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Abstract Title: Mercury Stable Isotope Fractionation During Aqueous Photoreduction in Sulfidic Environment
Presenter Name: Yaqiu Zhao
Company/Institution: School of Earth System Science, Institute of Surface-Earth System Science, Tianjin University
Session: Special Session - Assessing the effectiveness of the Minamata Convention on Mercury under climate uncertainties
Co-Authors: Yaqiu Zhao,Wang Zheng

Abstract Information :

Ocean redox conditions play a central role in the evolution of life and the habitability of Earth?s present and future. Anoxic and sulfidic conditions are expanding in many continental margins due to human- and climate-induced marine eutrophication and deoxygnation, threatening the marine ecosystem. Such changes are expected to significantly affect the cycling of mercury (Hg). The stable isotopes of Hg have been widely used as a powerful tracer of Hg cycling, for that they exhibit unique mass independent fractionation (MIF), which primarily occurs during photochemical reactions. However, the mechanism of Hg MIF is still poorly understood, and the current studies on MIF mechanism has been largely focusing on photo-reactions in oxygenated aqueous environment in the presence of organic matter, whereas the photochemical processes in other redox conditions (e.g., sulfidic conditions where Hg(II) is complexed by inorganic sulfide) are rarely explored, limiting our ability to accurately trace and predict the current and future Hg cycling impacted by the expanding marine anoxia. Here we experimentally investigate Hg isotope fractionation during photoreduction of Hg(II) in sulfide-rich water. We observed both mass dependent fractionation with enrichment of heavier isotopes in the reactant Hg(II) and significant negative MIF that caused depletion of odd mass isotopes in Hg(II). This negative MIF is opposite to the positive MIF previously observed in photoreduction in oxic waters, but is similar to the MIF during photoreduction of Hg(II) complexed by organic reduced thiol ligands. Surprisingly, our experiments observed extrodinarily large MIF values up to -15? (?201Hg), much higher than those found in natural samples and in other known photochemical processes. More details of the experiment are still in progress, but such a large negative MIF indicates that the photoreduction of Hg(II) under sulfidic water may significantly affect the isotopic signatures of Hg in the environment.

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