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

点击翻译按钮获取中文摘要

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 .