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MARCHE-02851; No of Pages 14 Marine Chemistry xxx (2011) xxx–xxx Contents lists available at ScienceDirect Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa Johann Bown a,⁎, Marie Boye a, Alexander Baker b, Eric Duvieilbourg a, François Lacan c, Frédéric Le Moigne a,1, Frédéric Planchon a,2, Sabrina Speich d, David M. Nelson a a Laboratoire des Sciences de l'Environnement Marin UMR6539, Institut Universitaire Européen de la Mer, Technopôle Brest Iroise, Place Nicolas Copernic, 29280 Plouzané, France School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom LEGOS, Observatoire Midi Pyrénées, 18 av. Edouard Belin, 31401 Toulouse Cedex 9, France d Laboratoire de Physique des Océans, Universite de Bretagne Occidentale — UFR Sciences, 6 av. Le Gorgeu, C.S. 93837, 29238 Brest Cedex 3, France b c a r t i c l e i n f o Article history: Received 7 September 2010 Received in revised form 27 January 2011 Accepted 25 March 2011 Available online xxxx Keywords: Dissolved cobalt Southeastern Atlantic Southern Ocean International Polar Year GEOTRACES a b s t r a c t The spatial distribution, biogeochemical cycle and external sources of dissolved cobalt (DCo) were investigated in the southeastern Atlantic and the Southern Ocean between 33°58′S and 57°33′S along the Greenwich Meridian during the austral summer 2008 in the framework of the International Polar Year. DCo concentrations were measured by flow-injection analysis and chemiluminescence detection in filtered (0.2 μm), acidified and UV-digested samples at 12 deep stations in order to resolve the several biogeochemical provinces of the Antarctic Circumpolar Current and to assess the vertical and frontal structures in the Atlantic sector of the Southern Ocean. We measured DCo ranging from 5.73 ± 1.15 pM to 72.9 ± 4.51 pM. The distribution of DCo was nutrient-like in surface waters of the subtropical domain with low concentrations in the euphotic layer due to biological uptake. The biological utilization of dissolved cobalt was proportional to that of phosphate in depletion ratio of ~ 44 μM M−1. In deeper waters the distribution the subtropical domain with a DCo:HPO2− 4 indicated remineralization of DCo and inputs from the margins of South Africa with lateral advection of enriched intermediate and deep waters to the southeastern Atlantic Ocean. In contrast the vertical distribution of DCo changed southward, from a nutrient-like distribution in the subtropical domain to scavenged-type behavior in the domain of the Antarctic Circumpolar Current and conservative distribution in the Weddell Gyre. There the cycle of DCo featured low biological removal by Antarctic diatoms with input to surface waters by snow, removal in oxygenated surface waters, and dissolution and stabilization in the low-oxygenated Upper Circumpolar Deep Waters. DCo distributions and physical hydro-dynamics features also suggest inputs from the Drake Passage and the southwestern Atlantic to the 0° meridian along the eastward flow of the Antarctic Circumpolar Current. Bottom enrichment of DCo in the Antarctic Bottom Waters was also evident, together with increasing water-mass pathway and aging, possibly due to sediment resuspension and/or mixing with North Atlantic Deep waters in the Cape Basin. Overall atmospheric input of soluble Co by dry aerosols to the surface waters was low but higher in the ACC domain than in the northern part of the section. At the highest latitudes, it is possible that snowfall could be a source of DCo to surface waters. Tentative budgets for DCo in the mixed layer of the subtropical and the ACC domains have been constructed for each biogeochemical region encountered during the cruise. The estimated DCo uptake flux was found to be the dominant cobalt flux along the section. This flux decreases southward, which is consistent with the observations that DCo shows a southward transition from nutrient-like towards conservative distribution in the mixed layer. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The abundance of cobalt (Co) in the upper continental crust (10 ppm) is much lower than that of iron (3.5 104 ppm), with a Fe/Co ⁎ Corresponding author. Tel.: + 33 2 98 49 88 38. E-mail address: [email protected] (J. Bown). 1 Now at: National Oceanography Centre, Southampton University of Southampton, Waterfront Campus European Way Southampton, SO14 3ZH, United Kingdom. 2 Previously at: Royal Museum for Central Africa (RMCF), 3080 Tervuren, Belgium. crustal abundance ratio of ~3500 (Taylor and McLennan, 1995). Dissolved cobalt (DCo) concentrations in the open ocean are low, typically b150 pM (Knauer et al., 1982; Martin et al., 1990; Fitzwater et al., 2000; Saito and Moffett, 2001; Saito et al., 2004; Ellwood, 2008; Noble et al., 2008; Pohl et al., 2011). Knowledge of Co distributions in the ocean has increased with the development of sensitive analytical techniques that are more accurate at such low concentrations (Vega and van den Berg, 1997; Cannizzaro et al., 2000; Saito and Moffett, 2001; Milne et al., 2010; Shelley et al., 2010) but data are still scarce in the ocean worldwide. Nevertheless oceanic distributions have suggested a 0304-4203/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2011.03.008 Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 2 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx nutrient-like cycle of DCo (Martin et al., 1993; Saito and Moffett, 2002; Saito et al., 2004; Jakuba et al., 2008; Noble et al., 2008; Saito and Goepfert, 2008), with additional (versus phosphate) Co removal from deep waters by scavenging as observed in the deep oligotrophic northeast Pacific and western Philippine Sea (Knauer et al., 1982; Wong et al., 1995). The elemental composition of phytoplankton suggests that Co is an important micronutrient for the growth of cyanobacteria (Sunda and Huntsman, 1995), coccolithophorids and dinoflagellates (Ho et al., 2003). Cobalt is involved in important biological functions and molecules such as vitamin B12 and carbonic anhydrase (Droop, 1957; Guillard and Cassie, 1963; Morel et al., 1994; Bertrand et al., 2007). Furthermore laboratory studies have shown that cobalt can interact with cadmium and zinc in cellular metabolism (Price and Morel, 1990; Sunda and Huntsman, 1995; Saito and Goepfert, 2008), although the specific impact of cobalt on phytoplankton growth and community composition is still not well understood. In addition to biological removal, internal processes and external sources such as remineralization, inputs from the atmosphere, deep inputs from the continental margin and physical hydro-dynamics may impact the oceanic distribution of cobalt. Sources of Co to surface waters are poorly constrained. Knauer et al. (1982) and Wong et al. (1995) have reported atmospheric inputs, whereas the impact of physical features and water masses on DCo distribution has been described by Wong et al. (1995), Saito et al. (2004) and Noble et al. (2008). However DCo distribution may reflect both the biogeochemical and physical characteristics of the studied regions (Wong et al., 1995; Saito and Moffett, 2002; Saito et al., 2004; Noble et al., 2008). In this study the vertical and meridional distributions of DCo are presented in the southeastern Atlantic Ocean and the Southern Ocean south off South Africa. The section includes contrasting biogeochemical domains, with an oligotrophic system in the subtropical northern region and a High Nutrient Low Chlorophyll region (HNLC; Minas et al., 1986) in the Antarctic domain in the southern part of the section. Furthermore, the transect crossed the zone where South Atlantic, Southern Ocean and Indian waters converge in one of the most dynamic and variable ocean domains in the world (Wunsch and Stammer, 1995; Boebel et al., 2003). DCo concentrations are discussed in combination with biogeochemical and physical features to further investigate how cobalt is distributed in these key ocean regions and to explain the distribution of DCo in terms of sources, sinks and internal processes that could impact its behavior within the water column. 2. Methods 2.1. The cruise Samples were collected during the multidisciplinary MD166 BONUSGoodHope cruise that took place during the International Polar Year in the austral summer 2008 (02/13/08–03/24/08) on board the French R/V Marion-Dufresne II which sailed from Cape Town, South Africa, and along the Greenwich Meridian to 57°S in the Southern Ocean (Fig. 1). Two main types of stations were sampled for trace metal analysis. These stations were defined by the number and depth of operations; LARGE (L) and SUPER (S) (Fig. 1). The distribution of DCo was studied at the five SUPER deep cast stations with ~20 sampling depths between 15 and 4000 m, and at the seven shallower LARGE stations with ~10 sampling depths between 15 and 2500 m. The position and designation of each station along the section and the position of the fronts are shown in Fig. 1. The exact position of the stations and depths sampled for DCo is reported in Table 1. 