Ca variability in the Atlantic and Pacific oceans

ELSEVIER

Earth and Planetary Science Letters 171 (1999) 623–634 www.elsevier.com/locate/epsl

Seawater strontium and Sr=Ca variability in the Atlantic and Pacific oceans Stephanie de Villiers * Department of Geological Sciences, Box 35-1310, University of Washington, Seattle, WA 98195-1310, USA Received 25 August 1998; accepted 8 July 1999

Abstract Seawater Sr and Sr=Ca exhibit spatial gradients of 2–3% globally, with the deep ocean more enriched relative to the surface. In latitudinal transects, the highest surface values are found at high latitudes and associated with areas of upwelling. A pronounced upper ocean vertical Sr gradient is attributable to the production of celestite skeletons by surface-dwelling acantharia, coupled to a shallow dissolution cycle. The upper ocean residence time of Sr with respect to celestite cycling is much shorter than its global oceanic residence time. Although the magnitude of seawater Sr=Ca variability is relatively small, it is significant with respect to high-precision paleoceanographic applications. Sr=Ca gradients in the contemporary ocean also complicates evaluating Quaternary changes in seawater Sr=Ca that may have resulted from other processes, such as aragonite recrystallization during sea-level low stands.  1999 Elsevier Science B.V. All rights reserved. Keywords: Sr=Ca; sea water; paleosalinity; Atlantic Ocean; Pacific Ocean

1. Introduction Understanding the oceanic strontium cycle is relevant to research areas as wide-ranging as ocean circulation, paleoclimatology and the chemical evolution of the ocean. Much of the earliest work on seawater Sr focussed on documenting the bomb 90 Sr signal, introduced during 1945–1962 [1,2]. The two aspects of seawater Sr geochemistry that have been the most intensively studied recently are the Sr isotopic history of seawater as a means of determining the variability of geochemical fluxes into the ocean [3–5], and application of the Sr=Ca composition of Ł Present

address: Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa. E-mail: [email protected]

marine biogenic carbonates in paleoclimate reconstructions [6–8]. The attractiveness of oceanic Sr as a proxy in paleoceanographic studies lies in its long residence time in the ocean, compared to the mixing time of the ocean, and its implied homogeneous distribution and conservative nature. It is perhaps surprising, given the far-reaching and often controversial implications of seawater Sr-based studies, that most of the focus has been on relating changes in seawater Sr to its external forcing functions, while comparatively little effort has gone into understanding its ‘internal’ cycle under present conditions. Early studies of seawater Sr were surrounded by considerable disagreement about its conservative nature [9]. The introduction of improved measurement techniques confirmed that

0012-821X/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 7 4 - 0

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surface ocean Sr concentrations are slightly lower than deeper values [10,11], and the proposed uptake of Sr into celestite (SrSO4 ) skeletons produced by surface-dwelling acantharia [11] has been supported by sediment trap studies [12,13]. The variabilities in seawater Sr and Sr=Ca that have been reported for the contemporary ocean are small [11,14], but equal in magnitude to changes observed in some paleoceanographic records [6,8,15,16] which are interpreted based on the assumption that seawater Sr and Sr=Ca are conservative. A prerequisite to the reliable interpretation of these high-precision paleoceanographic records is a thorough understanding of the behavior of oceanic Sr at the same level of resolution. The primary aim of this study was to provide a global database of high-precision (thermal-ionization mass-spectrometry) seawater Sr measurements, including surface ocean transects across latitudinal zones, in order to attempt a better understanding of its ‘internal’ oceanic cycle.

2. Sampling and analytical methods Surface (0–10 m) seawater samples were collected along two latitudinal transects, one in the Atlantic from 5ºS to 60ºN and another in the central Pacific from 10ºS to 45ºN, and also in the northeast Pacific Ocean (Fig. 1). Hydrocast seawater samples were collected at four stations, chosen to represent the flow and aging of oceanic deep water from the North Atlantic (60ºN, 20ºW), through the central Atlantic (5ºS, 25ºW) and Pacific (10ºS, 179ºE) to the North Pacific (45ºN, 179ºE), illustrated in Fig. 1. Samples were stored unfiltered and unacidified in polyethylene bottles, with Parafilm wrapped around the cap, and analyzed within a couple of months of sampling. Repeat analysis on selected samples showed no measurable signs of evaporation after one year in storage, and no measurable difference between filtered=unfiltered or acidified=unacidified samples. Strontium analyses (carried out simultaneously with Ca analyses [17]) were performed by isotope-dilution thermal-ionization mass-spectrometry (multi-collector VG Sector) at the University of Washington. About 1 g of the seawater sample was accurately weighed out, equi-

