Advection and scavenging: Effects on 230Th and 231Pa distribution off Southwest Africa

Earth and Planetary Science Letters 271 (2008) 159–169

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Advection and scavenging: Effects on Southwest Africa

230

Th and

231

Pa distribution off

Jan C. Scholten a,b,⁎, J. Fietzke b,c, A. Mangini d, C.-D. Garbe-Schönberg b, A. Eisenhauer c, R. Schneider b, P. Stoffers b er

a

International Atomic Energy Agency, Marine Environment Laboratories (IAEA-MEL), 4 Quai Antoine 1 , MC-98000 Monaco Institute of Geosciences, University Kiel, Olshausentr. 40, D-24118 Kiel, Germany c IFM-GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany d Heidelberger Akademie der Wissenschaften, University Heidelberg, D-69129 Heidelberg, Germany b

A R T I C L E

I N F O

Article history: Received 22 November 2006 Received in revised form 26 March 2008 Accepted 27 March 2008 Available online 11 April 2008 Edited by: H. Elderfield Keywords: natural radionuclides hydrography sediments Cape Basin Angola Basin, boundary scavenging fractionation tracer

A B S T R A C T We investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231Pa and 230Th in the water column and in surface sediments off Southwest Africa (Angola and Cape basins). Based on a vertical section of total 230Thex concentrations in the water column we show that small differences in the salinity between the North Atlantic Deep Water (NADW) in the Angola Basin and the NADW in the Cape Basin as well as the advection of NADW associated with the Namib Col Current are reflected in total 230 Thex concentrations. These variable total concentrations are believed to reflect the flow path and mixing history of NADW with the NADW in the Angola Basin being relatively older and 230Th enriched compared to the NADW in the Cape Basin. In the area investigated we found high 231Paex/230Thex ratios (231Paex/230Thex N 0.093) in surface sediments at the continental margin and lower ones (231Paex/230Thex b 0.093) in the open ocean. Such a distribution is normally interpreted to result from high particle flux at ocean margins (boundary scavenging). However, the lack of any significant depletion of dissolved 230Th and 231Pa in the water column does not indicate extensive scavenging at the continental margin. High 231Paex/230Thex ratios are constrained to shallow waters depths (b 2000 m) only and coincide with low fractionation between 231Pa and 230Th indicating that preferential scavenging of 231Pa on opal may have caused high 231Paex/230Thex ratios in the sediments. The observed close negative correlation (r2 = 0.82) between 231Paex/230Thex ratios in sediments and water depths is believed to reflect changes in the particle composition, i.e. a decrease in opal content with water depth. In the Angola and Cape basins the total 231Paex concentrations in NADW were the highest observed so far in the Atlantic Ocean, and they are attributed to the meridional export of 231Pa from the North Atlantic. This caused the average dissolved 231Pa/230Th in the Southeast Atlantic to be about a factor 2 higher when compared to the North Atlantic (Labrador Sea). These differences in the dissolved 231Pa/230Th were not reflected in 231Pa/230Th ratios of surface sediments because the fractionation is lower in the Labrador Sea compared to the Southeast Atlantic, i.e. fractionation counteracts changes in the dissolved 231Pa/230Th. This suggests that fractionation is more important for the determination of 231Paex/230Thex ratios in sediments than the meridional export of 231Pa from the North Atlantic. © 2008 Elsevier B.V. All rights reserved.

1. Introduction

adsorb on sinking particles and are removed to the sediments. Since Pa is less particle reactive than 230Th i.e. 231Pa has a longer residence time (τ) in the water column compared to 230Th (τ = 230Th: 20–40y; τ = 231Pa: 80–200y) 231Pa is preferentially removed in areas of higher particle flux, e. g. ocean margins (boundary scavenging); and the variable 231Pa/230Th ratios stored in the sediment record are believed to reflect changes in the paleofluxes during the geological past which can be interpreted as variations in paleoproductivity (Kumar et al., 1993). There is an on-going discussion as to what extent the composition of the vertical particle flux influences the 231Pa/230Th ratios in sediments (Chase and Anderson, 2004; Chase et al., 2002; Luo 231

The natural radionuclides 230Th (T1/2 = 75.6 ky) and 231Pa (T1/2 = 32.1 ky) are widely used as tracers for paleoceanographic process studies (Scholten et al., 1994; Henderson, 2002; McManus et al., 2004). Both isotopes are produced in seawater by radioactive decay of uranium isotopes (234U, 235U) and due to their particle reactivity they ⁎ Corresponding author. International Atomic Energy Agency, Marine Environment Laboratories (IAEA-MEL), 4 Quai Antoine 1er, MC-98000 Monaco. Tel./fax: +377 9797 7228. E-mail address: [email protected] (J.C. Scholten). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.03.060

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and Ku, 2004; Luo and Ku, 2004; Siddall et al., 2005; Scholten et al., 2005). Laboratory experiments have shown that fractionation between 230Th and 231Pa depends on the type of particles (Andersen et al., 1992; Geibert and Usbeck, 2004) with 231Pa having a higher tendency to adsorb on biogenic opal compared to 230Th. On a transect from the South Atlantic to the Weddell Sea it was shown that with increasing opal content the fractionation between 231Pa and 230Th decreases causing high particulate 231Pa/230Th ratios in areas where opal dominates the particle flux (Walter et al., 1997). Water mass advection may play an important role controlling the distribution of these radionuclides in those oceanic regions where water mass residence times are shorter or of the same order of magnitude as the scavenging residence time of 230Th and 231Pa (Moran et al., 1997). In the North Atlantic the deep thermohaline convection combined with rapid advection causes a reduction of 230Th concentrations in the deep-water column and subsequently reduced flux to the sediments (Vogler et al., 1998). Due to the relatively long residence time 231Pa is much more affected by lateral water mass transport. About 50% to 70% of the 231Pa produced in the North Atlantic is exported towards the South Atlantic via the North Atlantic Deep Water (NADW) and it is assumed that one of the reasons for the high 231Pa/230Th ratios in surface sediments of the Southern Ocean is this advection of 231Pa from the Atlantic (Yu et al., 1996). The observation that 231Pa and 230Th may respond to changes in oceanic circulation was used to infer from high 231Pa/230Th ratios in sediments a cessation in Atlantic meridional overturning circulation during the Heinrich event 1 (~17 ky) and the Younger Dryas (~12 ky) (McManus et al., 2004). However, a recent study challenges this interpretation and attributes some of the 231Pa/230Th variations in the Atlantic sediment record to changes in fractionation caused by variable opal productivity (Keigwin and Boyle, 2008). It is difficult to separate which process — particle flux, particle composition, and/or circulation — is most important for the distribution of 230 Th and 231Pa in the water column and in the sediments. For instance, high particle flux may reduce the water-column concentrations of 230Th (Scholten et al., 2005; Anderson et al., 1990). Such a reduction may, however, also result from a rapid advection of water masses depleted in 230 Th (Vogler et al., 1998). The export of 231Pa from the North Atlantic towards the Southern Ocean as postulated by Yu et al. (1996) may be significantly reduced if 231Pa is removed effectively at ocean boundaries. That study does not include the upwelling region off Namibia which is characterized by high bioproductivity and related high particle flux (Luthjeharms and Meeuwis,1987; Wefer and Fischer,1993). As a substantial part of NADW flows to the Southern Ocean on the eastern side of the South Atlantic (Reid,1989) this area may be an important sink for 231Pa. The unique coincidence of water mass flows (see more details below) as well as the high particle flux predestines the area off Southwest Africa to study the interplay between radionuclide scavenging and oceanic circulation. Based on water-column profiles of 230Th and 231Pa and 231Pa/230Th ratios in surface sediments obtained from the Angola and Cape basins (Fig. 1) we show that fractionation between 231Pa and 230Th constraints the distribution of sedimentary 231Pa/230Th in this area. 2. Hydrographic setting The Southeast Atlantic is an important area in the context of the global circulation (Reid, 1989): on the one hand water masses originating from the Indian Ocean flow north towards the equatorial Atlantic. On the other hand, a substantial part of the water mass formed in the North Atlantic, i.e. North Atlantic Deep Water (NADW), flows southwards to the Antarctic Circum Polar Current via the Angola and Cape basins. The water masses in the Angola and Cape basins can be divided into three major units: upper, intermediate and deep waters (Stramma and England, 1999; Arhan et al., 2003) (Fig. 2): In the upper water column (0 m–~ 1200 m depths) there is the northward flowing Benguela Current which consists of surface waters

(0 m–~150 m), South Atlantic Central Water (SACW, 150 m–500 m) and Antarctic Intermediate Waters (AAIW, 500 m–1200 m) (Mercier et al., 2003; Stramma and Peterson, 1989). The intermediate water mass in the Angola and Cape basins consists of North Atlantic Deep Water (NADW) (Fig. 2) and its poleward transport is about ~7–11 SV (Arhan et al., 2003) which is about a factor of 2 lower than NADW transport from the North Atlantic (~19 SV, Yu et al., 1996). NADW is observed in water depths between 1200 m and 3800 m in the Cape Basin and between 1200 m and the bottom in the Angola Basin. In the area investigated the Walvis Ridge, which is only about 2000 m deep to the northeast and gradually deepens towards the southwest, separates and restricts the water mass exchange between both basins. In the Angola Basin the NADW consists of two components: one is advected from the West Atlantic, the other is relatively old recirculated NADW advected from the northern Angola Basin (Fig. 1). Most of the NADW enters the Cape Basin through passages in the Walvis Ridge south of 28°S and originates from the West Atlantic (Reid, 1989). Slight differences in the salinity of NADW in the Cap and Angola basins (Fig. 2) reflect these different flow paths and mixing histories of NADW. A further flow path of NADW is associated with the Namib Col Current (Figs. 1, 2) (Speer et al., 1995). A saddle in the Walvis Ridge (Namib Col at about 21.5 S, 7.5 E) allows NADW from the Angola Basin to enter the Cape Basin, and this flow can be recognized in the Cape Basin by salinity around 34.88. Speer et al. (1995) assume that this current continues south in the Eastern Cape Basin. Below 3800 m water depth Antarctic Bottom Water (AABW) fills the Cape Basin (Fig. 2). According to its density (σ4 = N46.0) it can be recognized as Lower Circumpolar Deep Water (Arhan et al., 2003) which originates in the Antarctic Circumpolar Current (ACC). As the Walvis Ridge acts as a barrier, AABW cannot directly flow from the Cape Basin into the Angola Basin and a cyclonic flow of AABW in the Cape Basin of about 7–8 SV is suggested by Arhan et al. (2003) (Fig. 1). 3. Methods 3.1. Sample collection and preparation During Meteor cruise M48/4 (2000) water samples were obtained from 5 stations in the Angola Basin and 6 stations in the Cape Basin using Niskin bottles mounted on a CTD (Fig. 1). The sampling depths were chosen according to the salinity and temperature readings so that all major water masses at the locations investigated were sampled. For the determination of total (particulate and dissolved) thorium isotopes 2–4 l of water were filled in pre-cleaned polyethylene containers. The samples were weighed, spiked with 229Th and acidified with suprapure HNO3. After 24 h NH4OH was added so that Mg(OH)2 precipitated (Wu and Boyle, 1997). The precipitate was allowed to settle and after decanting the precipitate was transferred into 250 ml polyethylene bottles and was further processed in the home lab. For the determination of 231Pa 10 l of water was filled in pre-cleaned polyethylene containers, acidified and transported to the home lab. At locations 465, 466, 468, 471, 474 and 476 the particulate fraction (suspended particles) was sampled in the same depth as the total radionuclide fraction using in-situ filtration pumps equipped with 0.4 µm Nuclepore filters. The filtered water volume varied between 29.9 l and 710 l. Filters were sealed in plastic bags and transported to the home lab. Undisturbed surface sediment samples (0–1 cm) were obtained by multicorer and stored in plastic bags. 3.2. Purification methods for radionuclide analyses In the home lab 7 N HNO3 was carefully added to the thorium water sample to partially re-dissolve the precipitate and thus reduce the amount of precipitate. Samples had to be shaken after each acid addition in order to re-precipitate the nuclides released by the dissolved precipitate. The remaining precipitate was transferred into 50 ml

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Fig. 1. Locations investigated in the Angola and Cape basins; circles: sediment sampling; diamonds: water column and sediment sampling. Arrows indicate flow paths of North Atlantic Deep Water (NADW) (water depth N 1200 m, chequered arrows) and Antarctic Bottom Water (AABW) (N3800 m, black arrows) in the Angola and Cape basins (after Stramma and England, 1999; Arhan et al., 2003); the Namib Col allows NADW from the Angola Basin to enter the Cape Basin.

centrifuge tubes and further reduced by nitric acid addition to 2–5 ml. Samples were centrifuged, decanted and dissolved in 25 ml of 7 N HNO3. The main goal of the anion exchange column separation is the removal of Mg. Therefore BioRad 10 ml polypropylene columns were loaded with 2 ml BioRad AG 1 × 8 resin. The resin and column were cleaned twice (20 ml 7 N HNO3, 20 ml 7 N HCl, 20 ml H2O) and preconditioned (20 ml 7 N HNO3). The dissolved sample was put onto the column. Mg passes through the column and was completely removed with 20 ml 7 N HNO3. Thorium was eluted with 10 ml 7 N HCl into a Teflon beaker, and 50 µl of HClO4 was added to dissolve any residual resin. The solution was evaporated to dryness. After addition of 3 ml 5% HNO3 the samples were ready for the ICP-MS analysis. For the determination of total (dissolved and particulate) 231Pa about 8 l of seawater was spiked with appropriate amounts of 233Pa. The 233Pa spike was prepared by milking a 237Np solution using a column containing silica gel (R. F. Anderson, pers comm.). The 233Pa was eluted using 2 N HNO3 + 0.08 N HF and the solution was calibrated using UREM 11 uranium reference ore. The purification procedure for protactinium in seawater followed that for thorium with the exception that the sample was put over the column two times and that protactinium was eluted from the column with a mixture of 7 N HCl + 0.13 nHF.

For the determination of particulate 230Th, 232Th and 231Pa the Nuclepore filters were dissolved in HNO3 followed by further dissolution in a mixture of HF/HNO3/HCLO4. About 10% (amount gravimetrically determined) of the solution was used for measurements of thorium isotopes and this aliquot was spiked with appropriate amounts of 229Th. The remaining aliquot was spiked with 233Pa and was used for 231Pa determination. Further purification procedures were as described above. The determination of 232Th, 230Th and 231Pa in sediments followed the purification methods described previously using 229Th and 233Pa as yield monitors (same monitors as for the water samples) (Scholten et al., 2005). Measurements were conducted by alpha spectrometry (thorium isotopes and 231Pa) and by gamma counting (233Pa). The whole sample preparations were carried out in a clean lab environment. Ultra pure water (18.2 MΩ), suprapure HClO4, subboiled distilled HCl, HF and HNO3 were used for all chemical treatments. The total blank correction for thorium was between 0.5% and 10% of the total thorium content and for protactinium this correction was between 0.2 and 1% of the total concentration. The chemical yield for the procedures described above was between 70% and 90% for thorium and 50 and 80% for protactinium.

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Fig. 2. Vertical section of salinity along the transect from the Angola Basin (left) to the Cape Basin (right) obtained during Meteor M48/4 (using “Ocean Data View” Schlitzer et al., 2003). Location of section is displayed in the inset map. NADW fills the Angola Basin below ~ 1200 m and between ~ 1200m and ~ 3800 m in the Cape Basin; small differences in salinity of NADW between Angola and the Cape basins occur; the relatively high salinity (~ 34.88) in the Cape Basin in ~ 2200 m is associated with the Namib Col Current; AABW is present in the Cape Basin below 3800 m; locations of CTD profiles are indicated by vertical lines.

3.3. ICP-MS measurements The measurements were performed on a Micromass Plasma Trace 2, a double focussing ICP-MS with reverse Nier-Johnson geometry. Fully adjustable source and collector slits provide the opportunity to use this instrument in varying mass resolution modes (400…10,000Res,10% valley

definition). Desolvation was achieved with a Cetac MCN-6000. A self aspiring microconcentric glass nebulizer (“MicroMist”100 µl) with 120 µl/ min sample uptake rate introduces the aerosol into a heated PFA spray chamber. The aerosol is desolvated by a heated membrane and introduced into the plasma. For a more effective cone cooling Ni cones with a Cu core (Chilton RAC705 and RAC19) were used. Typical operating parameters and

Fig. 3. Concentration–depth profiles of total (dissolved and particulate) 230Thex, total 232Th and total average231Pa profile are from Henderson and Anderson (2003) and references therein.

231

Paex (with 2σ error bars) at the locations investigated; data for the global

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backgrounds are given in the Table S1 in the Appendix. The nebulizer and the spraychamber of the Cetac MCN6000 were cleaned after each measurement by uptake of 5% HNO3. Measurements of thorium isotopes were carried out in 400Res mass resolution modes to ensure maximum sensitivity. For seawater samples with their relatively high 230Th/232Th mass ratio of approx. 10− 4 abundance sensitivity is not critical. 229Th, 230Th and 232Th were measured in one scan. Each scan contained 4 points in the centre of the flat top region (centre±0.03 amu) of each peak. Integration time for one point was 1 s for 229Th and 232Th and 4 s for 230Th. About 50 scans were recorded for one measurement. Thus resulting acquisition time for one sample was about 25 min. A complete wash-out was controlled in order to avoid memory effects. For 231Pa determination 233Pa and 231Pa were measured in one scan. The instrument was used in 1000Res mass resolution mode to provide better abundance sensitivity (2⁎10− 6 1 amu to the high and low mass side) and to reduce possible peak tailing effects caused by the high 232Th beam. Integration times were 4 s per point for both Pa masses. 4 points were measured during each scan. 30 scans were measured for one analysis, starting with pure wash solution to obtain the actual background (about 5–8 scans), continued by the sample running for about 20 scans. Protactinium measurements were conducted within one day after column separation at the latest to minimize ingrowths of 233U from the

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decay of 233Pa. Nevertheless, 232Th and 235U were monitored in each sample to ensure the successful separation of uranium and thorium from the protactinium sample solution. None of the sample data presented was influenced by Th and U leakages into the Pa fraction. All measured 230Th and 231Pa concentrations (quoted as activities and activity ratios throughout the text) were corrected for detrital, Usupported 230Th and 231Pa concentrations, to obtain excess activities (230Thex and 231Paex) as follows: 230

 Thex ¼ 230 Thm  0:7F0:24232 ThÞ

ð1Þ

231

 Paex ¼ 231 Pam  0:0464 0:7F0:24232 ThÞ

ð2Þ

230

Thm, 231Pam and 232Th are the measured activities, 0.7 ± 0.2 is the average 238U/232Th ratio in detrital material (Marcantonio et al., 2001) and 0.046 is the natural abundance of 235U relative to 238U. The results of the measurements are listed in Tables S2 and S3 in the Appendix. All errors stated do include the individual contributions of spike calibration, blank subtraction, internal precision of the mass spectrometric measurements and weighting errors propagated as root of the sum of squares.

Fig. 4. 231Paex/230Thex ratios in surface sediments; grey circles are 231Paex/230Thex ratios higher than the production rate (PPa/Th) of these isotopes in the water column (PPa/Th = 0.093), black circles are 231Paex/230Thex ratios lower than PPa/Th.

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4. Results 4.1.

230

Thex in the water column

The total 230Thex concentrations in the water column at the locations investigated range between 0.125 dpm/m3 (2.74 fg/kg) and 2.96 dpm/m3 (65.0 fg/kg) (Fig. 3, Table S2). The total 232Th vary between 0.01 dpm/m3 (48.5 fg/kg) and 0.20 dpm/m3 (824 fg/kg) with highest concentrations observed in the upper (b1000 m) water column. Particulate 230Thex amounts between 2% and 44% of the total 230Thex concentrations. The continuous increase of total 230Thex concentrations with water depth observed in the area investigated (Fig. 3) is consistent with previous observations in other ocean regions (Rutgers van der Loeff and Berger, 1993; Scholten et al., 1995; Nozaki et al., 1987) and can be explained by the reversible scavenging model (Bacon and Anderson, 1982). This model neglects horizontal advection and the total 230Th concentration (C [dpm/m3]) is a function of an “apparent” sinking rate (S [m/y]): C¼

P z STK

ð3Þ

with P =production rate of 230Th in the water column (0.0252 dpm/m2 y), K =ratio of particulate and total 230Thex concentrations, and z = water depth [m]. From our data, excluding the high concentrations N4500 m (Fig. 3, locations 470, 471, 472, see discussion below), we obtain an average K = 0.10 and the resulting apparent sinking rate of ~800 m/y is within the range (500–1000 m/y) previously observed in other ocean regions (Rutgers van der Loeff and Berger, 1993; Scholten et al., 1995).

Fig. 6. Fractionation factors versus water depths; the fractionation factors calculated using 231Paex/230Thex ratios in suspended particles, FTh/Pa, shows a broad maximum between ~ 1000 m and 4400 m whereas the fractionation factors calculated using the sedimentary 231Paex/230Thex, FSed, increase with water depth.

the 231Pa distribution in deep waters is more influenced by lateral advection of water masses causing the 231Pa profile to deviate from the scavenging profile. 4.3.

4.2.

231

231

Paex/230Thex ratios in suspended particles and surface sediments

Paex in the water column 231

3

Paex concentrations range between 0.15 dpm/m The total (1.43 fg/kg) and 0.74 dpm/m3 (7.06 fg/kg) (Fig. 3, Table S2). The particulate 231Paex concentrations vary in between 0.3% and 4% of the total 231Paex concentrations. The vertical profiles of the total 231Paex display in the upper ~ 1500 m an increase with water depth i.e. a scavenging type distribution profile like 230Thex, whereas in deep water the concentrations show no clear trend and remain relatively constant. This structure and the concentration range of the measured total 231Paex profiles is comparable to the globally average 231Pa profile (Fig. 3; calculation and data from (Henderson and Anderson, 2003 and references therein). As a result of longer scavenging residence times

The 231Paex/230Thex ratios on suspended particles vary between 0.029 and 0.157 and between 0.023 and 0.137 in surface sediments (Table S3). The regional distribution of 231Paex/230Thex in surface sediments shows higher ratios, i.e. higher than the production rate (P) of these isotopes (PPa/Th = 0.093) at the ocean margin and at one location (1726) located on the Walvis Ridge, and lower ones in the open ocean (Fig. 4). A plot of all particulate (sediments and suspended particles) 231Paex/ 230 Thex ratios versus water depth (Fig. 5) indicates that N2500 m ratios on suspended particles and in surface sediments agree quite well (exemption: suspended particles at station 471: 4600 m, and station 468: 4800 m; surface sediment GKG 1024-3). They are generally higher in sediments than on suspended particles in water depths b2500 m (exemption station 468: 600 m). These differences may be explained by insufficient isotope equilibration between dissolved and suspended phases. As shown by Thomas et al. (2006) particles have to sink at least through a ~1000 m water column before they equilibrate with the dissolved 231Pa and 230Th concentrations of ambient waters. Surface sediments, in contrast, are in contact with the ambient bottom water for several hundred years (depending on sediment accumulation rates) allowing sufficient time for equilibration. As the gradients in the dissolved 230Th and 231Pa concentrations are steepest in the upper water-column differences between 231Paex/230Thex ratios on suspended particles and in surface sediments are expected to be most pronounced in the upper water column. 4.4. Fractionation factor The role of particle composition in the fractionation of 231Pa and Th can be quantified by the fractionation factor (FTh/Pa) (Anderson et al., 1983): 230

Fig. 5. Depth distribution of 231Paex/230Thex ratios on suspended particles and in surface sediments; below ~ 2500 m the ratios on suspended and in surface sediments match quite well.

FðTh=PaÞ ¼

Thpart =Papart Thdiss =Padiss

ð4Þ

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where Papart and Thpart are the suspended particle 231Paex and 230Thex concentrations in the water column, Padiss and Thdiss are the dissolved 231 Paex and 230Thex concentrations. For our data F(Th/Pa) varies in between 3 and 21 (average F(Th/Pa) ~12) with low values in the upper (~b1000 m) water column (stations 468 and 476) and a broad maximum between 1000 m and 4400 m (Fig. 6). Previous studies found F(Th/Pa) ~ 3 for the Labrador Sea and for the South Atlantic gyre; higher values were reported from the Equator (F(Th/Pa) ~ 11, (Moran et al., 2002)). These authors also note an increase in the F(Th/Pa) in the upper water column which they attribute to a change in the particle composition. The fractionation factor is calculated with the assumption of equilibrium conditions between both the dissolved and the particulate 231Paex/230Thex ratios. As already outlined above this is not necessarily the case. To overcome this problem we calculated the fractionation factor F(Sed) using the 231Paex/230Thex ratios in surface sediments instead of those on suspended particles (G. Henderson, pers comm.). We used 231Paex/230Thex of those surface sediments which where located +/−~300 m within the depth range of the dissolved ratio. When several sediment locations fulfilled this criterion we used the one nearest to the water-column location. The resulting F(Sed) shows a continuous increase with water depth (Fig. 6). 5. Discussion 5.1. Relation between water masses and total

230

Thex distribution

We plotted the total 230Thex concentrations in the water column in a vertical section in order to elaborate the regional differences (Fig. 7; using the program “Ocean Data View” (Schlitzer et al., 2003). This

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presentation of the 230Thex data displays several major characteristics of the total 230Thex distribution: In the upper water column (b1200 m) along the section no obvious horizontal gradients in the total 230Thex concentrations can be observed (Fig. 7). This section covers near-shore (locations 465, 476, 482) as well as off-shore areas (locations 468–471). We would expect differences in scavenging rates of 230Th caused by differences in the particle fluxes between these areas, e.g. lower total 230Thex concentrations in high particle flux areas (ocean margins). The fact that no such differences occur suggests that differences in the particle flux are not sufficient to maintain horizontal gradients of total 230Thex. At the pelagic locations in the Angola Basin (stations 467–469) total 230Thex in water depths between ~ 2000 m and 3000 m (NADW) is higher (average 230Thex = 1.04 +/− 0.05 dpm/m3) than south of the Walvis Ridge in the Cape Basin in similar water depths (stations 470, 471, average 230Thex = 0.83 +/− 0.06 dpm/m3). The slightly higher salinity of NADW in the Angola Basin compared to the NADW in the Cape Basin (Fig. 2) indicates that this difference is most likely related to different water mass characteristics. In the southern Angola Basin NADW is a mixture of two components: NADW which is advected from the northern Angola Basin and which represents a relative “old” NADW component and NADW advected from the West Atlantic which represents an relatively “young” NADW (Stramma and England, 1999). In the Cape Basin NADW consists only of the component advected directly from the West Atlantic (Stramma and England, 1999). Investigations of the meridional distribution of 230Th in the Atlantic show low 230Th concentrations in newly formed water masses in the Labrador Sea (Moran et al., 2002). Along the flow path of NADW towards the south the concentrations increase with water

Fig. 7. Vertical section of total 230Thex concentrations along the transect from the Angola Basin (stations 465–469) to the Cape Basin (stations 470–482); the location of the transect is displayed in the inset map. Differences in the concentrations in 2000 m–3000 m water depth (NADW) between the Angola and the Cape basins are attributed to differences in the water mass properties of NADW; the relatively high concentrations in about 2200 m water depth at stations 474 and 476 are associated with the Namib Col Current; the high total 230 Thex concentrations in the deep (N 4000 m) Cape Basin are found in water masses influenced by AABW.

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mass age. This increase is caused by ingrowth of 230Th from the decay of 234U; and several 230Th residence times (τ = 230Th: 20–40y) are required to reach a steady-state between production and removal of 230 Th through scavenging. If we assume the relation between water mass age and 230Th concentration in NADW to hold also for the Southeast Atlantic the different 230Th concentrations suggest that NADW in the Angola Basin is relatively older than in the Cape Basin. At locations in the Cape Basin near the continental margin (stations 474 and 476) in about 2000 m water depth elevated total 230Thex concentrations occur (Fig. 7). A possible explanation would be resuspension of ocean margin sediments. Such a process would increase the particulate fraction of 230Th as well as the 232Th concentration, the latter isotope being closely associated with detrital sediment particles (Huh et al., 1989). However, neither the particulate 230Thex concentrations nor the distribution of 232Th (Fig. 3) indicate that resuspension of sediments is responsible for these high total 230Thex concentrations. These high concentrations are associated with a water mass having salinity around 34.88 (Fig. 2). Warren and Speer (1991) point out that such high salinities in the north-eastern Cape Basin are related to the Namib Col Current which transport NADW from the Angola Basin to the Cape Basin. Thus, the high 230Th concentrations can be interpreted to be due to advection of NADW from the Angola basin which, as already discussed above, have slightly higher 230Th concentrations. In deep (N4500 m) waters in the Cape Basin at locations 470, 471 and 472 total 230Thex increases sharply with maximum concentrations of up to 2.96 dpm/m3 (Figs. 3, 7). According to the reversible scavenging model (3) such a high concentration would correspond to a water depth of about 8000 m, a depth not existent in the Atlantic and Southern Oceans. Sediments resuspended from the seafloor due to e.g. bottom nepheloid layers (McCave, 1986) may cause elevated concentrations of particulate 230Thex and 232Th. For those stations showing the high 230Thex concentrations in deep waters (470, 471, 472) only one particulate 230Thex data point is available (station 471, 4500 m water depth) but this does not show an increase of particulate 230Thex. The total 232Th concentrations at locations 470–471 are in the same range as at the others (e. g. 467 and 469, Fig. 3); so we have no indication that enhanced resuspension is responsible for the high deep-water 230Thex concentrations. An alternative explanation is advection of 230Th enriched water masses. The high total 230Thex concentrations observed in the Cape Basin are found in water depths influenced by AABW. As already pointed out, this water mass originates in the ACC. Several studies indicated that the ACC receives 230Th enriched waters from the Weddell Sea (Rutgers van der Loeff and Berger, 1993; Walter et al., 2000). Thus it is possible that the high total 230Thex concentrations observed in the Cape Basin result from the export of 230Th from the Weddell Sea. All the water-column 230Th concentrations available so far from the Southern Ocean are, however, lower than those observed in the deep Cape Basin. Thus, the origin of the 230 Th enriched waters remains unclear. 5.2. Controls of sedimentary

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Pa/230Th ratios

The 231Pa/230Th ratios in sediments are constrained by the fractionation between 231Pa and 230Th and by the dissolved 231Pa/ 230 Th ratio in the water column. The fractionation is determined by the composition of the particle flux. Variable dissolved ratios can be due to differences in the scavenging intensities (e.g. boundary scavenging) or due to differences in the isotope concentrations of water masses. In our study the spatial distribution of high 231Paex/230Thex ratios in surface sediments at the ocean margin off Namibia and lower ratios at open ocean sites (Fig. 4) is comparable to those observed in the Pacific and Arabian Sea. In those regions extensive scavenging of 230Th and 231Pa at the ocean margins is caused by high particle flux (boundary scavenging) (Scholten et al., 2005; Anderson et al., 1990; Walter et al., 1999). As a result the 230Th and 231Pa concentrations in the water column

Fig. 8. Relations between water depths and 231Paex/230Thex ratios in surface sediments: a) Southeast Atlantic (this study); b) Pacific and Arabian Sea. Negative relations with comparable gradients are found for all data sets; data for the Arabian Sea from (Scholten et al., 2005); data for the Pacific from (Lao et al., 1992) and (Yang et al., 1986).