2.2. Sample collection Samples were collected with 12 L acid-cleaned GO-FLO bottles (General Oceanics®) modified with PTFE rings, attached to a Kevlar Fig. 1. Location of the stations sampled for DCo during the MD166 BONUS-GoodHope cruise. Black circles designate the LARGE stations and white circles the SUPER stations. The positions of fronts are also shown, with the southern branch of the Subtropical Front (S-STF; ~42°2’S), the SubAntarctic Front (SAF; 44°2’S), the Polar Front (PF; 50° 22.4’S), the Southern ACC Front (SACCF; ~51°52’S) and the Southern Boundary of the ACC (SBdy; ~55°54.3’S). 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 Sby, and station S5 was in the northern branch of the Weddell Gyre. Map from ODV (Reiner Schlitzer). wire and closed using Teflon-coated messengers. The samples were then collected in acid-cleaned 250 mL LDPE Nalgene® bottles after online filtration with a 0.22 μm Sartobran® 300 (Sartorius) cartridge under pure N2 pressure (filtered 99.99% N2, 1 bar) in a pressurized clean container (class 100). Each sample was then acidified on board under a laminar flow hood (class 100) with hydrochloric acid (ultrapur HCl®, Merck) and stored at pH ~ 2 in double bags at dark and ambient temperature, until their analyses in the shore-based laboratory about 18 months after their collection. 2.3. Measurement of dissolved cobalt concentration 2.3.1. Method and equipment Dissolved cobalt concentrations were measured in acidified and UV-digested samples by Flow-Injection Analysis (FIA) and chemiluminescence detection, following the method of Shelley et al. (2010). The method includes UV oxidation of the acidified sample (30 mL) for 3 h in clean silica tubes using a 600 W high-pressure mercury-vapor lamp and an equilibration time of 48 h before analysis. An IDA-based resin (Toyopearl AF-650M Chelate) was used instead of immobilized 8-HQ resin (Sakamoto-Arnold and Johnson, 1987). The pre-concentration resin was conditioned and rinsed with ammonium acetate buffer, and the reaction temperature was maintained at 60 °C using a thermostatic bath to avoid interference due to bubbles passing through the detector (Shelley et al., 2010). Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx Table 1 Dissolved cobalt concentrations (DCo) and corresponding depths with the position of the stations and the biogeochemical domains. Errors on the DCo concentrations (Stdv) were calculated as the standard deviation of triplicate measurements of the same sample. nd means not determined. Domain Station Latitude (°S) Longitude (°E) Depth (m) DCo (pM) Stdv DCo (pM) STZ L1 34.43 14.4 20 40 60 80 200 700 800 1000 1200 2100 20 30 40 70 100 200 300 500 700 850 1000 1200 1400 1600 2000 2700 3050 3500 3800 4000 15 35 45 95 300 600 800 1200 1400 2100 15 30 35 80 120 196 314 461 598 809 1029 1250 1441 1764 2156 2548 2891 3245 3636 3950 30 100 150 270 400 600 1200 1400 2100 30 60 27.1 29.7 30.4 29.9 35.0 50.1 48.6 47.5 54.4 46.1 23.7 15.6 26.3 41.0 44.3 30.5 37.6 42.7 51.8 46.6 48.6 48.8 45.3 46.6 48.9 45.4 53.9 37.8 38.7 41.4 17.0 15.3 19.0 17.7 46.6 54.6 54.0 46.3 45.9 49.2 7.40 5.73 7.50 21.6 26.3 38.2 44.2 52.1 65.1 49.7 41.0 59.6 65.6 43.4 45.1 59.2 44.6 41.8 41.8 36.7 22.8 38.7 50.8 49.4 55.0 47.2 49.5 51.9 47.1 27.3 29.1 2.58 2.55 1.54 0.67 2.70 3.09 2.20 2.39 2.55 0.20 2.15 1.68 1.97 0.23 0.73 1.43 2.80 0.65 0.53 2.35 2.39 1.52 4.64 5.17 2.14 1.35 3.30 1.53 2.58 2.39 0.54 3.19 0.23 1.08 5.63 4.30 3.33 0.95 1.44 2.38 0.67 1.15 1.39 1.19 0.84 0.99 1.14 2.66 4.89 2.21 1.30 6.54 0.86 1.84 1.79 2.05 1.86 4.14 1.65 1.93 1.34 0.83 1.53 2.50 9.41 3.56 1.04 1.21 1.01 nd 1.66 STZ S1 36.50 13.1 STZ L2 41.18 9.92 S-STZ S2 42.47 8.93 SAZ SAZ L3 L4 44.88 46.01 6.88 5.87 (continued on next page) 3 Table 1 (continued) Domain Station Latitude (°S) Longitude (°E) SAZ L4 46.01 5.87 PFZ S3 47.55 4.37 PFZ L5 49.03 2.84 North PF L6 50.38 1.33 South PF S4 51.85 0 Sbdy L7 55.23 0.03 Weddell Gyre S5 57.55 0.03 Depth (m) DCo (pM) Stdv DCo (pM) 100 150 270 400 800 1300 1600 2050 20 30 40 70 100 200 300 450 600 800 1070 1500 2020 2500 3000 3500 3980 40 80 150 170 250 350 700 1000 1600 2200 30 60 100 135 180 300 600 850 1600 2100 30 60 130 160 180 250 300 350 400 500 700 900 1117 1950 2300 2490 30 60 100 120 200 300 650 1000 1500 2100 30 60 120 190 48.2 40.5 52.9 49.2 48.7 45.9 50.3 45.5 35.4 33.6 37.3 35.7 42.9 40.5 63.1 50.1 43.0 48.9 56.3 42.3 39.5 40.7 36.6 33.3 30.5 43.5 43.8 42.3 50.2 49.0 53.9 73.0 49.6 46.0 39.9 39.0 37.8 51.0 58.7 52.0 48.6 48.0 45.1 39.5 36.5 40.3 37.6 48.8 45.6 50.4 56.8 55.2 49.6 50.4 50.2 47.1 38.8 36.5 62.9 36.5 28.6 51.3 45.6 47.7 49.7 56.2 49.5 43.9 46.5 33.1 44.7 36.7 36.0 32.1 36.9 1.90 1.86 1.28 2.01 2.12 2.32 0.50 2.53 2.45 0.85 1.33 1.27 1.80 2.17 3.50 0.30 1.03 1.08 3.34 0.28 1.84 3.22 0.91 3.20 0.27 2.33 2.93 3.23 1.56 0.95 2.25 4.51 1.37 1.10 1.26 1.54 3.27 1.52 2.19 1.72 0.46 1.97 3.06 2.22 1.74 0.87 2.10 1.09 2.55 2.04 1.40 2.27 1.33 6.14 1.66 2.98 1.56 3.40 8.04 3.16 0.95 1.11 nd 2.54 1.88 4.46 3.15 3.79 2.79 1.89 0.59 0.49 0.60 1.86 1.05 (continued on next page) Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 4 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx Table 1 (continued) Domain Station Latitude (°S) Weddell Gyre S5 57.55 Longitude (°E) 0.03 Depth (m) DCo (pM) Stdv DCo (pM) 250 350 450 550 750 800 1250 1700 2150 2600 3050 3500 3840 43.4 36.4 39.7 34.4 50.4 37.6 30.1 38.4 32.9 33.3 24.8 29.1 30.5 1.23 2.22 2.74 4.52 nd 4.03 1.91 0.65 1.26 3.01 1.30 1.01 0.32 Briefly, the measurements are based on the oxidation reaction of pyrogallol with hydrogen peroxide in an alkaline solution in the presence of cetylmethylammonium bromide (CTAB) and methanol. The reaction is catalyzed by cobalt, hence chemiluminescent emission in the visible wavelengths that is proportional to the cobalt concentration is produced. The flow injection manifold consisted of a peristaltic pump (205 CA, Watson Marlow) used at 10 rpm, Tygon® pump tubing (internal diameter of 1.85 mm for all the reagents except for the buffer line which was 1.52 mm), two micro-electronically actuated, 6-port, 2 position, injection valves (SCIVEX valve and VICI valve from VALCO instruments), and connecting PTFE tubing of 0.75 mm diameter. The detection system is consisted of a photomultiplier detector (Hamamatsu, H9319 Series). The injection valves and the photomultiplier detector were connected to a computer by serial ports and operated by a Labview® 8.4 interface adapted from Bowie et al. (2005). The electronic devices were connected to a current modulator (ELLIPSE MAX 600, MGE/UPS Systems). The DCo concentration was calibrated against two calibration curves made with standard additions of cobalt of 0, 12.5, 25, 50 and 75 pM and performed before and after each series of 8 samples (see below). DCo concentrations and the standard deviation of the measurement were based on triplicate analyses of each sample using the peak height of the chemiluminescent signal. At a pump flow rate of 10 rpm using 1.85 mm internal diameter sample line tubing and a time of 5 min resulted in 6.25 mL of sample being preconcentrated. A complete run of triplicate analyses of each sample took ~23 min. Finally, the reaction buffer solution made up of sodium hydroxide (0.2 M, Sigma Ultra®, N98%) and 20% v/v methanol (Sigma, for trace analysis®) was prepared. Sodium hydroxide is required to reach the optimal chemiluminescent reaction pH of 10.4 and the addition of methanol leads to an increase of the chemiluminescence signal. The Toyopearl IDA AF-650M ion exchange resin was mounted in a 1 cm column device with metal-free frits (Global-FIA Inc.) and washed with 1 M HCl for 3 h and then rinsed with MilliQ for 3 h. Primary standards of Co(II) with concentrations of 10−6 M and −8 10 M were prepared weekly in acidified MilliQ water from serial dilution of a Spectrosol commercial solution (979 μg mL−1, Sigma Aldrich). Calibration was made daily with DCo standards containing 0, 12.5, 25, 50 and 75 pM of added DCo using the 10−8 M stock solution diluted in filtered and acidified seawater to pH ~ 2 with HCl (Ultrapur®, Merck). The seawater used for the calibrations was collected at station S5 (57.55°S 0.03°E at 30 m) and cleaned by passing through Toyopearl IDA AF-650M ion exchange resin. The background concentration of DCo in this seawater (at 0 addition of DCo) was 3.72 ± 1.36 pM (n = 40). 