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Deep-Sea Research II 58 (2011) 2661–2677

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Deep-Sea Research II

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Dissolved manganese in the Atlantic sector of the Southern Ocean

R. Middag a,n, H.J.W. de Baar a,b, P. Laan a, P.H. Cai c, J.C. van Ooijen a a Royal Netherlands Institute for Sea Research (Royal NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands b Department of Ocean Ecosystems, University of Groningen, Groningen, The Netherlands c State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China article info abstract

Article history: The first comprehensive dataset (492 samples) of dissolved Mn in the Southern Ocean shows extremely low Received 21 October 2010 values of 0.04 up to 0.64 nM in the surface waters and a subsurface maximum with an average concentration Accepted 21 October 2010 of 0.31 nM (n¼20; S.D.¼0.08 nM). The low Mn in surface waters correlates well with the nutrients PO4 and Available online 19 November 2010 NO3 and moderately well with Si(OH)4 and fluorescence. Furthermore, elevated concentrations of Mn in the Keywords: surface layer coincide with elevated Fe and light transmission and decreased export (234Th/238U deficiency) Trace metals and fluorescence. It appears that Mn is a factor of importance in partly explaining the HNLC conditions in the GEOTRACES Southern Ocean, in conjunction with significant controls by the combination of Fe limitation and light Southern Ocean limitation. No input of Mn from the continental margins was observed. This is ascribed to the protruding Dissolved manganese continental ice sheet that covers the shelf and shuts down the usual biological production, microbial Biological uptake breakdown and sedimentary geochemical cycling. The low concentrations of Mn in the deep ocean basins Mn nutrient relation (0.07–0.23 nM) were quite uniform, but some elevations were observed. The highest deep concentrations of Mn were observed at the Bouvet Triple Junction region and coincided with high concentrations of Fe and are deemed to be from hydrothermal input. The deep basins on both sides of the ridge were affected by this input. In the deep Weddell Basin the input of Weddell Sea Bottom Water appears to be the source of the slightly elevated concentrations of Mn in this water layer. & 2010 Elsevier Ltd. All rights reserved.

1. Introduction contribute. Manganese (and other trace metals) can also reach the open ocean via other pathways; notably Mn can enter the water Manganese (Mn) is the 12th most abundant element in the column by reductive dissolution from (anoxic or suboxic) sediments earth’s crust (Wedepohl, 1995), yet in the open ocean it only exists in (e.g., Froelich et al., 1979; Johnson et al., 1992; Heggie et al., 1987; nanomolar concentrations. It occurs in natural waters as insoluble Pakhomova et al., 2007), lateral transportation from the marginal Mn oxides and as soluble Mn ions (Sunda and Huntsman, 1994). sediments (e.g., Landing and Bruland, 1980; Martin and Knauer, The latter are thermodynamically unstable in oxic seawater, with 1984; Johnson et al., 1992) or hydrothermal input (Klinkhammer relatively slow inorganic oxidation kinetics. However, this process is et al., 1977; Klinkhammer and Bender, 1980). accelerated by microbial mediation (Sunda and Huntsman, 1988 and For the remote Southern Ocean the fluvial input is not of references therein; Tebo et al., 2007 and references therein), and significance and atmospheric dust input is believed to be the main therefore Mn ions should oxidise to insoluble Mn oxides and be lost source of trace metals for the surface water of the open Southern from the water column via particulate scavenging and sinking Ocean (Sedwick et al., 1997). Atmospheric dust input can be direct (Sunda and Huntsman, 1994). This is observed below the photic (either dry (aeolian) or wet (precipitation)) dust deposition or indirect zone where the dissolved Mn concentrations decrease quickly with from the dissolution of accumulated dust particles in melting sea ice. increasing depth. However, in the surface layer a typical dissolved Hydrothermal Mn input has been shownintheBransfieldStraitnear Mn profile shows elevated concentrations (e.g., Landing and the Antarctic Peninsula (Klinkhammer et al., 2001). Close to the Bruland, 1980). It has been shown that the elevated dissolved Mn Antarctic continent and the Antarctic Peninsula, reductive dissolution surface concentrations are mainly the result of photo reduction of from (anoxic) sediments or land run off (i.e., ice melt) can be a source Mn oxides (Sunda et al., 1983; Sunda and Huntsman, 1988, 1994), for the dissolved Mn. Icebergs can transport entrained particles from but atmospheric input (Landing and Bruland, 1980; Baker et al., the Antarctic continent and from accumulated atmospheric input, 2006) or fluvial input (Aguilar-Islas and Bruland, 2006) may also derived of other continents, into the Southern Ocean. The surface concentrations of dissolved Mn are known to be very low in the Southern Ocean (Klinkhammer and Bender, 1980, Martin et al., 1990; n ¨ Corresponding author. Tel.: +31 222 369 464. Westerlund and Ohman, 1991), but no comprehensive distributions E-mail address: [email protected] (R. Middag). over the entire basin have thus far been studied. The reason for the low

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2662 R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 concentrations of dissolved Mn in the Southern Ocean is believed to be 2004) and to some extent in the field (Buma et al., 1991; Coale, 1991a). its very limited input in combination with active removal by the However, the field experiments are ambiguous, as a response to Mn Antarctic algal community (Sedwick et al., 1997). addition is not always observed (Buma et al., 1991; Scharek et al., Mn is an essential trace nutrient, which is needed in enzymes for 1997; Sedwick et al., 2000), indicating that in some parts of the various biological processes in cells. Most notably Mn is needed in the Southern Ocean the ambient concentrations of dissolved Mn are in Photosystem II (PS II) for the splitting of water by photoautotrophs to adequate supply, i.e., not limited. To trace biological Mn uptake and supply electrons to the reaction centre of PS II (e.g., Sunda et al., 1983 subsequent export, a traditional tracer of particle export such as the and references therein; Raven, 1990). Recently it has been seen that disequilibrium between thorium-234 and uranium-238 (234Th/238U) Mn is also essential in the superoxide dismutase (SOD) enzymes of could be used. The 234Th/238U ratio is known equal to 1 at secular marine diatoms (Peers and Price, 2004; Wolfe-Simon et al., 2006). equilibrium while ratio values lower than 1 result from the removal of SOD destroys reactive oxygen species (ROS) such as superoxide and 234Th due to downward export of settling biogenic particles and hence hydroxyl radicals at the site of production, because these highly indicative of particle export. The 234Th is produced from radioac- reactive ROS cannot diffuse across cell membranes. Under iron (Fe) tive decay of the soluble 238U with a half-life of 4.47 109 years, while deficient conditions the efficiency of PS II decreases (Greene et al., the half-life of the very particle reactive 234Th is only 24.1 days. The 1992; McKay et al., 1997), which presumably increases the formation resulting disequilibrium between the soluble parent 238Uandthe of ROS due to the less efficient flow of electrons through the measured daughter 234Th activity reflects the net rate of particle photosystem (Peers and Price, 2004). Thus the availability of Mn is export from the upper ocean on timescales of days to weeks (Cai et al., essential especially under Fe-deficient circumstances. 2008 and references therein). The Southern Ocean is considered to be a so-called High Nutrient Low Chlorophyll (HNLC) ocean (De Baar et al., 1995), where despite the abundance of the major nutrients for phytoplankton relatively 2. Methods little primary production takes place. It has been shown that Fe is one of the limiting factors in the Southern Ocean by several iron- 2.1. Sampling and processing fertilisation experiments in bottles (review by De Baar, 1994)and in situ (review by De Baar et al., 2005). Dissolved Mn has been Samples were collected in February–March 2008 aboard Polar- suggested to be potentially (co-) limiting (Martin et al., 1990; Brand, stern during expedition ANT XXIV/3at22tracemetal(TM)stations 1991), and this has been shown in lab experiments (Peers and Price, and 30 regular stations (see Middag et al., 2011 forthecoordinatesof

Fig. 1. Trace metal sampling stations along the zero meridian and in the South East Atlantic Ocean during cruise ANT XXIV/3 from 10 February to 16 April 2008 from Cape Town (South Africa) to Punta Arenas (Chile) aboard R.V. Polarstern. Abbreviations in alphabetical order: AAZ: Antarctic Zone; ACC: Antarctic Circumpolar Current; PF: Polar Front; PFZ: Polar Frontal Zone; SAF: Sub Antarctic Front; SAZ: Sub Antarctic Zone; WF: Weddell Front; WG: Weddell Gyre. Author's personal copy

