b).
953
Among the water samples from GA03 considered in this study, those collected near the bottom at stations
954
GT11-04 and GT11-08 present particularly large anomalies in radionuclide activities. Measurements on
955
near-bottom samples at these stations display relatively low values of dissolved
956
and A5) and exceptionally high values of particulate
957
profiles of transmissometer data (voltage or particle beam attenuation coefficient) at GT11-04 and GT11-08
958
published in previous studies shows that these samples were collected in benthic nepheloid layers (e.g., Fig.
959
13 of Lam et al. (2015); Fig. 3 of Hayes et al. (2015a)). Here profiles of PM concentration at stations
960
GT11-04 and GT11-08 are derived from measurements of the beam attenuation coefficient (BAC) obtained
961
from transmissometry (WETLabs C-star 25-cm pathlength transmissometer with serial number CST-491DR;
962
transmissometer data from Schlitzer et al. (2018)), using the empirical relationship between PM and BAC
963
due to particles as reported by Gardner et al. (2018b). It is seen that both stations feature a well-defined
964
BNL in the lower part of the water column (Fig. 19).
230
51
Th and
231
230
Th and
231
Pa (Figs. A2
Pa (Figs. A3 and A6). Inspection of
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
3000
!"##$%(
2500
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
0
125 250 375 500
PM (mg m-3) 965
966
Fig. 19. Vertical profiles of optically derived particulate matter concentration at stations GT11-04,
967
GT11-06, and GT11-08 between the New England continental shelf and Bermuda. Particulate matter
968
concentration is estimated from beam attenuation coefficient measurements available in the GEOTRACES
969
Intermediate Data Product (Schlitzer et al. 2018) using the empirical relationship between PM
970
concentration and BAC due to particles as reported by Gardner et al. (2018b).
971
The concentration maxima amount to about 2100 and 450 mg m−3 near the bottom at GT11-04 and GT11-
972
08, respectively, and exceed those in the surface waters at each station. The upper boundary of the BNL
973
occurs at a water depth of about 3500 m at GT11-04 and about 4500 m at GT11-08, showing that samples
974
presenting relatively low
975
at these stations (Figs. A3 and A6) originated from a strong BNL. Deep water samples collected at other
976
stations along the western segment of GA03 and showing large radionuclide anomalies also originated from
977
a BNL. At station GT11-06, for example,
978
A2 and A5), where a BNL is also found (Fig. 19).
979
230
Thd and
231
Pad activities and exceptionally high
230
Thd and
231
230
Thp and
Pap activities
Pad activities show minima below 4000 m (Figs.
The foregoing observations lead to the hypothesis that intensified scavenging of 52
231
230
Th and
231
Pa may
980
occur in BNLs. The hypothesis is further suggested by a comparison of the horizontal distributions of PM
981
concentration,
982
main regions (Fig. 20): (i) along most of the continental slope and rise, (ii) the Sohm Abyssal Plain (Fig.
983
3), and (iii) the region between the New England Seamounts and Bermuda (Fig. 3).
230
Thd , and
231
Pad in our reference solution. Particle concentrations show maxima in three
PM (mg/m3)
z=3000 m
PM (mg/m3)
z=4000 m
z=5000 m
PM (mg/m3)
984
230Th (dpm/m3) d
z=3000 m
z=4000 m
230Th (dpm/m3) d
z=5000 m
230Th (dpm/m3) d
985
231Pa
z=3000 m
d (dpm/m
3)
z=4000 m
231Pa
d (dpm/m
3)
z=5000 m
231Pa
d (dpm/m
3)
986
Fig. 20. Distributions of particulate matter concentration as estimated from optical measurements
987
988
compiled by Gardner et al. (2017) (top three panels), 231
230
Thd activity as simulated in the reference solution
989
(middle panels), and
990
middle, and right panels show distributions at a depth of, respectively, 3000 m, 4000 m, and 5000 m.
991
The simulated distributions of 230 Thd and 231 Pad show minima in each of these regions (Fig. 20), suggesting
992
that the pattern of particle scavenging at least partly influences the distribution of both radionuclides. Notice
993
that regions of PM maxima and regions of (230 Thd ,231 Pad ) minima do not strictly coincide. For example,
994
995
996
230
Th and
231
Pad activity as simulated in the reference solution (bottom panels). The left,
Pa could be removed from solution in a region where PM levels are high and the resulting
(230 Thd ,231 Pad ) anomalies could be advected into a region where PM levels are lower, leading to 230 Thd and 231
Pad minima there. 53
Evidence for an influence of BNLs on the cycling of particle-reactive elements in the deep water column
997
998
has been reported in a number of studies, namely through the observed disequilibrium of
999
24.1 days) with its radioactive parent,
238
234
Th (half-life of
U (e.g., Bacon and Rutgers van der Loeff (1989); Turnewitsch and
1000
Springer (2001); Rutgers van der Loeff et al. (2002)). Using a model that describes the vertical distribution
1001
of
1002
in bottom waters can be linked to the distribution of excess
1003
particles. Recently, BNLs have been mapped using 6,392 full-depth profiles made during 64 cruises using
1004
transmissometers mounted on CTD in several programs including WOCE, SAVE, JGOFS, CLIVAR-Repeat
1005
Hydrography, and GO-SHIP over the last four decades (Gardner et al. 2018a). In addition to the western
1006
North Atlantic, areas of intense BNLs have been found in the Argentine Basin, the Southern Ocean, and the
1007
oceanic region around South Africa (see also Gardner et al. (2018b)). Future studies will need to establish the
1008
global significance of BNLs for the ocean biogeochemical cycles of
1009
elements in general.
1010
d. Temporal Variations of Particle Scavenging in the Deep Sea
1011
234
Th near the seabed, Rutgers van der Loeff and Boudreau (1997) showed that the depletion of
The possibility that
230
Th and
231
234
234
Th
Th in surface sediments and on resuspended
230
Th and
231
Pa and of adsorption-prone
Pa removal from solution can be enhanced in BNLs suggests that the
1012
scavenging of both nuclides may show significant temporal variations in the deep water column. Benthic
1013
nepheloid layers generally have a basal uniform region corresponding closely to the bottom mixed layer
1014
(typically 20–100 m thick) and an overlying region (typically 500–2000 m thick) where light scattering
1015
(or attenuation) falls off more or less logarithmically up to a clear water minimum (McCave 1986). The
1016
particulate material in each region is thought to originate from sediment resuspension followed by some
1017
combination of vertical turbulent mixing and lateral transport (McCave 1986). None of these processes are
1018
expected to be time-invariant, suggesting that the amount and distribution of particles within BNLs may
1019
both be variable.
1020
Dramatic evidence for the transient nature of BNLs in the western North Atlantic has been provided
1021
by a number of studies, most notably through the HEBBLE experiment on the Nova Scotian continental
1022
rise (HEBBLE 1988). Previous studies had established that intense BNLs in the western North Atlantic
1023
tend to form during episodes of strong abyssal currents and sediment resuspension, which have been picto-
1024
rially referred to as ‘benthic storms’ (Gardner and Sullivan 1981). Results from HEBBLE showed that fast
1025
currents, high concentrations of suspended sediment, and grooved mud beds are associated with erosion in
1026
frequent benthic storms (Hollister and McCave 1984). More recently, measurements of deep-sea currents,
1027
nephelometer-based particle concentration, and seafloor photographs gathered during science programs that
54
1028
spanned two decades in the western North Atlantic have been synthesized (Gardner et al. 2017). These au-
1029
thors concluded that benthic storms occurred in areas with high surface eddy kinetic energy, most frequently
1030
beneath the meandering Gulf Stream and its associated rings.
1031
Different mechanisms have been proposed to explain benthic storms, including (i) synoptic atmospheric
1032
events such as nor’easters, tropical storms, or hurricanes (Gardner and Sullivan 1981); and (ii) oceanic
1033
features generated from the Gulf Stream such as deep cyclones and topographic Rossby waves (Gardner
1034
et al. 2017). In this regard, it is instructive to consult the cruise report for the second leg of GA03 for
1035
information about weather conditions encountered during the leg (GA03 Shipboard Team 2016). After
1036
station GT11-04, where a very strong BNL was found (Fig. 19), weather conditions deteriorated, with
1037
a hurricane passing between the ship’s position and Bermuda, followed by three more days of sustained
1038
winds > 25 knots. At station GT11-06, a very strong BNL was observed, detectable below 4200 m but
1039
most strongly expressed below 4400 m (GA03 Shipboard Team (2016); Fig. 19). The BNL at GT11-08
1040
was even stronger (Fig. 19). Speculatively, the difficulty to simulate accurately radionuclide profiles at
1041
individual stations whilst the station-averaged profiles can be better reproduced (section 4.b) may be due
1042
to pronounced temporal variations in PM load and scavenging intensity at abyssal depths. Further work is
1043
needed to assess whether the strong BNLs observed at stations occupied along the western segment of GA03
1044
and showing large radionuclide anomalies near the bottom were related to the adverse weather conditions
1045
experienced during the cruise.
1046
e. Use of
1047
230
Th as a Ventilation Tracer
A few studies suggested that the low
230
Th and
231
Pa observed in the deep Atlantic compared to values
1048
expected from reversible exchange could be explained by deep ventilation through lateral advection and
1049
mixing (Moran et al. 1997; Vogler et al. 1998; Moran et al. 2002). These studies applied a model originally
1050
developed to interpret
1051
Water (Rutgers van der Loeff and Berger 1993). In this ‘mixing-scavenging’ model, the equations for re-
1052
versible exchange are extended to include a term that is intended to represent the additional influence of a
1053
source water,
230
Th data from the Weddell Sea in terms of upwelling of lower Circumpolar Deep
wp
A∗ − Atot ∂Ap = λAπ + , ∂z τ
(6)
1054
where Atot = Ad + Ap is the total activity, A∗ is the total activity of the source water, and τ is a time scale
1055
for water renewal. An analytical solution of (6) can be easily derived for uniform wp , A∗ , and τ , and fit to
1056
activity data to obtain an estimate of τ . From this approach, water renewal times from 3 to 140 yr have
1057
been reported, where the range reflects (i) estimates from different locations in the Atlantic Ocean and (ii)
55
1058
uncertainties associated with the variability of activity data from a single station (Moran et al. 1997; Vogler
1059
et al. 1998; Moran et al. 2002).
1060
Albeit instructive, the model (6) is not free of limitations. The effect of advection is represented in a
1061
crude manner and solutions obtained with uniform A∗ and τ assume that Atot is influenced by the same
1062
source water introduced at the same rate throughout the water column. More recently, an attempt to extract
1063
information about deep North Atlantic circulation from
1064
finite-difference geostrophic model (Marchal et al. 2007). These authors found that the addition of
1065
data to density and transport observations in the inversion leads to zonally integrated meridional transports
1066
below 1000 m which have larger amplitudes (by 2–9 Sv), where the range reflects the uncertainties in the
1067
large-scale
Th distribution and in the
230
Th data has been undertaken using an inverse 230
Th
Th balance equation.
The results reported in this paper, however, suggest that caution should be exercised when interpreting
1068
1069
230
230
230
Th and
231
Pa measurements from the western North Atlantic in terms of deep water ventilation, at least 230
in terms of ventilation from the DWBC. According to these results, the distributions of
1071
the western North Atlantic show on average only modest variations in response to a change by a factor of four
1072
in the strength of the DWBC (10 to 40 Sv) along 55◦ W (Fig. 15). Other factors, such as particle scavenging
1073
and the
1074
radionuclide distributions west of the MAR. This latter finding is consistent with a recent study by Deng
1075
et al. (2018), who found, using the ‘mixing-scavenging model’, that the
1076
water entering a deep basin significantly influence the
230
Th and
231
Th and
231
1070
Pa in
Pa activities in the inflowing waters, are shown here to have a larger influence on the
230
Th and
231
230
Th and
231
Pa activities of the
Pa activities within the basin.
1077
It is instructive to compare, for the reference solution, the residence time of the water in the domain
1078
with respect to the DWBC inflow with the residence of both radionuclides with respect to scavenging. The
1079
water residence time is computed from RRR
τwat = RR
dx dy dz , un (xDWBC , y, z)dy dz
(7)
1080
where un (xDWBC , y, z) is the zonal (westward) velocity at the DWBC inflow location. The triple integral in
1081
the numerator is over the entire volume of the domain and the double integral in the denumenator is over
1082
the surface area of the DWBC inflow.
1083
We find a value τwat = 21.0 yr, of the same order of magnitude as the water renewal times estimated
1084
from tritium and excess He-3 data (Doney and Jenkins 1994) and from CFC data (Rhein et al. 2015). On
1085
the other hand, we consider three different definitions of residence time for scavenging. First, we calculate
1086
the scavenging residence time as RRR τscav,1 =
Atot (x, y, z) dx dy dz RR , λAπ dx dy dz 56
(8)
where Atot is the total activity defined above. We find that τscav,1 = 19 yr for
1087
1088
231
230
Th and 91 yr for
Pa, consistent with the estimates for the whole Atlantic Ocean of, respectively, 26 yr and 111 yr, based
1089
on the same definition and reported by Yu et al. (1996). Although the above definition of residence time is
1090
convenient, it does rely on the assumption that the radionuclide balance in the water column is dominated
1091
by radioactive production and particle scavenging (Broecker and Peng 1982). We thus consider another
1092
definition, RRR τscav,2 =
A (x, y, z) dx dy dz RR tot , wp Ap,bot dx dy
(9)
1093
where Ap,bot is the activity of the radionuclide in the particulate phase just above the seafloor. The time scale
1094
as defined above could be regarded as a residence time with respect to the flux of particulate radionuclide to
1095
the seabed. We find that τscav,2 amounts to 8 yr for
1096
that Th is more sensitive to particles than Pa but noticeably smaller than the values of τscav,1 . Finally, we
1097
consider a third definition,
230
Th and 32 yr for
231
Pa, consistent with the notion
RRR
τscav,3 = RRR
Ad (x, y, z) dx dy dz , k1 (x, y, z)Ad (x, y, z) dx dy dz
(10)
1098
where Ad is the radionuclide activity in the dissolved phase and k1 is the adsorption rate constant as defined
1099
in section 2.3. This other time scale could be viewed as a residence time with respect to adsorption. Unlike
1100
τscav,1 but similarly to τscav,2 , it has the virtue that a specific removal process is identified in its definition.
1101
We find that τscav,3 amounts to 2 yr for
1102
sensitivity of Th but even smaller than the values of τscav,1 .
230
Th and 4 yr for
231
Pa, again consistent with the greater particle
A comparison of the values of τwat with those of (τscav,2 ,τscav,3 ) shows that
1103
230
Th is in general more
1104
rapidly removed by adsorption onto particles and by particle settling to the seafloor than it is transported
1105
by the DWBC. On the other hand, τwat is smaller than τscav,2 for
1106
1107
231
231
Pa, suggesting that the transport of
Pa is at least as important as its removal to sediments.
7. Summary and Perspectives
1108
In this study, we have first examined the extent to which a regional ocean circulation model with 1/4◦
1109
resolution and including a description of particle scavenging based on optically-derived particle concentration
1110
data can reproduce the circulation and the distributions of
1111
We found that some, though not all, elements of the general circulation and of the radionuclide distributions
1112
which are observed in the area can be reproduced. The model simulates a Gulf Stream displaying some
57
230
Th and
231
Pa in the western North Atlantic.
1113
degree of variability (e.g., meandering), a Deep Western Boundary Current along the continental slope
1114
and rise, and key features in the
1115
cruises, such as a reversal of the vertical
1116
the simulations presented in this paper differ in several respects from observations, e.g., the Gulf Stream
1117
separates from the western margin farther north than observed, the velocity maxima in the Gulf Stream
1118
(DWBC) are underestimated (overestimated) and occur too far to the northwest along line W, and the low
1119
1120
231
230
Thd,p and 230
231
Pad,p distributions observed during (pre-)GEOTRACES
Thd and
231
Pad gradients at mid-depth. On the other hand,
Pad activities observed in deep water are overpredicted. The model has then been applied to investigate the influences of particle scavenging and of DWBC
1121
inflow properties (strength and radionuclide contents) on the
230
1122
North Atlantic. We found that the vertical distributions of
230
1123
)GEOTRACES stations are more sensitive to particle scavenging than they are to the strength of the
1124
DWBC inflow. This result suggests that the distribution of particulate matter concentration, with (relative)
1125
maxima in benthic nepheloid layers, is more important than the strength of deep water ventilation in setting
1126
the basin-mean vertical distributions of both radionuclides in the study area. We also found that changes in
1127
the radionuclide activities of the DWBC inflow are more likely to produce variations in these distributions
1128
than changes in the strength of the DWBC inflow. A model solution with uniform rate constants of Th and
1129
Pa adsorption, with no enhancement in deep water, was unable to explain the observed inversion in
1130
and
1131
activities observed in deep waters in the western North Atlantic stem from intensified scavenging in thick,
1132
active benthic nepheloid layers rather than from ventilation by
1133
western boundary.
231
Th and
231
Thd,p and
Pa distributions in the western 231
Pad,p averaged for all (pre-
Pad profiles at mid-depth. Overall, these results suggest that the relatively low
230
Thd - and
231
230
Thd and
230
Thd
231
Pad
Pad -poor waters from the
1134
We clarify below what we perceive as being the main limitations of this study. Needless to say, the
1135
results reported in this paper are only as reliable as the model on which they are based. A number of
1136
factors could be responsible for the differences between model results and physical observations from satellite
1137
altimetry and repeat cruises along line W. These include, for instance, an inadequate horizontal resolution
1138
of the model, the surface forcing based on climatological fields of winds, temperature, and salinity, and the
1139
omission of shelf circulation. The factors responsible for the differences between the observed and modeled
1140
distributions of 230 Thd,p and 231 Pad,p could be due to errors in both the physical and geochemical components
1141
of the model. Although the present model represents explicitly the kinetics of sorption reactions through
1142
a separate equation for the dissolved and particulate phases, the treatment of
1143
the ocean remains relatively crude. While optical measurements provide important constraints on particle
1144
distribution, temporal variations in particle concentration, which can be dramatic near the bottom (e.g.,
1145
HEBBLE (1988)), are neglected in this study. While several light scattering time series from long-term
58
230
Th and
231
Pa cycling in
1146
nephelometer deployments indicate a strong positive correlation between the temporal standard deviation
1147
in near-bottom particle concentration and the eddy kinetic energy as derived from satellite altimetry, the
1148
temporal variability of particle concentration in the northwest Atlantic remains largely unknown.
1149
Given the above caveats, a number of perspectives for future modeling efforts can be outlined. First, a
1150
model with higher resolution would be desirable in order to simulate the western North Atlantic circulation
1151
in more detail. With a horizontal resolution of 1/4◦ , baroclinic eddies are allowed to grow but are only
1152
marginally resolved; to simulate the full dynamical and life cycle of baroclinic eddies, an ‘eddy-resolving’
1153
model, perhaps with a horizontal resolution approaching 1/10◦ , would be needed (Tr´eguier et al. 2014). Use
1154
of higher spatial resolution might also help (i) to better represent the separation of the Gulf Stream from
1155
the coast (Chassignet and Marshall 2008; Ezer 2016b) and (ii) to reduce the error in the evaluation of the
1156
horizontal pressure gradient (PG) in s-coordinate models. The error in the evaluation of the horizontal PG
1157
in s-coordinate models has been discussed at length in the literature (e.g., Shchepetkin and McWilliams
1158
(2003) and references therein; Ciappa (2008)). In POM, the initial density field area-averaged on z-levels
1159
and then transferred to s-coordinates is subtracted from the density field at each time step in an effort to
1160
reduce the truncation error associated with the calculation of the horizontal baroclinic PG (Mellor 2002).
1161
Theoretical analysis and numerical solutions obtained with POM have shown that velocity errors arising
1162
from the calculation of the horizontal PG vanish due to advection of the density field (Mellor et al. 1994),
1163
or do not vanish prognostically but are small, especially if a horizontally averaged density field is subtracted
1164
before computing the horizontal baroclinic PG (Mellor et al. 1998) as done in POM. Although these results
1165
appear encouraging, subtraction of a horizontally averaged density field may not yield particularly good
1166
results in a domain characterized by strong horizontal density gradients, such as the western North Atlantic,
1167
and represented with a coarse grid. Second, the model domain should be extended to include the continental
1168
shelf from Cape Hatteras to Nova Scotia. Consideration of the continental shelf would allow one to study the
1169
potential role of shelf-ocean exchange in the radionuclide distributions in the slope region where some of the
1170
230
Th and 231 Pa data used in this study originate. Finally, a more detailed description of particle scavenging 230
Th and
231
Pa in the study area. The model could incorporate
1171
would permit more credible simulations of
1172
an explicit representation of sediment resuspension, transport, and deposition, and of the influence of the
1173
resuspended sediment on the removal of both radionuclides from the deep water column (e.g., Rutgers van der
1174
Loeff and Boudreau (1997)). The model could be extended to include (i) the effect of particle composition
1175
on the removal of both radioisotopes (e.g., Hayes et al. (2015a)), and (ii) more than one particle class so as
1176
to account for the processes of particle (dis)aggregation, particularly in BNLs (e.g., McCave (1985); Hill and
1177
Nowell (1990)).
59
1178
Acknowledgments.
1179
We are grateful to John Toole for sharing velocity data from line W, and to both him and Magdalena
1180
Andres for useful discussions. We thank Jerry McManus for directing us to a recent compilation of surface
1181
sediment
1182
us to improve significantly the manuscript. This study could not have been possible without the dedication
1183
of researchers who collected samples at sea and generated the (pre-)GEOTRACES data analyzed in this
1184
study. This work has been supported by grant OCE-1556400 from the U.S. National Science Foundation.
231
Pa/230 Th data. Detailed and constructive comments by two anonymous reviewers have allowed
60
1185
APPENDIX A: Mapping of Particle Matter Concentration
1186
This appendix describes the procedure used to map the distribution of particulate matter (PM) concen-
1187
tration from optical data in the western North Atlantic. For convenience, the particle concentration derived
1188
from optical data and vertically interpolated on a model level is called cd below. The distributions of cd
1189
along the different s-surfaces of the model (31 surfaces) are mapped individually. The mean of cd for a given
1190
s-surface (¯ cd below) is computed and subtracted from the cd values along the surface to yield PM anomalies,
1191
c0d . The PM concentrations at the model grid points along the surface are then estimated from (i) the PM
1192
anomalies at the locations where optical data are available (‘data locations’ below) and (ii) the mean c¯d , ˆ = Ac0d + I¯ c cd .
(A1)
1193
ˆ is a vector including the PM estimates at the model grid points (c would include the true PM values Here c
1194
at these points), c0d is a vector including the PM anomalies at the data locations, and I is the identity matrix
1195
ˆ. The matrix A is derived such that the PM estimates at the model with order equal to the dimension of c
1196
grid points have minimum variance (e.g., Wunsch (2006)), −1
A = Rccd (Rcd cd + Ree )
,
(A2)
1197
where Rccd and Rcd cd are second-moment matrices for the PM field and Ree is a second-moment matrix for
1198
the errors in the elements of c0d .
1199
Consider first Rccd and Rcd cd . These matrices are specifically defined as Rccd = E[ccTd ] and Rcd cd =
1200
E[cd cTd ], where E[·] designates the expected value (mean) and T designates the transpose. The (i, j) element
1201
of Rccd , noted [Rccd ]i,j , is the covariance between the PM concentrations at model grid point location ˆri and
1202
data location rj . Similarly, the (i, j) element of Rcd cd , noted [Rcd cd ]i,j , is the covariance between the PM
1203
concentrations at data locations ri and rj . The following assumptions are made about the spatial covariances
1204
of PM concentration, |ˆri − rj | [Rccd ]i,j = σ 2 exp − L
and
|ri − rj | [Rcd cd ]i,j = σ 2 exp − , L
(A3)
1205
where σ 2 is a PM variance, L is a length scale, and | · | denotes the geodesic distance (taking the geodesic
1206
distance instead of the actual distance along the s-surface should incur only a small error given the smallness
1207
of bathymetric slopes). Equations (A3) entail that the covariance between the PM concentrations at two
1208
different locations decreases exponentionally with distance between these locations, dropping to 0.37σ 2 for
1209
a distance equal to L.
1210
Consider then Ree = E[eeT ], where e is the vector of errors in the elements of c0d . The diagonal elements
1211
of Ree are the variances of the errors in the PM anomalies in c0d , and the off-diagonal elements of Ree are 61
1212
the covariances between these errors. Here Ree is taken as diagonal (no error covariances) and its diagonal
1213
elements are set equal to σe2 = (8.5 mg m−3 )2 . This error variance is intended to account for the fact
1214
that, for many of the stations where transmissometry measurements were made, samples for PM were not
1215
collected, so the particle concentration in the water where the beam attenuation coefficient was minimum
1216
could not be identified. In fact, the minimum beam attenuation coefficient for particles for each cast was
1217
set to the cruise-average minimum taken as 0 m
1218
mg m−3 using the calibration equation. Thus, there is an error associated with the unknown concentration
1219
of particles at the clear water minima. Since these minima typically exhibit particle concentrations between
1220
5–12 mg m−3 in the western North Atlantic (Brewer et al. 1976; Gardner et al. 1985a), we use the median
1221
value of 8.5 mg m−3 as the uncertainty associated with the particle concentration in clear waters.
−1
(Gardner et al. 2018b), which results in PM value of 0
1222
The mapping as described above implies that the mapped PM field becomes smoother as the length scale
1223
L increases, and that mapped PM values at grid points that are very distant ( L) from data locations
1224
approach the mean value c¯d . In this study, we choose L = 100 km for all s-surfaces and σ 2 = σs2 , where
1225
σs2 is the variance of the PM field along the s-surface. With this choice, the mapped PM values at the
1226
data locations are generally consistent with the PM estimates at these locations, i.e., more than 0.4% of the
1227
mapped PM values are within 2σe of the PM estimates at the data locations (not shown).
1228
1229
1230
APPENDIX B: Horizontal Boundary Conditions This appendix describes the conditions imposed at the surface and at the bottom of the domain. Kinematic conditions are specified on the flow at the surface and at the bottom, w
=
w
=
∂η + uh · ∇η ∂t −uh · ∇h
at z = η(x, y, t),
(B1a)
at z = −h(x, y),
(B1b)
1231
where uh = (u, v) is the horizontal velocity, η(x, y, t) is the free surface elevation, and h(x, y) is the water
1232
depth. Annual mean wind stresses from SCOW (Risien and Chelton 2008), τSCOW , are prescribed at the
1233
surface and a shear stress τb is imposed at the bottom: ∂uh ∂z ∂uh ρo κm ∂z ρo κm
=
τSCOW
at z = η(x, y, t),
(B2a)
=
τb
at z = −h(x, y).
(B2b)
1234
Here ρo = 1025 kg m−3 is a reference density and κm is the (variable) vertical turbulent viscosity. The
1235
bottom stress is given by τb = ρo Cd |uh,b |uh,b .
62
(B3)
1236
In this expression, uh,b is the horizontal velocity at the bottom (the grid point nearest to the bottom) and
1237
Cd is a drag coefficient computed from Cd =
κ ln [(1 + sb ) h/zr ]
2 ,
(B4)
1238
where κ = 0.4 is the von K´ arm´ an constant, zr = 0.01 m is a bottom roughness parameter, and sb is the s
1239
coordinate of the deepest level above the bottom (Mellor 2002). The Cd values over the whole domain are
1240
tested such that 0.0025 ≤ Cd ≤ 1.
1241
1242
The turbulence variables at the surface and at the bottom are set as follows (Blumberg and Mellor 1987; Mellor 2002), 2/3 q 2 , q 2 l = b1 u2∗ , 0 ,
(B5)
1243
where b1 = 16.6 is an empirical constant and u∗ is the friction velocity associated with the surface stress or
1244
the bottom stress as denoted.
1245
Finally, fluxes of temperature and salinity at the surface are represented through a restoring to climato-
1246
logic annual mean values from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013), respectively,
1247
TWOA and SWOA , ∂T δ = (TWOA − T ) , ∂z τ δ ∂S κT = (SWOA − S) . ∂z τ
κT
(B6a) (B6b)
1248
Here δ is the thickness of the surface layer and τ = 14.4 days is a restoring time scale. A no-flux condition
1249
is specified for (Ad , Ap ) at the surface and for (T, S, Ad , Ap ) at the bottom.
1251 1250
APPENDIX C: Lateral Boundary conditions
1252
Conditions on Velocities
1253
The appendix describes the conditions applied at the lateral boundaries of the model domain. Conditions
1254
on both the horizontal velocities (internal mode) and their vertical averages (external mode) are needed. For
1255
convenience, the velocity component normal (tangent) to a lateral boundary is designated with subscript
1256
n (t), and the velocity averaged from the bottom to the surface is represented with an overbar. Thus, for
1257
example, un is the velocity normal to the boundary at a given depth and u ¯n is the vertically averaged velocity
1258
normal to the boundary. Different types of conditions are imposed along different segments of the lateral
1259
boundaries.
63
1260
Segments with Inflows and Outflows
1261
Deep Western Boundary Current The inflow of the DWBC is specified at the eastern boundary (55◦ W)
1262
along the segment between 42.5◦ –44.875◦ N. Conditions on both the distribution and the vertical average of
1263
(un , ut ) across the segment are specified. The distribution of (un , ut ) across the segment is set as follows, 2 ! 2 ! z − zi,DWBC y − yi,DWBC exp − . (C1a) un = ui,DWBC exp − ly,DWBC lz,DWBC ut
=
0.
(C1b)
1264
Here yi,DWBC (zi,DWBC ) is the meridional (vertical) coordinate of the core of the current and ly,DWBC (lz,DWBC )
1265
is a length scale for the meridional (vertical) extent of the current. We assume a value of yi,DWBC such
1266
that the core of the current is situated 120 km south of the northern boundary at 55◦ W, a value of 3500
1267
m for zi,DWBC , a value of 50 km for ly,DWBC , and a value of 500 m for lz,DWBC . The zonal velocity ui,DWBC
1268
is calculated such that the integral of un over the surface area of the segment is equivalent to a volume
1269
transport of 20 Sv (1 Sv = 106 m3 s−1 ). The above values assumed for (yi,DWBC , zi,DWBC , ly,DWBC , lz,DWBC ) and
1270
for the volume transport are broadly consistent with observations made in the area (Pickart and Smethie
1271
1998). The vertical average of un is prescribed through a radiation condition (Flather 1976), whereas the
1272
vertical average of ut is set to zero, r u ¯n = u ¯n,∗ +
g (η − η∗ ) h
and
u ¯t = 0.
