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230Th profiles in sediments
Earth and Planetary Science Letters 191 (2001) 219^230 www.elsevier.com/locate/epsl
Fluxes of 230Th and 231 Pa to the deep sea: implications for the interpretation of excess 230 Th and 231 Pa/230Th pro¢les in sediments E.-F. Yu a , R. Francois b; *, M.P. Bacon b , A.P. Fleer b b
a National Taiwan Normal University, Taipei, Taiwan Woods Hole Oceanographic Institution, Clark 4, MS 25, Woods Hole, MA 02543-1541, USA
Received 13 September 2000; accepted 12 June 2001
Abstract Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230 Th scavenged to the seafloor remains close to its production rate from the decay of 234 U in the overlying water column, and generally validates the use of 230 Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230 Th measured in the low flux regions amounts to 90 þ 6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230 Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230 Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231 Pa and the 231 Pa/230 Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231 Pa fluxes and 231 Pa/230 Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231 Pa/230 Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231 Pa/ 230 Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231 Pa/230 Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231 Pa/230 Th is more difficult and requires a more quantitative understanding of the influence of particle composition. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Th-230; Pa-231; Sediment traps
1. Introduction * Corresponding author. Tel.: +1-508-289-2637; Fax: +1-508-457-2193. E-mail address: [email protected] (R. Francois).
230
Th and 231 Pa are produced in seawater at a constant rate from the decay of dissolved uranium isotopes (LTh = 2.52U1032 dpm m33 yr31 ;
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 1 0 - 1
EPSL 5916 31-8-01
220
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
LPa = 2.33U1033 dpm m33 yr31 ). Both are rapidly scavenged from the water column into the underlying sediments, resulting in large 230 Th and 231 Pa de¢cits in the water column and large excesses in the underlying sediments [1^3]. In recent years, sediment activities of excess 230 Th (i.e. the activity of 230 Th in excess of that supported by the decay of U present in mineral lattices) and excess 231 Pa/ excess 230 Th (from here on: Pa/Th) have been widely applied to problems of paleoceanography. 230 Th is the most particle-reactive of the two, with very short residence times in the water column (ranging from 6 1 yr in surface water to a few decades in deep water), which limits redistribution by horizontal transport. Its £ux to the sea£oor should thus always approximate its wellconstrained production rate in the water column. Normalizing particle £ux to the known production rate of 230 Th in the water column thus provides a means of estimating preserved rain rates to the sea£oor (i.e. rain rates from the overlying water minus remineralization before burial) and syndepositional redistribution of sediments by bottom currents [4^13]. In contrast, 231 Pa is somewhat less particle-reactive. It has a longer residence with respect to scavenging in the water column (up to 200 yr in deep water; [14]), and is more e¡ectively transported to regions with high particle £ux and scavenging intensity (a process called `boundary scavenging'). As a result, the Pa/Th of settling particles and underlying sediments tends to increase with particle £ux [3,15^18]. Higher export production should thus translate into higher Pa/Th in the sedimentary record, and this principle has been used to reconstruct paleoproductivity ([9,19^21]. Because Pa/Th is not a¡ected by dissolution on the sea£oor, this ratio is preserved in sediments, recording the event even if postdepositional dissolution has erased the accumulation rate signature. The assumption that the £ux of scavenged 230 Th to the sea£oor is equal to its production rate in the overlying water column is only an approximation, as was recently highlighted by water column [22^24] and modeling [25] studies. The £ux of scavenged 230 Th to the sea£oor would be absolutely constant and equal to its produc-
tion rate over the entire ocean only if the scavenging of 230 Th was instantaneous, which would reduce its activity in seawater to zero. With a residence time in deep water amounting to several decades, some modi¢cation of this simple balance by lateral transport of 230 Th must be expected. Likewise, the relationship between Pa/Th and particle £ux can be obscured by changes in deepwater circulation, which a¡ect the lateral transport of the nuclides [14,22,26,27], and changes in particle composition [28^30]. We have thus measured the £uxes of 230 Th and 231 Pa with deep-sea moored sediment traps in a variety of oceanic conditions. The data are reported in a separate paper dealing with the collection e¤ciency of deep-sea sediment traps [31]. Here, we use the data to further assess the constancy of the 230 Th £ux to the sea£oor and the dependency of Pa/Th on particle £ux. 2. Materials and methods Large, conical, time-series sediment traps of similar design were deployed at sites ranging from oligotrophic central gyres to ocean margin regions (Fig. 1). To eliminate seasonal biases, composite annual samples were obtained by combining samples in proportion to the amount collected by each individual sampling cup and analyzed for radioisotopes (230 Th, 231 Pa and U isotopes), organic carbon, opal, carbonate and lithogenic material. Details are given in [31]. Trapping e¤ciencies were calculated from the £uxes of excess 230 Th and excess 231 Pa intercepted by the traps, the Pa/Th of the settling material (Rv ), and estimates of the ratio of the rates of lateral transport of the two radionuclides in the water column (Rh ) [31,32]. The paucity of water column data results in relatively large errors in Rh . Trapping-e¤ciency estimates for traps deployed in the open ocean, from where 231 Pa is exported (i.e. Rv 6 LPa /LTh ), are relatively insensitive to these errors [31]. On the other hand, errors in estimates of trapping e¤ciency for traps deployed in regions receiving lateral input of 231 Pa (e.g. Paci¢c margins, southern ocean) are much larger. These estimates and associated error bars
EPSL 5916 31-8-01
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
221
Fig. 1. Locations of the sediment trap moorings used in this study. Crosses indicate stations where trapping e¤ciency could not be calculated (no 231 Pa data available or di¤culties in constraining Rh [31]). Circles indicate stations where trapping e¤ciency was calculated; open and ¢lled circles are for deployment periods greater and smaller than 10 months, respectively.
are used in the following discussion to correct the measured 230 Th and 231 Pa £uxes. 3. Results and discussion Details of the deployments and results are listed in Table 1. Note that for a few traps, the total deployment period was less than 1 yr (Fig. 1). The samples from the Arabian Sea were obtained over a 6-month period including the summer monsoon, and thus, somewhat overestimate the annual £uxes. In contrast, the sediment trap mooring deployed in Panama Basin collected material spanning the lowest productivity period [33], so that the annual £ux may be somewhat underestimated. Results reported in the literature from shorterterm deployment in the tropical NW Atlantic [34] and the California margin [17] are also included to supplement our data set. 3.1. Vertical £ux of scavenged
230
Th
The basic assumption of the `constant
230
Th
£ux' model is that the £ux of scavenged 230 Th to the sea£oor is always equal to the production rate in the overlying water column. Since the latter increases linearly with water depth and the sediment traps were deployed at di¡erent depths (Table 1), we use the ratio of the £ux of 230 Th measured with the sediment traps (F) to its production rate (P) in the overlying water column (0.0252 dpm m33 yr31 UZ m; where Z is the depth of deployment of the sediment trap). These ratios are then plotted against total mass £ux measured by the corresponding traps. Particle £ux also decreases with depth, as a result of opal and carbonate dissolution and organic matter remineralization [31]. However, because this decrease is comparatively small below 1000 m and poorly constrained, it was not taken into consideration in the present analysis and the total mass £uxes used are those measured by the traps at their depth of deployment (corrected or not for trapping e¤ciency). The uncorrected data (F/P), show a general increase of 230 Th with total mass £ux (Fig. 2a). With 100% trapping e¤ciency and in the absence