R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 2663 the station positions) along a transect largely traversing the zero suspected outlier based on three criteria. When for the sampled GO- meridian (01W),withanextensionnortheast towards Cape Town FLO the nutrient data were anomalous for that depth, indicating (Fig. 1). At the TM stations seawater was collected using 24 internally closing at the wrong depth, the sample was marked as suspected Teflon-coated PVC 12 L GO-FLO samplers (General Oceanics Inc.) outlier. In case the trace metal data (Al, Mn and Fe) were elevated for mounted on an all-titanium frame that was connected to a Kevlar wire the same GO-FLO for more than one cast, indicating a contaminated andcontrolledfromonboard(De Baar et al., 2008). From these GO-FLO the sample was also marked as suspected outlier. The third samples, in addition to Mn and nutrients, Fe (Klunder et al., 2011), criterion was when one data point of Al gave anomalous result for its aluminium (Al) (Middag et al., 2011) and the speciation of Fe depth considering the data points at shallower and greater depths. (Thuro´czy et al., 2011) were assessed among other parameters. At This was done by visually inspecting the vertical profile for data points the regular stations seawater samples were collected with the regular that would not fit in the profile shape. Subsequently this data point CTD/Rosette, and were analysed for nutrients among other variables. was compared with potential temperature, salinity, nutrients, Mn and There were 18 TM stations on the zero meridian proper from 511570 to Fe to see if those showed a similar trend. If this was the case the data 691240S near the edge of the continental ice sheet, and four TM point was left in, if not the following test was applied; a linear stations on the extension towards Cape Town in the South East regression was determined between the concentrations of Al below Atlantic Ocean from 421200S, 091Eto501170S, 011270E(Fig. 1). Due to and above the suspect data point (two above and two below if the water requirement of other cruise participants and the occasional possible) versus depth. With the linear regression equation the malfunctioning of a GO-FLO sampler, 495 samples (instead of 528 ‘theoretical’ concentration was calculated for that depth. When this samples) were analysed for Mn from the 22 deployments at the TM calculated value was more than 25% lower than the measured value, it stations (Fig. 2). Of the 495 samples analysed for Mn, 27 samples was marked as a suspected outlier and not further used in the (5.5%) were suspected outliers and therefore not further used in the calculations or graphs. The complete relational database includes data analyses and figures here presented. Outliers were labelled as specific flags for suspect or rejected outlier values and will be available

Fig. 2. Concentrations of dissolved Mn (nM) over the entire water column at all 22 stations along the zero meridian and in the South East Atlantic Ocean. Upper panel shows the upper 1000 m, the lower panel the remainder of the water column with the 0.3 nM Mn isobar in the Bouvet region. Of the 495 samples analysed for Mn, 27 samples (5.5%) were suspected outliers and therefore not further used in the figures. Abbreviations in alphabetical order: AAZ: Antarctic Zone; ACC: Antarctic Circumpolar Current; PF: Polar Front; PFZ: Polar Frontal Zone; SAF: Sub Antarctic Front; SAZ: Sub Antarctic Zone; SB ACC: Southern Boundary of the Antarctic Circumpolar Current; WF: Weddell Front. Author's personal copy

2664 R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 at the international GEOTRACES datacentre (http://www.bodc.ac.uk/ line. The blank was determined by measuring column-cleaned sea- geotraces/) and can currently be obtained via http://gcmd.nasa.gov/ water. This was made by passing seawater with a low concentration of getdif.htm?Dissolved_Mn_Antarctic_Middag_IPY35_NL. Mn from the Southern Ocean (from mid-depth) over a Toyopearl The GO-FLO bottles from the TM stations were sampled in a AFChelate 650 M and an 8-hydroxyquinoline column, which retained class 100 clean-room environment. The water was filtered directly dissolved Mn and Fe. The average blank value was 0.020 nM from the GO-FLO sampler over a 0.2-mm filter capsule (Sartrobran- (S.D.¼0.004 nM, n¼22). The limit of detection defined as 3 times 300, Sartorius) under nitrogen pressure (1.5 atm). The filtered the standard deviation of the blank was o0.01 nM. The flow injection seawater for Mn analysis was subsampled into cleaned (see system was rinsed every day with a 0.5 M HCl solution. Middag et al., 2009, for cleaning procedure) low-density polyethy- An internal reference sample was measured in triplicate every day. lene (LDPE, Nalgene) bottles (125 mL) from each GO-FLO sampler. This internal reference sample was a sub-sample of a 25-L volume of All sample bottles were rinsed five times with the sample seawater filtered seawater that was taken at the beginning of the cruise. This prior to collection. Because all Mn data reported in this paper are internal reference sample was filtered and acidified as the samples. dissolved Mn, it will be hereafter referred to as Mn. This internal reference sample was analysed 40 times on different Underway surface sampling (target depth of 1.5 m) was con- days, and the relative standard deviation of the replicate analysis in ducted during transit between stations from south of 661010S triplicate was 3.2%. The relative standard deviation on the separate onwards until 691350S near the continental ice edge. This was done days was on average 1.3%. The average concentration of Mn of this with a towed epoxy-coated stainless steel torpedo deployed off a standard was 0.44 nM, and the deviation from this average for a given crane arm on the starboard side of the ship. Water was pumped measuring day was used as a correction factor. To verify whether this aboard with an Almatec A-15 Teflon diaphragm pump. A total of 24 correction was decreasing the inter-daily variability in the dataset, samples were tapped inside the same class 100 clean-room every day a sample that was collected and measured the previous environment (see above) by in-line filtration over a filter capsule measuring day, was analysed once again. The deviation between the containing a 0.4-mm pre-filter and a 0.2-mm end-filter (Sartrobran P concentrations measured on the different days decreased from 3.5% to 0.2 mm, Sartorius) 2.5%, indicating the data-correction procedure was beneficial. Unfiltered samples for nutrients were collected in high-density As an independent comparison, the certification samples collected polyethylene sample bottles, which were rinsed three times with on the Sampling and Analysis of Iron (SAFe) cruise (Johnson et al., 2007) sample water. Samples were stored in the dark at 4 1C prior to were analysed for Mn (Table 1). The resulting concentrations of Mn for analysis (see Section 2.4). Samples for the determination of the both SAFe Surface (S) (0.73 nM, n¼1) and SAFe deep (D2) from 1000 m 234Th/238U ratio were taken mostly at the regular stations and at (0.3270.01 nM, n¼37) agree with the values found by Mendez (pers. four TM stations (471400S, 41170E; 531S; 591S and 691240S). comm.) who uses an isotope dilution method with ICP-MS detection (Mendez et al, 2010). The D2 concentration of 0.3270.01 nM is slightly lower than the ‘consensus value’ (http://www.geotraces.org/Intercali 2.2. Analyses of dissolved Mn bration) of 11 participating laboratories of 0.3770.07 nM. However, the standard deviations for the D2 overlap and the relatively large Analyses of dissolved Manganese (Mn) were performed on standard deviation is caused by four labs that reported values that were board using the method developed by Doi et al. (2004), modified considerably higher (average 0.46 nM) than the remaining labs. When to buffer the samples in-line with and ammonium borate sample excluding these high values, the resulting average concentration of after Aguilar-Islas et al. (2006). Briefly, this is a sensitive flow injection 0.3270.03 nM matched the concentration here reported perfectly. analysis method based on the chemiluminescence from the reaction Moreover, the analyses of Mn performed were very consistent and between luminol and hydrogen peroxide, which is catalysed by Mn. some very strong correlations were found between the concentrations Samples were acidified to a pH of 1.8 (12 M Baseline Hydrochloric of Mn and other independent parameters (see Section 3.5). Acid, Seastar Chemicals Inc.) and subsequently buffered in-line to a pH of 8.570.2 with 0.5 M ammonium borate sample buffer that was 2.4. Additional analyses produced by dissolving 30.9 g of boric acid (Suprapure, Merck) in 1 L MQ water and adjusting the pH to 9.4 with ammonium hydroxide The salinity (conductivity), temperature and depth (pressure) (Suprapure, Merck). Dissolved Mn in the buffered sample was were measured with two different CTD’s of same type (Seabird SBE preconcentrated during 200 s on a Toyopearl AF-Chelate 650M 911+), one from the Netherlands Institute for Sea Research (NIOZ) (TesoHaas, Germany) column. The Mn is eluted from the column mounted on the all-titanium sampling system for the TM stations with a carrier solution and subsequently mixes with an ammonium and one from the Alfred Wegener Institute (AWI) mounted on the hydroxide and luminol solution (Doi et al., 2004)andpassesa3-m regular CTD/Rosette for the regular stations. Both were calibrated length mixing coil placed in a 25 1C water bath. The chemilumines- before and after the expedition by the company (Seabird) and the cence was detected with a Hamamatsu HC135 Photon counter. mutual agreement was excellent. Moreover, the conductivity Concentrations of dissolved Mn were calculated in nmol L1 (nM) sensors were calibrated during the cruise against salinity samples from the photon emission peak height (see below). Cleaned LDPE and measured onboard. Teflon bottles were used for all reagent storage (see Middag et al. The inorganic major nutrients nitrate (NO ), nitrite (NO ), (2009) for cleaning procedure). 3 2 phosphate (PO4) and silicate (Si(OH)4, from here on Si) were determined following improved procedures after (Grasshoff et al., 2.3. Calibration Mn 1983). The samples for nutrients were stored in a refrigerator and analysed usually within 10 h and always within 16 h on a Technicon The system was calibrated using standard additions from a TrAAcs 800 autoanalyser. The reproducibility of an internal labora-

