234Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE

Deep-Sea Research II 58 (2011) 2749–2766

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234

Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE Michiel Rutgers van der Loeff a,n, Pinghe H. Cai a,b, Ingrid Stimac a, Astrid Bracher a, Rob Middag c, Maarten B. Klunder c, Steven M.A.C. van Heuven c a b c

Alfred-Wegener Institute for Polar and Marine Research, PO Box 120161, D 27515 Bremerhaven, Germany State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, The Netherlands

a r t i c l e i n f o

abstract

Article history: Received 23 February 2011 Accepted 23 February 2011 Available online 3 March 2011

As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100 m, derived from the depletion of 234Th with respect to its parent 238 U, ranged from 621 7 105 to 1773 7 90 dpm m  2 d  1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2 mmol C m  2 d  1 (2.1–9.0 mmol C m  2 d  1) using POC/234Th ratio of bulk (respectively 4 50 mm) suspended particles at the export depth (100 m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–581S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–651S as studied during the previous leg had caused a high export flux at 64.51S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–691S evident from uptake of CO2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Thorium isotopes Trace elements Carbon cycle Algal blooms Export production GEOTRACES Southern Ocean Antarctic Circumpolar Current

1. Introduction Particle flux is a major factor in biogeochemical cycles. For the interpretation of the distribution of trace elements in the ocean, information on the geographical distribution of particle rain is therefore badly needed. Such information is available in a variety of time and space scales. n Correspondence address: Alfred-Wegener Institute for Polar and Marine Research, PO Box 120161, D 27570 Bremerhaven, Germany. Tel.: þ49 471 4831 1259; fax: þ49 471 4831 1425. E-mail address: [email protected] (M. Rutgers van der Loeff).

0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.02.004

The geological record shows us rain rates averaged over hundreds to thousands of years. The sedimentary record in the southern Atlantic shows large latitudinal changes across the Antarctic Circumpolar Current with maximum flux in the Last Glacial Maximum north of the present position of the Antarctic Polar Front (PF) shifting to a present maximum located south of the PF (Frank et al., 2000; Kumar et al., 1995). Most studies agree on very low present particle fluxes in the central Weddell Sea, supported by radionuclide data (Walter et al., 2000). Whereas we know that the sedimentary record is biased by sediment redistribution, a map of 230Th-based present particle rain rates supports the distribution with maximum rain rates in the zone

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between the PF and the Southern ACC Front and very low rain rates in the central Weddell Sea (Geibert et al., 2005). However: in the Weddell Sea a contradiction was observed between surface data and sedimentary record (Leynaert et al., 1993) which may be related to atypically shallow mineralization in this area (Usbeck et al., 2002). Averages over years to decades are obtained from compilations of hydrographic data. Schlitzer (2000) used inverse modeling to derive present net nutrient uptake and consequently POC export production from published oxygen, nutrient and carbon distributions. His analysis supports an enhanced production and export in the ACC around 45–551S at the zero meridian. The Dahlem map of global productivity (Berger, 1989), based on 14C primary production measurements, phosphate utilization and satellite observations, gives a productivity maximum in the 50–601S latitude band in the SE Atlantic. Distributions in a similar time frame are obtained from measurements of benthic fluxes (Jahnke, 1996) and oxygen penetration depth in the sediment (Sachs et al., 2009). The seasonal evolution of particle fluxes is shown by sediment trap deployments, from the high fluxes near the Polar Front to the extremely low fluxes in the Weddell Sea (Fischer et al., 1988; Fischer et al., 2000). Satellites give us the pigment distribution in the ocean on a daily basis, which can be converted to primary production rates (Antoine et al., 1996; Behrenfeld and Falkowski, 1997). Satellites frequently show thin filaments of high phytoplankton biomass sometimes associated with the fronts, as one might expect related to mesoscale eddies causing local upwelling of nutrients and shallow mixed layers (Strass et al., 2002). Export events of such filaments can even be observed on the sediment (Sachs et al., 2009). However Sokolov and Rintoul (2007), in their investigation of the relationship between satellite Chl-a and the location of fronts, find that productive regions are separated by fronts rather than associated with them, similar to the findings from the geological record. They also show the major importance of seafloor topography in stimulating production through upwelling (cf. KEOPS, Blain et al., 2007; CROZEX, Pollard et al., 2009).

For the modeling of the distribution of tracers in the deep ocean, where even the most particle reactive elements have residence times of the order of decades, the multi-year average rain rate fields are appropriate. But for the understanding of trace element behavior in the surface ocean where wax and wane of plankton blooms occur in time scales of days or weeks we need flux information at higher time and space resolution. This information cannot be obtained from the literature but must be measured along with the studies of the other trace elements. Fluxes based on 234Th disequilibrium have been measured in several Southern Ocean studies and were summarized by Savoye et al. (2008). During the expedition ZERO and Drake an extensive GEOTRACES program was executed in the Atlantic sector of the Southern Ocean along the Zero Meridian and across the Drake Passage. A major objective of the GEOTRACES program is the contemporaneous determination of a range of trace elements and isotopes in order to enable an integrated interpretation. We report here the distribution of 234Th and of the export rate of 234Th and of POC during this expedition. We investigate to what extent the 234Th-based fluxes are related to the development of phytoplankton as seen from space and to the removal of trace metals, notably iron and manganese, from the surface water.

2. Methods 2.1. Sampling The cruise track (Fig. 1) of Polarstern Expedition ANTXXIV/3 (February–March 2008, Fahrbach and De Baar, 2010) joined the zero meridian at 521S and followed this meridian to the ice shelf at 701S (Fig. 1). At this place the floating shelf ice extends far north, an extension of the Fimbul Ice Shelf called Trolltunga. Whereas the sea ice on the zero meridian section had disappeared before our expedition, the crossing of the Weddell Sea to the tip of the Antarctic Peninsula was influenced by sea ice which covered the eastern (up to 251W) and western (from 431W) part of the

Cape Town

102

SAF 104

PF 108

Punta Arenas

SAF 244 PF Drake Passage

SB ACC

113 118 125

SACCF SB ACC

131

230

140 210 222

201 204

Weddell Sea 196

149 157 169 178

Fig. 1. Map of Cruise track of Polarstern expedition ANTXXV/3 ‘‘Zero and Drake’’, which started in Cape Town and after the first few stations joined the zero meridian, with approximate positions where the Sub Antarctic Front (SAF), Polar Front (PF), Southern ACC Front (SACCF) and Southern Boundary of the ACC (SB ACC) were crossed. Stations where 234Th was sampled (dots) indicated by their numbers.

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

section. The Drake Passage crossing was set by the mooring program of our LOCEAN colleagues (Provost et al., 2011), which follows the track of the Jason altimetry satellite. We also report data from the same Drake Passage section measured 2 yr earlier on the ANTXXIII/3 expedition (January–February 2006, Provost et al., 2011). Salinity and transmission were obtained from the oceanography team. Beam attenuation (c) was derived from transmission (T) according to the formula T/Tcw ¼exp(  cL) where Tcw ¼transmission in clear water taken as 0.99 and L is the length of the light path of the transmissometer, 25 cm. For graphics the ODV software package (Schlitzer, 2010) was used.

2.2. Procedure total

234

Th

Sampling, analysis, calibration and data reduction followed Cai et al. (2006b) and Pike et al. (2005). Four-liter sample was acidified to a pH of o2.0, spiked with a known amount of 230Th. After 12–24 h, the pH was then brought up to 8.15–8.30, thorium was coprecipitated on MnO2 by adding 0.25 ml KMnO4 (3.0 g/L) and 0.25 ml of MnCl2 (8.0 g MnCl2  4H2O/L). The samples were heated in a water bath at 490 1C for 3 h, cooled down to room temperature and the MnO2 precipitate was then filtered onto a 25 mm, 1.0 mm QMA filter. The QMA filter with MnO2 precipitate was dried overnight in an oven, mounted under a layer of Mylar film and a layer of Al foil (8.00 mg/cm2), and counted on an RISØ beta counter onboard. After 6 months, the background of the MnO2 precipitate was recounted in the home laboratory. For the analysis of thorium recovery, the QMA filter with MnO2 precipitate was dismounted, and a known amount of 229Th was added as a second spike. The MnO2 precipitate was dissolved in 8 M HNO3 þH2O2 solution and sonicated for 20 min. Thorium isotopes were isolated and purified using classical column exchange chemistry. The 230Th/229Th ratio was measured on an ICP-MS. The parent 238U activity is obtained from the relationship 238 U (dpm L  1)¼0.0713  salinity, given by Pates and Muir (2007) based on Chen et al. (1986) (cf. Rutgers van der Loeff et al., 2006). The associated error is about 3% (Pates and Muir, 2007). All data of total 234Th are presented in Appendix A and are available in PANGAEA (doi:10.1594/PANGAEA.745479). For each station, a volume of 8 L seawater was collected at 100 m depth in order to determine the POC/234Th ratio on suspended particles. At selected stations, large volume (200–1000 L) seawater was also sampled from 100 m depth using in-situ pumps (Challenger Oceanic) equipped with 142 mm diameter filter holders. Seawater was pumped sequentially through a 50 mm Nitex screen, a 10 mm Nitex screen and a 1.0 mm (nominal pore size) quartz fiber filter (QMA, Whatman), and two 10-in MnO2-impregnated cartridges for parallel studies on Ac and Ra isotopes. After sample collection, particles on the 50 and 10 mm pore size Nitex screens were resuspended by approximately 5 min ultrasonication in filtered seawater, a mild procedure to avoid too much break-up of the large particles before their re-collection on 47-mm, 1.0 mm QMA filters. The 142-mm QMA filters as well as the 47-mm QMA filters were dried overnight, and a 22-mm subsample was punched from each filter with a steel punch on an aluminum-coated plate and prepared for beta counting. As with total 234Th in seawater, particulate samples were beta counted for initial 234Th activities and later for final background. Detector calibration for the total and particulate 234Th samples was conducted using aged seawater as described by Cai et al. (2006a). As a check, deep water samples (41000 m) were analyzed and they showed a 234Th/238U ratio of 1.00170.038 (n¼5). The estimated precision of the method is 3% at 2.50 dpm/L; the overall accuracy is estimated at 0.10 dpm/L.

