Filling the Gap of Mercury Isotopic Composition in the Ocean

Abstract

As one of the most important reservoirs of mercury (Hg) on earth, ocean plays a crucial role in mediating the global cycling of Hg. Mercury undergoes complex migration and transformation processes (e.g., adsorption/desorption, deposition/resuspension, oxidation/reduction, and methylation/demethylation) between multiple interfaces (e.g., atmosphere-seawater, land-seawater, seawater-sediment, and seawater/sediment-biota) of the ocean. Recently, Hg stable isotope approach has shown great advantages in studying the biogeochemical cycling of oceanic Hg, from tracing sources and processes to reconstructing paleoenvironment and paleoclimate. Here, we overviewed the accurate analytical methods for measuring Hg isotopes in different marine samples, summarized the reported Hg isotope dataset in seawater, sediment/particulate, and biological samples, and comprehensively analyzed the fractionation mechanisms of Hg isotopes and their potential applications in tracing marine Hg cycling. We found that 1) Hg isotope data in seawater and marine particles are very limited, 2) marine Hg isotope fractionation mechanisms, especially mass-independent fractionation mechanisms remain unclear, (3) studies on Hg transfer in marine food chain and Hg exchange amongst reservoirs are constricted only by limited Hg isotope data in single Hg species (e.g., Hg of different chemical forms in sediment and methylmercury in seawater/sediment/biota), thus with great uncertainity, and (4) reconstruction of paleo-marine environment using Hg isotopes is still in its infancy. As a result, the systematics of Hg in oceanic environment and the global Hg cycling model could not be accurately established using current marine Hg isotope data, requiring further systematic study to precisely verify and restrict the key processes and corresponding fluxes of marine and even global Hg cycling. This work includes, but not limited to 1) establish and/or improve isotope analysis method for single Hg species and in-situ or real-time measurement of seawater and important input sources like marine volcanic hydrothermal and submarine groundwater, 2) well investigate the mechanisms of Hg isotope fractionation in seawater matrix (e.g., organic and inorganic media, particulate matter, microorganism), 3) deeply explore Hg (especially MeHg) enrichment and transfer in marine food chain, and 4) Hg exchange between ocean-atmosphere and seawater-sediment, and further reconstruct ancient ocean evolution, in order to provide basic data and theoretical support for controlling marine Hg pollution and understanding global Hg cycling.