5000-nM Mn stock solution (Fluka) to filtered acidified seawater of tory standard mixture of Si, PO4 and NO3 was measured daily and low Mn concentration (o0.2 nM) that was collected in the Antarctic typically within 0.7% of its average value. This deviation from the Ocean. A five-point calibration line (0, 0.15, 0.36, 0.73 and 1.46 nM average was used as a correction factor. Moreover, a standard standard additions) and blank determination were made daily. The developed during an exercise for Reference Material of Nutrients in three lowest points (0, 0.15 and 0.36 nM) of the calibration line were Seawater organised by the Meteorological Research Institute (MRI) measured in triplicate and the two highest points (0.73 and 1.46 nM) of Japan (Aoyama et al., 2008) was measured daily and the average in duplicate to add more weight to the lower part of the calibration standard deviation between runs was 0.943, 0.012, 0.361 and Author's personal copy

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Table 1 on the water masses, fronts and their definitions can be found in Results of analysis of Mn (nM) for SAFe samples during the cruise ANT XXIV/3, 6 Middag et al. (2011). The most northerly TM station in the South February–16 April 2008 aboard Polarstern. Average concentration for the SAFe deep East Atlantic Ocean was located north of the ACC whereas (D2) sample is 0.3270.01 nM. the remaining three TM stations in the South East Atlantic Ocean Date SAFe sample Mn (nM) Standard (see Section 3.2) and the five TM stations in the Bouvet triple deviation (nM) junction region (see Section 3.3) were within the ACC. The thirteen TM stations in the deep Weddell Basin (see Section 3.4) were south 17/02/2008 SAFe S 426 0.73 0.004 of the WF and therefore in the Weddell Gyre. 20/02/2008 SAFe D2 342 0.32 0.003 21/02/2008 SAFe D2 342 0.32 0.004 Along the transect several major water masses were sampled, 22/02/2008 SAFe D2 342 0.31 0.009 which are described in Whitworth and Nowlin (1987) and briefly 23/02/2008 SAFe D2 406 0.31 0.010 summarised here (Fig. 3). Between the Subtropical Front and the 24/02/2008 SAFe D2 406 0.31 0.012 SAF (Sub Antarctic Zone; SAZ) the Subantarctic Surface Water 25/02/2008 SAFe D2 406 0.31 0.003 26/02/2008 SAFe D2 406 0.32 0.003 (SSW) is found above the salinity minimum of the Antarctic 27/02/2008 SAFe D2 406 0.32 0.014 Intermediate Water (AAIW). The transition surface waters between 28/02/2008 SAFe D2 406 0.35 0.012 the SAF and the PF (Polar Frontal Zone; PFZ) are marked by a steady 29/02/2008 SAFe D2 406 0.33 0.007 decrease in surface values of temperature and salinity and in this 01/03/2008 SAFe D2 406 0.34 0.003 paper are considered to be part of the SSW. South of the PF, in the 06/03/2008 SAFe D2 524 0.32 0.003 07/03/2008 SAFe D2 524 0.32 0.001 Antarctic Zone (AAZ) and Weddell Gyre the cold (yo5 1C) Antarctic 10/03/2008 SAFe D2 524 0.33 0.006 Surface Water (AASW) constitutes the upper water layer. The 11/03/2008 SAFe D2 524 0.33 0.013 Winter Water (WW) is the even colder water layer that shows as a 12/03/2008 SAFe D2 524 0.33 0.005 temperature minimum at the bottom of the AASW. The WW is a 13/03/2008 SAFe D2 121 0.32 0.002 14/03/2008 SAFe D2 121 0.33 0.003 remnant from the last winter before the overlying water was 16/03/2008 SAFe D2 121 0.30 0.003 warmed during the summer. In the deep South Atlantic Ocean the 18/03/2008 SAFe D2 121 0.32 0.006 relatively saline North Atlantic Deep Water (NADW) that flows in 19/03/2008 SAFe D2 121 0.31 0.002 from the north is found. The AAIW is relatively fresh (compared to 20/03/2008 SAFe D2 273 0.30 0.006 NADW) and sinks in between the SSW and NADW at the SAF and 22/03/2008 SAFe D2 273 0.32 0.002 24/03/2008 SAFe D2 273 0.32 0.005 flows northward. Within the ACC (Between the SAF and the WF) the 26/03/2008 SAFe D2 273 0.32 0.003 most extensive water mass is the Circumpolar Deep Water, which 28/03/2008 SAFe D2 273 0.32 0.003 is commonly distinguished between Upper Circumpolar Deep 29/03/2008 SAFe D2 273 0.31 0.001 Water (UCDW) and Lower Circumpolar Deep Water (LCDW). The 02/04/2008 SAFe D2 273 0.30 0.001 03/04/2008 SAFe D2 7 0.30 0.003 UCDW has a relative temperature maximum induced by the colder 06/04/2008 SAFe D2 7 0.30 0.011 overlying AAIW and WW. The LCDW has a salinity maximum due to 07/04/2008 SAFe D2 7 0.31 0.003 the influence from the relatively warm, saline NADW from the 08/04/2008 SAFe D2 7 0.30 0.003 north (Fig. 3). 09/04/2008 SAFe D2 7 0.29 0.003 In the deep Weddell Basin, below the AASW and WW, the Warm 11/04/2008 SAFe D2 19 0.29 0.003 12/04/2008 SAFe D2 19 0.29 0.002 Deep Water (WDW) is found that is largely replenished by LCDW 13/04/2008 SAFe D2 19 0.32 0.004 (Klatt et al., 2005). Underlying the WDW is the Weddell Sea Deep 14/04/2008 SAFe D2 8 0.34 0.005 Water (WSDW), which is more saline and has a lower potential temperature (0.7 1Coyo0.0 1C). Even colder (yo 0.7 1C) is the deepest water layer, the Weddell Sea Bottom Water (WSBW)

0.006 mM for Si, PO4, (NO2+NO3) and NO2, respectively. After the formed at the surface close to the Antarctic continent in winter correction based on the deviation from our laboratory standard, the by cooling and brine rejection due to sea ice formation. The WSDW standard deviation of the MRI standard between runs decreased to is believed to be formed by mixing between WDW and WSBW, but

0.464, 0.009 and 0.222 mM for Si, PO4 and (NO2+NO3) respectively. there is also evidence that WSDW is formed directly from surface Furthermore, the deepest sample analysed of a station of 24 waters and modified WDW (Klatt et al., 2005 and references samples, was kept and re-analysed within the next run of the next therein). station of 24 samples as an another verification for variability between runs. The amount of NO2 always was only minor (o2.5%) compared to the NO3 and in the remainder of this article only NO3 is 3.2. South East Atlantic Ocean mentioned, this actually comprising the sum of NO2 and NO3. Four TM stations were sampled in the South East Atlantic Ocean between Cape Town and the zero meridian, of which one was 3. Results located within the SAZ (421200S, 91E), two in the PFZ (461S, 51530E and 471400S, 41170E) and one just south of the PF in the AAZ 3.1. Hydrography (501160S, 11270E) (Fig. 2). A subsurface maximum of Mn was observed around 150 m depth and below this maximum the Several fronts exist within the sampled region. In the southward concentrations of Mn decreased with increasing depth. In the direction these are as follows: the Subtropical Front (STF), the Sub SSW above the subsurface maximum, the concentrations of Mn Antarctic Front (SAF), the Polar Front (PF), the Southern Boundary were around 0.2 nM. At the most northerly TM station at 421200S, of the Antarctic Circumpolar Current Front (SB ACC, also referred to 91E concentrations of Mn decreased to about 0.12 nM in the NADW as SBdy (e.g., Barre´ et al., 2008)) and the Weddell Front (WF). South and remained constant until close to the sediments at about of the SAF the Antarctic Circumpolar Current (ACC) flows eastward, 4350 m depth. The TM station south of the SAF at 461S, 51530E extending unbroken around the globe. Within the ACC the men- was somewhat shallower and extended only to about 3100 m tioned PF is found (Figs. 1 and 3). The SB ACC is considered to be a depth. At this station, concentrations of Mn decreased to approxi- part of the ACC at the zero meridian (Klatt et al., 2005) and south of mately 0.15 nM at 2000 m depth before increasing slightly to the ACC the sub polar cyclonic Weddell Gyre is found. More details about 0.2 nM at the greatest sampled depth. This profile shape of a Author's personal copy