(C2)
1273
Here u ¯n,∗ is the vertical average of the right-hand side of (C1a), h is the local water depth, η is the local
1274
free surface elevation simulated by the model, and η∗ is a value calculated geostrophically from u ¯n,∗ .
1275
The outflow of the DWBC is specified at the southern boundary (28◦ N) along the segment between
1276
75.5◦ –76.5◦ W. The condition (C1a) for the normal velocity is replaced by a radiation condition, and the
1277
velocity tangent to the boundary is set to zero: ∂un ∂un − ci =0 ∂t ∂n
and
ut = 0.
(C3)
1278
Here n is the coordinate normal to the boundary and ci > 0 is a speed for the internal mode, derived from
1279
the water depth at the boundary and a maximum depth, hmax (Mellor 2002). The differential equation
1280
(C3) is discretized with an upstream scheme, which implies that meridional momentum is always advected
1281
southwards (out of the domain) at the boundary. The vertical averages of (un , ut ) are specified as follows.
1282
The distribution of un along the segment is first taken as
z − zo,DWBC un = uo,DWBC exp − lz,DWBC
64
2 ! .
(C4)
1283
where zo,DWBC = 3000 m is the depth of the core of the current and lz,DWBC = 500 m. The meridional velocity
1284
uo,DWBC is determined such that the integral of un over the surface area of the segment is equivalent to a
1285
volume transport of 20 Sv, which is the volume transport set at the inflow location. The vertical averages
1286
of (un , ut ) are then prescribed similarly to (C2), r g u ¯n = u ¯n,∗ − (η − η∗ ) h
and
u ¯t = 0,
(C5)
1287
where u ¯n,∗ is the vertical average of the right-hand side of (C4) and η∗ is determined geostrophically from
1288
u ¯n,∗ .
1289
Gulf Stream The inflow of the Gulf Stream (GS), intended to represent the Florida Current, is applied
1290
at the southern boundary along the segment between 78◦ –80◦ W. In contrast to the DWBC inflow, only
1291
the vertical averages of (un , ut ) are specified. Conditions identical to (C3) are prescribed for (un , ut ). The
1292
vertical averages of (un , ut ) are prescribed as follows. The distribution of un along the segment is first taken
1293
as un = ui,GS exp −
z
2 ! .
lz,GS
(C6)
1294
where lz,GS = 200 m. The meridional velocity ui,GS is calculated such that the integral of un over the surface
1295
area of the segment is equivalent to a volume transport of 30 Sv. This value is consistent with the long-term
1296
mean transport of the Florida Current of about 32 Sv measured from daily cable observations and shipboard
1297
sections across the Straits of Florida (Baringer and Larsen 2001; Meinen et al. 2010). The vertical averages
1298
of (un , ut ) are then prescribed using conditions identical to (C5).
1299
1300
The outflow of the GS is applied at the eastern boundary along the segment between 39◦ –41◦ N. A radiation condition is used for un whilst ut is taken to vanish along the segment: ∂un ∂un + ci =0 ∂t ∂n
and
ut = 0.
(C7)
1301
As for (C7), the differential equation (C7) is approximated with an upstream scheme, so that zonal momen-
1302
tum is always advected eastwards (out of the domain) at the boundary. The vertical averages of (un , ut ) are
1303
specified as follows. The distribution of un along the segment is first taken as 2 ! z un = uo,GS exp − , lz,GS
(C8)
1304
where again lz,GS = 200 m. The zonal velocity uo,GS is computed such that the integral of un over the surface
1305
area of the segment is equivalent to a volume transport of 90 Sv, a value compatible with mooring-based
1306
estimates of the time-mean GS transport of 93 Sv at 55◦ W (Hogg 1992) and 88 Sv at 68◦ W (Johns et al.
1307
1995). The vertical averages of (un , ut ) are then prescribed using conditions identical to (C2).
1308
Sargasso Sea The inflow of Sargasso Sea water (SSW) is applied at the eastern boundary along the
1309
segment between 28◦ –39◦ N. In contrast to the DWBC inflow and similarly to the GS inflow, only the 65
1310
1311
vertical averages of (un , ut ) are specified. Conditions identical to (C7) are prescribed. The vertical averages of (un , ut ) are specified as follows. The distribution of un across the segment is first taken as 2 ! z un = ui,SSW exp − , lz,SSW
(C9)
1312
where lz,SSW = 1000 m. The zonal velocity ui,SSW is computed such that the integral of un over the surface
1313
area of the segment is equivalent to a volume transport of 60 Sv, which is the difference between the GS
1314
outflow (90 Sv) and GS inflow (30 Sv). Accordingly, the net inflow (or outflow) of water vanishes at the
1315
lateral boundaries of the domain. The vertical averages of (un , ut ) are then prescribed using conditions
1316
identical to (C2). No outflow condition is applied for SSW.
1317
Other Segments
1318
1319
Along segments where no inflow or outflow is prescribed, conditions (C3) or (C7) are applied for (un , ut ) and conditions (C2) or (C5) are applied for (¯ un , u ¯t ).
1320
Conditions on T, S, Ad , Ap , q 2 /2, q 2 l
1321
1322
The following radiation condition is imposed at all lateral boundaries, ∂T ∂T ± un = 0, ∂t ∂n
(C10)
1323
where T ∈ {T, S, Ad , Ap , q 2 /2, q 2 l}. As for (C3) and (C7), the differential equation (C10) is discretized with
1324
an upstream scheme. The variable T is advected out of the domain if the boundary velocity (with normal
1325
component un ) is directed outwards, and it is advected into the domain if the boundary velocity is directed
1326
inwards. Thus, boundary values of T must be prescribed for the case where the boundary velocity is directly
1327
inwards. Boundary values of (T, S) are derived from the World Ocean Atlas (Locarnini et al. 2013; Zweng
1328
et al. 2013) (section 2.a). The radionuclide activities (Ad , Ap ) are set to different values at different locations
1329
along the open boundaries. At the location of DWBC inflow (between 42.5◦ –44.875◦ N along 55◦ W),
1330
(230 Thp ) is set to 0.4 dpm m−3 (0.03 dpm m−3 ) and
1331
m−3 ). These values are the average activities below 1000 m in the Labrador Sea as reported by Moran
1332
et al. (2002)), except for
1333
this increase in
231
231
231
230
Thd
Pad (231 Pap ) is set to 0.22 dpm m−3 (0.003 dpm
Pad whose boundary value is set to double this average activity. We found that
Pad at the DWBC inflow produces a better fit to the observed radionuclide activities in
66
1334
the western North Atlantic. At all other locations, values of (Ad , Ap ) are set according to idealized profiles
1335
that broadly reproduce measured profiles at station GT11-14 (27.58◦ N, 49.63◦ W) situated to the east of the
1336
domain, Ad = Ad,o 1 − ez/lz and Ap = Ap,o 1 − ez/lz , 230
231
Pa. Note that z ≤ 0 in this study,
1337
where the length scales lz = 1600 m for
1338
which implies that Ad and Ap as calculated from (C11) increase monotonically with depth (Fig. A7). Finally,
1339
the boundary values of (q 2 /2, q 2 l) are set to small values.
APPENDIX D:
1340
1341
1342
230
Th and lz = 1132 m for
(C11)
Th and
231
Pa Profiles at Individual Stations
This appendix illustrates observed and simulated profiles of
230
Thd,p and
231
Pad,p at individual (pre-
)GEOTRACES stations (Figs. A1–A13).
1343
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77
1635
List of Tables
1636
1
Thorium-230 and Protactinium-231 Data Used in this Study
79
1637
2
Parameters of the physical model component
80
1638
3
Parameters of the geochemical model component
81
1639
4
Root Mean Square Difference Between Observed & Simulated Activities†
82
1640
5
Thorium-230 and Protactinium-231 Data Used in this Study
83
78
Table 1: Thorium-230 and Protactinium-231 Data Used in this Study
station
latitude
longitude
CMME-13 S1 EN407-3 EN407-4 EN407-6 BATS OC278-2 OC278-3 OC278-4 OC278-5 GT11-01 GT11-02 GT11-03 GT11-04 GT11-06 GT11-08 GT11-10 GT11-12 GT11-14
32.76◦ N 36.05◦ N 39.47◦ N 38.6◦ N 39.73◦ N 32◦ N 37◦ N 33◦ N 36◦ N 38◦ N 39.69◦ N 39.35◦ N 38.67◦ N 38.09◦ N 37.61◦ N 35.42◦ N 31.75◦ N 29.70◦ N 27.58◦ N
70.78◦ W 74.43◦ W 68.37◦ W 68.89◦ W 69.75◦ W 64◦ W 69◦ W 69◦ W 68◦ W 70◦ W 69.81◦ W 69.54◦ W 69.10◦ W 68.70◦ W 68.39◦ W 66.52◦ W 64.17◦ W 56.82◦ W 49.63◦ W
#a
230
8 11 11 19 19 19 11 11 10 11 25 17 20 16 20 17 28 18 21
Thd
#
230
Thp
8 10 0 0 0 0 0 0 0 0 10 12 12 12 12 12 12 12 12
a
#
231
Pad
11 19 0 0 0 0 0 0 0 0 25 17 20 16 21 17 28 18 21
#
231
Pap
0 0 0 0 0 0 0 0 0 0 10 12 12 12 12 12 11 12 11
errorb
reference
1σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ
Cochran et al. (1987) Guo et al. (1995) Luo et al. (2010) Luo et al. (2010) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a)
number of observations. b σ is the standard error for data from R. Fran¸cois (pers. com.) and the standard deviation for all other data.
79
Table 2: Parameters of the physical model component
Physical Parameters
ρo g C κ κu,o κT,o Pr zr τ
reference density acceleration due to gravity Smagorinsky coefficient von K´arm´an constant background vertical viscosity background vertical diffusivity turbulent Prandtl number bottom roughness parameter restoring time scale for SST and SSS
value
units
1025 9.806 0.2 0.4 0 0 5 0.01 14.4
kg m−3 m s−2 1 1 m2 s−1 m2 s−1 1 m d
value
units
15 450 5 200 100 0.1 0
s s 1 m m s−1 1 1
Numerical Parameters
∆tE ∆tI ∆s hmax umax c α a b
time step (external mode) time step (internal mode) step interval for advective termsa maximum depth in radiation condition maximum velocity for CFL violation constant of Asselin filter weight for surface slope termb
Step interval during which advective terms of the external mode are not updated Weight used for surface slope terms in the external mode equations
80
Table 3: Parameters of the geochemical model component
λTh-230 λPa-231 234 U 235 U k1 (Th) k1 (Pa) k1,b (Th) k1,b (Pa) k10 (Th) k10 (Pa) k−1 (Th) k−1 (Pa) wp (Th) wp (Pa)
radioactive decay constant of 230 Th radioactive decay constant of 231 Pa activity of 234 U activity of 235 U adsorption rate constant for Th adsorption rate constant for Pa background value of k1 (Th) background value of k1 (Pa) sensitivity of k1 (Th) to particle concentration sensitivity of k1 (Pa) to particle concentration desorption rate constant for Th desorption rate constant for Pa settling speed of 230 Thp settling speed of 231 Pap
81
value
units
9.17 × 10−6 2.12 × 10−5 2750 108 variable variable 0.4 0.04 0.04 0.02 3.69 18.45 1800 2400
yr−1 yr−1 dpm m−3 dpm m−3 yr−1 yr−1 yr−1 yr−1 −1 yr mg−1 m3 yr−1 mg−1 m3 yr−1 yr−1 m yr−1 m yr−1
Table 4: Root Mean Square Difference Between Observed & Simulated Activities†
230
n
Thd
230
Thp
231
Pad
231
Pap
238
100
161
84
reference solution
0.078
0.028
0.039
0.001
k10 / 2 k10 × 2
0.086 0.086
0.025 0.037
0.041 0.037
0.001 0.003
DWBC inflow = 10 Sv DWBC inflow = 40 Sv
0.081 0.076
0.030 0.029
0.040 0.037
0.002 0.002
Ad,p (DWBC inflow) / 2 Ad,p (DWBC inflow) × 2
0.078 0.165
0.025 0.041
0.033 0.110
0.001 0.002
uniform k1 (T h) & k1 (P a)
0.075
0.043
0.041
0.002
Ad,p (DWBC inflow) / 2, DWBC inflow = 10 Sv
0.093
0.027
0.041
0.002
†All values in dpm m−3
82
Table 5: Thorium-230 and Protactinium-231 Data Used in this Study
core
latitude
longitude
depth (m)
OCE152-BC1 OCE152-BC8 OCE152-BC9 OCE152-BC5 EN123-BC4 EN123-BC6 EN123-BC3 EN123-BC1 EN179-BC5 EN179-BC2 EN179-BC3 EN179-BC4 EN179-BC7 EN187-BC4 EN187-BC10 EN187-BC8 EN187-BC5 EN187-BC11 EN187-BC9 EN187-BC6 OCE325-GGC5 VM26-176
39.49◦ N 32.47◦ N 39.42◦ N 39.08◦ N 39.48◦ N 39.49◦ N 39.35◦ N 39.08◦ N 37.38◦ N 37.37◦ N 37.38◦ N 37.32◦ N 37.25◦ N 37.37◦ N 36.52◦ N 36.52◦ N 37.37◦ N 37.02◦ N 36.52◦ N 37.24◦ N 33.7◦ N 32.76◦ N
70.57◦ W 70.58◦ W 70.55◦ W 70.56◦ W 70.56◦ W 70.55◦ W 70.55◦ W 70.55◦ W 74.13◦ W 74.10◦ W 74.09◦ W 74.02◦ W 73.49◦ W 74.13◦ W 74.37◦ W 74.34◦ W 74.10◦ W 74.34◦ W 74.34◦ W 73.5◦ W 57.6◦ W 70.78◦ W
1126 1596 1981 2691 1280 1643 2344 2736 384 892 1031 1318 1989 512 580 1020 1045 1125 1165 2000 4550 1126
83
231
Pa/230 Th 0.082 0.071 0.091 0.063 0.076 0.066 0.061 0.053 0.127 0.050 0.075 0.071 0.051 0.063 0.089 0.053 0.069 0.062 0.075 0.055 0.054 0.065
reference Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) McManus et al. (2004) Yu (1994)
1641
1642
List of Figures 1
Figure A1. Profiles of dissolved and particulate 230 Th activities measured in the western North
1643
Atlantic prior to the GEOTRACES program. The horizontal bars show the measurement
1644
errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)),
1645
and the stations are identified with their names and with letters located in figure 3 (see Table
1646
1 for data sources).
1647
2
Figure A2. Profiles of dissolved
107 230
Th activity measured in the western North Atlantic along
1648
the GEOTRACES section GA03. The horizontal bars show the measurement errors (Table
1649
1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the
1650
stations are identified with their names and with numbers located in figure 3 (data from Hayes
1651
et al. (2015a)).
1652
3
108 230
Figure A3. Profiles of particulate
Th activity measured in the western North Atlantic
1653
along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table
1654
1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the
1655
stations are identified with their names and with numbers located in figure 3. Note that 230
1656
the other stations (data from Hayes et al. (2015a)).
1657
1658
Thp data from stations GT11-04 and GT11-08 are plotted with a different scale than for
4
Figure A4. Profiles of dissolved
231
109
Pa activity measured in the western North Atlantic prior
1659
to the GEOTRACES program. The horizontal bars show measurement errors (Table 1), the
1660
horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are
1661
identified with their names and with letters located in figure 3 (see Table 1 for data sources). 110
1662
5
Figure A5. Profiles of dissolved
231
Pa activity measured in the western North Atlantic along
1663
the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the
1664
horizontal dashed line indicates water depth (maximum depth from CTD), and the stations
1665
are identified with their names and with numbers located in figure 3 (data from Hayes et al.
1666
(2015a)).
1667
6
111
Figure A6. Profiles of particulate 231 Pa activity measured in the western North Atlantic along
1668
the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the
1669
horizontal dashed line indicates water depth (maximum depth from CTD), and the stations
1670
are identified with their names and with numbers located in figure 3. Note that
1671
from stations GT11-04 and GT11-08 are plotted with a different scale than for the other
1672
stations (data from Hayes et al. (2015a)).
231
Pap data
112
84
1673
7
Figure A7. Profiles of
230
Thd,p and
231
Pad,p measured at station GT11-14 (circles). The red
1674
circles are samples presumably influenced by the TAG hydrothermal vent, which is located
1675
to the east of GT11-14. The black lines are analytical profiles which approximate station
1676
GT11-14 measurements and are used as initial and lateral boundary conditions (Eq. C11).
1677
The horizontal bars show the measurement errors (Table 1; data from Hayes et al. (2015a)).
1678
8
230
Figure A8. Profiles of
Thd and
230
Thp as measured at pre-GEOTRACES stations (black
1679
circles) and as simulated in the reference solution near these stations (red solid line). The
1680
horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates
1681
water depth (from Ryan et al. (2009)), and the red dashed line is the 230 Thd or 230 Thp profile
1682
used as initial conditions and prescribed at the open boundaries. The stations are identified
1683
with their names and with letters located in figure 3.
1684
9
Figure A9. Profiles of
230
114
Thd as measured at GA03 stations (black circles) and as simulated
1685
in the reference solution near these stations (red solid line). The horizontal bars show the
1686
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1687
depth from CTD), and the red dashed line in each panel is the
1688
conditions and prescribed at the open boundaries. The stations are identified with their names
1689
and with numbers located in figure 3.
1690
10
Figure A10. Profiles of
230
230
Thd profile used as initial
115
Thp as measured at GA03 stations (black circles) and as simulated
1691
in the reference solution near these stations (red solid line). The horizontal bars show the
1692
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1693
depth from CTD), and the red dashed line in each panel is the
1694
conditions and prescribed at the open boundaries. The stations are identified with their
1695
names and with numbers located in figure 3. The extreme values of
1696
of stations GT11-04 and GT11-8 (Fig. A3) are excluded from this figure.
1697
11
Figure A11. Profiles of
113
231
230
Thp profile used as initial
230
Thp near the bottom 116
Pad as measured at pre-GEOTRACES stations (black circles) and
1698
as simulated in the reference solution near these stations (red solid line). The horizontal bars
1699
show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from
1700
Ryan et al. (2009)), and the red dashed line is the
1701
and prescribed at the open boundaries. The stations are identified with their names and with
1702
letters located in figure 3.
231
Pad profile used as initial conditions
117
85
1703
12
Figure A12. Profiles of
231
Pad as measured at GA03 stations (black circles) and as simulated
1704
in the reference solution near these stations (red solid line). The horizontal bars show the
1705
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1706
depth from CTD), and the red dashed line in each panel is the
1707
conditions and prescribed at the open boundaries. The stations are identified with their names
1708
and with numbers located in figure 3.
1709
13
Figure A13. Profiles of
231
231
Pad profile used as initial
118
Pap as measured at GA03 stations (black circles) and as simulated
1710
in the reference solution near these stations (red solid line). The horizontal bars show the
1711
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1712
depth from CTD), and the red dashed line in each panel is the
1713
conditions and prescribed at the open boundaries. The stations are identified with their
1714
names and with numbers located in figure 3. The extreme values of
1715
of stations GT11-04 and GT11-08 (A6) are excluded from this figure.
86
231
Pad profile used as initial
231
Pap near the bottom 119
230
0
0.2
Thd (dpm/m 3)
0.4
0.6
0.8
1
1.2
0 1000
depth (m)
2000 3000 4000 5000 6000 231
-0.2 0
Pad (dpm/m 3)
0
0.2
0.4
0.6
1000
depth (m)
2000 3000 4000 5000 6000 Fig. 1. Vertical profiles of dissolved 230 Th and 231 Pa activities in the North Atlantic. Data from stations west (east) of Bermuda are shown with black (blue) circles and data from station GT11-16, near the TAG hydrothermal vent, are shown with red circles. The horizontal bars show the measurement uncertainties (see Table 1 for data sources).
87
PM (mg m-3) 0
0
50
100
150
200
1000
depth (m)
2000 3000 4000 5000 6000
Fig. 2. Regionally averaged vertical profile of particulate matter (PM) concentration in the western North Atlantic. Each circle is an average based on (i) optical measurements converted empirically to PM concentration and (ii) a linear interpolation at the same vertical levels of the concentration estimates obtained from optical profiles at different stations. The horizontal bars show the standard errors of the averages (data compilation from Gardner et al. (2017)).
88
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
Fig. 3. Map of the study area showing the location of GEOTRACES stations (red triangles with numerals), pre-GEOTRACES stations (red triangles with letters), and nephelometer and transmissmeter stations (circles). Black lines are isobaths of 200, 1000, and 3000 m, and blue arrows show schematic pathways of the Gulf Stream (GS), Deep Western Boundary Current (DWBC), Northern Recirculation Gyre (NRG), and Subtropical Gyre (SG). Also shown are the approximate locations of Bermuda (BER), the New England Seamounts (NES), and the Sohm Abyssal Plain (SAP). The thin black line protruding from the continental shelf and slope south of New England is line W.
89
Mean kinetic energy (m2/s2)
Mean
Mean
230
230
Thp (dpm m-3)
Thd (dpm m-3)
days
days Pap (dpm m-3) Mean
Mean
231
231Pa
d
(dpm m-3)
days
days
days Fig. 4. Time series of the domain-averaged kinetic energy, 230 Thd , reference solution
90
230
Thp ,
231
Pad , and
231
Pap in the
mean elevation (1993-2012)
CH
model elevation
CH
Fig. 5. Averages of sea surface height (m) as observed from satellite altimeter data during the period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
91
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
Fig. 6. Standard deviation of sea surface height (m) as observed from satellite altimeter data during the period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
92
Fig. 7. Field of horizontal velocity in the surface layer (top) and at a depth of 3500 m (bottom) simulated in the reference solution. The horizontal arrow at the lower right outside each panel is the maximum speed in units of m s−1 in the corresponding field.
93
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
b
c
d
depth (m)
Model
a
Fig. 8. Distribution of horizontal velocity components between the New England continental shelf and Bermuda as measured during the line W program (top) and as simulated in the reference solution (bottom). At the top of each panel, red vertical lines show the position of GA03 stations GT11-01 to GT11-06, and grey vertical lines show the position of mooring locations. Coordinates along the horizontal axis are distances from 40.125◦ N,70.125◦ W (line W data from Toole et al. (2017)).
94
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th
Fig. 9. Profile of station-averaged 230 Thd (top) and 230 Thp (bottom) as calculated from pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles show averages of measurements from several stations with the following exceptions: for 230 Thd the shallowest circle is a measurement from a single station (OC278-5), and for 230 Thd,p the deepest circle is a measurement from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement and the deepest 230 Thd,p measurements; Table 1). The extreme values of 230 Thp near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements. 95
depth (m)
d
(dpm m-3)
depth (m)
231Pa
231Pa
p
(dpm m-3)
Fig. 10. Profile of station-averaged 231 Pad (top) and 231 Pap (bottom) as calculated from pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles show averages of measurements from several stations with the following exceptions: for 231 Pad (231 Pap ), the three (four) deepest circles show measurements from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 231 Pap near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
96
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3
0.4
230
0.5
0.6
0.7
0.06
0.04
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
0.12
(dpm m-3)
0.8
-0.02 -0.02
0.8
0.35
10
5
(dpm
0.1
p data
0.05
231Pa
0.15
3.5
Thp data (dpm/m 3)
0.2
0.08
0.1
Thp model (dpm/m 3) 230Th230model (dpm p
0.04
0.06
m-3)
0.12
0.14
-3
3 2.5 2
230
m-3)
0.25
Thd data (dpm/m 3)
0.02
r2 = 0.11 p<0.01 n = 83
4
230
(dpm m-3) d data
4.5
r2 = 0.81 p <0.01 n = 167
0.3
231Pa
0
3
Thd model (dpm/m ) -3 230Th model (dpm m ) d
1.5 1 0.5 0 0
0 0
0.05
0.1
231Pa
0.15
0.2
-3) Thd model (dpm/m 3m ) (dpm d model
0.25
0.3
230
0.5
1
1.5
231Pa
2 230
2.5
3
Thp model (dpm/m 3)
3.5
-3 p model (dpm m )
4
4.5
5 10
-3
Fig. 11. Scatter plots of measured radionuclide activities versus simulated radionuclide activities in the reference solution. Shown in each panel are the squared Pearson correlation coefficient (r2 ), the p value of the correlation, and the number of measurements (n). In each panel, the black line is the line of perfect agreement. For panel (b) and (d), the extreme measured values of 230 Thp and 231 Pap near the bottom of stations GT11-04 and GT11-08, along with the corresponding model values, are excluded from the scatter plot.
97
a a
c b
Fig. 12. Distribution of near-bottom 231 Pap /230 Thp in the reference experiment. The filled circles are surface sediment data (Table 5), and the solid black lines are the 200 m, 1000 m, and 3000 m isobath, respectively.
98
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
PaTh sed model modeled 231Pa/230Th Fig. 13. Scatter plot of surface sediment 231 Pa/230 Th data versus the near-bottom 231 Pap /230 Thp simulated near the corresponding data location in the reference experiment. The regression coefficient (slope) is 0.35 ± 0.36 (one standard error) and the Pearson correlation coefficient is 0.21 (n = 22). The black line is the line of perfect agreement.
99
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 14. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for k10 (Th) = 0.02 yr−1 mg−1 m3 and k10 (Pa) = 0.01 yr−1 mg−1 m3 (blue lines), k10 (Th) = 0.04 yr−1 mg−1 m3 and k10 (Pa) = 0.02 yr−1 mg−1 m3 (black, reference solution), and k10 (Th) = 0.08 yr−1 mg−1 m3 and k10 (Pa) = 0.04 yr−1 mg−1 m3 (green). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
100
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 15. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated when the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (black, reference solution), and 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
101
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 16. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated when the radionuclide activities at the DWBC inflow are halved (green lines) or doubled (blue) compared to their values in the reference solution (black). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
102
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 17. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for uniform k1 (Th) and k1 (Pa) when the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (red), and 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
103
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 18. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for the reference experiment (black), when the radionuclide activities at the DWBC inflow locations are halved (green), and when the radionuclide activities at the DWBC inflow locations are halved, and the DWBC at its inflow is 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
104
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
!"##$%(
2500
3000
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
0
125 250 375 500
PM (mg m-3) Fig. 19. Vertical profiles of optically derived particulate matter concentration at stations GT11-04, GT11-06, and GT11-08 between the New England continental shelf and Bermuda. Particulate matter concentration is estimated from beam attenuation coefficient measurements available in the GEOTRACES Intermediate Data Product (Schlitzer et al. 2018) using the empirical relationship between PM concentration and BAC due to particles as reported by Gardner et al. (2018b).
105
PM (mg/m3)
z=3000 m
z=3000 m
z=3000 m
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
PM (mg/m3)
z=4000 m
z=4000 m
z=4000 m
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
z=5000 m
z=5000 m
z=5000 m
PM (mg/m3)
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
Fig. 20. Distributions of particulate matter concentration as estimated from optical measurements compiled by Gardner et al. (2017) (top three panels), 230 Thd activity as simulated in the reference solution (middle panels), and 231 Pad activity as simulated in the reference solution (bottom panels). The left, middle, and right panels show distributions at a depth of, respectively, 3000 m, 4000 m, and 5000 m.
106
230Th (dpm d 0.2
0.4
0.6
0.8
1
0 0
1000
1000
2000
2000
( )
0
3000
p
p
( )
0
4000 5000
0
d
0.4
0.6
0.8
1
4000
(a)
CMME-13 0.2
0.4
0.6
0.8
OC278-2 (f)
5000
1
0
0.2
0.4
0.6
0.8
1
0 1000
500
2000
1000 3000
p
( )
0.2
3000
0
1500
S1 (b) 0
depth (m)
0.2
0.4
0.6
0.8
0
1
0
1000
1000
2000
2000
3000
3000
4000
4000
EN407-3 (c) 0
0.2
0.4
0.6
0.8
( ) p
0.6
0.8
1
0
0.2
0.4
0
0.2
0.4
0.6
0.8
1
0 1000
2000
2000
3000
3000
4000
4000
EN407-4 (d)
5000
0.2
0.4
0.6
0.8
0
1000
1000
2000
2000
3000
3000
4000
4000
BATS (e)
5000
OC278-5 (i)
5000
1
0
0.1
0.2
0.3
0.4
0
0
0.6
0.8
1
EN407-6 (j)
5000
230Th (dpm p 0
0.4
OC278-4 (h)
1
1000
( )
0.2
5000
0
0
(g)
5000
0
5000
p
OC278-3
4000
2000
m-3) 0.1
0.2
0.3
0.4
0
1000 500
( )
2000 3000
1500
4000 5000
1000
p
depth (m)
m-3)
CMME-13 (a)
2000
S1 (b)
Figure A1. Profiles of dissolved and particulate 230 Th activities measured in the western North Atlantic prior to the GEOTRACES program. The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are identified with their names and with letters located in figure 3 (see Table 1 for data sources).
107
230Th (dpm d 0
0.2
0.4
0.8
1
0
0.2
0.4
0
0.2
0.4
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
depth (m)
0.6
m-3)
GT11-01 (1) 0.2
0.4
0.6
0.8
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
GT11-02 (2) 0.2
0.4
0.6
0.8
5000
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
GT11-03 (3) 0
0.2
0.4
0.6
0.8
0.6
0.8
1
0.4
0.6
0.8
1
GT11-10 (10)
1
0 0
1000
1000
2000
2000
3000
3000
4000
4000
GT11-04 (4)
1
GT11-08 (8) 0.2
5000
0
5000
0.8
GT11-06 (6)
0
1
0.6
5000
0.2
0.4
0.6
0.8
1
GT11-12 (12)
Figure A2. Profiles of dissolved 230 Th activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3 (data from Hayes et al. (2015a)).