are lower at the ocean margin compared to the open ocean. Such regional differences are not observed in our total 230Thex and 231Paex concentration–depth profiles off Namibia (Figs. 3, 7) and thus they do not indicate extensive scavenging off the Namibian margin. There are further observations questioning boundary scavenging to be the appropriate explanation for the observed high 231Paex/230Thex in sediments. Whereas in areas of boundary scavenging like the Arabian Sea the 231Paex/230Thex ratios in surface sediments gradually decrease from the margin towards the open ocean (Scholten et al., 2005) we observe off Namibia a rapid change in the sedimentary ratios between nearby stations (e.g. by a factor 2 between location 3718 (231Paex/230Thex =0.11) and location 3720 (231Paex/230Thex = 0.05); Figs.1, 4). Furthermore 231Paex/230Thex N 0.093 are restricted to water depths ~b2000 m only whereas in the Arabian Sea ratios N0.93 are also observed off the shelf in deep waters. The 231Paex/230Thex ratios N0.093 may result from preferential scavenging of 231Pa over 230Th caused by high opal abundances. According to Bremner (1983) surface sediments at the inner shelf off Namibia are dominated by opal whereas further off-shore biogenic carbonate is the main particle type. Particle flux studies at the Walvis Ridge (near our station 465) and at the shelf off Walvis Bay revealed opal concentrations ranging between 2 and 19% (Treppke et al., 1996; Giraudeau et al., 2000). According to the relation between opal concentrations in sediment traps and fractionation between 231Pa and 230 Th (Scholten et al., 2005) opal concentrations of ~ 50% would result in a fractionation factor ~ 3 which is near to the FSed ~ 3–7 we observe in the upper water column (b2000 m) off Namibia (Fig. 6). Thus

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sedimentary 231Paex/230Thex N 0.093 at the Namibian margin are most likely not a result of boundary scavenging but due to low fractionation caused by high opal abundances. There is a close negative correlation (r2 = 0.82) between 231Paex/ 230 Thex ratios in sediments and water depth (Fig. 8a). As outlined above such a relationship requires either the dissolved 231Paex/230Thex or the fractionation factor to change accordingly. The dissolved 231 Paex/230Thex ratio remains relatively constant between ~ 1000 m and ~3500 m (Fig. 9) whereas the fractionation factor (FSed) increases with water depth (Fig. 6). This suggests that changes in the particle composition are responsible for the decrease of sedimentary 231Paex/ 230 Thex. A decrease in particulate 231Paex/230Thex ratios with increasing water depths is also reported from the Arabian Sea (Scholten et al., 2005) and from the Southern Ocean (Walter et al., 2000). Furthermore we find negative relationships between 231Paex/230Thex ratios in surface sediments and water depths in the Pacific (data sets from Lao et al., 1992; Yang et al., 1986) and in the Arabian Sea (data from Scholten et al., 2005) (Fig. 8b); however, the correlations are less strong here (R2 = 0.19), but the slope of the best fit regression line is quite similar for all data sets. It is not clear to what extent these correlations are biased by unrepresentative sampling of the areas considered. Nevertheless, it seems that in all data sets considered the sedimentary 231Pa/230Th ratios depend to some extent on the water depth. A possible explanation for this relationship may be changes in the particle composition with depth. It is well known that most of the sinking particle pool is remineralized during its descent in the water column (between 75% and 94%, Boyd and Trull, 2007). For instance, opal is strongly affected by dissolution as it is composed of biogenic silicate which is undersaturated with respect to seawater and thus opal tends to dissolve during settling through the water column (Nelson et al., 1995). Such a reduction in the opal content would probably increase the fractionation with water depth causing the observed depth-dependency of 231Paex/230Thex ratios in sediments. This interpretation needs, however, further verification by more detailed studies on changes in particle composition and fractionation with water depth. 5.3. Meridional transport of

231

Pa

When comparing the 231Paex concentrations along the meridional flow path of NADW starting in the Labrador Sea, over the Charly–Gibbs Fracture Zone to the western South Atlantic (IOC-8, 17°S, 25°W) and the Angola and Cape basins we find a successive increase in the deep-

Fig. 9. Dissolved 231Pa/230Th ratios versus water depth in the Southeast Atlantic; between ~ 1000 m −3500 m the ratios remain relatively constant.

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Fig. 10. Water-column concentration–depth profiles of total 231Paex. The increase of total 231 Paex concentrations along the flow path of NADW (Labrador Sea → Charly Gibbs Fracture Zone → Equatorial Atlantic → Angola/Cape basins) is a reflection of the export of 231Pa towards the south (data from Labrador Sea and Equatorial Atlantic: (Moran et al., 2002); data from the Charly–Gibbs Fracture Zone: own unpublished data).