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Fig. 3. Water masses and fronts along the entire transect (see Fig. 1). Salinity is represented in colour scale and isolines represent the potential temperature (1C). Upper panel shows the upper 1000 m, the lower panel the remainder of the water column. Abbreviations in alphabetical order: AAIW: Antarctic Intermediate Water (salinity minimum); AASW: Antarctic Surface Water (salinityo34.6); AAZ: Antarctic Zone; ACC: Antarctic Circumpolar Current; LCDW: Lower Circumpolar Deep Water (salinity maximum); NADW: North Atlantic Deep Water (salinity maximum); PF: Polar Front; PFZ: Polar Frontal Zone; SAF: Sub Antarctic Front; SAZ: Sub Antarctic Zone; SB ACC: Southern Boundary of the Antarctic Circumpolar Current; SSW: Subantarctic Surface Water (y44 1C); UCDW: Upper Circumpolar Deep Water (salinity4 34.4); WDW: Warm Deep Water (y40 1C, salinity434.6); WF: Weddell Front; WSBW: Weddell Sea Bottom Water (yo0.7 1C); WSDW: Weddell Sea Deep Water (0.7 1Coyo0.0 1C); WW: Winter Water (y minimum). mid-depth minimum of approximately 0.15 nM and an increase Ridge meet (Ligi et al., 1999). In this part of the transect the towards the sediments in the LCDW were also observed at the two concentrations of Mn were elevated in both the surface and deep remaining TM stations within the South East Atlantic Ocean layers, but the latitude of the surface and deep maxima were offset (471400S, 41170E and 501160S, 11270E). The increase in concentration (Fig. 2). At the most northerly TM station along the zero-meridian of Mn in deep waters was most pronounced at the most southerly transect (511570S), the concentrations of Mn decreased from the station (501160S, 11270E) with a concentration of 0.4 nM at the subsurface maximum to a mid-depth minimum (just below 0.15 nM) greatest sampled depth of 3500 m. at approximately 1250 m depth. Below the mid-depth minimum, the concentrations increased to around 0.25 nM in the deepest layer within the LCDW. The TM station at 531S also had a subsurface 3.3. Bouvet triple junction region maximum at approximately 150 m depth and below this maximum the concentrations of Mn decreased to a mid-depth minimum of Five TM stations were sampled with 11 resolution over the Bouvet 0.16 nM at approximately 1000 m depth. Below this minimum the Triple Junction ridge region on the zero meridian (Fig. 2), of which four concentrations of Mn increased steeply to a maximum of 1.6 nM at were in the AAZ (511570,531S, 541Sand551S) and one was between approximately 1750 m depth and values remained over 1 nM until the SB ACC and the WF (561S). This Bouvet triple junction region, the greatest sampled depth of 2350 m over the ridge crest. The TM hereafter called Bouvet region, is where the southernmost segment of station at 541S showed elevated concentrations of Mn in the surface the Mid-Atlantic Ridge, the westernmost segment of the Southwest layer (0.5–0.6 nM) and no distinct subsurface maximum was Indian Ridge and the easternmost segment of the American-Antarctic observed. Below the surface layer the concentrations of Mn did Author's personal copy

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depth. In the deepest part of the Weddell basin five stations (601S, 611S, 621S, 631S and 631580S) that were sampled with maximum sampled depths of around 5200 m. In the surface layer in this part of the transect, the concentrations of Mn decreased to concentrations below 0.1 nM and the subsurface maximum was observed between 100 and 150 m depth. Deeper, the concentra- tions of Mn decreased into the WDW to values of around 0.1 nM and stayed relatively constant throughout the WSDW until about 4000 m depth. Below 4000 m the concentrations of Mn increased again towards the sediments to values of around 0.15 nM in the WSBW. Further south the basin becomes less deep on the flank of Maud Rise and the maximum sampled depth at 651S was 3700 m. The subsurface maximum was observed somewhat shallower at 75 m depth and the values in the surface layer were below 0.1 nM. Below the surface layer the shape of the profile was very similar to what was observed in the deepest part of the basin with concen- trations of Mn decreasing to around 0.1 nM. Close to the sediments the concentrations of Mn increased slightly to 0.13 nM at the Fig. 4. Vertical depth profiles of dissolved Mn (nM) (circles), Fe (nM) (triangles) maximum sampled depth. The next three TM stations (661300,671S (Klunder et al., 2011) and Al (nM) (inverted open triangles) (Middag et al., 2011)at 1 the station at 541S over the Bouvet region. Error bars represent standard deviation of and 68 S) were deeper again with maximum sampled depths of triplicate measurement. Inset shows upper part of the water column at expanded about 4500 m. The subsurface maximum at these stations was scale. Elevated concentrations of Al in the surface layer of the Bouvet region are observed at approximately 100 m depth and the values in the discussed and can also be seen in Figs. 5 and 6AbyMiddag et al. (2011). Elevated surface layer were below 0.1 nM, except at 661300S (0.11 nM). concentrations of Fe in the surface layer of the Bouvet region are discussed and can Below the subsurface maximum the concentrations of Mn decrea- also be seen in Figs. 6 and 7 by Klunder et al. (2011). sed again to values around 0.1 nM in the WDW and WSDW and stayed relatively constant to about 3000 m depth. In the deepest decrease to a mid-depth minimum at approximately 1000 m depth, part of the water column the concentrations of Mn increased but this minimum value was with 0.25 nM, somewhat higher than slightly, to 0.23 nM at 661300S near Maud Rise and to values around that seen at the previous stations (o0.17 nM). In the deepest part of 0.12 nM at the other two stations (671S and 681S). The latter the water column the concentration of Mn peaked at 3.2 nM at concentration of 0.12 nM Mn is somewhat lower than what was 1750 m depth before decreasing to 1.6 nM at the greatest sampled observed in the deepest part of the basin in the WSBW north of depth of 2400 m (Fig. 4). Similarly, the shallowest TM station (551S) Maud Rise (0.15 nM). The two most southerly stations of the zero exhibited elevated concentrations of Mn in the surface layer meridian transect were sampled over the Antarctic continental (0.5 nM) and no subsurface maximum. Here, the concentrations slope with maximum sampled depths of 3300 m at 691S and of Mn decreased with depth to a minimum of just under 0.2 nM at 1900 m at 691240S, respectively. The shallowest station (691240S) 1250 m depth. Deeper, the concentrations increased again, to over was located as close to the edge of the continental ice sheet as 0.4 nM at the greatest sampled depth of 1600 m. At the deepest TM possible, but not yet over the continental shelf. The subsurface station (561S) between the SB ACC and the WF, a subsurface maximum at this station was again observed around 100 m depth maximum was observed again at 150 m depth. Below this maximum and the concentrations of Mn in the surface layer were below the concentration of Mn decreased through the WDW to approxi- 0.1 nM, as was observed on all stations south of 601S. The deepest mately 0.15 nM at 1000 m depth. Deeper than 1000 m depth, the station sampled on the continental slope at 691S had a profile shape concentration of Mn remained constant around 0.15 nM in the that was quite similar to what was observed in the deeper basin. WSDW, except for a slight maximum of 0.2 nM at 2500 m depth. Below the subsurface maximum the concentrations of Mn drop- ped to values of around 0.1 nM and stayed relatively constant to about 2500 m depth. Below that depth, however, there was a very 3.4. Deep Weddell Basin slight increase at about 3000 m, before the concentration of Mn before the concentrations of Mn decreased again at the maximum Another 13 TM stations were sampled in the southernmost part sampled depth of 3300 m. At the final station close to the con- of the zero-meridian transect that crossed the deep Weddell basin tinental ice edge at 691240S the concentrations of Mn decreased and extended onto the Antarctic continental slope until the below the subsurface maximum to approximately 0.12 nM at continental ice edge (Fig. 5). This part of the transect was also 1250 m. Deeper, the concentrations of Mn stayed relatively con- sampled with 11 resolution. However, due to adverse weather stant before the values increased slightly in the deepest 250 m to a circumstances and the edge of the continental ice sheet, two TM maximum value of 0.22 nM at the maximum sampled depth of stations (661300 and 691240S) did not follow the 11 spacing. At the 1900 m. most northerly TM station in this part of the transect (581S) a subsurface maximum was observed at 110 m depth. In the upper AASW of this station the concentrations of Mn were lower than 3.5. Mn and nutrients in the surface layer those in the surface layer of the Bouvet region, but still relatively high with a concentration of around 0.3 nM. Below the subsurface The concentrations of dissolved Mn in the upper surface layer maximum the concentrations of Mn decreased through the WDW (SASW and AASW) north of the PF and south of the WF (Fig. 6) were and WSDW to values below 0.15 nM except for a slight increase at generally below 0.2 nM (with the exception of the 52–581S region) 3000 m depth. One degree farther south at 591S the subsurface and increased towards a subsurface maximum before decreasing maximum was observed at 100 m depth and the concentrations of with depth (see Sections 3.2, 3.3 and 3.4). This subsurface max- Mn in the surface layer were below 0.15 nM. Deeper, the concen- imum of Mn coincided with a gradient in the potential density trations of Mn dropped to values around 0.1 nM except for a similar anomaly (sh). North of 581S the sh of the surface water was maximum as observed at the previous station, around 3500 m relatively constant to a depth of about 100 m before a steep Author's personal copy

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Fig. 5. Concentrations of dissolved Mn (nM) over the entire water column at 13 stations in the deep Weddell Basin. Upper panel shows the upper 1000 m, the lower panel the remainder of the water column. Note the different colour scale compared to Fig. 2. Of the 299 samples analysed for Mn, 17 samples (5.7%) were suspected outliers and therefore not further used in the figures.

