Mercury stable isotope composition of seawater suggests important net gaseous elemental mercury uptake

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.