2.3.3. Analytical performance Duplicate reagent blanks including the buffer blank were analyzed by measuring DCo with MilliQ water used instead of sample, at the beginning and at the end of DCo analyses (8 samples on average). The mean reagent blank (based on all blank determinations) was 5.90 ± 1.24 pM of DCo (n = 40). DCo concentrations presented in Table 1 are corrected with their respective reagent blanks. The limit of detection of the method was estimated as three times the standard deviation of the mean reagent blank and was thus on average 3.72 pM of DCo (n = 40). Each series of samples was calibrated by running one sample collected during the Sampling and Analysis of iron (SAFe) program, either the surface “S-SAFe” sample or the deep “D2-SAFe” sample, along with the samples. SAFe samples were UV-digested for 3 h prior to analyses and the results of DCo in S and D2 samples are reported in Table 2. Our measured DCo concentration in the D2-SAFe is within the range published (Johnson et al., 2007). Our S-SAFe value is slightly higher but this is more likely a reflection of the detection limit reported here (3.72 pM), which is close to the S-SAFe consensus concentration (4.20 ± 1.90 pM, Table 2). 2.4. Hydrography 2.3.2. Preparation of the reagents In order to reduce airborne contamination, sample handling in the home laboratory was carried out under a laminar flow hood (ADS Laminaire, ISO 5 class). All reagents were prepared in 1L LDPE Nalgene® bottles with MilliQ water the day before the analysis and kept overnight at room temperature for equilibration. A rinsing solution composed of HCl (0.05 M, Suprapur®, Merck) and ammonium acetate (0.05 M, Sigma Ultra®) was used to remove the seawater matrix ions from chelating resin; those ions would otherwise have interfered with the chemiluminescence detection. An ammonium acetate (0.3 M, Sigma Ultra®) buffer solution was used for in-line pH adjustment of samples up to pH ≥ 5.2 to keep within the optimal working pH of the chelating resin and ensure quantitative binding of cobalt. This solution was cleaned on line by passing through a column filled with IDA AF-650M chelate Toyopearl resin. Hydrochloric acid (0.1 M, Suprapur®, Merck) was used to elute the chelated cobalt from the resin column. The final volume of the eluted sample was 1.89 mL. The pyrogallol reagent solution was made up of pyrogallol (0.05 M, Sigma 99%, ACS reagent), CTAB (0.025 M, Sigma Ultra®, 99%) and 6% v/v, hydrogen peroxide (35%, Sigma Aldrich). CTAB was used to provide micellar environment for increasing the sensitivity of the reaction. Potential temperature (Θ), salinity (S) and dissolved oxygen (O2) were recorded using a SBE 911+ Seabird® probe, with respectively, SBE3+, SBE4 and SBE43 sensors (Branellec et al., 2010). Data recording and validation of potential temperature, salinity and oxygen are described by Branellec et al. (2010) and by Kermabon and Arhan (2008). 2.5. Macronutrient analyses Nutrient samples were collected using a CTD-rosette (SBE 32 Seabird®) equipped with Niskin bottles (Le Moigne, 2008; Le Moigne et al., in preparation). Silicate and nitrate were analyzed by standard Table 2 Comparison of dissolved cobalt analyses obtained for UV-oxidized samples by the FIAChemiluminescence method used in the present study with consensus values reported by the Sampling and Analysis of iron (SAFe) program. Water samples provided by SAFe from surface waters (SAFe S) and deep waters (SAFe D2) were analyzed. All ± terms represent standard deviation from average values. UV oxidized SAFe S SAFe D2 DCo consensus value (pM) DCo measured value (pM) 4.20 ± 1.90 43.1 ± 3.20 6.92 ± 0.40 (n = 6) 47.9 ± 1.94 (n = 6) Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx 5 Fig. 2. Potential temperature (°C) versus salinity (psu) in the whole water column (left panel) and in deep waters (right panel) at the LARGE and SUPER stations. (CTD data recording and validation as described by Branellec et al., 2010; Kermabon and Arhan, 2008). methods with a Bran+Luebbe AAIII autoanalyser as described by Tréguer and Le Corre (1979). Phosphate samples were analyzed manually by spectrophotometry as described by Murphy and Riley (1962). 2.6. Chlorophyll-a Chlorophyll-a concentration was determined after vacuum filtration of 1–2 L of seawater onto GF/F filters. The filters were placed in 90% (v/v) acetone/water and homogenized for 5 min (Guéneuguès and Boye, 2008). Measurements were performed with a TURNER Designs 10-AN fluorometer (Parsons and Strickland, 1963; Evans and Anderson, 1987). Detection limit and precision for chlorophyll-a were respectively 0.005 μg L−1 and 0.075 μg L−1 (Yentsch and Menzel, 1963; Neveux, 1976). 3. Results 3.1. Hydrography The cruise track crossed the subtropical domain, the Antarctic Circumpolar Current (ACC) and entered the Weddell Gyre. The subtropical domain southwest of Africa (stations L1, S1, and L2) extends to the southern branch of the Subtropical Front (S-STF), which was located at ~42°2′S during our cruise. In this study, the ACC (stations S2 to L7) is bounded to the north by the Subantarctic Front (SAF; Orsi et al., 1995; Belkin and Gordon, 1996) at ~44°50′S, and to the south by the Southern Boundary (SBy; Orsi et al., 1995) located at ~55°54.3′S during the cruise. The Weddell Sea Gyre corresponds to the southern end of the section (Station S5), as shown in Fig. 1. The T–S diagram for the SUPER and LARGE stations is presented in Fig. 2. The BONUS-GoodHope cruise crossed the subtropical domain in the Cape Basin region which is characterized by a very complex dynamical regime. Here warm and salty Indian Ocean water anticyclonic eddies commonly interacts strongly with shelf and slope waters, cyclones and filaments of South Atlantic origin. The anticyclones are generally referred to as “Agulhas rings” and are ejected from the western boundary current of the South Indian Ocean, the Agulhas Current, at its Retroflection (Lutjeharms and Vanballegooyen, 1988). The rings are constituted of Indian central water (Gordon et al., 1992) characterized by warm (potential temperature N 10 °C) and salty (S N 34.5) waters in the upper 800 m (Fig. 2). Station S2 is located just south of the S-STF and north of the SAF (hence north of the ACC). Its surface waters exhibited salinity and temperature characteristics of Indian subtropical waters (Fig. 2), which suggests advection from the S-STF region by a neighboring eddy. Hence this station is discussed here within the subtropical domain. Closer to Africa (stations L1 and S1) we observed Antarctic Intermediate Water of Indian Ocean origin (I-AAIW) between 800 and 1200 m which is transported in the Cape Basin region via the Agulhas current and the Agulhas rings (Gordon et al., 1992). I-AAIW is characterized by salinities N34.3 (Fig. 2). Another variety of AAIW was observed to the south (at L2 and S2) with salinities b34.3 (Fig. 2) featuring AAIW formed in the subantarctic region of the southwest Atlantic (A-AAIW; Piola and Gordon, 1989). Below AAIW, we observed Upper Circumpolar Deep Water (UCDW) that is characterized by an oxygen minimum (160–180 μmol kg−1) at Fig. 3. Vertical distributions of nitrate, phosphate, silicate (µM) in the upper 500 m at the LARGE and SUPER stations (Le Moigne, 2008; Le Moigne et al., in preparation). Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 6 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx Fig. 4. Chlorophyll-a (µg L-1) in the top 300 m at the LARGE and SUPER stations (Guéneuguès and Boye, 2008). ~ 1200–1500 m depth (Fig. 6). The sampled UCDW did not show welldefined varieties, but its northern part was also likely transported by the Agulhas Current and rings (L1 and S1), while it may originate from the southwest Atlantic (A-UCDW; Whitworth and Nowlin, 1987) at L2 and S2. At deeper depths, a diluted variety of North Atlantic Deep Water (SE-NADW) which has flowed along the southwest African continental shelf by following the “southeast route” (Arhan et al., 2003; Gladyshev et al., 2008) has been identified by its salinity signature N34.8 (Fig. 2), and an oxygen maximum at 2000–3200 m depth (Fig. 6) at stations S1 and S2. At the bottom, an old variety of Antarctic Bottom Water (AABW) likely formed in the Weddell Sea (Reid, 1989; Gladyshev et al., 2008) was observed in the Cape Basin abyssal plain at S1 below 3500 m depth which was characterized by low salinity and cold temperatures (Fig. 2). In the ACC south of the Agulhas Ridge, the Surface Water (SW) was marked by a southward decrease of temperature from 4 °C (L5) to 2 °C (S4) and by low salinity (b34), Fig. 2. Below lays the Atlantic variety of AAIW (A-AAIW) which was characterized by salinities b34.3 (Fig. 2). The isohaline layer was depressed at the northern edge, along the SAF, indicating the subduction of A-AAIW, which was further characterized by an oxygen maximum N250 μmol kg−1 (Fig. 6). The A-AAIW waters were detected at stations L3 to L5 between depths of 300 and 600 m. At greater depths the A-UCDW, which was colder than A-AAIW and marked by lower dissolved oxygen was found in the 1000–1500 m layer from station L3 to L5 (Fig. 6). North of the Polar Front (PF, Orsi et al., 1995; Belkin and Gordon, 1996), deep waters (1500–3000 m, stations L3 to L5) exhibit properties of diluted South West NADW (SW-NADW), which flows along the