EPSL 5916 31-8-01
Longitude
End date
Duration (days)
Aug. 83 Aug. 84 360
Start date
BOFS 47³53PN 19³32PW Apr. 89 Sep. 90 517 Apr. 89 Sep. 90 517 NABE 48 47³43PN 20³52PW Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 NABE 34 33³49PN 21³02PW Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 Sargasso 32³05PN 64³15PW Aug. 78 Jul. 81 774 Nares 23³16PN 63³55PW Aug. 83 Sep. 84 312 Aug. 84 Sep. 84 312 Tropical NE ^ oligotrophic 21³03PN 31³10PW Jun. 91 Sep. 91 90 Jun. 91 Sep. 91 90 Jun. 91 Sep. 91 90 Tropical NE ^ mesotrophic 18³30PN 21³06PW Jul. 92 Dec. 92 56 Southern ocean Antarctic circular current 50³09.0PS5³46.4PE Jan. 87 Mar. 88 361 BO 54³20.2PS3³20.2PW Dec. 90 Dec. 92 660 Jan. 91 Dec. 92 557 Brans¢eld Strait 62³15PS 57³31PW Dec. 83 Nov. 84 360 Weddell Sea ^ Polynia 64³54.1PS2³38.8PW Jan. 88 Feb. 89 384 Weddell Sea ^ AWI208 65³56.2PS35³30.2PW Oct. 89 Nov. 92 1035 Indian Arabian Sea ^ WAST 16³19PN 60³28PE May. 86 Oct. 86 162 May. 86 Oct. 86 162 Arabian Sea ^ EAST 15³27PN 68³44PE May. 86 Oct. 86 162 May. 86 Oct. 86 162
Atlantic Lofoten 69³30PN 10³00PE
Latitude
9.1 11.2 16.2 9.7 17.0 94.8
38.3 53.1 14.4 107.3 33.8 12.3
66.9 66.1 32.6 39.1
1464 4832 2500 4400 4590 2890
700 450 2194 1588 360 1090
1085 3020 1705 2773
19.4 22.4 21.2
1159 1981 4478 9.9
20.7 26.2
1110 3734
3200
22.6 32.8
22.8
3100 4465
2760
Depth Mass £ux (m) (g m32 yr31 ) (uncorrected)
^ ^
^
^ ^
EPSL 5916 31-8-01 ^
^
^ ^
s 87
0.90^1.01 32^36 1.14^1.37 29^34
0.81^0.88 76^83 0.89^1.07 62^74
0.88^1.00 12^14
^
^
^ ^
6 0.44
1.53^1.80 53^62
1.30^1.47 11^13 0.56^0.63 15^18 1.09^1.23 14^16
^ ^
0.75^0.93 11^13
0.64^0.75 26^30 0.83^1.03 22^27 0.96^1.22 17^22
0.38^0.43 48^54 0.70^0.88 30^37
^ ^
^
Trapping Mass £ux e¤ciency (E) (g m32 yr31 ) (corrected)
Th
0.49 0.56
0.17 0.22
^
^
^
^ ^
^
0.63
0.80 0.89 0.83
0.09 0.16
0.08
0.29 0.44 0.51
0.15 0.22
0.18 0.32
1.03
(dpm g
232
31
) (dpm g
)
^ ^
^
Th
V/P 230
^ ^
^
^
0.18
0.38^0.44
1.27 þ 0.05 0.119 þ 0.0070.94^1.08 2.03 þ 0.07 0.112 þ 0.0060.82^1.00
0.35 þ 0.01 0.036 þ 0.0020.96^1.07 1.08 þ 0.07 0.085 þ 0.0030.86^1.07
0.85
0.23 þ 0.01 0.064 þ 0.003^
0.54
0.10 þ 0.01 0.035 þ 0.004^ 0.36 þ 0.03 0.171 þ 0.018^
0.20 þ 0.01 0.065 þ 0.13 s 1
1.15 þ 0.06 0.052 þ 0.0110.82^0.99
4.55 þ 0.11 0.081 þ 0.0100.79^0.90 5.81 þ 0.13 0.144 þ 0.0150.80^0.91 6.79 þ 0.11 0.180 þ 0.0100.81^0.92
2.99 þ 0.36 ^ 7.76 þ 0.49 ^
6.06 þ 0.24 0.283 þ 0.0190.79^1.00
0.98 þ 0.04 0.030 þ 0.0050.86^1.03 1.80 þ 0.06 0.066 þ 0.0050.78^0.98 4.94 þ 0.16 0.135 þ 0.0090.76^0.97
0.52 þ .02 0.027 þ 0.0040.88^1.02 2.40 þ 0.07 0.060 þ 0.0040.86^0.96
1.51 þ 0.15 ^ 4.19 þ 0.21 ^
31
ex.231 Pa
2.89 þ 0.36 ^
) (dpm g
31
ex.230 Th
Table 1 Fluxes and composition of materials collected by the sediment traps used in this study
Pa
^ ^
^
Pa/Th
45 54
50
CaCO3 (%)
^ ^
49 42
0.28 þ 0.02
^
0.35 þ 0.05 0.47 þ 0.06
0.32 þ 0.07
18
8
5
13 5
26
0.94^1.11 0.094 þ 0.007 49 0.49^0.60 0.055 þ 0.004 50
1.05^1.21 0.101 þ 0.007 59 0.74^0.90 0.078 þ 0.006 54
0.88^1.00 0.21 þ 0.03
^
^
^ ^
s 3.5
0.34^0.54 0.045 þ 0.010 72
0.14^0.19 0.018 þ 0.002 ^ 0.20^0.26 0.025 þ 0.003 ^ 0.23^0.27 0.027 þ 0.002 ^
^ ^
0.40^0.51 0.047 þ 0.004 55
0.25^0.37 0.031 þ 0.005 68 0.30^0.39 0.037 þ 0.003 62 0.22^0.29 0.027 þ 0.002 61
0.45^0.62 0.053 þ 0.008 55 0.20^0.26 0.025 þ 0.002 58
^ ^
^
231
11 13
23 23
70
70
36
63 65
40
^
^ ^ ^
3 2
15
8 9 9
18 21
^ ^
5
Opal (%)
^ 24
^ 13
^
9
50
17 26
^
^
^ ^ ^
38 49
17
14 20 22
10 13
^ ^
31
terr (%)
^ 7
^ 5
^
7
4
3 2
7
2
^ ^ ^
6 4
5
5 4 4
7 4
^ ^
6
Corg (%)
[31] [31]
[31] [31]
[41]
[35]
[40]
[35] [35]
[35]
[34]
[34] [34] [34]
[42] [42]
[32]
[31] [31] [31]
[31] [31]
[39] [39]
M.P. Bacon (unpublished).
Reference
222 E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Longitude
Start date
End date
Duration (days)
3800 500 1000 2329 500 1750 2330 1000 3769
360 360 360
352 325 352
60
112
3800
3800
EPSL 5916 31-8-01 65.8
25.7
18 49 43
95.0 128 161
20.0
46.0
55.8
73.3
10.1
1000
3800
40.3 34.7
1000 2900
Depth Mass £ux (m) (g m32 yr31 ) (uncorrected)
311
Sep. 82 Apr. 84 560
Sep. 82 Sep. 83 360
Mar. 83 Oct. 83 180
Nov. 84 Oct. 85 California ^ near shore 42³05PN 125³46PW Sep. 87 Sep. 88 Sep. 87 Sep. 88 Sep. 87 Sep. 88 California ^ mid-way 42³11PN 127³35PW Sep. 87 Sep. 88 Sep. 87 Aug. 88 Sep. 87 Sep. 88 California ^ Gyre 41³33PN 132³00PW Feb. 88 Apr. 88 Panama 5³22PN 82³01PW Jul. 79 Dec. 79
PAPA 6^7
PAPA 1^3
PAPA 1^2
PAPA 2
Arabian Sea ^ CAST 14³29PN 64³46PE May. 86 Oct. 86 169 May. 86 Oct. 86 169 Paci¢c PAPA 2 50³00PN 145³00PW Mar. 83 Oct. 83 180
Latitude
Table 1 (continued)
^
^
^
^
0.75^0.99 66^88
0.31^0.41 62^83
0.25^0.35 51^71 0.30^0.68 72^163 0.25^0.56 77^172
1.04^1.80 53^91 1.28^2.02 63^100 0.77^1.60 101^209
0.96^1.15 17^21
^
^
^
^
0.93^1.07 38^43 0.98^1.17 30^35
Trapping Mass £ux e¤ciency (E) (g m32 yr31 ) (corrected)
Th
ex.230 Th
ex.231 Pa
^
0.16
0.27 0.39 0.53
0.54 0.67 0.62
0.15
^
0.01
0.01
0.04
0.34 0.53
230
Th
V/P
^
^
^
^
^
1.52
0.234
1.06^1.41
0.41 þ 0.02 0.061 þ 0.011 1.01^1.36
0.24 þ 0.02 0.046 þ 0.008 0.94^1.37 0.67 þ 0.03 0.148 þ 0.012 1.09^2.49 0.87 þ 0.02 0.196 þ 0.012 1.14^2.55
0.21 þ 0.04 0.040 þ 0.007 0.81^1.60 0.37 þ 0.05 0.076 þ 0.011 0.88^1.52 0.62 þ 0.04 0.130 þ 0.011 1.05^2.22
4.79 þ 0.30 0.371 þ 0.023 0.85^1.06
2.04
1.97 þ 0.06 ^
1.41 þ 0.04 ^
0.39 þ 0.02 ^
0.44 þ 0.03 0.030 þ 0.003 0.64^0.78 2.15 þ 0.11 0.155 þ 0.010 0.86^1.05
(dpm g31 ) (dpm g31 ) (dpm g31 )
232
Pa
CaCO3 (%)
^
^
^
^
36
34
^
35
1.75^2.36 0.15
1.47^2.34 0.15 þ 0.03
1.82^2.97 0.19 þ 0.04 2.58^5.96 0.22 þ 0.02 2.75^6.23 0.23 þ 0.01
1.68^3.27 0.19 þ 0.05 1.95^3.38 0.21 þ 0.04 2.57^4.07 0.21 þ 0.02
^
35
24 22 22
13 12 9
0.71^0.89 0.078 þ 0.007 40
^
^
^
^
0.46^0.59 0.070 þ 0.008 54 0.66^0.83 0.073 þ 0.006 66
231
Pa/Th
^
36
28 26 29
19 23 26
46
^
50
^
^
6 11
Opal (%)
^
13
26 43 46
58 63 63
^
^
^
^
^
^ 12
terr (%)
^
5
11 5 4
5 4 3
^
4
4
^
3
^ 6
Corg (%)
M.P. Bacon (unpublished).
[17]
[17] [17] [17]
[17] [17] [17]
[31]
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
[31] [31]
Reference
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230 223
224
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Fig. 2. Ratios of the £uxes of 230 Th (a, c) and 231 Pa (b, d) measured with sediment traps to the rates of 230 Th and 231 Pa produced in the overlying water (P) vs. corresponding total mass £uxes. (a,b) Fluxes (F) and mass £uxes are not corrected for trapping e¤ciency. (c, d) Fluxes (V) and mass £uxes are corrected for trapping e¤ciency according to [31]. Filled symbols are data taken from traps deployed for less than 10 months. They follow the general trend and are not biasing our conclusions. For the Antarctic circumpolar current (c, d), only a maximum trapping e¤ciency could be calculated [31]. The symbol represents V/P and mass £ux assuming this value and the line re£ects the trend obtained when assuming increasingly lower trapping e¤ciencies.
of signi¢cant lateral transport of 230 Th prior to scavenging, F/P should be constant and equal to one (i.e. F = P). The uncorrected data would thus suggest signi¢cant lateral transport of 230 Th, contrary to expectation. At low £uxes, however, we ¢nd a large range of F/P values from nearly zero to s 1, without a clear relationship to the measured total mass £ux. This scatter nearly disappears when the £uxes of 230 Th and total mass are corrected for trapping e¤ciency (V/P where V is the `true' corrected 230 Th £ux; Fig. 2c). Since we do not have 231 Pa data for all traps, we can
make the correction only for a subset of the uncorrected £ux data shown in Fig. 2a. Nonetheless, the results clearly indicate that the large apparent de¢cits in 230 Th £ux occasionally measured at low particle £uxes are largely due to low-trapping ef¢ciency and not a re£ection of lateral export of 230 Th to high particle £ux regions. Instead, V/P for 230 Th in regions where corrected total mass £ux is below 60 g m32 yr31 averages 0.90 þ 0.06, indicating only a small, but probably signi¢cant, lateral export from oligotrophic regions. Two notable exceptions were not included in this average.
