Distributions of dissolved trace metals (Cd, Cu, Mn, Pb, Ag) in the southeastern Atlantic and the Southern Ocean

Biogeosciences, 9, 3231–3246, 2012 www.biogeosciences.net/9/3231/2012/ doi:10.5194/bg-9-3231-2012 © Author(s) 2012. CC Attribution 3.0 License. Biogeosciences Distributions of dissolved trace metals (Cd, Cu, Mn, Pb, Ag) in the southeastern Atlantic and the Southern Ocean M. Boye1, B. D. Wake1,2, P. Lopez Garcia2,3, J. Bown1, A. R. Baker4, and E. P. Achterberg2 1Institut Universitaire Europ´ een de la Mer (IUEM) UMS3113, Laboratoire des Sciences de l’Environnement Marin UMR6539, Technopˆ ole Brest Iroise, 29280 Plouzan´ e, France 2School of Ocean & Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, UK 3Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas, Spain 4School of Environmental Sciences, University of East Anglia, Norwich, UK Correspondence to: M. Boye ([email protected]) Received: 8 March 2012 – Published in Biogeosciences Discuss.: 21 March 2012 Revised: 12 July 2012 – Accepted: 14 July 2012 – Published: 23 August 2012 Abstract. Comprehensive synoptic datasets (surface water down to 4000 m) of dissolved cadmium (Cd), copper (Cu), manganese (Mn), lead (Pb) and silver (Ag) are presented along a section between 34◦S and 57◦S in the southeastern Atlantic Ocean and the Southern Ocean to the south off South Africa. The vertical distributions of Cu and Ag display nutrient-like profiles similar to silicic acid, and of Cd similar to phosphate. The distribution of Mn shows a subsurface maximum in the oxygen minimum zone, whereas Pb concentrations are rather invariable with depth. Dry deposition of aerosols is thought to be an important source of Pb to surface waters close to South Africa, and dry deposition and snowfall may have been significant sources of Cu and Mn at the higher latitudes. Furthermore, the advection of water masses enriched in trace metals following contact with continental margins appeared to be an important source of trace elements to the surface, intermediate and deep waters in the southeastern Atlantic Ocean and the Antarctic Circumpolar Current. Hydrothermal inputs may have formed a source of trace metals to the deep waters over the Bouvet Triple Junction ridge crest, as suggested by relatively enhanced dissolved Mn concentrations. The biological utilization of Cu and Ag was proportional to that of silicic acid across the section, suggesting that diatoms formed an important control over the removal of Cu and Ag from surface waters. However, uptake by dinoand nano-flagellates may have influenced the distribution of Cu and Ag in the surface waters of the subtropical Atlantic domain. Cadmium correlated strongly with phosphate (P), yielding lower Cd/P ratios in the subtropical surface waters where phosphate concentrations were below 0.95µM. The greater depletion of Cd relative to P observed in the Weddell Gyre compared to the Antarctic Circumpolar Current could be due to increase Cd uptake induced by iron-limiting conditions in these high-nutrient–low-chlorophyll waters. Similarly, an increase of Mn uptake under Fe-depleted conditions may have caused the highest depletion of Mn relative to P in the surface waters of the Weddell Gyre. In addition, a cellular Mn-transport channel of Cd was possibly activated in the Weddell Gyre, which in turn may have yielded depletion of both Mn and Cd in these surface waters. 1 Introduction Our understanding of the biogeochemical cycles of trace elements and their influence on the oceanic productivity in the high-nutrient–low-chlorophyll region of the Southern Ocean is still limited, with perhaps the exception of iron (Fe) (Martin et al., 1990; de Baar et al., 2005; Boyd et al., 2007). Certain trace metals have profiles that are nutrient-like, like cadmium (Cd) and copper (Cu), which is indicative of their involvement in biological cycles (Boyle, 1988; Boyle and Edmond, 1975). In contrast, other metals have a scavenged-type behavior like lead (Pb; Flegal and Patterson, 1983) or behave in a conservative manner like uranium (Bruland and Lohan, 2003). Elements such as manganese (Mn) can be considered Published by Copernicus Publications on behalf of the European Geosciences Union. 