2.3. Particulate and dissolved

2751 234

Th in surface waters

In order to obtain a high-resolution distribution of 234Th in surface waters we applied the automated technique developed by Rutgers van der Loeff et al. (2006, 2004). Briefly, seawater from the ship’s seawater supply is filled into a 5-L container and filtered by air pressure through a 25 mm QMA filter. The filtrate is transferred to a column where a MnO2 precipitate is formed by addition of 1.2 ml 0.75 M ammonia, 2.4 ml KMnO4 (0.6 g/L) and 1.2 ml of MnCl2 (4 g MnCl2  4H2O/L). The MnO2 particles are allowed to grow during several hours before they are collected by filtration with a pneumatic pump over a second 25 mm QMA filter. Reported 234Th adsorption on QMA (Benitez-Nelson et al., 2001) may have shifted some activity from the dissolved to the particulate phase without affecting total activity. A second sample can be treated in parallel in a second column. Nevertheless, there is a trade-off between the requirements of high MnO2 recovery (long precipitation time) and spatial resolution. We selected a sampling frequency of once every 4 h, corresponding to a precipitate maturation time of 4 h. The MnO2 filter was rinsed with demineralized water. Both filters, one containing particulate matter and the other containing the MnO2 precipitate, were dried overnight in an oven, mounted under a layer of Mylar film and a layer of Al foil (8.00 mg/cm2), and counted for 700 min in an RISØ beta counter. Beta counting efficiency was determined from the procedure for total 234Th described above. The calibration of dissolved 234Th included the estimate of the precipitation efficiency of Th with MnO2. This efficiency was tested in two ways. First, we analyzed deep-water samples assumed to be in equilibrium with 238U. The precipitation efficiency of deep waters amounted to only 70.7 75.0% (n ¼5). Second, we performed duplicate analyses of surface water with the more accurate method of total 234Th using a yield tracer, described above. With this procedure we found a precipitation efficiency of 90 75% (n ¼9, see results of Weddell section below). We speculate that the seeding by a natural population of particles in surface waters, even passing the 1-mm QMA prefilter, speeds up the formation of the MnO2 precipitate, and we have used the higher precipitation efficiency for all surface water samples reported here. But clearly there remains an uncertainty in the cause of the depth dependence of precipitation efficiency. Thus, the higher spatial resolution achieved with the automated procedure was associated with a lower precision and accuracy. During the expedition ANTXXIII/3 (DRAKE) the automated procedure was used with 47-mm diameter filters in total mode, i.e. bypassing the first filter, thus yielding only total activities in order to improve counting statistics. The MnO2 precipitate together with other SPM was filtered over a 47 mm diameter QMA filter. 25-mm diameter subsamples were then counted in an RISØ beta counter. The efficiency of the MnO2 coprecipitation was checked with a 230Th spike, and turned out to be 9272% (n ¼4), comparable to the recovery found for surface waters from the Weddell Sea. The high recovery compared to deep-water calibration is likely due to the presence of natural particles as precipitation nuclei in these unfiltered surface samples.

2.4. Particulate organic carbon After beta counting, the particulate samples were then used to determine POC concentration. The POC concentrations were determined with a Eurovector C/N Element analyzer (Euroanalysator EA) according to the JGOFS protocols (Knap et al., 1996). All samples were treated with 0.1 M HCl solution to remove the carbonate phase. Each sample was corrected for a C blank. The C blank of the

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filter was less than 1.3 mmol C, which is generally o10% of the POC on the QMA filters. Based on replicate analyses, the precision for the POC determination was o5%. No correction was made for possible adsorption of DOC and 234Th on the QMA filters (Benitez-Nelson et al., 2001; Buesseler et al., 2006; Moran et al., 1999). 2.5. Calculation of

234

the SML was evaluated by Buesseler et al. (2005) and found to be insignificant. But if enhanced mixing causes a deepening of the SML this will not affect the integrated depletion. If we neglect advective and diffusive fluxes the export flux P follows from P ¼ lðAU ATh Þ ¼ lD

Th export flux

ð2Þ

2

The change in activity of (Savoye et al., 2006) by

234

Th in seawater is described

@ATh =@t ¼ lAU lATh P þ V

ð1Þ 238

234

where AU and ATh are the U and the total Th activities (expressed in dpm m  2), respectively, l is the decay constant of 234 Th ( ¼0.02876 d  1), P (expressed in dpm m  2 d  1) is the net removal flux of 234Th, and V is the sum of the advective and diffusive fluxes. The depletion of 234Th in the surface water is the cumulative result of export flux over the past several half lives of 234 Th. Flux variations in this time span can be significant, making a non-steady state calculation necessary (Buesseler et al., 1992; Savoye et al., 2006). Since for every station we have only one measurement in time, we will for a first calculation assume steady state. Advection and diffusion can play a role as the Antarctic Zone is a region of net upwelling. With an entrainment rate of deep water (with 234Th in equilibrium with 238U) into the surface mixed layer (SML) of the order of 50 m yr  1 (Geibert et al., 2003; Gordon and Huber, 1990) the 234Th deficit in a 50-m SML is flushed once per year (0.5 yr  1 in a 100-m SML), an order of magnitude slower than the 234Th ingrowth rate of 10.5 yr  1. Mixing at the bottom of

234

238

Where D (dpm m ) is the depletion of Th relative to U, which can be calculated in several ways (Moran et al., 2003). In order to calculate the export flux from the surface water, we integrate the deficiency D with trapezoidal integration. For the integration depth we choose 100 m, as the strongest depletion is confined to this layer (Table 1) and because the error in the derived flux rapidly increases when integration is proceeded to greater depths (Harada in Buesseler et al., 2007). We may use the history of ice distribution to make an estimate of the error we have introduced by making a steady state approach in the export calculation. The relation with ice cover, here and in earlier studies in the Antarctic (Rutgers van der Loeff and Berger, 1991) and in the Arctic (Cai et al., 2010; Moran et al., 1997), shows that Th is largely in equilibrium with U as long as the sea is ice-covered, although in the present study even under a closed ice cover some disequilibrium was observed (Table 1). We know that ice algae may accumulate 234Th (Rutgers van der Loeff et al., 2002). Upon ice melt, the release of this accumulated 234Th may cause locally an excess of 234Th which can bias calculation of export fluxes (Rodriguez y Baena et al., 2008). As this algaeaccumulated 234Th must have been derived from ice-enclosed seawater and from the water column below, we feel that it is unlikely that the process would produce disequilibrium after full

Table 1 Station list with steady state (SS) 234Th export, SS POC fluxes based on composition of bulk and 450 mm particles collected at 100 m, and non-steady state (NSS) 234Th and POC fluxes at 100 m depth with 1  s propagated counting errors. Station no. PS71/

Latitude

Zero Meridian 101 421 102 441 104 471 108 511 113 531 118 541 125 571 131 581 140 611 149 641 157 661 169 681 178 691

20.150 S 39.560 S 39.470 S 30.010 S 01.570 S 30.190 S 00.110 S 59.990 S 29.910 S 29.990 S 28.600 S 30.020 S 24.070 S

Longitude

081 071 041 001 001 001 001 001 001 001 001 001 001

59.910 E 05.77 E 16.77 E 00.570 E 03.540 E 01.760 E 00.170 W 00.100 E 00.350 W 00.040 E 01.850 W 00.120 E 00.180 W

Date Sampling

Date Ice melt

234 Th flux dpm m  2 d  1 SS

POC flux mmol C m  2 d  1 SS, total

13-02-08 15-02-08 16-02-08 19-02-08 20-02-08 21-02-08 23-02-08 24-02-08 27-02-08 28-02-08 08-03-08 10-03-08 11-03-08

Open Open Open Open Open 07-10-07 01-11-07 09-11-07 01-01-08 26-12-07 10-01-08 15-01-08 24-01-08

10067 94 16707 103 1476 7 85 1656 7 89 1613 7 96 10587 97 848 7 106 10317 89 912 7 105 1773 7 90 977 7 101 953 7 87 1737 7 101

3.37 0.4 28.37 2.7a 6.97 0.6 11.17 1.0 7.57 0.7 4.97 0.6 4.27 0.6 5.17 0.6 3.47 0.5 4.57 0.4 4.47 0.6 3.17 0.4 9.97 0.8

234

Th Flux dpm m  2 d  1 NSSb

POC flux mmol C m  2 d  1 NSSb

9.07 1.3

1006 1670 1476 1656 1613 1077 879 1077 1120 2080 1172 1182 2287

3.3 28.2a 6.9 11.1 7.5 5.0 4.3 5.4 4.2 5.3 5.3 3.8 13.0 3.4 5.5 5.2 4.1 4.3 8.7 3.1 8.5 7.0 13.2 8.7 7.2 3.7 4.3

POC flux mmol C m  2 d  1 SS, 450 mm

2.57 0.2 5.47 0.5

2.97 0.3

Weddell Sea Transect 184 69100.000 S 186 69103.860 S 192 66156.840 S 196 66100.500 S 201 65107.010 S 204 64147.980 S 210 64102.650 S 220 63128.17 S 222 63121.210 S

06158.330 W 17121.380 W 25117.270 W 32146.460 W 40119.310 W 42153.610 W 48116.020 W 52106.350 W 52151.030 W

13-03-08 15-03-08 17-03-08 20-03-08 22-03-08 23-03-08 25-03-08 28-03-08 29-03-08

Ice Ice 16-01-08 16-01-08 23-01-08 23-01-08 Ice Ice 01-02-08

8067 89 1148 7 101 9067 111 621 7 105 781 7 106 1521 7 92 872 7 104 10117 88 8707 95

3.47 0.5 5.57 0.7 4.47 0.6 3.57 0.7 3.57 0.6 7.27 0.8 3.17 0.5 8.57 1.4 5.77 1.0

5.57 4.7

806 1148 1078 730 944 1822 872 1011 1061

Drake Passage 230 60106.310 S 236 58157.790 S 241 57137.630 S 244 56151.500 S 250 55143.850 S

55116.640 W 58106.050 W 60154.420 W 62130.200 W 64126.580 W

02-04-08 05-04-08 07-04-08 09-04-08 11-04-08

15-08-07 Open Open Open Open

1579 7 89 16077 100 16707 99 1211 7 92 1534 7 90

13.27 1.6 8.77 1.0 7.27 0.6 3.77 0.4 4.37 0.4

4.37 1.5 4.47 1.4 3.87 0.3 2.17 0.3 2.77 0.3

1580 1607 1670 1211 1534

a b

Abundant zooplankton on filter used for POC/234Th determination. Assuming equilibrium at ice melt.