108
230Th (dpm p 0
0.1
0.2
0.3
m-3)
0.4
0 1000 2000 3000 4000 5000
GT11-01 (1) 0
0.1
0.2
GT11-06 (6)
0.3
0.4
0 1000 2000
depth (m)
3000 4000 5000
GT11-02 (2) 0
0.1
0.2
0.3
GT11-08 (8)
0.4
0 1000 2000 3000 4000 5000
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
Figure A3. Profiles of particulate 230 Th activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3. Note that 230 Thp data from stations GT11-04 and GT11-08 are plotted with a different scale than for the other stations (data from Hayes et al. (2015a)).
109
231Pa (dpm d
depth (m) p ( )
0
0.1
0.2
0.3
0.4
m-3) 0
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
EN407-3
5000
(c)
0.1
0.2
0.3
0.4
EN407-4 (d)
Figure A4. Profiles of dissolved 231 Pa activity measured in the western North Atlantic prior to the GEOTRACES program. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are identified with their names and with letters located in figure 3 (see Table 1 for data sources).
110
231Pa (dpm d
m-3)
GT11-06 (6)
depth p (m) ( )
GT11-01 (1)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-12
GT11-04 (4)
(12)
Figure A5. Profiles of dissolved 231 Pa activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3 (data from Hayes et al. (2015a)).
111
231Pa (dpm p
depth (m)
GT11-01 (1)
m-3)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
Figure A6. Profiles of particulate 231 Pa activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3. Note that 231 Pap data from stations GT11-04 and GT11-08 are plotted with a different scale than for the other stations (data from Hayes et al. (2015a)).
112
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
d
(dpm m-3)
230Pa
p
(dpm m-3)
depth (m)
231Pa
Figure A7. Profiles of 230 Thd,p and 231 Pad,p measured at station GT11-14 (circles). The red circles are samples presumably influenced by the TAG hydrothermal vent, which is located to the east of GT11-14. The black lines are analytical profiles which approximate station GT11-14 measurements and are used as initial and lateral boundary conditions (Eq. C11). The horizontal bars show the measurement errors (Table 1; data from Hayes et al. (2015a)).
113
Pre-GEOTRACES 230Thd CMME-13
depth (m)
(a)
S1
EN407-3
EN407-4
BATS
(c)
(d)
(e)
OC278-4
OC278-5
EN407-6
(h)
(i)
(j)
(b)
OC278-2
OC278-3
(f)
(g) 230Th
d
(dpm m-3)
CMME-13
(a) 230Th
p
S1
(b)
(dpm m-3)
Figure A8. Profiles of 230 Thd and 230 Thp as measured at pre-GEOTRACES stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the red dashed line is the 230 Thd or 230 Thp profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names 114 and with letters located in figure 3.
depth (m)
GEOTRACES 230Thd
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
d
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A9. Profiles of 230 Thd as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 230 Thd profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3.
115
depth (m)
GEOTRACES 230Thp
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
p
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A10. Profiles of 230 Thp as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 230 Thp profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3. The extreme values of 230 Thp near the bottom of stations GT11-04 and GT11-8 (Fig. A3) are excluded from this figure.
116
depth (m)
Pre-GEOTRACES 231Pad
EN407-3 (c)
EN407-4 (d)
Figure A11. Profiles of 231 Pad as measured at pre-GEOTRACES stations (black circles) and as simulated in 231line). The horizontal -3 bars show the measurement errors the reference solution near these stations (red solid d (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the red dashed line is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with letters located in figure 3.
Pa (dpm m )
117
depth (m)
GEOTRACES 231Pad
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8)
231Pa
GT11-10 (10)
d
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A12. Profiles of 231 Pad as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3.
118
depth (m)
GEOTRACES 231Pap
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8) 231Pa
p
GT11-03 (3)
GT11-04 (4)
GT11-10 (10)
GT11-12 (12)
(dpm m-3)
Figure A13. Profiles of 231 Pap as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3. The extreme values of 231 Pap near the bottom of stations GT11-04 and GT11-08 (A6) are excluded from this figure.
119
mean elevation (1993-2012)
CH
model elevation
CH
depth (m)
d
(dpm m-3)
depth (m)
231Pa
231Pa
p
(dpm m-3)
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3 230
0.4
0.6
0.7
0.04
-0.02 -0.02
0.8
(dpm m-3) 10
0.06
0.08 3
0.05
3.5 3
Thp data (dpm/m )
0.1
p data
0.15
0.12
0.14
m-3)
-3
3 2.5 2
230
(dpm m-3)
0.2
0.1
r2 = 0.11 p<0.01 n = 83
4
231Pa
Thd data (dpm/m 3)
4.5
230
(dpm m-3) d data 231Pa
0.04
Thp model (dpm/m ) 230Th model (dpm p
r2 = 0.81 p <0.01 n = 167
0.25
0.02
230
5
0.3
0
Thd model (dpm/m 3)
230Th model d
0.35
0.5
0.06
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
(dpm m-3)
0.8
0.12
1.5 1 0.5 0 0
0 0
0.05
0.1
231Pa
0.15
0.2
Thd model (dpm/m m ) (dpm d model 230
3
-3)
0.25
0.3
0.5
1
1.5
231Pa
2 230
2.5
3
Thp model (dpm/m 3)
p model
3.5
(dpm m-3)
4
4.5
5 10
-3
a a
c b
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.06
0.08
0.1
0.12
PaTh sed model modeled 231Pa/230Th
0.14
0.16
0.18
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
!"##$%(
2500
3000
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
PM (mg m-3)
0
125 250 375 500
230
0 0 1000
depth (m)
2000 3000 4000 5000 6000
0.2
0.4
Thd (dpm/m 3) 0.6
0.8
1
1.2
231
-0.2 0 1000
depth (m)
2000 3000 4000 5000 6000
0
Pad (dpm/m 3) 0.2
0.4
0.6
z=3000 m
PM (mg/m3)
z=4000 m
PM (mg/m3)
z=5000 m
PM (mg/m3)
z=3000 m
230Th (dpm/m3) d
z=4000 m
230Th (dpm/m3) d
z=5000 m
230Th (dpm/m3) d
z=3000 m
231Pa
d (dpm/m
3)
z=4000 m
231Pa
d (dpm/m
3)
z=5000 m
231Pa
d (dpm/m
3)
PM (mg m-3) 0 1000
depth (m)
2000 3000 4000 5000 6000
0
50
100
150
200
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
Mean
Mean
231
d
(dpm m-3)
Pap (dpm m-3)
231Pa
Mean
230
Mean
Thd (dpm m-3)
Thp (dpm m-3)
230
days
days
days
days
days
Mean kinetic energy (m2/s2)
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
b
c
d
depth (m)
Model
a
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th
depth (m)
GEOTRACES 230Thp
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
p
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
depth (m)
Pre-GEOTRACES 231Pad
EN407-3
EN407-4
(c)
231Pa
(d)
d
(dpm m-3)
depth (m)
GEOTRACES 231Pad
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8)
231Pa
GT11-10 (10)
d
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
depth (m)
GEOTRACES 231Pap
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8) 231Pa
p
GT11-03 (3)
GT11-04 (4)
GT11-10 (10)
GT11-12 (12)
(dpm m-3)
230Th (dpm d 0.2
0.4
0.6
0.8
1
0
1000
1000
2000
2000
( )
0
3000
p
p
( )
0 0
4000 5000
0
d
0.4
0.6
0.8
1
4000
(a)
CMME-13 0.2
0.4
0.6
0.8
OC278-2 (f)
5000
1
0
0.2
0.4
0.6
0.8
0.6
0.8
1
0 1000
500
2000
1000 3000
p
( )
0.2
3000
0
1500
depth (m)
0
0.2
0.4
0.6
0.8
0
1
0 1000
1000
2000
2000
3000
3000
4000
4000
EN407-3 (c) 0
0.2
0.4
0.6
0.8
( ) p
1
0
0.2
0.4
0
0.2
0.4
0.6
0.8
1
0 1000
2000
2000
3000
3000
4000
4000
EN407-4 (d)
5000
0.2
0.4
0.6
0.8
OC278-5 (i)
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
0.6
0.8
1
4000
BATS (e)
5000
0.1
0.2
0.3
EN407-6 (j)
5000
230Th (dpm p 0
0.4
OC278-4 (h)
1
0
( )
0.2
5000
1000
0
(g)
5000
0
5000
p
OC278-3
4000
S1 (b)
2000
0.4
0
0
m-3) 0.1
0.2
0.3
0.4
0
1000 500
( )
2000 3000
1500
4000 5000
1000
p
depth (m)
m-3)
CMME-13 (a)
2000
S1 (b)
230Th (dpm d 0
0.2
0.4
0.8
m-3)
1
0
0.2
0.4
0
0.2
0.4
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
depth (m)
0.6
GT11-01 (1) 0.2
0.4
0.6
0.8
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
GT11-02 (2) 0.2
0.4
0.6
0.8
5000
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
GT11-03 (3) 0
0.2
0.4
0.6
0.8
0.6
0.8
0
1000
1000
2000
2000
3000
3000
0.4
0.6
0.8
0.2
0.4
0.6
0.8
4000
GT11-04 (4)
1
1
GT11-10 (10)
1
0
5000
1
GT11-08 (8) 0.2
5000
0
4000
0.8
GT11-06 (6)
0
1
0.6
5000
GT11-12 (12)
1
230Th (dpm p 0
0.1
0.2
0.3
m-3)
0.4
0 1000 2000 3000 4000 5000
GT11-01 (1) 0
0.1
0.2
0
0.1
0.2
GT11-06 (6)
0.3
0.4
0 1000 2000
depth (m)
3000 4000 5000
GT11-02 (2) 0.3
GT11-08 (8)
0.4
0 1000 2000 3000 4000 5000
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
231Pa (dpm d
depth (m) p ( )
0
0.1
0.2
0.3
0.4
m-3) 0
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
EN407-3
(c)
5000
0.1
0.2
0.3
EN407-4 (d)
0.4
231Pa (dpm d
m-3)
GT11-06 (6)
depth p (m) ( )
GT11-01 (1)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12
(12)
231Pa (dpm p
depth (m)
GT11-01 (1)
m-3)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
d
(dpm m-3)
230Pa
p
(dpm m-3)
Pre-GEOTRACES 230Thd CMME-13
depth (m)
(a)
S1
EN407-3
EN407-4
BATS
(c)
(d)
(e)
OC278-4
OC278-5
EN407-6
(h)
(i)
(j)
(b)
OC278-2
OC278-3
(f)
(g) 230Th
d
(dpm m-3)
CMME-13
(a) 230Th
p
S1
(b)
(dpm m-3)
depth (m)
GEOTRACES 230Thd
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
d
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
230 Th and
Paul Lerner, Olivier Marchal, Phoebe J. Lam, Wilford Gardner, Mary Jo Richardson, Alexey Mishonov PII:
S0967-0637(19)30305-X
DOI:
https://doi.org/10.1016/j.dsr.2019.103159
Reference:
DSRI 103159
To appear in:
Deep-Sea Research Part I
Received Date: 18 October 2019 Accepted Date: 6 November 2019
Please cite this article as: Lerner, P., Marchal, O., Lam, P.J., Gardner, W., Richardson, M.J., Mishonov, 230 231 A., A Model Study of the Relative Influences of Scavenging and Circulation on Th and Pa in the western North Atlantic, Deep-Sea Research Part I, https://doi.org/10.1016/j.dsr.2019.103159. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
1
2
A Model Study of the Relative Influences of Scavenging and Circulation on
230
Th and
231
Pa in the western North Atlantic
3
Paul Lerner∗a , Olivier Marchalb , Phoebe J. Lamc , Wilford Gardnerd , Mary Jo Richardsond ,
4
and Alexey Mishonove
5
6
Goddard Institute for Space Studies, New York City, NY 10026, USA
b
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c
7
University of California Santa Cruz, Santa Cruz, CA 95064, USA d
8
9
a
e
Texas A&M University, College Station, TX 77843, USA
Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA
November 19, 2019
10
∗ Corresponding
Author. Address: Goddard Institute for Space Studies, 2880 Broadway, NYC, NY (Tel:1-212-678-5548)
email address: [email protected]
1
Generated using version 3.2 of the official AMS LATEX template
ABSTRACT
11
12
The oceanic cycles of thorium-230 and protactinium-231 are affected by a number of processes, such as
13
removal by adsorption to settling particles and transport by ocean currents. Measurements obtained as part
14
of GEOTRACES and earlier programs have shown that, in the North Atlantic, the activities of dissolved
15
230
Th (230 Thd ) and
231
Pa (231 Pad ) at abyssal depths are lower near the western margin than in the basin 230
interior. At least two factors could explain the lower
17
scavenging in benthic nepheloid layers (BNLs) extending a thousand meters or more above the seafloor; and
18
(ii) ventilation by relatively
19
Current (DWBC).
230
Thd - and
231
Thd and
231
16
Pad near the margin: (i) intensified
Pad -poor waters emanating from the Deep Western Boundary
20
Here a regional ocean circulation model with ‘eddy-permitting’ resolution (1/4◦ ) that incorporates 230 Th
21
and 231 Pa is used in an effort to reproduce the observed distributions of 230 Th and 231 Pa in the western North
22
Atlantic. In this model,
23
particulate matter that is derived from a recent synthesis of nephelometer and transmissometer data. The
24
model simulates a meandering Gulf Stream and a DWBC along the continental slope and rise, although
25
noticeable differences with physical observations also exist. A model solution is found that explains most
26
of the variance of
27
cruises. On the other hand, measurements of particulate
28
difficult to reproduce, with the same solution accounting for only 49% (11%) of the 230 Thp (231 Pap ) variance.
29
Sensitivity experiments suggest that the low
30
are due to enhanced removal rates of both nuclides in BNLs rather than to deep water ventilation from
31
the western boundary. The radionuclide activities present in the DWBC at its inflow location are also
32
found to strongly influence the basin-scale distributions of
33
BNLs as important sites of
34
simultaneously radionuclide measurements in dissolved and particulate forms in the studied area.
35
36
Keywords:
230
230
230
Th and
231
Pa removal from solution is governed by a prescribed distribution of
Thd measurements (85%) and
230
Th and
231
230
231
Pad measurements (81%) from (pre-)GEOTRACES
Thd and
231
230
Th (230 Thp ) and
231
Pa (231 Pap ) are more
Pad activities observed near the western margin
230
Th and
231
Pa. Overall, our study points to
Pa scavenging in the ocean and illustrates the difficulty to explain
Th, 231 Pa, regional ocean circulation model, Deep Western Boundary Current, reversible
exchange, particle concentration effect, nepheloid layer
1
37
1. Introduction
38
Thorium-230 (half-life of 75.6 kyr; Cheng et al. (2013)) and protactinium-231 (32.8 kyr; Robert et al.
39
(1969)) are two naturally-occurring radioisotopes that are produced from the radioactive decay of uranium-
40
234 and uranium-235, respectively. Uranium isotopes display quasi-conservative behaviour in seawater and
41
concentrations proportional to salinity (e.g., Owens et al. (2011)), so that the production rates of
42
and
43
thought to be removed from the water column through a reversible exchange (adsorption and desorption)
44
with settling particles (Nozaki et al. 1981; Bacon and Anderson 1982), a process often referred to as ‘particle
45
scavenging’. A convenient measure of the intensity of particle scavenging is provided by the distribution
46
coefficient, KD = Ap /(Ad P ), where Ad (Ap ) is the activity (in units of dpm) of the radionuclide in the
47
dissolved (particulate) phase per kg of seawater and P is the bulk particle concentration in units of grams
48
of particles per gram of seawater. In the open ocean, KD for
49
indicating a strong tendency for Th to adsorb onto marine particles, whereas KD for 231 Pa is generally lower
50
by one order of magnitude (Moran et al. 2002; Hayes et al. 2015a). As a result of their different sensitivities
51
to particle scavenging, Th and Pa represent two model types for the fate of trace metals, Th being removed
52
from the open ocean primarily by settling particles and Pa largely by lateral transport to other environments
53
(Anderson et al. 1983b,a; Henderson and Anderson 2003).
54
231
230
Pa are expected to be approximately uniform in the world’s oceans. Both
230
Th and
231
230
Th
Pa are
Th is typically of the order of 107 g g−1 ,
The distributions of 230 Th and 231 Pa in the North Atlantic have been documented in a number of studies. 230
Th (230 Thd ) or dissolved
231
Pa (231 Pad ) from distant
55
Early work showed that the activities of dissolved
56
locations exhibit a similar increase with depth in the upper ∼ 1000 m of the water column but present
57
large horizontal variations in the abyssal region below (for syntheses see Marchal et al. (2007); Luo et al.
58
(2010)). More recently, the distributions of
59
been determined with unprecedented horizontal and vertical resolutions along the U.S. GEOTRACES North
60
Atlantic section GA03 (Hayes et al. 2015a). Consistent with pre-GEOTRACES data,
61
distant locations showed a similar increase with depth in the upper 1000 m but large lateral variations at
62
greater depths. Below about 1000 m, stations west of Bermuda displayed lower
63
stations east of Bermuda (Fig. 1).
230
Th and
2
231
Pa in both dissolved and particulate forms have
230
230
Thd or 231 Pad from
Thd and
231
Pad than
230
0
0.2
Thd (dpm/m 3)
0.4
0.6
0.8
1
1.2
0 1000
depth (m)
2000 3000 4000 5000 6000
64
231
-0.2 0
Pad (dpm/m 3)
0
0.2
0.4
0.6
1000
depth (m)
2000 3000 4000 5000
65
66
6000 Fig. 1. Vertical profiles of dissolved
230
Th and
231
Pa activities in the North Atlantic. Data from stations
67
west (east) of Bermuda are shown with black (blue) circles and data from station GT11-16, near the TAG
68
hydrothermal vent, are shown with red circles. The horizontal bars show the measurement uncertainties
69
(see Table 1 for data sources). 230
Thd and
231
70
At stations west of Bermuda,
Pad increased with depth down to about 2000–4000 m but
71
decreased with depth below. Whereas the activity increase with depth observed in the upper part of the
72
water column is consistent with a reversible exchange of Th and Pa with settling particles (Bacon and
73
Anderson 1982), the reversal in the vertical activity gradient at mid-depth and the large horizontal activity
74
gradients in the deeper part await further investigation. 3
75
At least three factors could explain the departures of
230
Thd and
231
Pad measurements from the profiles
76
expected from reversible exchange: (i) spatial variations in particle scavenging due to variations in bulk
77
particle concentration, (ii) spatial variations in particle scavenging due to variations in particle composition,
78
and (iii) the ventilation of deep layers by components of the North Atlantic Deep Water (NADW). Each of
79
these factors is introduced in turn below.
80
Particle scavenging in oceanic waters appears to vary with bulk particle concentration (P ). Two observed
81
characteristics of metal sorption in natural waters are in apparent contradiction with physico-chemical ad-
82
sorption theory: (i) the time scales of sorption reactions are long, reaching days to weeks and more, and (ii)
83
KD is negatively correlated with particle concentration – the so-called “particle concentration effect” (e.g.,
84
Honeyman and Santschi (1989)). Colloids have for a long time been invoked to explain the slow Th sorption
85
kinetics (e.g., Tsunogai and Minagawa (1978)). In the model developed by these authors, dissolved Th
86
species rapidly and irreversibly adsorb onto colloidal particles, which then reversibly aggregate with larger
87
filterable particles. Santschi et al. (1986) proposed that if the sorption of a metal ion has particle aggregation
88
as a rate-limiting step, then the rate of sorption should vary with particle number or particle concentration.
89
In the Brownian-pumping model, both characteristics (i-ii) result from the same underlying processes: the
90
rapid formation of metal-colloid surface site complexes (adsorption) and the slow coagulation of colloids with
91
filterable particles (Honeyman and Santschi 1989). More generally, the negative correlation between KD and
92
P could arise from at least two factors: (i) higher concentrations of Th-binding colloids, small enough to
93
pass through typical filters, may keep apparent Th concentrations high in the ‘dissolved’ phase, and (ii) the
94
surface area per mass of particles may decrease at greater particle mass, which would reduce the number of
95
available binding sites (Henderson et al. 1999; Pavia et al. 2018). Owing to their high specific surface area
96
and complexation capacity, colloidal particles may be of great importance in the cycling of particle-reactive
97
elements (e.g., Guo et al. (1997)), and laboratory studies suggested that they could significantly influence
98
Th scavenging in oceanic waters (e.g., Guo et al. (1997); Quigley et al. (2001, 2002)).
99
The 230 Th, 231 Pa, and particle data collected along GA03 indicated that particle concentration influences
100
solid-solution partitioning and sorption rates in North Atlantic waters (Hayes et al. 2015b; Lerner et al. 2017).
101
Hayes et al. (2015b) found that KD for 230 Th and 231 Pa generally decreased with particle concentration along
102
GA03. Lerner et al. (2017) derived estimates of the apparent first-order rate constant for Th adsorption onto
103
particles (k1 ) from an inversion of size-fractionated 228,230,234 Th and particle data gathered at eleven stations
104
east of Bermuda. They found that k1 generally decreases with depth, with the time scale 1/k1 averaging
105
1.0 yr in the upper 1000 m and (1.4 – 1.5) yr below. A positive relationship between k1 and P was found,
106
consistent with the notion that k1 increases with the number of surface sites available for adsorption (e.g.,
107
Honeyman et al. (1988)). In contrast to the influence of colloids as envisioned by the Brownian pumping
4
108
hypothesis, Lerner et al. (2017) provided evidence that the particle concentration effect may arise from the
109
joint effect of P on the rate constants for Th attachment to, and detachment from, particles.
110
Particle scavenging could be intensified at abyssal depths in the western North Atlantic due to the
111
presence of benthic nepheloid layers (BNLs) (Fig. 2). Vertical traces from nephelometers have long revealed
112
that BNLs are common near the western margin in depths greater than 3000 m where a clear water minimum
113
is typically found; in contrast to the western basins, the eastern basins generally show low turbidity (Eittreim
114
et al. 1969, 1976; Biscaye and Eittreim 1977). A recent synthesis of light scattering and attenuation data
115
illustrated that the particle load in excess of the concentration at the clear water minimum decreases eastward
116
in the western North Atlantic by up to one order of magnitude (Gardner et al. 2017). In line with this result,
117
particulate matter concentrations estimated from size-fractionated particle composition data featured intense
118
BNLs along GA03 in the western North American margin with P > 1000 mg m−3 , two to three orders of
119
magnitude higher than in surrounding waters (Lam et al. 2015). These observations suggest that intensified
120
scavenging due to high particle concentrations could contribute to the relatively low
121
the deep waters west of Bermuda.
5
230
Thd and
231
Pad in
PM (mg m-3) 0
0
50
100
150
200
1000
depth (m)
2000 3000 4000 5000 6000 122
123
Fig. 2. Regionally averaged vertical profile of particulate matter (PM) concentration in the western North
124
Atlantic. Each circle is an average based on (i) optical measurements converted empirically to PM
125
concentration and (ii) a linear interpolation at the same vertical levels of the concentration estimates
126
obtained from optical profiles at different stations. The horizontal bars show the standard errors of the
127
averages (data compilation from Gardner et al. (2017)).
128
Particle scavenging in the ocean seems to vary also with the chemical composition of particles (e.g.,
129
Anderson et al. (1983a); Chase et al. (2002); Geibert and Usbeck (2004); Scholten et al. (2005); Roberts
130
et al. (2009); Chuang et al. (2013); Lin et al. (2014)). A number of studies provided evidence for a role
131
of particle composition on the sorption of
132
(2005) reported that
133
is positively correlated with Mn, Ba, and lithogenic fractions and does not covary with CaCO3 or biogenic
134
silica (bSi). Radioisotope and particle data from section GA03 provided further insight into the effects
135
of particle type on Th and Pa scavenging in the North Atlantic (Hayes et al. 2015b; Lerner et al. 2018).
230
230
Th and
231
Pa in North Atlantic waters. Roy-Barman et al.
Th of particles collected by sediment traps deployed in the eastern North Atlantic
6
136
Hayes et al. (2015b) inferred enhanced scavenging by authigenic Fe and Mn (hydr)oxides in the form of
137
hydrothermal particles emanating from the Middle Atlantic Ridge (MAR) and particles resuspended from
138
reducing environments near the seabed off the West African coast. Biogenic silica was not found to be a
139
significant scavenging phase for Th and Pa, presumably because of its low abundance and small variance
140
at the sampled stations. Lerner et al. (2018) examined the dependence of their k1 estimates (below about
141
100 m) on particle composition (particulate organic C, CaCO3 , bSi, lithogenic material, and Fe and Mn
142
(hydr)oxides) at stations east of Bermuda. Particle composition was found to explain a larger fraction
143
of k1 variance for stations within the Mauritanian upwelling region (‘eastern stations’) than for stations
144
west of that region (‘western stations’). The k1 variance explained by particle composition was mainly due
145
to biogenic particles at the ‘eastern stations’ and to Mn oxides at the ‘western stations’. Interestingly, the
146
proportions of k1 variance explained by particle composition and particle concentration were not significantly
147
different (Lerner et al. 2018).
148
In addition to variations in particle scavenging intensity, ventilation of the deep basins by components 230
Thd and
231
149
of NADW has been postulated to impact the distributions of
Pad in the North Atlantic
150
(e.g., Moran et al. (1997); Vogler et al. (1998); Moran et al. (2002)). The Deep Western Boundary Current
151
(DWBC) in the western North Atlantic transports cold waters from the northern North Atlantic toward the
152
equatorial region (e.g., Schmitz and McCartney (1993)) and appears to act as a major conduit from which
153
deep layers in the North Atlantic are ventilated. The dynamical processes responsible for the ventilation of
154
deep basins from the western boundary are varied and complex, involving, e.g., recirculation gyres (e.g., Hogg
155
et al. (1986); Pickart and Hogg (1989)), mesoscale eddies (e.g., Pickart et al. (1997)), offshore entrainment in
156
the region where the DWBC and the Gulf Stream cross over (e.g., Bower and Hunt (2000a,b)), deep cyclones
157
accompanying meander troughs in the Gulf Stream path (e.g., Andres et al. (2016)), and stirring between
158
the boundary and the interior (e.g., Le Bras et al. (2017, 2018)).
159
Tracer measurements have shed considerable light on deep ventilation in the North Atlantic. Tritium
160
and excess helium-3 data indicated that the deep water in the subpolar gyre is ventilated by NADW on
161
time scales of about 10–15 yr (Doney and Jenkins 1994). Chlorofluorocarbon (CFC) data showed that deep
162
layers in the North Atlantic are renewed on decadal time scales by Labrador Sea Water (LSW) and Denmark
163
Strait Overflow Water (DSOW) originating from the western boundary (e.g., Rhein et al. (2015)). From
164
multiple tracer data collected along GA03, Holzer et al. (2018) estimated that abyssal waters have a mean
165
age ranging from 200 to 400 yr and are younger in the western basins. Overflow waters last ventilated in
166
the Arctic Ocean and in the Norwegian and Greenland Seas would contribute about 40% of waters present
167
in the western basins where ∼60% of these overflow waters are younger than 160 yr (Holzer et al. 2018).
168
Early measurements of
230
Thd in the Labrador Sea and the Denmark Strait presented relatively small 7
169
values compared to those observed at more southern locations in the Atlantic Ocean (Moran et al. 1995,
170
1997, 2002), suggesting that ventilation by LSW and DSOW would tend to decrease
171
North Atlantic. Similarly, recent measurements obtained on filtered seawater collected in the Labrador Sea,
172
Irminger Sea, Iceland Basin, and West European Basin along GEOTRACES section GA01 (Deng et al.
173
2018) showed
174
Ocean. The low
175
related to the presence of young (e.g., CFC-rich) waters, whilst enhanced scavenging of both nuclides near
176
the seafloor would also occur in overflow waters (Deng et al. 2018).
230
Thd and 230
231
Thd and
Thd in the deep
Pad activities that are lower than those measured further south in the Atlantic 231
Pad near the bottom in the Labrador Sea and Irminger Sea appeared to be
In summary, at least two processes could contribute to the relatively low
177
230
230
Thd and
231
Pad observed at
178
abyssal depths in the western North Atlantic: (i) intensified particle scavenging in thick or rapidly cycling
179
BNLs and (ii) ventilation by 230 Thd - and 231 Pad -poor components of NADW from the western boundary. In
180
this paper, we apply a regional ocean circulation model in order to study the effects of these two processes on
181
the distributions of both radionuclides. The model is configured to represent the western North Atlantic with
182
‘eddy-permitting’ (1/4◦ ) resolution and includes a description of 230 Th and 231 Pa cycling. In this description,
183
the rates of 230 Th and 231 Pa removal from solution are determined by a prescribed distribution of particulate
184
matter concentration based on a recent compilation of nephelometer and transmissometer data. Results from
185
the circulation-scavenging model are then compared to (i) physical oceanographic observations provided from
186
satellite altimetry and repeat sections between the New England continental shelf and Bermuda; and (ii)
187
radionuclide measurements obtained from (pre-)GEOTRACES cruises. Finally, numerical experiments are
188
performed with the model in order to evaluate the sensitivity of the simulated distributions of
189
190
231
230
Th and
Pa in both dissolved and particulate forms to (i) the intensity of particle scavenging, (ii) the strength of
the DWBC, and (iii) the radionuclide activities that are present in the DWBC.
191
Thorium-230 and protactinium-231 have been incorporated in ocean models in previous studies. Both
192
tracers have been included in three-dimensional (3D) models of global ocean circulation (Henderson et al.
193
1999; Siddall et al. 2005; Dutay et al. 2009; Rempfer et al. 2017; Gu and Liu 2017; van Hulten et al. 2018),
194
in a zonally-averaged circulation model (Marchal et al. 2000), in a 2D (latitude-depth) box model (Luo et al.
195
2010), and in 3D regional inverse models (Marchal et al. 2007; Burke et al. 2011). Most of these studies were
196
aimed at testing paleoceanographic applications of
197
230
230
Th and
231
Pa, more specifically the use of sediment
Th activity data to correct accumulation rates for the effects of sediment redistribution (Bacon 1984) and 231
Pa/230 Th ratio from Atlantic sediments as a paleocirculation indicator (Yu et al. 1996).