increase in sh with depth was observed. The Mn subsurface surface layer were considerably higher than 0.2 nM with values up to maximum coincided with this pycnocline (Fig. 7A). South of 581S 0.6 nM (Fig. 6). North and south (from about 521 to about 581S) the sh remained constant until a much shallower depth of about of these two stations the concentrations of Mn in the surface layer 30 m. Below this wind-mixed layer the sh increased with depth were also elevated but decreased with distance away from quite steeply until a depth between 70 and 150 m. Deeper, the sh the 54–551S region to the very low Mn surface layer concentrations increase with depth was much less steep and the Mn subsurface encountered in the remainder of the transect. The elevated concen- maximum coincided with this reduction in the density gradient trations of Mn in the 52–581S region in the surface layer coincided (Fig. 7B). A relation was seen between the 234Th/238U ratio and the with several parameters that indicated a decrease in biomass. sh gradient, which was similar to the one observed for Mn. The Elevated light transmission values detected with the conventional 234Th/238U ratio was usually below 1 in the wind-mixed layer north CTD/Rosette system (Fig. 6) indicated that there were fewer particles of 581S, and south of 581S the 234Th/238U ratio was usually below 1 in the surface waters, lower fluorescence in this region (Fig. 6) 234 238 until the reduction in the sh density gradient (Rutgers van der Loeff indicated less phytoplankton, and the higher Th/ U ratio in this et al., 2011). At the four TM stations where thorium samples were region indicated less biogenic export (Rutgers van der Loeff et al., obtained (471400S, 41170E; 531;591 and 691240S), depleted con- 2011). Furthermore, with the exception of the TM at 541 and 551S, the centrations of Mn corresponded with low 234Th/238U ratios concentrations of Mn showed a nutrient-like profile in the surface

(Fig. 7C), and the Mn subsurface maximum was observed at the layer (Fig. 7D), correlated convincingly well with the nutrients PO4 234 238 depth at which the Th/ U ratio increased steeply to a value and NO3 (Fig. 8A and B) and correlated moderately with Si and near or over 1. Moreover, the depleted concentrations of Mn fluorescence (Fig. 8C and D). For the relation between the Mn and the corresponded with low concentrations of PO4 and at the depth nutrient concentrations as well as the relation between Mn and where the Mn subsurface maximum was observed, also the fluorescence all data shallower than the Mn subsurface maximum concentrations of PO4 increased (Fig. 7D). were used (see Section 4.4). At the TM stations at 541Sand551S there was no subsurface The samples collected using the torpedo (target depth of 1.5 m) maximum of Mn encountered and the concentrations of Mn in the south of 661010S (see Section 2.1), gave very similar results to the Author's personal copy

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Fig. 6. Concentrations of dissolved Mn (nM) (upper graph), the light transmission (%) (middle graph) and the fluorescence (lower graph) in the upper 250 along the entire transect. Note the different colour scale for Mn compared to the previous figures. Of the 163 samples analysed for Mn, 3 samples (1.8%) were suspected outliers and therefore not further used in the figures. Abbreviations in alphabetical order: AAZ: Antarctic Zone; ACC: Antarctic Circumpolar Current; PF: Polar Front; PFZ: Polar Frontal Zone; SAF: Sub Antarctic Front; SAZ: Sub Antarctic Zone; SB ACC: Southern Boundary of the Antarctic Circumpolar Current; WF: Weddell Front. shallowest (10 m) samples from the all-titanium CTD sampling hour in transit between stations, resulting in about 2–4 samples. system (except for the station at 661300S) with almost all concen- The concentrations of Mn ranged from 0.050 to 0.11 nM with the trations below 0.1 nM (Fig. 9). Surface samples were taken at every highest values closest to the continental ice edge. However, another Author's personal copy

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3 Fig. 7. (A) Profiles of dissolved Mn (nM) (filled circles), dissolved oxygen (mM) (filled triangles) and sy (kg dm ) over depth at the station at 531S along the zero-meridian 3 transect. sy¼(density1) 1000 (density in kg dm ). Error bars represent standard deviation of triplicate measurement. (B) Profiles of dissolved Mn (nM) (filled circles), 3 dissolved oxygen (mM) (filled triangles) and sy (density1) 1000 (density in kg dm ) over depth at the station at 651S along the zero-meridian transect. 3 sy¼(density1) 1000 (density in kg dm ). Error bars represent standard deviation of triplicate measurement. (C) Profiles of dissolved Mn (nM) (filled circles) and the 234Th/238U ratio (inverted filled triangles) over depth at the station at 531S along the zero-meridian transect. Error bars represent standard deviation of triplicate measurement. (D) Profiles of dissolved Mn (nM) (filled circles) and dissolved PO4 (mM) (filled diamonds) over depth at the station at 651S along the zero-meridian transect. Error bars represent standard deviation of triplicate measurement. maximum of over 0.10 nM was observed between 671S and 681S, values of dissolved Mn. This new high-quality, high-resolution data which was not in the proximity of the continental ice sheet. set reveals interesting features of the distribution of Mn in the Southern Ocean such as the surface depletion of Mn and the correlation with nutrients as discussed below. Previously limited 4. Discussion data have been available about the distributions of dissolved Mn in the Southern Ocean. Martin et al. (1990) had reported one profile in 4.1. Comparison with previously reported Mn in the Southern Ocean the South Drake Passage and observed a similar surface value of 0.08 nM compared to the surface values here presented. However, The complete section (Fig. 2), combined with the torpedo data, is in the remainder of their profile until 1400 m depth the concen- the first comprehensive dataset (492 data points) of dissolved Mn trations of Mn slowly increase with depth to 0.3 nM, instead of the in the Southern Ocean. Together with sections in the Weddell Sea clear subsurface maximum that was observed in the data presented and Drake Passage (Middag, 2010) this comprises almost 900 new here. Nevertheless, this increase with depth (Martin et al., 1990)is Author's personal copy

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Fig. 8. (A) Concentrations of dissolved Mn (nM) versus concentrations of PO4 (mM) above the Mn subsurface maximum north of 581S (ACC, filled circles; Mn¼0.36PO4–0.34; 2 2 R ¼0.87; n¼33; Po0.0001), South of 581S (Weddell Gyre, open circles; Mn¼0.39PO4–0.60; R ¼0.90; n¼47; Po0.0001) and at the stations at 541S and 551S (filled triangles). Of the 91 samples analysed for Mn, 2 samples (2.2%) were suspected outliers and therefore not further used in the figures. (B) Concentrations of dissolved Mn (nM) versus 2 concentrations of NO3 (mM) above the Mn subsurface maximum north of 581S (ACC, filled circles; Mn¼0.022NO3–0.27; R ¼0.78; n¼33; Po0.0001), South of 581S (Weddell 2 Gyre, open circles; Mn¼0.026NO3–0.56; R ¼0.70; n¼47; Po0.0001) and at the stations at 541S and 551S (filled triangles). (C) Concentrations of dissolved Mn (nM) versus concentrations of Si (mM) above the Mn subsurface maximum north of 581S (ACC, filled circles; Mn¼0.0023Si+0.20; R2¼0.79; n¼29; Po0.0001), South of 581S (Weddell Gyre, open circles; Mn¼0.0067Si–0.29; R2¼0.38; n¼47; Po0.0001) and at the stations at 541S and 551S (filled triangles). (D) Concentrations of dissolved Mn (nM) versus fluorescence above the Mn subsurface maximum north of 581S (ACC, filled circles; Mn¼0.29Fl+0.35; R2 ¼0.40; n¼33; Po0.0001), South of 581S (Weddell Gyre, open circles; Mn¼0.08 Fl+0.18; R2 ¼0.26; n¼47; P¼0.003) and at the stations at 541S and 551S (filled triangles).

similar to what we observed in the south Drake Passage (Middag, O¨ hman (1991) are generally higher than those in the Weddell Gyre 2010) on the later part of our expedition and is therefore most likely along the zero meridian but the profile shapes are consistent. related to that region. Perhaps the higher values observed previously can be attributed to Westerlund and O¨ hman (1991) have reported a larger dataset of lateral advection sources as suggested for the 52–581S region (see dissolved Mn for the Weddell Sea with some stations in the deep Section 4.4) or are related to other regional differences. Weddell basin (within the Weddell Gyre) somewhat west of the Concentrations of dissolved Mn in the shallow (350–800 m) zero meridian transect here presented. Some, but not all of their part of the Ross Sea (Sedwick et al., 2000) were as low as 0.03 nM in stations in this region, do show a subsurface maximum. However, the surface layer. The surface layer was relatively depleted in Mn at the sampling resolution over depth is much less and the sampled about half of the stations, but a clear subsurface maximum was depths are not always consistent between stations and the Mn usually not observed. This is most likely related to the shallow subsurface maximum that we have observed could therefore easily water-column depth and shelf water processes in this shelf sea. In have been missed. Surface concentrations of Mn were higher than the Australian sub Antarctic region east and southeast of Tasmania the concentrations here presented for the Weddell Gyre, but with subsurface maxima were encountered at about 150 m depth at depth the Mn values reach concentrations as low as 0.15 at 1600 m 451S and farther south (Sedwick et al., 1997). This depth of 150 m is depth, which is only slightly higher than the values of around similar to the depths at which the subsurface maximum is observed 0.1 nM presented here. Overall, the values of Westerlund and at similar latitudes in the dataset here presented. Furthermore, the Author's personal copy