3.1.1. Total 234Th in the upper 200 m and export There is a clear latitudinal pattern in the distribution of the depletion of 234Th with respect to its parent 238U in the surface waters (Fig. 2A). The northernmost station at 421S, north of the SAF, showed in the surface water (10 m) a depletion of 3473%. Southward, this depletion gradually declined and in the region 54–591S it was reduced to 10–21%. Then further south, in the central Weddell Gyre, the depletion increased again with a maximum of 4273% observed at 64.51S. The same latitudinal pattern is observed at a depth of 25 m, as expected based on the distribution of the mixed layer depth, but it can no longer be seen at 50 m depth. The deficit of 234 Th is confined to the upper about 100 m, which coincides in the north with the SML but extends somewhat deeper than the SML in the southern part of the section. Below 100 m, 234Th approaches equilibrium with 238U in most stations. At 591S (200 m) and 61.51S (150 m) 234Th/238U reaches values 41.1, which is significantly above equilibrium and infers remineralization (Buesseler et al., 2008; Savoye et al., 2004; Usbeck et al., 2002). This corresponds with the subsurface maximum in dissolved Mn observed at

Depth (m)

30

2500

25

2000

20 1500 15 1000 10 500

C export (mmol C m-2d-1)

The Cape Town—zero meridian section (Fig. 1) crossed the Subantarctic Front (SAF) at about 451S, the Polar Front (PF) at about 491S and the southern boundary of the ACC (SB ACC) at about 561S (Middag et al., 2011). The surface mixed layer (SML) was 80–120 m thick in the northern region from 421S to 551S and decreased to 25–50 m in the latitude range 60–691S (Klunder et al., 2011, cf. Fig. 2A). Close to the edge of the Fimbul Ice Shelf (near the protruding floating shelf ice called Trolltunga) a fresh, cold and very clear water mass was reached belonging to the coastal current and influenced by contact with the shelf ice.

Fig. 2. Total 234Th/238U in upper 200 m with schematic representation of the Mixed Layer Depth in (A) SE Atlantic section across the Antarctic Circumpolar Current, from 501S following the zero meridian, (B) section across the Weddell Sea and (C) Drake Passage section. In (A) and (C) the approximate location of major fronts is indicated.

5 Zero

Weddell

Drake 0

0 230 236 241 244 250

3.1. Zero meridian

Latitude

184 186 192 196 201 204 210 220 222

3. Results

Longitude

101 102 104 108 113 118 125 131 140 149 157 169 178

Mean monthly values of marine phytoplankton Chl-a concentrations in the investigated area prior and during the ANTXXIV-3 cruise were derived from the merged daily Full Product Set (FPS) of the GlobColour Archive (http://hermes.acri.fr/). This data set is based on the merging of MERIS, SeaWiFS and MODIS level-2 data with the GSM model and algorithm, developed by (Maritorena and Siegel, 2005) over the whole globe with the best resolution of 4.6 km. The GlobColour Chl-a product has undergone an extensive validation based on a validation protocol (ACRI-STLOV et al., 2006) derived from the Sensor Intercomparison for Marine Biological and Interdisciplinary Ocean Studies (SIMBIOS) protocol (http://oceancolor. gsfc.nasa.gov/MEETINGS/simbios_ref.html).

Depth (m)

2.6. Remote sensing

Latitude

234Th export (dppm m-2d-1)

ice melt and surface layer mixing. Although we realize that the development of 234Th depletion between ice melt and our measurements is uncertain, the best guess appears to be an equilibrium situation at the time of ice retreat. From the satellite based ice distributions (data from the University of Bremen Sea ice Group, http://www.iup.uni-bremen.de/iuppage/satellite_in dex_e.html) we have estimated the date of ice retreat from each station. Assuming equilibrium 234Th activity at that date and a constant export rate after that date, we have calculated the nonsteady state (NSS) export rate. The NSS rates are clearly higher than the steady state (SS) approximations, but the general trend of the results does not change (Table 1). If we do not assume equilibrium but use the depletion observed at the ice-covered stations as the situation during ice melt, the NSS estimates would be much closer to the SS estimates.

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Depth (m)

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station number

Fig. 3. Steady state (open circles) and non-steady state (crosses) 234Th export flux at 100 m and corresponding (steady state) carbon flux using POC/234Th ratio in bulk particles (squares) and in 4 50 mm particles (triangles). Exceptionally high (station 102) and low (station 149) POC/234Th ratios discussed in text.

100–150 m depth and explained as resulting from mineralization (Middag et al., 2011). The resulting steady state flux of 234Th out of the surface 100 m layer ranges on this section from 8487106 dpm m  2 d  1 at 571000 S (station 125) to 1773790 dpm m  2 d  1 at 641300 S (station 149, Table 1, Fig. 3). The export 234Th flux shows the abrupt southward decrease from 1600 dpm m  2 d  1 at 45–531S (stations 104, 108, 113) to less than 1100 dpm m  2 d  1 at 541300 –611300 S (stations 118, 125, 131, 140). This southward gradient in export is

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under the prevailing easterly winds, as is also seen in the deeper penetration of the surface turbidity (transmission data shown by Middag et al., 2011). The moderate depletion found in the surface water continues to much greater depth than further north, resulting in the appreciable integrated 234Th depletion. In this case, the assumption of negligible advection, on which Eq. (2) is based, is not valid and the derived export rate must be considered an overestimate.

enhanced because the mixed layer shoals in this interval and the depletion does not penetrate as deep as north of 531S (Fig. 2A). South of 591S, the export flux increases and reaches its maximum value, 1773790 dpm m  2 d  1 at 641300 S (station 149) in the region where, as we will show, a bloom had been observed in the previous cruise leg. The export here is even higher than in the ACC north of 541S. South of 641300 S (stations 157, 169) the integrated (steady state) picture is not as pronounced as the surface water distribution, because the depletion is only very shallow.

3.2. Weddell Sea section 3.1.2. High resolution particulate and dissolved 234Th in the surface water Particulate 234Th (plotted as 234Th/238U ratio) decreased gradually southward until it suddenly increased at 651S (Fig. 4A). The clear water very close to the shelf ice also had very low particulate 234Th concentrations. Total 234Th (particulateþdissolved, obtained from the automated analysis, Fig. 4A) and the corresponding depletion relative to its parent 238U showed a latitudinal pattern in qualitative agreement with the more accurate 4-L method (Fig. 2A). The depletion was lowest between 521S and 591S, then increased again in the Weddell Gyre. Only in the clear water adjacent to the ice edge 234 Th was again close to equilibrium with 238U.

The east–west section through the Weddell Basin is characterized by the ice conditions. During the eastern part of the section, up to around 251W, the sea was largely covered by sea ice. The central part of the section was ice free, but at 431W (station 204) the ice edge was reached again and the western part (43–531W) was again ice covered. Transmission in ice-covered surface waters around 201W and 501W was high, while the surface waters especially in the ice-free range 30–431W were more turbid (Fig. 5). The mixed layer depth in this section increased westwards from 30 m in the ice covered east to 60 m in the open water at 301W, then gradually decreased to 25 m at 471W to increase again towards the shelf of the Peninsula (Fig. 2B). The surface water with lowest salinity (33.31) and density (s0 ¼26.8 kg m  3, not shown) was found near the ice edge at 451W. 3.2.1. Total 234Th in the upper 200 m and export flux The entire section shows less export than at the zero meridian with a depletion of at most 20% at the surface, declining rapidly to about 5% at 50 m depth (Fig. 2B). In the ice-covered westernmost part of the section the surface depletion was further reduced. The total 234Th/238U ratio did not exceed 1.06 (200 m depth at stations 201, 210), which implies that there is no evidence of shallow mineralization. The steady state export flux in this area was low (Table 1, Fig. 3), including at station 196 the lowest flux of the entire

1.0

1.0

0.8

0.8

234Th/238U

234Th/238U

3.1.3. Coastal current At the final station of this section (station 178), situated in the coastal current, the calculated 234Th export, 1737 7101 mmol m  2 d  1, is among the highest of the expedition (Fig. 3). Increased export by blooms developing in the coastal current is likely and has been observed before in this area (Bathmann et al., 1992). But the contrast with the low 234Th depletion found in the surface water makes it unlikely that such a high export had occurred and asks for a different explanation for the high depth-integrated depletion. This station is in the influence of the coastal current where isopycnals are depressed because of shoreward deflection of the surface water by Ekman transport

0.6 0.4 0.2 0.0 38

0.6 0.4 0.2

43

48

53 58 latitude °S

63

0.0 55

68

56

57

1.0

0.8

0.8

234Th/238U

234Th/238U

SAF 1.0

0.6 0.4

PF

60

61

60

61

SACCF

0.6 0.4 0.2

0.2 0.0 -60

58 59 latitude °S

-50

-40 -30 -20 longitude °E

-10

0

0.0 55

56

57

58 59 latitude °S

Fig. 4. Distribution of particulate (green filled triangles), dissolved (black open squares) and total 234Th/238U ratio (black filled diamonds) in surface water measured with automated procedure (A) along zero meridian section, (B) along Weddell Sea section, (C) Drake passage 2008 expedition (particulate data only) and (D) Drake passage 2006 expedition. Results in (B) and (C) are compared with analyses of duplicate samples for total 234Th/238U with the yield-calibrated procedure (back squares: surface samples of Table 1 and Fig. 2, yellow circles: additional samples from surface seawater intake).

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Longitude Fig. 5. Light transmission (%) in upper water layer of the section across the Weddell Sea.

study (6217105 dpm m  2 d  1). These low fluxes result from low 234 Th depletion and a shallow penetration of this depletion (Fig. 2B). The highest depletion and export (1521792 dpm m  2 d  1) on this section was observed at the ice edge at station 204. 3.2.2. High-resolution particulate and dissolved 234Th in the surface water Particulate 234Th/238U reached a maximum around 40% at a longitude of about 26–351W (Fig. 4B). It then decreased westward to values below 10% W of 521W. Total 234Th/238U (using the sum of particulate and dissolved activities from the automated procedure and confirmed by the intercalibration with the yieldcorrected 4-L method, Fig. 4B) was about 15–30% depleted in the entire section apart from the ice-covered region west of 471W where 234Th was close to equilibrium with 238U. 3.3. Drake Passage 3.3.1. Total 234Th in the upper 200 m and export In the Drake Passage we crossed the southern boundary of the ACC (SB ACC) at about 60.51S, the southern ACC front at 58.51S, the Polar Front (PF) at about 571S and the Subantarctic Front (SAF) at about 55.51S (Middag et al., 2011). No clear latitudinal trend can be observed in the 5 stations analyzed on this section (Fig. 2C). Surface depletion (10 m depth) over the entire section is around 25%. The highest depletion (32%) is observed at the southernmost station north of Elephant Island, but such a difference is no longer present at 25 m depth. In all five stations on this section the depletion is rather uniform in the upper 75 m and is still significant at 100 m (except station 244 near 571S which is in equilibrium at 100 m). This distribution is comparable to the stations with highest export at the zero meridian (stations 104–113, 47.6–531S), but the depletion extends significantly deeper than in the Weddell Sea. The moderate steady state export flux at this section (1200–1700 dpm m  2 d  1) corresponds to a surface depletion of 234Th of 15–25% at the surface throughout the Drake Passage and observable down to 100 m (Table 1, Fig. 3). 3.3.2. High-resolution particulate and dissolved 234Th in the surface water Particulate 234Th/238U is below 10% south of 60.21S, i.e. close to the Bransfield Strait, increasing to a rather constant 25% over Drake Passage (Fig. 4C). We do not report the total 234Th values obtained in the Drake Passage with the automated procedure during ANTXXIV/3 because a technical problem with the second filtration unit caused the MnO2 precipitation efficiency to become insufficient and variable at this section.

