Mercury in the North Atlantic Ocean: The U.S. GEOTRACES zonal and meridional sections

Mercury (Hg) in the ocean undergoes many chemical transformations, including in situ production of monomethylmercury (MMHg), the form that biomagnifies in marine food webs. Because the ocean is a primary and dynamic reservoir of Hg cycling at earth׳s surface and the principal source of human MMHg exposures through seafood, it is important to understand the distribution of Hg and its chemical species in marine environments. We examined total Hg, elemental Hg (Hg0), MMHg, and dimethylmercury (DMHg) with fully resolved high-resolution profiles during the U.S. GEOTRACES zonal and meridional sections of the North Atlantic Ocean (GEOTRACES GA03). Total Hg in filtered water had both scavenged- and nutrient-type vertical distributions, whereas concentrations of DMHg, Hg0, and filtered MMHg were increased in the oxygen deficient zone of the permanent thermocline across the basin, relative to water above and often below. Total Hg and MMHg on suspended particles accounted for less than 10% of total concentrations. The TAG hydrothermal vent on the Mid-Atlantic Ridge (MAR) was a source of total Hg and MMHg to nearby waters with apparent scavenging and Hg transformation occurring in the buoyant plume. Uniquely, we observed significant horizontal segregation of filtered total Hg and MMHg, DMHg, and Hg0 in North Atlantic Deep Water (NADW) between younger water on the western and older water on the eastern side of the MAR. Relative to eastern NADW, Hg concentrations in western NADW were greater, on average, by 1.14× for filtered total Hg, 1.6× for Hg0, 2.5× for filtered MMHg, and 2.6× for DMHg. Total Hg enrichment in deep water of the western basin may have resulted from downwelling of anthropogenic Hg during NADW formation. Enrichment of MMHg, DMHg, and Hg0 in western basin NADW may be explained by either greater Hg substrate availability or greater methylation and reduction potentials in younger deep waters.