198
the use of the
199
The present work complements these earlier studies by isolating the relative influences of particle scavenging
200
and ocean circulation on radionuclide distributions in the western North Atlantic using a model with higher
201
horizontal resolution and a treatment of sorption kinetics that relates specific rates of adsorption to a particle
8
202
concentration field derived from optical data. On the other hand, our model does not account for fluxes
203
of radionuclides from the seafloor such as due to sediment resuspension (e.g., Rutgers van der Loeff and
204
Boudreau (1997)). Emphasis is placed on understanding water column measurements, although our results
205
may also have implications for the interpretation of sediment records of
230
Th and
231
Pa.
206
The remainder of this paper is organized as follows. In section 2, the physical, radionuclide, and optical
207
data sets used in this study are briefly described. Nephelometer and transmissometer data are converted
208
into estimates of particulate matter (PM) concentration using published calibrations, and an estimate of the
209
3D distribution of PM concentration in the study area is derived. The ocean model is presented in section 3.
210
The physical and geochemical components of the model are detailed, together with the conditions imposed
211
at the surface and at the open boundaries of the regional domain. In section 4, a particular model solution
212
is presented that shows relatively good agreement with physical oceanographic and radionuclide data. The
213
influences of particle scavenging intensity and of DWBC properties (strength and radionuclide contents) are
214
then illustrated in section 5 through a number of sensitivity experiments. Our results are discussed in section
215
6, with emphasis on the role of BNLs in particle scavenging. A summary and perspectives follow in section
216
7.
217
2. Data
218
a. Physical Data
219
This study relies on a number of physical datasets. Bathymetric data used to establish the model
220
bottom topography are obtained from ETOPO2 version 2 (ETOPO2v2 2006), a gridded product with 2-min
221
resolution and derived from satellite altimeter and shipboard sounding data. Temperature and salinity data
222
employed to constrain the initial and boundary conditions of the model (section 4 and appendix C) are
223
obtained from the 1/4◦ resolution climatology (decadal averages, objectively analyzed) of the World Ocean
224
Atlas (Locarnini et al. 2013; Zweng et al. 2013). Climatologic values of potential temperature are first derived
225
from the values of in situ temperature, salinity, and depth of the Atlas using the algorithm developed by
226
Jackett et al. (2004) and assuming 1 dbar = 1 m. Potential temperature and salinity data at model levels
227
are then obtained from their respective climatologic values by linear interpolation (and extrapolation, where
228
needed). Note that the longitudes and latitudes of model grid points are set to coincide with the longitudes
229
and latitudes of the climatologic grid, so no horizontal interpolation is necessary.
230
Two types of satellite data sets are used in this study. First, surface wind stresses used to provide the
231
mechanical forcing to the model come from the Scatterometer Climatology of Ocean Winds, SCOW (Risien
9
232
and Chelton 2008). The monthly means of zonal and meridional wind stresses, available at 1/4◦ resolution,
233
are averaged to produce an annual mean field. The annual mean wind stresses at a given model grid point are
234
then obtained from the closest values of the climatologic fields. Second, observations of dynamic topography
235
are used in order to test the modeled circulation. Monthly observations of sea surface height (SSH) between
236
1993–2012 are derived from satellite altimetry maps prepared by Ssalto/Duacs and distributed by Aviso+,
237
with support from CNES (https://www.aviso.altimetry.fr). These observations are averaged to produce
238
a multi-year annual mean field which is compared to model results.
239
Finally, horizontal velocity data from line W, between the New England continental shelf and Bermuda,
240
and coinciding with the western segment of GA03 (Fig. 3), are used to provide another test of the modeled
241
circulation. Velocity measurements along line W were obtained from shipboard deployments of Lowered
242
Accoustic Doppler Current Profilers and from moored profiler and current meters between 2004 and 2014
243
(Toole et al. 2017). The velocity data used in this study are 10-yr averages derived by averaging daily profiles
244
of subinertial-filtered velocity (note that the eastern-most mooring only contains data between 2008 and
245
2014, so only 6-yr averages are used for that mooring). The daily profiles were constructed by combining
246
moored profiler and current meter data from six moorings deployed along line W (John Toole, personal
247
communication).
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
248
249
Fig. 3. Map of the study area showing the location of GEOTRACES stations (red triangles with
250
numerals), pre-GEOTRACES stations (red triangles with letters), and nephelometer and transmissmeter
251
stations (circles). Black lines are isobaths of 200, 1000, and 3000 m, and blue arrows show schematic
10
252
pathways of the Gulf Stream (GS), Deep Western Boundary Current (DWBC), Northern Recirculation
253
Gyre (NRG), and Subtropical Gyre (SG). Also shown are the approximate locations of Bermuda (BER),
254
the New England Seamounts (NES), and the Sohm Abyssal Plain (SAP). The thin black line protruding
255
from the continental shelf and slope south of New England is line W.
256
257
258
b. Radionuclide Data The
230
Th and
231
Pa activity data considered in this study originate from both pre-GEOTRACES
programs and the GA03 section (Table 1).
11
Table 1. Thorium-230 and Protactinium-231 Data Used in this Study
259
#a
230
errorb
reference
0
1σ
Cochran et al. (1987)
19
0
2σ
Guo et al. (1995)
0
0
0
2σ
Luo et al. (2010)
19
0
0
0
2σ
Luo et al. (2010)
69.75◦ W
19
0
0
0
2σ
R. Fran¸cois (pers. com.)
32◦ N
64◦ W
19
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-2
37◦ N
69◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-3
33◦ N
69◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-4
36◦ N
68◦ W
10
0
0
0
2σ
R. Fran¸cois (pers. com.)
OC278-5
38◦ N
70◦ W
11
0
0
0
2σ
R. Fran¸cois (pers. com.)
GT11-01
39.69◦ N
69.81◦ W
25
10
25
10
1σ
Hayes et al. (2015a)
GT11-02
39.35◦ N
69.54◦ W
17
12
17
12
1σ
Hayes et al. (2015a)
GT11-03
38.67◦ N
69.10◦ W
20
12
20
12
1σ
Hayes et al. (2015a)
GT11-04
38.09◦ N
68.70◦ W
16
12
16
12
1σ
Hayes et al. (2015a)
GT11-06
37.61◦ N
68.39◦ W
20
12
21
12
1σ
Hayes et al. (2015a)
GT11-08
35.42◦ N
66.52◦ W
17
12
17
12
1σ
Hayes et al. (2015a)
GT11-10
31.75◦ N
64.17◦ W
28
12
28
11
1σ
Hayes et al. (2015a)
GT11-12
29.70◦ N
56.82◦ W
18
12
18
12
1σ
Hayes et al. (2015a)
GT11-14
27.58◦ N
49.63◦ W
21
12
21
11
1σ
Hayes et al. (2015a)
Thd
#
230
Thp
#
231
station
latitude
longitude
Pad
CMME-13
32.76◦ N
70.78◦ W
8
8
11
S1
36.05◦ N
74.43◦ W
11
10
EN407-3
39.47◦ N
68.37◦ W
11
EN407-4
38.6◦ N
68.89◦ W
EN407-6
39.73◦ N
BATS
#
231
Pap
260
261
262
263
264
a
number of observations.
b
σ is the standard error for data from R. Fran¸cois (pers. com.) and the standard
deviation for all other data.
Radionuclide data in both dissolved and particulate forms are used and displayed in Appendix D (Fig. A1–A7).
12
265
1) Thorium-230
266
Measurements of
230
Thd from pre-GEOTRACES cruises generally show an increase with depth in the
267
upper 2500–3000 m of the water column (Fig. A1). Below 2500–3000 m, stations sampled during these cruises
268
feature either uniform
269
only two studies reported
270
published
271
measurements from a station off Cape Hatteras in the Middle Atlantic Bight. The 230 Thp activities measured
272
at both stations generally increase with depth. The
273
230
230
230
Thd or a 230
230
Thd decrease with depth. To our knowledge, prior to GEOTRACES,
Thp measurements in the investigated region (Fig. A1). Cochran et al. (1987)
Thp data for a station in the Hatteras Abyssal Plain, and Guo et al. (1995) reported
230
Thp
Thd,p data collected along GA03 and used in this study have been described by Hayes et al. 230
274
(2015a), so only a brief overview is provided below. Broadly,
275
of the water column at all stations (Fig. A2). At the deepest stations, the
276
increase with depth down to 3000–4000 m, where vertical gradients of
277
are particularly noticeable at stations GT11-04 to GT11-10. At station GT11-12 (east of Bermuda),
278
below ∼ 4000 m tend to be more uniform compared to the activities measured at stations west of Bermuda.
279
Similarly to 230 Thd data, 230 Thp data from GA03 generally show an increase with depth (Fig. A3). Stations
280
GT11-06, GT11-10, and GT11-12 show indications of a reversal in the vertical
281
3000 m, with measurements presenting a local maximum in the deepest sample(s), although 230 Thp data from
282
GT11-10 and GT11-12 also display relatively large dispersion and (or) uncertainties. Particularly spectacular
283
are the extremely large
284
GT11-08. The
285
two orders of magnitude than
286
230
230
230
Thd increased with depth in the upper part
230
230
Thd data show a continued
Thd change sign. These inversions
230
230
Thd
Thp gradient near 2000–
Thp activities measured on four near-bottom samples at stations GT11-04 and
Thp activities in these samples approach or exceed 1 dpm m−3 , which is larger by one to 230
Thp activities measured at similar depths at other stations and larger than
Thd activities measured on the same or nearby samples at the same stations.
287
2) Protactinium-231
288
Only a few measurements of
231
Pa in the western North Atlantic existed prior to GEOTRACES (Fig.
231
Pad profiles at two locations at 38.5◦ N and 39.5◦ N near the western
289
A4). Luo et al. (2010) reported
290
boundary, which generally show an increase with depth down to 2000–2500 m and a decrease with depth
291
below. Similarly to 230 Thd,p data, the 231 Pad,p data obtained along GA03 and used in this study have already
292
been discussed (Hayes et al. 2015a). As for
293
upper part of the water column at all stations (Fig. A5). At the deepest stations, measurements of
294
present maxima between 2000–3000 m and a decrease with depth below. Measurements of 231 Pap display an
295
increase with depth down to 2000–3000 m and a decrease with depth below except for the deepest samples
230
Thd data,
13
231
Pad data show an increase with depth in the 231
Pad
230
296
(Fig. A6). As for
Thp , stations GT11-04 and GT11-08 show extreme
297
samples which largely exceed
298
c. Optical Data
231
Pap and
231
231
Pap values in near-bottom
Pad measured on the same or nearby samples.
299
A relatively large number of optical measurements from nephelometers and beam transmissometers are
300
available in the western North Atlantic (for a recent synthesis, see Gardner et al. (2017)). These instruments
301
can be used to measure the distribution of, respectively, light attenuation and light scattering with vertical
302
resolutions of approximately 2 m and 100-250 m, respectively (note that the range for the light scattering
303
resolution reflects the dependence of the resolution on the rate at which the nephelometer is lowered in
304
the water column;Gardner et al. (1985b)). From measurements of these optical properties, the vertical
305
distribution of PM concentration (P , mg m−3 ) can be estimated using empirical calibrations.
306
The following approach is applied to derive a field of PM concentration for use in the geochemical
307
component of the model. First, the light scattering and light attenuation data compiled by Gardner et al.
308
(2017, 2018b) are converted into P estimates using calibration formulae reported by Gardner et al. (2017)
309
for light scattering and Gardner et al. (2018b) for light attenuation. In addition, the light attenuation data
310
from GA03 and available from the GEOTRACES Intermediate Data Product (Schlitzer et al. 2018) are used
311
and converted to P estimates following Gardner et al. (2018b). Second, the PM concentrations at locations
312
where optical data are available are interpolated linearly at the model grid levels. The errors produced
313
by vertical interpolation are expected to be generally small given the high vertical resolution of the optical
314
profiles and are neglected in this work. Note that the model is based on terrain-following (s) coordinates, not
315
depth coordinates (section 3), so that the levels at which P values are interpolated occur along s-surfaces,
316
not depth surfaces. Finally, the interpolated P values are mapped along each s-surface using a minimum
317
variance (Gauss-Markov) procedure in order to obtain an estimate of PM concentration at each model grid
318
point along that surface (for details see Appendix A). Note that in this study the PM concentrations at
319
model grid points are fixed and not allowed to be transported by advection or mixing.
320
3. Ocean Model
321
a. Domain
322
The model domain considered in this study is the western North Atlantic north of 28◦ N and west of
323
55◦ W (Fig. 3). It is bounded in the west by the 200-m isobath, which is taken as a closed boundary. The
324
other two boundaries – the latitude of 28◦ N and the longitude of 55◦ W – are open. The model grid has a
14
325
horizontal resolution of 1/4◦ and includes 31 vertical levels. It is determined by a trade-off between a desire
326
to simulate the circulation with greatest detail possible and the large computational cost associated with
327
the simulation of steady-state distributions of
328
are s-coordinates defined from
230
Th and
s=
231
Pa (section 4). The vertical levels of the model
z−η . h+η
(1)
329
Here z is the local depth, η is the free surface elevation, and h = h(x, y) is the water depth where x and y
330
denote, respectively, the longitude and latitude coordinates in the domain. The model topography h(x, y)
331
is derived by averaging the bathymetric data from ETOPO2v2 (2006) in model grid cells.
332
b. Physical Component
333
The physical component of the model is the Princeton Ocean Model (POM) – a primitive-equation model
334
based on s-coordinates and a free surface (Blumberg and Mellor 1987; Mellor 2002). The computer code
335
applied in this study is pom2k.f. Note that in the present application, the variation of the Coriolis parameter
336
with latitude is taken into account according to f = fo + β(y − yo ), where fo = 2Ω sin φo , β = (2Ω/r) cos φo ,
337
Ω = 7.29 × 10−5 s−1 is Earth angular velocity, r = 6371 km is the Earth radius, φo = 36.375◦ N, and yo is the
338
value of y corresponding to the latitude φo . The basic equations of POM are the governing equations for the
339
zonal velocity component (u) and the meridional velocity component (v), the hydrostatic approximation, the
340
governing equations for (potential) temperature (T ) and salinity (S), and the condition of incompressibility.
341
These equations are complemented with a nonlinear equation of state (Mellor 1991).
342
c. Geochemical Component
343
The geochemical component of the model includes governing equations for
230
Th and
231
Pa in both the
344
dissolved phase and the particulate phase. The dissolved and particulate activities in the model are intended
345
to represent the dissolved and particulate activities as measured on water samples and defined operationally
346
by filtration. The basic equations of the geochemical component are ∂Ad + u · ∇Ad ∂t ∂Ap ∂Ap + u · ∇Ap + wp ∂t ∂z 230
Th or
= = 231
∂ ∂Ad λAπ + k−1 Ap − k1 Ad + κT + Fd , ∂z ∂z ∂ ∂Ap k1 Ad − k−1 Ap + κT + Fp . ∂z ∂z
(2a) (2b)
Pa in dissolved (particulate) form expressed in dpm m−3 , u is
347
Here Ad (Ap ) is the activity of
348
the fluid velocity, wp is the particle settling speed, λ is the radioactive decay constant for
349
Aπ is the activity of the radioactive parent (234 U for
15
230
Th and
235
U for
231
230
Th or
231
Pa,
Pa), k1 (k−1 ) is a first-order
350
apparent rate constant for Th or Pa adsorption onto particles (Th or Pa desorption from particles), κT
351
is a vertical turbulent diffusivity, and the terms (Fd , Fp ) denote the effects of horizontal mixing. Clearly,
352
t is time and ∇ = x ˆ(∂/∂x) + yˆ(∂/∂y) + zˆ(∂/∂z) is the gradient operator where (ˆ x, yˆ, zˆ) are unit vectors.
353
Vertical and horizontal mixing processes are represented as for temperature and salinity: the diffusivity
354
κT is obtained from a turbulent closure scheme (Mellor and Yamada 1982) which involves the solution of
355
governing equations for turbulent kinetic energy (q 2 /2) and for q 2 l, where l is a turbulence length scale, and
356
the terms (Fd , Fp ) are parameterized according to Smagorinsky (1963).
357
Note that the radioactive decay terms are omitted in (2a-2b). The half-lives of
230
Th and
231
Pa exceed
358
by several orders of magnitude the time scales of solid-solution equilibration of Th and Pa in laboratory
359
experiments (e.g., Nyffeler et al. (1984); Geibert and Usbeck (2004)). Likewise, the value of λ = 9.17 × 10−6
360
yr−1 for
361
in oceanic waters (for a compilation see Marchal and Lam (2012); Lerner et al. (2017)).
230
Th is very small compared to observational estimates of k1 and k−1 of ≥ O(10−1 yr
−1
) for Th
362
The influence of particle concentration on particle scavenging is accounted for in the model as follows.
363
Both theoretical considerations (e.g., Honeyman et al. (1988)) and empirical evidence (e.g., Lerner et al.
364
(2017)) suggest that the apparent rate constant for metal adsorption onto particles, k1 , increases with the
365
(bulk) concentration of particles. In our model, k1 is set to vary with particle concentration according to k1 (x, y, z) = k1,b + k10 P (x, y, z).
(3)
366
Here P (x, y, z) is the particle concentration estimated from the optical measurements (Appendix A), k10
367
describes the variation of the adsorption rate constant with particle concentration, and k1,b is a background
368
value intended to account for the effect of particles that are not detected by the optical instruments (for a
369
discussion of instrumental sensitivities, see, e.g., McCave (1986) and Boss et al. (2009)).
370
Three comments are in order regarding the geochemical component of the model. First, in a more
371
general treatment, k1 would be a function of P b where the exponent b could be different from one. From
372
a compilation of k1 estimates reported in the literature, Honeyman et al. (1988) showed that k1 appears
373
to be proportional to P b with b = 0.51 (their Table 2). These authors argued that such a variation of
374
the adsorption rate constant with particle concentration could explain the particle concentration effect. In
375
contrast, in their analysis of Th isotope and particle data collected east of Bermuda along GA03, Lerner
376
et al. (2017) estimated that b ranges from 0.95 to 1.62 (their Table 4), depending on the technique used to
377
regress lnk1 against lnP and on the application of a smoothing condition on the vertical distribution of the
378
rate parameters. Consistent with the latter result, a value of b = 1 is assumed in this study.
379
Second, in contrast to k1 , the desorption rate constant (k−1 ) and the particle settling speed (wp ) are
380
assumed to be independent of particle concentration or any other particle (or water) property. The k−1 16
381
values estimated from Th isotope and particle data at GA03 stations east of Bermuda (Lerner et al. 2017)
382
do not exhibit a consistent vertical trend, except at four stations where k−1 appears to decrease with depth.
383
Near-surface values of k−1 estimated at these stations are also high relative to the other stations. The wp
384
values estimated by Lerner et al. (2017) tend to be larger and display enhanced vertical variability below
385
2000 m, except at the TAG hydrothermal vent of the MAR, where their wp estimates are low relative to
386
the other stations. In the present study, k−1 and wp are taken as uniform throughout the model domain for
387
simplicity. Finally, the rate parameters (k1,b , k10 , k−1 ) are allowed to be different for
388
230
Th and
231
Pa in the model
389
in order to account for the differences in the sorption kinetics of Th and Pa in oceanic waters (e.g., Moran
390
et al. (2002); Hayes et al. (2015a)). The particle settling speed, wp , is also allowed to be different for the two
391
nuclides, a treatment suggested by the fact that (i) Th and Pa may be carried by different particulate phases
392
in the ocean (e.g., Chase et al. (2002); Geibert and Usbeck (2004); Roberts et al. (2009); Chuang et al.
393
(2013)), and (ii) different particles types may have different sinking speeds (e.g., biogenic silica, lithogenic
394
material, and calcium carbonte may sink much more rapdily than particulate organic carbon (Armstrong
395
et al. 2001; Klaas and Archer 2002)). The value of wp for
396
least-squares) the equation
397
230
230
Thp (z) =
230
230
Th is determined by fitting (using ordinary
Thp (0) + λ234 Th-230 Uz/wp to a composite profile compiled from all
Thp data in the upper 3000 m for stations GT11-01 to GT11-12 (not shown). This popular approach to 230
398
constrain wp (Th) from
399
Rutgers van der Loeff and Berger (1993); Scholten et al. (1995); Venchiarutti et al. (2008)) yields an estimate
400
of wp (Th) of 1800 m yr−1 , the value used in our simulations. In contrast, the value of wp for
401
generally less sensitive to particles and thus more strongly influenced by circulation than 230 Th, is determined
402
by a trial-and-error approach until good agreement with
403
for
404
231
230
Thp data (e.g., Bacon and Anderson (1982); Krishnaswami et al. (1976, 1981);
231
231
Pa, which is
Pad,p data is achieved. Thus, the value of wp
Thp is determined by disregarding the effects of advection and mixing, whereas the value of wp for
Pap is determined by taking such effects into account.
405
A trial-and-error approach is also used to determine the value of the other geochemical parameters with
406
two exceptions: k1,b for Th is fixed to 0.4 yr−1 , a value consistent with k1 estimates for GA03 stations east
407
of Bermuda in waters with low particle concentration (Lerner et al. 2017), and k1,b for Pa is fixed to a lower
408
value, 0.04 yr−1 , consistent with the different particle sensitivities of Pa and Th in seawater (e.g., Moran
409
et al. (2002); Hayes et al. (2015a)). Values for each parameter in the geochemical component of the model
410
are listed in Table 3.
17
411
d. Boundary Conditions
412
The conditions imposed at the horizontal and lateral boundaries of the model domain are summarized
413
below (for details see appendices B-C). Consider first the horizontal boundaries. At the sea surface, the
414
model is forced with an annual mean wind field derived from satellite scatterometry (Risien and Chelton
415
2008), and the simulated values of temperature and salinity in the surface layer are restored to annual mean
416
fields from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013) (section 2a). The restoring
417
approach implies that the surface (T, S) fields simulated by the model will not depart too markedly from the
418
climatologic fields – a desirable result – with the caveat that the modeled variability due to eddy activity
419
will be muted to some degree. At the bottom, a shear stress condition is applied and a condition of no flux
420
is specified for
230
Thd,p and
231
Pad,p .
421
Consider then the lateral boundaries. The western boundary (aligned with the 200-m isobath) is a
422
closed boundary. This treatment implies that the model simulations presented in this paper will not rep-
423
resent the exchanges of momentum, energy, and material between the continental shelf and the continental
424
slope (e.g., Garvine et al. (1988); Lozier and Gawarkiewicz (2001); Gawarkiewicz et al. (2004); Churchill
425
and Gawarkiewicz (2009)). At the other two lateral boundaries (along 28◦ N and along 55◦ W), radiation
426
conditions are specified in order to minimize the reflection of perturbations generated within the domain.
427
Along specific segments of these boundaries, inflows and outflows representing the Gulf Stream, the DWBC,
428
and Sargasso Sea water entering or exiting the domain are also applied, following previous regional model
429
studies of the western North Atlantic (e.g., Thompson and Schmitz (1984); Ezer (2016a,b)). Note that, at
430
the DWBC inflow, the boundary values of
431
the Labrador Sea, except for
432
reference solution that more closely agreed with the available radionuclide measurements. At all other lo-
433
cations along the open boundaries,
434
which is situated to the east of the model domain (Appendix C).
435
e. Method of Solution
231
230
Th and
231
Pa are based on measurements in deep waters in
Pad whose value exceeds these measurements, which we found produced a
230
Th and
231
Pa values are based on measurements at station GT11-14,
436
The differential equations of the model are solved as follows (for details see Blumberg and Mellor (1987);
437
Mellor (2002)). First, the differential equations are expressed in terrain-following (s) coordinates. The
438
particle settling speed wp , which appears in equation (2b), is converted into an equivalent speed in s-
439
coordinates using the relationship, ωp = wp cos θ,
18
(4)
440
where θ is the angle between the vertical axis and the normal to the s-surface. Here cos θ is approximated
441
from cos θ = p
1 (∂z/∂x)2s + (∂z/∂y)2s + 1
,
(5)
442
where the partial derivatives are evaluated using central differences. Second, the dynamical equations of
443
POM are separated into equations for slow motions (internal mode) and fast motions (external mode). This
444
‘mode splitting’ technique allows the calculation of the free surface elevation at relatively small computational
445
cost. Finally, the differential equations represented in s-coordinates are solved using finite differences on a
446
staggered C grid. The advection terms in the governing equations for (T, S, Ad , Ap , q 2 /2, q 2 l) are solved using
447
a central difference scheme, and these equations are integrated forward in time using a leap frog scheme with
448
an Asselin filter. The equations for the external mode are integrated with a time step of 15 s and the
449
equations for the internal mode, including the activity equations (2a–2b), are integrated with a time step of
450
450 s. Unless stipulated otherwise, the model parameters take the values listed in Tables 2–3.
19
Table 2. Parameters of the physical model component
451
Physical Parameters
value
units
ρo
reference density
1025
kg m−3
g
acceleration due to gravity
9.806
m s−2
C
Smagorinsky coefficient
0.2
1
κ
von K´arm´an constant
0.4
1
κu,o
background vertical viscosity
0
m2 s−1
κT,o
background vertical diffusivity
0
m2 s−1
Pr
turbulent Prandtl number
5
1
zr
bottom roughness parameter
0.01
m
τ
restoring time scale for SST and SSS
14.4
d
value
units
452
Numerical Parameters
453
454
∆tE
time step for external mode
15
s
∆tI
time step for internal mode
450
s
∆s
step interval for advective termsa
5
1
hmax
maximum depth in radiation condition
200
m
umax
maximum velocity for CFL violation
100
m s−1
c
constant of Asselin filter
0.1
1
α
weight for sea surface slope termb
0
1
a
Step interval during which the advective terms of the external mode are not updated
b
Weight used in the external mode equations 20
Table 3. Parameters of the geochemical model component
455
λTh-230
radioactive decay constant of
230
λPa-231
radioactive decay constant of
231
Th Pa
value
units
9.17 × 10−6
yr−1
2.12 × 10−5
yr−1
234
U
activity of
234
U
2750
dpm m−3
235
U
activity of
235
U
108
dpm m−3
k1 (Th)
adsorption rate constant for Th
variable
yr−1
k1 (Pa)
adsorption rate constant for Pa
variable
yr−1
k1,b (Th)
background value of k1 (Th)
0.4
yr−1
k1,b (Pa)
background value of k1 (Pa)
0.04
yr−1
k10 (Th)
sensitivity of k1 (Th) to particle concentration
0.04
yr−1 mg−1 m3
k10 (Pa)
sensitivity of k1 (Pa) to particle concentration
0.02
yr−1 mg−1 m3
k−1 (Th)
desorption rate constant for Th
3.69
yr−1
k−1 (Pa)
desorption rate constant for Pa
18.45
yr−1
1800
m yr−1
2400
m yr−1
456
457
wp (Th)
settling speed of
230
wp (Pa)
settling speed of
231
Thp Pap
4. Reference Solution
458
A large number of experiments have been conducted with the model in an effort to reproduce the
459
circulation and the radionuclide distributions observed in the western North Atlantic. In this section, we
460
describe a particular model solution, obtained with parameter values listed in Tables 2–3, which shows
461
relatively good agreement with the observations. This solution is called the ‘reference’ solution below. The
462
word ‘reference’ is meant to imply that this solution is used as a pivot against which results from other
463
simulations are compared, not that it is the most accurate simulation that could be obtained from the
464
model.
465
The model is integrated forward in time until the circulation and the radionuclide distributions reach
466
quasi-steady states. The initial conditions of the model are the following: (i) the ocean is at rest (u = 0,
467
q 2 = 0, and q 2 l = 0) with a flat surface (η = 0), (ii) the (T, S) distributions are set to the annual mean
468
distributions from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013), and (iii) the
21
230
Thd,p
231
469
and
Pad,p distributions are specified from idealized vertical profiles which broadly reproduce data from
470
station GT11-14 (located east of the domain along GA03) and which are also used as lateral boundary
471
conditions (Appendix C; Fig. A7). The model is first integrated diagnostically for a period of 10 days, with
472
(T, S, 230 Thd,p , 231 Pad,p ) fixed to their initial values, and then prognostically for a period of 2545 days (∼
473
7 yr), with (T, S, 230 Thd,p , 231 Pad,p ) allowed to vary according to their respective governing equations. At
474
this time, the domain averages of kinetic energy ρo |u2 |/2,
475
values (Fig. 4).
Thd,p , and
231
Pad,p have reached quasi-steady
Mean kinetic energy (m2/s2)
230
Mean
Mean
230
230
Thp (dpm m-3)
Thd (dpm m-3)
days
days
476
477
478
Pap (dpm m-3) Mean
Mean
231
231Pa
d
(dpm m-3)
days
days
days Fig. 4. Time series of the domain-averaged kinetic energy,
230
reference solution
22
Thd ,
230
Thp ,
231
Pad , and
231
Pap in the
230
Notice that the integration time needed to attain quasi-steady state is longer for
480
than it is for the mean kinetic energy (∼ 1 yr). Hence the model results reported in this paper are averages
481
for the last 2190 days (6 yr) of the integration for sea surface elevation and velocities, and for the last 365
482
d (1 yr) of the integration for
483
a. Circulation
230
Thd,p and
231
Th and
231
479
Pa (∼ 6 yr)
Pad,p .
484
1) Sea Surface Elevation
485
The distribution of the sea surface elevation simulated by the model is compared to the distribution of
486
the multi-year annual mean SSH derived from satellite altimetry (Fig. 5).
23
mean elevation (1993-2012)
CH
model elevation
CH
487
488
Fig. 5. Averages of sea surface height (m) as observed from satellite altimeter data during the period
489
1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf
490
Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
491
The SSH distribution from satellite altimeters reveals the average pathway of the Gulf Stream during the
24
492
observation period (1993–2012). Maxima in SSH occur just south of the Gulf Stream, downstream of the
493
region where this separates from the margin near Cape Hatteras, and minima in SSH occur in the region
494
between the Gulf Stream and the continental slope east of Cape Hatteras. The northward depression in SSH
495
across the Gulf Stream is of the order of 1 m and occurs on an horizontal scale of O(100 km). The pattern
496
of sea surface elevation simulated by the model is broadly consistent with the observed pattern, although
497
the magnitude of the SSHs and the gradients of SSH across the Gulf Stream as observed by altimeters are
498
generally underestimated by the model. The relatively small SSH gradients in the simulation implies that the
499
strength of the Gulf Stream as predicted by the model may be lower than observed, an inference consistent
500
with a comparison with velocity data along line W (see next section).