3.3.3. ANTXXIII/3 Total 234Th (automated procedure) was depleted by 20 710% in surface water of the Drake Passage (56–611S) in February 2006 (Fig. 4D). There was no apparent change in export related to the crossing of oceanographic fronts. The only clear latitudinal signal was the increased scavenging on the approach of the South American continent. Particulate 234Th in surface water reached a maximum (38% of parent 238U activity) in the southern part at 601S.

4. Discussion The 234Th depletion is an integral result of export processes in the preceding approximately 2 months. For the interpretation of the 234Th results we must therefore consider the development of phytoplankton in the few months before our sampling. We discuss here data of Chl-a from remote sensing and data from a preceding Polarstern expedition to the zero meridian. For the phytoplankton distribution during our own sampling campaign we use data of beam attenuation and of particulate 234Th in surface waters. 4.1. Particulate

234

Th as tracer of phytoplankton distribution

Thorium is highly reactive to organic and inorganic particles. As most particles in the remote ocean area of this study are biogenic (Hegner et al., 2007), binding with organic surfaces can be expected to be dominant. Thorium is strongly complexed to Acid Polysaccharides (APS) (cf. Robert et al., unpublished manuscript) and the extent of aggregation determines whether the complex is found in the colloidal or particulate phase (Quigley et al., 2002). Through this strong affinity of thorium to organic surfaces it can be expected that the abundance of plankton is reflected in particulate 234Th or rather, to account for the availability of 234Th for adsorption, in the ratio of particulate to dissolved 234Th. Latter 234Thpart/234Thdiss decreases from 1 to 1.5 in the northern part of the section to low values around 0.25 in the latitude range 54–64.51S, then increases sharply to values of 0.6–1.1 between 651S and 691S and finally falls down to 0.3 in the clearest water near the shelf ice edge. Particulate 234Th distribution is related to particle distribution as derived from beam attenuation in the surface water at the nearest hydrographic station (Fig. 6). We have no explanation for the distinctly higher particulate 234Th values north of 451S. For samples taken from 451S southward on the zero meridian the correlation coefficient is reasonable (R2 ¼0.83), considering that the parameters were not measured in the same water mass, and

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we conclude that particulate abundance.

234

Th indeed mirrors the plankton

4.2. Zero meridian 4.2.1. Bloom development An analysis of Chl-a distribution from remote sensing (Fig. 7) reveals that only a few features are related to the position of the hydrographic fronts. In December 2007, blooms occurred westward (‘‘upstream’’) of our section in the zone 44–511S with meandering bands of high productivity continuing eastward in the region of the SAF around 451S and close to the PF around 501S. But in January these blooms had disappeared. If the end of these blooms in the SAF and PF areas had been associated with export, this had occurred 1–2 months before our passing of the area midFebruary, a delay that would have been long enough to allow for a significant ingrowth of the associated Th depletion. In January an intensive bloom developed in the Weddell Gyre (60–661S), an area that was still ice-covered in December (Fig. 7). This bloom was studied along the same section during the preceding leg of Polarstern (IPY/SCACE expedition ANTXXIV/2; 1.4

1.2

234Thpart/234Thdiss

1.0

0.8

0.6

0.4

0.2

0.0 0

0.1

0.2

0.3

0.4

0.5

December 2007–January 2008). On their first crossing in midDecember the ice edge was at 601S. On their second crossing 17–28 January, the sea ice had disappeared from the section down to the shelf ice and the strong bloom in the latitude range 60–651S was observed as a strong reduction in transmission and pCO2 in surface waters with a maximum turbidity and pCO2 reduction at about 63.51S (Strass, personal communication). There are indications that the bloom here and at the parallel section along 31W were related to the stable shallow mixed layer resulting from the ice melt (Strass, personal communication). In February, just before our sampling, this bloom had disappeared. 4.2.2. North of 591S Surface waters north of the SAF were high in particulate 234Th and depleted in total 234Th (Figs. 2A, 4A). There was a gradual transition across the SAF and PF to the situation further south, where surface waters in the zone of approximately 54–591S were low in particulate 234Th and beam attenuation (i.e. high transmission, Fig. 8A), indications of low algal biomass. Although fluorescence is not a direct measure of Chl-a due to the sometimes strong daylight quenching, the very low fluorescence (Fig. 8B) supports the conclusion of very low algal biomass in this zone. In the area 52–591S, fCO2 was at or above saturation, but south of 591S it dropped to a clear undersaturation (around 350 matm) in the Weddell Sea (Fig. 8D), a situation very similar to the one described by Hoppema et al. (2000) for April 1996. This distribution agrees with the average pattern of Chl-a concentrations measured in February by remote sensing (Fig. 7): sustained high values from December through February north of the SAF (39–431S), very low values o0.5 mg/L in the latitude band 53–611S, increasing south of 611S. Nevertheless we must consider whether the high pCO2 values in the zone of 52–591S could be due to upwelling of CDW in this region. Similarly, the high surface concentrations of Fe and Mn at 541S and 551S (Klunder et al., 2011; Middag et al., 2011; also Fig. 8E and F) could suggest upwelling of hydrothermally influenced deep waters. But the correlation with dissolved Al shows that these metals are derived from atmospheric input, not upwelling, an analysis confirmed by back trajectories of air masses (Klunder et al., 2011). Also, upwelling occurs in the entire Antarctic Zone (Geibert et al., 2002; Hoppema et al., 1999), and there is no indication of stronger upwelling north of 591S than further south. We conclude that the zone 54–591S was indeed characterized by low algal abundance.

beam attenuation (m-1) Fig. 6. Relationship in surface waters of zero meridian section between beam attenuation and particulate 234Thpart/234Thdiss ratio at nearest station. Excluding the distinct observations N of 451S (open circles), R2 ¼ 0.83.

4.2.3. 591S to 691S In the zone 59–691S we observed the strongest signals in growth and export. The drop of pCO2 south of 591S is an

Fig. 7. Monthly means of satellite Chl-a distributions from the merged daily Full Product Set (FPS) of the GlobColour Archive around the Zero Meridian section for December 2007–March 2008. All stations of the ANTXXIV-3 cruise within this area are plotted with black circles. Thorium stations are marked by white-filling. The position of the Subantarctic Front (SAF), the Polar Front (PF) and the southern boundary of the ACC (SB) are highlighted with red lines.

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Fig. 8. Distribution of (A) transmission (%), (B) fluorescence, (C) 234Thtot/238U, (D) fCO2 (matm), (E) dissolved Fe (nM), (F) dissolved Mn (nM) in upper 150 m of Weddell Sea section of zero meridian with southward shift of bloom indicated by comparison with zone of low transmission as observed 2 months earlier (black bar; Strass, personal communication).

indication of southward increasing algal abundance and growth. The low fluorescence from 541S to 611S and high values in the range 65–691S support the distribution of biomass as derived from particulate 234Th, pCO2 and light transmission. The bloom we encountered in the southern part of the section had depleted 234 Th in the dissolved phase to 234Thd/238U of 0.4 at 65–691S, the lowest values in the Antarctic Zone of the section. However the highest export was not associated with this bloom. In the present study, we observed on 28 February the maximum 234Th surface depletion (42%) and steady state flux (1773790 dpm m  2 d  1) in the Weddell gyre at 641300 S (station PS71/149, Figs. 2A, 3), the center of the bloom in January studied during the preceding leg and well visible in Fig. 7. We conclude from 234Th data (Figs. 2A, 4A) that the biomass produced in the bloom at 60–651S in December 2007–January 2008 had already sunk out in February/March, leaving a low particulate load and a significant Th depletion. In February/March 2008 the bloom had now shifted south, giving low transmission, high particulate 234Th and undersaturated pCO2 in the latitude range 65–691S (Fig. 8). 4.2.4. Comparison with other studies The distribution of 234Th was determined in parallel by the team of Frank Dehairs on the same transect from Cape Town down to 571S during the Bonus Good Hope expedition on board RV Marion Dufresne. Their results of 234Th export (and on 234Thbased carbon export) compare well with ours (Frank Dehairs, personal communication). Here we compare the distribution from the present study with the situation we observed on two earlier expeditions in spring (October/November 1992, 61E) (Rutgers van der Loeff et al., 1997) and summer (December 1995, ca. 151E) (Rutgers van der Loeff et al., 2002). In the northern part of the zero meridian section the high depletion in the present study agrees with previous observations earlier in the season (December 1995). In the Polar Front area we had observed a much higher export rate in spring (October/November 1995). When 3 yr later we revisited the area in December (Rutgers van der Loeff et al., 2002) we observed a

lower but steady export rate near the Polar Front, with depletions of approximately 20%. This depletion was also found in the waters further south in the ACC until we reached the ice edge at 591S. Interestingly, when we resampled the section mid-January 1996, the ice had melted and we observed at 641S a 234Th depletion of 40% in surface waters, the largest value in surface waters south of the Polar Front and at the same latitude where we observed the largest export in February 2008. The ice-edge bloom we described here also developed in a way very similar to the bloom observed by Buesseler et al (2003) in the AESOPS study at 1701W, where an ice-edge bloom in October was followed by an enhanced export 1 month later. 4.2.5. Relationship with inorganic carbon cycling The parallel uptake of 234Th and CO2 by phytoplankton on the prime meridian is illustrated by reduced fCO2 values along with enhanced particulate 234Th/238U ratios in surface water in the bloom areas (see Fig. 9A). Whereas fCO2 in the zone 42.3–591S, with only moderate particulate 234Th activities, is close to equilibrium with the atmosphere (380 matm, blue dots in Fig. 9A), fCO2 is reduced to an average of 335 matm north of the SAF (39–42.31S, black dots) and in the Weddell Gyre 65–69.41S (green dots in Fig. 9A). The strong CO2 uptake by the January bloom in the area 59–651S stands out in this figure, but here the low particulate 234Th/238U ratios (red dots in Fig. 9A) show that the particles have sunk, leaving a clear and CO2-depleted water mass, in perfect agreement with the depletion in total 234Th (Figs. 2A, 8). The reduced fCO2 values at low particulate 234Th/238U around 48.5–49.51S may be related to export subsequent to the bloom in the meandering high-productivity band at the latitude of the PF (satellite Chl-a seen in Fig. 7). We have no full station with a profile of total 234Th activity to confirm this export while the automated procedure yielded just one slightly reduced value of total 234Th at 49.01S. 4.2.6. Relationship with uptake of Fe and Mn Low growth and particle export rates, as observed in the zone 54–591S, does not only cause low uptake of carbon and 234Th by