continental slope of South America, down to the Argentinean Basin before being injected in the ACC in the southwestern Atlantic (Whitworth and Nowlin, 1987). In the deepest sampled layers at station S3, below 3250 m on the northern flank of the Mid Atlantic Ridge, we observed a variety of bottom water fresher and colder than AABW observed in the Cape Basin Abyssal plain (Fig. 2). South of the PF another variety of UCDW, which has passed through the Drake Passage (DP-UCDW) was identified by a core of low oxygen water (Whitworth and Nowlin, 1987) at depths between 250 and 700 m for stations L6, S4 and L7 (Fig. 6). There, deep waters exhibited properties of Lower Circumpolar Deep Water (LCDW), with lower salinity than SW-NADW. South of the ACC domain (station S5), the whole water column was impacted by Weddell Gyre waters: this water is much colder (Fig. 2) with a younger variety of AABW observed at the bottom than that at S3 and in the Cape Basin at S1. These Weddell Gyre waters were characterized by higher dissolved O2 concentrations and lower Fig. 5. Contour plot of dissolved cobalt concentrations (DCo; pM) versus depth (m) and latitude along the MD166 BONUS-GoodHope section. The positions of LARGE (L) and SUPER (S) stations and the fronts are indicated. Black dots represent sampling depths for DCo measurements. The colour mapping extrapolation is based on the sampling resolution along the section of ~2600 km that was achieved with 12 stations separated on average by ~2.2° latitude (~240 km), with a total of 160 sampling depths. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Figure from ODV (Reiner Schlitzer). Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx 7 Fig. 6. DCo (pM) and O2 (µmol kg-1) versus depth (m) at the LARGE and SUPER stations. (O2 data recording and validation as described by Branellec et al., 2010; Kermabon and Arhan, 2008). salinities and temperature (indicating a less important mixing with nearby waters than the two varieties of AABW sampled in the Cape Basin and along the Mid-Atlantic Ridge; Fig. 6). 3.2. Biogeochemical features along the section Contrasting biogeochemical provinces were crossed along the section, generally characterized by the southward increase of macronutrient concentrations (Fig. 3) and associated with the frontal boundaries. The northern part of the subtropical domain (stations L1 and S1) was characterized by low nitrate and silicate concentrations, typically ≤5 μM (Fig. 3) and with low phosphate ≤0.5 μM (Fig. 3). Here there was a subsurface chlorophyll-a (Chl-a) maximum with concentrations ranging between 0.3 and 0.5 μg L−1 (Fig. 4). These features indicate oligotrophic conditions (Le Moigne et al., in preparation). In the southern part of the subtropical domain (L2 and S2), surface silicate concentrations remained low (≤5 μM) but nitrate (5–20 μM) and phosphate (0.5−1.2 μM) increased as compared with the northern subtropical region (Fig. 3). Surface waters of these stations (L2 and S2) were also marked by a subsurface maximum in Chl-a concentrations with the highest values recorded along the section, ranging from 0.4 μg L−1 at S2 to 0.6 μg L−1 at L2 (Fig. 4). In the Polar Frontal Zone, on the northern side of the PF, nitrate and phosphate concentrations increased to 20 μM and 1 μM respectively, while silicate remained ≤10 μM (Fig. 3). Ammonium exhibited a subsurface maximum of ~1.25 μM near the PF between 70 and 100 m depth (Le Moigne et al., in preparation) and Chl-a concentrations ranged between 0.2 and 0.4 μg L−1 (Fig. 4), while phaeopigments, a degradation product of chlorophyll-a, displayed the highest values recorded along the section (Le Moigne et al., in preparation). All these features indicate a bloom of diatoms, which probably occurred before the start of our observations in the vicinity of the PF (Le Moigne et al., in preparation). In the HNLC area between SACCF (S4) and Sby (L7), silicate and nitrate concentrations both increased with values ranging from 20 to 70 μM for silicate and near 30 μM for nitrate as well as phosphate levels that increased up to 1.5 μM (Fig. 3). Chl-a concentrations were low (b0.3 μg L−1; Fig. 4). A slight increase of Chl-a concentration to 0.3 μg L−1 (Fig. 4), associated with an increase of phaeopigments and a slight decrease of silicate concentrations, was recorded at the southern end of the section at station S5 located in the Weddell Gyre. 3.3. The meridional and vertical distributions of dissolved cobalt The distribution of DCo along the section is presented in Figs. 5 and 6, and the data are provided in Table 1. Dissolved cobalt concentrations ranged from 5.73 ± 1.15 to 72.9 ± 4.51 pM along the section (all ± terms reported represent one standard deviation of triplicate analyses of a single sample unless otherwise indicated). The lowest concentration was recorded in surface waters south of the S-STF (5.73 ± 1.15 pM at 30 m depth at station S2), whereas the highest level was recorded in the deeper waters north of the PF (72.9 ± 4.51 pM at station L5 at 700 m). The distribution of DCo at depth exhibited vertical gradients that corresponded to different biogeochemical provinces. Indeed the distribution of DCo shows a nutrient-like profile in the subtropical domain as well as in the subantarctic region and beyond it (e.g., stations L1 to L4), with surface depletion in the euphotic layer and increasing concentrations below 250 m depth (Figs. 5–6). The lowest DCo concentrations detected in the euphotic layer along the whole section coincided with the Chl-a maximum at stations L2 and S2 (Fig. 5). In these Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 8 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx domains DCo concentrations then decreased again toward the bottom below 3000 m depth (e.g., as exemplified by the stations S1 and S2, Figs. 5–6). In contrast, DCo was not depleted in surface waters of the Polar Frontal Zone and between PF and SBdy (stations S3 to L7, Figs. 5–6). Here the concentrations were relatively high in surface waters (from 35.4 ± 2.5 pM at S3 to 51.3 ± 1.1 pM at L7) and in the core of UCDW (A-UCDW and DP-UCDW) whereas DCo concentrations occasionally decreased with depth below these layers to values in the range or slightly lower than the surface concentrations (Figs. 5–6). In the Weddell Gyre (station S5) the distribution of DCo was fairly homogeneous throughout the water column, although the lowest values were observed at the bottom in the AABW (Figs. 5–6). In the subtropical domain DCo exhibited concentrations ranging from 15.6 ± 1.68 pM to 44.3 ± 0.73 pM in the surface Indian central waters at L1 and S1, and the lowest concentrations encountered in the surface waters of the section at L2 and S2 (Figs. 5–6). At intermediate depths (e.g., 200–1000 m) DCo was higher than in surface waters, with a mean DCo of 49.3 ± 2.04 pM (standard deviation for all samples measured, n = 4) in the I-AAIW and of 55.8 ± 3.68 pM (n = 4) in the A-AAIW (Fig. 6, Table 3). Beneath the intermediate depths DCo concentrations were also high, with a mean value of 57.3 ± 2.18 pM (n = 14) found in the core of the A-UCDW (Figs. 5–6). In deeper waters DCo had relatively high values, ranging between 40 and 50 pM, with the exception of higher DCo imprints in the core of the SE-NADW sampled at S1 (53.9 ± 3.30 pM, at 3050 m depth) and S2 (59.2 ± 2.05 pM at 2548 m depth), Figs. 5 and 6. In the ACC domain surface DCo concentrations generally increased southward, from 22.8 ± 3.71 pM at station L3 to 51.3 ± 1.11 pM at L7, and the depletion of DCo in the euphotic layer became less and less pronounced southward (Fig. 6). At intermediate and deep depths north of the Polar Front, DCo concentrations in the A-AAIW and AUCDW were in the same range as at the more northerly stations (L2 and S2), whereas there was no imprint of high levels in the core of the SW-NADW were DCo could decrease to 39.5 ± 1.84 pM at 2020 m depth (Fig. 6). South of the Polar Front DCo reached the highest concentrations at depth in the core of the DP-UCDW (50.0 ± 2.97 pM, mean and standard deviation of 7 samples), while underlying deep DCo values could exhibit lower concentrations (i.e. 36.5 ± 3.40 pM at 1117 m depth, S4). A discrete deep concentration showed, however, a high value at 1950 m at station S4 (62.9 ± 8.04 pM, Fig. 6). In the Weddell Gyre (S5) DCo concentrations were ~35 pM throughout most of the water-column, and lower in the AABW (28.1 ± 0.87 pM). Thus, DCo concentrations in the bottom waters of the Weddell Gyre were lower than those observed in the older variety of the AABW Table 3 Comparison and ranges of dissolved cobalt (DCo) in the water column of the Southern Ocean reported for different sectors: (a) this study; (b) Fitzwater et al., 2000; (c) Sanudo-Wilhelmy et al., 2002; (d) Ellwood et al., 2005; (e) Ellwood, 2008; (f) Westerlund and Ohman, 1991; (g) Martin et al., 1990. All seasons indicated are for the Southern Hemisphere. Location Position Season Depth (m) DCo (pM) 42–57°S; 0–15°E Summer 72°S–76°S; 180°W 63–65°S; 41–64°W 48–50°S; 20°E Summer Summer Autumn e South East Atlantic Sector Ross Sea Weddell Sea Atlantic sector across the Polar Front Subantarctic zone 40–52°S; 155–160°E Winter f Weddell Sea 60–80°S; 10–60°W Summer g Drake Passage 56°S; 65°W Summer Gerlarche Strait 64°S; 63°W Summer 15–200 200–4000 0.5–375 Surface 0–200 200–1000 0–1200 200–1200 50–200 200–4000 0–200 200–1850 0–200 5–59 24–74 5–41 20–172 15–40 30–45 10–40 20–50 12–46 13–58 ~25 20–30 58–82 a b c d flowing north of the Agulhas Ridge (40.0 ± 2.49 pM, mean and standard deviation of 2 different samples). 