Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)

Accepted Manuscript Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)

Michelle G Nerentorp Mastromonaco, Katarina Gårdfeldt, Karen M. Assmann, Sarka Langer, Tulasi Delali, Yaroslav M. Shlyapnikov, Igor Zivkovic, Milena Horvat PII: DOI: Reference:

S0304-4203(17)30072-5 doi: 10.1016/j.marchem.2017.03.001 MARCHE 3431

To appear in:

Marine Chemistry

Received date: Revised date: Accepted date:

19 April 2016 26 February 2017 2 March 2017

Please cite this article as: Michelle G Nerentorp Mastromonaco, Katarina Gårdfeldt, Karen M. Assmann, Sarka Langer, Tulasi Delali, Yaroslav M. Shlyapnikov, Igor Zivkovic, Milena Horvat , Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Marche(2017), doi: 10.1016/ j.marchem.2017.03.001

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ACCEPTED MANUSCRIPT

Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas

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(Southern Ocean)

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Michelle G Nerentorp Mastromonaco*a, Katarina Gårdfeldta, Karen M. Assmannb, Sarka Langerc, Tulasi Delali d, Yaroslav M. Shlyapnikovd, Igor

Department of Chemistry and Chemical Engineering, Chalmers University of Technology SE-412 96 Göteborg,

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Zivkovicd, Milena Horvatd

Sweden

*Corresponding author: [email protected] ,+46761707005

Department of Marine Sciences, University of Gothenburg, Sweden

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Department of Environmental Sciences, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

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IVL Swedish Environmental Research Institute, P.O. Box 53021, SE-400 14 Gothenburg, Sweden

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b

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[email protected]

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ACCEPTED MANUSCRIPT Abstract Despite the distance from large anthropogenic emission sources, toxic mercury is transported via the atmosphere and oceans to the Southern Ocean. Seawater samples were collected at selected stations and were analysed for total mercury (HgT) (8 stations), dissolved gaseous mercury (DGM) (62 stations) and methylmercury (12 stations) during winter (Weddell Sea), spring (Weddell Sea) and summer (Amundsen and Ross Seas) in the Southern Ocean. The HgT distribution in water columns was found to not vary significantly with depth. In the Weddell Sea the average column concentration

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was higher in spring (2.6 ± 1.3 pM, 2 stations) than in winter (2.0 ± 1.0 pM, 6 stations). We hypothesize that the seasonal HgT increase is due to atmospheric deposition of particulate Hg(II)

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formed during atmospheric mercury depletion events (AMDEs), as well as the addition of inorganic

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mercury species from melting sea ice and snow. Furthermore, HgT concentrations found in this study were significantly higher than previously measured in the Southern Ocean (Cossa et al., 2011), which

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was hypothesized to be due to seasonal variations in atmospheric deposition. The average water column DGM concentration in the Weddell Sea was 454 ± 254 fM in winter and 384 ± 239 fM in spring. The lowest average DGM concentration was found in summer in the

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Amundsen and Ross Seas (299 ± 137 fM). The highest observed concentration in winter was hypothesized to be caused by the larger sea ice coverage, which is known to reduce the evasion of Hg(0) from the sea surface.

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The average monomethylmercury (MMHg) concentration in the Weddell Sea was 60 ± 30 fM in

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winter (6 stations) and 95 ± 85 fM in spring (2 stations), showing no significant seasonal difference. In the Amundsen and Ross Seas the summer average concentration of MeHg (MMHg and dimethylmercury ;DMHg) was 135 ± 189 fM (4 stations). The highest MeHg concentration was found

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in modified circumpolar deep water, which is known to have high primary production.

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ACCEPTED MANUSCRIPT 1. Introduction The levels of mercury in the environment have increased since the industrialization and high concentrations of this globally dispersed contaminant have been found in remote polar regions (Bargagli et al., 2005; Bargagli et al., 2007; AMAP, 2011). The bioaccumulation of the neurotoxin monomethylmercury (MMHg) in biota is of major concern for the Inuit people in the Arctic because of their primarily marine diet (AMAP, 2011). Similarly, Antarctic biota is exposed to large quantities of MMHg and humans are at risk when fishing activities are extended to these areas (Soerensen et al.,

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2014; Cossa et al., 2011).

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Mercury in polar regions originates from long range transportation in air (in a matter of days) and in water (in a matter of decades) released from areas having significantly higher anthropogenic and

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natural emissions (Corbitt et al., 2011; Schroeder & Munthe, 1998; UNEP, 2013; AMAP, 2011). Atmospheric mercury is generally deposited onto polar oceans mainly as Hg(II) but also as gaseous

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elemental mercury (GEM) by wet and dry processes (Zhang et al., 2012; Skov et al., 2004; Steffen et al., 2008). Atmospheric deposition in polar regions is especially enhanced in springtime during atmospheric mercury depletion events (AMDEs). During these events GEM is oxidized by halogen

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radicals formed by photolysis from halogens released from newly formed sea ice (Nghiem et al., 2012; Skov et al., 2004; Steffen et al., 2008, Schroeder et al., 1998).

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MMHg in seawater was generally believed to originate from in situ production (via microorganisms

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from dissolved inorganic Hg(II)) in sediments and in the water column (King et al., 2000). Organic mercury present in surface water could originate from coastal upwelling processes, as proposed for the Pacific Ocean (Conaway et al., 2009). However, recently it has been found that the largest net MMHg

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production in Arctic water columns occur in the subsurface layer (20 - 200 m; Soerensen et al., 2016). Despite the proposed in situ production, they suggested that the largest net input of MMHg to the Arctic Ocean is via atmospheric deposition. It has previously been proposed that tropospheric MMHg

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originates from sewage treatment plants/municipal waste emissions, or via methylation of Hg(II) by acetate ligands in hydrometeors (Gårdfeldt et al., 2003; Lindberg et al., 2001). Soerensen et al. (2016), however, suggested that the main source of MMHg in the Arctic atmosphere is via the recycling and decomposition of short-lived dimethylmercury (DMHg). In the Arctic Ocean, DMHg is produced via the conversion of MMHg in the subsurface region where it diffuses to the surface and is emitted to air (Soerensen et al., 2016; Baya et al., 2015). Analogous to the Arctic Ocean, similar processes could also apply to the Southern Ocean. Additional inputs of MMHg to the upper layers of the Southern Ocean could be melting glaciers and sea ice, release from sediments and upwelling from subsurface layers (Turner et al., 2005; Cossa et al., 2009; Jonsson et al., 2016; Soerensen et al., 2010; Strode et al., 2007). 3

ACCEPTED MANUSCRIPT MMHg formation is limited by the reduction of Hg(II) to dissolved elemental mercury (Hg(0)) via biological or photochemical processes (Amyot et al., 1997; Allard & Arsenie, 1991; Fitzgerald et al., 1984; Strode et al., 2007). A fast equilibrium exists between Hg(II) and Hg(0) in seawater and Hg(0) can be re-oxidized by photochemical or dark oxidation (e.g. Whalin et al., 2007; Mason et al., 2001; Amyot et al., 2000; Lalonde et al., 2001). The reducible fraction of Hg(II) is affected by the stability of Hg(II) complexes depending on the inorganic and organic ligands (Whalin et al., 2007; Lamborg et al., 2004).

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Dissolved gaseous mercury (DGM) in seawater at midlatitudes consists primarily of Hg(0) and a few percent of DMHg (Horvat et al., 2003; Bowman et al., 2015; Hammerschmidt et al., 2012). At the

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surface layer of the Arctic Ocean the share of DMHg has been found to be substantially higher (~20%) (Soerensen et al., 2016). Sea surfaces are globally often supersaturated with respect to DGM and the

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subsequent re-evasion of Hg(0) to air plays a role in the global mercury cycle due to an increased atmospheric distribution (Amos et al., 2013; Fitzgerald et al., 2007; Andersson et al., 2011). Sea ice

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act as a cap, reducing this re-evasion which leads to a build-up of DGM under sea ice (Andersson et al., 2008). In the Arctic the sea ice coverage is rapidly decreasing as a consequence of climate change. This might lead to the uncovering of DGM built up under sea ice and subsequent re-evasion. A recent

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model study estimated a yearly decline of -0.67% in mercury concentration in the surface layers of the Arctic Ocean due to an increased re-evasion (Chen et al., 2015). In Antarctica the sea ice coverage between 1978 and 2010 was observed to have increased on average with the largest increase observed

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in the Ross Sea. On the contrary, a small decrease was observed in the Bellingshausen and Amundsen

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Seas (Parkinson & Cavalieri 2012). The implications of future sea ice changes in Antarctica and its effects on the mercury cycle are not yet studied. The Southern Ocean is highly under-sampled with respect to mercury speciation in seawater columns.

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In 2008, Cossa et al. (2011) measured mercury species in water columns during a campaign taking place in a southerly transect from Tasmania to Antarctica (SR3 Geotraces, Fig. 1). Total mercury

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(HgT), DGM and MMHg concentrations in water columns were found in ranges of 0.6 – 2.8 pM, 10 – 588 fM and 20 - 862 pM, respectively (Cossa et al., 2011). Mercury was also measured in water columns by Bratkic et al., (2016) at a 40oS latitudinal transect in the South Atlantic Ocean (GA10 Geotraces, Fig. 1), revealing average values of HgT and DGM of 1.4 ± 0.6 pM and 224 ± 145 fM, respectively. MMHg concentrations in water columns were measured in a range from below the detection limit up to 250 fM. Here we present HgT, DGM [Hg(0) + DMHg] and methylated mercury (MMHg + DMHg) concentrations in seawater columns, sampled during three campaigns in Antarctic Seas as part of the European project: Global Mercury Observation System (GMOS, www.gmos.eu). The objective of this study was to sample and analyze seawater for mercury species in water columns in the ordinarily

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ACCEPTED MANUSCRIPT inaccessible waters of the Southern Ocean. The aim was to identify the water masses and relate them to the measured mercury species in order to increase the knowledge about the distribution and mechanisms of mercury in deep Antarctic water columns.

2. Materials and methods 2.1. Study area and sampling stations

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Seawater samples were collected at a total of 63 CTD/Rosette stations during three Antarctic oceanographic expeditions. A winter (8 June - 12 August 2013, ANTXXIX/6) and a spring campaign

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(14 August - 16 October 2013, ANTXXIX/7) were carried out in the Weddell Sea onboard the R/V Polarstern. A summer expedition was completed in the Amundsen and Ross Seas onboard the IB Oden

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between 8 December 2010 and 14 January 2011. The locations and labels of the winter, spring and

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stations can be found in Appendix, Table S1.

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summer stations are presented in Fig. 1. A detailed description including coordinates and depths of the

Fig. 1. Sampling stations during the winter, spring and summer campaigns in the Southern Ocean and the cruise tracks from the Geotraces campaigns, presented for comparison (Cossa et al., 2011; Bratkic et al., 2016).

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ACCEPTED MANUSCRIPT 2.2. Oceanographic data and seawater sampling R/V Polarstern was equipped with a CTD system using the Sea-bird program type SBE 911+ to log oceanographic data such as temperature, practical salinity, σθ and oxygen concentration (O2). Seawater samples were collected from 24 Niskin bottles (12 L) closed at selected depths. IB Oden was equipped with 21 Niskin bottles and used the Sea-bird SBE 9 program with the Seasave V. 7.18 software to collect oceanographic data. Comparisons between surface water samples collected in Niskin bottles and collected manually in FEP bottles on ice floes showed good correlation (R2 = 0.9, n = 8).

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Contamination due to the use of Niskin bottles has previously been found to be insignificant for mercury analysis and the use of trace metal Niskin bottles has been shown to be of minor importance

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(Kotnik et al., 2007; MERCYMS Final Report, 2006).

2.3. Total mercury (HgT) analysis

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Seawater samples for HgT and methylmercury (MeHg) analyses were collected from Niskin bottles at selected stations in acid-cleaned 500 mL FEP bottles. During the winter and spring expeditions, the

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filled FEP bottles were kept in a freezer storage room (-20oC) prior being shipped frozen to Ljubljana (Slovenia) for analysis.

HgT concentrations were determined using cold vapor atomic fluorescence spectrometry (CV-AFS),

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using a Tekran 2600 system. Analyses were carried out by means of the US EPA Method 1631 (2002).

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Calculations of achieved concentrations were based on the calibration equation using seven standard solutions with known concentrations in the range of 2.5 – 12.5 pM. The R-factor for the calibration curve was in the range of 0.9995 – 0.9999. The detection limit was calculated as three times the

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blank).

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standard deviation of the seven calibration blanks (0.3 – 1.2 pM, depending on the repeatability of the

2.4. Dissolved gaseous mercury (DGM) analysis Seawater samples for DGM [DGM = Hg(0) + DMHg] analysis were collected directly from the Niskin bottles in 1 L clean and conditioned glass bottles filled to the brim to avoid head space. Prior to analysis, the samples were put in darkness in a cold temperate bath at constant surface seawater temperature. All water samples were analysed within 24 hours after sampling. Around 0.4 L of the water sample was gently poured into a conditioned bubble flask. The sample was purged with mercury-free air for 9 min at a flow rate of 1 L min-1. The purge time was chosen according to calculations presented in Gårdfeldt et al. (2002), allowing all DGM to be released into the outgoing air. Mercury in outgoing air was detected with a Tekran 2537A mercury analyzer (CV-AFS). 6

ACCEPTED MANUSCRIPT The detection limit of the Tekran 2537A instrument was estimated by Poissant et al. (2005) to be 0.06 pg L-1. The purge blanks were checked regularly and adjusted according to the used bubble flask and active gold trap (the Tekran 2537A instrument uses dual cartridges). The detection limit of the purge and trap method was estimated to 1.5 fM, calculated as three times the standard deviation of the total blanks. The precision measured in field varied between 1 to 20%. The statistical reproducibility of the method has previously been reported to be ± 6 % when measuring DGM concentrations from 75 to 100 fM (Wängberg et al., 2001).

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Calibrations of the Tekran 2537A instrument were performed regularly by manual injections and by using the internal calibration source of the instrument. Manual calibrations were performed by syringe

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injections with known amounts of mercury vapour withdrawn from a mercury source kept at 4 oC. All maintenance and procedures for the Tekran 2537A instrument were performed according to the

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recommendations and directives of the standard operational procedure (SOP) from GMOS (Global Mercury Observation System). All cleaning procedures in the lab were performed using Milli-Q water.

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2.5. Methylmercury (MeHg) analysis

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No speciation was performed between Hg(0) and DMHg.

Methylmercury concentrations were determined using hydride generation cold vapor atomic

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fluorescence spectrometry (HG-CV-AFS), or aqueous phase ethylation cold vapor atomic fluorescence spectrometry (Eth-CV-AFS). The frozen seawater samples collected during winter and spring were

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analysed with HG-CV-AFS, optimized for MMHg. The instrumentation was not calibrated for DMHg and DMHg could therefore not be detected. Acid was added to the reaction vessel during MMHg analysis by hydride generation. However, the short time period of a few minutes was probably not

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enough to allow significant amounts of DMHg in the samples to decompose to MMHg (Black et al., 2009). The analysis results of samples from the winter and spring expeditions could therefore be read

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as MMHg concentrations.

During the summer expedition, samples for methylmercury analysis were collected in acid-cleaned 125 mL FEP bottles and were acidified and stored at 5°C prior analysis in Ljubljana, Slovenia. The acidification and storage time could have led to a decomposition of present DMHg to MMHg in the samples. Hence, the results should be read as total methylmercury concentrations (MMHg + DMHg). The majority of the summer samples were analysed using HG-CV-AFS and only samples from station 14 and the four top samples from station 31 were analysed using Eth-CV-AFS. The reason for introducing a second analytical approach was the inability to detect peaks with hydride generation for these samples.

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ACCEPTED MANUSCRIPT Calibration of the HG-CV-AFS system was performed several times per day by measuring 10 pg of MMHg standard. The repeatability and reproducibility of the method were 10% and 14%. Spike recoveries were calculated every day and were on average 99.3 ± 4.24%. The method used was a modification of what has been described in the literature (Craig et al., 1999; Ritsema & Donard, 1994; Stoichev et al., 2004). When peak separation between Hg(0) and MMHg was poor using HG-CV-AFS (in samples from stations 14 and 31), the samples were analysed by solvent extraction, aqueous phase ethylation and

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cold vapor atomic fluorescence spectrometry (Eth-CV-AFS) using a Brooks Rand III detector. The detection limit of this method (5 – 6.5 fM) was calculated as three times the standard deviation of the

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sample blanks. The repeatability and reproducibility of this method was 7.4% and 9.4%, respectively. Spike recoveries were calculated for each batch and ranged from 73.3 to 95.5%. The results were

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corrected for the corresponding recovery factors. This method is further described in the literature (Horvat et al., 1993; Liang et al., 1994).

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The conformation of the two analytical methods were on average 5% for higher methylmercury concentrations (50 -745 fM) and for lower concentrations (< 50 fM) the average difference was

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16.5%. No bias was observed and possible errors are expected to be random. No Certified Reference Materials (CRM) were used for the HgT, DGM and MMHg determinations. Further details about the methods used for HgT and MMHg analysis are given in Appendix, S2 and

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S3.

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3. Results

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3.1. Identification of water masses According to measured temperature, practical salinity and dissolved O2, different water masses in the Weddell, Amundsen and Ross Seas were identified, see Fig. 2. T-S diagrams for the three campaigns are presented in Fig. 3.

Antarctic Surface Water (AASW) is cold, light and low in salinity (Fig. 3) and was defined for any water at depths shallower than 35 m. Cold subsurface water (~100 m depth), a remnant of the deep winter mixed layer, is often referred to as Winter Water (WW) and was defined as when T < -1.7oC. The northern end of the WW extent marks the position of the Polar Front (see Fig. S1, Appendix). Weddell Sea Bottom Water (WSBW) originates mainly from the Filchner-Ronne shelf and is cold, salty and oxygen-rich. The WSBW is considered being too dense to leave the Weddell Sea. Moreover 8

ACCEPTED MANUSCRIPT Weddell Sea Deep Water (WSDW) originates from descending shelf water in the Weddell Sea and fills together with WSBW the basin below the Winter Deep Water (WDW) core. WSDW is lighter than WSBW and can leave the Weddell Sea, filling the abyssal Atlantic up to 30oN where it is known as Antarctic Bottom Water (AABW). In this study AABW, WSDW and WSBW were combined into a single water mass which covers all water with practical salinity (S) > 34.6 and T < 0 oC) (Schröder and Fahrbach, 1999). WDW [or Circumpolar Deep Water (CDW) as it is called outside the Weddell Sea] forms the main

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source water mass in the Southern Ocean. The core is warm and salty and is visible in any Southern Ocean profile as a temperature maximum and oxygen minimum between 300 and 1000 m depth.

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Antarctic Intermediate Water (AAIW) is believed to be a mix of AASW and CDW, upwelled along

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the Antarctic Circumpolar Current (further described in S1 and Fig. S1, Appendix). AAIW was defined here as Smin between Z = 600-1000 m, σθ = 27 - 27.2 kg m-3, T > 2oC (Hartin et al., 2011) and was only found at stations 519-1 and 521-1. In addition, High Salinity Shelf Water (HSSW) was

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identified at station 43 on the western Ross Sea shelf with temperatures near the surface freezing point and S > 34.6, see Fig. 2 and 3 (Schröder and Fahrbach, 1999).

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More information about currents and fronts present in the Southern Ocean is provided in Appendix, section S1 and Fig. S1. Measured temperature, practical salinity and O2 are presented for all stations in

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Fig. S2-S4 in Appendix.

Fig. 2. Identified borders of water masses found at stations during the a) winter, b) spring and c) summer expeditions, approximated from measuring points of temperature, salinity and O 2 (here presented as measuring points of salinity). Yellow dotted areas mark regions of mixed water from WW and WDW/CDW.

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Fig. 3. T-S diagrams for stations sampled during a) winter and b) spring in the Weddell Sea and c) summer in the Amundsen and Ross Seas. Water masses were defined as: Circumpolar Deep Water (CDW)/Warm Deep Water (WDW): S > 34.55, T > 0oC. Warm, salty water on the Amundsen shelf was identified as Modified Circumpolar

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Deep Water (MCDW). Antarctic Bottom Water (AABW)/Weddell Sea Deep Water (WSDW)/Weddell Sea Bottom Water (WSBW): S > 34.6, T < 0 oC. Winter Water (WW): T < -1.7oC. In summer, water at station 43 on the western Ross shelf with T < -1.7oC, S > 34.6 was identified as High Salinity Shelf Water (HSSW). Antarctic

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Surface Water (AASW) was defined as any water shallower than 35m. Antarctic Intermediate Water (AAIW) was apparent in spring data and was defined as T > 2oC, 27.0 < σθ < 27.2. Measured DGM concentrations [fM] are plotted along the T-S profiles.

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3.2. Depth profiles of HgT Samples for HgT analysis were collected at six stations during winter (Fig. 3) and at two stations in spring (Fig.4). Depth profiles of HgT showed the largest variations in the top 500 m (all profiles) and near the bottom (mainly visible in winter). These observations describe typical HgT profiles in

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seawater columns according to e.g. Cossa et al. (2011; Southern Ocean) and Laurier et al. (2004; North Pacific Ocean). The observed HgT variabilities in the surface could be due to reduction and

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evasion processes and/or atmospheric deposition. Variabilities in surface waters could also be explained by the potential presence of Hg associated plankton and/or due to the remineralization of

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sinking particulates (Zhang et al., 2014 and references therein).

Near the bottom the HgT concentration could be affected by sediment flux or particulate mercury

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associated to the nepheloid layer (Zhang et al., 2014; Cossa et al., 2011). The four winter stations 4901, 498-1, 500-3 and 503-3, located close to each other in the vicinity of Cape Norwegia showed

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variable HgT profiles. They showed however similar temperature, practical salinity and O2 profiles and the reason for the varying HgT profiles could thus not be explained by auxiliary data. Winter stations 509-1 and 515-1 situated on the tip of the Larsen Shelf showed however comparable HgT

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concentrations, as did the two spring stations 555-2 and 566-16.

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Compared to the average HgT concentrations measured during the SR3 Geotraces transect in autumn 2008 (1.3 ± 0.4 pM) and the GA10 Geotraces transect in summer 2011/2012 (1.5 ± 0.6 pM; Fig. 1), HgT measured in the Weddell Sea in this study was in general 50 - 90% higher (2.6 ± 1.3 pM, spring;

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2.0 ± 1.0 pM, winter; Cossa et al., 2011; Bratkic et al., 2016).

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3.3. HgT in identified water masses The average HgT concentrations calculated in identified water masses are presented in Table 1. Statistical probabilities were calculated to study if variations in HgT concentrations in different water masses were significant. The variations were found to mainly be insignificant (p > 0.05), showing that the HgT concentrations throughout the water columns of the Weddell, Amundsen and Ross Sea were relatively well-mixed. Similar well-mixed columns of HgT concentrations were also observed by Bratkic et al. (2016) who found no correlations between HgT and temperature, salinity or nutrients. The uniform mixing of HgT throughout the water column was explained by the strong bonding of mercury to organic matter that is generally long-lived in the water column (Bratkic et al., 2016).

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ACCEPTED MANUSCRIPT Cossa et al. (2011) measured 1.4 ± 0.4 pM in the AABW layer, 1.2 ± 0.3 pM in CDW and 1.2 ± 0.2 pM in AAIW. They found no significant differences within a 95% confidence level. The higher HgT found in the AABW and CDW layers in this study indicate that spatial variations could exist within the same water masses in the Southern Ocean. 3.4. Depth profiles of DGM The average DGM column concentration during the winter expedition was 454 ± 254, (35 - 2692) fM,

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which was found to be significantly higher than during the spring (384 ± 239, 15 - 1241 fM, p < 0.01) and summer expeditions (299 ± 140, 45 - 813 fM, p < 0.01). All 62 DGM profiles showed a generally lower surface concentration which increased along the thermocline to a higher and relatively stable

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deep column concentration (Fig. 7). Along the prime meridian a subsurface DGM maxima was

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observed for all depth profiles during winter and spring at a depth of around 400 - 500 m, see Fig. 7 aI and bIII. Similar subsurface maxima were observed at stations 566-16 and 3 (Fig. 7bII and 6).

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Along the Greenwich meridian in winter (Fig. 7aI) the sea ice edge occurred at around 64oS, resulting in increased surface concentrations for stations starting from station 490-1 and moving south. The same pattern was also observed when moving north along the prime meridian transect in spring (Fig

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7bIII). Also spring stations 555-2, 565-3 and 566-16 (situated within the sea ice region) showed higher surface concentrations. The higher observed surface concentrations observed at stations within the sea ice margin were mainly due to the capsuling effect of sea ice, hindering evasion (Andersson et al.,

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2011).

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At the Larsen Shelf stations 509-1 to 518-4 in Fig 7aIII, the depth profiles showed uniform concentrations except for a few remarkably high peaks which could not be explained by oceanographic physiology. The DGM concentrations at the Larzen Shelf and at the Dotzon and Getz

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ice shelfs were found to be comparable.

During the spring expedition, the majority of the casts were performed down to a depth of 1000 m

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disregarding bottom depth. Stations 519-1 and 521-1 showed lower DGM concentrations than the rest of the western stations (Fig. 7bI), possibly due to the warmer and fresher AAIW observed here, see Fig. 2 and 3 (and Fig. S3 and Table S2 in Appendix). The DGM:HgT ratio in deep water profiles (> 30 m) was around 30% in the Weddell Sea during both winter and spring. In the surface AASW water the share of DGM was significantly lower both during winter (23 ± 11%, p < 0.01) and spring (15 ± 8%, p < 0.01), possibly due to the evasion of DGM from the sea surface. Cossa et al. (2011) measured an average DGM concentration of 190 (10 - 588 fM), having an average DGM:HgT ratio of 14%. Bratkic et al. (2016) measured an average concentration of 224 ± 145 fM with a DGM:HgT ratio varying between 5 and 56% in the water column. Sorensen et al. (2016) presented a review over mercury species measurements in water columns performed during 12

ACCEPTED MANUSCRIPT several campaigns in the Canadian Arctic Archipelago and found an average share of DGM (Hg(0) + DMHg) of 11.5% in surface water (< 20 m) and 25.4% in the subsurface, which are in good correlation to ratios observed in this study.

3.5. DGM in identified water masses

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DGM partitioned in different water masses are presented in Fig. 3 and Table 1. The distinctions of DGM concentrations in different water masses were statistically evaluated and p-values are presented

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in Table 2. The majority of all comparisons resulted in low p-values, showing that the DGM concentrations varied significantly (95% confidence interval) between different water masses.

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Generally, the DGM concentration in the AASW was found to be consistently lower than in the rest of the water column.

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During the dark winter expedition, DGM was found to be regularly higher in the deeper water column in the CDW/WDW and WSDW/AABW/WSBW layers compared to the shallower WW and AASW

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layers. No significant difference in DGM concentration was observed between the two deeper layers. During spring, the DGM concentrations were found to be lower in the CDW/WDW and WSDW/AABW/WSBW than during winter (p = 0.026, 0.005) and a significantly lower concentration

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was observed in the deeper WSDW/AABW/WSBW than in the CDW/WDW. Besides, DGM seemed

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to be relatively well-mixed in the subsurface layers having only significant differences with the surface AASW.

In the Amundsen Sea, DGM concentrations were found to be significantly different between identified

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water masses with higher concentrations in the deep CDW/WDW layer. Similarly at the Dotson and Getz ice shelfs the DGM concentration was found to vary between different water masses, except for the MCDW layer that showed similar concentrations as the CDW/WDW layer. In the west section

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including stations 32 and 33, the surface AASW was relatively well-mixed with the underlying WW, as can be also observed in Fig. 7cII. Cossa et al. (2011) partitioned mercury species into the water masses of AAIW, CDW and AABW and found average DGM concentrations of 239 ± 30, 219 ± 70 and 239 ± 40 fM, respectively, showing no significant variations between water masses. Bratkic et al. (2016) found an average DGM concentration of 434 fM in AABW in summer which is in good agreement with the spring average obtained in AABW in this study (Table 1).

3.6. Depth profiles of MeHg 13

ACCEPTED MANUSCRIPT Methylmercury (MeHg) concentrations were measured in water samples collected at six CTD stations during winter (Fig. 4), two stations during spring (Fig. 5) and four stations during summer (Fig. 6). In samples from the summer expedition the long storage time probably allowed present DMHg to decompose to MMHg, see section 2.5. Hence, MeHg measured in the Amundsen and Ross Seas is presented as MMHg + DMHg. As observed for the HgT profiles during the winter expedition, the largest variations of MMHg were observed within the top 500 m and close to the bottom, possibly due to similar reasons as for HgT

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(described in section 3.2).

The average MMHg concentration in water columns in the Weddell Sea in winter was 60 ± 30 (15 -

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179) fM and in spring 70 ± 95 (15 - 454) fM, having similar shares of HgT (4 ± 3% and 5 ± 4%),

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respectively. The percental share of MMHg in the top 20 m was 2.3 ± 0.9% (winter) and 1.4 ± 0.9% (spring) and in the deeper layers (>20 m) 3.7 ± 2.4% (winter) and 4.9 ± 14% (spring). During summer

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in the Amundsen and Ross Seas the average concentration of MMHg + DMHg was 135 ± 189 fM. The measured MeHg concentrations in this study were in in the same order of magnitude as previously measured in the Southern Ocean (20 – 857 fM; Cossa et al., 2011) and in the Arctic Ocean (90 ± 70

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fM; Soerensen et al., 2016). Cossa et al. (2011) measured higher average share of HgT (up to 78% south of the South Polar Front) than in the Weddell Sea in this study. This is, however, mainly due to higher measured HgT in this study. In the Arctic MMHg was found to have a share of around 7% in

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the surface layer (<20 m) and 20 - 25% in the deeper ocean (>20 m) (Soerensen et al., 2016).

3.7. MeHg in identified water masses

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MeHg in different water masses is presented in Table 1 and statistical p-values are presented in Table 4. In winter in the Weddell Sea, a significant differentiation was observed between all water masses with highest MMHg concentrations in the WDW/CDW layer and lowest in the surface AASW. No

during

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significant variations were found between the WSDW/AABW/WSBW layer and other water masses spring.

The

higher

average

MMHg

concentration

reported

in

Table

1

for

WSDW/AABW/WSBW is explained by the large observed peak of all measured mercury species at 2500 m at station 566-16. The highest average MeHg concentration was found in the warm and salty MCDW layer on the Dotson and Getz sea ice shelfs. Cossa et al. (2011) measured an average MeHg concentration of 479 ± 179, 439 ± 170 and 518 ± 110 fM in the CDW, AAIW and AABW, respectively, having a significant difference only between the AAIW and AABW. In the study by Bratkic et al. (2016) an average MMHg concentration of 56 fM was measured in the upper 500 m, which is in the same order of magnitude as found in the upper layers of the Weddell Sea (35 ± 10 fM). 14

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Fig. 4 HgT, DGM, MMHg, practical salinity, temperature and oxygen profiles sampled at 6 CTD stations during the winter campaign in the Weddell Sea.

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Fig. 5. HgT, DGM, MMHg, practical salinity, temperature and oxygen profiles sampled at 2 CTD stations during the spring campaign in the Weddell Sea.

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ACCEPTED MANUSCRIPT Table 1. Average concentrations of DGM, MMHg and HgT partitioned in identified water masses, seasons and seas, (n = no. of samples).

Weddell Sea

HgT [pM] (n) Winter Spring Winter Spring

WW Open Sea

Winter Spring Winter Spring

WDW/CDW Open Sea

AAIW Open Sea

2.0 ± 1.0 (44) 3.7 ± 3.4 (3)

1.9 ± 0.8 (28) 1.9 ± 0.4 (7)

Winter Spring Winter Spring

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AASW (under sea ice) HSSW

50 ± 20 (44) 44 ± 11 (3)

612 ± 205 (46) 570 ± 92 (101) 641 ± 113 (46) 670 ± 198 (20)

82 ± 9 (28) 72 ± 33 (7)

374 ± 4 (2)

606 ± 5 (41) 133 ± 158 (10) 616 ± 302 (56) 653 ± 220 (11) DGM [fM] (n)

64 ± 26 (36) 111 ± 169 (6) MMHg + DMHg [fM] (n)

2171 ± 77 (12) 335 ± 34 (3) 597 ± 99 (4)

52 ± 22 (6) 76 ± 18 (3) 48 ± 25 (9)

Summer Summer Summer Summer

151 ± 48 (18) 293 ± 116 (27) 363 ± 91 (4) 391 ± 93 (28)

30 (1) 46 ± 36 (6)

Summer Summer Summer Summer

170 ± 83 (6) 186 ± 112 (5) 305 ± 200 (11) 344 ± 165 (5)

Summer Summer

281 ± 43 (9) 361 ± 71 (9)

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Section (stations) Amundsen Sea (2, 3, 4) AASW (under sea ice) WW WDW/CDW Dotson and Getz Ice shelfs (10, 14, 16, 18, 19, 25) AASW (under sea ice) WW WDW/CDW MCDW West Section (32, 33) AASW (under sea ice) WW WDW/CDW MCDW Ross Sea (43)

2.1 ± 1.1 (36) 3.1 ± 1.0 (6)

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Amundsen and Ross Seas

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Under sea ice

375 ± 278 (11) 444 ± 266 (6)

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Spring

WSDW/AABW/WSBW Open Sea

39 ± 11 (24) 28 ± 9 (5)

226 ± 150 (83)

Winter Spring Winter Spring

Under sea ice

MMHg [fM] (n)

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Under sea ice

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2.0 ± 1.1 (24) 2.8 ± 1.4 (5)

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Under sea ice

103 ± 30 (26) 69 ± 30 (53) 334 ± 74 (48) 349 ± 203 (10)

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AASW Open Sea

DGM [fM] (n)

285 ± 90 (27)

ACCEPTED MANUSCRIPT Table 2. One-tailed T-test significance probabilities for the differences in HgT in identified water masses during the winter and spring expeditions in the Weddell Sea (WS). *Significant at 95% confidence interval. WSDW/ WW CDW/WDW AABW/WSBW 0.4723 0.3954 0.3073 0.4723 0.3366 0.2959 0.3954 0.3366 0.1795 0.3073 0.2959 0.1795

HgT Winter WS AASW WW CDW/WDW WSDW/AABW/WSBW

AASW

HgT Spring WS AASW WW CDW/WDW WSDW/AABW/WSBW

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WSDW/ WW CDW/WDW AABW/WSBW 0.3533 0.1026 0.3869 0.3533 0.3850 0.3850 0.1026 0.2223 0.0173* 0.3869 0.3850 0.0173*

Fig. 6. DGM, MMHg, practical salinity, temperature and oxygen profiles sampled at 4 CTD stations during the summer campaign in the Amundsen and Ross Seas.

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Fig. 7. Water column profiles of DGM during the a) winter, b) spring and c) summer expeditions in the Southern Ocean.

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ACCEPTED MANUSCRIPT Table 3. One-tailed T-test significance probabilities for the differences in DGM in identified water masses during the winter, spring and summer expeditions in the Weddell (WS), Amundsen and Ross Sea. *Significant at 95% confidence interval.

DGM Amundsen Sea (2, 3, 4) AASW WW

0.0324*

CDW/WDW

3.30E-07*

AASW

AASW

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DGM Ross Sea (43) HSSW

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0.1252 0.3853 0.2727 WW 0.0324*

AASW

0.3967 0.0393* 0.0352* AASW 0.0284*

0.0101* 0.3312

WSDW/ AABW/WSBW 0.0004* 0.3853 0.0101* 0.1186

CDW/WDW 3.30E-07* 2.29E-05*

2.290E05*

WW 1.39E-06* 0.0002* 0.0898

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1.39E-06* 2.64E-15* 0.0098*

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DGM Dotson and Getz Ice Shelfs (10, 14, 16, 18, 19, 25) AASW WW MCDW CDW/WDW

CDW/WDW 6.45E-44* 0.1252

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0.0159* 6.45E-44* 0.0004* 5.84E-22*

WW 0.0159*

WSDW/ AABW/WSBW 5.47E-27* 3.46E-11* 0.3158

0.3158

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AASW

CDW/WDW 7.03E-37* 8.36E-14*

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0.0006* 7.03E-37* 8.36E-14* 5.47E-27* 3.46E-11*

DGM Spring WS AASW WW CDW/WDW WSDW/AABW/WSBW AAIW

DGM West section (32, 33) AASW WW MCDW CDW/WDW

WW 0.0006*

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AASW

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DGM Winter WS AASW WW CDW/WDW WSDW/AABW/WSBW

WW 0.3967 0.0627 0.0767

MCDW 2.64E-15* 0.0002*

CDW/WDW 0.0098* 0.0898 0.3004

0.3004 MCDW 0.0393* 0.0627 0.3567

CDW/WDW 0.0352* 0.0767 0.3567

AAIW 5.84E-22* 0.2727 0.3312 0.1186

ACCEPTED MANUSCRIPT Table 4. One-tailed T-test significance probabilities for the differences in methylmercury in identified water masses during the winter, spring and summer expeditions in the Weddell (WS), Amundsen and Ross Seas. *Significant at 95% confidence interval. WSDW/ WW CDW/WDW AABW/WSBW 0.0010* 4.74E-06* 8.06E-07* 0.0010* 0.0004* 0.0049* 4.74E-06* 0.0004* 0.0275* 8.06E-07* 0.0049* 0.0275*

MMHg Winter WS AASW WW CDW/WDW WSDW/AABW/WSBW

AASW

MMHg + DMHg Amundsen Sea (3, 4) AASW WW CDW/WDW

0.0705 0.3689

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MMHg + DMHg Dotson and Getz Ice Shelfs (14) AASW WW MCDW

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WW CDW/WDW 0.0705 0.3689 0.0437* 0.0437*

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AASW

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0.0439* 0.0060* 0.1439

WSDW/ WW CDW/WDW AABW/WSBW 0.0439* 0.0060* 0.1439 0.0418 0.1932 0.0418 0.3038 0.1932 0.3038

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AASW

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MMHg Spring WS AASW WW CDW/WDW WSDW/AABW/WSBW

AASW 0.2568 0.4167

WW 0.2568 0.0027*

MCDW 0.4167 0.0027*

ACCEPTED MANUSCRIPT 4. Discussion 4.1. Variations in total Hg Average HgT concentrations showed significantly higher HgT in all water masses during spring (2.6 ± 1.3 pM) than during winter (2.0 ± 1.0 pM; p < 0.01). Additionally, about 50 - 90% higher average HgT concentrations were found in the Weddell Sea in this study compared to previous campaigns in

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the Southern Ocean and the South Atlantic Ocean (Cossa et al., 2011; Bratkic et al., 2016). Bratkic et al. (2016) showed that spatial variations exists in the South Atlantic Ocean when measuring around

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14% higher HgT concentrations in water columns on the Western compared to the Eastern side of the South Atlantic transect at 40oS.

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Mercury measured in the Southern Ocean could originate from long range oceanic transportation. The main source of oceanic mercury is originally atmospheric deposition into surface waters (Mason et al.,

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2012). The CDW waters in the Southern Ocean originates from North Atlantic Deep Water (NADW) that is formed from sinking surface water at the Labrador and Norwegian Seas (McCartney and Talley, 1984). Bowman et al. (2015) measured an average HgT concentration of 0.9 pM in older NADW in

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the eastern North Atlantic at 20oN and 1.0 pM in younger NADW in the western basin. Further South, Mason and Sullivan (1999) measured higher HgT in the South Atlantic at around 16 oS (~1.8 pM) and Bratkic et al. (2016) measured between 1.0 and 2.5 pM at 40oS which is in good agreement with our

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measurements in the CDW (~1.8 pM).

The observed increase in HgT in the water column from winter to spring in this study also signify seasonal variations. Atmospheric mercury depletion events (AMDEs) were observed during late winter and spring and increased HgT concentrations were found in the surface layer at stations 500-3,

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515-1 and 566-16, of which sampling of the last two stations coincided with AMDEs (Nerentorp Mastromonaco et al., 2016a). A large part of the formed oxidized mercury during the winter and

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spring AMDEs was found to be associated to particles (Nerentorp Mastromonaco et al., 2016a). Since particulate mercury more easily transport downwards in the water column (Heimbürger et al., 2010), atmospheric deposition could be a source of the higher HgT measured in water columns in this study. Since no AMDEs have been reported occurring during summer and winter, the lower HgT concentrations measured by Cossa et al. (2011) and Bratkic et al. (2016) could be due to seasonal variations in atmospheric deposition. The two spring stations (555-2 and 566-16) were sampled close to the sea ice border. Sea ice and snow have been found to contain significant amounts of Hg and could be an additional source of Hg to seawater during the melting period, possibly explaining the higher HgT concentrations observed in

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ACCEPTED MANUSCRIPT spring (Gionfriddo et al., 2016; Chaulk et al., 2011; Cossa et al., 2011; Nerentorp Mastromaco et al., 2016b).

4.2. Variations in DGM The largest variation in DGM concentration between expeditions was observed in the top 100 m of the water column with highest average in winter (289 ± 162 fM), followed by summer (219 ± 90 fM, p <

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0.01), and lowest in spring (145 ± 150 fM, p < 0.01). During the winter expedition 13 of 20 stations were sampled within the sea ice margin and in spring only 3 of 25 stations were ice covered. In the

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Amundsen and Ross Seas during summer, all stations were sampled under sea ice. The formation of Hg(0) in seawater is the product of photochemical and biological reduction of Hg(II) species and

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demethylation of MeHg. Photochemical reduction dominates in the surface and lead to a supersaturation of Hg(0) and subsequent re-evasion (Andersson et al., 2011; Bowman et al., 2015 and

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references therein). Higher DGM in surface water measured under sea ice has previously been explained being due to the fact that sea ice acts as a cap preventing DGM from re-evading to the atmosphere, resulting in a build-up of DGM under sea ice (Andersson et al., 2011). Seasonal variation

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in irradiation and sea ice cover could explain the observed variations in surface DGM concentrations in this study.

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The general DGM profile showed an increase along the thermocline from a lower surface concentration to a higher concentration deeper in the water column. The DGM subsurface maxima was

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found to be highest in the warm Upper Circumpolar Deep Water (UCDW) core in the Weddell and Ross Gyre. The UCDW is oxygen-poor and nutrient rich and is one of the oldest water masses in the Southern Ocean (Rintoul, 2006). DGM concentrations in the subsurface were found to often peak at

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low O2, showing a negative correlation between DGM and O2 during all campaigns (R2 = 0.6, p < 0.01). Hg(0) present in deeper water columns represents the balance concentration of in situ abiotic

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and microbial reduction and oxidation processes (Bowman et al., 2015). Mason et al. (1998) found that Hg(0) in the North Atlantic Sea was produced in the mixed layer and was conserved when the NADW water mass was forming and sinking. This suggests that the high measured DGM in the CDW and WSDW layers, which originates from NADW, could be transported from the Atlantic Ocean. However, the low measured DGM concentrations in the NADW by Bratkic et al. (2016) in the South Atlantic Ocean contradicts with our observations. The subsurface maxima of Hg(0) was proposed by Bowman et al. (2015) to indicate a dark reaction which leads to a deep formation coinciding with the remineralization of organic matter. They also proposed a deep water formation via abiotic and microbial reduction which could vary spatially due to differences in reactivity or reduction capacity and activity of e.g. microbes.

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ACCEPTED MANUSCRIPT Speciated mercury has previously been shown to vary seasonally due to a seasonal change in redox reactions, temperatures, bioavailability and productivity (Horvat et al., 2003). Our results indicate a potential seasonal variation of the DGM distribution in the deep water columns of the Weddell Sea, having higher deep DGM concentrations in winter than spring, see Table 1.

4.3. Variations in MeHg

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Significantly higher average MeHg (MMHg + DMHg) was found in the Amundsen and Ross Seas compared to MMHg measured in the Weddell Sea (p = 0.047). It suggests that either a significant part

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of the measured MeHg was DMHg or that the Amundsen and Ross Seas have a higher production of MMHg. No HgT measurements were performed during the summer expedition. Hence, it cannot be

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excluded that the encountered higher MeHg could be due to higher HgT. DGM, however, showed an opposite trend with lowest water column concentrations in the Amundsen and Ross Seas, indicating a

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different ratio between DGM and MeHg and thus spatial variations in methylation. Cossa et al. (2011) measured a higher share of MMHg than DGM on the eastern side of the Antarctic

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continent. Water temperature and the concentration of organic matter are two factors found to influence methylation activity (Fitzgerald and Lamborg, 2007). The seawater temperature during the SR3 Geotraces transect was generally higher ranging from -2.5oC to 15 oC compared to the Weddell

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Sea where temperatures ranged from -2oC to 5 oC (Fig. S2 and S3 in SI). Cossa et al. (2011) hypothesized that due to a high productivity and the occurrence of organic matter in the Antarctic

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zone, a high net methylation occurs here in the hypoxic zones. During winter along and nearby the prime meridian in the Weddell Sea subsurface MMHg maxima were generally found at O2 - min at around 200 m depth (Fig. 4). A negative correlation between

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MMHg and O2 was observed during winter (R2 = 0.24, p < 0.01) with a regression coefficient of 0.01 pM MMHg /mL L-1 O2. High MMHg has previously been found at low O2 which can be explained by

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the connection to the microbial respiration of organic matter (Mason and Fitzgerald, 1990; Morel et al., 1998; Cossa et al., 2011; Bratkic et al., 2016). The regression coefficient has been proposed by Cossa et al. (2011) to be related to the bioavailability of mercury for methylation and has been used as a proxy for in situ methylation. Oxygen and the phosphate concentration in surface and intermediate seawater have been proposed as indicators for organic carbon remineralization, associating Hg(II) methylation with microbial decomposition of particulate organic carbon. However, MMHg is not only produced in low oxygen zones but also at the subsurface chlorophyll maximum, thus relating the formation of MMHg to primary production (Heimburger et al., 2010). In a Southern Ocean productivity study by Constable et al. (2003) it was found that the Ross and Weddell Seas and the area north of the Weddell Sea have generally higher chlorophyll levels 25

ACCEPTED MANUSCRIPT compared to the South East Pacific. The Amundsen and Ross Seas are acknowledged to have high biological productivity which could explain the higher MeHg concentrations found here than in the Weddell Sea (Arrigo et al., 2012). However, without measurements of HgT, chlorophyll or nutrients in the Amundsen and Ross Seas, it is hard to draw any conclusions about geographical variations in methylation in the Southern Ocean. Small-scale spatial differences were found among stations. During the winter expedition the highest water column MMHg concentrations were found at stations 498-1 and 500-3 in conformity with high

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HgT, see Fig. 4. During the summer campaign the MMHg + DMHg concentration was on average higher at station 14 at the Dotson and Getz ice shelf (194 ± 229 (15 - 783) fM) and at the shallow

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station 31 (304 ± 299 (65 - 847) fM) than at the Amundsen Sea stations (3 and 4) where the average concentration was 60 ± 30 (25 - 150) fM (p = 0.034, 0.048). Coastal regions are generally the most

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productive areas which could explain the higher MeHg concentrations found closer to the coast line (Constable et al., 2003)

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Spatial variations of MeHg concentrations in water columns were found to be more substantial than seasonal variations. Geographical variations in physiological parameters of the water column (Fig. S2-

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S5, Appendix) and the movement of water masses, oceanic currents and gyres could possibly explain

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the observed variations in MeHg concentration between stations.

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4.4. Implications for the mercury cycle and global modeling The bioaccumulation of MeHg in Antarctic food chains is if major concern both to wildlife and humans via extended fisheries. Around 47% of present MMHg in polar seawater columns was estimated by Lehnherr et al. (2011) to originate from in situ production, which shows that methylated

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mercury also can be transported from other geographical locations or enter via atmospheric deposition through DMHg recycling (Soerensen et al., 2016). The stability of MMHg in surface water before

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decomposition was according to Lehnherr et al. (2011) relatively short, resulting in a maximum transportation distance of around 2000 km. The measured MeHg in the water column in this study could thus originate from both an in situ production and via oceanic transport. MMHg in deeper waters of the Southern Ocean was hypothesized by Cossa et al. (2011) to exist as stable complexes in the absence of light, explaining their higher measured MMHg concentrations in deeper waters. Despite large seasonal differences in irradiation in the Southern hemisphere, no significant seasonal variations in MeHg concentrations were found. On contrary, geographical differences in measured MeHg concentrations were found in the Southern Ocean, which could speculatively lead to similar geographical variations in MeHg uptake in biota.

