Award: OCE-1132480

Methylmercury accumulating in wildlife is the major public health concern associated with mercury in the environment. Not only is methylmercury quite toxic but it can be produced and bioaccumulate within aqueous systems, especially marine. Indeed, consumption of seafood has resulted in about 5–10% of American women of childbearing age having blood methylmercury levels that pose an increased risk for fetal neurodevelopment problems. Knowledge about the distribution, behavior, and fate of mercury and methylmercury in the oceans is limited. The major sources of methylmercury to the ocean and its biota are neither well quantified nor are the marine biogeochemical reactions leading to its synthesis well understood. Key questions include whether internal or external sources are more important and what is the impact of human-related mercury contamination. Co-investigators Chad Hammerschmidt and Carl Lamborg, in addition to HammerschmidtÆs Ph.D. student, have been studying the distribution of mercury and its chemical forms in the North Atlantic Ocean as part of the U.S. GEOTRACES Program. This NSF-Chemical Oceanography Program supported study has sought to uniquely reveal how mercury, two forms of methylmercury (monomethylmercury and gaseous dimethylmercury), and gaseous elemental mercury are distributed vertically in the water column of the North Atlantic as well as how the concentrations of these mercury forms vary spatially in seawater, rain, and atmospheric particles sampled between the United States and the coast of western Africa. Their prior research and that of others have pointed to mercury methylation in coastal sediments and oceanic water column production at depths shallower than 1000 m as the major sources of methylmercury to most marine organisms. Collaborative efforts at sea and in the lab have provided new information as to how methylmercury and the other forms of mercury enter and are transformed and distributed in waters of the North Atlantic Ocean. This research shows that the concentrations and forms of mercury differ considerably with depth in the water column and between deep waters in the western and eastern basins of the North Atlantic Ocean, which are separated by the Mid-Atlantic Ridge system. Vertical variations in mercury concentrations are attributed to gas exchange between the ocean and atmosphere, particle scavenging and remineralization, and biological and abiotic processes that transform mercury among its major forms at varying depths in the water column. Concentration maxima of total mercury, mono- and di-methylmercury, and elemental mercury coincide with minima in dissolved oxygen levels in the thermocline region, typically at between 500 and 1000 m depth in the North Atlantic. Remarkable differences in the concentrations of mercury species in deep water (> 2000 m) appear to be related to changing biogeochemical characteristics of the water as it ages and moves from the western to eastern basin of the North Atlantic. Increased knowledge of biogeochemical processes affecting the production, destruction, and uptake of methylmercury is required to manage and diminish the impact of mercury contamination. This project has shown that monomethylmercury and dimethylmercury are formed and transformed throughout the marine water column and in the upper ocean in regions affected by human-related mercury contamination. This implies that reductions of mercury inputs to the ocean, mostly from human sources via atmospheric deposition, would result is less methylmercury production and accumulation in fish. Last Modified: 04/30/2013 Submitted by: Chad R Hammerschmidt