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Human exposure to toxic mercury (Hg) is dominated by the consumption of seafood. Earth system models suggest that Hg in marine ecosystems is supplied by Hg(II) deposition, with a 3x smaller contribution from gaseous Hg(0) uptake, and that photochemical reduction of marine Hg(II) drives important Hg(0) evasion to the atmosphere. Observations of marine Hg(II) deposition and gas exchange are sparse however, leaving the suggested importance of air-sea exchange unconstrained. Here we present the first Hg stable isotope measurements of total Hg (tHg) in surface and deep Atlantic and Mediterranean seawater. We use an isotope mass balance to estimate that sea water tHg can be explained by the mixing of 41% atmospheric Hg(II) deposition and 59% Hg(0) uptake. In the particulate Hg (pHg) fraction, which includes phytoplankton at the base of the marine food web, and in a compilation of marine fish Hg isotope data, we estimate similarly important marine Hg(0) uptake fractions of 73% and 49%. We observe no photochemical odd Hg isotope anomalies in tHg, which calls into question the large model Hg(0) evasion flux. Our findings indicate that direct atmospheric Hg(0) uptake is important and has implications for our understanding of atmospheric Hg dispersal and marine ecosystem recovery.

Even mercury (Hg) isotope mass independent fractionation (MIF), observed in rainfall globally, is used to quantify atmospheric Hg deposition pathways. The underlying reaction and MIF mechanism are unknown however. Here we investigate the Hg isotope composition of free tropospheric gaseous elemental Hg0 and HgII forms. We find that gaseous oxidized HgII has positive Δ199Hg, Δ201Hg, Δ200Hg, and negative Δ204Hg signature, similar to rainfall HgII, and we document rainfall HgII Δ196Hg to be near-zero. Cloud water and rainfall HgII show enhanced odd MIF compared to gaseous HgII, indicating in-cloud HgII photoreduction. Hg MIF observations of free tropospheric Hg0 dynamics show how net Hg0 oxidation leads to opposite MIF in Hg0 and HgII. A Δ200Hg mass balance for Hg0 and HgII forms suggests that measurements and models underestimate the tropospheric HgII pool.

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Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ199Hg values. These Δ199Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.