(Invited Presentation) Monomethyl and Dimethyl Mercury Cycling on The Continental Margin of The Northwest Atlantic Ocean

Principal and unifying findings from three comprehensive oceanographic cruises (2008-10) relating to the biogeochemical cycling of Hg and methylated Hg species on the continental margin (NW Atlantic) are presented. Vertical gradients are evident for mono and dimethylmercury (i.e., MMHg and DMHg, respectively) in waters on the shelf and upper slope (< 1000m) with substantial enhancements near the sediments. These regions are affected by pollution-borne mercury, principally from atmospheric sources. Diffusional estimates (pore water gradients) range from 0.1 to 45 pmol m^-2 d^-1 for MMHg ( average of 10 pmol m^-2 d^-1), while measurements with benthic chambers are typically 3-5x greater. Estimated vertical fluxes of DMHg based on water column distributions range from DL to ca. 40 pmol m^-2 d^-1. These inputs are quite significant, likely present in other comparable oceanic settings, and represent a potentially large sedimentary source of methylated Hg to the marine environment including the open ocean. Further, upper ocean maxima in MMHg, DMHg, and filtered total Hg, which correlate with the oxygen distributions (e.g., minimum zone) and isopycnal surfaces, are found at the more remote and deeper stations on the slope. This studies are showing quantitatively that MMHg and DMHg are formed on the continental margin and in ocean waters (<1000m), which are affected by atmospheric Hg deposition from natural and anthropogenic sources.