Mercury cycling in sediments of Chesapeake Bay and the mid-Atlantic continental shelf and slope

Date of Completion

January 2010

Keywords

Biogeochemistry|Sociology, Organizational|Geochemistry

Degree

Ph.D.

Abstract

Human activities have significantly altered the mercury (Hg) cycle, increasing elemental Hg emissions to the atmosphere and inorganic Hg loadings to aquatic systems. Increased production of methylmercury (MeHg), a neurotoxin formed in aquatic systems, and its bioaccumulation into aquatic foodwebs, is likely, but the potential magnitude of the increase is not known. With fish MeHg levels as the main driver of the Hg-based US EPA and FDA fish consumption advisories, and given the importance of ocean fisheries as a source of MeHg exposure to humans, a comprehensive understanding of the behavior and fate of MeHg in the marine environment is necessary. This dissertation was designed to enhance knowledge of the biogeochemical factors affecting MeHg production and distribution in coastal marine sediments and to determine the importance of these sediments as a source of MeHg to the coastal ocean. ^ Although the biogeochemical characteristics of the suboxic-anoxic sediments of the mid-Atlantic continental shelf and slope were significantly different than those of the productive Chesapeake Bay, the controls on Hg distribution and MeHg production were consistent. Hg concentration and partitioning in the bulk-phase were strongly controlled by the organic matter quantity and quality of the sediment. MeHg production was related to the concentration of aqueous neutrally charged Hg-S complexes in the porewater and, to a lesser extent, the activity of methylating bacteria. In offshore sediments, high MeHg production coincided with a decrease in the strength of Hg partitioning to the bulk-phase, a relative increase in Hg porewater concentrations, and an expansion of the oxic-anoxic transition zone down-core. ^ All shelf and slope sites were sources of MeHg to the water column over all seasons studied, with an average diffusive flux of 0.8 pmol m−2 day−1. By integrating the flux over the global area of the continental margin, the coastal diffusive flux of MeHg was estimated to be 0.01 Mmol per year. This flux, which is a minimum estimate and likely at least ten times higher due to bioturbation/biorirrigation, is on the same order as other quantified MeHg inputs to the ocean, and therefore is consistent with the hypothesis that the continental margin is a significant source of MeHg to the global ocean, and must be included in future ocean MeHg budgets. ^

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