Atmospheric source of mercury to the ocean constrained by isotopic model

Mercury is a potent neurotoxin that poses significant health risks to humans, primarily through seafood consumption. Atmospheric deposition is the largest source of oceanic mercury, in either oxidized (Hg^II) or elemental (Hg⁰) form. Understanding the relative contributions of atmospheric Hg^II and Hg⁰ to the ocean is essential for accurately assessing global mercury budgets. Earlier even mercury isotope (Δ²⁰⁰Hg) analyses suggested equivalent Hg^II/Hg⁰ contributions but neglected spatial variations in atmospheric Δ²⁰⁰Hg signatures. Here, we developed a 3D atmospheric model incorporating mercury chemistry and isotopic fractionation to address this limitation. Our simulations reveal distinct atmospheric Δ²⁰⁰Hg patterns and quantify their deposition to the ocean. Constrained by observed Δ²⁰⁰Hg data in the ocean, we propose an updated deposition ratio of atmospheric Hg^II to Hg⁰ to the ocean, which may exceed 2:1, higher than the previously reported 1:1. Our findings are crucial for assessing atmospheric mercury dispersal and predicting the recovery of marine ecosystems.