3232 M. Boye et al.: Distributions of dissolved trace metals hybrid-type metals at high latitudes, as their distribution is controlled by both biological uptake and scavenging processes (Bruland and Lohan, 2003). Furthermore, the oceanic behavior of other trace metals like silver (Ag) is still not well understood, with little data on Ag distributions in the global ocean, notably in the Southern Hemisphere (Zhang et al., 2004). Despite major advances on the biological involvement of trace metals and their geochemical dynamics in the ocean, basic knowledge is still lacking on their biogeochemical cycles. For instance, there are only a few comprehensive datasets of Mn (Middag et al., 2011a), and of Cu, Cd and Pb (L¨ oscher, 1999; Ellwood, 2008) for the Southern Ocean. The external sources of trace metals to the southeastern Atlantic and the Southern Ocean are not well constrained. Recent work suggests the importance of advection of water masses enriched in trace metals following contact with continental margins (Bown et al., 2011; Chever et al., 2010), in addition to atmospheric depositions to surface waters and inputs from hydrothermal vents to bottom waters (Middag et al., 2011a; Klunder et al., 2011). The coupling between trace and major nutrients cycles, such as the correlations between Cd and phosphate (PO4) in the Southern Ocean (de Baar et al., 1994; Elderfield and Rickaby, 2000), indicates the removal of trace nutrients in surface waters due to phytoplankton uptake and their later sinking and remineralisation in deep waters. However, our knowledge of this coupling is still limited to a small number of trace elements. Such correlations have been used, alongside Cd/Ca ratios preserved in foraminifera tests, to reconstruct past changes in ocean circulation and nutrients distributions (Boyle, 1988). Furthermore, laboratory studies (Sunda and Huntsman, 2000) and shipboard incubation experiments (Cullen et al., 2003) have demonstrated the relationships between certain trace nutrients for phytoplankton uptake. For example, the depletion of Cd before that of PO4 appears to be linked to Fe limitation in the Southern Ocean surface waters (Ellwood, 2008). Thus, a synoptic observation of trace nutrient distributions will increase our understanding of the coupling between the biogeochemical cycles of the trace nutrients in the Southern Ocean. The emergence of new methods for trace metals analysis, such as isotope dilution-inductively coupled plasma mass spectrometry (ID-ICPMS) now allows us to achieve multielement analyses with low limits of detection (Milne et al., 2010). The ID-ICPMS method has been employed here to further assess the biogeochemical cycles of Cd, Cu, Pb, Mn and Ag along a section in the southeastern Atlantic Ocean and the Southern Ocean south off South Africa during the International Polar Year in 2008. The survey area includes contrasting biogeochemical domains with the oligotrophic subtropical South Atlantic in the north, through to the highnutrient–low-chlorophyll region of the Southern Ocean in the southern part of the section (Le Moigne et al., 2012). Furthermore, the section crosses one of the most dynamic and variable ocean domains in the world where the South Atlantic, Southern Ocean and Indian waters converge (Boebel et al., 2003). The distributions of the dissolved trace metals Cd, Cu, Mn, Pb, and Ag are discussed in combination with their atmospheric inputs, and the biogeochemical and physical features of the study region in order to determine the sources of these trace elements, their coupling with the major nutrients and the connection between their biogeochemical cycles. 2 Methods 2.1 Study area Water samples were collected at twelve deep stations located in the southeast Atlantic and Atlantic sector of the Southern Ocean during the multidisciplinary MD 166 BONUSGoodHope cruise. The cruise took place during the International Polar Year in the austral summer 2008 (8 February– 24 March 2008) aboard the French R/V Marion Dufresne II (Fig. 1). The samples were collected from the shelf region off the South African coast to 57◦S in the Weddell Gyre at the southern end of the section. The twelve stations consisted of seven LARGE stations (L#), sampled at up to ten depths to ∼2200m, and five SUPER stations (S#), sampled at up to twenty depths to ∼4000m. The station locations, oceanographic fronts and regions are marked on Fig. 1. 2.2 Sample collection Samples were collected using acid-cleaned 12l GO-FLO bottles (General Oceanics) modified with PTFE O-rings, mounted on a Kevlar wire and triggered using Teflon-coated messengers. The GO-FLO bottles were immediately transferred into a pressurized clean container (class 100) for subsampling. The samples were then collected in acid-cleaned 250ml low-density polyethylene bottles (LDPE, Nalgene) after online filtration with a 0.22µm Sartobran 300 (with 0.4µM pre-filter, Sartorius) cartridge filter under pure N2 pressure (filtered 99.99% N2, 1bar). Each sample was then acidified to pH∼1.9 on board in a laminar flow hood (class 100) with nitric acid (ultrapure HNO® 3, Merck) and stored in double bags in the dark at ambient temperature until their analyses in the shore-based laboratory about 22months after their collection. 