EPSL 5916 31-8-01
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Low 230 Th £uxes were found in the middle of the Weddell Sea, where particle £ux and scavenging intensity are very low because of ice cover [35]. Relatively low 230 Th £ux was also found with a trap deployed at 1000 m depth outside the Arabian Sea upwelling region (Fig. 2c), possibly re£ecting the proximity of a zone with a large horizontal gradient in particle £ux. In high-£ux regions, the £ux of scavenged 230 Th may exceed production by more than 50%. At the moment, however, the only available data showing such excess (V/P s 1.5) come from the California margin and the Atlantic sector south of the Polar Front, where sediment trap e¤ciency is poorly constrained [31] and the resulting error on V/P is large (Table 1). Fig. 2b thus validates the use of the 230 Th constant £ux model in most open ocean settings, but the extent of deviations at the margins and south of the Polar Front still needs to be better constrained and documented by detailed water column pro¢les of the two nuclides. Our results are consistent with a recent study [25] in which a particle ¢eld was introduced into a global ocean circulation model to model scavenging and produce synoptic maps of 230 Th £uxes to
the sea£oor. There is a good general agreement between our sediment trap £ux data and the model estimates at the same sites (Table 2), supporting the conclusions derived from these two independent studies. Although an approximation, the constant 230 Th £ux model can thus be applied to reconstruct past changes in preserved rain rates over large areas of the ocean. The model is useful even in areas where the scavenged £ux of 230 Th may exceed production signi¢cantly (e.g. Paci¢c margins, Polar Front in the Atlantic sector; Fig. 2c), as the corrections required for syndepositional redistribution of sediment in these regions are often much larger than the uncertainty associated with the scavenged £ux of 230 Th ([9,11,19^21]. Nonetheless, in regions where the vertical £ux of 230 Th can be signi¢cantly lower than the production rate, the model must be used with caution. This is the case for the Weddell Sea, where the scavenged £ux of 230 Th appears to only be 40% of the production rate [22,31,35]. We could expect to ¢nd similar de¢cits near regions of deep-water formation in the north Atlantic [23,24].
Table 2 Comparison between the ratios of the trapping-e¤ciency corrected £ux of and the ratios obtained in an ocean circulation model [25] Location Atlantic NABE 48 NABE 34 Sargasso Tropical NE ^ Oligotr. Tropical NE ^ Mesotr. Southern ocean Polar Front Weddell Sea Indian Arabian Sea ^ WAST Arabian Sea ^ EAST Arabian Sea ^ CAST Paci¢c PAPA 6^7 California ^ near shore California ^ mid-way California ^ Gyre Panama
Latitude
225
230
Th to production rate (V/P) estimated in this study
Longitude
V/P (230 Th) this work
[25]
47³43PN 33³49PN 32³05PN 21³03PN 18³30PN
20³52PW 21³02PW 64³15PW 31³10PW 21³06PW
0.94 þ 0.08 0.90 þ 0.13 0.90 þ 0.11 0.86 þ 0.07 0.91 þ 0.09
0.9 0.9 0.8 0.7 0.9
50³09PS 66³56PS
05³46PE 35³30PW
s1 0.41 þ 0.03
1.3 ?
16³19PN 15³27N 14³29PN
60³28PE 68³44PE 64³46PE
0.97 þ 0.11 0.95 þ 0.13 0.85 þ 0.20
1.0 1.0 1.0
50³00PN 42³05PN 42³11PN 41³33PN 05³22PN
145³00PW 125³46PW 127³35PW 132³00PW 82³01PW
0.96 þ 0.11 1.5 þ 0.7 1.7 þ 0.8 1.2 þ 0.2 1.2 þ 0.2
1.1 1.3 1.2 1.0 1.2
EPSL 5916 31-8-01
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3.2. Vertical £ux of ex.231 Pa and changes in ex.231 Pa/ex.230 Th with particle £ux The scavenged vertical £ux of 231 Pa is strongly dependent on particle £ux (Fig. 2b), and unlike for 230 Th, correcting for trapping e¤ciency does not alter this conclusion (Fig. 2d). Corrected 231 Pa £uxes measured in the north Atlantic average only 1/3 of the production rate in the overlying water, while at the Polar Front and at the California margin of the Paci¢c 231 Pa £uxes exceed production by more than a factor of three (Table 1). As anticipated, for a similar total £ux, the vertical £uxes of 231 Pa tend to be lower in the Atlantic Ocean, re£ecting the e¡ect of thermohaline circulation. Using water column data, Yu et al. [14] calculated that approximately 50% of the 231 Pa produced in the Atlantic north of 25³N is laterally exported with NADW to the south. This is reasonably similar to the 64 þ 12% (n = 6) estimated from the sediment traps deployed in the Sargasso Sea and during the North Atlantic Bloom Experiment (Table 1). The 231 Pa exported from the Atlantic Ocean is e¡ectively scavenged by the opal £ux in the southern ocean [26,27], and very little appears to reach the Paci¢c Ocean [14]. The relatively high 231 Pa trap £uxes measured in the Paci¢c Ocean are mainly from margin regions (Table 1) and thus do not necessarily indicate thermohaline input of 231 Pa in this ocean basin. A more comprehensive Paci¢c data set (from surface sediment or sediment traps) is required to con¢rm the extent to which 231 Pa may be exported to the Paci¢c. Pa/Th variations essentially re£ect variations in 231 Pa £ux (Fig. 3). In the Atlantic, Pa/Th in settling particles is invariably low, consistent with the low Pa/Th also found in Atlantic sediments and the export of 231 Pa into the southern ocean ([14,18,26,27]. Although no measurements of 231 Pa and 230 Th £uxes were made in the Atlantic with particle £ux exceeding 60 g m2 y31 , extrapolation of the available sediment trap data suggests that Pa/Th would not rise much above the production rate ratio of 0.0925, even at very high particle £ux. This is consistent with Pa/Th measured in surface sediments in the upwelling region o¡ Africa [14,18,27,34]. Boundary scavenging of
Fig. 3. Pa/Th in sediment trap material vs. corrected total mass £ux. Filled symbols are data taken from traps deployed for less than 10 months. The line through the Atlantic Ocean data set is a linear regression. The lines through the Paci¢c Ocean and Arabian Sea data are polynomial ¢ts (see text for equations). 231
Pa cannot be fully expressed in the Atlantic Ocean because the mean residence time of deep water, which was estimated at V100 yr [36] or V275 yr [37], is equivalent to the time required for basin-wide lateral mixing [16]. As a result, the increase in Pa/Th with particle £ux, while still recognizable, is subdued (linear regression on the Atlantic data set in Fig. 3: Pa/Th = (0.00043UTMF)+0.022; r2 = 0.37) In the Paci¢c, the slope of the Pa/Th vs. total mass £ux relationship is steeper than that in the Atlantic, and appears to plateau at higher mass £ux. A polynomial ¢t (Pa/Th = 30.0000107(TMF)2 +0.00285 (TMF)+0.0290; r2 = 0.82) is used to illustrate this trend (Fig. 3). Here, there is a paucity of trap data from the oligotrophic region, but data from surface sediments show clearly that the Pa/Th in these regions can be as low as in the Atlantic [15,18]. The higher dependency of Pa/Th on particle £ux in this ocean re£ects the longer residence time of deep water, which allows a full expression of boundary scav-
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enging. Lower particle £ux and scavenging intensity also enhance lateral transport by increasing residence times of the radionuclides in the water column. Limits to the lateral supply rate of 231 Pa by eddy di¡usion must be responsible for the Pa/Th plateau observed at higher mass £ux. The Arabian Sea appears to re£ect a situation intermediate between the Atlantic and Paci¢c Ocean. Here, a polynomial ¢t with a forced intercept has been used to highlight the trend (Pa/Th = 30.0000101(TMF)2 +0.00167(TMF)+0.025; r2 = 0.32) In the southern ocean south of the Polar Front, Pa/Th is signi¢cantly higher than in other oceanic regions and remains well above the production rate ratio even at low particle £ux (e.g. Weddell Sea; Fig. 3). It is thus evident that boundary scavenging is not the only process controlling the relative scavenging of 231 Pa and 230 Th in this region. Instead, particle composition must also play an important role in controlling the fractionation between 231 Pa and 230 Th during scavenging [29]. The fractionation factor (fTh=Pa = KdTh /KdPa ; where Kd = [M]p/[M]d; [M]p,d are activities (dpm g31 ) in particles and seawater) determines the relative residence time of the two nuclides with respect to scavenging, and thus in£uences the slope of response of Pa/Th to particle £ux. When fTh=Pa s 1, 231 Pa has a longer residence time than 230 Th and is more e¡ectively transported to high£ux regions. When fTh=Pa = 1, however, the residence time with respect to scavenging of both nuclides is the same and, even though boundary scavenging may still operate, it will not be expressed in the Pa/Th ratio since scavenging will a¡ect both nuclides similarly. In this case, Pa/Th of particles simply re£ects the Pa/Th of seawater and is independent of particle £ux. fTh=Pa W10 when particle composition is dominated by clay, and approaches 1 when adsorption occurs on amorphous silica [29,30,35]. Opal concentration in particles settling in the southern ocean is highly variable, especially in the Polar Frontal zone, where particle composition gradually shifts from carbonate-dominated to opaldominated [38]. Measurements across the PF in the Atlantic sector have documented a pronounced decrease in fTh=Pa with increasing opal
227
content in suspended and sinking particles, from typical values of V10 north of the PF to values approaching 1 in the Weddell Sea [22,29]. Settling materials intercepted by traps deployed in the Weddell Sea have the highest opal content of all samples analyzed in this study (70%; Table 1). Their mean Pa/Th (0.21) is only slightly lower than the dissolved Pa/Th measured in Weddell seawater (0.28 þ 0.06 ; [22,35]). This observation con¢rms that little fractionation occurs during scavenging by opal and explains the high Pa/Th measured at relatively low particle £ux [29]. On the other hand, the trap deployed further north, just south of the Polar Front, collected material with signi¢cantly lower opal content (40%) and a signi¢cantly higher Pa/Th. fTh=Pa in this region has been estimated at 2 [29]. Boundary scavenging could thus play a role in establishing the Pa/Th of settling particles in this region. Another impor-
Fig. 4. Diamonds = Atlantic Ocean; Triangles = Indian Ocean; Circles = Paci¢c Ocean. Open symbols = when corrected total mass £uxes are s 50 g m2 yr31 . Closed symbols = when corrected total mass £uxes are 6 50 g m2 yr31 . gray symbols are for stations where corrected mass £uxes are not available. Open square = Antarctic circular current; ¢lled square = Weddell Sea; ¢lled circle = station PAPA.