501
502
The standard deviations of SSH values derived by satellite altimetry are compared with the standard deviations of sea surface elevation calculated by the model (Fig. 6).
25
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
503
504
Fig. 6. Standard deviation of sea surface height (m) as observed from satellite altimeter data during the
505
period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the
506
Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
507
As expected, the standard deviations observed by altimeters show the largest values along the average
26
508
pathway of the Gulf Stream, which reflects the meandering and eddy activity of the Gulf Stream. Maxima
509
in standard deviations are also predicted in the vicinity of the Gulf Stream by the model, although the
510
standard deviations from the model are in general strongly underestimated compared to those observed.
511
Eddy variability is thus underpredicted by the model. Model-data differences in SSH (mean and standard
512
deviation) could be ascribed to various factors, such as insufficient spatial resolution of the model, the
513
restoring of surface (T, S) to climatologic fields, and the fact that model averages (6 yr) and data averages
514
(20 yrs) are not calculated over the same time span.
515
2) Horizontal Velocity
516
The horizontal velocities simulated by the model in the surface layer and at a depth of 3500 m are
517
displayed in Figure 7.
27
518
519
520
Fig. 7. Field of horizontal velocity in the surface layer (top) and at a depth of 3500 m (bottom) simulated
521
in the reference solution. The horizontal arrow at the lower right outside each panel is the maximum speed
28
522
in units of m s−1 in the corresponding field.
523
In the surface layer, the model simulates a strong current – the Gulf Stream – which flows northward along
524
the margin. The simulated current detaches from the margin north of Cape Hatteras, i.e., too far to the north
525
compared to the observations (Fig. 5), and then flows to the northeast as a meandering structure. Inaccurate
526
Gulf Stream separation has been a long-standing problem with many models, although simulations generally
527
improve when high resolution is used (e.g., Ezer (2016b); and references therein). The largest speeds in the
528
modeled Gulf Stream reach O(1 m s−1 ), which is consistent with observations from extensive field programs
529
(e.g., Meinen and Luther (2016)). At a depth of 3500 m, the model predicts a relatively strong current
530
flowing to the southwest along the boundary – the Deep Western Boundary Current. The modeled DWBC
531
presents distinct cores at some locations but remains generally noticeable all along the western boundary.
532
533
The simulated horizontal velocities between the New England continental shelf and Bermuda are compared to the velocities observed during the line W program (Fig. 8).
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
a
b
c
d
depth (m)
Model
534
535
Fig. 8. Distribution of horizontal velocity components between the New England continental shelf and
536
Bermuda as measured during the line W program (top) and as simulated in the reference solution
537
(bottom). At the top of each panel, red vertical lines show the position of GA03 stations GT11-01 to
538
GT11-06, and grey vertical lines show the position of mooring locations. Coordinates along the horizontal
29
539
axis are distances from 40.125◦ N,70.125◦ W (line W data from Toole et al. (2017)).
540
The observed velocity components both parallel and perpendicular to line W show the largest magnitudes in
541
the vicinity of the Gulf Stream, i.e., at a distance of ≥ 200 km from the shelf break. The Gulf Stream is the
542
most conspicuous feature along the section, showing flow to the northeast with velocity maxima in the upper
543
∼ 1000 m. Below the Gulf Stream and along the continental rise is the Deep Western Boundary Current,
544
which flows to the southwest and is characterized by much lower vertical shears and speeds compared to the
545
Gulf Stream. In accordance with these observations, the model simulates strong currents in the upper ∼
546
1000 m and along the western boundary. Whereas the simulated speeds in the Gulf Stream and DWBC have
547
the observed orders of magnitude, the horizontal velocities in the Gulf Stream (DWBC) are underestimated
548
(overestimated) by the model compared to the observations. Moreover, both currents in the model simulation
549
occur north of their respective observed locations. Again, model errors such as due to too coarse spatial
550
resolution and surface thermodynamical forcing may contribute to the differences between the simulation
551
and the observations.
552
b. Radionuclide Activities
553
We compare in this section the distributions of 230 Thd,p and 231 Pad,p calculated in the reference solution to
554
those observed from pre-GEOTRACES campaigns and along the western segment of GA03. Model results are
555
compared with both (i) station-averaged profiles of 230 Thd,p and 231 Pad,p computed by linearly interpolating
556
all radionuclide data at the same vertical levels and averaging the interpolated data along each vertical
557
level, and (ii) measured radionuclide profiles at individual stations. The rationale for considering both (i)
558
and (ii) is that a model driven by climatologic forcing may better reproduce station-averaged profiles than
559
profiles measured at specific locations and specific times. Notice that in the comparison between measured
560
and simulated radionuclide activities, the extreme
561
stations GT11-04 and GT11-08 (Figs. A3 and A6) are excluded.
562
The station-averaged profiles of
230
Thd and
231
230
Thp and
231
Pap values measured on two samples at
Pad display a reversal in activity gradients at mid-depth
563
(Figs. 9 and 10), as expected from the inspection of the profiles measured at the individual stations (Figs.
564
A1-A2 and Figs. A4-A5). Note also the relatively high
565
levels, below 4500 m. These averages are based on data from a relatively small number of stations and are
566
strongly influenced by data from station GT11-12, situated at > 500 km to the southeast of Bermuda (Fig.
567
3). Notably, the 230 Thd activities measured below ∼ 4500 m at station GT11-12 exceed 0.8 dpm m−3 , which
568
is larger by a least a factor of two than the
569
stations of GA03 (Fig. A2). The deepest value of 230 Thd shown in figure 9 comes from station GT11-12 only,
230
230
Thd and
231
Pad averages at the three deepest
Thd activities measured below ∼ 4500 m at the other deep
30
570
whilst the two values directly above are averages of data from GT11-12 as well as a few other stations. As a
571
result, the deepest value
572
by large standard errors (Fig. 9). Speculatively, the much higher 230 Thd and 231 Pad activities measured near
573
the bottom at GT11-12 compared to other stations of GA03 stem from the much weaker BNL at GT11-12,
574
as revealed by the sections of the beam attenuation coefficient for particles (Fig. 3 of Hayes et al. (2015a))
575
and of the bulk particle concentration (Fig. 5 of Lam et al. (2015)).
230
Thd is high, and the other two values are also relatively high and characterized
31
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th 576
230
Fig. 9. Profile of station-averaged
578
and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles
579
580
Thd (top) and
230
577
Thp (bottom) as calculated from pre-GEOTRACES
show averages of measurements from several stations with the following exceptions: for shallowest circle is a measurement from a single station (OC278-5), and for
32
230
230
Thd the
Thd,p the deepest circle is a
581
582
583
measurement from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230
measurements; Table 1). The extreme values of
230
Thd measurement and the deepest
230
Thd,p
Thp near the bottom of stations GT11-04 and GT11-08
are excluded from the station-averaged profile of the measurements.
depth (m)
584
d
(dpm m-3)
depth (m)
231Pa
585
586
587
231Pa Fig. 10. Profile of station-averaged
231
p
(dpm m-3)
Pad (top) and
231
Pap (bottom) as calculated from
pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution 33
588
589
590
591
592
(black line). The circles show averages of measurements from several stations with the following exceptions: for
231
Pad (231 Pap ), the three (four) deepest circles show measurements from a single station
(GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
231
Pap near the bottom
of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
593
1) Distribution of
230
594
The vertical profile of
230
Th
Thd obtained by averaging the available data in the study area is reproduced 230
595
rather closely in the reference solution (Fig. 9). The solution shows both the
596
in the upper ∼ 3000 m and the
597
holds over most of the water column, except for the three deepest observations. In contrast to
598
and consistent with the observed averages, the station-averaged
599
generally downwards over the entire ocean depth, although the observed
600
by the model (Fig. 9).
230
Thd increase with depth
Thd decrease below. The good agreement of the model with the data
230
230
Thd ,
Thp simulated by the model increases 230
Thp are generally overestimated
601
Similarly to the station averages, the simulated 230 Thd and 230 Thp profiles compare in general favourably
602
with the observed profiles at individual stations (Figs. A8 and A10). Noticeable differences with the
603
observations occur at pre-GEOTRACES station S1 (Guo et al. 1995) where
604
m of the water column are underestimated (Fig. A8), and at station GT11-12 where
605
3000 m are also underpredicted (Fig. A9). The reference solution generally overestimates
606
stations, particularly (i) below ∼ 2000 m at stations GT11-03, GT11-04, GT11-06, and GT11-08 and (ii)
607
over most of the water column at the deeper stations GT11-10 and GT11-12 (Fig. A10). Thus, whereas the
608
reference solution captures reasonably well the station-averaged profiles of
609
Atlantic, it is less successful at explaining the observations made at individual stations.
610
611
230
230
Thp data in the upper 1000 230
Thd data below ∼ 230
Thp at GA03
Thd,p in the western North
For future reference, we provide measures of the (dis)agreement of the reference solution with activity measurements from pre-GEOTRACES campaigns and the western segment of GA03 (Fig. 11).
34
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3 230
0.4
0.6
0.7
0.04
-0.02 -0.02
0.8
10
(dpm p data
0.05
3.5
Thp data (dpm/m 3)
0.1
0.04
0.06
0.1
m-3)
0.12
0.14
-3
3 2.5 2
230
m-3)
0.15
231Pa
Thd data (dpm/m 3)
0.2
0.08
Thp model (dpm/m 3) 230Th230model (dpm p
r2 = 0.11 p<0.01 n = 83
4
230
(dpm m-3) d data 231Pa
4.5
r2 = 0.81 p <0.01 n = 167
0.25
0.02
(dpm m-3) 5
0.3
0
Thd model (dpm/m 3)
230Th model d
0.35
0.5
0.06
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
0.12
(dpm m-3)
0.8
1.5 1 0.5 0 0
0 0
612
0.05
0.1
231Pa
0.15
0.2
-3) Thd model (dpm/m 3m ) (dpm d model
0.25
0.5
0.3
1
1.5
2
231Pa
230
230
2.5
3
3.5
Thp model (dpm/m 3)
-3 p model (dpm m )
4
4.5
5 10
-3
613
Fig. 11. Scatter plots of measured radionuclide activities versus simulated radionuclide activities in the
614
reference solution. Shown in each panel are the squared Pearson correlation coefficient (r2 ), the p value of
615
the correlation, and the number of measurements (n). In each panel, the black line is the line of perfect
616
agreement. For panel (b) and (d), the extreme measured values of
230
Thp and
231
Pap near the bottom of
617
stations GT11-04 and GT11-08, along with the corresponding model values, are excluded from the scatter
618
plot. 230
619
We find that the reference solution accounts for 85% of the variance in the
620
and for 49% of the variance in the
621
simulated activities is very significant for both the dissolved phase and the particulate phase (p < 0.01). The
622
root mean square difference between the measured and simulated activities amounts to 0.078 dpm m−3 for
623
624
230
Thd and to 0.028 dpm m−3 for
230
230
Thd measurements (n = 254)
Thp measurements (n = 98). The correlation between measured and
Thp (Table 4).
Table 4. Root Mean Square Difference Between Observed & Simulated Activitiesa
35
230
n
Thd
230
Thp
231
Pad
231
Pap
238
100
161
84
reference solution
0.078
0.028
0.039
0.001
k10 / 2
0.086
0.025
0.041
0.001
k10 × 2
0.086
0.037
0.037
0.003
DWBC inflow = 10 Sv
0.081
0.030
0.040
0.002
DWBC inflow = 40 Sv
0.076
0.029
0.037
0.002
Ad,p (DWBC inflow) / 2
0.078
0.025
0.033
0.001
Ad,p (DWBC inflow) × 2
0.165
0.041
0.110
0.002
uniform k1 (T h) & k1 (P a)
0.075
0.043
0.041
0.002
Ad,p (DWBC inflow) / 2, DWBC inflow = 10 Sv
0.093
0.027
0.041
0.002
625
626
a
All values in dpm m−3
627
2) Distribution of
628
The station-averaged
231
231
Pa
Pad for (pre-)GEOTRACES data are broadly reproduced in the reference solu-
629
tion in the upper 3000 m of the water column, but they are overpredicted at greater depths except near 5000
630
m (Fig. 10). The reversal in the vertical
631
m) than in the observations (∼ 3000 m). As for
632
data tend to be overestimated at most levels in the reference solution, except between about 2000–3000 m
633
where the simulation agrees closely with the observational averages. Note that the station-averaged profiles
634
of 231 Pad,p are largely unaltered if wp for 231 Pap is decreased from its reference value of 2400 m yr−1 to 1800
635
m yr−1 , the value used for
636
agreement with the measured
230
231
Pad gradient occurs deeper in the model simulation (at ∼ 4000 231
Pad , the station-averaged
Thp (not shown). The simulated 231
231
231
Pap computed from GA03
Pad profiles show in general the closest
Pad profiles at individual stations in the upper ∼ 3000 m of the water 36
637
column (Figs. A11 and A12). Below this depth, the model overestimates the 231 Pad measurements, with the
638
notable exception of station GT11-12, where the observed 231 Pad are rather closely reproduced below 3000 m
639
and underestimated between about 2000–3000 m. Inspection of
640
that the largest differences between the simulated and observed
641
GT11-03 to GT11-08 and in the upper ∼ 2000 m at station GT11-10 (Fig. A13). 230
642
As for
643
surements of
644
231
231
Pap profiles at individual stations shows
231
Pap occur near the seafloor at stations
Thd,p , we quantify the (dis)agreement of the reference solution with (pre-)GEOTRACES mea231
Pad,p activities (Fig. 11). The reference solution “explains” 81% of the variance in the
Pad measurements (n = 167) and only 11% of the variance in the
231
Pap measurements (n = 83). The
645
correlation between measured and simulated activities is very significant, even for the particulate phase
646
(p < 0.01). The root mean square difference between the simulated and observed activities amounts to 0.039
647
dpm m−3 for
231
Pad and to 0.001 dpm m−3 for
231
231
Pap (Table 4).
Pa/230 Th
648
3) Comparison to Sediment
649
The enhanced deposition of particle-reactive substances in ocean-margin sediments is often referred to as
650
‘boundary scavenging’ (Spencer et al. 1981). Anderson et al. (1994) found that deposition rates of 230 Th and
651
231
Pa measured during the SEEP-I and SEEP-II programs in the Middle Atlantic Bight exceed their local
652
rates of supply, consistent with boundary scavenging. However, they also found that the 231 Pa/230 Th activity
653
ratios of surface sediments are consistently less than the
654
odds with the notion that
655
is generally less prone to particle scavenging than
656
its production site). The authors postulated that the anomalous boundary scavenging of 230 Th and 231 Pa in
657
the Middle Atlantic Bight could be explained by the export from the region of fine-grained Mn-rich particles
658
that would scavenge greater portions of
231
231
Pa/230 Th production ratio of 0.093, which is at
Pa should be preferentially deposited over
231
230
230
Th in marginal sediments (231 Pa
Th and thus more likely to be transported away from
Pad than of
230
Thd .
659
In this section, the distribution of 231 Pap /230 Thp near the bottom (deepest grid point) which is simulated
660
in the reference solution is compared to surface sediment 231 Pa/230 Th data which are available in the western
661
North Atlantic (Table 5).
37
Table 5. Surface sediment
662
231
Pa/230 Th data in the western North Atlantic.
231
Pa/230 Th
core
latitude
longitude
depth (m)
reference
OCE152-BC1
39.49◦ N
70.57◦ W
1126
0.082
Anderson et al. (1994)
OCE152-BC8
32.47◦ N
70.58◦ W
1596
0.071
Anderson et al. (1994)
OCE152-BC9
39.42◦ N
70.55◦ W
1981
0.091
Anderson et al. (1994)
OCE152-BC5
39.08◦ N
70.56◦ W
2691
0.063
Anderson et al. (1994)
EN123-BC4
39.48◦ N
70.56◦ W
1280
0.076
Anderson et al. (1994)
EN123-BC6
39.49◦ N
70.55◦ W
1643
0.066
Anderson et al. (1994)
EN123-BC3
39.35◦ N
70.55◦ W
2344
0.061
Anderson et al. (1994)
EN123-BC1
39.08◦ N
70.55◦ W
2736
0.053
Anderson et al. (1994)
EN179-BC5
37.38◦ N
74.13◦ W
384
0.127
Anderson et al. (1994)
EN179-BC2
37.37◦ N
74.10◦ W
892
0.050
Anderson et al. (1994)
EN179-BC3
37.38◦ N
74.09◦ W
1031
0.075
Anderson et al. (1994)
EN179-BC4
37.32◦ N
74.02◦ W
1318
0.071
Anderson et al. (1994)
EN179-BC7
37.25◦ N
73.49◦ W
1989
0.051
Anderson et al. (1994)
EN187-BC4
37.37◦ N
74.13◦ W
512
0.063
Anderson et al. (1994)
EN187-BC10
36.52◦ N
74.37◦ W
580
0.089
Anderson et al. (1994)
EN187-BC8
36.52◦ N
74.34◦ W
1020
0.053
Anderson et al. (1994)
EN187-BC5
37.37◦ N
74.10◦ W
1045
0.069
Anderson et al. (1994)
EN187-BC11
37.02◦ N
74.34◦ W
1125
0.062
Anderson et al. (1994)
EN187-BC9
36.52◦ N
74.34◦ W
1165
0.075
Anderson et al. (1994)
EN187-BC6
37.24◦ N
73.5◦ W
2000
0.055
Anderson et al. (1994)
OCE325-GGC5
33.7◦ N
57.6◦ W
4550
0.054
McManus et al. (2004)
VM26-176
32.76◦ N
70.78◦ W
1126
0.065
Yu (1994)
663
664
The sediment
231
Pa/230 Th data used for this comparison originate from the compilation reported by
665
Henry et al. (2016) and come for the most part from the SEEP-I and SEEP-II programs (Anderson et al.
666
1994).
667
It is seen that the near-bottom 231 Pap /230 Thp ratio in our reference solution show relative minima between 231
Pap /230 Thp present
668
1000–3000 m in the Middle Atlantic Bight; east of the 3000-m isobath, the simulated
669
maxima in multiple regions predominately located in the northern part of the domain (Fig. 12). 38
a a
c b
670
231
Pap /230 Thp in the reference experiment. The filled circles are
671
Fig. 12. Distribution of near-bottom
672
surface sediment data (Table 5), and the solid black lines are the 200 m, 1000 m, and 3000 m isobath,
673
respectively. 231
Pap /230 Thp ratios below the production ratio of 0.093 along the margin is consistent
674
The simulation of
675
with measurements of surface sediment
676
port of fine-grained Mn-rich particles (Anderson et al. 1994) may not be necessary to explain the anomalous
677
boundary scavenging observed in the Bight. In other words, sediment
678
duction ratio may also arise from the joint effects of ocean circulation and particle scavenging as represented
679
in our model. The dissolved and particulate
680
tively, 0.22/0.40 = 0.55 and 0.003/0.03 = 0.10 (Appendix C), i.e., to values that exceed the
681
production ratio. Consequently, the near-bottom 231 Pap /230 Thp ratios below the production ratio which are
682
simulated in the Middle Atlantic Bight do not reflect the
683
ary but should rather result from processes operating within the model domain. On the other hand, the
684
sediment
685
13), indicating that the ability of the model to explain the variability of sediment
686
Bight is very limited. Various factors could explain the small correlation between the simulated near-bottom
687
231
231
231
Pa/230 Th in the Middle Atlantic Bight, suggesting that the ex-
231
231
Pap /230 Thp ratios below the pro-
Pa/230 Th ratios at the DWBC inflow are set to, respec-
230
Thd,p and
231
231
Pa/230 Th
Pad,p values set at the bound-
Pa/230 Th show a positive but insignificant relationship with the simulated 231
231
Pap /230 Thp (Fig.
Pa/230 Th within the
Pap /230 Thp and the measured sediment 231 Pa/230 Th, such as (i) model errors, including errors due to the
688
omission of the process postulated by Anderson et al. (1994), (ii) bioturbation within the sediment column,
689
and (iii) sediment lateral redistribution (e.g., Kretschmer et al. (2010, 2011)).
39
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.08
0.1
0.12
0.14
0.16
0.18
PaTh sed model modeled 231Pa/230Th
690
691
0.06
Fig. 13. Scatter plot of surface sediment
231
Pa/230 Th data versus the near-bottom
231
Pap /230 Thp
692
simulated near the corresponding data location in the reference experiment. The regression coefficient
693
(slope) is 0.35 ± 0.36 (one standard error) and the Pearson correlation coefficient is 0.21 (n = 22). The
694
black line is the line of perfect agreement.
695
696
5. Sensitivity Experiments In this section, we explore the sensitivity of the
230
Thd,p and
231
Pad,p distributions simulated by the
697
circulation-geochemical model to the intensity of particle scavenging and to properties of the DWBC. Specif-
698
ically, numerical experiments are conducted with varying values of (i) the sensitivity of the adsorption rate
699
constant k1 to particle concentration (k10 ), (ii) the volume transport of the Deep Western Boundary Current
700
at its inflow location, or (iii) the 230 Thd,p and 231 Pad,p activities of the DWBC at its inflow location. For each
701
experiment, the other model parameters and boundary conditions are the same as those for the reference
702
solution (section 4).
40
703
a. Particle Scavenging Intensity
704
We consider two numerical experiments where the sensitivity of k1 to P is either halved or doubled
705
relatively to the values of k10 assumed for Th and Pa in the reference solution (Table 3). Consider first the
706
effect of changing k10 for Th, k10 (Th), from its reference value of 0.04 yr−1 mg−1 m3 to either 0.02 or 0.08
707
yr−1 mg−1 m3 . A larger value of k10 means that a larger influence of PM concentration and hence of BNLs
708
on chemical scavenging is incorporated into the model. As expected, 230 Thd is found to decrease and 230 Thp
709
is found to increase as k10 (Th) is enhanced (top panels of Fig. 14). d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
710
Fig. 14. Profile of station-averaged
711
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
712
(pre-)GEOTRACES measurements (black circles) and as simulated for k10 (Th) = 0.02 yr−1 mg−1 m3 and
713
k10 (Pa) = 0.01 yr−1 mg−1 m3 (blue lines), k10 (Th) = 0.04 yr−1 mg−1 m3 and k10 (Pa) = 0.02 yr−1 mg−1 m3
714
(black, reference solution), and k10 (Th) = 0.08 yr−1 mg−1 m3 and k10 (Pa) = 0.04 yr−1 mg−1 m3 (green).
715
The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for
716
717
718
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages
(measurement error for the shallowest three (four) deepest measurements of
230
231
Thd measurement, the deepest
230
Thd,p measurements, and the
Pad (231 Pap ); Table 1). The extreme values of
41
230
Thp (Fig. 3) and
719
231
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
720
230
721
The agreement with
722
compared to the reference solution: the root mean square difference between the observed and simulated
723
230
Thd measured at (pre-)GEOTRACES stations, however, is only weakly modified
Thd , rmsd(230 Thd ), amounts to 0.086 dpm m−3 for both k10 (Th) = 0.02 and 0.08 yr−1 mg−1 m3 (Table
724
4). The solution with k10 (Th) = 0.02 yr−1 better describes the observed
725
solution, although the agreement with the observed 230 Thd averages deteriorates (Fig. 14) and rmsd(230 Thp )
726
is only slightly reduced compared to the reference solution (Table 4). The solution with k10 (Th) = 0.08 yr−1
727
mg−1 m3 strongly overestimates the observed
728
the surface and the bottom of the profile (Fig. 14), with rmsd(230 Thp ) reaching 0.037 dpm m−3 (Table 4).
729
230
230
Thp averages than the reference
Thp averages over most of the water column, except near
Consider then the effect of altering k10 (Pa) from its reference value of 0.02 yr−1 mg−1 m3 to either 0.01 or
730
0.04 yr−1 mg−1 m3 (bottom panels of Fig. 14). Similarly to
230
731
decrease (increase) as k10 (Pa) is enhanced. Interestingly, the
231
732
a small sensitivity to k10 (Pa) in the upper ∼ 3000 m, which suggests that particle scavenging is in general a
733
small term in the budget of
734
the low
735
particles is assumed to be more sensitive to particle concentration, although rmsd(231 Pad ) is reduced only
736
slightly (Table 4). However, in this case, the model strongly overestimates the 231 Pap averages observed over
737
the entire column (Fig. 14), which illustrates a difficulty to explain simultaneously the observed
738
dissolved and particulate forms. As shown above, a similar difficulty arises for 230 Th, although the reference
739
solution appears to better reproduce the observed
740
(Fig. 14).
741
b. Strength of the DWBC Inflow
742
231
231
Thd,p , the station-averaged
231
Pad (231 Pap )
Pad averages simulated by the model show
Pad in this part of the water column. Compared to the reference solution,
Pad averages observed below 3000 m are better reproduced by the model if Pa adsorption onto
230
Thd,p averages than the observed
231
231
Pa in
Pad,p averages
We now examine how the strength of the Deep Western Boundary Current at its inflow location (between
743
42.5–44.875◦ N along 55◦ W; Appendix C) affects the distributions of
744
particulate forms in the western North Atlantic domain. Observations along the continental slope and rise in
745
the western North Atlantic suggest considerable temporal variations in the volume transport of the DWBC.
746
Four repeat hydrographic sections across the DWBC at 55◦ W were occupied in order to investigate the inter-
747
annual variability of the deep flow (Pickart and Smethie 1998). The volume transport was estimated from
748
geostrophic velocities in four density layers corresponding to Upper Labrador Sea Water (ULSW), Classical
749
Labrador Sea Water (CLSW), Iceland-Scotland Overflow Water (ISOW), and Denmark Strait Overflow
42
230
Th and
231
Pa in both dissolved and
750
Water (DSOW). The 4-layer summed transport during occupations in 1991, 1994, and 1995 was estimated
751
to range from 12.6 to 25.2 Sv, with an average of 18.8±6.3 Sv. More recently, current meter observations
752
collected along line W from May 2004 to April 2008 showed that the 5-d averaged transport summed in the
753
ULSW, CLSW, ISOW, and DSOW layers ranged from 3.5 Sv to 79.9 Sv, with a record mean of 25.1 Sv and
754
standard deviation of 12.5 Sv (Toole et al. 2011). Bias adjustment to account for the finite width of the
755
mooring array increased the mean transport estimate to 28.7 Sv (Toole et al. 2011).
756
Here two numerical experiments are considered, where the volume transport of the DWBC inflow is
757
changed from its value of 20 Sv in the reference solution to either 10 Sv or 40 Sv. These experiments are not
758
intended to be realistic; their sole purpose is to document the sensitivity of the radionuclide distributions
759
to sizeable variations in the strength of the DWBC inflow in the model. It is seen that the station-averaged
760
profiles of
761
10 to 40 Sv (Fig. 15).
230
Thd,p and
Pad,p experience only modest changes if the DWBC inflow is strengthened from
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231
231Pa
p
(dpm m-3)
depth (m)
231Pa
762
763
Fig. 15. Profile of station-averaged
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
764
(pre-)GEOTRACES measurements (black circles) and as simulated when the strength of the DWBC at its
765
inflow location is set to 10 Sv (green lines), 20 Sv (black, reference solution), and 40 Sv (blue). The circles
766
show averages of measurements from several stations, with the exceptions listed in Figure 9 for
43
230
Thd,p
767
and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages (measurement 230
768
error for the shallowest
769
deepest measurements of
Thd measurement, the deepest
231
230
Thd,p measurements, and the three (four)
Pad (231 Pap ); Table 1). The extreme values of
230
Thp (Fig. 3) and
231
Pap
770
(Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile
771
of the measurements.
772
The differences between the simulated radionuclide activities and the measured radionuclide activities from
773
(pre-)GEOTRACES cruises are not greatly altered by changes in the DWBC inflow: as the DWBC inflow is
774
increased from 10 to 40 Sv, rmsd(230 Thd ) changes from 0.081 to 0.076 dpm m−3 , rmsd(230 Thp ) from 0.030
775
to 0.029 dpm m−3 , and rmsd(231 Pad ) from 0.040 to 0.037 dpm m−3 , whereas rmsd(231 Pap ) amounts to the
776
same value of 0.002 dpm m−3 (Table 4).
777
Notice that the small variations of the station-averaged profiles with DWBC inflow do not imply that the
778
DWBC inflow has also a modest influence on local radionuclide profiles. A change in DWBC inflow from 10
779
to 40 Sv does produce appreciable changes at some locations, particularly in the vicinity of the inflow and
780
near the depth of the current core (not shown).
781
c. Radionuclide Activities in the DWBC Inflow
782
Finally, we explore the sensitivity of the 230 Thd,p and 231 Pad,p distributions to the radionuclide activities 230
which are assumed at the location of the DWBC inflow. Although
784
available to constrain the variability of the two radionuclides near this location, hydrographic observations
785
indicate that water properties of the DWBC can exhibit significant inter-annual variations. In their analysis
786
of repeat sections along 55◦ W, Pickart and Smethie (1998) reported that the largest property variability
787
between 1983–1995 occurred in the CLSW, which in the 1990s became markeldy colder, fresher, and richer
788
in dissolved oxygen and CFCs, all suggestive of ‘new ventilation’. Observations along line W from 1995 to
789
2014 revealed water mass changes that are consistent with changes in source water properties upstream in
790
the Labrador Sea (Le Bras et al. 2017). Particularly evident was the cold, dense, and deep class of Labrador
791
Sea Water (dLSW) that was presumably created by recurring convection events during severe winters in
792
1987–1994. From 2010 to 2014, the density of DSOW within the DWBC along line W was found to decrease
793
as a result of warming overcompensating a slight salinity increase (Andres et al. 2016).
794
Here we consider two numerical experiments where the
230
Th and
231
783
Thd,p and
231
Pa measurements are not
Pad,p activities at the DWBC
795
inflow location are halved or doubled, compared to their respective values in the reference solution (Appendix
796
C). It is seen that the station-averaged
797
variations in radionuclide activities at the DWBC inflow (Fig. 16).