2.3 Trace metal determination Concentrations of dissolved trace metals (Cd, Pb, Cu) were determined by ID-ICPMS, and the mono-isotopic element Mn was analysed using a standard addition approach followed by ICPMS detection, following the methods described in Milne et al. (2010). The analysis was conducted following an offline preconcentration/matrix removal step on a Toyopearl AF-650M chelate resin column (Milne et al., 2010). Briefly, 15ml of the acidified sample in an acidcleaned 30ml FEP bottle (Nalgene) was spiked with 100µl of a multi-element standard of isotopes (containing the stable Biogeosciences, 9, 3231–3246, 2012 www.biogeosciences.net/9/3231/2012/ M. Boye et al.: Distributions of dissolved trace metals 3233 Fig. 1. Location of the stations sampled for dissolved Ag, Cd, Cu, Mn and Pb during the MD166 BONUS-GoodHope cruise. Black circles designate the LARGE stations (L) and white circles the SUPER stations (S). The positions of fronts are also shown, with the southern branch of the Subtropical Front (S-STF, ∼42◦20S), the Subantarctic Front (SAF, 44◦20S), the Polar Front (PF, 50◦22.40S), the Southern ACC Front (SACCF, ∼51◦520S) and the Southern Boundary of the ACC (Sbdy, ∼55◦54.30S). From their geographical positions, stations L1, S1 and L2 were in the Subtropical Zone (STZ), S2 was on the northern side of the Subantarctic Zone (SAZ), stations L3, L4, S3 and L5 were within the Polar Frontal Zone, station L6 was on the northern flank of PF, S4 was on the SACCF, station L7 was at the SBdy, and station S5 was in the northern branch of the Weddell Gyre. Figure prepared using Ocean Data View (Schlitzer, 2012). isotopes 65Cu, 111Cd and 207Pb; ISOFLEX, San Francisco, CA, USA) enriched over their natural abundance. A working solution of 65Cu, 111Cd and 207Pb was prepared in 0.024M ultrapure HNO3(Romil, Cambridge UK), and the exact concentration of the enriched isotope spikes in the mixed solution was determined by ICP optical emission spectroscopy (OES) against known natural standards (10mgl−1, ICP High Purity Standards) (Milne et al., 2010). Addition of the isotope spike to the sample contributed 60pMCd, 37pMPb and 1.13nMCu. In addition, standard additions of Mn were performed on sub-sets of the same seawater sample. The spiked samples were left for overnight equilibration. Subsequently, the samples were buffered to pH∼6.2 using 2M ammonium acetate, prepared with ultrapure acetic acid and ammonia (Romil, Cambridge, UK). The buffered sample was then pumped over the preconcentration column, at 2mlmin−1. The column was rinsed with 1ml de-ionised water to remove salts, and subsequently the metals were eluted using 1 ml of 1MHNO3(Romil ultrapure HNO3). The eluent was collected into acid cleaned autosampler polypropylene vials (OmniVials; 4ml) and capped. Prior to loading of the next sample, the column was washed with an acid solution (1.5MHCl) to remove residual trace elements. The extracted samples were analysed using a ThermoFisher Scientific Element II ICP-MS (E2 Bremen, Germany). The sample was introduced via a 100µl Teflon nebuliser connected to a quartz spray chamber. Measurements for Cd and Pb were performed in low resolution mode (R=300); whereas all other elements were measured in medium resolution (R=4000). Silver (Ag) was measured separately by ID-ICPMS with preconcentration on an anion-exchange column (Yang and Sturgeon, 2002; Barriada et al., 2007). In this method about 20ml of the acidified sample was placed in an acid cleaned vial and spiked with 109Ag stable isotope (75µl of ∼5nM solution in 0.024MHNO3per 20ml sample, 18.7pM). The solution was allowed to equilibrate overnight. The sample was then pumped at 3mlmin−1through a 1cm long, 85µM internal volume minicolumn (Global FIA, Inc., Fox Island, WA, USA) filled with a strong anion-exchange resin (Dowex 1X8, 200–400mesh; Supelco, Bellefonte, CA, USA). The sample was eluted using 2ml of 1.2MHNO3(Romil SpA). Measurements were performed using an X-SERIES 2 ICPMS (Thermo Fisher Scientific, Bremen, Germany) in the standard configuration, interfaced with an ASX-510 autosampler (Cetac, Omaha, Nebraska, USA). The concentration of the 109Ag spike solution was determined using ICPOES. The accuracy and precision of the methods were assessed by analyses of SAFe (Sampling and Analysis of iron) and GEOTRACES reference samples (http://www.geotraces.org/science/intercalibration/ 