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tant factor is lateral input of 231 Pa with NADW from the Atlantic [22,27]. Three factors thus combine to yield the high Pa/Th in the Atlantic sector near the Polar Front: thermohaline circulation, high particle £ux and high opal content in the settling material. Fig. 4 further illustrates the interplay between these three factors. The general increasing trend in the Pa/Th of settling particles with increasing percent of opal supports the importance of opal for scavenging 231 Pa from the water column. However, this trend is also in part due to the general coincidence between high total mass £ux and high percent of opal. The only exceptions to this rule are station PAPA and the Weddell Sea. At these two sites, particle £uxes are comparatively low, and accordingly, the Pa/Th of settling particles is lower compared to other stations with similar percent of opal. Also, Pa/Th in the material settling in the Atlantic sector of the ACC (0.32 þ 0.07) is much higher than in the material collected at the California Gyre station (0.15 þ 0.03), even though both have nearly similar percentages of opal (40 vs. 36%) and total mass £uxes ( s 87 g m2 yr31 vs. 62^83 g m2 yr31 ). Although corrected £uxes at the ACC can only give a minimum value, this di¡erence could re£ect 231 Pa advection from the Atlantic Ocean. In the Atlantic, settling material has invariably low opal concentration, which could contribute to the low Pa/Th in this ocean basin. If the settling £ux in the Atlantic Ocean was dominated by opal with fTh=Pa W1, the residence time of 231 Pa in the Atlantic water column would be much shorter, resulting in little thermohaline export to the southern ocean, little or no fractionation between 231 Pa and 230 Th, and similar Pa/Th in both ocean basins that would be close to the production rate ratio of 0.093. 4. Conclusions We have shown that sediment trap data validate the use of 230 Th normalization for the paleoceanographic reconstruction of past changes in preserved rain rates over large sections of the ocean £oor, and are consistent with recent results
obtained with a general circulation model [25]. Both approaches show that scavenging rates of 230 Th exceeding the rate of production by more than a factor of two do not occur. However, the constant 230 Th £ux model can overestimate rain rates, possibly by as much as a factor of two, when applied to sediments deposited in regions where deep-water ventilation ages are younger than the mean residence time with respect to Th scavenging (e.g. Nordic and Labrador seas; NW Atlantic; Weddell Sea). In these regions, the scavenging rates of 230 Th can be signi¢cantly lower than the production rate, due to lateral export. Additional water column measurements are necessary to better document these deviations [22^ 24]. Sediment trap data also clearly illustrate the complexity of the relationship between particle £ux and the Pa/Th of settling material. Although Pa/Th is a potentially useful tool for exploring past changes in particle £ux, interpretation of its sedimentary record requires that the location of the study, general circulation patterns, and the chemical composition of the settling material be taken into account. Pa/Th is best suited as a synoptic tool to constrain past changes in relative particle £ux between regions or to document changes in deep-water circulation patterns. In the Atlantic, Pa/Th depends mostly on the strength of the `conveyor belt' circulation, which exports 231 Pa from the Atlantic into the southern ocean [14,26,27]. Pa/Th in Atlantic sediment cores can be used to estimate past changes in particle £ux, but only if it can be ascertained that the strength of the thermohaline circulation changed insigni¢cantly during the period considered. At the modern rate of NADW formation, however, the sensitivity of Pa/Th to changes in particle £ux in the Atlantic is small (Fig. 4). In this basin, sedimentary Pa/Th is thus better suited to quantify past changes in the strength of the conveyor belt circulation [14], which is best estimated from the mean sediment Pa/Th over the entire north Atlantic [27]. In the Paci¢c, on the other hand, baring major changes in deep-water circulation and particle composition, Pa/Th will be a more sensitive tracer of relative changes in particle £ux, and should prove a useful tool for re-
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constructing past changes in export £ux in Paci¢c upwelling regions. In the southern ocean, quantitative interpretation of Pa/Th is more di¤cult because it is affected not only by particle £ux, but also by global circulation and particle composition. Sedimentary Pa/Th has been used as a qualitative paleoproductivity proxy in the southern ocean [9,19^21], where high Pa/Th generally coincides with high 230 Th-normalized opal £ux and high opal concentration. Qualitatively, it could be argued that changes in particle composition have ampli¢ed changes in Pa/Th resulting from changes in particle £ux, but it cannot be ruled out that the increase in Pa/Th is entirely due to the decrease in fTh=Pa . Unless the e¡ect of variable fTh=Pa can be taken into account, it will be impossible to quantify particle £ux from Pa/Th in this and other regions where biogenic opal dominates the settling material.
[10]
Acknowledgements
[11]
Financial support for this work was provided by grants from the U.S. National Science Foundation. E.-F. Yu also acknowledges support from the Chinese National Science Council and National Taiwan Normal University. The MIT reactor used for producing the 233 Pa spike is supported by the U.S. DOE Reactor Sharing Grant No. DE-FG07-80ER10770.A020. We are grateful to S. Honjo and S. Manganini for providing sediment trap samples, and to R.F. Anderson and G. Henderson for providing constructive reviews. This is U.S. JGOFS Contribution No. 651 and WHOI contribution No. 10409.[EB]
[4] [5]
[6]
[7] [8] [9]
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Boundary e¡ects and large scale chemical fractionation, Philos. Trans. R. Soc. London 320 (1988) 187^200. M.P. Bacon, Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230 Th measurements, Isot. Geosci. 2 (1984) 97^111. D.O. Suman, M.P. Bacon, Variations in Holocene sedimentation in the North American basin determined from 230 Th measurements, Deep-Sea Res. I 36 (1989) 869^ 878. R. Francois, M.P. Bacon, D.O. Suman, Th-230 pro¢ling in deep-sea sediments: High-resolution records of £ux and dissolution of carbonate in the equatorial Atlantic during the last 24 000 years, Paleoceanography 5 (1990) 761^787. R. Francois, M.P. Bacon, Variations in terrigenous input to the deep equatorial Atlantic during the past 24 000 years, Science 251 (1991) 1473^1476. R. Francois, M.P. Bacon, Heinrich events in the North Atlantic: radiochemical evidence, Deep-Sea Res. 41 (1994) 315^334. N. Kumar, R.F. Anderson, R.A. Mortlock, P.N. Froelich, P.W. Kubik, B. Dittrich-Hannen, M. Suter, Iron fertilization of glacial-age Southern Ocean productivity, Nature 378 (1995) 675^680. J. Thomson, S. Colley, R. Anderson, G.T. Cook, A.B. MacKenzie, A comparison of sediment accumulation chronologies by the radiocarbon and 230 Thex methods, Earth Planet. Sci. Lett. 133 (1995) 59^70. M. Frank, R. Gersonde and A. Mangini, Sediment redistribution, 230 Thex -normalization and implications for the reconstruction of particle £ux and export paleoproductivity, in: G. Fischer and G. Wefer (Eds.), Proxies in Paleoceanography. University of Bremen, Bremen, 1999, pp. 409^426. C.C. Veiga-Pires, C. Hillaire-Marcel, U and Th isotope constraints on the duration of Heinrich events H0^H4 in the southeastern Labrador Sea, Paleoceanography 14 (1999) 187^199. I.R. Hall, I.N. McCave, Paleocurrent reconstruction, sediment and thorium focusing on the Iberian margin over the last 140 ka, Earth Planet. Sci. Lett. 178 (2000) 151^ 164. E.-F. Yu, R. Francois, M.P. Bacon, Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data, Nature 379 (1996) 689^ 694. H.-S. Yang, Y. Nozaki, H. Sakai, A. Masuda, The distribution of 230 Th and 231 Pa in the deep-sea surface sediments of the Paci¢c Ocean, Geochim. Cosmochim. Acta 50 (1986) 81^89. R.F. Anderson, Y. Lao, W.S. Broecker, S.E. Trumore, H.J. Ho¡man, W. Wol¢, Boundary scavenging in the Paci¢c Ocean: a comparison of 10 Be and 31 Pa, Earth Planet. Sci. Lett. 96 (1990) 287^304. Y. Lao, R.F. Anderson, W.S. Broecker, H.J. Hofmann, W. Wol¢, Particulate £uxes of 230 Th, 231 Pa and 10 Be in the northeastern Paci¢c, Geochim. Cosmochim. Acta 57 (1993) 205^217.