230
Thd,p and
231
44
Pad,p vary markedly at most depths in response to
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
798
799
Fig. 16. Profile of station-averaged
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
800
(pre-)GEOTRACES measurements (black circles) and as simulated when the radionuclide activities at the
801
DWBC inflow are halved (green lines) or doubled (blue) compared to their values in the reference solution
802
(black). The circles show averages of measurements from several stations, with the exceptions listed in
803
Figure 9 for
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the
804
averages (measurement error for the shallowest
230
Thd measurement, the deepest
805
and the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
806
(Fig. 3) and
231
230
Thd,p measurements, 230
Thp
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
807
230
As a result, the agreement with observational averages is noticeably modified: as
809
the DWBC inflow are changed from half to twice their reference values, rmsd(230 Thd ) (rmsd(230 Thp )) varies
810
from 0.078 and 0.165 dpm m−3 (0.025 to 0.041 dpm m−3 ), whereas rmsd(231 Pad ) (rmsd(231 Pap )) varies
811
from 0.033 and 0.110 dpm m−3 (0.001 to 0.002 dpm m−3 ) (Table 4). Thus, in the model, the radionuclide
812
contents of the DWBC inflow more strongly influence the station-averaged profiles than the strength of the
813
DWBC inflow.
45
Thd,p and
231
808
Pad,p at
814
815
6. Discussion The results presented in section 4 show that the circulation-geochemical model considered in this study 230
can capture most (> 80%) of the variance in the measurements of
817
obtained from (pre-)GEOTRACES cruises in the western North Atlantic. Notably, the reversal in the 230 Thd
818
and
819
m on average for
820
and 231 Pa in the particulate phase could also be broadly reproduced, although a model simulation could not
821
be found that closely replicates radionuclide measurements in the two phases simultaneously. Conceivably, a
822
combination of model parameters and (or) boundary conditions different than the ones that have been tested
823
could lead to a significantly better fit of model results with radionuclide measurements in the two phases.
824
Such a combination, however, is probably best sought by using an inverse procedure, which is beyond the
825
scope of this study.
231
Th and
231
816
Pa in the dissolved phase
Pad gradients observed at mid-depth can be reproduced by the model, though too deeply by ∼ 1000 231
Pad . Results from sensitivity experiments (section 5) show that measurements of
230
Th
826
Despite the difficulty experienced to closely replicate 230 Th and 231 Pa data in both the dissolved phase and
827
the particulate phase, the present model does seem to be appropriate for investigating important questions
828
about the observed distributions of 230 Th and 231 Pa in the western North Atlantic. Of particular significance
829
are the observed reversals in the vertical 230 Thd and 231 Pad gradients at mid-depth, leading to radionuclides
830
activities in deep waters that are markedly lower near the western margin than far from the margin (Fig. 1).
831
The model experiments illustrated in section 5 suggest that the elevated particle concentrations in benthic
832
nepheloid layers could enhance chemical scavenging and hence produce the low
833
which have been measured in these waters (Fig. 14).
230
Thd and
231
Pad activities
834
In the remainder of this section, we first examine in more detail the importance of particle scavenging
835
relatively to DWBC inflow in setting the radionuclide distributions in the western North Atlantic. Our
836
results are then compared to those obtained in previous model studies. The potential of BNLs as sites of
837
intensified scavenging and the possibility of significant temporal variations in chemical scavenging in the
838
deep western North Atlantic are subsequently discussed. Finally, we clarify the implications of our results
839
for the use of
840
(2002)).
841
a. Importance of Particle Scavenging
842
230
Th as a ventilation tracer (e.g., Moran et al. (1997); Vogler et al. (1998); Moran et al.
We consider another experiment that further illustrates the potential role of spatial variations in particle 230
scavenging, and of BNLs in particular, in setting the distributions of
844
Atlantic. In this experiment, the apparent rate constants for Th and Pa adsorption are uniform and equal
46
Th and
231
843
Pa in the western North
845
to the domain-averaged values of k1 (Th) and k1 (Pa) in the reference solution. This treatment ensures that
846
the differences in the radionuclide distributions between this experiment and the reference experiment are
847
due to different assumptions about spatial variations in k1 (uniform versus non-uniform), and not due to
848
different domain-averaged k1 values. We find that, with uniform k1 (Th) and k1 (Pa),
849
850
851
230
Thd and
231
Pad averages increase monotonically with
depth, in contrast to the reference solution (and the observations), which shows inversions in 231
230
Thd and
Pad profiles at mid-depth (Fig. 17). d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
852
Fig. 17. Profile of station-averaged
853
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
854
(pre-)GEOTRACES measurements (black circles) and as simulated for uniform k1 (Th) and k1 (Pa) when
855
the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (red), and 40 Sv (blue).
856
The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230
857
858
859
860
861
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the averages
(measurement error for the shallowest three (four) deepest measurements of 231
230
231
Thd measurement, the deepest
230
Thd,p measurements, and the
Pad (231 Pap ); Table 1). The extreme values of
230
Thp (Fig. 3) and
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements. 47
230
As a result, the low
863
The root mean square differences for
864
or higher than those for variable k1 (Th) and k1 (Pa) (Table 4). Experiments with a DWBC inflow of 10, 20,
865
or 40 Sv all show a monotonic increase of
866
(Fig. 17). Overall, these results suggest that the low 230 Thd and 231 Pad activities observed near the western
867
margin are more likely due to enhanced scavenging in the deep water column than to ventilation by
868
and
231
Thd and
231
862
Pad activities observed in deep waters are not reproduced by the model. 230
Thd,p and
230
231
Thd and
Pad,p for uniform k1 (Th) and k1 (Pa) are comparable
231
Pad with depth if k1 (Th) and k1 (Pa) are uniform
230
Thd -
Pad -poor waters from the western boundary.
869
The small sensitivity to changes in DWBC inflow (section 5b) may be a consequence of our choice of
870
lateral boundary conditions. To test this possibility, we consider an experiment where two modifications to
871
the reference experiment are brought simultaneously: (i) the DWBC inflow is increased from 20 Sv to 40
872
Sv and (ii) the radionuclide activities at the DWBC inflow location are halved compared to the reference
873
values. The resulting values of the rmsd between observed and simulated activities are listed in Table 4. It
874
is seen that these values are generally larger than those for the reference experiment and for the experiment
875
where the activities at the DWBC inflow are halved but the strength of the DWBC inflow is kept equal to 20
876
Sv. Compared to these two experiments, strengthening the DWBC inflow and lowering the activities at the
877
DWBC inflow noticeably lowers the average
878
m (Fig. 18). However, the dissolved and particulate activities below ∼ 3500 m remain largely unaltered in
879
the experiment where (i) and (ii) are both implemented, suggesting that the small sensitivity of radionuclide
880
activities in deep waters to changes in DWBC inflow is a robust result of the model.
d
(dpm m-3)
d
(dpm m-3)
Thd,p and
231
Pad,p in the upper water column above ∼ 3500
230Th
p
(dpm m-3)
depth (m)
230Th
230
231Pa
depth (m)
231Pa
881
48
p
(dpm m-3)
Fig. 18. Profile of station-averaged
882
230
Thd,p (top) and
231
Pad,p (bottom) as calculated from
883
(pre-)GEOTRACES measurements (black circles) and as simulated for the reference experiment (black),
884
when the radionuclide activities at the DWBC inflow locations are halved (green), and when the
885
radionuclide activities at the DWBC inflow locations are halved, and the DWBC at its inflow is 40 Sv
886
(blue). The circles show averages of measurements from several stations, with the exceptions listed in
887
Figure 9 for
230
Thd,p and in Figure 10 for
231
Pad,p . The horizontal bars show the standard errors of the
888
averages (measurement error for the shallowest
230
Thd measurement, the deepest
889
and the three (four) deepest measurements of
231
Pad (231 Pap ); Table 1). The extreme values of
890
(Fig. 3) and
231
230
Thp
station-averaged profile of the measurements.
b. Comparison to Previous Model Studies In this section, we briefly put our results in the context of three recent model studies on the distribution
893
894
Thd,p measurements,
Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the
891
892
230
231
Pa and
230
Th in the global ocean (Rempfer et al. 2017; Gu and Liu 2017; van Hulten et al. 2018).
895
Some of the key differences between these previous models and the one considered in this study are worth
896
mentioning. In these previous models, particle scavenging is determined from the distribution of different
897
particle types simulated by the models, in contrast to our model which includes a single particle field directly
898
based on observations. Moreover, the models of Gu and Liu (2017) and van Hulten et al. (2018) simulate
899
total activities, not dissolved and particulate activities separately. This approach has the benefits that the
900
radionuclide distributions can be calculated by solving a single equation and that sorption reactions need
901
not be treated explictly, although it does require assumptions about solid-solution partitioning. Perhaps
902
more important, none of these previous models represent the large particle concentrations that characterize
903
benthic (and intermediate) nepheloid layers.
904
Interestingly, the difficulty to produce simulations that agree simultaneously with
230
Th and
231
Pa mea-
905
surements in dissolved and particulate forms is a common result of all three models (Rempfer et al. 2017;
906
Gu and Liu 2017; van Hulten et al. 2018). Rempfer et al. (2017) acknowledged the difficulty to produce a
907
simulation that simultaneously accords with 230 Th and 231 Pa data in the two phases, and found that a better
908
agreement is obtained by representing in their model additional sinks at the sea floor (‘bottom scavenging’)
909
and at continental boundaries (‘boundary scavenging’). van Hulten et al. (2018) reported that the
910
and
911
of the ocean, but that
912
nepheloid layers. Finally, Gu and Liu (2017) stated that their model can simulate
231
230
Thd
Pad distributions simulated by their model compare ‘well’ with GEOTRACES data in many parts 230
Thp and
231
Pap activities are underpredicted because of missing particles from
49
230
Thd and
231
Pad dis-
230
Thp and
231
913
tributions that are in ‘good agreement’ with observations but that
Pap are generally smaller
914
than observed in the deep ocean (and generally larger than observed in the surface ocean).
915
The difficulty to reproduce in the same simulation the observed radionuclide distributions in the two
916
phases is also encountered in this work, although it is perhaps less severe owing to the consideration of BNLs
917
and of the associated enhancement in particle scavenging in deep water. Rempfer et al. (2017) included
918
simplified representations of ‘bottom scavenging’ and ‘boundary scavenging’ in their computationally efficient
919
model with coarse horizontal resolution (36×36 grid cells). ‘Bottom scavenging’ was represented by applying
920
a globally uniform concentration of resuspended particles to grid cells adjacent to the bottom, and ‘boundary
921
scavenging’ was represented by increasing by a uniform factor the adsorption rate constant for
922
cells adjacent to continents. The simulation representing both forms of scavenging and illustrated in their
923
paper (Rempfer et al. (2017); their Fig. 2) shows that the deep water 230 Thd and 231 Pad activities are reduced
924
compared to a simulation where ’bottom scavenging’ and ‘boundary scavenging’ are absent. However, in this
925
simulation, both
926
to present the
927
apparent lack of
928
be due to the fact that ‘bottom scavenging’ was represented only in the deepest grid cells of their model,
929
whereas we allow for enhanced k1 over the entire thickness of BNLs as estimated from the optically-derived
930
particle field.
931
c. BNLs as Sites of Intensified Scavenging
230
230
Thd and
230
231
Thd and 231
Thd and
231
Pa in grid
Pad appear to increase monotonically with depth, i.e., they did not seem
Pad inversions at mid-depth as shown in observations and in our model. The
231
Pad inversions in the simulation illustrated by Rempfer et al. (2017) could
932
Whereas boundary scavenging is expected to affect all particle-reactive species with long residence times,
933
the processes leading to enhanced deposition of these species in marginal sediments are not completely
934
understood. One process, however, could operate at many margins: the intensified scavenging in particle-
935
rich nepheloid layers (benthic and intermediate) that may be found near continental slopes (for reviews
936
see McCave (1986); Gardner et al. (2018a)). Benthic nepheloid layers near the western North American
937
margin are characterized by particle concentrations that are much higher than, and particle compositions
938
that are distinct from, those in surrounding waters (e.g., Lam et al. (2015); Gardner et al. (2017)). Particles
939
sampled from BNLs near the western margin along GA03 presented concentration levels of up to 1648 mg
940
m−3 and were dominated by lithogenic material (Lam et al. 2015). If the adsorption rates of particle-reactive
941
metals increase with particle abundance, as postulated on theoretical grounds (e.g., Honeyman et al. (1988))
942
and supported by observational evidence (e.g., Honeyman et al. (1988); Lerner et al. (2017)), then these
943
layers would be characterized by relatively large removal rates of Th and Pa from solution. Less obvious,
50
944
however, is the effect of the chemical composition of resuspended sediment on Th and Pa scavenging in the
945
water column. Lithogenic material was not found to strongly contribute to k1 variance at GA03 stations
946
east of Bermuda (Lerner et al. 2017), although this analysis focused on data collected outside BNLs. More
947
generally, particles resuspended from the seabed would lead to scavenging rates that are different from those
948
in surrounding waters if their chemical composition differs from that of the primary settling material from
949
surface waters. Various factors could produce such differences: (i) temporal variations in the composition of
950
the primary flux, (ii) diagenetic changes in the sediment (later resuspended), (iii) differential resuspension of
951
particles by type and size, and (iv) different original sources for the resuspended material and the primary
952
material (Gardner et al. 1985b).
953
Among the water samples from GA03 considered in this study, those collected near the bottom at stations
954
GT11-04 and GT11-08 present particularly large anomalies in radionuclide activities. Measurements on
955
near-bottom samples at these stations display relatively low values of dissolved
956
and A5) and exceptionally high values of particulate
957
profiles of transmissometer data (voltage or particle beam attenuation coefficient) at GT11-04 and GT11-08
958
published in previous studies shows that these samples were collected in benthic nepheloid layers (e.g., Fig.
959
13 of Lam et al. (2015); Fig. 3 of Hayes et al. (2015a)). Here profiles of PM concentration at stations
960
GT11-04 and GT11-08 are derived from measurements of the beam attenuation coefficient (BAC) obtained
961
from transmissometry (WETLabs C-star 25-cm pathlength transmissometer with serial number CST-491DR;
962
transmissometer data from Schlitzer et al. (2018)), using the empirical relationship between PM and BAC
963
due to particles as reported by Gardner et al. (2018b). It is seen that both stations feature a well-defined
964
BNL in the lower part of the water column (Fig. 19).
230
51
Th and
231
230
Th and
231
Pa (Figs. A2
Pa (Figs. A3 and A6). Inspection of
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
3000
!"##$%(
2500
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
0
125 250 375 500
PM (mg m-3) 965
966
Fig. 19. Vertical profiles of optically derived particulate matter concentration at stations GT11-04,
967
GT11-06, and GT11-08 between the New England continental shelf and Bermuda. Particulate matter
968
concentration is estimated from beam attenuation coefficient measurements available in the GEOTRACES
969
Intermediate Data Product (Schlitzer et al. 2018) using the empirical relationship between PM
970
concentration and BAC due to particles as reported by Gardner et al. (2018b).
971
The concentration maxima amount to about 2100 and 450 mg m−3 near the bottom at GT11-04 and GT11-
972
08, respectively, and exceed those in the surface waters at each station. The upper boundary of the BNL
973
occurs at a water depth of about 3500 m at GT11-04 and about 4500 m at GT11-08, showing that samples
974
presenting relatively low
975
at these stations (Figs. A3 and A6) originated from a strong BNL. Deep water samples collected at other
976
stations along the western segment of GA03 and showing large radionuclide anomalies also originated from
977
a BNL. At station GT11-06, for example,
978
A2 and A5), where a BNL is also found (Fig. 19).
979
230
Thd and
231
Pad activities and exceptionally high
230
Thd and
231
230
Thp and
Pap activities
Pad activities show minima below 4000 m (Figs.
The foregoing observations lead to the hypothesis that intensified scavenging of 52
231
230
Th and
231
Pa may
980
occur in BNLs. The hypothesis is further suggested by a comparison of the horizontal distributions of PM
981
concentration,
982
main regions (Fig. 20): (i) along most of the continental slope and rise, (ii) the Sohm Abyssal Plain (Fig.
983
3), and (iii) the region between the New England Seamounts and Bermuda (Fig. 3).
230
Thd , and
231
Pad in our reference solution. Particle concentrations show maxima in three
PM (mg/m3)
z=3000 m
PM (mg/m3)
z=4000 m
z=5000 m
PM (mg/m3)
984
230Th (dpm/m3) d
z=3000 m
z=4000 m
230Th (dpm/m3) d
z=5000 m
230Th (dpm/m3) d
985
231Pa
z=3000 m
d (dpm/m
3)
z=4000 m
231Pa
d (dpm/m
3)
z=5000 m
231Pa
d (dpm/m
3)
986
Fig. 20. Distributions of particulate matter concentration as estimated from optical measurements
987
988
compiled by Gardner et al. (2017) (top three panels), 231
230
Thd activity as simulated in the reference solution
989
(middle panels), and
990
middle, and right panels show distributions at a depth of, respectively, 3000 m, 4000 m, and 5000 m.
991
The simulated distributions of 230 Thd and 231 Pad show minima in each of these regions (Fig. 20), suggesting
992
that the pattern of particle scavenging at least partly influences the distribution of both radionuclides. Notice
993
that regions of PM maxima and regions of (230 Thd ,231 Pad ) minima do not strictly coincide. For example,
994
995
996
230
Th and
231
Pad activity as simulated in the reference solution (bottom panels). The left,
Pa could be removed from solution in a region where PM levels are high and the resulting
(230 Thd ,231 Pad ) anomalies could be advected into a region where PM levels are lower, leading to 230 Thd and 231
Pad minima there. 53
Evidence for an influence of BNLs on the cycling of particle-reactive elements in the deep water column
997
998
has been reported in a number of studies, namely through the observed disequilibrium of
999
24.1 days) with its radioactive parent,
238
234
Th (half-life of
U (e.g., Bacon and Rutgers van der Loeff (1989); Turnewitsch and
1000
Springer (2001); Rutgers van der Loeff et al. (2002)). Using a model that describes the vertical distribution
1001
of
1002
in bottom waters can be linked to the distribution of excess
1003
particles. Recently, BNLs have been mapped using 6,392 full-depth profiles made during 64 cruises using
1004
transmissometers mounted on CTD in several programs including WOCE, SAVE, JGOFS, CLIVAR-Repeat
1005
Hydrography, and GO-SHIP over the last four decades (Gardner et al. 2018a). In addition to the western
1006
North Atlantic, areas of intense BNLs have been found in the Argentine Basin, the Southern Ocean, and the
1007
oceanic region around South Africa (see also Gardner et al. (2018b)). Future studies will need to establish the
1008
global significance of BNLs for the ocean biogeochemical cycles of
1009
elements in general.
1010
d. Temporal Variations of Particle Scavenging in the Deep Sea
1011
234
Th near the seabed, Rutgers van der Loeff and Boudreau (1997) showed that the depletion of
The possibility that
230
Th and
231
234
234
Th
Th in surface sediments and on resuspended
230
Th and
231
Pa and of adsorption-prone
Pa removal from solution can be enhanced in BNLs suggests that the
1012
scavenging of both nuclides may show significant temporal variations in the deep water column. Benthic
1013
nepheloid layers generally have a basal uniform region corresponding closely to the bottom mixed layer
1014
(typically 20–100 m thick) and an overlying region (typically 500–2000 m thick) where light scattering
1015
(or attenuation) falls off more or less logarithmically up to a clear water minimum (McCave 1986). The
1016
particulate material in each region is thought to originate from sediment resuspension followed by some
1017
combination of vertical turbulent mixing and lateral transport (McCave 1986). None of these processes are
1018
expected to be time-invariant, suggesting that the amount and distribution of particles within BNLs may
1019
both be variable.
1020
Dramatic evidence for the transient nature of BNLs in the western North Atlantic has been provided
1021
by a number of studies, most notably through the HEBBLE experiment on the Nova Scotian continental
1022
rise (HEBBLE 1988). Previous studies had established that intense BNLs in the western North Atlantic
1023
tend to form during episodes of strong abyssal currents and sediment resuspension, which have been picto-
1024
rially referred to as ‘benthic storms’ (Gardner and Sullivan 1981). Results from HEBBLE showed that fast
1025
currents, high concentrations of suspended sediment, and grooved mud beds are associated with erosion in
1026
frequent benthic storms (Hollister and McCave 1984). More recently, measurements of deep-sea currents,
1027
nephelometer-based particle concentration, and seafloor photographs gathered during science programs that
54
1028
spanned two decades in the western North Atlantic have been synthesized (Gardner et al. 2017). These au-
1029
thors concluded that benthic storms occurred in areas with high surface eddy kinetic energy, most frequently
1030
beneath the meandering Gulf Stream and its associated rings.
1031
Different mechanisms have been proposed to explain benthic storms, including (i) synoptic atmospheric
1032
events such as nor’easters, tropical storms, or hurricanes (Gardner and Sullivan 1981); and (ii) oceanic
1033
features generated from the Gulf Stream such as deep cyclones and topographic Rossby waves (Gardner
1034
et al. 2017). In this regard, it is instructive to consult the cruise report for the second leg of GA03 for
1035
information about weather conditions encountered during the leg (GA03 Shipboard Team 2016). After
1036
station GT11-04, where a very strong BNL was found (Fig. 19), weather conditions deteriorated, with
1037
a hurricane passing between the ship’s position and Bermuda, followed by three more days of sustained
1038
winds > 25 knots. At station GT11-06, a very strong BNL was observed, detectable below 4200 m but
1039
most strongly expressed below 4400 m (GA03 Shipboard Team (2016); Fig. 19). The BNL at GT11-08
1040
was even stronger (Fig. 19). Speculatively, the difficulty to simulate accurately radionuclide profiles at
1041
individual stations whilst the station-averaged profiles can be better reproduced (section 4.b) may be due
1042
to pronounced temporal variations in PM load and scavenging intensity at abyssal depths. Further work is
1043
needed to assess whether the strong BNLs observed at stations occupied along the western segment of GA03
1044
and showing large radionuclide anomalies near the bottom were related to the adverse weather conditions
1045
experienced during the cruise.
1046
e. Use of
1047
230
Th as a Ventilation Tracer
A few studies suggested that the low
230
Th and
231
Pa observed in the deep Atlantic compared to values
1048
expected from reversible exchange could be explained by deep ventilation through lateral advection and
1049
mixing (Moran et al. 1997; Vogler et al. 1998; Moran et al. 2002). These studies applied a model originally
1050
developed to interpret
1051
Water (Rutgers van der Loeff and Berger 1993). In this ‘mixing-scavenging’ model, the equations for re-
1052
versible exchange are extended to include a term that is intended to represent the additional influence of a
1053
source water,
230
Th data from the Weddell Sea in terms of upwelling of lower Circumpolar Deep
wp
A∗ − Atot ∂Ap = λAπ + , ∂z τ
(6)
1054
where Atot = Ad + Ap is the total activity, A∗ is the total activity of the source water, and τ is a time scale
1055
for water renewal. An analytical solution of (6) can be easily derived for uniform wp , A∗ , and τ , and fit to
1056
activity data to obtain an estimate of τ . From this approach, water renewal times from 3 to 140 yr have
1057
been reported, where the range reflects (i) estimates from different locations in the Atlantic Ocean and (ii)
55
1058
uncertainties associated with the variability of activity data from a single station (Moran et al. 1997; Vogler
1059
et al. 1998; Moran et al. 2002).
1060
Albeit instructive, the model (6) is not free of limitations. The effect of advection is represented in a
1061
crude manner and solutions obtained with uniform A∗ and τ assume that Atot is influenced by the same
1062
source water introduced at the same rate throughout the water column. More recently, an attempt to extract
1063
information about deep North Atlantic circulation from
1064
finite-difference geostrophic model (Marchal et al. 2007). These authors found that the addition of
1065
data to density and transport observations in the inversion leads to zonally integrated meridional transports
1066
below 1000 m which have larger amplitudes (by 2–9 Sv), where the range reflects the uncertainties in the
1067
large-scale
Th distribution and in the
230
Th data has been undertaken using an inverse 230
Th
Th balance equation.
The results reported in this paper, however, suggest that caution should be exercised when interpreting
1068
1069
230
230
230
Th and
231
Pa measurements from the western North Atlantic in terms of deep water ventilation, at least 230
in terms of ventilation from the DWBC. According to these results, the distributions of
1071
the western North Atlantic show on average only modest variations in response to a change by a factor of four
1072
in the strength of the DWBC (10 to 40 Sv) along 55◦ W (Fig. 15). Other factors, such as particle scavenging
1073
and the
1074
radionuclide distributions west of the MAR. This latter finding is consistent with a recent study by Deng
1075
et al. (2018), who found, using the ‘mixing-scavenging model’, that the
1076
water entering a deep basin significantly influence the
230
Th and
231
Th and
231
1070
Pa in
Pa activities in the inflowing waters, are shown here to have a larger influence on the
230
Th and
231
230
Th and
231
Pa activities of the
Pa activities within the basin.
1077
It is instructive to compare, for the reference solution, the residence time of the water in the domain
1078
with respect to the DWBC inflow with the residence of both radionuclides with respect to scavenging. The
1079
water residence time is computed from RRR
τwat = RR
dx dy dz , un (xDWBC , y, z)dy dz
(7)
1080
where un (xDWBC , y, z) is the zonal (westward) velocity at the DWBC inflow location. The triple integral in
1081
the numerator is over the entire volume of the domain and the double integral in the denumenator is over
1082
the surface area of the DWBC inflow.
1083
We find a value τwat = 21.0 yr, of the same order of magnitude as the water renewal times estimated
1084
from tritium and excess He-3 data (Doney and Jenkins 1994) and from CFC data (Rhein et al. 2015). On
1085
the other hand, we consider three different definitions of residence time for scavenging. First, we calculate
1086
the scavenging residence time as RRR τscav,1 =
Atot (x, y, z) dx dy dz RR , λAπ dx dy dz 56
(8)
where Atot is the total activity defined above. We find that τscav,1 = 19 yr for
1087
1088
231
230
Th and 91 yr for
Pa, consistent with the estimates for the whole Atlantic Ocean of, respectively, 26 yr and 111 yr, based
1089
on the same definition and reported by Yu et al. (1996). Although the above definition of residence time is
1090
convenient, it does rely on the assumption that the radionuclide balance in the water column is dominated
1091
by radioactive production and particle scavenging (Broecker and Peng 1982). We thus consider another
1092
definition, RRR τscav,2 =
A (x, y, z) dx dy dz RR tot , wp Ap,bot dx dy
(9)
1093
where Ap,bot is the activity of the radionuclide in the particulate phase just above the seafloor. The time scale
1094
as defined above could be regarded as a residence time with respect to the flux of particulate radionuclide to
1095
the seabed. We find that τscav,2 amounts to 8 yr for
1096
that Th is more sensitive to particles than Pa but noticeably smaller than the values of τscav,1 . Finally, we
1097
consider a third definition,
230
Th and 32 yr for
231
Pa, consistent with the notion
RRR
τscav,3 = RRR
Ad (x, y, z) dx dy dz , k1 (x, y, z)Ad (x, y, z) dx dy dz
(10)
1098
where Ad is the radionuclide activity in the dissolved phase and k1 is the adsorption rate constant as defined
1099
in section 2.3. This other time scale could be viewed as a residence time with respect to adsorption. Unlike
1100
τscav,1 but similarly to τscav,2 , it has the virtue that a specific removal process is identified in its definition.
1101
We find that τscav,3 amounts to 2 yr for
1102
sensitivity of Th but even smaller than the values of τscav,1 .
230
Th and 4 yr for
231
Pa, again consistent with the greater particle
A comparison of the values of τwat with those of (τscav,2 ,τscav,3 ) shows that
1103
230
Th is in general more
1104
rapidly removed by adsorption onto particles and by particle settling to the seafloor than it is transported
1105
by the DWBC. On the other hand, τwat is smaller than τscav,2 for
1106
1107
231
231
Pa, suggesting that the transport of
Pa is at least as important as its removal to sediments.
7. Summary and Perspectives
1108
In this study, we have first examined the extent to which a regional ocean circulation model with 1/4◦
1109
resolution and including a description of particle scavenging based on optically-derived particle concentration
1110
data can reproduce the circulation and the distributions of
1111
We found that some, though not all, elements of the general circulation and of the radionuclide distributions
1112
which are observed in the area can be reproduced. The model simulates a Gulf Stream displaying some
57
230
Th and
231
Pa in the western North Atlantic.
1113
degree of variability (e.g., meandering), a Deep Western Boundary Current along the continental slope
1114
and rise, and key features in the
1115
cruises, such as a reversal of the vertical
1116
the simulations presented in this paper differ in several respects from observations, e.g., the Gulf Stream
1117
separates from the western margin farther north than observed, the velocity maxima in the Gulf Stream
1118
(DWBC) are underestimated (overestimated) and occur too far to the northwest along line W, and the low
1119
1120
231
230
Thd,p and 230
231
Pad,p distributions observed during (pre-)GEOTRACES
Thd and
231
Pad gradients at mid-depth. On the other hand,
Pad activities observed in deep water are overpredicted. The model has then been applied to investigate the influences of particle scavenging and of DWBC
1121
inflow properties (strength and radionuclide contents) on the
230
1122
North Atlantic. We found that the vertical distributions of
230
1123
)GEOTRACES stations are more sensitive to particle scavenging than they are to the strength of the
1124
DWBC inflow. This result suggests that the distribution of particulate matter concentration, with (relative)
1125
maxima in benthic nepheloid layers, is more important than the strength of deep water ventilation in setting
1126
the basin-mean vertical distributions of both radionuclides in the study area. We also found that changes in
1127
the radionuclide activities of the DWBC inflow are more likely to produce variations in these distributions
1128
than changes in the strength of the DWBC inflow. A model solution with uniform rate constants of Th and
1129
Pa adsorption, with no enhancement in deep water, was unable to explain the observed inversion in
1130
and
1131
activities observed in deep waters in the western North Atlantic stem from intensified scavenging in thick,
1132
active benthic nepheloid layers rather than from ventilation by
1133
western boundary.
