Mercury species concentrations near Casey station, Antarctica and along the SR3 CASO-GEOTRACES transect

We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the 'snow-sea ice-seawater' continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg(T)) concentrations varied from 0.63 to 2.76 pmol/L with 'transient-type' vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg(T) concentrations (1.35 ± 0.39 pmol/L) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol/L). Labile Hg (Hg(R)) concentrations varied from 0.01 to 2.28 pmol/L, with a distribution showing that the Hg(T) enrichment south of the SPF consisted mainly of Hg(R) (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg(T)) concentrations ranged from 0.02 to 0.86 pmol/L. All vertical MeHg(T) profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg(T), low mean MeHg(T) concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg(T) concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg(T) vs Apparent Oxygen Utilization (AOU) relationship (p <0.001). The proportion of Hg(T) as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg(T) and Hg(R) concentrations found in the 'snow-sea ice-seawater' continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg(T) there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg(T) concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol/L. The MeHg(T) vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg(T) 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; doi:10.1021/es903839n), the Hg species distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition on surface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylation of Hg south of the SPF.本研究首次针对南大洋水柱开展汞形态分析，沿140°E经线在南极洲与澳大利亚塔斯马尼亚岛之间布设高分辨率南北向断面（27个站位，纬度范围65.50°S至44.00°S），每个站位设置最多15个采样深度（0~4440 m）。此外，为探究海冰在汞循环中的作用，本研究在澳大利亚凯西站（66.40°S；101.14°E）附近的沿岸站点开展了"雪-海冰-海水"连续体的汞形态分析。在开阔大洋水域中，总汞（Hg(T)）浓度介于0.63~2.76 pmol/L之间，呈现瞬态型垂直剖面特征；其纬度分布表明，南极极锋（SPF）以南存在大气汞来源，而亚南极锋（SAF）以北区域则发生表层汞移除。南极底层水（AABW）中的平均总汞浓度（1.35 ± 0.39 pmol/L）略高于南极中层水（AAIW）的1.15 ± 0.22 pmol/L。活性汞（Hg(R)）浓度范围为0.01~2.28 pmol/L，分布特征显示，SPF以南的总汞富集主要由活性汞贡献（占比67 ± 23%）；与之相反，极锋区（PFZ）以北表层水中该占比仅为其一半（33 ± 23%）。甲基汞形态（MeHg(T)）浓度介于0.02~0.86 pmol/L之间。所有站位的甲基汞垂直剖面均呈现相似规律：表层浓度较低，随深度增加浓度逐渐升高，在中层深度达到峰值。与总汞一致，南极中层水中的平均甲基汞浓度较低，而南极底层水中的平均甲基汞浓度更高。各站位的甲基汞浓度峰值均系统性地出现在氧最低区，且甲基汞与表观耗氧量（AOU）之间存在显著统计相关性（p <0.001）。总汞中甲基汞形态的占比在表层水中低于5%，在1000 m以下的深层水中约为50%，在SPF以南区域最高可达78%。在凯西沿岸站点，"雪-海冰-海水"连续体中测得的总汞与活性汞浓度较开阔大洋水域高出一个数量级。该区域总汞分布表明，大气汞随雪沉降，且在海冰形成过程中发生分馏作用：海冰排斥汞，同时使卤水中汞富集，这与开阔大洋中观测到的南极底层水汞富集现象可能存在关联。与之相反，海冰环境中的甲基汞浓度与开阔大洋水域处于同一范围，均低于0.45 pmol/L。"雪-海冰-海水"连续体的甲基汞垂直剖面表明其存在多种来源，包括大气、海水以及底层冰中的甲基化作用。本次在南极大陆与塔斯马尼亚岛之间采集的水样中总汞浓度，与全球其他海域近年的观测结果（例如Soerensen等，2010；doi:10.1021/es903839n）基本可比；但汞形态分布特征表明，南大洋汞循环存在独特性：(i) 冰缘附近表层水中的大气汞净沉降，(ii) 海冰形成过程中卤水中的汞富集，以及(iii) SPF以南区域的汞净甲基化。