Mercury stable isotopes reveal atmospheric oxidation and removal processes to high and mid-latitude oceans

Mercury (Hg) stable isotopes have become a powerful tracer for understanding Hg sources and complex biogeochemical processes in the natural environment. Anomalies of even mass number mercury isotopes (even-MIF; Δ²⁰⁰Hg, Δ²⁰⁴Hg), in particular, have enabled the differentiation of Hg chemical forms (Hg0 vs. HgII) and their depositional pathways. This is because even-MIF occurs exclusively via upper atmospheric photo-oxidation, leaving HgII with a positive Δ²⁰⁰Hg value and Hg0 with a negative Δ²⁰⁰Hg value. Over the past several years, my research group has characterized even-MIF anomalies in atmospheric samples (gaseous Hg0, precipitation), seawater, zooplankton, and fish from high (Beaufort, Chukchi Sea) and mid-latitude oceans (West to Central Pacific Ocean). Our goal was to comprehensively trace sources, oxidation/removal pathways, and fate of Hg to open ocean food web. The results depict a clear Δ²⁰⁰Hg dichotomy, in which all samples from mid-latitude oceans have positive Δ²⁰⁰Hg (reflecting HgII) and the samples from high-latitude oceans have negative Δ²⁰⁰Hg (reflecting Hg0). The δ²⁰²Hg, which has been used to trace Hg sources (types of anthropogenic, natural sources) across a large spatial scale, show that, while high-latitude oceans exhibit values similar to that of background Hg, mid-latitude oceans have δ²⁰²Hg consistent with anthropogenic Hg. There is also a gradual dilution of zooplankton Hg concentration and anthropogenic δ²⁰²Hg signals from West to the Central Pacific Ocean. We summarize Hg sources and oxidation pathways as such, by using an isotope mixing model: In the West and Central Pacific, 52-60% of Hg0 emitted from anthropogenic sources is first circulated to the upper atmosphere for photo-oxidation prior to oxidation and removal to the open ocean. The remainder of Hg comes from riverine Hg export. In the Arctic, >70% of Hg is oxidized near the biosphere, not in the upper atmosphere, thereby conserving the even-MIF of Hg0 even upon oxidation. We speculate that the presence of abundant halogens and sea salt aerosols (SSA) is responsible for rapid Hg0 oxidation and removal to the open ocean. Our study showcases that Hg stable isotopes can be used to differentiate sources, pathways of removal, and fate of Hg across a large spatial scale. After compiling further dataset, we identify that near-surface Hg0 oxidation mediated by halogens and SSA in the Arctic explains elevated Hg levels reported in the Arctic fish, mammals, and polar bears. The pathway of anthropogenic Hg0 emission to bioaccumulation detected in the West and Central Pacific suggests that anthropogenic Hg0 abatement from continental Asia would lower Hg levels in the adjacent marine ecosystems.