231
Th and
231
Thd,p and
Pa distributions in the western 231
Pad,p averaged for all (pre-
Pad profiles at mid-depth. Overall, these results suggest that the relatively low
230
Thd - and
231
230
Thd and
230
Thd
231
Pad
Pad -poor waters from the
1134
We clarify below what we perceive as being the main limitations of this study. Needless to say, the
1135
results reported in this paper are only as reliable as the model on which they are based. A number of
1136
factors could be responsible for the differences between model results and physical observations from satellite
1137
altimetry and repeat cruises along line W. These include, for instance, an inadequate horizontal resolution
1138
of the model, the surface forcing based on climatological fields of winds, temperature, and salinity, and the
1139
omission of shelf circulation. The factors responsible for the differences between the observed and modeled
1140
distributions of 230 Thd,p and 231 Pad,p could be due to errors in both the physical and geochemical components
1141
of the model. Although the present model represents explicitly the kinetics of sorption reactions through
1142
a separate equation for the dissolved and particulate phases, the treatment of
1143
the ocean remains relatively crude. While optical measurements provide important constraints on particle
1144
distribution, temporal variations in particle concentration, which can be dramatic near the bottom (e.g.,
1145
HEBBLE (1988)), are neglected in this study. While several light scattering time series from long-term
58
230
Th and
231
Pa cycling in
1146
nephelometer deployments indicate a strong positive correlation between the temporal standard deviation
1147
in near-bottom particle concentration and the eddy kinetic energy as derived from satellite altimetry, the
1148
temporal variability of particle concentration in the northwest Atlantic remains largely unknown.
1149
Given the above caveats, a number of perspectives for future modeling efforts can be outlined. First, a
1150
model with higher resolution would be desirable in order to simulate the western North Atlantic circulation
1151
in more detail. With a horizontal resolution of 1/4◦ , baroclinic eddies are allowed to grow but are only
1152
marginally resolved; to simulate the full dynamical and life cycle of baroclinic eddies, an ‘eddy-resolving’
1153
model, perhaps with a horizontal resolution approaching 1/10◦ , would be needed (Tr´eguier et al. 2014). Use
1154
of higher spatial resolution might also help (i) to better represent the separation of the Gulf Stream from
1155
the coast (Chassignet and Marshall 2008; Ezer 2016b) and (ii) to reduce the error in the evaluation of the
1156
horizontal pressure gradient (PG) in s-coordinate models. The error in the evaluation of the horizontal PG
1157
in s-coordinate models has been discussed at length in the literature (e.g., Shchepetkin and McWilliams
1158
(2003) and references therein; Ciappa (2008)). In POM, the initial density field area-averaged on z-levels
1159
and then transferred to s-coordinates is subtracted from the density field at each time step in an effort to
1160
reduce the truncation error associated with the calculation of the horizontal baroclinic PG (Mellor 2002).
1161
Theoretical analysis and numerical solutions obtained with POM have shown that velocity errors arising
1162
from the calculation of the horizontal PG vanish due to advection of the density field (Mellor et al. 1994),
1163
or do not vanish prognostically but are small, especially if a horizontally averaged density field is subtracted
1164
before computing the horizontal baroclinic PG (Mellor et al. 1998) as done in POM. Although these results
1165
appear encouraging, subtraction of a horizontally averaged density field may not yield particularly good
1166
results in a domain characterized by strong horizontal density gradients, such as the western North Atlantic,
1167
and represented with a coarse grid. Second, the model domain should be extended to include the continental
1168
shelf from Cape Hatteras to Nova Scotia. Consideration of the continental shelf would allow one to study the
1169
potential role of shelf-ocean exchange in the radionuclide distributions in the slope region where some of the
1170
230
Th and 231 Pa data used in this study originate. Finally, a more detailed description of particle scavenging 230
Th and
231
Pa in the study area. The model could incorporate
1171
would permit more credible simulations of
1172
an explicit representation of sediment resuspension, transport, and deposition, and of the influence of the
1173
resuspended sediment on the removal of both radionuclides from the deep water column (e.g., Rutgers van der
1174
Loeff and Boudreau (1997)). The model could be extended to include (i) the effect of particle composition
1175
on the removal of both radioisotopes (e.g., Hayes et al. (2015a)), and (ii) more than one particle class so as
1176
to account for the processes of particle (dis)aggregation, particularly in BNLs (e.g., McCave (1985); Hill and
1177
Nowell (1990)).
59
1178
Acknowledgments.
1179
We are grateful to John Toole for sharing velocity data from line W, and to both him and Magdalena
1180
Andres for useful discussions. We thank Jerry McManus for directing us to a recent compilation of surface
1181
sediment
1182
us to improve significantly the manuscript. This study could not have been possible without the dedication
1183
of researchers who collected samples at sea and generated the (pre-)GEOTRACES data analyzed in this
1184
study. This work has been supported by grant OCE-1556400 from the U.S. National Science Foundation.
231
Pa/230 Th data. Detailed and constructive comments by two anonymous reviewers have allowed
60
1185
APPENDIX A: Mapping of Particle Matter Concentration
1186
This appendix describes the procedure used to map the distribution of particulate matter (PM) concen-
1187
tration from optical data in the western North Atlantic. For convenience, the particle concentration derived
1188
from optical data and vertically interpolated on a model level is called cd below. The distributions of cd
1189
along the different s-surfaces of the model (31 surfaces) are mapped individually. The mean of cd for a given
1190
s-surface (¯ cd below) is computed and subtracted from the cd values along the surface to yield PM anomalies,
1191
c0d . The PM concentrations at the model grid points along the surface are then estimated from (i) the PM
1192
anomalies at the locations where optical data are available (‘data locations’ below) and (ii) the mean c¯d , ˆ = Ac0d + I¯ c cd .
(A1)
1193
ˆ is a vector including the PM estimates at the model grid points (c would include the true PM values Here c
1194
at these points), c0d is a vector including the PM anomalies at the data locations, and I is the identity matrix
1195
ˆ. The matrix A is derived such that the PM estimates at the model with order equal to the dimension of c
1196
grid points have minimum variance (e.g., Wunsch (2006)), −1
A = Rccd (Rcd cd + Ree )
,
(A2)
1197
where Rccd and Rcd cd are second-moment matrices for the PM field and Ree is a second-moment matrix for
1198
the errors in the elements of c0d .
1199
Consider first Rccd and Rcd cd . These matrices are specifically defined as Rccd = E[ccTd ] and Rcd cd =
1200
E[cd cTd ], where E[·] designates the expected value (mean) and T designates the transpose. The (i, j) element
1201
of Rccd , noted [Rccd ]i,j , is the covariance between the PM concentrations at model grid point location ˆri and
1202
data location rj . Similarly, the (i, j) element of Rcd cd , noted [Rcd cd ]i,j , is the covariance between the PM
1203
concentrations at data locations ri and rj . The following assumptions are made about the spatial covariances
1204
of PM concentration, |ˆri − rj | [Rccd ]i,j = σ 2 exp − L
and
|ri − rj | [Rcd cd ]i,j = σ 2 exp − , L
(A3)
1205
where σ 2 is a PM variance, L is a length scale, and | · | denotes the geodesic distance (taking the geodesic
1206
distance instead of the actual distance along the s-surface should incur only a small error given the smallness
1207
of bathymetric slopes). Equations (A3) entail that the covariance between the PM concentrations at two
1208
different locations decreases exponentionally with distance between these locations, dropping to 0.37σ 2 for
1209
a distance equal to L.
1210
Consider then Ree = E[eeT ], where e is the vector of errors in the elements of c0d . The diagonal elements
1211
of Ree are the variances of the errors in the PM anomalies in c0d , and the off-diagonal elements of Ree are 61
1212
the covariances between these errors. Here Ree is taken as diagonal (no error covariances) and its diagonal
1213
elements are set equal to σe2 = (8.5 mg m−3 )2 . This error variance is intended to account for the fact
1214
that, for many of the stations where transmissometry measurements were made, samples for PM were not
1215
collected, so the particle concentration in the water where the beam attenuation coefficient was minimum
1216
could not be identified. In fact, the minimum beam attenuation coefficient for particles for each cast was
1217
set to the cruise-average minimum taken as 0 m
1218
mg m−3 using the calibration equation. Thus, there is an error associated with the unknown concentration
1219
of particles at the clear water minima. Since these minima typically exhibit particle concentrations between
1220
5–12 mg m−3 in the western North Atlantic (Brewer et al. 1976; Gardner et al. 1985a), we use the median
1221
value of 8.5 mg m−3 as the uncertainty associated with the particle concentration in clear waters.
−1
(Gardner et al. 2018b), which results in PM value of 0
1222
The mapping as described above implies that the mapped PM field becomes smoother as the length scale
1223
L increases, and that mapped PM values at grid points that are very distant ( L) from data locations
1224
approach the mean value c¯d . In this study, we choose L = 100 km for all s-surfaces and σ 2 = σs2 , where
1225
σs2 is the variance of the PM field along the s-surface. With this choice, the mapped PM values at the
1226
data locations are generally consistent with the PM estimates at these locations, i.e., more than 0.4% of the
1227
mapped PM values are within 2σe of the PM estimates at the data locations (not shown).
1228
1229
1230
APPENDIX B: Horizontal Boundary Conditions This appendix describes the conditions imposed at the surface and at the bottom of the domain. Kinematic conditions are specified on the flow at the surface and at the bottom, w
=
w
=
∂η + uh · ∇η ∂t −uh · ∇h
at z = η(x, y, t),
(B1a)
at z = −h(x, y),
(B1b)
1231
where uh = (u, v) is the horizontal velocity, η(x, y, t) is the free surface elevation, and h(x, y) is the water
1232
depth. Annual mean wind stresses from SCOW (Risien and Chelton 2008), τSCOW , are prescribed at the
1233
surface and a shear stress τb is imposed at the bottom: ∂uh ∂z ∂uh ρo κm ∂z ρo κm
=
τSCOW
at z = η(x, y, t),
(B2a)
=
τb
at z = −h(x, y).
(B2b)
1234
Here ρo = 1025 kg m−3 is a reference density and κm is the (variable) vertical turbulent viscosity. The
1235
bottom stress is given by τb = ρo Cd |uh,b |uh,b .
62
(B3)
1236
In this expression, uh,b is the horizontal velocity at the bottom (the grid point nearest to the bottom) and
1237
Cd is a drag coefficient computed from Cd =
κ ln [(1 + sb ) h/zr ]
2 ,
(B4)
1238
where κ = 0.4 is the von K´ arm´ an constant, zr = 0.01 m is a bottom roughness parameter, and sb is the s
1239
coordinate of the deepest level above the bottom (Mellor 2002). The Cd values over the whole domain are
1240
tested such that 0.0025 ≤ Cd ≤ 1.
1241
1242
The turbulence variables at the surface and at the bottom are set as follows (Blumberg and Mellor 1987; Mellor 2002), 2/3 q 2 , q 2 l = b1 u2∗ , 0 ,
(B5)
1243
where b1 = 16.6 is an empirical constant and u∗ is the friction velocity associated with the surface stress or
1244
the bottom stress as denoted.
1245
Finally, fluxes of temperature and salinity at the surface are represented through a restoring to climato-
1246
logic annual mean values from the World Ocean Atlas (Locarnini et al. 2013; Zweng et al. 2013), respectively,
1247
TWOA and SWOA , ∂T δ = (TWOA − T ) , ∂z τ δ ∂S κT = (SWOA − S) . ∂z τ
κT
(B6a) (B6b)
1248
Here δ is the thickness of the surface layer and τ = 14.4 days is a restoring time scale. A no-flux condition
1249
is specified for (Ad , Ap ) at the surface and for (T, S, Ad , Ap ) at the bottom.
1251 1250
APPENDIX C: Lateral Boundary conditions
1252
Conditions on Velocities
1253
The appendix describes the conditions applied at the lateral boundaries of the model domain. Conditions
1254
on both the horizontal velocities (internal mode) and their vertical averages (external mode) are needed. For
1255
convenience, the velocity component normal (tangent) to a lateral boundary is designated with subscript
1256
n (t), and the velocity averaged from the bottom to the surface is represented with an overbar. Thus, for
1257
example, un is the velocity normal to the boundary at a given depth and u ¯n is the vertically averaged velocity
1258
normal to the boundary. Different types of conditions are imposed along different segments of the lateral
1259
boundaries.
63
1260
Segments with Inflows and Outflows
1261
Deep Western Boundary Current The inflow of the DWBC is specified at the eastern boundary (55◦ W)
1262
along the segment between 42.5◦ –44.875◦ N. Conditions on both the distribution and the vertical average of
1263
(un , ut ) across the segment are specified. The distribution of (un , ut ) across the segment is set as follows, 2 ! 2 ! z − zi,DWBC y − yi,DWBC exp − . (C1a) un = ui,DWBC exp − ly,DWBC lz,DWBC ut
=
0.
(C1b)
1264
Here yi,DWBC (zi,DWBC ) is the meridional (vertical) coordinate of the core of the current and ly,DWBC (lz,DWBC )
1265
is a length scale for the meridional (vertical) extent of the current. We assume a value of yi,DWBC such
1266
that the core of the current is situated 120 km south of the northern boundary at 55◦ W, a value of 3500
1267
m for zi,DWBC , a value of 50 km for ly,DWBC , and a value of 500 m for lz,DWBC . The zonal velocity ui,DWBC
1268
is calculated such that the integral of un over the surface area of the segment is equivalent to a volume
1269
transport of 20 Sv (1 Sv = 106 m3 s−1 ). The above values assumed for (yi,DWBC , zi,DWBC , ly,DWBC , lz,DWBC ) and
1270
for the volume transport are broadly consistent with observations made in the area (Pickart and Smethie
1271
1998). The vertical average of un is prescribed through a radiation condition (Flather 1976), whereas the
1272
vertical average of ut is set to zero, r u ¯n = u ¯n,∗ +
g (η − η∗ ) h
and
u ¯t = 0.
(C2)
1273
Here u ¯n,∗ is the vertical average of the right-hand side of (C1a), h is the local water depth, η is the local
1274
free surface elevation simulated by the model, and η∗ is a value calculated geostrophically from u ¯n,∗ .
1275
The outflow of the DWBC is specified at the southern boundary (28◦ N) along the segment between
1276
75.5◦ –76.5◦ W. The condition (C1a) for the normal velocity is replaced by a radiation condition, and the
1277
velocity tangent to the boundary is set to zero: ∂un ∂un − ci =0 ∂t ∂n
and
ut = 0.
(C3)
1278
Here n is the coordinate normal to the boundary and ci > 0 is a speed for the internal mode, derived from
1279
the water depth at the boundary and a maximum depth, hmax (Mellor 2002). The differential equation
1280
(C3) is discretized with an upstream scheme, which implies that meridional momentum is always advected
1281
southwards (out of the domain) at the boundary. The vertical averages of (un , ut ) are specified as follows.
1282
The distribution of un along the segment is first taken as
z − zo,DWBC un = uo,DWBC exp − lz,DWBC
64
2 ! .
(C4)
1283
where zo,DWBC = 3000 m is the depth of the core of the current and lz,DWBC = 500 m. The meridional velocity
1284
uo,DWBC is determined such that the integral of un over the surface area of the segment is equivalent to a
1285
volume transport of 20 Sv, which is the volume transport set at the inflow location. The vertical averages
1286
of (un , ut ) are then prescribed similarly to (C2), r g u ¯n = u ¯n,∗ − (η − η∗ ) h
and
u ¯t = 0,
(C5)
1287
where u ¯n,∗ is the vertical average of the right-hand side of (C4) and η∗ is determined geostrophically from
1288
u ¯n,∗ .
1289
Gulf Stream The inflow of the Gulf Stream (GS), intended to represent the Florida Current, is applied
1290
at the southern boundary along the segment between 78◦ –80◦ W. In contrast to the DWBC inflow, only
1291
the vertical averages of (un , ut ) are specified. Conditions identical to (C3) are prescribed for (un , ut ). The
1292
vertical averages of (un , ut ) are prescribed as follows. The distribution of un along the segment is first taken
1293
as un = ui,GS exp −
z
2 ! .
lz,GS
(C6)
1294
where lz,GS = 200 m. The meridional velocity ui,GS is calculated such that the integral of un over the surface
1295
area of the segment is equivalent to a volume transport of 30 Sv. This value is consistent with the long-term
1296
mean transport of the Florida Current of about 32 Sv measured from daily cable observations and shipboard
1297
sections across the Straits of Florida (Baringer and Larsen 2001; Meinen et al. 2010). The vertical averages
1298
of (un , ut ) are then prescribed using conditions identical to (C5).
1299
1300
The outflow of the GS is applied at the eastern boundary along the segment between 39◦ –41◦ N. A radiation condition is used for un whilst ut is taken to vanish along the segment: ∂un ∂un + ci =0 ∂t ∂n
and
ut = 0.
(C7)
1301
As for (C7), the differential equation (C7) is approximated with an upstream scheme, so that zonal momen-
1302
tum is always advected eastwards (out of the domain) at the boundary. The vertical averages of (un , ut ) are
1303
specified as follows. The distribution of un along the segment is first taken as 2 ! z un = uo,GS exp − , lz,GS
(C8)
1304
where again lz,GS = 200 m. The zonal velocity uo,GS is computed such that the integral of un over the surface
1305
area of the segment is equivalent to a volume transport of 90 Sv, a value compatible with mooring-based
1306
estimates of the time-mean GS transport of 93 Sv at 55◦ W (Hogg 1992) and 88 Sv at 68◦ W (Johns et al.
1307
1995). The vertical averages of (un , ut ) are then prescribed using conditions identical to (C2).
1308
Sargasso Sea The inflow of Sargasso Sea water (SSW) is applied at the eastern boundary along the
1309
segment between 28◦ –39◦ N. In contrast to the DWBC inflow and similarly to the GS inflow, only the 65
1310
1311
vertical averages of (un , ut ) are specified. Conditions identical to (C7) are prescribed. The vertical averages of (un , ut ) are specified as follows. The distribution of un across the segment is first taken as 2 ! z un = ui,SSW exp − , lz,SSW
(C9)
1312
where lz,SSW = 1000 m. The zonal velocity ui,SSW is computed such that the integral of un over the surface
1313
area of the segment is equivalent to a volume transport of 60 Sv, which is the difference between the GS
1314
outflow (90 Sv) and GS inflow (30 Sv). Accordingly, the net inflow (or outflow) of water vanishes at the
1315
lateral boundaries of the domain. The vertical averages of (un , ut ) are then prescribed using conditions
1316
identical to (C2). No outflow condition is applied for SSW.
1317
Other Segments
1318
1319
Along segments where no inflow or outflow is prescribed, conditions (C3) or (C7) are applied for (un , ut ) and conditions (C2) or (C5) are applied for (¯ un , u ¯t ).
1320
Conditions on T, S, Ad , Ap , q 2 /2, q 2 l
1321
1322
The following radiation condition is imposed at all lateral boundaries, ∂T ∂T ± un = 0, ∂t ∂n
(C10)
1323
where T ∈ {T, S, Ad , Ap , q 2 /2, q 2 l}. As for (C3) and (C7), the differential equation (C10) is discretized with
1324
an upstream scheme. The variable T is advected out of the domain if the boundary velocity (with normal
1325
component un ) is directed outwards, and it is advected into the domain if the boundary velocity is directed
1326
inwards. Thus, boundary values of T must be prescribed for the case where the boundary velocity is directly
1327
inwards. Boundary values of (T, S) are derived from the World Ocean Atlas (Locarnini et al. 2013; Zweng
1328
et al. 2013) (section 2.a). The radionuclide activities (Ad , Ap ) are set to different values at different locations
1329
along the open boundaries. At the location of DWBC inflow (between 42.5◦ –44.875◦ N along 55◦ W),
1330
(230 Thp ) is set to 0.4 dpm m−3 (0.03 dpm m−3 ) and
1331
m−3 ). These values are the average activities below 1000 m in the Labrador Sea as reported by Moran
1332
et al. (2002)), except for
1333
this increase in
231
231
231
230
Thd
Pad (231 Pap ) is set to 0.22 dpm m−3 (0.003 dpm
Pad whose boundary value is set to double this average activity. We found that
Pad at the DWBC inflow produces a better fit to the observed radionuclide activities in
66
1334
the western North Atlantic. At all other locations, values of (Ad , Ap ) are set according to idealized profiles
1335
that broadly reproduce measured profiles at station GT11-14 (27.58◦ N, 49.63◦ W) situated to the east of the
1336
domain, Ad = Ad,o 1 − ez/lz and Ap = Ap,o 1 − ez/lz , 230
231
Pa. Note that z ≤ 0 in this study,
1337
where the length scales lz = 1600 m for
1338
which implies that Ad and Ap as calculated from (C11) increase monotonically with depth (Fig. A7). Finally,
1339
the boundary values of (q 2 /2, q 2 l) are set to small values.
APPENDIX D:
1340
1341
1342
230
Th and lz = 1132 m for
(C11)
Th and
231
Pa Profiles at Individual Stations
This appendix illustrates observed and simulated profiles of
230
Thd,p and
231
Pad,p at individual (pre-
)GEOTRACES stations (Figs. A1–A13).
1343
REFERENCES
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
Anderson, R. F., M. P. Bacon, and P. G. Brewer, 1983a: Removal of
230
Th and
231
Pa at ocean margins.
Earth Planet. Sci. Lett., 66, 73–90. Anderson, R. F., M. P. Bacon, and P. G. Brewer, 1983b: Removal of
230
Th and
231
Pa from the open ocean.
Earth Planet. Sci. Lett., 62, 7–23. Anderson, R. F., M. Q. Fleisher, P. E. Biscaye, N. Kumar, B. Dittrich, P. Kubik, and M. Suter, 1994: Anomalous boundary scavenging in the Middle Atlantic Bight: Evidence from 210
230
Th,
231
Pa,
10
Be and
Pb. Deep Sea Res., 41, 537–561.
Andres, M., J. Toole, D. Torres, W. Smethie, T. Joyce, and R. Curry, 2016: Stirring by deep cyclones and the evolution of Denmark Strait Overflow Water observed at line W. Deep Sea Res. I, 109, 10–26.
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Tsunogai, S. and M. Minagawa, 1978: Settling model for the removal of insoluble chemical elements in seawater. Geochim. J., 12, 483–490. Turnewitsch, R. and B. M. Springer, 2001: Do bottom mixed layers influence 234 Th dynamics in the abyssal near-bottom water column? Deep Sea Res., 48, 1279–1307.
1621
van Hulten, M., J.-C. Dutay, and M. Roy-Barman, 2018: A global scavenging and circulation ocean model of
1622
thorium-230 and protactinium-231 with realistic particle dynamics (NEMO-ProThorP 0.1). Geosci. Model
1623
Dev., 11, 3537–3556.
1624
1625
1626
1627
Venchiarutti, C., C. Jeandel, and M. Roy-Barman, 2008: Particle dynamics study in the wake of Kerguelen Island using thorium isotopes. Deep Sea Res. I, 55, 1343–1363. Vogler, S., J. Scholten, M. Rutgers van der Loeff, and A. Mangini, 1998: The importance of water mass ventilation in the balance of
230
230
Th in the eastern North Atlantic:
Th. Earth Planet. Sci. Lett., 156, 61–74.
1628
Wunsch, C., 2006: Discrete inverse and state estimation problems. Cambridge University Press, 371 pp.
1629
Yu, E.-F., 1994: Variations in the particulate flux of
1630
1631
1632
1633
1634
of the
231
230
Th and
231
Pa and paleoceanographic applications
Pa/230 Th ratio. Ph.D. thesis, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
Yu, E.-F., R. Fran¸cois, and M. P. Bacon, 1996: Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data. Nature, 379, 689–694. Zweng, M., et al., 2013: World Ocean Atlas 2013, Volume 2: Salinity. NOAA Atlas NESDIS 74, Levitus, S (Ed.) and Mishonov, A. (Tech. Ed.), 39 pp.
77
1635
List of Tables
1636
1
Thorium-230 and Protactinium-231 Data Used in this Study
79
1637
2
Parameters of the physical model component
80
1638
3
Parameters of the geochemical model component
81
1639
4
Root Mean Square Difference Between Observed & Simulated Activities†
82
1640
5
Thorium-230 and Protactinium-231 Data Used in this Study
83
78
Table 1: Thorium-230 and Protactinium-231 Data Used in this Study
station
latitude
longitude
CMME-13 S1 EN407-3 EN407-4 EN407-6 BATS OC278-2 OC278-3 OC278-4 OC278-5 GT11-01 GT11-02 GT11-03 GT11-04 GT11-06 GT11-08 GT11-10 GT11-12 GT11-14
32.76◦ N 36.05◦ N 39.47◦ N 38.6◦ N 39.73◦ N 32◦ N 37◦ N 33◦ N 36◦ N 38◦ N 39.69◦ N 39.35◦ N 38.67◦ N 38.09◦ N 37.61◦ N 35.42◦ N 31.75◦ N 29.70◦ N 27.58◦ N
70.78◦ W 74.43◦ W 68.37◦ W 68.89◦ W 69.75◦ W 64◦ W 69◦ W 69◦ W 68◦ W 70◦ W 69.81◦ W 69.54◦ W 69.10◦ W 68.70◦ W 68.39◦ W 66.52◦ W 64.17◦ W 56.82◦ W 49.63◦ W
#a
230
8 11 11 19 19 19 11 11 10 11 25 17 20 16 20 17 28 18 21
Thd
#
230
Thp
8 10 0 0 0 0 0 0 0 0 10 12 12 12 12 12 12 12 12
a
#
231
Pad
11 19 0 0 0 0 0 0 0 0 25 17 20 16 21 17 28 18 21
#
231
Pap
0 0 0 0 0 0 0 0 0 0 10 12 12 12 12 12 11 12 11
errorb
reference
1σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 2σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ 1σ
Cochran et al. (1987) Guo et al. (1995) Luo et al. (2010) Luo et al. (2010) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) R. Fran¸cois (pers. com.) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a) Hayes et al. (2015a)
number of observations. b σ is the standard error for data from R. Fran¸cois (pers. com.) and the standard deviation for all other data.
79
Table 2: Parameters of the physical model component
Physical Parameters
ρo g C κ κu,o κT,o Pr zr τ
reference density acceleration due to gravity Smagorinsky coefficient von K´arm´an constant background vertical viscosity background vertical diffusivity turbulent Prandtl number bottom roughness parameter restoring time scale for SST and SSS
value
units
1025 9.806 0.2 0.4 0 0 5 0.01 14.4
kg m−3 m s−2 1 1 m2 s−1 m2 s−1 1 m d
value
units
15 450 5 200 100 0.1 0
s s 1 m m s−1 1 1
Numerical Parameters
∆tE ∆tI ∆s hmax umax c α a b
time step (external mode) time step (internal mode) step interval for advective termsa maximum depth in radiation condition maximum velocity for CFL violation constant of Asselin filter weight for surface slope termb
Step interval during which advective terms of the external mode are not updated Weight used for surface slope terms in the external mode equations
80
Table 3: Parameters of the geochemical model component
λTh-230 λPa-231 234 U 235 U k1 (Th) k1 (Pa) k1,b (Th) k1,b (Pa) k10 (Th) k10 (Pa) k−1 (Th) k−1 (Pa) wp (Th) wp (Pa)
radioactive decay constant of 230 Th radioactive decay constant of 231 Pa activity of 234 U activity of 235 U adsorption rate constant for Th adsorption rate constant for Pa background value of k1 (Th) background value of k1 (Pa) sensitivity of k1 (Th) to particle concentration sensitivity of k1 (Pa) to particle concentration desorption rate constant for Th desorption rate constant for Pa settling speed of 230 Thp settling speed of 231 Pap
81
value
units
9.17 × 10−6 2.12 × 10−5 2750 108 variable variable 0.4 0.04 0.04 0.02 3.69 18.45 1800 2400
yr−1 yr−1 dpm m−3 dpm m−3 yr−1 yr−1 yr−1 yr−1 −1 yr mg−1 m3 yr−1 mg−1 m3 yr−1 yr−1 m yr−1 m yr−1
Table 4: Root Mean Square Difference Between Observed & Simulated Activities†
230
n
Thd
230
Thp
231
Pad
231
Pap
238
100
161
84
reference solution
0.078
0.028
0.039
0.001
k10 / 2 k10 × 2
0.086 0.086
0.025 0.037
0.041 0.037
0.001 0.003
DWBC inflow = 10 Sv DWBC inflow = 40 Sv
0.081 0.076
0.030 0.029
0.040 0.037
0.002 0.002
Ad,p (DWBC inflow) / 2 Ad,p (DWBC inflow) × 2
0.078 0.165
0.025 0.041
0.033 0.110
0.001 0.002
uniform k1 (T h) & k1 (P a)
0.075
0.043
0.041
0.002
Ad,p (DWBC inflow) / 2, DWBC inflow = 10 Sv
0.093
0.027
0.041
0.002
†All values in dpm m−3
82
Table 5: Thorium-230 and Protactinium-231 Data Used in this Study
core
latitude
longitude
depth (m)
OCE152-BC1 OCE152-BC8 OCE152-BC9 OCE152-BC5 EN123-BC4 EN123-BC6 EN123-BC3 EN123-BC1 EN179-BC5 EN179-BC2 EN179-BC3 EN179-BC4 EN179-BC7 EN187-BC4 EN187-BC10 EN187-BC8 EN187-BC5 EN187-BC11 EN187-BC9 EN187-BC6 OCE325-GGC5 VM26-176
39.49◦ N 32.47◦ N 39.42◦ N 39.08◦ N 39.48◦ N 39.49◦ N 39.35◦ N 39.08◦ N 37.38◦ N 37.37◦ N 37.38◦ N 37.32◦ N 37.25◦ N 37.37◦ N 36.52◦ N 36.52◦ N 37.37◦ N 37.02◦ N 36.52◦ N 37.24◦ N 33.7◦ N 32.76◦ N
70.57◦ W 70.58◦ W 70.55◦ W 70.56◦ W 70.56◦ W 70.55◦ W 70.55◦ W 70.55◦ W 74.13◦ W 74.10◦ W 74.09◦ W 74.02◦ W 73.49◦ W 74.13◦ W 74.37◦ W 74.34◦ W 74.10◦ W 74.34◦ W 74.34◦ W 73.5◦ W 57.6◦ W 70.78◦ W
1126 1596 1981 2691 1280 1643 2344 2736 384 892 1031 1318 1989 512 580 1020 1045 1125 1165 2000 4550 1126
83
231
Pa/230 Th 0.082 0.071 0.091 0.063 0.076 0.066 0.061 0.053 0.127 0.050 0.075 0.071 0.051 0.063 0.089 0.053 0.069 0.062 0.075 0.055 0.054 0.065
reference Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) Anderson et al. (1994) McManus et al. (2004) Yu (1994)
1641
1642
List of Figures 1
Figure A1. Profiles of dissolved and particulate 230 Th activities measured in the western North
1643
Atlantic prior to the GEOTRACES program. The horizontal bars show the measurement
1644
errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)),
1645
and the stations are identified with their names and with letters located in figure 3 (see Table
1646
1 for data sources).