Fig. 1. Hydrocast stations and latitudinal surface transects in the Atlantic and Pacific oceans. Station locations are tabulated in Tables 1 and 2.

librated with an appropriate amount of 84 Sr spike solution, and the Sr (plus Ca) fraction separated from other sea salts using a cation-exchange resin (Biorad AG-50, ð8) [18]. The Sr fraction was then eluted, dried and redissolved in HCl before loading onto Ta filaments. Mass fractionation was corrected for by an exponential law [19]. Repeat analysis of an internal standard over a 2-year period yielded an external measurement precision of 0.13% (1¦ , n D 15). Duplicate analyses of seawater samples typically yielded an error less than 0.05%. All Sr results are reported as concentration values normalized to a salinity of 35.

3. Results 3.1. Surface ocean Sr and Sr=Ca variability The average Sr (normalized to salinity) and Sr=Ca content of all surface ocean water samples analyzed are 87.40 µM (š0.56%) and 8.539 mmol=mole (š0.45%), respectively (Table 1). The total ranges in surface ocean Sr and Sr=Ca are ¾2.8% and ¾1.4%, respectively, with some noteworthy latitudinal gradients. In this study, the lowest surface ocean Sr and Sr=Ca values are found in the Pacific Ocean, associated with the mid-latitude (20–35ºN) surface salinity

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Table 1 Surface seawater Sr (normalized to a salinity of 35) and Sr=Ca values along latitudinal transects in the Atlantic and Pacific oceans Lat.=Long.

T (ºC)

Salinity

Sr (µM)

Sr=Ca (mmol=mol)

Si (µM)

PO (µM)

Atlantic 05ºS=25ºW 00º=25ºW 05ºN=25ºW 10ºN=25ºW 15ºN=28ºW 20ºN=24ºW 25ºN=21ºW 30ºN=19ºW 34ºN=22ºW 40ºN=20ºW 45ºN=20ºW 50ºN=20ºW 55ºN=20ºW 60ºN=20ºW

26.68 25.16 27.90 27.44 26.27 22.99 22.80 21.40 21.07 20.10 19.08 17.50 14.21 11.04

36.019 36.139 35.898 35.647 36.281 36.341 37.124 36.924 36.573 35.902 35.786 35.684 35.400 35.175

87.55 87.30 87.19 87.34 87.65 87.95 87.59 87.48 87.21 87.26 87.07 86.97 87.56 87.54

8.493 8.565 8.562 8.549 8.579 8.605 8.569 8.547 8.548 8.548 8.538 8.519 8.559 8.554

1.0 1.5 0.7 0.2

0.10 0.11 0.02 0.00

8.7 2.8 2.8 2.9 3.6 4.0

0.23 0.05 0.07 0.13 0.26 0.39

Western Pacific 10ºS=179ºE 05ºS=179ºE 00º=179ºE 05ºN=179ºE 15ºN=179ºE 20ºN=179ºE 25ºN=179ºE 35ºN=179ºE 40ºN=179ºE 45ºN=179ºE

28.61 29.81 30.06 29.43 27.83 27.80 27.91 23.60 15.06 9.50

34.390 34.324 34.122 33.969 34.777 35.151 35.380 34.869 34.034 33.085

87.16 87.17 87.32 87.27 87.23 86.92 86.82 86.99 87.60 88.05

8.525 8.514 8.524 8.546 8.524 8.501 8.489 8.487 8.552 8.612

2.7 2.5 2.8 1.2 3.4 3.1 2.9 2.9 10.4 28.7

0.19 0.20 0.18 0.17 0.09 0.06 0.04 0.03 0.41 1.33

Eastern Pacific 24ºN=154ºW 29ºN=150ºW 37ºN=138ºW 50ºN=151ºW 54ºN=158ºW 56ºN=136ºW

25.79 21.38 16.71 6.79 8.18 13.10

35.960 35.280 33.910 32.703 32.285 31.976

87.13 87.24 86.50 88.98 88.23 88.33

8.506 8.501 8.535 8.591 8.597 8.544

0.8 0.5 4.0 22.9 13.5 17.0

0.25 0.02 0.19 1.24 0.85 0.49

The nutrient hydrocast data are from samples taken from the same Niskin sample bottles.