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[18] H.-J. Walter, M.M. Rutgers van der Loe¡ and R. Francois, Reliability of the 231 Pa/230 Th activity ratio as a tracer for bioproductivity of the ocean, in G. Fischer and G. Wefer (Eds.), Proxies in Paleoceanography, University of Bremen, Bremen, 1999, pp. 393^408. [19] R. Francois, M.P. Bacon, M.A. Altabet, L.D. Labeyrie, Glacial/interglacial changes in sediment rain rate in the S.W. Indian sector of subantarctic waters as recorded by 230 Th, 231 Pa, U and N15 N, Paleoceanography 8 (1993) 611^629. [20] R. Francois, M.A. Altabet, E.-F. Yu, D. Sigman, M.P. Bacon, M. Frank, G. Bohrmann, G. Bareille, L.D. Labeyrie, Contribution of southern ocean surface water strati¢cation to low atmospheric CO2 concentrations during the last glacial period, Nature 389 (1997) 929^935. [21] M. Frank, J.-D. Eckardt, A. Eisenhauer, P.W. Kubik, B. Dittrich-Hannen, M. Segl, A. Mangini, Berylium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications for hydrothermal activity and paleoproductivity changes during the last 100 000 years, Paleoceanography 9 (1994) 559^578. [22] M.M. Rutgers van der Loe¡, G.W. Berger, Scavenging of 230 Th and 231 Pa near the Antarctic Polar Front in the South Atlantic, Deep-Sea Res. I 40 (1993) 339^357. [23] S.B. Moran, M.A. Charette, J.A. Ho¡, R.L. Edwards, W.M. Landing, Distribution of 230 Th in the Labrador Sea and its relation to ventilation processes constraints on 231 Pa/230 Th as a paleocirculation tracer in the deep Atlantic, Earth Planet. Sci. Lett. 150 (1997) 151^160. [24] S. Vogler, J.C. Scholten, M.M. Rutgers van der Loe¡, A. Mangini, 230 Th in the eastern North Atlantic: The importance of water mass ventilation in the balance of 230 Th, Earth Planet. Sci. Lett. 156 (1998) 61^74. [25] G.M. Henderson, C. Heinze, R.F. Anderson, A.M.E. Winguth, Global distribution of the 230 Th £ux to the ocean sediments constrained by GCM modeling, DeepSea Res. 46 (1999) 1861^1893. [26] T. Asmus, M. Frank, C. Koschmieder, N. Frank, R. Gersonde, G. Kuhn, A. Mangini, Variations of biogenic particle £ux in the southern Atlantic section of the Subantarctic zone during the late Quaternary: Evidence from sedimentary 231 Paex and 230 Thex , Mar. Geol. 159 (1999) 63^78. [27] O. Marchal, R. Francois, T.F. Stocker, F. Joos, Ocean thermohaline circulation and sedimentary 231 Pa/230 Th ratio, Paleoceanography 15 (2000) 625^641. [28] G.B. Shimmield, J.W. Murray, J. Thomson, M.P. Bacon, R.F. Anderson, N.B. Price, The distribution and behaviour of Th-230 and Pa-231 at an ocean margin, Baja California, Mexico, Geochim. Cosmochim. Acta 50 (1986) 2499^2507. [29] H.-J. Walter, M.M. Rutgers van der Loe¡, H. Hoeltzen, Enhanced scavenging of 231 Pa relative to 230 Th in the
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South Atlantic south of the Polar Front: Implication for the use of the 231 Pa/230 Th ratio as a paleoproductivity proxy, Earth Planet. Sci. Lett. 149 (1997) 85^100. S. Luo, T.-L. Ku, Oceanic 231 Pa/230 Th ratio in£uenced by particle composition and remineralization, Earth Planet. Sci. Lett. 167 (1999) 183^195. E.-F. Yu, R. Francois, M.P. Bacon, S. Honjo, A.P. Fleer, S.J. Manganini, M.M. Rutgers van der Loe¡, V. Ittekot, Trapping e¤ciency of bottom-tethered sediment traps estimated from the intercepted £uxes of 230 Th and 231 Pa, Deep-Sea Res. I 48 (2001) 865^889. M.P. Bacon, C.-A. Huh, A.P. Fleer, W.G. Deuser, Seasonality in the £ux of natural radionuclides and plutonium in the deep Sargasso Sea, Deep-Sea Res. I 32 (1985) 273^286. S. Honjo, Seasonality of biogenic and lithogenic £uxes in the Panama Basin, Science 218 (1982) 883^884. F. Legeleux, J.-L. Reyss, S. Floris, Entrainement des metaux vers les sediments sur les marges continentales de l'Atlantic Est, C.R. Acad. Sci. Paris 320 (serie II a) (1995) 1195^1202. H.J. Walter, M.M. Rutgers van der Loe¡, H. Hoeltzen, U. Bathmann, Reduced scavenging of 230 Th in the Weddell Sea: Implications for Paleoceanographic reconstructions in the South Atlantic, Deep-Sea Res. I 47 (2000) 1369^1387. W.S. Broecker, A revised estimate for the radiocarbon age of the North Atlantic deep water, J. Geophys. Res. 84 (1979) 3218^3226. M. Stuiver, P.D. Quay, H.G. Ostlund, Abyssal water carbon-14 distribution and the age of the world oceans, Science 219 (1983) 849^851. S. Honjo, R. Francois, A. Manganini, J. Dymond, R. Collier, Particle £uxes to the interior of the Southern Ocean in the western Paci¢c sector along 170³W, DeepSea Res. II 47 (2000) 3521^3548. S. Colley, J. Thomson, P.P. Newton, Detailed 230 Th, 232 Th and 210 Pb £uxes recorded by the 1989/90 BOFS sediment trap time-series at 48³N, 20³W, Deep-Sea Res. 42 (1995) 833^848. M.M.R. Rutgers van der Loe¡, G.W. Berger, Scavenging and particle £ux seasonal and regional variations in the Southern Ocean (Atlantic sector), Mar. Chem. 35 (1991) 553^568. H.J. Walter, W. Geibert, M.M. Rutgers van der Loe¡, G. Fischer, U. Bathmann, Shallow vs. deep water scavenging of 231 Pa and 230 Th in radionuclide enriched waters of the Atlantic sector of the Southern Ocean, Deep-Sea Res. I 48 (2001) 471^493. N.S. Fisher, J.K. Cochran, S. Krishnaswami, H.D. Livingston, Predicting the oceanic £ux of radionuclides on sinking biogenic debris, Nature 335 (1988) 622^625.
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Fluxes of 230Th and 231 Pa to the deep sea: implications for the interpretation of excess 230 Th and 231 Pa/230Th pro¢les in sediments E.-F. Yu a , R. Francois b; *, M.P. Bacon b , A.P. Fleer b b
a National Taiwan Normal University, Taipei, Taiwan Woods Hole Oceanographic Institution, Clark 4, MS 25, Woods Hole, MA 02543-1541, USA
Received 13 September 2000; accepted 12 June 2001
Abstract Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230 Th scavenged to the seafloor remains close to its production rate from the decay of 234 U in the overlying water column, and generally validates the use of 230 Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230 Th measured in the low flux regions amounts to 90 þ 6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230 Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230 Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231 Pa and the 231 Pa/230 Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231 Pa fluxes and 231 Pa/230 Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231 Pa/230 Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231 Pa/ 230 Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231 Pa/230 Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231 Pa/230 Th is more difficult and requires a more quantitative understanding of the influence of particle composition. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Th-230; Pa-231; Sediment traps
1. Introduction * Corresponding author. Tel.: +1-508-289-2637; Fax: +1-508-457-2193. E-mail address: [email protected] (R. Francois).
230
Th and 231 Pa are produced in seawater at a constant rate from the decay of dissolved uranium isotopes (LTh = 2.52U1032 dpm m33 yr31 ;
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 1 0 - 1
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LPa = 2.33U1033 dpm m33 yr31 ). Both are rapidly scavenged from the water column into the underlying sediments, resulting in large 230 Th and 231 Pa de¢cits in the water column and large excesses in the underlying sediments [1^3]. In recent years, sediment activities of excess 230 Th (i.e. the activity of 230 Th in excess of that supported by the decay of U present in mineral lattices) and excess 231 Pa/ excess 230 Th (from here on: Pa/Th) have been widely applied to problems of paleoceanography. 230 Th is the most particle-reactive of the two, with very short residence times in the water column (ranging from 6 1 yr in surface water to a few decades in deep water), which limits redistribution by horizontal transport. Its £ux to the sea£oor should thus always approximate its wellconstrained production rate in the water column. Normalizing particle £ux to the known production rate of 230 Th in the water column thus provides a means of estimating preserved rain rates to the sea£oor (i.e. rain rates from the overlying water minus remineralization before burial) and syndepositional redistribution of sediments by bottom currents [4^13]. In contrast, 231 Pa is somewhat less particle-reactive. It has a longer residence with respect to scavenging in the water column (up to 200 yr in deep water; [14]), and is more e¡ectively transported to regions with high particle £ux and scavenging intensity (a process called `boundary scavenging'). As a result, the Pa/Th of settling particles and underlying sediments tends to increase with particle £ux [3,15^18]. Higher export production should thus translate into higher Pa/Th in the sedimentary record, and this principle has been used to reconstruct paleoproductivity ([9,19^21]. Because Pa/Th is not a¡ected by dissolution on the sea£oor, this ratio is preserved in sediments, recording the event even if postdepositional dissolution has erased the accumulation rate signature. The assumption that the £ux of scavenged 230 Th to the sea£oor is equal to its production rate in the overlying water column is only an approximation, as was recently highlighted by water column [22^24] and modeling [25] studies. The £ux of scavenged 230 Th to the sea£oor would be absolutely constant and equal to its produc-
tion rate over the entire ocean only if the scavenging of 230 Th was instantaneous, which would reduce its activity in seawater to zero. With a residence time in deep water amounting to several decades, some modi¢cation of this simple balance by lateral transport of 230 Th must be expected. Likewise, the relationship between Pa/Th and particle £ux can be obscured by changes in deepwater circulation, which a¡ect the lateral transport of the nuclides [14,22,26,27], and changes in particle composition [28^30]. We have thus measured the £uxes of 230 Th and 231 Pa with deep-sea moored sediment traps in a variety of oceanic conditions. The data are reported in a separate paper dealing with the collection e¤ciency of deep-sea sediment traps [31]. Here, we use the data to further assess the constancy of the 230 Th £ux to the sea£oor and the dependency of Pa/Th on particle £ux. 2. Materials and methods Large, conical, time-series sediment traps of similar design were deployed at sites ranging from oligotrophic central gyres to ocean margin regions (Fig. 1). To eliminate seasonal biases, composite annual samples were obtained by combining samples in proportion to the amount collected by each individual sampling cup and analyzed for radioisotopes (230 Th, 231 Pa and U isotopes), organic carbon, opal, carbonate and lithogenic material. Details are given in [31]. Trapping e¤ciencies were calculated from the £uxes of excess 230 Th and excess 231 Pa intercepted by the traps, the Pa/Th of the settling material (Rv ), and estimates of the ratio of the rates of lateral transport of the two radionuclides in the water column (Rh ) [31,32]. The paucity of water column data results in relatively large errors in Rh . Trapping-e¤ciency estimates for traps deployed in the open ocean, from where 231 Pa is exported (i.e. Rv 6 LPa /LTh ), are relatively insensitive to these errors [31]. On the other hand, errors in estimates of trapping e¤ciency for traps deployed in regions receiving lateral input of 231 Pa (e.g. Paci¢c margins, southern ocean) are much larger. These estimates and associated error bars
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221
Fig. 1. Locations of the sediment trap moorings used in this study. Crosses indicate stations where trapping e¤ciency could not be calculated (no 231 Pa data available or di¤culties in constraining Rh [31]). Circles indicate stations where trapping e¤ciency was calculated; open and ¢lled circles are for deployment periods greater and smaller than 10 months, respectively.
are used in the following discussion to correct the measured 230 Th and 231 Pa £uxes. 3. Results and discussion Details of the deployments and results are listed in Table 1. Note that for a few traps, the total deployment period was less than 1 yr (Fig. 1). The samples from the Arabian Sea were obtained over a 6-month period including the summer monsoon, and thus, somewhat overestimate the annual £uxes. In contrast, the sediment trap mooring deployed in Panama Basin collected material spanning the lowest productivity period [33], so that the annual £ux may be somewhat underestimated. Results reported in the literature from shorterterm deployment in the tropical NW Atlantic [34] and the California margin [17] are also included to supplement our data set. 3.1. Vertical £ux of scavenged
230
Th
The basic assumption of the `constant
230
Th
£ux' model is that the £ux of scavenged 230 Th to the sea£oor is always equal to the production rate in the overlying water column. Since the latter increases linearly with water depth and the sediment traps were deployed at di¡erent depths (Table 1), we use the ratio of the £ux of 230 Th measured with the sediment traps (F) to its production rate (P) in the overlying water column (0.0252 dpm m33 yr31 UZ m; where Z is the depth of deployment of the sediment trap). These ratios are then plotted against total mass £ux measured by the corresponding traps. Particle £ux also decreases with depth, as a result of opal and carbonate dissolution and organic matter remineralization [31]. However, because this decrease is comparatively small below 1000 m and poorly constrained, it was not taken into consideration in the present analysis and the total mass £uxes used are those measured by the traps at their depth of deployment (corrected or not for trapping e¤ciency). The uncorrected data (F/P), show a general increase of 230 Th with total mass £ux (Fig. 2a). With 100% trapping e¤ciency and in the absence
EPSL 5916 31-8-01
Longitude
End date
Duration (days)
Aug. 83 Aug. 84 360
Start date
BOFS 47³53PN 19³32PW Apr. 89 Sep. 90 517 Apr. 89 Sep. 90 517 NABE 48 47³43PN 20³52PW Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 NABE 34 33³49PN 21³02PW Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 Apr. 89 Apr. 90 365 Sargasso 32³05PN 64³15PW Aug. 78 Jul. 81 774 Nares 23³16PN 63³55PW Aug. 83 Sep. 84 312 Aug. 84 Sep. 84 312 Tropical NE ^ oligotrophic 21³03PN 31³10PW Jun. 91 Sep. 91 90 Jun. 91 Sep. 91 90 Jun. 91 Sep. 91 90 Tropical NE ^ mesotrophic 18³30PN 21³06PW Jul. 92 Dec. 92 56 Southern ocean Antarctic circular current 50³09.0PS5³46.4PE Jan. 87 Mar. 88 361 BO 54³20.2PS3³20.2PW Dec. 90 Dec. 92 660 Jan. 91 Dec. 92 557 Brans¢eld Strait 62³15PS 57³31PW Dec. 83 Nov. 84 360 Weddell Sea ^ Polynia 64³54.1PS2³38.8PW Jan. 88 Feb. 89 384 Weddell Sea ^ AWI208 65³56.2PS35³30.2PW Oct. 89 Nov. 92 1035 Indian Arabian Sea ^ WAST 16³19PN 60³28PE May. 86 Oct. 86 162 May. 86 Oct. 86 162 Arabian Sea ^ EAST 15³27PN 68³44PE May. 86 Oct. 86 162 May. 86 Oct. 86 162
Atlantic Lofoten 69³30PN 10³00PE
Latitude
9.1 11.2 16.2 9.7 17.0 94.8
38.3 53.1 14.4 107.3 33.8 12.3
66.9 66.1 32.6 39.1
1464 4832 2500 4400 4590 2890
700 450 2194 1588 360 1090
1085 3020 1705 2773
19.4 22.4 21.2
1159 1981 4478 9.9
20.7 26.2
1110 3734
3200
22.6 32.8
22.8
3100 4465
2760
Depth Mass £ux (m) (g m32 yr31 ) (uncorrected)
^ ^
^
^ ^
EPSL 5916 31-8-01 ^
^
^ ^
s 87
0.90^1.01 32^36 1.14^1.37 29^34
0.81^0.88 76^83 0.89^1.07 62^74
0.88^1.00 12^14
^
^
^ ^
6 0.44
1.53^1.80 53^62
1.30^1.47 11^13 0.56^0.63 15^18 1.09^1.23 14^16
^ ^
0.75^0.93 11^13
0.64^0.75 26^30 0.83^1.03 22^27 0.96^1.22 17^22
0.38^0.43 48^54 0.70^0.88 30^37
^ ^
^
Trapping Mass £ux e¤ciency (E) (g m32 yr31 ) (corrected)
Th
0.49 0.56
0.17 0.22
^
^
^
^ ^
^
0.63
0.80 0.89 0.83
0.09 0.16
0.08
0.29 0.44 0.51
0.15 0.22
0.18 0.32
1.03
(dpm g
232
31
) (dpm g
)
^ ^
^
Th
V/P 230
^ ^
^
^
0.18
0.38^0.44
1.27 þ 0.05 0.119 þ 0.0070.94^1.08 2.03 þ 0.07 0.112 þ 0.0060.82^1.00
0.35 þ 0.01 0.036 þ 0.0020.96^1.07 1.08 þ 0.07 0.085 þ 0.0030.86^1.07
0.85
0.23 þ 0.01 0.064 þ 0.003^
0.54
0.10 þ 0.01 0.035 þ 0.004^ 0.36 þ 0.03 0.171 þ 0.018^
0.20 þ 0.01 0.065 þ 0.13 s 1
1.15 þ 0.06 0.052 þ 0.0110.82^0.99
4.55 þ 0.11 0.081 þ 0.0100.79^0.90 5.81 þ 0.13 0.144 þ 0.0150.80^0.91 6.79 þ 0.11 0.180 þ 0.0100.81^0.92
2.99 þ 0.36 ^ 7.76 þ 0.49 ^
6.06 þ 0.24 0.283 þ 0.0190.79^1.00
0.98 þ 0.04 0.030 þ 0.0050.86^1.03 1.80 þ 0.06 0.066 þ 0.0050.78^0.98 4.94 þ 0.16 0.135 þ 0.0090.76^0.97
0.52 þ .02 0.027 þ 0.0040.88^1.02 2.40 þ 0.07 0.060 þ 0.0040.86^0.96
1.51 þ 0.15 ^ 4.19 þ 0.21 ^
31
ex.231 Pa
2.89 þ 0.36 ^
) (dpm g
31
ex.230 Th
Table 1 Fluxes and composition of materials collected by the sediment traps used in this study
Pa
^ ^
^
Pa/Th
45 54
50
CaCO3 (%)
^ ^
49 42
0.28 þ 0.02
^
0.35 þ 0.05 0.47 þ 0.06
0.32 þ 0.07
18
8
5
13 5
26
0.94^1.11 0.094 þ 0.007 49 0.49^0.60 0.055 þ 0.004 50
1.05^1.21 0.101 þ 0.007 59 0.74^0.90 0.078 þ 0.006 54
0.88^1.00 0.21 þ 0.03
^
^
^ ^
s 3.5
0.34^0.54 0.045 þ 0.010 72
0.14^0.19 0.018 þ 0.002 ^ 0.20^0.26 0.025 þ 0.003 ^ 0.23^0.27 0.027 þ 0.002 ^
^ ^
0.40^0.51 0.047 þ 0.004 55
0.25^0.37 0.031 þ 0.005 68 0.30^0.39 0.037 þ 0.003 62 0.22^0.29 0.027 þ 0.002 61
0.45^0.62 0.053 þ 0.008 55 0.20^0.26 0.025 þ 0.002 58
^ ^
^
231
11 13
23 23
70
70
36
63 65
40
^
^ ^ ^
3 2
15
8 9 9
18 21
^ ^
5
Opal (%)
^ 24
^ 13
^
9
50
17 26
^
^
^ ^ ^
38 49
17
14 20 22
10 13
^ ^
31
terr (%)
^ 7
^ 5
^
7
4
3 2
7
2
^ ^ ^
6 4
5
5 4 4
7 4
^ ^
6
Corg (%)
[31] [31]
[31] [31]
[41]
[35]
[40]
[35] [35]
[35]
[34]
[34] [34] [34]
[42] [42]
[32]
[31] [31] [31]
[31] [31]
[39] [39]
M.P. Bacon (unpublished).
