Mercury (Hg) in marine shallow-water hydrothermal systems - an overlooked source of Hg in the global cycle

随着2013年联合国环境规划署《水俣公约》的签署，全球各国政府都承认汞是有毒的。尽管对汞的研究已经进行了几十年，但我们仍然缺乏一些关于汞在环境中的流动和转化的最基本问题的答案。例如，热液喷口的通量被怀疑是对全球汞循环最重要的自然贡献，然而通量的估计范围从20到2000 t -1不等。尽管深海热液系统存在一些相互矛盾的数据，但浅海、近岸环境的热液系统在很大程度上被忽视了，尽管它们对沿海地表水的化学成分和生物多样性产生了公认的影响。它们不仅将硫化氢和砷等有毒化合物直接引入生产性地表水，还提供铁和碳等营养物质。目前，海洋浅水热液系统（MSWHS）对全球汞循环的贡献尚不清楚。尽管已经有一些研究试图将MSWVS作为潜在的汞源，但没有一项研究能够提供实际的流体测量数据，并调查热液汞排放对全球汞循环的可能贡献。部分原因是由于海洋热液喷口流体具有挑战性的基质在分析上存在困难。此外，不同地质/地球化学背景的MSWHS对全球汞循环的贡献可能不同。汞在喷口流体中的化学形态及其沿喷口海水梯度的行为和转变尚不清楚。其中一个基本的转变是汞的细菌甲基化转化为甲基汞（MeHg），然后在鱼类体内进行生物积累和生物放大，最终进入人类消费者的体内。甲基汞的产生是一种遍及海洋的现象，但由于开放海洋中初始汞浓度较低，因此难以精确研究这一反应。因此，MSWHS中较高的初始汞浓度应使其成为确定转化率及其对环境变量依赖性的理想天然实验室。因此，我们建议对MSWHS中汞的赋存、形态和通量进行研究，为全球汞循环的研究提供更好的估计。计划的工作包括四个部分：(1)利用水肺潜水设备对选定的MSWHS站点进行测绘和采样；(2)完成释放汞种类（无机汞、甲基汞、气态元素汞）的表征；(3)甲基化率的测定；(4)估计MSWHS中总汞和甲基化汞的通量。

With the signing of the UNEP Minamata Convention in 2013, governments have globally accepted that mercury (Hg) is toxic. Despite decades of Hg research, we still lack answers to some of the most fundamental questions concerning Hg flux and transformation in the environment. The flux from hydrothermal vents, for example, is suspected to be the single most important natural contribution to the global Hg cycle, yet flux estimates range several orders of magnitude from 20 to 2000 t y-1. While some contradictory data exists for hydrothermal systems in the deep ocean, hydrothermal systems in shallow, near shore environments have been largely ignored, despite their recognized impact on the chemical composition and biodiversity of coastal surface waters. They not only introduce toxic compounds, such as hydrogen sulfide and arsenic directly into productive surface waters, they also provide nutrients such as iron and carbon. Today, the contribution of marine shallow-water hydrothermal systems (MSWHS) to the global Hg cycle remains unknown. Though, a few studies have already tried to address MSWVS as potential Hg source, none of them could provide actual fluid measurements and investigate the possible contribution hydrothermal Hg emissions to the global Hg cycle. Part of this stems from analytical difficulties with the challenging matrix of marine hydrothermal vent fluids. Moreover MSWHS of different geological/geochemical settings may contribute differently to the global Hg cycle. The chemical speciation of Hg in vent fluids and its behavior and transformation along the vent seawater gradient remains unknown. One of those fundamental transformations would be the bacterial methylation of Hg to methylmercury (MeHg), which then bioaccumulates and biomagnifies in fish, eventually finding its way to the human consumer. MeHg production is an ocean-wide phenomenon, but the low initial Hg concentrations in the open ocean make it difficult to precisely study this reaction. Thus higher initial Hg concentrations in MSWHS should make them ideal natural laboratories to determine transformation rates and their dependence on environmental variables. Thus, we propose to investigate the occurrence, speciation and flux of Hg in MSWHS to provide better estimates for the study of the global Hg cycle.The planned work consists of four parts: (1) Mapping and sampling of selected MSWHS sites utilizing SCUBA diving equipment, (2) complete characterization of the released Hg species (inorganic Hg, MeHg, gaseous elemental Hg), (3) determination of methylation rates and (4) estimating the flux of total and methylated Hg from MSWHS .