4. Discussion Relatively few studies of DCo distributions in the Southern Ocean have been reported, but they provide ranges of DCo of the same general magnitude as that reported here (Table 3). More precisely, DCo concentrations reported here from the upper 1000 m on both sides of the PF were somewhat higher (35–75 pM) than those reported by Ellwood et al. (2005) along the 20°E meridian (15–50 pM). Differences in the pretreatment of the samples might cause such discrepancy since the samples were acidified (pH ~ 2) before the UVdigestion step in the present study whereas they were UV-digested without acidification in the work of Ellwood et al. (2005) possibly yielding lower recovery of DCo. The acidification of the sample coupled to the UV-oxidation may promote the release of DCo from strong organic ligands, thus increasing the recovery of DCo (Donat and Bruland, 1988; Vega and van den Berg, 1997; Saito and Moffett, 2001; Noble et al., 2008; Shelley et al., 2010). In the Weddell Sea western rim, previously reported DCo concentrations in surface waters ranged between 18.4 and 71.1 pM, with a mean of 53.0 ± 16.7 pM (Sanudo-Wilhelmy et al., 2002). The concentrations detected in the present study, further east in the northern side of the Weddell Gyre, were somewhat lower, with a mean of 36.90 ± 2.70 pM (n = 6) between 0 and 250 m and 32.10 ± 4.50 pM (n = 8) in deeper waters (800–3840 m) at station S5 (Fig. 6, Table 1). However the DCo concentrations we report here compare well with values observed in the surface waters of the Weddell Sea (30–40 pM), as well as with deep concentrations, which were fairly uniform around 26 pM (Westerlund and Ohman, 1991) (Table 3). In the present study the vertical distribution of DCo evolved southward from a nutrient-like behavior in the subtropical domain and the subantarctic zone, to more complex distributions within the Polar Frontal Zone including scavenging-type behavior, and to a conservative behavior in the Weddell Gyre region. These distributions reflect the several biogeochemical and physical processes and the different external sources that play important roles in the oceanic cycle of DCo. 4.1. Biogeochemical cycle of cobalt in the subtropical region The distribution of DCo in the subtropical domain was nutrientlike, with low levels in surface waters and the lowest concentrations recorded where Chl-a levels were highest (Figs. 6 and 4). This suggests biological uptake of cobalt in this region. Similar surface depletion of DCo has been reported in the subtropical region of the South Pacific (Ellwood, 2008), in the northeast Pacific (Martin et al., 1989), in the North Atlantic (Martin et al., 1993), and in the Sargasso Sea (Saito and Moffett, 2001), as well as in other oligotrophic systems (Martin et al., 1989; Saito and Moffett, 2002), upwelling waters (Saito et al., 2004) or near islands (Noble et al., 2008), all indicative of biological utilization. For instance the cyanobacteria Prochlorococcus sp. and Synechococcus sp., which often dominate the picophytoplankton assemblage in oligotrophic regions, have an absolute cobalt requirement for growth (Saito and Moffett, 2002). Biological uptake by cyanobacteria can be the dominant removal mechanism for cobalt in surface waters of the oligotrophic Sargasso Sea (Saito and Moffett, 2001). Similarly uptake by cyanobacteria may be sufficient to cause the observed depletion of DCo in the oligotrophic region on the northern side of the section. Furthermore the strongest removal of DCo was observed in the southernmost subtropical region where nanoflagellates (b5 μm, mostly unidentified species) dominated the phytoplankton biomass and dinoflagellates were also abundant (B. Becker, LEMAR, France, pers. comm.). Dinoflagellates can have a relatively high Co cellular quota compared to other phyla such as diatoms and chlorophyceae sp. (Ho et al., 2003), but in the range of the Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx Fig. 7. [HPO24 ] (µM) versus DCo (pM) in the top 150 m at the LARGE and SUPER stations (Le Moigne, 2008; Le Moigne et al., in preparation). Co quota of cyanobacteria (e.g., Co:C of 0.08 to 1.43 μmol mol−1 for Synechococcus bacillaris; Sunda and Huntsman, 1995). Nanoflagellates, such as prasinophyceae, can also have Co quota similar than those of diatoms, although their Co quota are lower compared to dinoflagellates (Ho et al., 2003). It is thus possible that Co was used by dinoflagellates and nanoflagellates in addition to cyanobacteria. Moreover the uptake of DCo may be proportional to that of major nutrient phosphate, as previously observed in other oceanic areas (Martin et al., 1989; Saito and Moffett, 2002; Saito et al., 2004; Jakuba et al., 2008; Noble et al., 2008). In the present study a linear relationship between DCo and phosphate concentrations was observed in the euphotic layer (0–150 m) of the whole section. Although the correlation is relatively weak when all data are considered (r2 = 0.424, n = 45; Fig. 7), a much stronger relationship was detected in the oligotrophic region (r2 = 0.914; n = 5 at S1) with a DCo:HPO2− 4 depletion ratio of 49.3 μM M−1, as well as in the southern side of the subtropical domain (r2 = 0.988, n = 5 at S2) with a DCo:HPO2− 4 depletion ratio of 44.0 μM M−1 (Fig. 7). In this region the DCo:HPO2− 4 depletion ratio is in the same range as that reported in the northeast −1 Pacific (DCo:HPO2− ; Sunda and Huntsman, 1995; Martin 4 ~ 40 μM M et al., 1989) and near the Hawaiian Islands (37 μM M−1; Noble et al., 2008). By contrast, a much higher ratio has been reported in the Equatorial Atlantic (DCo:HPO2− depletion ratio = 560 μM M−1) 4 where concentrations of phosphate are in the low nanomolar range (40–140 nmol L−1) and DCo range from 5 to 87 pM at 5 m depth (Saito and Moffett, 2002) compared to 0.1 to 1.0 μM and 5 to 45 pM for phosphate and DCo levels respectively observed in the present study. Such differences in phosphate and DCo concentrations could explain the contrasted DCo:HPO2− 4 depletion ratio observed in the Equatorial Atlantic and those found in the subtropical south-west Atlantic. Furthermore, several factors could account for regional differences in the DCo:HPO2− depletion ratio, such as differences in the biological 4 community composition, biochemical substitution of zinc for cobalt in eukaryotic phytoplankton and changes in the bioavailability of cobalt (Noble et al., 2008). In deeper waters the nutrient-like distribution of DCo observed in the subtropical domain (Fig. 6) suggests remineralization of DCo. For instance the greatest increase of DCo with depth was observed in the mesopelagic zone (200–600 m) of the southern side of the subtropical region, where remineralization of particulate matter was found to be intense as exemplified by high levels of particulate barite (D. Cardinal, RMCA, Belgium, pers. comm.). The increase in DCo in the mesopelagic zone was not proportional to that 9 of phosphate, suggesting a decoupling between the cobalt and the phosphate cycles in intermediate and deep waters. Furthermore the DCo:HPO2− depletion ratios are lower at depth than in surface layer, 4 indicating that the apparent remineralization of DCo is lower than that of phosphate in intermediate and deep waters. In turn the internal cycle of cobalt in the subtropical domain may be described by combining a nutrient-like cycling like that of major nutrient phosphate, with a lower regeneration of Co compared to P and/or with additional (versus P) removal of Co in the mesopelagic zone. Both later processes would account for the decoupling between Co and P in deep waters. The internal processes of Co cycling within the ocean furthermore interact with external sources. There are potentially at least two such sources in the subtropical domain, aeolian input of terrestrial dust into the surface waters and enrichment from continental shelf and slope waters. Aeolian deposition of cobalt may generate a near-surface DCo maximum as reported in the western Philippine Sea (N100 pM at 48 m depth; Wong et al., 1995) and recently in the Sargasso Sea at the Bermuda Atlantic Time-series Study (BATS) site (Milne et al., 2010). However at the northern stations likely to be the most affected by aerosol deposition