Controls of Primary Production on Southern Ocean Mercury Cycling (COPPSOM)

汞(Hg)是一种有毒元素，从自然和人为来源扩散到全球。在全球生物地球化学汞循环中，大量汞通过大气沉积进入海洋，海洋沉积物被认为是最终的汞汇。海洋汞在海洋食物链中以有毒的甲基汞的形式在海洋食物链中生物放大，甲基汞通过微生物过程或海洋鱼类的摄取在水相或沉积物中形成，是汞进入人类的重要途径。对陆地环境中生物地球化学汞循环的了解相对较多，而对海洋中汞的去向，特别是海洋生物泵的作用则知之甚少。以前的研究表明，汞在水体中表现出类似营养物的行为，藻类对汞的清除是将汞输出到沉积物中的重要载体。然而，汞在高产地区的清除、汞形态转化和出口的影响只得到了很少的调查，还没有被量化。在这个项目中，我们的目标是调查海水、悬浮物、浮游植物和浮游动物以及沉积物岩心中的总汞和甲基汞。样本是在南乔治亚州附近南大西洋的一个高产地区，在“岛屿撞击”号极地研究巡航期间(2022年10月/11月)在几个深度和加工站采集的。海水被采样到最大限度。深度7000米，同时对悬浮物和浮游生物进行最大限度采样。350米和600米，分别采用网状和大流量原地泵。沉积物岩心直接位于取样水柱下方，最大深度为8000米(三明治海沟)。汞和甲基汞浓度的数据将结合基本的和扩展的水化学和物理参数，如叶绿素a、营养物质、氧化还原指示剂、颗粒转化和通量，以及对化学成分和生物来源(浮游植物、大型或中型浮游动物、粪便颗粒等)的分析来解释。将由岛屿影响项目的其他参与者提供的悬浮物和沉积物。有了这个全面的数据集，我们的目标是解决汞和甲基汞的生物地球化学循环，并量化高产量地区的汞出口。我们推测，浮游植物水华导致水体中汞的枯竭，是甲基汞形成和汞向沉积物输出的热点。此外，我们想要证明，有机沉积物在穿过水柱时的分解是否会导致下沉颗粒中甲基汞的枯竭和总汞的浓缩。我们将进一步破译浮游动物中的甲基汞和三氢呋喃是否比营养不良区浮游动物中的甲基汞和三氢呋喃含量更高。

Mercury (Hg) is a toxic element that is globally dispersed from natural and anthropogenic sources. In the global biogeochemical Hg cycle, a substantial amount of Hg emitted to the atmosphere reaches the ocean through atmospheric deposition and ocean sediments are regarded as the ultimate Hg sink. Hg in the oceans biomagnifies in the marine food chain as toxic methyl-Hg which is formed in the water phase or sediment by microbial processes and consumption of marine fish is the crucial pathway of Hg to humans. The biogeochemical Hg cycle in the terrestrial environment is comparatively well understood, much less is known about the fate of Hg in the oceans, specifically about the role of the marine biological pump. Previous studies, most of them from oligotrophic areas, suggested that Hg shows a nutrient like behaviour in the water column and that Hg scavenging by algae is an important vector for the export of Hg to the sediments. However, the effect of Hg scavenging, Hg species transformation and export in highly productive areas has only been sparsely investigated and not yet been quantified. In this project we aim to investigate total Hg and methyl Hg in seawater, suspended matter, phyto- and zooplankton as well as in sediment cores. Samples were collected in a highly productive area in the South Atlantic around South Georgia during the “ISLAND IMPACT” Polarstern research cruise (Oct./Nov. 2022) at several Deep- and Process Stations. Seawater was sampled down to a max. depth of 7000 m while suspended matter and plankton was sampled to max. 350 and 600 m respectively using nets and large volume in-situ pumps. Sediment cores were taken directly below the sampled water column at a maximum depth of 8000 m (Sandwich Trench). The data of Hg and methyl Hg concentrations will be interpreted in context with basic and extended hydrochemical- and physical parameters such as chlorophyll a, nutrients, redox indicators, particle transformation and fluxes as well as analyses of the chemical composition and the biological origin (phytoplankton, macro or mesozooplankton, fecal pellets etc.) of suspended matter and sediments which will be provided by other participants of the ISLAND IMPACT project. With this comprehensive data set we aim resolve the biogeochemical cycling of Hg and methyl Hg and to quantify the Hg export in highly productive areas. We hypothesize that phytoplankton blooms cause depletion of Hg in the water column and are hot spots for methyl-Hg formation and Hg export to the sediment. Moreover, we want to prove if decomposition of organic seston on its way through the water column causes depletion of methyl Hg and enrichment of total Hg in sinking particles. We will further decipher if MeHg and THg in zooplankton of areas of intense phytoplankton blooms becomes higher enriched compared to zooplankton of oligotrophic zones.