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42.3 -59°S

400

47.5 -58°W

400

fCO2 fCO2 µatm

fCO2 µatm

21.8-40.2°W

350

39-42.3 °S

48.5-49.5°S 59-65°S 65-69.4 °S

300

350 17-21.8 °W

300

40.2-47°W Coast; 0-17°W

Coastal water

250

Weddell

250 0

0.1

0.3 0.2 part234Th/238U

0.4

0.5

0

0.7

0.1

0.2 0.3 part234Th/238U

0.4

0.5

0.1

0.2 0.3 part234Th/238U

0.4

0.5

0.8

0.6

Mn

dissolved Mn (nM)

dissolved Mn (nM)

fCO2

0.5 0.4 (C)

0.3 0.2 0.1 0

0.7 Fe

0.6 0.5 0.4 0.3 0.2 0.1 0

0

0.1

0.2 0.3 part234Th/238U

0.4

0.5

0

Fig. 9. (A) fCO2, (B) dissolved Mn and (C) dissolved Fe as function of 234Thp/238U ratio in surface water along the zero meridian at nearest available position distinguishing the latitude zones as indicated. (D) fCO2 as function of 234Thp/238U ratio in surface water of Weddell section at nearest available position distinguishing longitude zones as indicated. Blue symbols are interpreted to represent areas with low plankton biomass, green symbols areas with active plankton biomass and red symbols areas where plankton has disappeared by sinking. Light blue are coastal waters influenced by cooling and ice melt.

algae, but under these conditions also the scavenging of other trace metals is inefficient. The high concentrations of dissolved Fe and Mn observed in the surface mixed layer at 54–591S (Fig. 9C and B, respectively) could only be maintained by the low scavenging flux as a result of low algal abundance (Klunder et al., 2011; Middag et al., 2011). In the zone 65–691S, however, the algal bloom that had reduced fCO2 and depleted 234Th in the dissolved phase to 234Thd/238U of 0.4 in the surface water was also associated with the removal of Fe and Mn from the dissolved phase (Klunder et al., 2011; Middag et al., 2011) (Fig. 8). This uptake can be caused by either adsorption to reactive particle surfaces, as is probably the case for Th, or active incorporation in the cell for metabolic functions as hypothesized for Mn by Middag (2010) and shown for Fe by Sunda (2001). The uptake of trace metals is seen in low dissolved Mn and 234Th (Fig. 8C) concentrations in surface waters, associated with high particulate 234Th/238U ratios (green symbols in Fig. 9B). Fe concentrations are generally low in this productive zone as well (green symbols in Fig. 9C) but there are notable exceptions of two samples collected with the towed fish with high Fe but low Mn contents. The erratic iron distribution in the surface water suggests that local Fe inputs, e.g. from ice melt, (Klunder et al., 2011), can cause transient and short-lived patches of enhanced Fe concentrations even during a bloom period. We must assume that the preceding bloom in the zone 59–651S that had caused export of 234Th had also removed Fe and Mn. Indeed, dissolved Mn is very low here despite low particulate 234Th (Fig. 9B, red symbols): the particles that had accumulated 234Th (and carbon, Fig. 9A) and had sunk out already must also have adsorbed and removed Mn. But for Fe the situation is again different. Fe concentrations were low at a depth of 100 m in the range south of 621S (Fig. 8). But Fe values in the upper approximately 50 m at 63–651S were about 0.3 nM, higher than in the present bloom (661S

to 691S). This is also seen in a plot of dissolved Fe vs. particulate 234 Th/238U (Fig. 9C, red symbols), where the export stations in the zone 60–651S do not stand out as they did for fCO2 and Mn. The higher Fe values suggest a rapid input of dissolved Fe from external inputs or remineralization before sinking, thus re-establishing the pre-bloom Fe concentrations. Clearly, the turnover of Fe is more rapid than of Th or Mn. 4.2.7. Coastal water The section ended at 69.61S at the edge of the Fimbul Ice Shelf in the coastal current with strong influence of ice melt. The last hydrographical stations showed the transition to fresh, cold and clear surface waters with surface temperature and salinity dropping to  1.84 1C and 33.50, respectively. The particulate 234 Th/238U ratio had decreased rapidly southward from 0.36 at 691S to 0.19 at 69.41S along with the change in light transmission and fluorescence, confirming the transition to clear water with few algae, but this reduction was compensated by an increase in dissolved 234Th, leaving total 234Th unaffected (Fig. 4A). This implies that the very low fCO2 values (light blue in Fig. 9A) did not result from growth and export but must have been caused by cooling and mixing with melt water. 4.3. Weddell Sea section In line with the particle distribution derived from transmissometry (Fig. 5), particulate 234Th was enhanced in the ice-free central area (234Thpart/238U about 0.35) indicating the presence of particles (Fig. 4B), which in this offshore area of the Weddell Sea are largely phytoplankton. Satellite Chl-a is also enhanced in this section between approximately 301W and 381W (Fig. 10). These values are indicative of a bloom situation, but this bloom in the

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2759

bloom that developed after the disintegration of the ice cover in the central part of the section had not or not yet reached the stage of an appreciable particle export. In fact, this bloom developed much later, it was not yet visible in February but only developed in March (Fig. 10). Chl-a concentrations were here more diluted by the deeper mixed layer compared to the stratified conditions typical of the ice edge. This sequence of growth and export is again illustrated by a plot of fCO2 vs. particulate 234Th/238U (Fig. 9D). The ice covered western part (47.5–581W) is characterized by low particulate 234Th and high fCO2, close to equilibrium with the atmosphere (blue dots in Fig. 9D). The bloom in the ice-free section (22–401W) with its enhanced particulate 234Th and reduced CO2 (around 350 matm) shows CO2 uptake by plankton (green dots in Fig. 9D). The area 40–471W, encompassing the long-stable ice edge, stands out (red symbols in Fig. 9D) with yet lower CO2 values (300–350 matm), but low particulate 234Th. This is consistent with the uptake of CO2 by plankton and subsequent export removal and agrees with the maximum export flux on this section at 421W. The westernmost station of the section (PS71/222) was located at the shelf edge, depth 447 m, under the influence of the coastal current. A significant depletion of 234Th was observed here down to the maximum sampling depth of 200 m, which can be due to depression of isopycnals in the coastal current similar to the situation at station 178. The deeper penetration of depletion can also be related to enhanced scavenging by interaction with the shelf sediments. The strong influence of shelf sediments in this water mass is shown by the distribution of Mn (Middag et al., 2011), 232Th (Venchiarutti et al., 2011) and Fe (Klunder et al., 2011). The deeper penetration of the depletion does not translate into a higher export flux in this westernmost station than at other stations on this transect (Fig. 3). 4.4. Drake Passage

Fig. 10. Monthly mean satellite Chl-a distributions for December 2007–March 2008 and station positions of the ANTXXIV-3 cruise within the Drake Passage/ Weddell Sea area, shown as in Fig. 7.

ice-free Weddell section was not associated with a large export. The highest export on this section was observed at the ice edge at station 204, in contrast to the situation at the Zero Meridian where we had observed the highest export in the open water, not at the ice edge. This discrepancy may be due to the development of ice conditions in the weeks and months before sampling, the time frame covered by our 234Th tracer. On 1 January, the entire Weddell Sea was still icecovered, but by mid-January the ice in the central part rapidly disintegrated, leaving the area 20–451W of our section ice-free from the end of January. But in the west, the ice edge remained stationary around 43–451W from the beginning of February until our sampling on 23 March when the surface mixed layer was only 25 m thick with very low salinity. We speculate that this stationary ice edge had allowed the prolonged existence of a classical ice-edge bloom (Nelson et al., 1989), prominently visible at 41–441W in the February Chl-a data (Fig. 10). This ice-edge bloom had largely disappeared in March, which we interpret to be the cause of the relatively high export measured at station 204. On the contrary, the

The enhanced scavenging close to the South American coast, clearly observed in the high resolution 2006 section (Fig. 4D), was not observed in the lower resolution section of 2008 (Fig. 4C). There is some indication of enhanced production and flux in the south. Particulate 234Th in the February 2006 section increased in the southern part of the section in the Ona Basin at 59–601S. This fits with the blooms frequently observed in the Ona Basin and also observed during our 2006 expedition (Barre´, pers. comm.) in the turbulent region of the SACCF after it passed the Shackleton Fracture Zone (Barre´ et al., 2008). Surface depletion of total 234Th in the March 2008 section was highest (3373%) in the southernmost station at 601060 S (station 230) (Fig. 2). However, because of the shallow surface mixed layer this did not result in a higher total export flux, which is based on the depth-integrated depletion of 234Th (Table 1, Fig. 3). This southernmost station is on the northern edge of the region where enhanced productivity, frequently visible on satellite-based Chl-a distributions, is attributed to a control by (micro) nutrient supply and surface water mixing (i.e. light) conditions (Hewes et al., 2008) (Dulaiova, personal communication). During our sampling in 2008 and the few months before, Chl-a values witnessed a bloom situation at station 230 while Chl-a and particulate 234Th decreased southward towards the end of the section (Figs. 4C, 10). In the present study it was not possible to include a survey of the more productive area around the tip of the peninsula and the Weddell-Scotia Confluence Zone. 4.5. Latitudinal trends compared to literature data In many previous studies using 234Th as export tracer it has been attempted to characterise latitudinal trends in production and export fluxes. Enhanced fluxes near the ACC fronts as found

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by Usbeck et al. (2002) were not observed in this study or by Savoye et al. (2004) in the Australian sector. However in the present study we find that fluxes reach higher values in the Antarctic Zone than in the northern part of the ACC, in agreement with many previous studies (Buesseler et al., 2001, 2003; Coppola et al., 2005; Savoye et al., 2004). Nevertheless, there is large variability, which is what should be expected considering the patchiness in space and time of Chl-a distributions as seen from space, and the long time averages of the geological record will never be reproduced during a single expedition. The spatial distribution of export fluxes during one particular expedition per se is not of general interest. But just because of that patchiness the interpretation of other tracers requires not the long time average but the current export flux during a multi-tracer expedition. 4.6. POC/234Th ratio in total suspended matter and in size-fractionated particles POC and 234Th were determined in the total suspended material obtained by filtration of 8 L of seawater collected at 100 m depth, and in the size fractions 1–10, 10–50 and 450 mm collected with in-situ pumps deployed at the same depth. It is known that 234Th and DOC adsorb to GFF (Moran, 1991) and QMA (Benitez-Nelson et al., 2001; Gardner et al., 2003; Rutgers van der Loeff et al., 2006). This may have caused a bias in the POC/234Th ratios reported here. Table 2 POC and