across the section (stations L1, S1, L2 and S2) the low surface DCo concentrations (≤ 45 pM in the upper 100 m, Fig. 6, Table 3) do not indicate a significant atmospheric input of cobalt, originating either from the South Africa or from Patagonia (Li et al., 2008), unlike dissolved iron (Chever et al., 2010). Nevertheless at stations L1 and S1, DCo distribution displayed relative DCo maxima in the top 100 m depth, especially at S1 where the relative maximum ranged between 40 and 45 pM at 70–100 m depth (Fig. 6). Although the biological uptake could be lower at those stations compared to the southern subtropical region (stations L2 and S2) according to the Chl-a levels (Fig. 4), which in turn may generate a relative maximum, it is possible that the DCo inputs were comparatively higher in the surface waters of the northernmost stations. Particulate aluminum distributions indeed shown enrichment in surface waters at station S1 compared to stations located more in the south, but it was not possible to discern whether the lithogenic inputs there resulted from atmospheric deposition and/or from the transportation of particles from the Agulhas Bank originated either from river discharges or from sediment resuspension above the margin and the continental slope (Jeandel et al., 2010; F. Lacan, LEGOS, France, pers. comm.). On the other hand, the stronger depletion of DCo observed in surface waters near the S-STF (stations L2 and S2) (Fig. 6) is marked by the highest subsurface maximum Chl-a concentrations encountered along the section (Fig. 4). There, near-surface DCo removal by biological utilization could potentially mask any lithogenic inputs from the atmosphere or from the advection of enriched surface waters. Nevertheless, analyses of dry aerosols sampled during the cruise gave an estimation of the dry deposition flux of soluble Co of 0.16–0.33 nmol m−2 d−1 in surface waters of the subtropical domain (A. Baker, UEA, United Kingdom, pers. comm.). Considering a 4 month deposition episode, such input would contribute for an increase of 0.40 to 0.80 pM DCo in the mixed layer, hence representing ≤2% of the concentration measured in the mixed layer. Another study showed comparable estimates and suggested that the dissolution of cobalt from a natural dust event may account for an enrichment of only ~0.01 to 0.43 pM DCo to the mixed layer (Thuróczy et al., 2010); that would represent ≤1.2% of the concentration measured in the mixed layer of the northern part of the section. Thus, aeolian inputs would not be a sufficient Co input to generate a distinct surface maximum unless they were much greater than those occurring during the cruise or previously estimated by Thuróczy et al. (2010), especially when phytoplankton uptake is also occurring. In turn the relative DCo maxima observed in surface waters of the northern subtropical domain may further originate from the advection of Indian waters that interact strongly with shelf and slope waters, as well as with river discharges, of the Agulhas Bank. Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 10 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx Maxima of DCo were observed in intermediate and deep waters in the cores of I-AAIW and SE-NADW, as well as those of A-AAIW and A-UCDW (Fig. 6), suggesting lateral advection of Co-enriched waters at those depths. Benthic and sediment inputs, and transport from continental shelf and slope waters towards the open ocean, may be important sources of trace metals to the deep ocean (Elderfield et al., 1981; Bowie et al., 2002; Noble et al., 2008; Zhang et al., 2008b), notably for DCo (Noble et al., 2008; Huang and Conte, 2009). Moreover deep Co inputs might be stabilized as Co organic complexes and generate high deep DCo concentrations (Saito and Moffett, 2002). The organic complexation of DCo along the section will be examined elsewhere (Bown et al., in prep.). The I-AAIW had flowed along the southern margin of South Africa (Gordon et al., 1992) whereas the SE-NADW followed the southwestern African margin before passing by the southeast Atlantic (Arhan et al., 2003). Hence it is possible that the high DCo found in these water-masses was transported from the sediments of the South African margins. Particulate aluminum (Jeandel et al., 2010) and dissolved iron (Chever et al., 2010) also displayed high values in these water masses, further supporting the hypothesis of a significant lithogenic source from the margins of South Africa in this region. Similarly distinct depth maxima in DCo were observed in the Pacific waters surrounding Hawaiian Islands, suggestive of the release of cobalt from areas of comparable depth that surrounds those islands (Noble et al., 2008). A-UCDW also exhibited high DCo concentrations that are close to those found in the A-AAIW; both of these water masses were formed in the southwest Atlantic (Whitworth and Nowlin, 1987) suggesting a source of cobalt in the region of formation and/or an enrichment of DCo between the southwest Atlantic and the southeast Atlantic due to remineralization at intermediate depths. 4.2. The cycle of cobalt in the ACC and the Weddell Gyre The distribution of DCo changed from a nutrient-like behavior north of the Polar Frontal Zone toward a more conservative profile in the Weddell Gyre (Fig. 6). This trend was mostly attributable to a southward increase of DCo in the euphotic layer, as there was no significant meridional gradient in the deep concentrations (Fig. 6). In the northern ACC domain (stations L3 and L4), depletion of DCo in surface waters (~25 pM) was observed within the relative Chl-a maxima (e.g., 0.25 μg L−1 at L3; 0.4 μg L−1 at L4) and there, nanoflagellates still dominated the phytoplankton assemblage (B. Beker, pers. comm.). Dinoflagellate abundance was slightly higher at L4 and nearly twice as high at L3 as those observed in the subtropical domain. Hence biological uptake may generate the nutrient-like distribution of Co north of the ACC. Here, the uptake of DCo was proportional to that of phosphate, since the correlation between DCo and phosphate was significant (r2 = 0.870, n = 4 at L4) with a DCo:HPO2− 4 depletion ratio of 48.0 μM M−1 (Fig. 7). The DCo:HPO2− depletion ratio observed at 4 station L4 was in the same range than those recorded on the southern side of the subtropical domain, indicating that DCo and phosphate could be biologically used in the same proportions in both biogeochemical provinces. In the PF region and the Weddell Gyre, there was no significant 2− correlation between DCo and HPO2− 4 and no clear trend in the DCo:HPO4 depletion ratio associated with the change of DCo behavior. The PF region has been characterized by a vast post-diatom-bloom condition (Le Moigne et al., in preparation). Diatoms dominated the phytoplankton biomass (B. Becker, pers. comm.), and in the Weddell Gyre this assemblage contained degraded frustules with small or absent chloroplasts also suggesting a late stage of a relatively minor diatom bloom (Le Moigne et al., in preparation). These observations suggest that biological uptake, especially by diatoms, may not be the dominant mechanism for removal of DCo from surface waters at those latitudes. This observations contrasts with the relatively strong depletion of silicate (Le Moigne et al., in preparation) and iron (Chever et al., 2010) observed on the southern side of the Polar Front, but it is consistent with culture experiments, which have shown that diatoms use much more iron, and to a lesser extent zinc, than cobalt to grow (Sunda and Huntsman, 1995). For instance zinc distributions were nutrient-like in the PF region indicating biological utilization of zinc, although zinc concentrations still kept at high values (≥2 nM) in the surface waters south of the PF at the Greenwhich Meridian (Baars and Croot, in press). So there would be no reason for a high cellular demand for Co (Ho et al., 2003; Baars and Croot, in press). On the other hand the rather low DCo observed in the subsurface waters south of the PF suggests that another process was at work to remove DCo from the surface. For instance these subsurface waters were characterized by relatively high O2 levels (N300 μmol kg−1, Fig. 6), at which cobalt may be in the less soluble Co(III) state (Saito and Moffett, 2002). The highest DCo concentrations in surface waters were recorded at the SBdy latitudes (e.g., L7 station, Fig. 6). The low biological uptake of cobalt may have led to relative cobalt accumulation there, although snow that fell at those latitudes during the expedition could be a source of DCo in surface waters. The concentrations of cobalt in snow could indeed be 4–5 times higher than in surface waters, with mean values of 137 pM measured in snow depositing on the Wasa Shelf in Antarctica (Westerlund and Ohman, 1991). In addition to the input by snow, dry aerosol deposition flux was 1.3 to 4 times higher at the polar latitudes