maximum and depleted nutrient levels (Fig. 2). In the equatorial Pacific (5ºS to 15ºN) Sr and Sr=Ca levels are ¾0.7% higher than at mid-latitudes, probably related to equatorial upwelling as reflected in slightly enhanced nutrient levels (Fig. 2). The most noticeable feature of the latitudinal surface transect is the ¾1.5% increase in Sr and Sr=Ca values at high latitudes, which coincides with the high nutrient levels of these low-salinity surface water masses (Fig. 2). Along the Atlantic Ocean surface transect Sr and Sr=Ca values are on average slightly higher than

in the Pacific Ocean (Table 1), and the co-variation with nutrients and salinity observed in the Pacific is absent (Fig. 3). An interesting feature in the Atlantic Ocean surface transect is enhanced Sr concentrations off the North African coast in the 15ºN to 25ºN latitudinal belt (Fig. 3). Similarly elevated surface concentrations have been reported for Ca, Mg and K [20] and also trace metal (e.g. Fe, Al, Cu, Zn and Cd) in this ocean area, and attributed to dust input from the Sahara desert [21]. It has been suggested that “the atmospheric flux of soluble calcium may represent a major portion of the total flux of soluble

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Fig. 2. Surface water values for (a) temperature and salinity, and (b) Sr=Ca and PO4 along a latitudinal transect in the central Pacific Ocean (data in Table 1). Positive values for latitudinal degrees correspond to the Northern Hemisphere.

calcium to the ocean” in “certain regions where the eolian transport is unusually high and the stream discharge slight (e.g. off the west coast of North Africa)” [22]. It may therefore be instructive to evaluate the possibility that continental dust may provide a quantitatively significant source of Sr to surface ocean water. The 5–10 g m 2 y 1 [23] Saharan dust flux off North Africa typically consists of quartz, feldspars and 10–50% carbonates [22,24,25]. Consideration of only the carbonate fraction, with a presumed Sr content of 1500 µg=g, translates into a ‘Sr dust’

flux of 8.6 to 86 µmol m 2 y 1 if the carbonate fraction undergoes complete dissolution. If this Sr is not scavenged or removed by other processes, a simplistic mass balance calculation suggests that it would take 8.7 to 87 years to produce a 0.75 µmol=kg increase in Sr per meter water depth in the upper ocean. This speculative calculation implies that dust inputs may result in measurable enhanced surface ocean Sr concentrations in this area and its plausibility can be tested in future studies with detailed high-precision analysis of Sr variations in the mixed layer.

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Fig. 3. Surface water variations in Sr=Ca, versus that in salinity, in the Atlantic Ocean latitudinal transect (data in Table 2).

Fig. 4. Vertical profiles of (a) dissolved Sr (normalized to a salinity of 35) and (b) Sr=Ca at the Atlantic and Pacific hydrocast stations shown in Fig. 1, tabulated in Table 2.

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Table 2 The Sr composition of Atlantic and Pacific Ocean hydrocast stations Station

Depth (m)

Salinity

T (ºC)

Sr (µM)

Sr=Ca (mmol=mol)

Si (µM)

PO4 (µM)

60ºN, 20ºW

0 100 800 1300 1700 2500

35.175 35.159 35.084 34.941 34.901 34.971

11.04 8.64 6.30 3.92 3.30 2.72

87.54 87.51 87.64 87.79 87.82 87.79

8.55 8.57 8.59 8.59 8.59 8.60

3.9 4.1 3.5 2.3 2.3 1.5

0.39

5ºS, 25ºW

0 100 200 450 825 1050 1300 1500 1700 2100 2500 3000 3500 4000 4500

36.02 35.97 35.04 34.68 34.47 34.63 34.83 34.94 34.97 34.96 34.93 34.91 34.91 34.84 34.74

26.68 18.80 11.08 7.49 4.49 4.16 4.21 4.13 3.80 3.30 2.79 2.42 2.26 1.55 0.61

87.55 87.74 88.20 88.57 88.62 88.50 88.36 88.08 87.77 88.01 88.16 87.97 87.88 88.00 88.37

