Seasonal Variability in Marine Atmospheric Mercury Isotope Signatures and Environmental Drivers

Atmospheric deposition is the dominant source of mercury (Hg) to the ocean, shaping its transport, transformation, and potential exposure to global populations. Mercury isotopes offer a powerful tool for evaluating these processes; however, the isotopic composition of atmospheric Hg over the ocean remains poorly understood. This study applies a three-dimensional isotopic model to investigate the mass-independent fractionation (MIF, represented by Δ¹⁹⁹Hg and Δ²⁰⁰Hg) signatures in atmospheric Hg within the marine boundary layer. The model reveals marked spatial and seasonal variations in Δ¹⁹⁹Hg and Δ²⁰⁰Hg of oxidized Hg species. These variations are driven by sea salt aerosol debromination, influenced by environmental factors such as solar radiation, sea surface temperature, and wind speed, which modulate Hg redox chemistry and deposition processes. Simulations indicate that seasonal changes in atmospheric MIF signatures substantially influence the isotopic composition of Hg deposited to the ocean, particularly through dry and wet deposition of oxidized Hg species. These findings underscore the importance of accounting for spatial and temporal variability in atmospheric end-members when tracing marine Hg sources and interpreting sedimentary Hg isotope records under a changing climate.