1647
2
Figure A2. Profiles of dissolved
107 230
Th activity measured in the western North Atlantic along
1648
the GEOTRACES section GA03. The horizontal bars show the measurement errors (Table
1649
1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the
1650
stations are identified with their names and with numbers located in figure 3 (data from Hayes
1651
et al. (2015a)).
1652
3
108 230
Figure A3. Profiles of particulate
Th activity measured in the western North Atlantic
1653
along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table
1654
1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the
1655
stations are identified with their names and with numbers located in figure 3. Note that 230
1656
the other stations (data from Hayes et al. (2015a)).
1657
1658
Thp data from stations GT11-04 and GT11-08 are plotted with a different scale than for
4
Figure A4. Profiles of dissolved
231
109
Pa activity measured in the western North Atlantic prior
1659
to the GEOTRACES program. The horizontal bars show measurement errors (Table 1), the
1660
horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are
1661
identified with their names and with letters located in figure 3 (see Table 1 for data sources). 110
1662
5
Figure A5. Profiles of dissolved
231
Pa activity measured in the western North Atlantic along
1663
the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the
1664
horizontal dashed line indicates water depth (maximum depth from CTD), and the stations
1665
are identified with their names and with numbers located in figure 3 (data from Hayes et al.
1666
(2015a)).
1667
6
111
Figure A6. Profiles of particulate 231 Pa activity measured in the western North Atlantic along
1668
the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the
1669
horizontal dashed line indicates water depth (maximum depth from CTD), and the stations
1670
are identified with their names and with numbers located in figure 3. Note that
1671
from stations GT11-04 and GT11-08 are plotted with a different scale than for the other
1672
stations (data from Hayes et al. (2015a)).
231
Pap data
112
84
1673
7
Figure A7. Profiles of
230
Thd,p and
231
Pad,p measured at station GT11-14 (circles). The red
1674
circles are samples presumably influenced by the TAG hydrothermal vent, which is located
1675
to the east of GT11-14. The black lines are analytical profiles which approximate station
1676
GT11-14 measurements and are used as initial and lateral boundary conditions (Eq. C11).
1677
The horizontal bars show the measurement errors (Table 1; data from Hayes et al. (2015a)).
1678
8
230
Figure A8. Profiles of
Thd and
230
Thp as measured at pre-GEOTRACES stations (black
1679
circles) and as simulated in the reference solution near these stations (red solid line). The
1680
horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates
1681
water depth (from Ryan et al. (2009)), and the red dashed line is the 230 Thd or 230 Thp profile
1682
used as initial conditions and prescribed at the open boundaries. The stations are identified
1683
with their names and with letters located in figure 3.
1684
9
Figure A9. Profiles of
230
114
Thd as measured at GA03 stations (black circles) and as simulated
1685
in the reference solution near these stations (red solid line). The horizontal bars show the
1686
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1687
depth from CTD), and the red dashed line in each panel is the
1688
conditions and prescribed at the open boundaries. The stations are identified with their names
1689
and with numbers located in figure 3.
1690
10
Figure A10. Profiles of
230
230
Thd profile used as initial
115
Thp as measured at GA03 stations (black circles) and as simulated
1691
in the reference solution near these stations (red solid line). The horizontal bars show the
1692
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1693
depth from CTD), and the red dashed line in each panel is the
1694
conditions and prescribed at the open boundaries. The stations are identified with their
1695
names and with numbers located in figure 3. The extreme values of
1696
of stations GT11-04 and GT11-8 (Fig. A3) are excluded from this figure.
1697
11
Figure A11. Profiles of
113
231
230
Thp profile used as initial
230
Thp near the bottom 116
Pad as measured at pre-GEOTRACES stations (black circles) and
1698
as simulated in the reference solution near these stations (red solid line). The horizontal bars
1699
show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from
1700
Ryan et al. (2009)), and the red dashed line is the
1701
and prescribed at the open boundaries. The stations are identified with their names and with
1702
letters located in figure 3.
231
Pad profile used as initial conditions
117
85
1703
12
Figure A12. Profiles of
231
Pad as measured at GA03 stations (black circles) and as simulated
1704
in the reference solution near these stations (red solid line). The horizontal bars show the
1705
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1706
depth from CTD), and the red dashed line in each panel is the
1707
conditions and prescribed at the open boundaries. The stations are identified with their names
1708
and with numbers located in figure 3.
1709
13
Figure A13. Profiles of
231
231
Pad profile used as initial
118
Pap as measured at GA03 stations (black circles) and as simulated
1710
in the reference solution near these stations (red solid line). The horizontal bars show the
1711
measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum
1712
depth from CTD), and the red dashed line in each panel is the
1713
conditions and prescribed at the open boundaries. The stations are identified with their
1714
names and with numbers located in figure 3. The extreme values of
1715
of stations GT11-04 and GT11-08 (A6) are excluded from this figure.
86
231
Pad profile used as initial
231
Pap near the bottom 119
230
0
0.2
Thd (dpm/m 3)
0.4
0.6
0.8
1
1.2
0 1000
depth (m)
2000 3000 4000 5000 6000 231
-0.2 0
Pad (dpm/m 3)
0
0.2
0.4
0.6
1000
depth (m)
2000 3000 4000 5000 6000 Fig. 1. Vertical profiles of dissolved 230 Th and 231 Pa activities in the North Atlantic. Data from stations west (east) of Bermuda are shown with black (blue) circles and data from station GT11-16, near the TAG hydrothermal vent, are shown with red circles. The horizontal bars show the measurement uncertainties (see Table 1 for data sources).
87
PM (mg m-3) 0
0
50
100
150
200
1000
depth (m)
2000 3000 4000 5000 6000
Fig. 2. Regionally averaged vertical profile of particulate matter (PM) concentration in the western North Atlantic. Each circle is an average based on (i) optical measurements converted empirically to PM concentration and (ii) a linear interpolation at the same vertical levels of the concentration estimates obtained from optical profiles at different stations. The horizontal bars show the standard errors of the averages (data compilation from Gardner et al. (2017)).
88
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
Fig. 3. Map of the study area showing the location of GEOTRACES stations (red triangles with numerals), pre-GEOTRACES stations (red triangles with letters), and nephelometer and transmissmeter stations (circles). Black lines are isobaths of 200, 1000, and 3000 m, and blue arrows show schematic pathways of the Gulf Stream (GS), Deep Western Boundary Current (DWBC), Northern Recirculation Gyre (NRG), and Subtropical Gyre (SG). Also shown are the approximate locations of Bermuda (BER), the New England Seamounts (NES), and the Sohm Abyssal Plain (SAP). The thin black line protruding from the continental shelf and slope south of New England is line W.
89
Mean kinetic energy (m2/s2)
Mean
Mean
230
230
Thp (dpm m-3)
Thd (dpm m-3)
days
days Pap (dpm m-3) Mean
Mean
231
231Pa
d
(dpm m-3)
days
days
days Fig. 4. Time series of the domain-averaged kinetic energy, 230 Thd , reference solution
90
230
Thp ,
231
Pad , and
231
Pap in the
mean elevation (1993-2012)
CH
model elevation
CH
Fig. 5. Averages of sea surface height (m) as observed from satellite altimeter data during the period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
91
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
Fig. 6. Standard deviation of sea surface height (m) as observed from satellite altimeter data during the period 1993-2012 (top) and as simulated in the reference solution (bottom). The average pathway of the Gulf Stream coincides with the yellow band (upper panel) and “CH” stands for Cape Hatteras.
92
Fig. 7. Field of horizontal velocity in the surface layer (top) and at a depth of 3500 m (bottom) simulated in the reference solution. The horizontal arrow at the lower right outside each panel is the maximum speed in units of m s−1 in the corresponding field.
93
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
b
c
d
depth (m)
Model
a
Fig. 8. Distribution of horizontal velocity components between the New England continental shelf and Bermuda as measured during the line W program (top) and as simulated in the reference solution (bottom). At the top of each panel, red vertical lines show the position of GA03 stations GT11-01 to GT11-06, and grey vertical lines show the position of mooring locations. Coordinates along the horizontal axis are distances from 40.125◦ N,70.125◦ W (line W data from Toole et al. (2017)).
94
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th
Fig. 9. Profile of station-averaged 230 Thd (top) and 230 Thp (bottom) as calculated from pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles show averages of measurements from several stations with the following exceptions: for 230 Thd the shallowest circle is a measurement from a single station (OC278-5), and for 230 Thd,p the deepest circle is a measurement from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement and the deepest 230 Thd,p measurements; Table 1). The extreme values of 230 Thp near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements. 95
depth (m)
d
(dpm m-3)
depth (m)
231Pa
231Pa
p
(dpm m-3)
Fig. 10. Profile of station-averaged 231 Pad (top) and 231 Pap (bottom) as calculated from pre-GEOTRACES and GA03 measurements (black circles) and as simulated in the reference solution (black line). The circles show averages of measurements from several stations with the following exceptions: for 231 Pad (231 Pap ), the three (four) deepest circles show measurements from a single station (GT11-12). The horizontal bars show the standard errors of the averages (measurement error for the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 231 Pap near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
96
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3
0.4
230
0.5
0.6
0.7
0.06
0.04
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
0.12
(dpm m-3)
0.8
-0.02 -0.02
0.8
0.35
10
5
(dpm
0.1
p data
0.05
231Pa
0.15
3.5
Thp data (dpm/m 3)
0.2
0.08
0.1
Thp model (dpm/m 3) 230Th230model (dpm p
0.04
0.06
m-3)
0.12
0.14
-3
3 2.5 2
230
m-3)
0.25
Thd data (dpm/m 3)
0.02
r2 = 0.11 p<0.01 n = 83
4
230
(dpm m-3) d data
4.5
r2 = 0.81 p <0.01 n = 167
0.3
231Pa
0
3
Thd model (dpm/m ) -3 230Th model (dpm m ) d
1.5 1 0.5 0 0
0 0
0.05
0.1
231Pa
0.15
0.2
-3) Thd model (dpm/m 3m ) (dpm d model
0.25
0.3
230
0.5
1
1.5
231Pa
2 230
2.5
3
Thp model (dpm/m 3)
3.5
-3 p model (dpm m )
4
4.5
5 10
-3
Fig. 11. Scatter plots of measured radionuclide activities versus simulated radionuclide activities in the reference solution. Shown in each panel are the squared Pearson correlation coefficient (r2 ), the p value of the correlation, and the number of measurements (n). In each panel, the black line is the line of perfect agreement. For panel (b) and (d), the extreme measured values of 230 Thp and 231 Pap near the bottom of stations GT11-04 and GT11-08, along with the corresponding model values, are excluded from the scatter plot.
97
a a
c b
Fig. 12. Distribution of near-bottom 231 Pap /230 Thp in the reference experiment. The filled circles are surface sediment data (Table 5), and the solid black lines are the 200 m, 1000 m, and 3000 m isobath, respectively.
98
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
PaTh sed model modeled 231Pa/230Th Fig. 13. Scatter plot of surface sediment 231 Pa/230 Th data versus the near-bottom 231 Pap /230 Thp simulated near the corresponding data location in the reference experiment. The regression coefficient (slope) is 0.35 ± 0.36 (one standard error) and the Pearson correlation coefficient is 0.21 (n = 22). The black line is the line of perfect agreement.
99
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 14. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for k10 (Th) = 0.02 yr−1 mg−1 m3 and k10 (Pa) = 0.01 yr−1 mg−1 m3 (blue lines), k10 (Th) = 0.04 yr−1 mg−1 m3 and k10 (Pa) = 0.02 yr−1 mg−1 m3 (black, reference solution), and k10 (Th) = 0.08 yr−1 mg−1 m3 and k10 (Pa) = 0.04 yr−1 mg−1 m3 (green). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
100
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 15. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated when the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (black, reference solution), and 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
101
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 16. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated when the radionuclide activities at the DWBC inflow are halved (green lines) or doubled (blue) compared to their values in the reference solution (black). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
102
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 17. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for uniform k1 (Th) and k1 (Pa) when the strength of the DWBC at its inflow location is set to 10 Sv (green lines), 20 Sv (red), and 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
103
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
231Pa
p
(dpm m-3)
depth (m)
231Pa
Fig. 18. Profile of station-averaged 230 Thd,p (top) and 231 Pad,p (bottom) as calculated from (pre-)GEOTRACES measurements (black circles) and as simulated for the reference experiment (black), when the radionuclide activities at the DWBC inflow locations are halved (green), and when the radionuclide activities at the DWBC inflow locations are halved, and the DWBC at its inflow is 40 Sv (blue). The circles show averages of measurements from several stations, with the exceptions listed in Figure 9 for 230 Thd,p and in Figure 10 for 231 Pad,p . The horizontal bars show the standard errors of the averages (measurement error for the shallowest 230 Thd measurement, the deepest 230 Thd,p measurements, and the three (four) deepest measurements of 231 Pad (231 Pap ); Table 1). The extreme values of 230 Thp (Fig. 3) and 231 Pap (Fig. 6) near the bottom of stations GT11-04 and GT11-08 are excluded from the station-averaged profile of the measurements.
104
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
!"##$%(
2500
3000
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
0
125 250 375 500
PM (mg m-3) Fig. 19. Vertical profiles of optically derived particulate matter concentration at stations GT11-04, GT11-06, and GT11-08 between the New England continental shelf and Bermuda. Particulate matter concentration is estimated from beam attenuation coefficient measurements available in the GEOTRACES Intermediate Data Product (Schlitzer et al. 2018) using the empirical relationship between PM concentration and BAC due to particles as reported by Gardner et al. (2018b).
105
PM (mg/m3)
z=3000 m
z=3000 m
z=3000 m
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
PM (mg/m3)
z=4000 m
z=4000 m
z=4000 m
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
z=5000 m
z=5000 m
z=5000 m
PM (mg/m3)
230Th (dpm/m3) d
231Pa
d (dpm/m
3)
Fig. 20. Distributions of particulate matter concentration as estimated from optical measurements compiled by Gardner et al. (2017) (top three panels), 230 Thd activity as simulated in the reference solution (middle panels), and 231 Pad activity as simulated in the reference solution (bottom panels). The left, middle, and right panels show distributions at a depth of, respectively, 3000 m, 4000 m, and 5000 m.
106
230Th (dpm d 0.2
0.4
0.6
0.8
1
0 0
1000
1000
2000
2000
( )
0
3000
p
p
( )
0
4000 5000
0
d
0.4
0.6
0.8
1
4000
(a)
CMME-13 0.2
0.4
0.6
0.8
OC278-2 (f)
5000
1
0
0.2
0.4
0.6
0.8
1
0 1000
500
2000
1000 3000
p
( )
0.2
3000
0
1500
S1 (b) 0
depth (m)
0.2
0.4
0.6
0.8
0
1
0
1000
1000
2000
2000
3000
3000
4000
4000
EN407-3 (c) 0
0.2
0.4
0.6
0.8
( ) p
0.6
0.8
1
0
0.2
0.4
0
0.2
0.4
0.6
0.8
1
0 1000
2000
2000
3000
3000
4000
4000
EN407-4 (d)
5000
0.2
0.4
0.6
0.8
0
1000
1000
2000
2000
3000
3000
4000
4000
BATS (e)
5000
OC278-5 (i)
5000
1
0
0.1
0.2
0.3
0.4
0
0
0.6
0.8
1
EN407-6 (j)
5000
230Th (dpm p 0
0.4
OC278-4 (h)
1
1000
( )
0.2
5000
0
0
(g)
5000
0
5000
p
OC278-3
4000
2000
m-3) 0.1
0.2
0.3
0.4
0
1000 500
( )
2000 3000
1500
4000 5000
1000
p
depth (m)
m-3)
CMME-13 (a)
2000
S1 (b)
Figure A1. Profiles of dissolved and particulate 230 Th activities measured in the western North Atlantic prior to the GEOTRACES program. The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are identified with their names and with letters located in figure 3 (see Table 1 for data sources).
107
230Th (dpm d 0
0.2
0.4
0.8
1
0
0.2
0.4
0
0.2
0.4
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
depth (m)
0.6
m-3)
GT11-01 (1) 0.2
0.4
0.6
0.8
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
GT11-02 (2) 0.2
0.4
0.6
0.8
5000
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
GT11-03 (3) 0
0.2
0.4
0.6
0.8
0.6
0.8
1
0.4
0.6
0.8
1
GT11-10 (10)
1
0 0
1000
1000
2000
2000
3000
3000
4000
4000
GT11-04 (4)
1
GT11-08 (8) 0.2
5000
0
5000
0.8
GT11-06 (6)
0
1
0.6
5000
0.2
0.4
0.6
0.8
1
GT11-12 (12)
Figure A2. Profiles of dissolved 230 Th activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3 (data from Hayes et al. (2015a)).
108
230Th (dpm p 0
0.1
0.2
0.3
m-3)
0.4
0 1000 2000 3000 4000 5000
GT11-01 (1) 0
0.1
0.2
GT11-06 (6)
0.3
0.4
0 1000 2000
depth (m)
3000 4000 5000
GT11-02 (2) 0
0.1
0.2
0.3
GT11-08 (8)
0.4
0 1000 2000 3000 4000 5000
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
Figure A3. Profiles of particulate 230 Th activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3. Note that 230 Thp data from stations GT11-04 and GT11-08 are plotted with a different scale than for the other stations (data from Hayes et al. (2015a)).
109
231Pa (dpm d
depth (m) p ( )
0
0.1
0.2
0.3
0.4
m-3) 0
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
EN407-3
5000
(c)
0.1
0.2
0.3
0.4
EN407-4 (d)
Figure A4. Profiles of dissolved 231 Pa activity measured in the western North Atlantic prior to the GEOTRACES program. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the stations are identified with their names and with letters located in figure 3 (see Table 1 for data sources).
110
231Pa (dpm d
m-3)
GT11-06 (6)
depth p (m) ( )
GT11-01 (1)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-12
GT11-04 (4)
(12)
Figure A5. Profiles of dissolved 231 Pa activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3 (data from Hayes et al. (2015a)).
111
231Pa (dpm p
depth (m)
GT11-01 (1)
m-3)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
Figure A6. Profiles of particulate 231 Pa activity measured in the western North Atlantic along the GEOTRACES section GA03. The horizontal bars show measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the stations are identified with their names and with numbers located in figure 3. Note that 231 Pap data from stations GT11-04 and GT11-08 are plotted with a different scale than for the other stations (data from Hayes et al. (2015a)).
112
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
d
(dpm m-3)
230Pa
p
(dpm m-3)
depth (m)
231Pa
Figure A7. Profiles of 230 Thd,p and 231 Pad,p measured at station GT11-14 (circles). The red circles are samples presumably influenced by the TAG hydrothermal vent, which is located to the east of GT11-14. The black lines are analytical profiles which approximate station GT11-14 measurements and are used as initial and lateral boundary conditions (Eq. C11). The horizontal bars show the measurement errors (Table 1; data from Hayes et al. (2015a)).
113
Pre-GEOTRACES 230Thd CMME-13
depth (m)
(a)
S1
EN407-3
EN407-4
BATS
(c)
(d)
(e)
OC278-4
OC278-5
EN407-6
(h)
(i)
(j)
(b)
OC278-2
OC278-3
(f)
(g) 230Th
d
(dpm m-3)
CMME-13
(a) 230Th
p
S1
(b)
(dpm m-3)
Figure A8. Profiles of 230 Thd and 230 Thp as measured at pre-GEOTRACES stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the red dashed line is the 230 Thd or 230 Thp profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names 114 and with letters located in figure 3.
depth (m)
GEOTRACES 230Thd
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
d
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A9. Profiles of 230 Thd as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 230 Thd profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3.
115
depth (m)
GEOTRACES 230Thp
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
p
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A10. Profiles of 230 Thp as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 230 Thp profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3. The extreme values of 230 Thp near the bottom of stations GT11-04 and GT11-8 (Fig. A3) are excluded from this figure.
116
depth (m)
Pre-GEOTRACES 231Pad
EN407-3 (c)
EN407-4 (d)
Figure A11. Profiles of 231 Pad as measured at pre-GEOTRACES stations (black circles) and as simulated in 231line). The horizontal -3 bars show the measurement errors the reference solution near these stations (red solid d (Table 1), the horizontal dashed line indicates water depth (from Ryan et al. (2009)), and the red dashed line is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with letters located in figure 3.
Pa (dpm m )
117
depth (m)
GEOTRACES 231Pad
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8)
231Pa
GT11-10 (10)
d
GT11-04 (4)
GT11-12 (12)
(dpm m-3)
Figure A12. Profiles of 231 Pad as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3.
118
depth (m)
GEOTRACES 231Pap
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8) 231Pa
p
GT11-03 (3)
GT11-04 (4)
GT11-10 (10)
GT11-12 (12)
(dpm m-3)
Figure A13. Profiles of 231 Pap as measured at GA03 stations (black circles) and as simulated in the reference solution near these stations (red solid line). The horizontal bars show the measurement errors (Table 1), the horizontal dashed line indicates water depth (maximum depth from CTD), and the red dashed line in each panel is the 231 Pad profile used as initial conditions and prescribed at the open boundaries. The stations are identified with their names and with numbers located in figure 3. The extreme values of 231 Pap near the bottom of stations GT11-04 and GT11-08 (A6) are excluded from this figure.
119
mean elevation (1993-2012)
CH
model elevation
CH
depth (m)
d
(dpm m-3)
depth (m)
231Pa
231Pa
p
(dpm m-3)
1
0.14
r2 = 0.49 p <0.01 n = 98
0.9
r2 = 0.85 p <0.01 n = 254
Thp data (dpm/m 3)
p data
0.4
230
0.5
230
Thd data (dpm/m 3)
0.6
0.1
0.2 0.1
0.1
0.2
0.3 230
0.4
0.6
0.7
0.04
-0.02 -0.02
0.8
(dpm m-3) 10
0.06
0.08 3
0.05
3.5 3
Thp data (dpm/m )
0.1
p data
0.15
0.12
0.14
m-3)
-3
3 2.5 2
230
(dpm m-3)
0.2
0.1
r2 = 0.11 p<0.01 n = 83
4
231Pa
Thd data (dpm/m 3)
4.5
230
(dpm m-3) d data 231Pa
0.04
Thp model (dpm/m ) 230Th model (dpm p
r2 = 0.81 p <0.01 n = 167
0.25
0.02
230
5
0.3
0
Thd model (dpm/m 3)
230Th model d
0.35
0.5
0.06
0
0 0
0.08
0.02
230Th
0.3
230Th
d data
(dpm m-3)
0.7
(dpm m-3)
0.8
0.12
1.5 1 0.5 0 0
0 0
0.05
0.1
231Pa
0.15
0.2
Thd model (dpm/m m ) (dpm d model 230
3
-3)
0.25
0.3
0.5
1
1.5
231Pa
2 230
2.5
3
Thp model (dpm/m 3)
p model
3.5
(dpm m-3)
4
4.5
5 10
-3
a a
c b
surface sediment 231Pa/230Th PaTh sed data
0.18 0.16
r2 = 0 .05 p = 0.34 n=22
0.14 0.12 0.1 0.08 0.06 0.04 0.04
0.06
0.08
0.1
0.12
PaTh sed model modeled 231Pa/230Th
0.14
0.16
0.18
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
d
(dpm m-3)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
231Pa
p
(dpm m-3)
0
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
500
1000
depth (m)
1500
2000
!"##$%(
2500
3000
!"##$%'
3000
3500
3500
4000
4000
4500
4500
!"##$%&
3000
3500
5000
4000 0
625 1250 1875 2500
5000 0
125 250 375 500
PM (mg m-3)
0
125 250 375 500
230
0 0 1000
depth (m)
2000 3000 4000 5000 6000
0.2
0.4
Thd (dpm/m 3) 0.6
0.8
1
1.2
231
-0.2 0 1000
depth (m)
2000 3000 4000 5000 6000
0
Pad (dpm/m 3) 0.2
0.4
0.6
z=3000 m
PM (mg/m3)
z=4000 m
PM (mg/m3)
z=5000 m
PM (mg/m3)
z=3000 m
230Th (dpm/m3) d
z=4000 m
230Th (dpm/m3) d
z=5000 m
230Th (dpm/m3) d
z=3000 m
231Pa
d (dpm/m
3)
z=4000 m
231Pa
d (dpm/m
3)
z=5000 m
231Pa
d (dpm/m
3)
PM (mg m-3) 0 1000
depth (m)
2000 3000 4000 5000 6000
0
50
100
150
200
j2 i
1 c 3 d 4
6
f b
h
a
8
g
e 10 12
Mean
Mean
231
d
(dpm m-3)
Pap (dpm m-3)
231Pa
Mean
230
Mean
Thd (dpm m-3)
Thp (dpm m-3)
230
days
days
days
days
days
Mean kinetic energy (m2/s2)
elevation std deviation (1993-2012 2012)
CH
model elevation std deviation
CH
along-line W velocities (cm/s)
cross-line W velocities (cm/s)
depth (m)
Data
b
c
d
depth (m)
Model
a
depth (m)
d
(dpm m-3)
p
(dpm m-3)
depth (m)
230Th
230Th
depth (m)
GEOTRACES 230Thp
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
p
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
depth (m)
Pre-GEOTRACES 231Pad
EN407-3
EN407-4
(c)
231Pa
(d)
d
(dpm m-3)
depth (m)
GEOTRACES 231Pad
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8)
231Pa
GT11-10 (10)
d
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
depth (m)
GEOTRACES 231Pap
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8) 231Pa
p
GT11-03 (3)
GT11-04 (4)
GT11-10 (10)
GT11-12 (12)
(dpm m-3)
230Th (dpm d 0.2
0.4
0.6
0.8
1
0
1000
1000
2000
2000
( )
0
3000
p
p
( )
0 0
4000 5000
0
d
0.4
0.6
0.8
1
4000
(a)
CMME-13 0.2
0.4
0.6
0.8
OC278-2 (f)
5000
1
0
0.2
0.4
0.6
0.8
0.6
0.8
1
0 1000
500
2000
1000 3000
p
( )
0.2
3000
0
1500
depth (m)
0
0.2
0.4
0.6
0.8
0
1
0 1000
1000
2000
2000
3000
3000
4000
4000
EN407-3 (c) 0
0.2
0.4
0.6
0.8
( ) p
1
0
0.2
0.4
0
0.2
0.4
0.6
0.8
1
0 1000
2000
2000
3000
3000
4000
4000
EN407-4 (d)
5000
0.2
0.4
0.6
0.8
OC278-5 (i)
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
0.6
0.8
1
4000
BATS (e)
5000
0.1
0.2
0.3
EN407-6 (j)
5000
230Th (dpm p 0
0.4
OC278-4 (h)
1
0
( )
0.2
5000
1000
0
(g)
5000
0
5000
p
OC278-3
4000
S1 (b)
2000
0.4
0
0
m-3) 0.1
0.2
0.3
0.4
0
1000 500
( )
2000 3000
1500
4000 5000
1000
p
depth (m)
m-3)
CMME-13 (a)
2000
S1 (b)
230Th (dpm d 0
0.2
0.4
0.8
m-3)
1
0
0.2
0.4
0
0.2
0.4
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
depth (m)
0.6
GT11-01 (1) 0.2
0.4
0.6
0.8
5000
1
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
0
GT11-02 (2) 0.2
0.4
0.6
0.8
5000
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
GT11-03 (3) 0
0.2
0.4
0.6
0.8
0.6
0.8
0
1000
1000
2000
2000
3000
3000
0.4
0.6
0.8
0.2
0.4
0.6
0.8
4000
GT11-04 (4)
1
1
GT11-10 (10)
1
0
5000
1
GT11-08 (8) 0.2
5000
0
4000
0.8
GT11-06 (6)
0
1
0.6
5000
GT11-12 (12)
1
230Th (dpm p 0
0.1
0.2
0.3
m-3)
0.4
0 1000 2000 3000 4000 5000
GT11-01 (1) 0
0.1
0.2
0
0.1
0.2
GT11-06 (6)
0.3
0.4
0 1000 2000
depth (m)
3000 4000 5000
GT11-02 (2) 0.3
GT11-08 (8)
0.4
0 1000 2000 3000 4000 5000
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
231Pa (dpm d
depth (m) p ( )
0
0.1
0.2
0.3
0.4
m-3) 0
0
0
1000
1000
2000
2000
3000
3000
4000
4000
5000
EN407-3
(c)
5000
0.1
0.2
0.3
EN407-4 (d)
0.4
231Pa (dpm d
m-3)
GT11-06 (6)
depth p (m) ( )
GT11-01 (1)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12
(12)
231Pa (dpm p
depth (m)
GT11-01 (1)
m-3)
GT11-06 (6)
GT11-02 (2)
GT11-08 (8)
GT11-03 (3)
GT11-10 (10)
GT11-04 (4)
GT11-12 (12)
d
(dpm m-3)
230Th
p
(dpm m-3)
depth (m)
230Th
depth (m)
231Pa
d
(dpm m-3)
230Pa
p
(dpm m-3)
Pre-GEOTRACES 230Thd CMME-13
depth (m)
(a)
S1
EN407-3
EN407-4
BATS
(c)
(d)
(e)
OC278-4
OC278-5
EN407-6
(h)
(i)
(j)
(b)
OC278-2
OC278-3
(f)
(g) 230Th
d
(dpm m-3)
CMME-13
(a) 230Th
p
S1
(b)
(dpm m-3)
depth (m)
GEOTRACES 230Thd
GT11-01 (1)
GT11-06 (6)
GT11-02 (2)
GT11-03 (3)
GT11-08 (8) 230Th
GT11-10 (10)
d
(dpm m-3)
GT11-04 (4)
GT11-12 (12)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: