Mercury in the Southern Ocean

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Cossa, D, Heimburger, LE, Lannuzel, D, Rintoul, SR, Butler, ECV, Bowie, AR, Averty, B, Watson, RJ, Remenyi, TA
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2011
• Subjects: Environmental Sciences; Ecological Applications; Ecosystem Function; Antarctic; Southern Ocean; The Antarctic; Antarc*; Antarctica; Sea ice
• Online Access: https://doi.org/10.1016/j.gca.2011.05.001; http://ecite.utas.edu.au/72217

We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolutionsouth-to-north section (27 stations from 65.50!S to 44.00!S) with up to 15 depths (04440 m) between Antarctica andTasmania (Australia) along the 140!E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a studyof mercury speciation in the snowsea iceseawater continuum was conducted at a coastal site, near the AustralianCasey station (66.40!S; 101.14!E). In the open ocean waters, total Hg (HgT) concentrations varied from 0.63 to2.76 pmol L!1 with transient-type vertical profiles and a latitudinal distribution suggesting an atmospheric mercurysource south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightlyhigher mean HgT concentrations (1.35 0.39 pmol L!1) were measured in Antarctic Bottom Water (AABW) compared toAntarctic Intermediate water (AAIW) (1.15 0.22 pmol L!1). Labile Hg (HgR) concentrations varied from 0.01 to2.28 pmol L!1, with a distribution showing that the HgT enrichment south of the SPF consisted mainly of HgR(67 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 23%). Methylatedmercury species (MeHgT) concentrations ranged from 0.02 to 0.86 pmol L!1. All vertical MeHgT profiles exhibitedroughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depthup to an intermediate depth maximum. As for HgT, low mean MeHgT concentrations were associated with AAIW, andhigher ones with AABW. The maximum of MeHgT concentration at each station was systematically observed within theoxygen minimum zone, with a statistically significant MeHgT vs Apparent Oxygen Utilization (AOU) relationship(p < 0.001). The proportion of HgT as methylated species was lower than 5% in the surface waters, around 50% in deepwaters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station HgT and HgR concentrationsfound in the snowsea iceseawater continuum were one order of magnitude higher than those measured in openocean waters. The distribution of HgT there suggests an atmospheric Hg deposition with snow and a fractionation processduring sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurringwith the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHgT concentrations in thesea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol L!1. The MeHgTvertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basalice. Whereas HgT concentrations in the water samples collected between the Antarctic continent and Tasmania are comparable to recent measurements made in the other parts of the World Ocean (e.g., Soerensen et al., 2010), the Hgspecies distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition onsurface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylationof Hg south of the SPF.