322-standards-and-reference-materials), and results are reported in Table 1. The values determined using the ID-ICPMS method show good consistency with the reported consensus values for dissolved Cd and Pb (Table 1). There is no consensus value for dissolved Ag yet, hence the data recorded in this work may provide a basis for later analyses of the reference samples. Dissolved Cu concentrations determined by ID-ICPMS were lower compared to the consensus values, especially in the deep samples (Table 1), possibly because the samples were not exposed to UV-irradiation prior their analyses (Milne et al., 2010). Dissolved Mn detected by ICPMS yielded higher concentrations than the consensus values (Table 1), in line with the general trend that shows higher dissolved Mn concentrations using methods based upon ICP-MS as compared to those based upon catalytic-enhanced flow injection (http://www. geotraces.org/images/stories/documents/intercalibration/ Files/Reference Samples November11/SAFe Ref Mn.pdf). The complete database of the dissolved concentrations of Cd, Cu, Mn, Ag and Pb at the stations LARGE and SUPER can be obtained via http://www.obs-vlfr.fr/proof/php/x datalist.php?xxop=bonusgh&xxcamp=bonusgh, and will be available at the international GEOTRACES datacenter (http: //www.bodc.ac.uk/geotraces/). www.biogeosciences.net/9/3231/2012/ Biogeosciences, 9, 3231–3246, 2012 ,," ". 3234 M. Boye et al.: Distributions of dissolved trace metals Table 1. Comparison of dissolved Cd, Cu, Pb and Ag analyses obtained in the present study by the ID-ICPMS method and of dissolved Mn obtained by the internal standard addition and ICPMS detection, with the consensus values reported by the SAFe project (in the surface-S and deep-D2 samples) and in the North Atlantic GEOTRACES (in the surface-GS and deep-GD samples) Reference Samples (updated in November 2011). All ±terms represent standard deviation from average values. SAFe concentration Consensus value SAFe GEOTRACES concentration Consensus value GEOTRACES S D2 S D2 GS GD GS GD Cd 0.77 ±0.96 pM 1034 ±15 pM 1.00 ±0.2 pM 986 ±27 pM 2.86 ±0.37 pM 287 ±7 pM 2.4 ±0.4 pM 273 ±6 pM (n=4) (n=8) (n=3) (n=4) Cu 0.56 ±0.03 nM 1.39 ±0.13 nMa0.51 ±0.05 nM 2.25 ±0.11 nM 0.72 ±0.07 nMa1.12 ±0.04 nMa0.83 ±0.08 nM 1.55 ±0.13 nM (n=10) (n=9) (n=4) (n=4) Mn 0.93 ±0.06 nM 0.49 ±0.07 nM 0.79 ±0.06 nMb0.35 ±0.06 nMbNot measured Not measured 1.45 ±0.17 nM 0.21 ±0.04 nM (n=4) (n=4) Pb 51.2 ±2.2 pM 36.3 ±1.4 pM 47.6 ±2.4 pM 27.7 ±1.8 pM 31.1 ±1.6 pM 43.8 ±3.2 pM 29.5 ±2.1 pM 42.2 ±1.3 pM (n=11) (n=7) (n=4) (n=4) Ag 3.5 ±0.7 pM 26.2 ±0.9 pM No value No value 2.7 ±0.9 pM 13.1 ±1.0 pM No value No value (n=3) (n=5) (n=3) (n=4) aThe UV oxidation of the samples might be required to obtain an accurate value for dissolved Cu using this method (Milne et al., 2010). bIn general, methods based upon ICP-MS yield higher dissolved Mn concentrations than methods based upon catalytic-enhanced flow injection. Furthermore, there are significant differences between UV treatment and non-UV treated samples for dissolved Mn http://www.geotraces.org/images/stories/documents/intercalibration/Files/Reference Samples November11/SAFe Ref Mn.pdf. 2.4 Aerosol sampling and analysis Aerosol samples were collected onto single 20×25cm Whatman 41 filters using a Tisch TSP high volume aerosol collector operating at a flow rate of ∼1m3min−1(Baker et al., 2007). Due to the very low aerosol concentrations in the study region, collection times for each sample were relatively long (2–3days), and the collector was only used when the ship was heading into the prevailing wind in order to avoid contamination from the ship. Collection filters were extensively acid-washed before use, with 0.5MAristarHCl, 0.1MAristarHCl (Rickli et al, 2010), followed by a further wash with 0.1MSuprapureHCl, which was carried out in a trace metal clean laboratory. After collection, samples were sealed in plastic bags and immediately frozen at −16◦C for return to the shore laboratory. One quarter of each aerosol filter was extracted into 1M ammonium acetate solution for between 1 and 2h and the solution filtered through 0.2µM minisart filters (Sartorius), as described in Baker et al. (2007). Soluble Mn, Cu, Pb, Cd and Ag were then determined by ICP-OES (Mn and Cu) and ICP-MS (Pb, Cd, Ag). Concentrations of Cd and Ag were below analytical detection limits (0.05–0.2pmolm−3) in all cases. Concentrations ranges for the other elements were 0.8–4.8pmolm−3(Mn), 4.3–13pmolm−3(Cu) and 0.27– 0.83pmolm−3(Pb). Here we estimate the dry deposition flux of these elements as the product of their aerosol concentrations and a dry deposition