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4.3. Subsurface Mn maximum

The distinct subsurface maximum of dissolved Mn was observed just below the wind-mixed layer in the ACC, with the exception of two stations (541S and 551S). At these two stations there was an apparent recent atmospheric input of Mn that led to higher Mn in the wind-mixed layer such that the subsurface Mn was no longer a distinct maximum (see Section 4.4). In the Weddell Gyre the wind mixed layer was very shallow and the Mn subsurface maximum coincided with the reduction in the density gradient (see Section 3.5). At four stations (471400S, 41170E; 531S; 591S and 691240S) where also the vertical distribution of the 234Th/238U ratio was studied, the Mn subsurface maximum coincided with an increase towards values near or over 1 of the 234Th/238U ratio (see Fig. 7C), indicating that biological removal occurs at shallower depths. It should be noted that the 234Th/238U ratios smaller than 1 are attributed to extra-cellular adsorption of 234Th onto settling biogenic particles (see Section 1). Therefore, the similarity between Fig. 9. Concentrations of Mn (nM) in the upper surface layer (target depth of 1.5 m) 234 238 sampled with the towed torpedo (filled circles) and with the all-titanium CTD the Mn depletion and the Th/ U ratio shows Mn is depleted in sampling system (filled triangles, 10 m depth) versus latitude south of 661S. Error the surface layer by biological removal, but does not distinguish bars represent standard deviation of triplicate measurement. between intra-cellular uptake of Mn and extra-cellular adsorption of Mn onto settling biogenic particles. However, as given below (Section 4.4) we will argue that the depletion of Mn in the surface surface layer concentrations of 0.09 nM Mn are very similar to the layer is most likely due to intra-cellular uptake. surface layer concentrations presented here. Otherwise no full Below the subsurface Mn maximum the concentrations decreased water-column profiles were taken by Sedwick et al. (1997). with depth towards the low uniform deep concentrations of Mn. To account for the Mn subsurface maximum there has to be a source of Mn. The Mn subsurface maximum was observed as deep as 150 m, 4.2. Comparison with Mn in other ocean basins and at this depth photo reduction can hardly be of significant importance. Therefore remineralisation of biogenic particles settling The general profile shape of dissolved Mn here presented and by from above is the most likely explanation for the observed Mn Westerlund and O¨ hman (1991) for the Atlantic section of the subsurface maximum. Moreover, a value of over 1 of the 234Th/238U Southern Ocean is different from the general profile shape of ratio was observed at several stations (Rutgers van der Loeff et al., dissolved Mn in the other major ocean basins. In the Pacific Ocean 2011) at similar depth as the Mn subsurface maximum. A value of (e.g., Landing and Bruland, 1980, 1987), Atlantic Ocean (e.g., over 1 of the 234Th/238U ratio is also indicative of particle reminer- Bruland and Franks, 1983; Jickells and Burton, 1988; Landing alisation (Cai et al., 2008 and references therein), providing additional et al., 1995; Saager et al., 1997; Shiller, 1997; Statham and evidence for the source of the Mn subsurface maximum being particle Burton, 1986), Indian Ocean (e.g., Saager et al., 1989; Morley remineralisation. et al., 1993) and Arctic Ocean (e.g., Yeats, 1988) the Mn generally shows elevated surface concentrations and low uniform deep values. Moreover, elevated concentrations are associated with 4.4. Depletion of Mn and major nutrients in surface waters the oxygen minimum in the Pacific Ocean (Landing and Bruland, 1980, 1987) and the Indian Ocean (Saager et al., 1989; Morley et al., The high vertical and lateral resolution and accuracy of the 1993). In the Southern Ocean, however, the Mn surface concentra- datasets of Mn and nutrients reveal a strong correlation between tions are depleted, followed by a subsurface maximum and Mn and the nutrients in the upper waters as well as an inverse trend generally low and uniform deep values with a slight increase in between Mn and fluorescence in the surface layer (Fig. 8A–D and the deepest water layer. The low uniform deep concentrations of Table 2). Moreover, the trends for dissolved Mn are consistent with Mn appear to be an ocean-wide phenomenon, most likely due to the trends for the 234Th/238U ratio in the same upper waters. For the oxidative scavenging reactions, removing dissolved Mn as Mn relation between the Mn and nutrient concentrations as well as the oxide particles from the water column (Sunda et al., 1983; Sunda relation between Mn and fluorescence, all data shallower than the Mn and Huntsman, 1988 and references therein; Statham et al., 1998). subsurface maximum were used. The rationale behind this was that at Elevations in the deep concentrations of dissolved Mn are usually the Mn subsurface maximum the 234Th/238U ratio was near or above explained by the hydrothermal input (e.g., Klinkhammer et al., 1, indicating no export but net remineralisation at this depth. The Mn 1977, 2001; Klinkhammer and Bender, 1980), or related to low and nutrient concentrations at this depth were therefore most likely oxygen concentrations like in the Pacific Ocean and Indian Ocean not representative of the biological uptake, but of the remineralisation (see above), or completely anoxic conditions like in the Black Sea instead. To the best of our knowledge, this is the first direct correlation (e.g., Haraldsson and Westerlund, 1988; Lewis and Landing, 1991; between the trace nutrient element Mn and the major nutrients Yemenicioglu et al., 2006) and the Cariaco Trench (e.g., Jacobs et al., observed in the oceans. 1987; Percy et al., 2008). In the Southern Ocean a slight oxygen Between about 521S until about 581S, the concentrations of Mn minimum occurred around 300–400 m depth, but contrarily, no in the surface layer were elevated compared to the remainder of the coinciding Mn maximum was observed (Fig. 7A and B). Moreover, transect. This coincided with elevated light transmission (Fig. 6) elevated concentrations of Mn were observed in the WSBW (see i.e., less (biogenic) particles, lowered fluorescence (Fig. 6) and Section 4.8) that is newly formed and has relatively high concen- lower export based on the 234Th/238U ratio (Rutgers van der Loeff trations of dissolved oxygen. This indicates that the oxidative et al., 2011). Besides elevated concentrations of Mn, also concen- scavenging is not the most important factor determining the trations of dissolved Fe were elevated in the upper water column of distribution of Mn in the Southern Ocean. this same 52–581S region (Klunder et al., 2011). The higher Author's personal copy

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Table 2 The correlations between the concentrations of Mn (nM) and the concentrations of the nutrients (mM) and between the concentrations of Mn (nM) and the fluorescence (Fl). Separate correlations are given for the surface layer north of 581S (ACC) and south of 581S (Weddell Gyre). The standard error of the slope is reported in the middle and last columns.

North of 581S Std. err. South of 581S Std. err.

Relation R2 P value Slope Relation R2 P value Slope

Mn/PO4 Mn¼0.36 PO4–0.34 0.87 o0.0001 0.025 Mn¼0.39 PO4–0.60 0.90 o0.0001 0.020 Mn/NO3 Mn¼0.022 NO3–0.27 0.78 o0.0001 0.0021 Mn¼0.026 NO3–0.56 0.70 o0.0001 0.0025 Mn/Si Mn¼0.0023 Si+0.20 0.79 o0.0001 0.00023 Mn¼0.0067 Si–0.29 0.38 o0.0001 0.0013 Mn/Fl Mn¼0.29 Fl+0.35 0.40 o0.0001 0.064 Mn¼0.08 Fl+0.18 0.26 0.0003 0.019

concentrations of Mn and Fe in this region of apparent lower Peers and Price (2004) showed that diatoms under iron-deplete primary productivity, in combination with the strong correlation of conditions require more Mn than diatoms under replete conditions. Mn with the nutrients in the adjacent regions (at lower and higher Moreover, Twining et al. (2004) observed a decrease in the Mn/P latitudes), is a strong indication that Mn is an important trace ratio for diatoms from 0.42 103 to 0.28 103 mol mol1 after nutrient for the plankton community in the Southern Ocean. iron fertilisation. When looking at the concentrations of Fe in the A distinct difference was observed between the Mn-nutrient surface layer from our section (Klunder et al., 2011), the concen- relations north and south of 581S(Table 2). The concentrations trations of Fe are slightly lower (0.1970.08 nM) in the part of the of Mn in the surface layer north of 581Swerehigherthanthe transect south of 581S (Weddell Gyre) compared to the northern concentrations of Mn south of 581Swhilethereverseholdsfor part (0.2470.1 nM) (ACC). Diatoms were the dominant phyto- the nutrients, causing a difference in the offset of the two correlations. plankton class in the ACC and Weddell Gyre (Alderkamp et al.,