8.49 8.57 8.63 8.64 8.64 8.63 8.62 8.60 8.58 8.59 8.59 8.59 8.59 8.59 8.61

1.0 3.7 10.2 19.5 31.9 32.4 25.2 19.0 17.6 21.0 28.9 35.6 33.6 55.8 97.5

0.10 0.93 1.61 2.27 2.31 2.19 1.84 1.50 1.40 1.34 1.41 1.45 1.39 1.62 2.08

10ºS, 179ºE

0 100 250 500 800 1000 1600 2000 2600 3000 3600 4200

34.39 36.08 35.20 34.56 34.51 34.52 34.60 34.63 34.66 34.67 34.69 34.70

28.61 26.68 15.22 7.36 5.18 4.30 2.58 2.08 1.64 1.42 1.17 0.95

87.16 87.78 88.27 88.61 88.72 88.67 88.65 88.55 88.53 88.71 88.75 88.73

8.53 8.58 8.63 8.64 8.64 8.64 8.62 8.61 8.60 8.62 8.63 8.65

2.7 2.7 8.4 29.7 55.4 70.3 103.7 118.0 133.5 138.4 135.2 125.3

0.19 0.62 1.34 2.10 2.50 2.56 2.55 2.56 2.50 2.55 2.43 2.30

45ºN, 179ºE

0 90 250 500 1000 1200 1600 1800 2000 2600 3000 3600 4500 5100 5400 5740

33.09 33.63 33.86 34.10 34.38 34.44 34.53 34.56 34.59 34.64 34.65 34.67 34.68 34.68 34.68 34.69

9.50 6.26 4.79 3.95 2.82 2.51 2.07 1.90 1.77 1.46 1.39 1.22 1.11 1.10 1.10 1.10

88.05 87.97 88.10 88.37 88.72 88.94 89.16 89.14 88.94 88.96 88.84 88.63 88.65 88.65 88.63 88.67

8.62 8.61 8.62 8.63 8.64 8.64 8.65 8.64 8.62 8.62 8.61 8.61 8.61 8.60 8.60 8.60

28.7 28.4 66.7 103.2 145.1 155.2 166.0 168.4 170.3 169.1 167.6 161.1 159.7 161.5 161.2 161.7

1.33 1.33 2.34 2.89 3.13 3.13 3.03 2.98 2.94 2.77 2.74 2.62 2.54 2.53 2.53 2.54

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Fig. 5. Depth profiles of salinity and salinity-normalized Sr at the equatorial Atlantic station. The salinity minimum at around 800 m represents the core of Antarctic Intermediate Water.

3.2. The deep ocean Sr gradient The deep ocean is enriched in Sr and Sr=Ca relative to average surface ocean levels by ¾2% (Fig. 4; Table 2). A horizontal gradient of similar magnitude exists at depth between the Atlantic and Pacific oceans. The vertical Sr profiles are to some extent shaped by the horizontal flow of water masses with variable pre-formed values: Antarctic Intermediate Water (AAIW at ¾450 m depth in the South Pacific and 500–1000 m in the South Atlantic) and Antarctic

Bottom Water (AABW below 4000 m) are distinguished by higher Sr concentrations (Fig. 4a). North Atlantic Deep Water (NADW at ¾3500 m in the South Atlantic) on the other hand is characterized by low Sr concentrations, comparable to surface values at the North Atlantic station. The semi-conservative nature of seawater Sr is illustrated in a comparison with salinity at the South Atlantic station in Fig. 5. Insight into the processes responsible for variable Sr concentrations can be gained from the classical approach of comparison with ‘analogue’ species.

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The most appropriate comparison appears to be with nutrient proxies (PO34 and NO3 ), based on similar distribution patterns along latitudinal surface transects (Fig. 2) and the intermediate depth concentration maximum (Fig. 4). The assumed common source is therefore remineralization associated with biogenic fluxes in the upper ocean. A simple comparison of the relative change in seawater Sr and PO34 with the Sr:PO34 content of organic material, implies that organic matter remineralization can account for at most 5% of the observed Sr variation [11,14]. Another potential Sr transport mechanism to the deep ocean is biogenic calcium carbonate dissolution. The upper ocean (0–1600 m) increase in dissolved Ca (Table 3) explains 26–76% of the observed increase in Sr if it represents aragonite dissolution