velocity. We used an estimated dry deposition velocity of 0.3cms−1, a value appropriate for elements likely to be chiefly associated with fine mode aerosol (Duce et al., 1991). We emphasize that dry deposition velocities are probably uncertain to plus or minus a factor of 2–3 (Duce et al., 1991), hence our calculated dry fluxes should be considered as order of magnitude estimates. Atmospheric deposition of trace metals is also likely to occur through wet deposition. Attempts to sample rain and snow during the cruise were unsuccessful and we are therefore unable to make direct estimates of the magnitude of this wet flux. However, the northern part of the transect is relatively dry (Xie and Arkin, 1997) and wet deposition likely makes a less significant contribution there than in the south (Baker et al., 2010). 3 Results 3.1 Hydrography A comprehensive description of the hydrography along the meridional section of this voyage is given in Bown et al. (2011). Further details are provided for the 2004 occupation of this CLIVAR GoodHope Line in Gladyshev et al. (2008). Briefly, the Subtropical Zone (STZ) extended to the southern branch of the Subtropical Front (S-STF; ∼42◦S), including stations L1, S1 and L2 (Fig. 1). Despite its geographical location, S2 exhibited Sand Tsignatures of subtropical waters (Bown et al., 2011), and this station will be discussed within the subtropical domain. The subtropical domain in the Cape Basin region is characterized by a very complex dynamic regime (Lutjeharms and Vanballegooyen, 1988; Gordon et al., 1992). The central water layer was mainly occupied by waters of Indian origin (Gordon et al., 1992). Antarctic Intermediate Water of Indian Ocean origin (I-AAIW) was observed at depths between 800 and 1200m in the region close to Africa (stations L1 and S1), whereas another variety of AAIW was observed to the south (at L2 and S2) featuring AAIW formed in the subantarctic region of the southwest Atlantic (A-AAIW; Piola Biogeosciences, 9, 3231–3246, 2012 www.biogeosciences.net/9/3231/2012/ M. Boye et al.: Distributions of dissolved trace metals 3235 Fig. 2. Diagrams of potential temperature (◦C) versus salinity in the whole water column (left panel) and in deep waters (right panel) at the stations sampled for trace metals determination (CTD data and validation from Branellec et al., 2010; Kermabon and Arhan, 2008). and Gordon, 1989) (Fig. 2). The Upper Circumpolar Deep Water was found at a depth range of ∼1200–1500m, and characterized by its oxygen minimum (160–180µmolkg−1). The complex water mass structure observed at L1 and S1 was likely transported by the Agulhas Current and rings, whereas it may originate from the southwest Atlantic (A-UCDW) at L2 and S2. At greater depths, a diluted variety of the North Atlantic Deep Water was observed which was transported along the southwest African continental shelf (SE-NADW; Arhan et al., 2003; Gladyshev et al., 2008) and characterised by its S signature (Fig. 2) and an oxygen maximum at 2000– 3200m depth at S1 and S2. Near the seafloor an old variety of Antarctic Bottom Water (AABW) was observed in the Cape Basin abyssal plain at S1. In the Antarctic Circumpolar Current (ACC; Fig. 1), the surface water was marked by a southward decrease of temperature from 4◦C (L5) to 2◦C (S4) and low S(Fig. 2). Below the surface mixed layer, AAAIW was observed; this water mass subducted northward along the Subantarctic Front (SAF). At greater depths the Antarctic Winter Waters (AAWW; characterized by a Tminimum; Fig. 2), the A-UCDW, and a diluted variety of NADW whichflows along thecontinentalslope of SouthAmerica before being injected in the ACC in the southwestern Atlantic (SW-NADW; Whitworth and Nowlin, 1987) were observed north of the Polar Front (PF). South of the PF, another variety of UCDW which had passed through the Drake Passage (DP-UCDW) was identified by a core of low oxygen water (stations L6, S4 and L7). There deep waters exhibited properties of Lower Circumpolar Deep Waters (LCDW; Fig. 2). Finally, south of the Southern Boundary of the ACC (SBdy), the near surface waters may have been in contact with the western continental margin of the Antarctic Peninsula, while the deeper waters may have been in contact with the northern topographic limit of the Weddell Basin (Orsi et al., 1993; Meredith et al., 2000). Near the seafloor a fresher and colder variety of AABW was observed south of the ACC domain, as compared to AABW observed in the Cape Basin abyssal plain and on the northern flank of the Mid-Atlantic Ridge (Fig. 2). 3.2 Dissolved copper The vertical distributions of dissolved Cu displayed nutrientlike behavior with lower concentrations in surface waters and increasing levels with depth (Figs. 3 and 4). There was a clear north–south gradient in the ACC and the Weddell Gyre waters, with the southern waters having higher concentrations compared to the north of the area (Fig. 3). This gradient followed that of silicate (Fig. 4). The highest Cu concentrations in surface waters were recorded in the northern branch of the Weddell Gyre (Fig. 3). In contrast, in the subtropical waters the dissolved Cu concentrations decreased in southward direction, and were lowest in the south of this domain (at stations L2 and S2) where enhanced chlorophyll aconcentrations were observed along the section (Le Moigne et al., 2012). Deep maxima of Cu were observed in the cores of IAAIW and SE-NADW (at S1), A-AAIW (at L3, L4 and L5), A-UCDW (at L3), and DP-UCDW (at S4) (Figs. 3 and 4). 3.3 Dissolved silver Vertical distributions of dissolved Ag generally showed lower concentrations in surface waters and increasing concentrations with depth (Fig. 3), similar to silicate (Fig. 4). There was a general north–south trend in the Ag concentrations along the section, with the concentrations of Ag being generally lower in the subtropical domain than in the ACC (Fig. 3). Relative Ag minima were observed at several stations at depths between 150 and 350m (Figs. 3 and 4) coinciding with oxygen minima (see dissolved O2profiles in Bown et al., 2011), whereas relative maxima were recorded www.biogeosciences.net/9/3231/2012/ Biogeosciences, 9, 3231–3246, 2012 ~ E " • ~ " • ~ E ~ " ro " e • , O ~ , 33.0 m 34.11 34. 5 35.0 15.5 Sallnlty Salinity 3236 M. Boye et al.: Distributions of dissolved trace metals Fig. 3. Contour plots of dissolved copper (Cu, in nM), silver (Ag, in pM), lead (Pb, in pM), cadmium (Cd, in pM) and manganese (Mn, in nM) along the MD166 BONUS-GoodHope section between the southeastern Atlantic and the Southern Ocean. The positions of L and S stations and the fronts are indicated. The colour map gridding is based on the sampling resolution along the section of ∼2600km that was achieved with 12 stations separated on average by ∼2.2◦latitude, with a total of 160 sampling depths. Figure prepared using Ocean Data View (Schlitzer, 2012). in deeper waters in the cores of the different varieties of UCDW – UCDW which was transported by the Agulhas Current (at S1), of southwest Atlantic origin (at S2), and which had passed through the Drake Passage (at S4 and L7). Conversely, Ag was relatively depleted in the cores of SE-NADW (at S1 and S2) and SW-NADW (L3 to L5). 3.4 Dissolved lead The distributions of Pb were rather constant with depth across the section (with concentrations ranging between 10.2 and 32.3pM), with the exception of the two northern stations closest to South Africa (L1 and S1) where Pb concentrations were much higher in the top 100m depth (e.g. 37–55pM) than in deeper waters (Fig. 3). Enrichment in the core of IAAIW was observed at S1 station (Fig. 3). The lowest concentrations of Pb at depth (7.1–12.0pM) were observed at the Southern Boundary of the ACC (station L7; Fig. 3). 3.5 Dissolved cadmium Dissolved Cd concentrations increased markedly with depth (Fig. 3), similar to the major nutrient phosphate (Fig. 4). There was an obvious north–south gradient in the surface concentrations of Cd, similar to phosphate (Fig. 4), with lower concentrations in the northern waters (as low as 3.8pM) as compared to much higher Cd concentrations in the southern waters of up to 895pM at the ACC Southern Boundary (Figs. 3 and 4). However, the surface Cd concentrations in the Weddell Gyre were different from this pattern, since they were generally lower than the values recorded on the southern side of the ACC, south of the PF (Fig. 3). 3.6 Dissolved manganese The vertical distributions of dissolved Mn generally showed low concentration in surface waters, a subsurface maximum in the oxygen minimum zone deepening southwards, and rather constant and low deep water concentrations in the range of or below the surface values (Figs. 3 and 4). Biogeosciences, 9, 3231–3246, 2012 www.biogeosciences.net/9/3231/2012/ 1 1 I I 1 ! M. Boye et al.: Distributions of dissolved trace metals 3237 Fig. 4. Vertical distributions of dissolved copper (a) and silver (b), silicic acid (c), dissolved cadmium (d), manganese (e), and phosphate (f) at selected L and S stations (the data of major nutrients are from Le Moigne et al., 2012). However, enhanced Mn concentrations (e.g. 0.8–1.3nM) were recorded in the upper 100m at the northern stations close to South Africa (L1 and S1; Figs. 3 and 4). In the ACC the highest surface concentrations were observed on the southern side of the ACC south of PF, particularly at the ACC Southern Boundary (at L7; Fig. 4), whereas comparatively low Mn concentrations occurred in the Weddell Gyre (Fig. 3). Enhanced Mn concentrations in bottom waters were observed above the Mid-Atlantic Ridge (at S4; Figs. 3 and 4). 4 Discussion 4.1 Comparison of datasets of trace metal concentrations obtained at two crossover stations During the MD166 BONUS-GoodHope cruise, two crossover stations were occupied with the Zero & Drake expedition aboard Polarstern (ANT XXIV/3 cruise; Fahrbach et al., 2011) at about 2week intervals to allow comparison of trace metals sampling and analyses (Fig. 5). The samples collected during the Zero & Drake expedition were taken using internally Teflon-coated PVC GO-FLO samplers (General Oceanic Inc.) mounted on an all-titanium frame connected to a Kevlar wire (de Baar et al., 2008), as compared to this study, where samples were collected with GO-FLO bottles attached to a Kevlar line and triggered using Teflon-coated messengers. During both cruises, sub-sampling was done after online filtration through 0.2µM filter cartridges (Sartorius) under pure N2pressure, in a class 100 clean room container. The upper 1000m showed considerable variability in potential temperature at the northern crossover station (e.g. BGH#S2 versus Z&D#101), probably caused by the strong dynamic in this region, and silicate showed offsets of 5–10% between 1500 and 3000m at this station (Fig. 5). At the southern crossover station (e.g. BGH#S3 versus Z&D#104), the temperature profiles were very similar, but still an offset of 5–10% in silicate concentrations was observed in deep waters (Fig. 5). In spite of these differences in hydrology and probably in methodology for silicate analyses, the linear regression www.biogeosciences.net/9/3231/2012/ Biogeosciences, 9, 3231–3246, 2012 3238 M. Boye et al.: Distributions of dissolved trace metals Fig. 5. Comparison of the vertical profiles of temperature (◦C) and silicate concentrations (µM) (top panel), and of dissolved copper (a1) and manganese (b1) concentrations at the two crossover stations of the MD166 BONUS-GoodHope cruise (BGH#S2 at 42.47◦S, BGH#S3 at 47.55◦S) and the ANT XXIV/3 cruise (Z&D#101 at 42.34◦S, Z&D#104 at 47.66◦S) along the Greenwhich Meridian. Linear regression between the two datasets of copper concentrations (a2) (in nmolkg−1) and of manganese concentrations (b2) (in nmoll−1) obtained at the crossover stations (the Z&D dataset of Mn is from Middag et al., 2011a, and of Cu from Heller et al., 2012). between the two datasets of Cu concentrations is significant (r2=0.86, n=5; Fig. 5a2), with the concentrations determined by graphite furnace atomic absorption (ETAAS: Perkin-Elmer Model 4100 ZL) after pre-concentration by simultaneous dithiocarbamate-freon extraction (Heller et al., 2012) being ca. 0.11nmolkg−1(n=5) higher than those determined by ID-ICPMS (this study), notably in the deep waters below 1000m (Fig. 5a1). Measurements of the SAFe intercalibration samples S and D2 by graphite furnace atomic absorption yield Cu concentrations of 0.40±0.02nM (n=2) and 2.22±0.08nM (n=2), respectively (Heller et al., 2012). These values are lower than consensus values in the surface (Table 1), and in the range of the consensus values in the deep D2 sample (Table 1), again suggesting that the samples may need to be exposed to UV-irradiation prior to analysis (Milne et al., 2010). The comparison of the vertical profiles of Mn generally showed similar shapes at both crossover stations (Fig. 5b1). However, the Mn concentrations determined by ICPMS (this study) yield ca. 0.045nM (n=25) higher values than those determined by flow-injection analysis and chemiluminescence detection (Middag et al., 2011a) (Fig. 5b2). The factor of 0.808nMnM−1difference between the ICPMS and FIA-chemiluminescence methods (Fig. 5b2) also falls in the 4 to 9% offsets of the Mn values we determined by ICPMS in the SAFe Sand D2 samples compared to the Biogeosciences, 9, 3231–3246, 2012 www.biogeosciences.net/9/3231/2012/ M. Boye et al.: Distributions of dissolved trace metals 3239 consensus values (Table 1). Measurements of the SAFe intercalibration samples S and D2 by FIA-chemiluminescence method yield Mn concentrations of 0.73±0.004nM (n=1) and 0.32±0.01nM (n=37), respectively (Middag et al., 2011a), in excellent agreement with the consensus values (Table 1); yet, the consensus values are partly dependent on the values reported by Middag et al. (2011a), whereas they are independent of the ones we measured (Table 1). Comparison of two Cd datasets at the southern crossover station showed a good agreement between the isotope dilution thermal ionization mass spectrometry method (O. Baars, personnal communication, 2012) and the ID-ICPMS method (this study) (data not shown). Overall, there is fairly good agreement for Cu and Mn (and Cd) between the two cruises, with some further investigation needed to determine the sources of difference. 