The general southward increasing trend of major nutrients, not only 2010); thus the trend of a higher Mn/PO4 ratio (derived from the silicate but also nitrate and phosphate, is well known in the Antarctic slope of the Mn–PO4 relation) with lower Fe concentrations in the Ocean and due to the relatively low uptake by phytoplankton (the southern part of the transect could potentially be explained by the High Nutrient Low Chlorophyll, HNLC, conditions) in combination higher Mn requirement of the diatom community. with water-mass circulation and mixing. We argue that the opposite In field or bottle experiments there is no consistent response to trend of general northward increasingMninsurfacewatersisdueto the addition of Mn (Peers and Price, 2004 and references therein; the general closer proximity to land sources (e.g., Patagonia) of see Section 1). This could be related to the initial concentration of atmospheric dust input (Gehlen et al., 2003; Han et al., 2008; Li both Mn and Fe, as the Mn requirement of diatoms increases under et al., 2008; Klunder et al., 2011). lower Fe conditions (Peers and Price, 2004). The surface concen-

The Mn/PO4 ratio derived from the slope of the correlation trations of Mn in the surface layer here reported, in combination 3 1 between Mn and PO4 was 0.36 10 mol mol north of 581S and with the low concentrations of Fe would be stressful for especially 0.39 103 mol mol1 to the south of 581S, but the difference in the diatoms. Overall, based on the data and correlations here slopes was not significant (P¼0.4, 2-tailed t-test after Zar, 1999). presented, Mn appears to be of high importance for the phyto- The station at 561S with relatively elevated Mn surface concentra- plankton community, especially in the Weddell Gyre. tions fell within the ACC whereas the first station in the deep At two stations at 541S and 551S the concentrations of Mn in the Weddell Basin at 581S is located further south than the presumed surface layer were extremely elevated and fell completely outside 0 00 location of the WF at 56122 30 S. Therefore all but one station north the ranges of the correlations between Mn and the nutrients PO4, of 581S are part of the ACC and all stations south of 581S are part of NO3 and Si at the other stations (Fig. 8). This indicates that these the Weddell Gyre. Based on the Mn-nutrient correlations it appears high concentrations were not solely due to less uptake by the the surface layer of the station at 581S is affected significantly by phytoplankton community, but that an additional source term is the ACC. The previously reported Mn/P elemental ratio measured needed to explain these elevated values. At these two stations also with bulk analysis in natural phytoplankton assemblages as the concentrations of dissolved aluminium (Al) (Middag et al., selected by Bruland et al. (1991) (also shown by Twining et al., 2011) and dissolved iron (Fe) (Klunder et al., 2011) were elevated in 2004) ranges from values of 0.34 103 mol mol1 (Collier and the surface layer (Fig. 4), indicating an atmospheric source for the Edmond, 1984) to 0.38 103 and 0.39 103 mol mol1 (Martin elevated trace metals at these stations. This was further confirmed et al., 1976; Martin and Knauer, 1973, respectively). Cullen et al. by Klunder et al. (2011) based on air mass back-trajectories using (2003) found a somewhat higher ratio of 1.68 103 mol mol1. the NOAA HYSPLIT model. However, the average concentration of Elemental ratio values of Mn/P measured in individual plankton Mn in the surface mixed layer at 541S (100 m, Klunder et al., 2011) cells in the Southern Ocean by Twining et al. (2004) range from was 0.57 nM. To take the lower uptake by phytoplankton into 3 1 0.14 10 mol mol in heterotrophic flagellated cells to account, the PO4 concentration and the Mn–PO4 relation can be 3 1 0.42 10 mol mol in diatoms. The dissolved Mn/PO4 ratio used to calculate an expected concentration of Mn of 0.33 nM. This values derived from the slope of the Mn–PO4 relation for the surface 0.24 nM enrichment of Mn in the surface mixed layer would need a layer reported in this paper (Table 2), agree very well with the range first order estimate input of Mn of 1.8 g m2 (crustal abundance of for plankton reported in the literature, hence perfectly match the Mn 0.072%, Wedepohl, 1995). This implies dust input in the range extended Redfield ratio value concept of Mn/P¼0.4 103 mol of 3–15 g m2 based on a range 12–56% of Mn dissolving from dust mol1 by Bruland et al. (1991). This is another strong indication the (Baker et al., 2006; Mendez et al., 2010). This is unrealistically high Mn depletion in the surface layer is due to intra-cellular uptake. as the atmospheric dust input to the Southern Ocean derived from 2 Using the average (of north and south of 581S) observed Mn/PO4 the global dust entrainment and deposition model is 0.29 g m 3 1 3 1 ratio (0.38 10 mol mol ) and Mn/NO3 ratio (0.024 10 year (Han et al., 2008). Furthermore, the concentrations of Mn mol mol1) to calculate the N/P ratio results in a N/P ratio of 15. were also relatively elevated below the surface mixed layer This is consistent with the value of 14 one would expect in compared to the remainder of the zero meridian transect Antarctic waters and the average value of 15 for the world oceans (Fig. 2). Therefore lateral advection has also to be taken into (De Baar et al., 1997). account to explain the higher concentrations of Mn observed in Author's personal copy

2674 R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 the 52–581S region. Elevated concentrations of Fe and Mn have been attributed to the advection from the plateau of the Kerguelen (Bucciarelli et al., 2001) and Crozet islands (Planquette et al., 2007). However, Bouvet Island is more than 31 to the east (Ligi et al., 1999, Fig. 1) and thus ‘downstream’ of the ACC, making it an unlikely candidate. We suggest that next to atmospheric deposition, upwelling of water with elevated concentrations of Mn (perhaps of hydrothermal origin) and subsequent lateral advection is the only likely source of the observed elevated (surface and subsurface) concentrations of Mn in the 52–581S region along the zero meridian.

4.5. Mn supply from the edge of the continental ice sheet

There was no directly observed unambiguous influence of the proximity of the edge of the continental ice sheet on the concen- trations of Mn (Fig. 9). The continental ice sheet might be a source of Mn and other trace metals, but biological uptake, melting ice bergs and melting sea ice (with accumulated dust) in general would obscure any signal from the edge of the continental ice sheet that might be there. Atmospheric dust input and lateral advection appear to be locally important (see Section 4.4) for explaining the distribution of Mn in the surface layer. Nevertheless, the highest fluorescence (indicating more phytoplankton biomass) was observed close to the ice edge (Fig. 6) where the salinity was lower (Fig. 3). This increase in phytoplankton biomass must have resulted in an increased uptake of macro- and micro-nutrients, among which Mn. At the station near the ice edge 0.5 mgL1 chlorophyll a and at the station at 661300S 0.25 mgL1 chlorophyll a was measured (Timmermans et al., in preparation). This can be con- verted to mg Carbon (C) using 1 mg chlorophyll a¼20 mg C Fig. 10. Thick continental ice sheet covers the continental shelf and part of the really (Veldhuis and Timmermans, 2007) to calculate the C uptake by steep slope; figure courtesy of A. Humbert. Upper figure shows a satellite image the phytoplankton. Using the ratio C:P:Mn¼106:1:0.39 103 to from MODIS (03 September 2003) from the Fimbul ice shelf region. Superimposed calculate a first order estimate of the uptake of Mn in the upper on the satellite image are the floating continental ice sheet and the bathymetry in 2 colour scale. The colour scale represents the vertical water-column thickness. Red 25 m results in 77 nmol Mn m for the ice edge station and dot is the most southerly trace metal station on the zero-meridian transect, red 2 0 38 nmol Mn m at 66130 S. The concentrations of Mn in the crosses indicate locations of towed torpedo samples. Lower figure shows the cross upper 25 m at the ice-edge station and at 661300S were very similar section as indicated by the red line in the upper figure (along the flow line of the ice (0.09 and 0.1 nM, respectively) and thus an additional source is stream). The cross section shows the protruding continental ice sheet that covers the needed to explain the difference of 40 nmol Mn m2. The lower entire slope and part of the really steep slope. Ice sheet protrudes further to about 691350S at the zero meridian where the water-column depth is also somewhat 0 salinity near the ice edge compared to 66130 S(Fig. 3) indicates this shallower than at the location of the cross section (upper figure). The red arrow 2 40 nmol Mn m difference could very well be due to melt water indicates the latitude of the most southerly trace metal station on the zero meridian input from the continental ice edge. If only 1% of the total water transect to the east (upper figure). Bathymetry (water-column thickness) under the volume in the upper 25 m would constitute the input of melt water ice is from Nøst (2004), bathymetry north of the continental ice edge is from the GEBCO data base and the ice surface elevation and ice draft were compiled from with a concentration of 0.3 nM Mn, this would account for the different sources by A. Humbert from the University of Munster.¨ 40 nmol Mn m2 input. However, this modest precision esti- mate only accounts for the Mn uptake of the standing stock of phytoplankton at the time of sampling. To calculate the total input export of organic matter to the sediments is possible over the from the continental ice edge, production and export estimates for continental shelf and slope. Some organic matter that was formed the entire growing season would be needed. elsewhere might be deposited under the continental ice by currents. However, the data here presented show only a minor 4.6. Absence around Antarctica of a continental margin source of Mn elevation of Mn near the continental slope (Fig. 5). This indicates that under the floating continental ice sheet hardly any Mn (and Fe) Usually one finds elevated concentrations of Mn near the gets mobilised and transported from the shelf and slope sediments. continental shelf (see Section 1) as was also observed near the Therefore, the usual input of Mn (and Fe) from the shelf seas is Antarctic Peninsula (Middag, 2010). These elevated concentrations apparently absent at the Antarctic end of the zero meridian transect are related to the biological activity over the continental shelves due to the protruding ice sheet that covers the shelf and shuts down and subsequent input of organic matter into the sediments the usual biological production, microbial breakdown and sedi- followed by microbial breakdown. This microbial breakdown mentary reductive geochemistry. consumes O2 and creates reducing conditions in the sediments, favourable for the dissolution and mobilisation of Mn and Fe from the sediments. At our most southerly station at 691240S close to the 4.7. Hydrothermal vent supply over the Bouvet Triple Junction Ridge continental ice edge the water depth to the seafloor still was Crest 2005 m. The thick (200 m) continental ice sheet covers the water column over the entire continental shelf and part of the continental The low and generally uniform concentrations of Mn in the slope (Fig. 10). Therefore no surface ocean biology and subsequent deep basins were elevated at some locations along the transect. In Author's personal copy

