Near surface oxidation of elemental mercury explains high mercury levels in the Arctic Ocean biota

Abstract It has been well-established that mercury oxidized (Hg2+) in the atmosphere and those exported from rivers are the main sources to the open ocean waters and biota. A recent modelled result of gaseous elemental Hg (Hg0) invasion into the Arctic Ocean seawater raises ecological concern, given the prevalence of Hg0 driven by long-range transport and surface re-emissions. We measured Hg isotope ratios in zooplankton, Arctic cod, and in samples of total gaseous Hg, sediment, seawater, snowpack in the Bering Strait, Chukchi Sea, and the Beaufort Sea. The Δ200Hg, used to differentiate between Hg0 and Hg2+, show, on average, 70% of Hg0 in all sampled biota and environmental matrices, except for the seawater (Hg2+). This is intriguing given that plankton cells cannot passively uptake Hg0. We show that this isotopic pattern can only be explained by near-surface Hg0 oxidation via terrestrial vegetation and coastally evaded halogen and sea salt aerosols, which preserve near-zero Δ200Hg of Hg0 upon oxidation. Our results contrast positive Δ200Hg anomalies reported from mid-latitude oceans, where upper atmospheric oxidation supplies the majority of bioavailable Hg2+. Our study highlights the importance of near-surface Hg0 oxidation to explain the high Hg levels reported in the Arctic Ocean biota.