Ocean plays a crucial role in mediating the global cycling of mercury (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. These complex and coupled processes make it difficult to distinguish Hg sources, transformation pathways, and exchange fluxes using concentration data alone. The Hg stable isotope approach has developed rapidly and shown great advantages in studying the biogeochemical cycling of oceanic Hg, from tracing sources and processes to reconstructing paleoenvironment and paleoclimate. Hg isotopes have a unique three-dimensional tracing system involving MDF, odd-MIF, and even-MIF. MDF, expressed as δ²⁰²Hg, can be caused by various physical and chemical processes, whereas odd-MIF, expressed as Δ¹⁹⁹Hg and Δ²⁰¹Hg, is mainly induced by photochemical reactions. In contrast, even-MIF, expressed as Δ²⁰⁰Hg and Δ²⁰⁴Hg, is more closely related to atmospheric Hg oxidation and deposition processes, and thus serves as a relatively conservative tracer for atmospheric Hg.
Herein, we examined the reported Hg isotope datasets in seawater, fish, and sediment/particulate samples from different oceanic environments, and evaluated their potential applications in tracing marine Hg cycling and constraining mass balance across different interfaces. Seawater Hg isotopes can provide constraints on atmospheric deposition, water-mass mixing and oceanic Hg reemission. Fish, which mainly accumulate methylmercury (MeHg), generally preserve the isotopic signals of MeHg sources and transformation histories, especially trophic transfer and exposure pathways. Sedimentary and particulate Hg isotopes integrate terrestrial input, atmospheric deposition, redox conditions and diagenesis, and therefore are useful for source apportionment and paleo-marine reconstruction. We found that i) Hg isotope data in seawater, suspended particles, and important input sources such as marine volcanic hydrothermal fluids and groundwater discharge are still very limited; ii) the processes and mechanisms controlling marine Hg isotope fractionation in complex seawater matrices remain poorly understood; and iii) studies on Hg transfer in marine food chains and Hg exchange across oceanic interfaces are rarely performed and lack of species-specific isotope data. As a result, the systematics of Hg in oceanic environments and global Hg cycling models cannot yet be accurately established, requiring future work to improve isotope methods for ultra-low-concentration seawater, single Hg species, to trace important source inputs and fluxes. Such an integrated isotope mass-balance framework will help verify and constrain the processes and corresponding fluxes of global Hg cycling.
Keywords: Seawater; Marine sediment; Marine biota; Hg stable isotopes

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