(Sr=Ca ³ 8.6 mmol=mole), and 5–14% in the case of calcite dissolution (1.5 mmol=mole) (Table 3) [7,26]. The upper water column Ca gradient most probably results from the dissolution of aragonitic small pteropod species [27]. The measured increase in seawater Sr=Ca in the upper ocean therefore requires an additional source of Sr, with little or no Ca associated with it (Table 3). The most likely explanation for the pronounced upper ocean dissolved Sr gradient is its proposed transport as celestite produced by surface-dwelling acantharia [11]. Sediment trap studies indicate almost complete dissolution in the upper 900 m of the water [12,13,28], consistent with the location of the most pronounced vertical dissolved Sr gradient (Fig. 4). Quantitative evaluation of the vertical Sr

Table 3 Comparison of the measured change in Sr and Sr=Ca with depth, with that expected based on the assumption that the corresponding measured increase in dissolved Ca is the result of either biogenic aragonite (with a Sr=Ca content of 8.6 mmol=mol) or calcite (with Sr=Ca 1.5 mmol=mol) dissolution 1Sr (µM)

CaN (µM)

Southwest Pacific: 5ºS, 179ºE 0 87.16 8.525 1000 88.67 8.636 3000 88.71 8.618

1.51 0.04

North Pacific: 45ºN, 179ºE 0 88.05 1600 89.16 4500 88.65 5400 88.67

1.11 0.60 0.62

Depth (m)

SrN (µM)

Sr=Ca (ð10 3 )

8.620 8.646 8.611 8.600

Aragonite dissolution

Calcite dissolution

1Sr (µM)

Sr=Ca

1Sr (µM)

Sr=Ca

10.223 10.268 10.293

0.39 0.60

8.526 8.526

0.07 0.11

8.495 8.478

10.215 10.313 10.294 10.301

0.84 0.68 0.86

8.619 8.620 8.631

0.15 0.12 0.13

8.552 8.565 8.560

The change (1) at depth is calculated relative to the surface ocean values at each of the two Pacific Ocean stations. Also given are the calculated seawater Sr=Ca values expected if calcium carbonate dissolution is the only source of deep water Sr.

Table 4 Model results for particulate Sr fluxes at the South Pacific and North Pacific stations Area (depth)

J =w (µmol kg

South Pacific (125–800 m) South Pacific (1600–4200 m) North Pacific (500–3600 m)

1.0 ð 10 5.0 ð 10 1.0 ð 10

4

3.5 ð 10 7.0 ð 10

4

a Lower

5

b

4

estimate: K v D 10 Upper estimate: K v D 10

m2 =s. m2 =s.

J (µmol kg 1

4 4

4

m 1)

lower a

1

y 1)

Integrated J (mmol m 2 y 1 )

upper b

0.16 ð 10 0.83 ð 10 0.02 ð 10

3

0.10 ð 10 0.20 ð 10

3

3 3

3

1.6 ð 10 8.2 ð 10 0.2 ð 10

3

1.0 ð 10 2.0 ð 10

3

3 3

3

0.1 –1 0.5 –5 0.05–0.5 0.3 –3 0.6 –6

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Fig. 6. Comparison of measured and model-generated upper and deep water profiles of dissolved Sr at the South Pacific hydrocast station. Values for J =w are in µmol Sr kg 1 m 1 . The error bars represent a 1¦ Sr measurement error.

profiles with a one-dimensional advection-diffusion model (Appendix A; Figs. 6 and 7) produces integrated Sr particulate fluxes of 0.05 to 6 mmol Sr m 2 y 1 (Table 4), almost identical to sediment trap flux estimates of 0.012 to 6 mmol Sr m 2 y 1 at 400 m and 0.06 to 0.74 mmol Sr m 2 y 1 at 900 m [12]. It would be advantageous to provide a tighter constraint on the vertical Sr particulate flux out of the surface ocean, but it is limited by uncertainties in model parameters such as vertical eddy diffusivity [31], as well as the inherent limitations of sediment trap fluxes [12]. The above flux range of 0.05 to 6 mmol Sr m 2 y 1 translates into a Sr residence in the upper 400 m of the water column of 5800 to 700,000 years,

considerably shorter than the 2–5 m.y. residence time of Sr in the global ocean [5,6]. More sediment trap studies may be helpful in overcoming uncertainties related to sediment trap efficiency and spatial and temporal variability in acantharia fluxes, to produce a better constraint on the upper ocean Sr residence time.