water (~ N1200 m) concentrations with highest concentrations in the Southeast Atlantic (Fig. 10). This increase can be explained by an export of 231Pa from the northern latitudes towards the south and is due to the relatively rapid water mass advection rates in comparison to the scavenging rates of 231Pa (Yu et al., 1996; Moran et al., 2002). This export of 231Pa causes a successive increase in the average dissolved 231Paex/230Thex ratios with lowest in the Labrador Sea (N1000 m average dissolved 231Pa/230Th = 0.27 +/− 0.03, data from Moran et al., 2002) increasing towards the equatorial and western South Atlantic (N1000 m average dissolved 231Pa/230Th = 0.38 +/− 0.10, stations IOC-6, IOC-RFZ, IOC-8 in Moran et al., 2002) and the highest ratio in NADW in the Southeast Atlantic (1200 m–3500 m average dissolved 231Paex/230Thex = 0.70 +/− 0.1). These changes in the dissolved ratios should be reflected in the 231Paex/230Thex ratios of surface sediments if, as previously proposed (McManus et al., 2004; Yu et al., 1996), this isotope ratio is sensitive to the meridional circulation in the Atlantic, i.e. to the export of 231Pa from the North Atlantic. For the area north of 40°N covering the Labrador Sea and the Iceland Basin recent investigations revealed an average 231Paex/230Thex ratio in surface sediments near the production ratio of these isotopes (231Paex/230Thex ~0.094; R. F. Anderson, pers comm.). This ratio is higher than previous estimates for the Atlantic (average 231Paex/230Thex = 0.065 +/− 0.005, for sediments north of 45°S Yu et al., 1996; Marchal et al., 2000) but these studies did not include data from in the Labrador Sea and Iceland Basin. In the Southeast Atlantic we find an average 231Paex/ 230 Thex = 0.052 +/− 0.023 for sediments N2000 m water depths (for sediments in NADW average 231Paex/230Thex = 0.061 +/− 0.031). So there is a decrease in the average sedimentary ratios between the Labrador Sea and the Southeast Atlantic. The comparatively higher ratio in the Labrador Sea can be explained by a lower fractionation factor in the Labrador Sea (FTh/Pa ~ 3) than in the Southeast Atlantic (average fractionation factor N1200 m: FSed ~ 12; FTh/Pa ~ 14). Thus, the relatively high dissolved 231Paex/230Thex in the Southeast Atlantic does not cause higher sedimentary ratios because of the strong fractionation here. It seems that fractionation compensates for changes in the dissolved 231 Paex/230Thex between the Labrador Sea and the Southeast Atlantic. The important point is that the differences in sedimentary 231Paex/ 230 Thex ratios between the Labrador Sea and the Southeast Atlantic

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can solely be explained by different fractionation and that variable sedimentary 231Paex/230Thex ratios cannot unambiguously be related to the export of 231Pa, i.e. the meridional circulation. 6. Conclusions We investigated the distribution of 230Th and 231Pa in the water column and in surface sediments in the Angola and Cape basins in order to reveal the relative importance of scavenging and/or water mass advection for the distribution of these radionuclides. The vertical section of total 230Thex in the water column covering the Angola and Cape basins indicates that even small differences in the salinity of NADW between the Angola and Cape basins caused by variable water mass flow paths and water mass ages are mirrored in the total 230Thex distribution. This result indicates that the 230Th distribution in the water column is much more controlled by hydrography than previously suggested. The very high total 230Thex concentrations in AABW in the Cape Basin are interpreted to originate from the lateral advection of 230Th enriched water masses (AABW) from the ACC. However, as such high concentrations have not yet been observed anywhere in the Southern Ocean the source of this 230Th enrichment remains obscure. High 231Paex/230Thex ratios in sediments, i.e. ratios exceeding the production ratio of these isotopes (PPa/Th N 0.093) in the water column are found at locations at the ocean margin and ratios lower than the production ratio at open ocean sites. In previous studies such a distribution was interpreted to be due to boundary scavenging. However, the lack of any significant depletion of 231Pa and 230Th in the water column at the near-shore sites suggests that boundary scavenging is not very intense in the area investigated. Instead low fractionation between 230Th and 231Pa in shallow water depths (b2000 m) is believed to have caused high 231Paex/230Thex ratios at the ocean margin. The close relation between the 231Paex/230Thex ratios in sediments and water depth observed in our data set is possibly a reflection of the changes in the particle composition with water depth. The total 231Paex concentrations in NADW in the Southeast Atlantic are higher than those observed upstream in the western South and North Atlantic (Labrador Sea) which is a result of an export of 231Pa from northern latitudes. This export causes a relatively high average dissolved 231 Pa/230Th ratio in the area investigated (231Pa/230Th= 0.70 +/− 0.1) compared to the Labrador Sea (231Pa/230Th= 0.27 +/− 0.03, Moran et al., 2002). The differences in the dissolved ratios are not reflected in surface sediments: in the area investigated the average 231Paex/230Thex ratio = 0.052 +/− 0.023 (for sediments N2000 m water depths) is lower compared to the Labrador Sea and Iceland Basin (average 231Paex/ 230 Thex ~ 0.094; R. F. Anderson, pers. comm.). Thus, the differences in dissolved ratios are not mirrored in the sediments because fractionation between 231Pa and 230Th is lower in the Labrador Sea (FTh/Pa ~ 3, Moran et al., 2002) than in the Southeast Atlantic (FTh/Pa ~ 14). Therefore the different fractionation compensates for changes in the dissolved 231Pa/ 230 Th ratios. This suggests that fractionation between 231Pa and 230Th is more important in determining the 231Paex/230Thex ratios in sediments of the Atlantic Ocean than the dissolved ratios and the export of 231Pa from the North Atlantic. This result challenges the use of 231Paex/230Thex ratios in sediments to serve as a proxy for the intensity of the meridional circulation in the Atlantic. Variable 231Paex/230Thex ratios in sediments during the geological past can only be interpreted as indicative for changes in the meridional circulation in cases where fractionation between 231Pa and 230Th remains relatively constant through time. Techniques to estimated fractionation from sediment cores during the geological past have to be developed yet. Acknowledgements We thank the captain and the crew of METEOR 48/4 for their support during the expedition. We are grateful to W. Balzer, S. Reuter,

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