Reference
222 E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Longitude
Start date
End date
Duration (days)
3800 500 1000 2329 500 1750 2330 1000 3769
360 360 360
352 325 352
60
112
3800
3800
EPSL 5916 31-8-01 65.8
25.7
18 49 43
95.0 128 161
20.0
46.0
55.8
73.3
10.1
1000
3800
40.3 34.7
1000 2900
Depth Mass £ux (m) (g m32 yr31 ) (uncorrected)
311
Sep. 82 Apr. 84 560
Sep. 82 Sep. 83 360
Mar. 83 Oct. 83 180
Nov. 84 Oct. 85 California ^ near shore 42³05PN 125³46PW Sep. 87 Sep. 88 Sep. 87 Sep. 88 Sep. 87 Sep. 88 California ^ mid-way 42³11PN 127³35PW Sep. 87 Sep. 88 Sep. 87 Aug. 88 Sep. 87 Sep. 88 California ^ Gyre 41³33PN 132³00PW Feb. 88 Apr. 88 Panama 5³22PN 82³01PW Jul. 79 Dec. 79
PAPA 6^7
PAPA 1^3
PAPA 1^2
PAPA 2
Arabian Sea ^ CAST 14³29PN 64³46PE May. 86 Oct. 86 169 May. 86 Oct. 86 169 Paci¢c PAPA 2 50³00PN 145³00PW Mar. 83 Oct. 83 180
Latitude
Table 1 (continued)
^
^
^
^
0.75^0.99 66^88
0.31^0.41 62^83
0.25^0.35 51^71 0.30^0.68 72^163 0.25^0.56 77^172
1.04^1.80 53^91 1.28^2.02 63^100 0.77^1.60 101^209
0.96^1.15 17^21
^
^
^
^
0.93^1.07 38^43 0.98^1.17 30^35
Trapping Mass £ux e¤ciency (E) (g m32 yr31 ) (corrected)
Th
ex.230 Th
ex.231 Pa
^
0.16
0.27 0.39 0.53
0.54 0.67 0.62
0.15
^
0.01
0.01
0.04
0.34 0.53
230
Th
V/P
^
^
^
^
^
1.52
0.234
1.06^1.41
0.41 þ 0.02 0.061 þ 0.011 1.01^1.36
0.24 þ 0.02 0.046 þ 0.008 0.94^1.37 0.67 þ 0.03 0.148 þ 0.012 1.09^2.49 0.87 þ 0.02 0.196 þ 0.012 1.14^2.55
0.21 þ 0.04 0.040 þ 0.007 0.81^1.60 0.37 þ 0.05 0.076 þ 0.011 0.88^1.52 0.62 þ 0.04 0.130 þ 0.011 1.05^2.22
4.79 þ 0.30 0.371 þ 0.023 0.85^1.06
2.04
1.97 þ 0.06 ^
1.41 þ 0.04 ^
0.39 þ 0.02 ^
0.44 þ 0.03 0.030 þ 0.003 0.64^0.78 2.15 þ 0.11 0.155 þ 0.010 0.86^1.05
(dpm g31 ) (dpm g31 ) (dpm g31 )
232
Pa
CaCO3 (%)
^
^
^
^
36
34
^
35
1.75^2.36 0.15
1.47^2.34 0.15 þ 0.03
1.82^2.97 0.19 þ 0.04 2.58^5.96 0.22 þ 0.02 2.75^6.23 0.23 þ 0.01
1.68^3.27 0.19 þ 0.05 1.95^3.38 0.21 þ 0.04 2.57^4.07 0.21 þ 0.02
^
35
24 22 22
13 12 9
0.71^0.89 0.078 þ 0.007 40
^
^
^
^
0.46^0.59 0.070 þ 0.008 54 0.66^0.83 0.073 þ 0.006 66
231
Pa/Th
^
36
28 26 29
19 23 26
46
^
50
^
^
6 11
Opal (%)
^
13
26 43 46
58 63 63
^
^
^
^
^
^ 12
terr (%)
^
5
11 5 4
5 4 3
^
4
4
^
3
^ 6
Corg (%)
M.P. Bacon (unpublished).
[17]
[17] [17] [17]
[17] [17] [17]
[31]
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
M.P. Bacon (unpublished).
[31] [31]
Reference
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230 223
224
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Fig. 2. Ratios of the £uxes of 230 Th (a, c) and 231 Pa (b, d) measured with sediment traps to the rates of 230 Th and 231 Pa produced in the overlying water (P) vs. corresponding total mass £uxes. (a,b) Fluxes (F) and mass £uxes are not corrected for trapping e¤ciency. (c, d) Fluxes (V) and mass £uxes are corrected for trapping e¤ciency according to [31]. Filled symbols are data taken from traps deployed for less than 10 months. They follow the general trend and are not biasing our conclusions. For the Antarctic circumpolar current (c, d), only a maximum trapping e¤ciency could be calculated [31]. The symbol represents V/P and mass £ux assuming this value and the line re£ects the trend obtained when assuming increasingly lower trapping e¤ciencies.
of signi¢cant lateral transport of 230 Th prior to scavenging, F/P should be constant and equal to one (i.e. F = P). The uncorrected data would thus suggest signi¢cant lateral transport of 230 Th, contrary to expectation. At low £uxes, however, we ¢nd a large range of F/P values from nearly zero to s 1, without a clear relationship to the measured total mass £ux. This scatter nearly disappears when the £uxes of 230 Th and total mass are corrected for trapping e¤ciency (V/P where V is the `true' corrected 230 Th £ux; Fig. 2c). Since we do not have 231 Pa data for all traps, we can
make the correction only for a subset of the uncorrected £ux data shown in Fig. 2a. Nonetheless, the results clearly indicate that the large apparent de¢cits in 230 Th £ux occasionally measured at low particle £uxes are largely due to low-trapping ef¢ciency and not a re£ection of lateral export of 230 Th to high particle £ux regions. Instead, V/P for 230 Th in regions where corrected total mass £ux is below 60 g m32 yr31 averages 0.90 þ 0.06, indicating only a small, but probably signi¢cant, lateral export from oligotrophic regions. Two notable exceptions were not included in this average.
EPSL 5916 31-8-01
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
Low 230 Th £uxes were found in the middle of the Weddell Sea, where particle £ux and scavenging intensity are very low because of ice cover [35]. Relatively low 230 Th £ux was also found with a trap deployed at 1000 m depth outside the Arabian Sea upwelling region (Fig. 2c), possibly re£ecting the proximity of a zone with a large horizontal gradient in particle £ux. In high-£ux regions, the £ux of scavenged 230 Th may exceed production by more than 50%. At the moment, however, the only available data showing such excess (V/P s 1.5) come from the California margin and the Atlantic sector south of the Polar Front, where sediment trap e¤ciency is poorly constrained [31] and the resulting error on V/P is large (Table 1). Fig. 2b thus validates the use of the 230 Th constant £ux model in most open ocean settings, but the extent of deviations at the margins and south of the Polar Front still needs to be better constrained and documented by detailed water column pro¢les of the two nuclides. Our results are consistent with a recent study [25] in which a particle ¢eld was introduced into a global ocean circulation model to model scavenging and produce synoptic maps of 230 Th £uxes to
the sea£oor. There is a good general agreement between our sediment trap £ux data and the model estimates at the same sites (Table 2), supporting the conclusions derived from these two independent studies. Although an approximation, the constant 230 Th £ux model can thus be applied to reconstruct past changes in preserved rain rates over large areas of the ocean. The model is useful even in areas where the scavenged £ux of 230 Th may exceed production signi¢cantly (e.g. Paci¢c margins, Polar Front in the Atlantic sector; Fig. 2c), as the corrections required for syndepositional redistribution of sediment in these regions are often much larger than the uncertainty associated with the scavenged £ux of 230 Th ([9,11,19^21]. Nonetheless, in regions where the vertical £ux of 230 Th can be signi¢cantly lower than the production rate, the model must be used with caution. This is the case for the Weddell Sea, where the scavenged £ux of 230 Th appears to only be 40% of the production rate [22,31,35]. We could expect to ¢nd similar de¢cits near regions of deep-water formation in the north Atlantic [23,24].
Table 2 Comparison between the ratios of the trapping-e¤ciency corrected £ux of and the ratios obtained in an ocean circulation model [25] Location Atlantic NABE 48 NABE 34 Sargasso Tropical NE ^ Oligotr. Tropical NE ^ Mesotr. Southern ocean Polar Front Weddell Sea Indian Arabian Sea ^ WAST Arabian Sea ^ EAST Arabian Sea ^ CAST Paci¢c PAPA 6^7 California ^ near shore California ^ mid-way California ^ Gyre Panama
Latitude
225
230
Th to production rate (V/P) estimated in this study
Longitude
V/P (230 Th) this work
[25]
47³43PN 33³49PN 32³05PN 21³03PN 18³30PN
20³52PW 21³02PW 64³15PW 31³10PW 21³06PW
0.94 þ 0.08 0.90 þ 0.13 0.90 þ 0.11 0.86 þ 0.07 0.91 þ 0.09
0.9 0.9 0.8 0.7 0.9
50³09PS 66³56PS
05³46PE 35³30PW
s1 0.41 þ 0.03
1.3 ?