4. Discussion and conclusions The Sr and Sr=Ca contents of the contemporary ocean exhibit ‘labile nutrient-like’ behavior, that is depleted surface values relative to the deep ocean and elevated surface values at high latitude and

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Fig. 7. Comparison of measured and model-generated profiles of dissolved Sr in the North Pacific. Water profiles of dissolved Sr at the South Pacific hydrocast station. Values for J =w are in µmol Sr kg 1 m 1 . The error bars represent a 1¦ Sr measurement error.

upwelling areas. The most likely explanation for the pronounced upper ocean Sr gradient is the shallow dissolution cycle of celestite skeletons produced by surface-dwelling acantharia. A secondary influence on seawater Sr=Ca is calcium carbonate cycling. In some areas dust inputs may contribute to elevated surface Sr levels. These factors need to be studied more intensively to quantify their relative roles in Sr cycling. A vertical celestite flux provides a mechanism for the transport of Sr, and therefore 90 Sr, from the surface to the deeper ocean, and questions the accuracy of upper ocean mixing rates based on an assumption of transport of 90 Sr by water masses only [1,2]. It may also explain the apparent contradictory deep ocean mixing rates obtained by 90 Sr versus 14 C methods [1,2].

Sr and Sr=Ca gradients in the contemporary ocean pose a problem to paleoceanographic studies that require simplifying assumptions such as homogeneous global oceanic Sr distributions. A particular troublesome aspect is the very pronounced vertical gradient in the upper 200 m, and the implied susceptibility of especially low-latitude areas to changes in surface Sr=Ca values related to upwelling events of even short duration. This is supported by a calculated residence time in the upper 400 m of the water column of 5800 to 700,000 years. The implications of variable seawater Sr=Ca will depend on the paleoceanographic proxy in question, and the study location. The magnitude of variability documented in this study, around 2 to 3%, is much smaller than for example Cenozoic variations in seawater Sr=Ca as recorded in foraminiferal shells [26], but equiv-

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alent to Quaternary changes recorded in coralline aragonite [6,15,16]. Also, this study considered open ocean sites only. Inclusion of near-shore sites subject to for example river discharge, or Sr fluxes from the recrystallization of shelf carbonates during sea-level low stands [32], can be expected to widen this range. Given the important role that acantharia production plays in the oceanic Sr cycle, high priority should be given to continued studies of the biogenic fluxes involved and the factors controlling its spatial and temporal variability. Coupled to a more comprehensive database of oceanic Sr distributions, it will allow a more robust evaluation of the implications for paleoceanographic studies.

Acknowledgements I would like to thank Bruce Nelson for his invaluable contribution to this work. The manuscript benefited greatly from comments by Dr. R.H. Byrne, as well as two anonymous reviewers. Numerous people helped in sample collection, including Steve Covey. The laboratory assistance of Jerry Hinn is much appreciated. Funding was provided by NSF. [MK]

Appendix A The amount of Sr dissolved in the water column can be calculated by applying a one-dimensional advection-diffusion model [29], if “the horizontal flux, the product of the horizontal diffusion constant and the concentration gradient, is much smaller than the vertical flux” [30]. The potential temperature– salinity relationship is approximately linear in the 100–500 m and 1600–4200 m depth range at the South Pacific station, and 500–3600 m in the North Pacific station, implying negligible horizontal mixing. For a non-conservative element the balance between vertical diffusion and vertical advection can be expressed by the equation: ŽC=Žt D K v .Ž 2 C=Žz 2 /

w.ŽC=Žz/ C J

(1)

with C D dissolved Sr concentration (µmol=kg), K v D vertical eddy diffusion coefficient (m2 =y), w D upwelling velocity (m=y, positive downward), z D depth (m), J D particulate Sr flux (µmol kg 1 y 1 ), zero if conservative element, with K v , w and J assumed to be constant with depth. At steady state the solution to Eq. 1 is: [Sr] D c1 C c2 exp[. w=K v /z]

.J =w/z

(2)

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with c1 and c2 integral constants determined from the boundary values. The scale height w=K v is determined from vertical profiles of the conservative properties salinity and=or potential temperature. Given measured Sr profile data (Table 2), values for J =w can be determined from best model fits to the data (Table 4). Calculation of the particulate Sr flux (J ) involves estimates of either w or K v . In this case, recently proposed values for K v in the deep ocean is used [31], with lower and upper estimates of 10 5 m2 =s and 10 4 m2 =s. The order of magnitude range in model estimates of the Sr particulate flux (Table 4) reflects this uncertainty in the deep ocean vertical eddy diffusivity.

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