than in the subtropical domain, with soluble Co deposition flux ranging from 0.42 to 0.66 nmol m−2 d−1 (A. Baker, pers. comm.). Such deposition could increase DCo concentrations in the mixed layer by 0.60 to 1.40 pM over a 4 months dust deposition event, representing only ~2–4% of DCo recorded in the mixed layer. Comparison of DCo in surface waters previously found in the northern (27.1 pM) and southern (25.1 pM) Drake Passage (Martin et al., 1990; Table 3) with, respectively, those recorded at L6 (39.0 ± 1.54 pM) and at stations S4 and L7 (45.8 ± 1.0 pM, n = 2) indicates an enrichment from the Drake Passage to the 0° meridian. It suggests that the transport of Patagonian dust by the westerly winds and their deposition in the surface ACC can still be a source of DCo, similar to iron (Cassar et al., 2007). In addition the higher surface DCo concentrations recorded at Gerlache Strait (58– 82 pM at 15–200 m depth; Martin et al., 1990; Table 3) were probably due to the proximity of the Antarctic Peninsula, which could be identified as another potential DCo source in surface waters of the ACC. At intermediate depths, the nutrient-like profiles of DCo observed north of the PFZ furthermore indicate remineralization of cobalt in the nutricline (Figs. 3–6); this is consistent with the particulate barite distribution (D. Cardinal, pers. comm.). In contrast, intermediate and deep concentrations of DCo in the central and southern PFZ and between PF and SBdy decreased with depth, as magnified at the SACCF (Fig. 6). This suggests that cobalt may be scavenged in the water column of the central and the southern ACC domains by adsorption onto living, dead and mineral particulate material (Brown and Parks, 2001). For instance the strongest removal of DCo in deep waters at the SACCF is in line with the most intense export of particles recorded 234 238 along the section inferred by Th/ U measurements (F. Planchon and F. Dehairs, VUB, Belgium, pers. comm.). Scavenging-type distribution has been previously suggested for cobalt in ocean waters receiving atmospheric dust (Wong et al., 1995). Dissolved cobalt concentrations were high in the fast eastwardflowing jet of the ACC in the cores of A-AAIW and A-UCDW north of the PF, as well as DP-UCDW south of the PF (Fig. 6). These waters have been flowing along the South American continental slope before being injected in the ACC in the southwestern Atlantic (Whitworth and Nowlin, 1987), and the DP-UCDW passed through the Drake Passage (Whitworth and Nowlin, 1987). Those water masses may thus have received sedimentary inputs from the Drake Passage as well as inputs from the margins of South America for those flowing north of the PF, and of the Antarctic Peninsula for those flowing south of the PF. Hence it is possible that the high DCo measured in these water-masses was transported from these source regions within the rapidly flowing ACC, which displays eastward velocities of ~0.16 m sec−1 in the Atlantic sector (Cunningham and Pavic, 2007; Gladyshev et al., 2008). The Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx transport of DCo from the source regions towards the 0° meridian is conceivable since the mean oceanic residence time estimate of 40– 150 year for DCo (Saito and Moffett, 2002) is 20 to 75 times longer than a mean transit time estimate of 2 year from the Drake passage to 1°E (Blanke et al., 2006). Comparison of DCo concentrations measured in the core of DP-UCDW at S4 and L7 stations (50.0 ± 2.97 pM, n = 7) with those recorded in the same water-mass at the Drake Passage (~24 pM, Martin et al., 1990; Table 3) suggests an enrichment of DCo in the DP-UCDW from the Drake Passage eastward towards the 0° meridian. Hence in addition to the advection from the source regions, DCo may also be regenerated along the transit of these water-masses between the Drake Passage and the 0° meridian, as it may stabilize to a more soluble redox state in the A-UCDW and DP-UCDW which are characterized by an oxygen minimum of ~ 170–180 μmol kg−1. Indirect evidences based on the chemical similarities between Co and manganese (Mn), especially considering the redox speciation, can further support the stabilization of Co in less oxygenated waters. For instance both Co and Mn could be co-oxidized via a common microbial pathway in regions of high microbial Mn oxidizing activity (Moffett and Ho, 1996). Furthermore comparable maximum concentrations of Mn and Co have been reported near the oxygen minimum layer in deep waters (Johnson et al., 1996; Pohl et al., 2011). The reduction of Mn oxides from margin sediments that intersect the oxygen minimum, followed by the transport of soluble Mn into the open ocean could generate such maxima (Johnson et al., 1996). It is 11 therefore possible that the maximum of DCo observed in the A-UCDW and DP-UCDW resulted from the reduction of Co(III) oxides from margin sediments and transport of the more soluble Co(II) in those less oxygenated water masses. In the bottom waters lower DCo concentrations (20–30 pM) were observed in the recently formed AABW near the Weddell Gyre seafloor as compared to the higher values (30–40 pM) observed in the older variety of AABW that flowed north of the Agulhas Ridge (Fig. 6). This may reflect enrichment with DCo as AABW flows from south to north across topographic features like the Agulhas Ridge, possibly from abyssal sediments as bottom waters flow northward, and perhaps from hydrothermal vents, which are a source of trace metals such as iron (Bennett et al., 2008). However there was no record of high DCo at the bottom of the Mid Atlantic Ridge (Fig. 5), whereas high values of dissolved iron concentrations suggested hydrothermal inputs (Chever et al., 2010). This suggests that hydrothermal activity may not be acting as a significant source of DCo as it is for Ca, Mn and Fe (German et al., 1991; Tagliabue et al., 2010). 4.3. Comparative biogeochemical Co budgets in the subtropical South Atlantic and the Southern Ocean Tentative budgets for DCo in the mixed layer of the subtropical and ACC domains are presented in Table 4. The budgets are based on the S stations located in the STZ (S1 and S2), in the PFZ (S3), at the SACCF Table 4 Summary of cobalt pools and fluxes for the surface mixed layer at each SUPER (S) stations during BONUS-Goodhope expedition. STZ MLD (m) Permanent pycnocline (m) Mean DCo in the mixed layer (pM) (1) PCoN 0.4 μm (pmol L−1) (2) TPOC (fine particles 1–53 μm) + (large particles N53 μm) (μmol L−1) (2) POCN 53 μm (μmol L−1) (3) PCoN 53 μm estimation (pmol L−1) (4) C export from the mixed layer (mmol m−2 d−1) Fluxes (nmol m−2 d−1) (A) Vertical diffusive DCo supply (B) Lateral advective DCo supply (C) Soluble cobalt supply from dry aerosols (D) Downward PCo export flux (E) Cobalt uptake (estimated from cobalt uptake rate by phytoplankton) Cobalt uptake (estimated from NPP) Net cobalt uptake (estimated from Co net seasonal depletion) (F) Vertical advective DCo supply PFZ SACCF Weddell Gyre S1 S2 S3 S4 S5 54 585 21.9 ± 1.93 2.30 ± 0.18 1.11 0.13 0.28 1.7 ± 0.1 51 775 6.88 ± 1.10 1.69 ± 0.17 3.60 0.40 0.19 1.7 ± 0.2 60 139 35.4 ± 1.54 1.49 ± 0.17 3.57 0.25 0.10 3.3 ± 0.2 88 169 38.9 ± 1.49 0.53 ± 0.18 1.74 0.26 0.08 6.0 ± 1.4 90 110 36.4 ± 0.55 1.27 ± 0.21 2.12 0.55 0.33 3.3 ± 1.4 (+) 0.04 (+) 17–28 (+) 0.2–0.3 (−) 2.5 (−) 47–94 (−) 55–73 nd Negligible (+) 0.20 nd (+) 0.2–0.3 (−) 0.8 (−) 47–94 (−) 55–73 (−) 6.0 Negligible (+) 0.12 ± 0.02 (−) 0.87 (+) 0.4–0.7 (−) 1.4 (−) 13 (−) 18–36 nd Negligible (+) 0.46 ± 0.08 (−) 1.7 (+) 0.4–0.7 (−) 1.8 (−) b 13 (−) 9.0–18 (−) 3.3 (+) 6.4 (+) 0.33 ± 0.06 negligible (+)0.4–0.7 (−) 2.0 (−) b 13 (−) 9.0–18 nd Negligible Note: nd means not determined (1) 1 data point available in the mixed layer (data from F. Lacan et al.). (2) 1 data point available in the mixed layer (data from F. Planchon et al.). (3) Using the relationship: PCoN 53 μm = PCoN 0.4 μm (POCN 53 μm/TPOC) considering that PCo and POC are covarying in each size-fraction of the particulate matter. (4) Estimated from total 234 234 Th export fluxes (0–100 m) and POC/ Th ratios of sinking particles (N53 μm) (data from F. Planchon et al.). (A) Calculated from DCo gradients across the pycnocline and a Kz of 0.3 cm2 s−1 (Kawabe, 2008) in the subtropical domain (S1 and S2) and a Kz of 0.66 ± 0.11 cm2 s−1 (Croot et al., 2007) in the ACC domain (S3 and S4) and the Weddell Sea gyre (S5). (B) In the PFZ and SACCF: estimated from DCo concentrations north and south at the Drake Passage (Table 3; Martin et al., 1990), a velocity of 0.17 m s−1 (Cunningham and Pavic, 2007; Gladyshev et al., 2008) and a distance of 10 575 km between Drake Passage and the 0° meridian. It is also considered that mixed layer depths are similar at the Drake Passage and at stations S3 and S4. This flux is negligible at S5 in the light of surface concentrations previously measured in the Weddell Sea (Table 3). In the STZ, see details of the