16³19PN 15³27N 14³29PN
60³28PE 68³44PE 64³46PE
0.97 þ 0.11 0.95 þ 0.13 0.85 þ 0.20
1.0 1.0 1.0
50³00PN 42³05PN 42³11PN 41³33PN 05³22PN
145³00PW 125³46PW 127³35PW 132³00PW 82³01PW
0.96 þ 0.11 1.5 þ 0.7 1.7 þ 0.8 1.2 þ 0.2 1.2 þ 0.2
1.1 1.3 1.2 1.0 1.2
EPSL 5916 31-8-01
226
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
3.2. Vertical £ux of ex.231 Pa and changes in ex.231 Pa/ex.230 Th with particle £ux The scavenged vertical £ux of 231 Pa is strongly dependent on particle £ux (Fig. 2b), and unlike for 230 Th, correcting for trapping e¤ciency does not alter this conclusion (Fig. 2d). Corrected 231 Pa £uxes measured in the north Atlantic average only 1/3 of the production rate in the overlying water, while at the Polar Front and at the California margin of the Paci¢c 231 Pa £uxes exceed production by more than a factor of three (Table 1). As anticipated, for a similar total £ux, the vertical £uxes of 231 Pa tend to be lower in the Atlantic Ocean, re£ecting the e¡ect of thermohaline circulation. Using water column data, Yu et al. [14] calculated that approximately 50% of the 231 Pa produced in the Atlantic north of 25³N is laterally exported with NADW to the south. This is reasonably similar to the 64 þ 12% (n = 6) estimated from the sediment traps deployed in the Sargasso Sea and during the North Atlantic Bloom Experiment (Table 1). The 231 Pa exported from the Atlantic Ocean is e¡ectively scavenged by the opal £ux in the southern ocean [26,27], and very little appears to reach the Paci¢c Ocean [14]. The relatively high 231 Pa trap £uxes measured in the Paci¢c Ocean are mainly from margin regions (Table 1) and thus do not necessarily indicate thermohaline input of 231 Pa in this ocean basin. A more comprehensive Paci¢c data set (from surface sediment or sediment traps) is required to con¢rm the extent to which 231 Pa may be exported to the Paci¢c. Pa/Th variations essentially re£ect variations in 231 Pa £ux (Fig. 3). In the Atlantic, Pa/Th in settling particles is invariably low, consistent with the low Pa/Th also found in Atlantic sediments and the export of 231 Pa into the southern ocean ([14,18,26,27]. Although no measurements of 231 Pa and 230 Th £uxes were made in the Atlantic with particle £ux exceeding 60 g m2 y31 , extrapolation of the available sediment trap data suggests that Pa/Th would not rise much above the production rate ratio of 0.0925, even at very high particle £ux. This is consistent with Pa/Th measured in surface sediments in the upwelling region o¡ Africa [14,18,27,34]. Boundary scavenging of
Fig. 3. Pa/Th in sediment trap material vs. corrected total mass £ux. Filled symbols are data taken from traps deployed for less than 10 months. The line through the Atlantic Ocean data set is a linear regression. The lines through the Paci¢c Ocean and Arabian Sea data are polynomial ¢ts (see text for equations). 231
Pa cannot be fully expressed in the Atlantic Ocean because the mean residence time of deep water, which was estimated at V100 yr [36] or V275 yr [37], is equivalent to the time required for basin-wide lateral mixing [16]. As a result, the increase in Pa/Th with particle £ux, while still recognizable, is subdued (linear regression on the Atlantic data set in Fig. 3: Pa/Th = (0.00043UTMF)+0.022; r2 = 0.37) In the Paci¢c, the slope of the Pa/Th vs. total mass £ux relationship is steeper than that in the Atlantic, and appears to plateau at higher mass £ux. A polynomial ¢t (Pa/Th = 30.0000107(TMF)2 +0.00285 (TMF)+0.0290; r2 = 0.82) is used to illustrate this trend (Fig. 3). Here, there is a paucity of trap data from the oligotrophic region, but data from surface sediments show clearly that the Pa/Th in these regions can be as low as in the Atlantic [15,18]. The higher dependency of Pa/Th on particle £ux in this ocean re£ects the longer residence time of deep water, which allows a full expression of boundary scav-
EPSL 5916 31-8-01
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
enging. Lower particle £ux and scavenging intensity also enhance lateral transport by increasing residence times of the radionuclides in the water column. Limits to the lateral supply rate of 231 Pa by eddy di¡usion must be responsible for the Pa/Th plateau observed at higher mass £ux. The Arabian Sea appears to re£ect a situation intermediate between the Atlantic and Paci¢c Ocean. Here, a polynomial ¢t with a forced intercept has been used to highlight the trend (Pa/Th = 30.0000101(TMF)2 +0.00167(TMF)+0.025; r2 = 0.32) In the southern ocean south of the Polar Front, Pa/Th is signi¢cantly higher than in other oceanic regions and remains well above the production rate ratio even at low particle £ux (e.g. Weddell Sea; Fig. 3). It is thus evident that boundary scavenging is not the only process controlling the relative scavenging of 231 Pa and 230 Th in this region. Instead, particle composition must also play an important role in controlling the fractionation between 231 Pa and 230 Th during scavenging [29]. The fractionation factor (fTh=Pa = KdTh /KdPa ; where Kd = [M]p/[M]d; [M]p,d are activities (dpm g31 ) in particles and seawater) determines the relative residence time of the two nuclides with respect to scavenging, and thus in£uences the slope of response of Pa/Th to particle £ux. When fTh=Pa s 1, 231 Pa has a longer residence time than 230 Th and is more e¡ectively transported to high£ux regions. When fTh=Pa = 1, however, the residence time with respect to scavenging of both nuclides is the same and, even though boundary scavenging may still operate, it will not be expressed in the Pa/Th ratio since scavenging will a¡ect both nuclides similarly. In this case, Pa/Th of particles simply re£ects the Pa/Th of seawater and is independent of particle £ux. fTh=Pa W10 when particle composition is dominated by clay, and approaches 1 when adsorption occurs on amorphous silica [29,30,35]. Opal concentration in particles settling in the southern ocean is highly variable, especially in the Polar Frontal zone, where particle composition gradually shifts from carbonate-dominated to opaldominated [38]. Measurements across the PF in the Atlantic sector have documented a pronounced decrease in fTh=Pa with increasing opal
227
content in suspended and sinking particles, from typical values of V10 north of the PF to values approaching 1 in the Weddell Sea [22,29]. Settling materials intercepted by traps deployed in the Weddell Sea have the highest opal content of all samples analyzed in this study (70%; Table 1). Their mean Pa/Th (0.21) is only slightly lower than the dissolved Pa/Th measured in Weddell seawater (0.28 þ 0.06 ; [22,35]). This observation con¢rms that little fractionation occurs during scavenging by opal and explains the high Pa/Th measured at relatively low particle £ux [29]. On the other hand, the trap deployed further north, just south of the Polar Front, collected material with signi¢cantly lower opal content (40%) and a signi¢cantly higher Pa/Th. fTh=Pa in this region has been estimated at 2 [29]. Boundary scavenging could thus play a role in establishing the Pa/Th of settling particles in this region. Another impor-
Fig. 4. Diamonds = Atlantic Ocean; Triangles = Indian Ocean; Circles = Paci¢c Ocean. Open symbols = when corrected total mass £uxes are s 50 g m2 yr31 . Closed symbols = when corrected total mass £uxes are 6 50 g m2 yr31 . gray symbols are for stations where corrected mass £uxes are not available. Open square = Antarctic circular current; ¢lled square = Weddell Sea; ¢lled circle = station PAPA.
EPSL 5916 31-8-01
228
E.F. Yu et al. / Earth and Planetary Science Letters 191 (2001) 219^230
tant factor is lateral input of 231 Pa with NADW from the Atlantic [22,27]. Three factors thus combine to yield the high Pa/Th in the Atlantic sector near the Polar Front: thermohaline circulation, high particle £ux and high opal content in the settling material. Fig. 4 further illustrates the interplay between these three factors. The general increasing trend in the Pa/Th of settling particles with increasing percent of opal supports the importance of opal for scavenging 231 Pa from the water column. However, this trend is also in part due to the general coincidence between high total mass £ux and high percent of opal. The only exceptions to this rule are station PAPA and the Weddell Sea. At these two sites, particle £uxes are comparatively low, and accordingly, the Pa/Th of settling particles is lower compared to other stations with similar percent of opal. Also, Pa/Th in the material settling in the Atlantic sector of the ACC (0.32 þ 0.07) is much higher than in the material collected at the California Gyre station (0.15 þ 0.03), even though both have nearly similar percentages of opal (40 vs. 36%) and total mass £uxes ( s 87 g m2 yr31 vs. 62^83 g m2 yr31 ). Although corrected £uxes at the ACC can only give a minimum value, this di¡erence could re£ect 231 Pa advection from the Atlantic Ocean. In the Atlantic, settling material has invariably low opal concentration, which could contribute to the low Pa/Th in this ocean basin. If the settling £ux in the Atlantic Ocean was dominated by opal with fTh=Pa W1, the residence time of 231 Pa in the Atlantic water column would be much shorter, resulting in little thermohaline export to the southern ocean, little or no fractionation between 231 Pa and 230 Th, and similar Pa/Th in both ocean basins that would be close to the production rate ratio of 0.093. 4. Conclusions We have shown that sediment trap data validate the use of 230 Th normalization for the paleoceanographic reconstruction of past changes in preserved rain rates over large sections of the ocean £oor, and are consistent with recent results
obtained with a general circulation model [25]. Both approaches show that scavenging rates of 230 Th exceeding the rate of production by more than a factor of two do not occur. However, the constant 230 Th £ux model can overestimate rain rates, possibly by as much as a factor of two, when applied to sediments deposited in regions where deep-water ventilation ages are younger than the mean residence time with respect to Th scavenging (e.g. Nordic and Labrador seas; NW Atlantic; Weddell Sea). In these regions, the scavenging rates of 230 Th can be signi¢cantly lower than the production rate, due to lateral export. Additional water column measurements are necessary to better document these deviations [22^ 24]. Sediment trap data also clearly illustrate the complexity of the relationship between particle £ux and the Pa/Th of settling material. Although Pa/Th is a potentially useful tool for exploring past changes in particle £ux, interpretation of its sedimentary record requires that the location of the study, general circulation patterns, and the chemical composition of the settling material be taken into account. Pa/Th is best suited as a synoptic tool to constrain past changes in relative particle £ux between regions or to document changes in deep-water circulation patterns. In the Atlantic, Pa/Th depends mostly on the strength of the `conveyor belt' circulation, which exports 231 Pa from the Atlantic into the southern ocean [14,26,27]. Pa/Th in Atlantic sediment cores can be used to estimate past changes in particle £ux, but only if it can be ascertained that the strength of the thermohaline circulation changed insigni¢cantly during the period considered. At the modern rate of NADW formation, however, the sensitivity of Pa/Th to changes in particle £ux in the Atlantic is small (Fig. 4). In this basin, sedimentary Pa/Th is thus better suited to quantify past changes in the strength of the conveyor belt circulation [14], which is best estimated from the mean sediment Pa/Th over the entire north Atlantic [27]. In the Paci¢c, on the other hand, baring major changes in deep-water circulation and particle composition, Pa/Th will be a more sensitive tracer of relative changes in particle £ux, and should prove a useful tool for re-
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constructing past changes in export £ux in Paci¢c upwelling regions. In the southern ocean, quantitative interpretation of Pa/Th is more di¤cult because it is affected not only by particle £ux, but also by global circulation and particle composition. Sedimentary Pa/Th has been used as a qualitative paleoproductivity proxy in the southern ocean [9,19^21], where high Pa/Th generally coincides with high 230 Th-normalized opal £ux and high opal concentration. Qualitatively, it could be argued that changes in particle composition have ampli¢ed changes in Pa/Th resulting from changes in particle £ux, but it cannot be ruled out that the increase in Pa/Th is entirely due to the decrease in fTh=Pa . Unless the e¡ect of variable fTh=Pa can be taken into account, it will be impossible to quantify particle £ux from Pa/Th in this and other regions where biogenic opal dominates the settling material.
[10]
Acknowledgements
[11]
Financial support for this work was provided by grants from the U.S. National Science Foundation. E.-F. Yu also acknowledges support from the Chinese National Science Council and National Taiwan Normal University. The MIT reactor used for producing the 233 Pa spike is supported by the U.S. DOE Reactor Sharing Grant No. DE-FG07-80ER10770.A020. We are grateful to S. Honjo and S. Manganini for providing sediment trap samples, and to R.F. Anderson and G. Henderson for providing constructive reviews. This is U.S. JGOFS Contribution No. 651 and WHOI contribution No. 10409.[EB]
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