234

1 mm ISP

234 Th dpm/L

POC mmol/L

POC/234Th

Zero Meridian 101 0.577 0.025 102 0.407 0.020 104 0.417 0.020 108 0.337 0.018 113 0.547 0.021 118 0.307 0.020 125 0.367 0.020 131 0.297 0.020 140 0.537 0.025 149 0.587 0.030 157 0.307 0.024 161 169 0.477 0.041 178 0.687 0.023

1.9 6.8 1.9 2.2 2.5 1.4 1.8 1.5 2.0 1.5 1.4

3.3 7 0.2 16.9 7 1.2a 4.7 7 0.3 6.7 7 0.5 4.7 7 0.3 4.6 7 0.4 4.9 7 0.4 5.0 7 0.5 3.8 7 0.3 2.5 7 0.2 4.5 7 0.5

1.5 3.9

3.2 7 0.4 5.7 7 0.4

Weddell Sea Section 184 0.257 0.017 186 0.417 0.030 193 0.327 0.019 196 0.407 0.042 201 0.40 7 0.036 204 0.287 0.020 210 0.297 0.019 220 0.147 0.017 222 0.167 0.017

1.1 2.0 1.5 2.2 1.8 1.3 1.0 1.2 1.0

4.3 7 0.4 4.8 7 0.5 4.8 7 0.4 5.6 7 0.7 4.5 7 0.5 4.8 7 0.4 3.5 7 0.3 8.4 7 1.1 6.6 7 0.8

Drake Passage 230 0.197 0.017 236 0.267 0.018 241 0.667 0.021 244 0.467 0.020 250 0.587 0.022

1.6 1.4 2.9 1.4 1.6

8.3 7 0.9 5.4 7 0.5 4.3 7 0.3 3.1 7 0.2 2.8 7 0.2

Average SD SD of average a

4.7. Carbon export In order to convert the 234Th export rates to carbon fluxes we must select the proper POC/234Th ratio. The best procedure to derive the POC/234Th ratio in sinking particles is the use of sediment traps. In the absence of such data, we must use the next best solution, which is the POC/Th ratio in suspended particles. Literature values vary widely (Buesseler et al., 2006) but the values we determined in our study were remarkably constant. The POC/234Th ratio in bulk suspended material was 4.871.5 mmol dpm  1, indistinguishable from the ratio in the 1–10 mm fraction. With the POC/234Th ratio measured on every station (Table 1), the carbon export (both for steady state and non-steady state) ranges from 3.1 to 13.2 mmol C m  2 d  1 (Fig. 3) well in the range of other studies in the Southern Ocean (reviewed by Savoye et al., 2008). In two aspects this distribution deviates from the distribution of 234Th export. First, in the Drake Passage, there is no latitudinal gradient in

Th in total suspended particulate matter (SPM) and in size-fractionated particles collected with in situ pump (ISP). Total SPM in 8-L sample

Station PS71/

The 1–10 mm fraction accounted for 74711 and 78 713 % of particulate 234Th and POC, respectively. The POC/234Th ratio in the total SPM is 4.8 71.5 mmol dpm  1 (Table 2). The POC/234Th ratio in the size-fractionated samples decreased with particle size, from 4.772.8 mmol dpm  1 (1–10 mm) and 3.9 72.4 mmol dpm  1 (10–50 mm) to 2.9 71.4 mmol dpm  1 ( 450 mm) (Fig. 11). The POC/234Th ratio of the sum of the size fractions, dominated by the composition of the 1–10 mm fraction, was 4.472.6 mmol dpm  1, indistinguishable from the results of the bulk.

10 mm ISP

234 Th dpm/L

POC mmol/L

0.0347 0.002

0.11

0.0327 0.004

234 Th dpm/L

mmol/L

234 Th dpm/L

POC mmol/L

POC/234Th

3.2 70.3

0.00167 0.0003

0.00

2.2 70.5

0.0087 0.000

0.01

1.77 0.1

0.09

2.9 70.4

0.0172 7 0.0005

0.07

4.1 70.2

0.017 7 0.001

0.06

3.37 0.2

0.0267 0.003

0.08

3.1 70.4

0.00167 0.0003

0.01

3.3 70.7

0.0077 0.000

0.02

2.87 0.2

0.111 7 0.007

0.39

3.5 70.3

0.0145 7 0.0009

0.05

3.6 70.3

0.0067 0.001

0.01

2.47 0.4

0.156 7 0.010

1.18

7.6 70.6

0.0574 7 0.0014

0.28

4.9 70.3

0.0097 0.001

0.05

5.27 0.7

0.133 7 0.009

0.41

3.1 70.3

0.0572 7 0.0013

0.11

1.9 70.1

0.0087 0.001

0.02

2.37 0.3

0.0687 0.012

0.58

8.5 71.6

0.00927 0.0014

0.09

10.2 71.7

0.0027 0.001

0.01

6.47 5.4

0.0927 0.009 0.139 7 0.012 0.516 7 0.022 0.877 7 0.046 0.279 7 0.019

1.00 0.63 1.77 1.90 0.82

10.8 71.2 4.5 70.5 3.4 70.2 2.2 70.2 2.9 70.3

0.01097 0.0011 0.0898 7 0.0018 0.0858 7 0.0029 0.21007 0.0038 0.0575 7 0.0015

0.07 0.26 0.33 0.34 0.14

6.2 70.7 2.9 70.2 3.9 70.2 1.6 70.1 2.4 70.1

0.0037 0.001 0.0057 0.001 0.0707 0.003 0.038 7 0.003 0.029 7 0.001

0.01 0.01 0.16 0.07 0.05

2.77 0.9 2.87 0.9 2.37 0.2 1.87 0.2 1.77 0.1

4.8 1.5 0.3

Abundant zooplankton. Not included in average.

POC/234Th

50 mm ISP

4.7 2.8 0.8

POC

POC/234Th

3.9 2.4 0.7

2.9 1.4 0.4

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

In this study, we have combined the analysis of precise 234Th profiles at a 1–21 (100–200 km) horizontal resolution with data of surface water at a substantial higher resolution.

0.6 1000

0.4

500

0.2

40

45

50

55 60 Latitude °S

65

surface

0 70

2500

1.2

2000

1 0.8

1500 0.6 1000 0.4 500

surface

0

234Th/238U

0.8 1500

234Th/238U

flux dpm m-2 d-1

1

2000

0 -60

5. Conclusions

1.2

2500

234Th

Th flux, but due to a large N–S increase in POC/234Th ratio there is a large southward increase in carbon export. The southward increase in POC/234Th ratio towards the more productive tip of the Antarctic Peninsula is in line with the conclusion of Buesseler et al. (2006) that productive coastal waters are characterized by particles that have elevated (and variable) POC/234Th ratios. Second, the bloom-related 234 Th export maximum at 641300 S (station 149) on the zero meridian does not exist in the carbon export. This discrepancy results from an anomalously low POC/234Th ratio (2.5) based on the analysis of POC and 234Th on one single filter. It is unlikely for this anomalous composition to be representative for the flux of the past 2 months. If instead we use the average POC/234Th value (4.8) to convert 234Th to POC fluxes, the POC export at 641S is 8.5 mmol C m  2 d  1. Although this flux may have been enhanced by iron inputs from melting sea ice (Geibert et al., 2010; Klunder et al., 2011), the flux is not high compared to flux enhancements around 10 mmol C m  2 d  1 that are found after artificial or natural iron fertilizations (cf. Savoye et al., 2008). The high carbon export at station 102 was obtained in a sample with much zooplankton on the filter that we could not completely remove and we consider this value an artefact. As the coarsest fraction is often assumed to be responsible for the bulk of the particle flux (Buesseler et al., 2006 and references therein), we also calculate the export based on the POC/234Th ratio in that fraction. In the coarsest fraction, 450 mm, the ratio was somewhat smaller (2.971.4 mmol dpm  1) than in the bulk (Fig. 11). This decrease with particle size has been observed in other studies (e.g. Coppola et al., 2005), but contrasts with most other studies where particles were collected with in-situ pumps and where the POC/234Th either increases or is relatively invariant with particle size (Buesseler et al., 2006). Size-fractionated filtration of large water volumes obtained with a hose pump showed a clear increase of POC/234Th with particle size (Savoye et al., 2008). One reason for the low POC/234Th ratio in the 450 mm fraction may be absorption of 234Th onto the QMA filters used to collect the particles sonicated from the screens. Using the POC/234Th ratio we observed in the 450 mm particles, we find a SS carbon export that ranges from 2.1 to 5.5 mmol C m  2 d  1 (Fig. 3).

flux dpm m-2 d-1

234

The high-resolution distribution of particulate 234Th in surface water helps to characterize the distribution of phytoplankton and complements data of light transmission and Chl-a (shipboard pigment analyses or satellite-derived). The total 234Th monitoring in surface waters with the automated procedure gives a higher horizontal resolution (Fig. 12), shows the horizontal extent of changes in export in better detail, or even reveals features that are missed in the sampling on discrete stations. The details in Fig. 12B show that the largest export in the Weddell Sea was probably at 461W rather than at 431W where the 234Th profile was measured. But the automated surface measurement cannot replace the full profiles. The calculated export at 641S on the Zero Meridian is high not only because the depletion at the surface is high but also because this depletion penetrates well below 50 m depth. Clearly, the vertical structure of the 234Th profile is essential to obtain reliable export flux data. A 234Th section yields a snapshot of the 234Th and POC export during the preceding month, a flux that can be very different from the longer-time averages as they are derived from long term sediment trap, inverse modeling or sediment accumulation studies. But these actual 234Th-derived fluxes yield the background needed for the interpretation of other tracers in the surface ocean. The most important application of the 234Th-based fluxes is therefore the common interpretation of data of other tracers that are measured in parallel. This is the general approach used in the framework of GEOTRACES. By distinguishing where a plankton bloom is developing and where it has already sunk, the 234Th data can be very helpful in interpreting data of other particle-reactive tracers in the surface water column, like in this case the distribution of Mn and Fe along the zero meridian.

234Th

Fig. 11. POC/234Th ratio (mean with 1  s standard deviation) in bulk particles collected with bottles (left: 8-L samples) compared with size-fractionated particles collected with in situ pumps.

2761

0.2 0 -50

-40

-30 -20 Longitude °E

-10

0

Fig. 12. Steady state (pink squares) and non-steady state (blue-gray diamonds) 234 Th export flux at 100 m compared with high-resolution total 234Th/238U in surface water (red diamonds) along zero meridian (A) and Weddell Sea section (B).