4.2 Imprints of the continental margin–open ocean exchanges and external sources Concentrations of dissolved Pb and Mn in surface waters were the highest close to South Africa where these waters were of Indian Ocean origin (Fig. 3). The waters in this region interact with shelf and slope waters, cyclones and filaments of South Atlantic origin, and are ejected as Agulhas rings from the western boundary current of the South Indian Ocean, the Agulhas Current, at its retroflection (Lutjeharms and Vanballegooyen, 1988). They can be imprinted with external inputs of trace metals from the shelf and slope waters of the South African margin. The waters may also receive atmospheric inputs from Southern Africa-derived sources along their transport route from the Indian Ocean (Piketh et al., 2002). In addition, the Patagonian desert may constitute a dust source (Chever et al., 2010) to the waters west of South Africa. Furthermore, benthic and fluvial sources, such as on the Agulhas Bank, can increase Mn concentrations (Landing and Bruland, 1980; Aguilar-Islas and Bruland, 2006). Atmospheric inputs (Baker et al., 2006), reductive dissolution and subsequent diffusion from sediments above margin and slope (Pakhomova et al., 2007), and photoreduction of Mn oxides in surface waters (Sunda and Huntsman, 1994) also contribute to enhance dissolved Mn concentrations in surface waters. The dry deposition flux of Mn was relatively high in the northernmost part of the section (e.g. ∼0.8nmolm−2d−1; Fig. 6), whereas enhanced fluxes were also recorded on the southern side of the ACC (e.g. up to 1.2nmolm−2d−1; Fig. 6). In addition, Pb has been identified as an excellent tracer for anthropogenic sources of dust and particles to surface waters (Boyle, 2001), and these sources are likely higher on the South African margin than in the remote Southern Ocean. For instance, the highest dry deposition flux of Pb (0.14–0.22nmolm−2d−1; Fig. 6) was found in the northernmost side of the section. Other trace metals have also shown enrichments in the surface waters close to Africa, such as particulate aluminium (Al) (JeanFig. 6. Dry deposition flux of Mn (blue squares), Cu (orange triangles) and Pb (green circles) (in nmolm−2d−1) as a function of latitude along the section in the southeastern subtropical Atlantic and the Southern Ocean. An estimated dry deposition velocity of 0.3cms−1was used to calculate the fluxes. Ag and Cd dry deposition fluxes were extremely low along the section: from <0.06nmolm−2d−1in the northernmost part of the section to <0.02nmolm−2d−1in the southern side of the ACC for Ag, and below 0.05nmolm−2d−1in the northernmost area to <0.01nmolm−2d−1in the southern side of the ACC for Cd. del et al., 2010), dissolved cobalt (Co) (Bown et al., 2011), and dissolved and total iron (Fe) (Chever et al., 2010). The isotopic composition of dissolved Fe was also imprinted by the proximity of the African continent in the northern part of the section (Lacan et al., 2008). This further suggests that advection of Indian Ocean waters that interact strongly with shelf and slope waters of the South African margin, as well as with atmospheric inputs and river discharges on the Agulhas Bank, can form a source of trace elements to the southeastern Atlantic Ocean. Relatively high concentrations of Cu, Ag, Cd and Mn were also observed in surface waters on the southern side of the ACC, notably at the ACC Southern Boundary, whereas the lowest Pb concentrations were recorded there (Fig. 3). These relatively enhanced concentrations coincided with enhanced dry deposition fluxes of Cu and Mn, which were 1.4 to 2.8 times higher on the southern side of the ACC compared those in the central subtropical domain (Fig. 6), and were observed during snow fall at those latitudes during the expedition. Such snow inputs may have been a source of these trace metals to surface waters at the polar latitudes, which www.biogeosciences.net/9/3231/2012/ Biogeosciences, 9, 3231–3246, 2012 3246 M. Boye et al.: Distributions of dissolved trace metals Saito, M. A., Goepfert, T. J., Noble, A. E., Bertrand, E. M., Sedwick, P. N., and DiTullio, G. 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