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

Abstract

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.

Dynamic mercury methylation and demethylation in oligotrophic marine water

“…The enhanced demethylation in oxygen minimum waters is in contrast to the general trend of elevated [MeHg] 215 in regions of net remineralization that is widespread in ocean basins (Mason and Fitzgerald, 1993;Cossa et al, 2009;Sunderland et al;Cossa et al, 2011;Bowman et al, 2015;Bowman et al, 2016). In addition, demethylation rate constants were enhanced in unfiltered waters (0.50 % d -1 , n = 4) relative to filtered waters (0.26 % d -1 , n = 7).…”

“…Because methylated Hg (collectively 35 abbreviated as MeHg) accumulates more efficiently than inorganic Hg, (Hg(II)), transformations into MeHg ultimately controls burdens in upper trophic level biota. These methylated species, monomethylmercury (MMHg) and dimethylmercury (DMHg), can be found throughout the water column, with concentration maxima often found at depths of rapid oxygen consumption (Mason and Fitzgerald, 1993;Cossa et al, 2009;Sunderland et al;Cossa et al, 2011;Bowman et al, 2015;Bowman et al, 40 2016). At these depths, production of methylated Hg has been linked to active organic matter remineralization (Cossa et al, 2009;Sunderland et al, 2009).…”

“…The 305 use of dual tracers in this time course experiment reveals that demethylation controls [MeHg] as the incubation approaches steady-state, indicating an important role of demethylation in determining [MeHg] in the marine water column and its availability for bioaccumulation. (Mason and Fitzgerald, 1993;Cossa et al, 2009;Sunderland et al;Cossa et al, 2011;Bowman et al, 2015;Bowman et al, 2016), gaps persist in our understanding of how organic matter remineralization influences methylation. While low [MeHg] in surface and deep waters are thought to be the result of active demethylation by both photodemethylation and biological demethylation (Cossa et al, 315 2009;Sunderland et al, 2009;Monperrus et al, 2007;Lehnherr et al, 2011) subsurface peaks in [MeHg] are thought to be controlled by two factors: 1) the release of Hg(II) substrate from sinking organic matter remineralization (Cossa et al, 2009;Sunderland et al, 2009), and 2) enhancement of microbial loop activity (Heimbürger et al, 2010).…”

Trace metal concentration and partitioning in the first 1.5 m of hydrothermal vent plumes along the Mid-Atlantic Ridge: TAG, Snakepit, and Rainbow

“…In particular, pyrite nanoparticles oxidize slowly , so the presence of trace metals means that pyrite nanoparticles may serve not only as a source of Fe to the larger oceans (Yücel et al, 2011), but also as a source of other trace metals as demonstrated by the 2011 North Atlantic GEOTRACES cruises for total Fe (Hatta et al, 2015), particulate Fe (Ohnemus and Lam, 2015), soluble Fe(II) , and soluble Hg (Bowman et al, 2015) near the TAG vent field.…”

Air-sea exchange of gaseous mercury in the East China Sea

“…Generally, dissolved gaseous mercury (DGM) in seawater includes both Hg 0 and dimethylmercury, but little dimethylmercury (generally < 5%) can be found in the surface seawater (Bowman et al, 2015;Hammerschmidt and Bowman, 2012). DGM is only a small fraction of total Hg (THg) in seawater, but plays an important role in the oceanic Hg cycle (Fu et al, 2010).…”