2762

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

Acknowledgments We are grateful to captain Schwarze and the crew of FS Polarstern for their support during the expedition. We thank Eberhard Fahrbach for the way in which he prepared and managed the expedition, Gerd Rohardt and Sven Ober for their help in collecting and providing the hydrographic data and the entire GEOTRACES team under guidance of Hein de Baar for excellent cooperation. We acknowledge thoughtful comments by Brad Moran and an anonymous reviewer. Remote sensing data of chlorophyll concentrations were supplied for SeaWiFS and MODIS by NASA and

Table A1 Depth, Rosette bottle number, potential temperature, salinity,

238

U, total

234

for MERIS by ESA and merged to one data product by the GlobColour project financed by ESA. Services and data delivery are highly acknowledged. PC thanks the Humboldt foundation and AB the Helmholtz Initiative and Networking Fund for their support.

Appendix A Potential temperature, salinity, 238U, total 234Th and the total Th/238U ratio for all profiles sampled for total 234Th are shown in Table A1. 234

Th and the total

234

Th/238U ratio with 1  s on the expedition Zero and Drake.

Salinity

238

Station ANTXXIV/3-101-1, 42120.150 S, 081 59.910 E, 2530 m 10 24 11.18 25 19 11.13 50 15 11.08 75 14 10.70 100 9 9.58 150 8 8.91 200 1 7.95

34.041 34.041 34.569 34.681 34.521 34.492 34.347

2.43 7 0.07 2.43 7 0.07 2.46 7 0.07 2.47 7 0.07 2.46 7 0.07 2.46 7 0.07 2.45 7 0.07

1.611 7 0.049 1.669 7 0.049 2.188 7 0.055 2.4407 0.059 2.614 7 0.064 2.4037 0.062 2.581 7 0.062

0.664 70.028 0.688 70.029 0.888 70.035 0.987 70.038 1.062 70.041 0.977 70.039 1.054 70.040

Station ANTXXIV/3-102-2, 44139.560 S, 07105.770 E, 4613 m 10 24 9.11 25 23 9.10 50 22 9.09 75 21 8.54 100 20 7.38 150 19 6.96 200 18 6.63

33.930 33.928 33.928 34.033 34.264 34.315 34.331

2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.43 7 0.07 2.44 7 0.07 2.45 7 0.07 2.45 7 0.07

1.718 7 0.067 1.793 7 0.069 1.663 7 0.066 1.861 7 0.071 2.394 7 0.083 2.579 7 0.086 2.5507 0.085

0.710 70.035a 0.741 70.036a 0.687 70.034a 0.767 70.037a 0.980 70.045a 1.054 70.047a 1.042 70.047a

Station ANTXXIV/3-104-1, 47139.470 S, 04116.770 E, 4547 m 7 ssw 6.47 25 19 6.49 50 15 6.49 75 14 6.49 100 9 4.45 150 8 3.96 200 1 3.59

33.720 33.726 33.726 33.725 33.807 33.854 33.956

2.407 0.07 2.407 0.07 2.407 0.07 2.407 0.07 2.41 7 0.07 2.41 7 0.07 2.42 7 0.07

1.747 7 0.038 1.772 7 0.041 1.958 7 0.047 1.852 7 0.040 2.224 7 0.048 2.3907 0.061 2.386 7 0.057

0.727 70.027 0.737 70.028 0.814 70.031 0.770 70.028 0.923 70.034 0.990 70.039 0.986 70.038

Station ANTXXIV/3-108-1, 51130.010 S, 00100.570 E, 2764 m 10 22 3.02 25 21 3.01 50 9 3.00 75 8 2.91 100 6 2.21 150 5 0.90 200 1 1.65

33.732 33.733 33.733 33.735 33.783 33.997 34.274

2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.42 7 0.07 2.44 7 0.07

1.743 7 0.046 1.779 7 0.051 1.8307 0.049 1.831 7 0.050 2.0157 0.050 2.2407 0.058 2.424 7 0.062

0.725 70.029 0.740 70.031 0.761 70.030 0.761 70.031 0.836 70.033 0.924 70.037 0.992 70.039

Station ANTXXIV/3-113-7, 53101.570 S, 00103.540 E, 2429 m 10 22 1.42 25 17 1.39 50 16 1.25 75 15 1.23 100 9 1.20 150 8 0.20 200 1 0.65

33.787 33.787 33.788 33.788 33.789 34.059 34.301

2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.43 7 0.07 2.45 7 0.07

1.8087 0.059 1.849 7 0.060 1.8027 0.059 1.871 7 0.061 1.935 7 0.063 2.445 7 0.081 2.484 7 0.082

0.751 70.033a 0.768 70.034a 0.748 70.033a 0.776 70.034a 0.803 70.035a 1.007 70.045a 1.016 70.045a

Station ANTXXIV/3-118-2, 54130.190 S, 00101.760 E, 1733 m 10 24 0.98 25 23 0.98 50 21 0.91 75 19 0.85 100 17 0.29 150 16 0.45 200 14 1.27

33.842 33.839 33.850 33.852 33.977 34.234 34.442

2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.42 7 0.07 2.44 7 0.07 2.46 7 0.07

1.914 7 0.047 1.985 7 0.049 2.0277 0.067 2.164 7 0.070 2.1077 0.069 2.425 7 0.075 2.5107 0.077

0.793 70.031 0.823 70.032 0.840 70.038a 0.896 70.040a 0.870 70.039a 0.994 70.043a 1.022 70.044a

Station ANTXXIV/3-125-1, 57100.110 S, 00100.170 W, 3768 m 10 23 0.54 25 22 0.53 50 7 0.50 75 6 0.45 100 5  0.05 150 4  0.93

34.140 34.143 34.154 34.160 34.213 34.423

2.43 7 0.07 2.43 7 0.07 2.44 7 0.07 2.44 7 0.07 2.44 7 0.07 2.45 7 0.07

2.161 7 0.075 2.1017 0.074 2.0747 0.073 2.168 7 0.075 2.277 7 0.078 2.3907 0.077

0.888 70.041a 0.863 70.040a 0.852 70.039a 0.890 70.041a 0.934 70.043a 0.974 70.043a

Depth (m)

bottle #

Pot temp. (1C)

U (dpm/L)

234

Th (dpm/L)

234

Th/238U

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

2763

Table A1 (continued ) Salinity

238

234

234

34.016 34.008 34.015 34.211 34.344 34.537 34.639 34.683 34.686 34.680 34.675 34.667 34.663 34.655 34.649

2.43 7 0.07 2.42 7 0.07 2.43 7 0.07 2.44 7 0.07 2.45 7 0.07 2.46 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07

1.920 70.046 2.017 70.043 1.870 70.049 2.234 70.045 2.423 70.060 2.545 70.060 2.695 70.064 2.565 70.062 2.497 70.056 2.847 70.061 2.378 70.057 2.587 70.060 2.473 70.061 2.386 70.059 2.550 70.065

0.792 70.030 0.832 70.031 0.771 70.031 0.916 70.033 0.990 70.038 1.034 70.039 1.091 70.042 1.037 70.040 1.010 70.038 1.152 70.043 0.962 70.037 1.047 70.040 1.001 70.039 0.966 70.037 1.032 70.041

Station ANTXXIV/3-140-1, 61129.910 S, 00100.350 W, 5378 m 10 24 0.35 25 23 0.33 50 22  1.58 75 21  1.80 100 20  1.64 150 19 0.38 200 18 0.52 500 16 0.34

33.968 33.968 34.191 34.295 34.360 34.655 34.680 34.684

2.42 7 0.07 2.42 7 0.07 2.44 7 0.07 2.45 7 0.07 2.45 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07

1.552 70.060 1.653 70.062 2.198 70.075 2.610 70.084 2.473 70.081 2.892 70.092 2.572 70.085 2.462 70.082

0.641 70.031a 0.683 70.033a 0.901 70.041a 1.067 70.047a 1.010 70.045a 1.170 70.051a 1.040 70.046a 0.995 70.045a

Station ANTXXIV/3-149-1, 64129.990 S, 0010 0.040 E, 4661 m 10 24 0.41 25 23 0.39 50 22  0.20 75 21  1.53 100 20 0.16 150 19 0.95 200 18 1.06

33.963 33.964 34.062 34.352 34.562 34.656 34.681

2.42 7 0.07 2.42 7 0.07 2.43 7 0.07 2.45 7 0.07 2.46 7 0.07 2.47 7 0.07 2.47 7 0.07

1.395 70.042 1.485 70.049 1.675 70.049 2.156 70.056 2.526 70.053 2.399 70.049 2.407 70.047

0.576 70.025 0.613 70.027 0.690 70.029 0.880 70.035 1.025 70.038 0.971 70.035 0.973 70.035

Station ANTXXIV/3-157-1, 66128.600 S, 00101.850 W, 4585 m 10 24  0.80 25 19  0.90 50 14  1.75 75 13  0.66 100 7 0.65 150 1 1.02 200 1 1.04

33.839 33.906 34.333 34.519 34.616 34.671 34.682

2.41 7 0.07 2.42 7 0.07 2.45 7 0.07 2.46 7 0.07 2.47 7 0.07 2.47 7 0.07 2.47 7 0.07

1.564 70.066 1.757 70.057 2.287 70.069 2.326 70.070 2.511 70.074 2.343 70.072 2.566 70.077

0.648 70.034a 0.727 70.032a 0.934 70.040a 0.945 70.040a 1.017 70.043a 0.948 70.041a 1.038 70.044a

Station ANTXXIV/3-169-1, 68130.020 S, 00100.120 E, 7 ssw 25 23 50 22 75 21 100 20 150 19 200 18

34.002 34.002 34.334 34.441 34.565 34.647 34.675

2.42 7 0.07 2.42 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07 2.47 7 0.07 2.47 7 0.07

1.760 70.040 1.981 70.045 2.182 70.045 2.185 70.045 2.438 70.045 2.343 70.059 2.636 70.058

0.726 70.027 0.817 70.031 0.891 70.032 0.890 70.032 0.989 70.035 0.948 70.037 1.066 70.040

Station ANTXXIV/3-178-1, 69124.070 S, 001 00.180 W, 2007 m 10 24  1.48 25 23  1.46 50 22  1.34 75 21  1.27 100 20  1.29 150 19  1.71 200 18  1.40

34.004 34.013 34.012 34.103 34.136 34.393 34.427

2.42 7 0.07 2.43 7 0.07 2.43 7 0.07 2.43 7 0.07 2.43 7 0.07 2.45 7 0.07 2.45 7 0.07

1.695 70.064 1.866 70.068 1.868 70.068 1.708 70.065 2.010 70.072 2.327 70.068 2.415 70.070

0.699 70.034a 0.770 70.036a 0.770 70.036a 0.702 70.034a 0.826 70.039a 0.949 70.040a 0.984 70.041a

Station ANTXXIV/3-184-1, 69100.000 S, 06158.330 W, 2950 m 10 24  1.83 25 23  1.83 50 22  1.70 75 21  1.73 100 19  1.67 150 18  1.27 200 17  0.61

