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Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36-1,313 million moles since the 1500s. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.

shelf-and river-derived elements to the central Arctic Ocean • The TPD is rich in dissolved organic matter (DOM), which facilitates long-range transport of trace metals that form complexes with DOM • Margin trace element fluxes may increase with future Arctic warming due to DOM release from permafrost thaw and increasing river discharge

a b s t r a c tMercury (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 (Hg 0 ), 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 scavengedand nutrient-type vertical distributions, whereas concentrations of DMHg, Hg 0 , 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 Hg 0 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 Â for filtered total Hg, 1.6 Â for Hg 0 , 2.5 Â for filtered MMHg, and 2.6 Â 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 Hg 0 in western basin NADW may be explained by either greater Hg substrate availability or greater methylation and reduction potentials in younger deep waters.

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