R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 2675 the Bouvet region a distinct Mn maximum was observed in the in the surface layer were extremely low and correlate well

1000–2400 m depth range with values as high as 3.2 nM (see with the nutrients PO4 and NO3 and moderately well with Si Section 3.3). These elevated concentrations of Mn coincided with and fluorescence, indicating intense involvement of Mn in elevated concentrations of Fe while the concentrations of Al the plankton biological cycle. The observed Mn/PO4 ratio remained relatively constant (Fig. 4). Elevated concentrations of perfectly matches the extended Redfield ratio value concept of Mn in the deep ocean near a mid-ocean ridge are a clear indicator of Mn/P¼0.4 103 mol mol1 (Bruland et al., 1991). Furthermore hydrothermal input (e.g., Klinkhammer et al., 1985, 2001; Coale et al., at other stations, the opposite observation of elevated concentra- 1991b; Kawagucci et al., 2008). As there are no other obvious sources tions of Mn in the surface layer coincided with elevated concen- of both Mn and Fe but not Al, the elevated concentrations of Fe and Mn trations of Fe and light transmission and decreased export observed over the Bouvet Triple Junction ridge must be of hydro- (234Th/238U deficiency) and fluorescence. The low concentrations thermal origin as also shown by Klunder et al. (2011) based on the Al/ of Mn and Fe in most of the surface waters would be especially (Al+Fe+Mn) ratio. The highest deep concentrations of Mn were stressful for diatoms due to their increased Mn demand under low observed in this Bouvet region, but the hydrothermal influence is also concentrations of Fe (Peers and Price, 2004). As the diatoms were visible in the South East Atlantic. There the deep concentrations of Mn the dominant phytoplankton class, the increased Mn demand were elevated towards the Bouvet region while still on the flank of the under lower concentrations of Fe would also explain the trend of ridge (Fig. 2). On the other side of the ridge, into the deep Weddell the higher Mn/nutrient ratios (derived from the slope of the Basin the hydrothermal influence also shows in elevated concentra- dissolved Mn-nutrient relations) in the southern part (Weddell tions of Mn that were observed between 3000 and 3500 m depth as Gyre) of the transect where ambient concentrations of Fe tended to far as 601S(Figs. 2 and 5). be lower. This is indicative of co-limitation by Mn and Fe of phytoplankton growth in the Southern Ocean. 4.8. Deep lateral supply of Mn from the West with Weddell Sea Bottom Highest surface concentrations of Mn were observed at two Water stations at 541S and 551S. These values of Mn fell completely outside the ranges of the correlations between Mn and the The concentrations of Mn were slightly elevated to around nutrients PO4,NO3 and Si at the other stations (see Section 3.5). 0.15 nM below 4000 m depth (Fig. 5 and Section 3.4) in the WSBW We suggest that the elevated concentrations of Mn at these two between the Bouvet region and Maud Rise. The WSBW is relatively stations and the adjacent region (52–581S) are due to atmospheric young and formed by cold dense winter water that sinks along the deposition in combination with upwelling (with elevated Mn, continental slope of the Weddell Sea into the deep Weddell Basin perhaps of hydrothermal origin) and subsequent lateral advection. (Klatt et al. (2005) and references therein, see Section 3.1). This The concentrations of Mn in the deep ocean basins were low and water was recently in contact with the surface and continental quite uniform around 0.1 nM, but some elevations were observed. sources, like land run off and melting ice and therefore is likely The highest deep concentrations of Mn were observed at the Bouvet enriched in trace metals (Lannuzel et al., 2008). This is confirmed by Triple Junction region where three mid-ocean ridges meet. The the trace metal concentration data collected in the continental shelf coinciding high concentrations of Fe without a matching elevation sea of the Antarctic Peninsula in the same cruise (Middag, 2010). in the concentrations of Al point out hydrothermal input as the Presumably the inflow of WSBW brings in elevated concentrations source of these elevated concentrations of Mn. The hydrothermal of Mn in the deep Weddell Basin. A similar increase in the vent supply of the dissolved Mn to the Southern Ocean is much concentrations of Al in the deep Weddell Basin was observed more significant than hitherto realised and the deep basins on both (Middag et al., 2011), providing additional evidence for this sides of the Bouvet Triple Junction ridge were affected by this input. explanation. South of Maud Rise the concentrations of Mn were In contrast, the extended ice sheet of the Antarctic continent proper also slightly elevated close to the sediments, especially near largely prevents the usual reductive dissolution Mn supply from Maud Rise, but also farther south in the WSDW to concentrations continental-margin sediments, but by first-order estimate it can be around 0.12 nM (Fig. 5 and Section 3.4). This could be attributed calculated there has to be a small flux of Mn from the continental to the influence of the WSBW on the WSDW (see Section 3.1) ice edge to sustain the higher biomass encountered near this ice and sediment resuspension as suggested by Middag et al. (2011) edge. The more extensive shelf region around the Antarctic based on elevated concentrations of Al near sea-floor elevations. Peninsula may be a more normal source of Mn from reductive sediments, which in combination with winter formation of deep waters and the general eastward flow of Weddell Sea Bottom Water 5. Conclusions is transported to the zero-meridian section of the Weddell Basin. This explains the slightly elevated concentrations of Mn in this The first comprehensive section and dataset of dissolved Mn for deepest water layer. the Southern Ocean here presented is internally consistent and either similar or somewhat lower than the concentrations previously reported. The unprecedented high resolution and the quality of the Acknowledgments data allowed for new insights to be revealed in the (biological) cycling of Mn in the Southern Ocean. The oceanographic consistency between The authors are very much grateful to the master S. Schwarze two independent analysed tracers Mn and PO4 supports the overall and crew of R.V. Polarstern for their splendid support during the quality of the data in terms of both contamination-free sampling and cruise. Special thanks to Sven Ober, for the operation of the all- reliable accurate chemical analyses. This is one of the first examples titanium CTD sampling system for trace metals. Sven Ober was that demonstrates what the international GEOTRACES effort will be assisted by Willem Polman who, together with the pilot, tragically able to accomplish. Furthermore, it shows the importance of high- lost his live during this expedition in a helicopter accident. His help resolution sampling and precise analysis of multiple parameters for a and support were invaluable, as were his personality and presence. better understanding of the oceans. Willem Polman will therefore live on in our memories. After this It is proposed that Mn is a factor of importance in partly tragic loss Michal Stimac stepped up to help with the operation of explaining the HNLC conditions in the Southern Ocean, next to the TM sampling system. For this we are most grateful, as without significant controls by the combination of light limitation and Fe his efforts we would not have been able to continue our work in the limitation (De Baar et al., 2005). The concentrations of dissolved Mn memory of the colleagues we lost. Much gratitude to Loes Gerringa Author's personal copy

2676 R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 for her constructive comments, my colleagues Maarten Klunder facility for ultraclean sampling of trace elements and isotopes in the deep oceans and Charles-Edouard Thuro´ czy for their help with sampling and in the international Geotraces program. Marine Chemistry 111 (1–2), 4–21. Doi, T., Obata, H., Maruo, M., 2004. Shipboard analysis of picomolar levels of general support and Angelika Humbert for providing Figure 10. manganese in seawater by chelating resin concentration and chemilumines- Finally, I would like to express a lot of gratitude to all participants cence detection. Analytical and Bioanalytical Chemistry 378 (5), 1288–1293. of cruise ANT XXIV/3, for their help and support during this Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., memorable expedition. Constructive comments of three reviewers Dauphin, P., Hammond, D., Hartman, B., Maynard, V., 1979. 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