estimation of the flux in the text. (C) Data from A. Baker. (D) Using the relation: PCoexported flux = Cexport (PCoN 53 μm/POCN 53 μm). (E) In the subtropical domain, the flux was estimated from the mean cobalt uptake rate of Prochlorococcus (Saito and Moffett, 2002) of 14.3 atom cell−1 h−1 obtained at [Co2+] b 1 pM, and from a cells density ranging between 50 000 and 100 000 cell mL−1 (Partensky et al., 1999; Zhang et al., 2008a). In the Polar Front region (S3), the cobalt uptake rate was estimated from a fucoxanthine concentration of 63.7 ng L−1 similar to the recorded field values (J. Ras and H. Claustre, pers. comm.) and the corresponding cobalt uptake rate of 0.21 pmol L−1 d−1 reported by Saito et al. (2010). At S4 and S5, the fucoxanthine concentrations were too low (J. Ras and H. Claustre, pers. comm.) to find a corresponding cobalt uptake rate. The second estimation of the cobalt uptake flux was based on New Primary Production values in the study area (Field et al., 1998) and an assumed phytoplankton Co/C ratio of 1.6 × 10−6 (mol mol−1), a value based on data reported for oceanic species (Ho et al., 2003). A third estimate is given for station S2 and S4. At S2 the DCo net seasonal depletion was estimated from DCo concentration found south of the STF during winter (Ellwood, 2008) whereas at S4 DCo net depletion was estimated from DCo concentration recorded in Antarctic Winter Waters. (F) The supply of DCo by upwelling at S4 was defined as vertical velocity (0.13 m d−1, de Baar et al., 1995) deep DCo mean concentration. This term includes contribution from remineralization. Please cite this article as: Bown, J., et al., The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem. (2011), doi:10.1016/j.marchem.2011.03.008 12 J. Bown et al. / Marine Chemistry xxx (2011) xxx–xxx (S4) and in the Weddell Gyre (S5), since the full set of cruise parameters (chemical, biological, geochemical and hydrographic) was available only at those stations. Some fluxes have been estimated using parameters from the literature. Nevertheless some parameters cannot be estimated. For example, we are aware of no information on rates of DCo regeneration in the mixed layer, and the budgets described below suggest that this was probably a significant source of DCo. The estimated cobalt uptake flux was estimated in two ways. One estimate was derived from phytoplankton biomass, as estimated from measured photosynthetic pigment concentrations, and literature values for uptake of Co by phytoplankton (Saito et al., 2010). The other was derived from satellite-based estimates of annual net primary productivity in the study area (Field et al., 1998) and an assumed phytoplankton Co/C ratio of 1.6 × 10−6 (mol mol−1), a value based on data reported for oceanic species (Ho et al., 2003). These two independent estimates agree reasonably well (Table 4) and each indicates that uptake by phytoplankton was the dominant Co flux at all stations considered. In addition, both estimation methods show Co uptake by phytoplankton decreasing from north to south on the section (Table 4). This is in line with the observation that DCo showed a nutrient-like vertical distribution in the STZ, and that this changed gradually to a more conservative distribution in the Weddell Gyre. All the budgets were negative, indicating missing or underestimated source terms, overestimated removal fluxes or some combination of those two effects. It seems likely that the most important Co source not accounted for in these budgets is the biological regeneration of DCo within the upper water column. We have estimates of seasonal net Co removal (biological uptake minus regeneration) in the mixed layer at stations S2 and S4, where winter DCo values are available from comparable surface waters (Ellwood, 2008). At S2, the net removal rate estimated in that way is only ~10% of the uptake rate estimated by each of the two methods described above, while at S4 it is ~ 30% of those rates. This excess of biological uptake over seasonal net removal implies that regeneration in surface waters is a very important Co flux throughout the study area, ranging from ~ 70% of biological uptake at the SAACF to ~90% in the STZ. The budgets become less negative from north to south, reflecting the decrease of the estimated cobalt uptake flux from the STZ to the Weddell Gyre. At station S1 the estimated lateral advective DCo supply ranges from 17 to 28 nmol m−2 d−1, based on estimated inputs from the South African continental shelf (Table 4). This flux was estimated from a hypothetical coastal DCo concentration of 100 pM, based on coastal concentrations measured by Saito and Moffett (2002) and Kremling and Streu (2001), a distance of ~650 km between the continental shelf and station S1 and a surface water velocity ranging from 0.03 to 0.05 m s−1 according to Agulhas Rings velocities of translation (Boebel et al., 2003). Supply from the nearshore waters could thus be a significant source term at this station. Lateral advective sources appear to be only minor terms in the Co budget at the other sites considered (Table 4). Vertical diffusive supply, aeolian supply from dry aerosols and downward export of particulate Co all appear to be minor terms in the regional DCo budget (Table 4). Vertical supply via upwelling also appears to be minor at most stations, but at S4 this flux is estimated to be sufficient to supply approximately half of the DCo taken up by phytoplankton (Table 4). 5. Conclusion The biogeochemical cycle of DCo differs in the subtropical domain of the southeastern Atlantic and in the Southern Ocean. The biological uptake of DCo proportional to that of phosphate, and the remineralization of cobalt in the mesopelagic zone mostly generate the nutrient-like distribution of DCo in the subtropical domain. In the ACC domain, the uptake by diatoms was apparently not a significant sink of DCo in surface waters and this feature was also evident in budgets. There, scavenging of DCo onto particles may occur at intermediate depths. However the removal by adsorption onto particles was difficult to discern, as deep profiles were strongly impacted by different water masses which were identified as potential sources of DCo. The external sources of DCo may be driven by margin input and transport in both the subtropical and ACC domains. South African margins have indeed been identified as potential sources of DCo into the intermediate and deep waters of the subtropical domain with additional input in surface waters at the northernmost stations. Furthermore DCo transport from source regions in the Drake Passage and southwestern Atlantic, carried by the eastward flow of the Antarctic Circumpolar Current, could bring DCo at the Greenwich Meridian. In addition, A-UCDW and DP-UCDW exhibited the highest DCo concentrations encountered in this study. Thus, those water masses could be considered as the main reservoirs of DCo along the section. Each of those water masses was characterized by relatively low O2, which may have promoted stabilization of DCo into the more soluble Co(II) state in the eastward flow of the ACC. Similarly the low DCo concentrations observed in surface waters both north and south of the PF may be partly due to the decrease in the solubility of cobalt as the surface waters there had higher dissolved oxygen concentrations. Water mass characteristics, especially the absence of oxygenation, may hence play a crucial role in DCo solubility and residence time thus requiring further investigations. Acknowledgments The Captain P. Courtes, the officers and the crew members of the French research vessel Marion Dufresne II are warmly acknowledged for their wonderful work at sea during the IPY MD166 BONUSGOODHOPE cruise. S. Speich and M. Boye were co-chief scientists on the cruise, and we thank as well their collaborators M. Arhan (LPO, Brest, FR) and F. Dehairs (VUB, Brussels, Belgium). We are grateful to the colleagues of our GO-FLO sampling team (Fanny Chever, Bronwyn Wake, Eva Buciarrelli, Géraldine Sarthou, and Amandine Radic). We thank Beatriz Becker, Damien Cardinal, Frank Dehairs, Marie Labatut, Hervé Claustre and Joséphine Ras for providing access to their unpublished data on phytoplankton species composition, barite 234 238 distributions, Th/ U, pigments and particulate cobalt, respectively. We also thank the two anonymous reviewers of this paper for constructive comments which significantly improve this manuscript. This investigation was supported by the BONUS-GOODHOPE project and cruise funded by the French LEFE National Program of INSU, the National Agency of French Research (ANR–07–BLAN–0146–01) and the Paul Emile Victor Institut. It is a contribution to the International Polar Year and the international GEOTRACES programs. We also thank the European COST Action ES801 for funding a Short Term Scientific Mission to J. 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