33.992 33.991 33.997 34.393 34.405 34.456 34.515

2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.45 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07

1.785 70.044 1.834 70.045 2.340 70.052 2.335 70.049 2.434 70.053 2.443 70.053 2.563 70.055

0.736 70.029 0.757 70.029 0.965 70.036 0.952 70.035 0.992 70.037 0.995 70.037 1.041 70.039

Station ANTXXIV/3-186-1,69103.860 S, 17121.380 W, 4765 m 10 24  1.84 25 23  1.84 50 22  1.72 75 21  1.75 100 20  1.71

33.933 33.933 34.246 34.431 34.450

2.42 7 0.07 2.42 7 0.07 2.44 7 0.07 2.45 7 0.07 2.46 7 0.07

1.764 70.061 1.758 70.061 2.014 70.066 2.316 70.073 2.376 70.075

0.729 70.033a 0.727 70.033a 0.825 70.037a 0.944 70.041a 0.967 70.042a

Depth (m)

bottle #

Station ANTXXIV/3-131-1, 5819.990 S, 00100.100 E, 10 24 25 23 50 22 75 21 100 20 150 19 200 18 300 17 500 15 750 14 1000 13 1500 12 2000 11 3000 9 4000 7

Pot temp. (1C) 4585 m 0.23 0.15 0.12  0.91  1.23  0.48 0.25 0.50 0.41 0.23 0.10  0.13  0.29  0.53  0.72

4257 m  0.79  0.79  1.48  1.57  0.46 0.68 0.93

U (dpm/L)

Th (dpm/L)

Th/238U

2764

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

Table A1 (continued ) bottle #

Pot temp. (1C)

Salinity

238

19 18

 0.78 0.36

34.533 34.643

2.46 7 0.07 2.47 7 0.07

2.461 7 0.082 2.567 7 0.084

1.000 70.045a 1.039 70.046a

Station ANTXXIV/3-192-1,66156.840 S, 25117.270 W, 4851 m 10 24  1.82 50 23  1.62 75 22  1.73 100 20  1.73 150 19  1.16 200 18  0.11

33.964 34.343 34.460 34.472 34.525 34.622

2.42 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07 2.47 7 0.07

1.794 7 0.047 2.163 7 0.050 2.3047 0.053 2.352 7 0.058 2.447 7 0.055 2.391 7 0.048

0.741 70.029 0.884 70.033 0.938 70.035 0.957 70.037 0.994 70.037 0.969 70.035

Station ANTXXIV/3-196-1,66100.500 S, 32146.460 W, 4791 m 10 24  1.55 25 23  1.57 50 22  1.72 75 21  1.78 100 20  1.77 150 19  1.58 200 18  0.10

33.996 33.998 34.473 34.478 34.486 34.525 34.637

2.42 7 0.07 2.42 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07 2.47 7 0.07

2.0017 0.067 2.0827 0.069 2.398 7 0.076 2.252 7 0.073 2.3707 0.076 2.511 7 0.080 2.577 7 0.082

0.825 70.037a 0.859 70.038a 0.976 70.043a 0.916 70.041a 0.964 70.042a 1.020 70.045a 1.043 70.046a

Station ANTXXIV/3-201-1,65107.010 S, 40119.310 W, 4774 m 10 24  1.53 25 23  1.52 50 22  1.67 75 21  1.79 100 20  1.75 150 19  1.39 200 18 0.09

33.997 34.000 34.314 34.485 34.498 34.542 34.654

2.42 7 0.07 2.42 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07 2.47 7 0.07

1.878 7 0.067 1.855 7 0.067 2.229 7 0.075 2.454 7 0.080 2.414 7 0.080 2.561 7 0.085 2.628 7 0.086

0.775 70.036a 0.765 70.036a 0.911 70.041a 0.998 70.044a 0.981 70.044a 1.040 70.046a 1.063 70.047a

Station ANTXXIV/3-204-1,64147.980 S, 42153.610 W, 4676 m 10 21  1.78 25 20  1.79 50 10  1.69 75 9  1.77 100 7  1.79 150 6  1.38

33.909 33.910 34.215 34.482 34.497 34.530

2.42 7 0.07 2.42 7 0.07 2.44 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07

1.654 7 0.051 1.724 7 0.050 1.754 7 0.052 2.222 7 0.055 2.222 7 0.057 2.3047 0.058

0.684 70.029 0.713 70.030 0.719 70.030 0.904 70.035 0.904 70.036 0.936 70.037

Station ANTXXIV/3-210-2,64102.650 S, 48116.020 W, 4016 m 10 21  1.75 25 20  1.81 50 10  1.64 75 9  1.56 100 7  1.39 150 6  0.72 200 1 0.26

33.746 33.785 34.370 34.473 34.505 34.570 34.645

2.41 7 0.07 2.41 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07 2.46 7 0.07 2.47 7 0.07

1.824 7 0.064 2.0527 0.070 2.189 7 0.073 2.233 7 0.074 2.3047 0.076 2.425 7 0.076 2.623 7 0.081

0.758 70.035a 0.852 70.039a 0.893 70.040a 0.909 70.041a 0.936 70.042a 0.984 70.043a 1.062 70.046a

Station ANTXXIV/3-220-2,63128.170 S, 52106.350 W, 943 m 10 24  1.86 25 23  1.86 50 15  1.86 75 14  1.86 100 12  1.74 150 11  1.69 200 10  1.63 300 9  1.24 600 6  0.69

34.171 34.172 34.172 34.182 34.322 34.411 34.421 34.463 34.589

2.44 7 0.07 2.44 7 0.07 2.44 7 0.07 2.44 7 0.07 2.45 7 0.07 2.45 7 0.07 2.45 7 0.07 2.46 7 0.07 2.47 7 0.07

2.131 7 0.041 2.1207 0.047 2.0777 0.043 1.987 7 0.051 2.192 7 0.044 2.296 7 0.055 2.3607 0.056 2.393 7 0.058 2.187 7 0.055

0.875 70.031 0.870 70.032 0.853 70.031 0.815 70.032 0.896 70.032 0.936 70.036 0.962 70.037 0.974 70.038 0.887 70.035

Station ANTXXIV/3-222-1,63121.210 S, 52151.030 W, 444 m 10 24  1.82 25 12  1.82 50 11  1.80 75 10  1.54 100 8  1.55 150 7  1.33 200 6  1.01

34.071 34.072 34.093 34.338 34.401 34.466 34.510

2.43 7 0.07 2.43 7 0.07 2.43 7 0.07 2.45 7 0.07 2.45 7 0.07 2.46 7 0.07 2.46 7 0.07

1.9807 0.057 2.0107 0.057 2.162 7 0.060 2.264 7 0.057 2.225 7 0.055 2.427 7 0.061 2.261 7 0.059

0.815 70.034 0.828 70.034 0.890 70.036 0.925 70.036 0.907 70.035 0.988 70.039 0.919 70.036

Station ANTXXIV/3-230-2,60106.310 S, 55116.640 W, 3521 m 10 23 1.29 25 22 1.12 50 21 0.81 75 7 0.69 100 5 0.62 150 4 0.82 200 1 0.17

33.897 33.994 34.153 34.237 34.283 34.351 34.375

2.42 7 0.07 2.42 7 0.07 2.44 7 0.07 2.44 7 0.07 2.44 7 0.07 2.45 7 0.07 2.45 7 0.07

1.611 7 0.045 1.9107 0.050 1.874 7 0.048 1.915 7 0.048 2.0637 0.051 2.321 7 0.048 2.1047 0.051

0.666 70.027 0.788 70.031 0.770 70.030 0.784 70.031 0.844 70.033 0.948 70.035 0.859 70.033

Station ANTXXIV/3-236-1,58157.790 S, 58106.050 W, 3773 m 10 23 2.47 25 22 2.47 50 21 2.46 75 7 2.17

33.869 33.869 33.869 33.970

2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.42 7 0.07

1.766 7 0.064 1.863 7 0.066 1.829 7 0.065 1.784 7 0.064

0.731 70.034a 0.771 70.036a 0.757 70.035a 0.737 70.035a

Depth (m) 150 200

U (dpm/L)

234

Th (dpm/L)

234

Th/238U

M. Rutgers van der Loeff et al. / Deep-Sea Research II 58 (2011) 2749–2766

2765

Table A1 (continued ) Salinity

238

234

234

34.081 34.302 34.436

2.43 7 0.07 2.45 7 0.07 2.46 7 0.07

2.161 70.073 2.283 70.076 2.402 70.079

0.889 70.040a 0.933 70.042a 0.978 70.043a

Station ANTXXIV/3-241-1,57137.630 S, 60154.420 W, 3461 m 7 ssw 2.88 25 24 2.87 50 23 2.79 75 22 2.79 100 21 0.54 150 20  0.06 200 19 0.32

33.780 33.780 33.780 33.782 33.814 33.948 34.014

2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.41 7 0.07 2.42 7 0.07 2.43 7 0.07

1.823 70.066 1.903 70.068 1.714 70.060 1.802 70.063 1.964 70.070 2.434 70.078 2.458 70.079

0.757 70.036a 0.790 70.037a 0.712 70.033a 0.748 70.034a 0.815 70.038a 1.006 70.044a 1.013 70.045a

Station ANTXXIV/3-244-6,56151.500 S, 62130.200 W, 4158 m 7 ssw 5.39 25 24 5.38 50 23 5.38 75 22 5.22 100 21 4.30 150 20 3.69 200 19 3.45

33.902 33.902 33.907 33.924 33.985 34.016 34.036

2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.43 7 0.07 2.43 7 0.07

1.913 70.054 1.900 70.058 1.916 70.054 2.000 70.052 2.435 70.057 2.255 70.054 2.171 70.055

0.791 70.033 0.786 70.034 0.793 70.033 0.827 70.033 1.005 70.038 0.930 70.036 0.894 70.035

Station ANTXXIV/3-250-5,55143.850 S, 64126.580 W, 3819 m 7 ssw 5.10 25 23 5.07 50 17 5.08 75 15 5.08 100 9 4.57 150 8 3.55 200 1 3.45

33.882 33.882 33.882 33.883 33.908 34.011 34.013

2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.42 7 0.07 2.43 7 0.07

1.779 70.047 1.898 70.050 1.963 70.052 1.778 70.052 2.004 70.052 2.277 70.048 2.416 70.047

0.736 70.029 0.786 70.031 0.813 70.032 0.736 70.031 0.829 70.033 0.939 70.035 0.996 70.036

Depth (m) 100 150 200

bottle # 5 4 1

Pot temp. (1C) 0.76 0.93 1.25

U (dpm/L)

Th (dpm/L)

Th/238U

ssw: ship’s seawater intake. a

No spike recovery determined; average recovery of 0.957 7 0.022 is applied.

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