Diagenetic behavior of methylmercury in organic-rich coastal sediments

Conditions regulating the formation and diagenetic behavior of methylmercury, CH3Hg(II), in marine sediments are poorly characterized, although understanding these conditions is essential to understanding the modes of transfer of this contaminant to the water column and biota. We report the first high-resolution profiles of CH3Hg(II) concentrations in pore waters extracted under rigorous oxygen-free and trace metal clean conditions from organic-rich contaminated marine sediments. Methylmercury was present at concentrations reaching 10 ng liter(-1) as Hg (50 pM) in pore waters extracted from the anoxic sediment layers but was below our detection limit in pore waters recovered from the surficial oxic layers. Dissolved CH3Hg(II) in the anoxic layers accounts for up to 30% of the total dissolved Hg but < 1% of the CH3Hg(II) adsorbed onto particles. Our data suggest that the surficial oxic sediment layer serves as a geochemical barrier to the diffusion of dissolved CH3Hg(II) accumulated in anoxic sediments to the overlying waters. Burrowing organisms, however, likely assimilate CH3Hg(II) directly from the underlying anoxic sediments and become active vectors of methylmercury to benthic predators, such as nordic shrimp and cod.