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ACCEPTED MANUSCRIPT Climate change could lead to a changing sea ice extent and temperature in polar regions. Sea ice is a major production zone and is a source of phytoplankton. It has recently also been found to contain methylating bacteria, being a source of MeHg in the Southern Ocean (Gionfriddo et al., 2016). When sea ice or icebergs melts, MeHg and nutrients such as iron and organic matter could be released into surface water, increasing the productivity and surface MeHg concentration (Constable et al., 2003). A diminishing sea ice extent and higher water temperature could however have a positive effect on the total mercury cycle in Antarctic water columns due to an increased re-evasion from open sea surfaces,

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as was suggested by Outridge et al. (2008) for the Arctic. However, the transition from older sea ice to newly formed sea ice could promote more formation of halogen radicals (Nghiem et al., 2012) leading

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to a more extensive atmospheric deposition. Melting sea ice and snow (shown to be significant reservoirs of mercury) would have a negative effect, leading to an initial increased input of mercury

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into Antarctic waters (Chaulk et al., 2011; Nerentorp Mastromonaco et al., 2016b). A lower sea ice coverage, higher temperature and an increased productivity would affect mercury interconversions by

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causing a higher photo-reduction and evasion, as well as a stronger methylation. Zhang et al. (2014) performed global modeling of the mercury cycle by using coupled OFFTRAC and

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GEOS-Chem models. Their results for the Southern Ocean showed lower calculated values than what was measured by Cossa et al. (2011). The differences were largest for younger water masses and was hypothesized to be due to a missing anthropogenic factor. Our higher measured HgT concentrations in

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Ocean, possibly due to an underestimation of atmospheric deposition during winter and spring, found to be enhanced over sea ice areas in the Weddell Sea (Nerentorp Mastromonaco et al., 2016a).

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5. Conclusions

The distribution of HgT in the water column showed no significant vertical variations, signifying a

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relatively even distribution of mercury in identified water masses of the Weddell Sea. The general vertical HgT profile showed largest observable variations in the surface and bottom layers, indicating a loss due to Hg(II) reduction and re-evasion of Hg(0) at the surface and influences of sediment resuspension/nepheloid layer at the bottom. The general DGM profile showed an increase along the thermocline from a lower surface concentration to a higher deep column concentration. A significantly higher average DGM column concentration was found in winter compared to spring due to a larger sea ice coverage which capsulate Hg(0), preventing re-evasion. DGM concentrations were found to have significant variations in different water masses with lower values in the surface AASW due to evasion losses from non-icecovered surfaces. 27

ACCEPTED MANUSCRIPT MMHg showed the largest variations in the top 500 m of the water column and close to the bottom. No seasonal variation of MMHg was found in the Weddell Sea between the winter and spring expeditions. However, spatial variations among stations were found such as for HgT. As for DGM the lowest MMHg concentration was found in the ASSW layer in the Weddell Sea, possibly due to photoinduced demethylation. The highest MeHg concentration was found in the MCDW layer at the Dotson and Getz ice shelfs. MCDW contains melted sea ice and snow and is enriched in iron and is associated with high phytoplankton blooms (Arrigo et al., 2012).

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Compared to measurements performed during the Geotraces transects (SR3 and GA10) HgT was found to be significantly higher in this study (50 - 90%). During the same campaigns in the Weddell

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Sea, LGGE in Grenoble measured high HgT concentrations in sea ice (48 pM in winter and 23 pM in spring) (Nerentorp Mastromonaco et al., 2016b), which could indicate higher HgT concentrations in

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the Weddell Sea compared to study sites visited during the Geotraces campaigns. In this study a significantly higher average HgT water column concentration was measured in spring

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(2.6 ± 1.3 pM) compared to winter (2.0 ± 1.0 pM), which could be explained by melting sea ice and snow, an increased input of atmospheric mercury due to observed AMDEs or due to spatial differences

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(Chaulk et al., 2011; Nerentorp Mastromonaco et al., 2016a+b). The unchanged shares of DGM and MMHg measured in the Weddell Sea suggest no significant changes in methylation and reduction

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rates between winter and spring.

Acknowledgements

This study was performed within the European Union’s Seventh Programme for Research: GMOS

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(Global Mercury Observation System) Project. Funding was provided by the GMOS project, the Swedish Polar Research Secretariat (Svenska Polarforskningssekritariatet) and CNRS-INSU through the program LEFE, and IPEV. We thank the Alfred Wegner Institute for Polar and Marine Research,

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the Swedish Polar Research Secretariat and the crews aboard R/V Polarstern and IB Oden who made our participation in the cruises possible.

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ACCEPTED MANUSCRIPT References Allard, B., Arsenie, I., 1991. Abiotic reduction of mercury by humic substances in aquatic systems – an important process for the mercury cycle. Water, air and soil Pollution. 56, 457-464. AMAP., 2011. Arctic Pollution 2011. Arctic Monitoring and Assessment Programme (AMAP), Oslo,

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Amos, H. M., Jacob, D. J., Streets, D. G., Sunderland, E. M., 2013. Legacy Impacts of All-Time

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Anthropogenic Emissions on the Global Mercury Cycle. Biogeochem. Cycles. 27, 1−12.

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Amyot, M., Gill, G.A., Morel, F.M.M., 1997. Production and loss of dissolved gaseous mercury in

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Amyot, M., Lean D.R.S., Poissant, L., Doyon, M-R., 2000. Distribution and transformation of elemental mercury in the St.Lawrence River and Lake Ontario. Can. J. Fish. Aquat. Sci. 57(Suppl.1),

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155 –163.

Andersson, M.E., Sommar, J., Gårdfeldt, K., Jutterström, S., 2011. Air-sea exchange of volatile mercury in the North Atlantic Ocean. Marine Chemistry. 12, 1-7.

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Andersson, M.E., Sommar, J., Gårdfeldt, K., Lindquist, O., 2008. Enhanced concentrations of

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dissolved gaseous mercury in the surface waters of the Arctic Ocean. Marine Chemistry, 110: 190194.

Arrigo, K.R., Lowry, K.E., van Dijken, G.L., 2012. Annual changes in sea ice and phytoplankton in

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polynyas of the Amundsen Sea, Antarctica. Deep-Sea Research II. 71-76, 5-15. Bargagli, R., Agnorelli, C., Borghini, F., Monaci, F., 2005. Enhanced deposition and bioaccumulation

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ACCEPTED MANUSCRIPT Title: Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean) Authors: Michelle G Nerentorp Mastromonaco, Katarina Gårdfeldt, Karen M. Assmann, Sarka Langer, Talasi Dulali, Yaroslav M. Shlyapnikov, Igor Zivkovic, Milena Horvat

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Significant variations of mercury were found between different water masses. Seasonal variation in HgT in the water column was found in the Weddell Sea. Atmospheric deposition and melting sea ice contribute to mercury in seawater. Highest MMHg concentration was found in modified circumpolar deep water.

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