Understanding sources and cycling of mercury using mercury stable isotopes

Mercury (Hg) is a neurotoxin that humans are exposed to primarily through consumption of fish. Significant gaps remain in our knowledge of Hg biogeochemistry and measurement of natural abundance Hg stable isotopes is beginning to answer some of the outstanding questions. Mass-dependent fractionation (MDF, reported as δ202Hg) and mass-independent fractionation (MIF, reported as Δ199Hg) of Hg isotopes occur during many biogeochemical processes.;To better understand Hg cycling in volcanic systems we first investigated the isotopic composition of Hg emissions from Yellowstone volcanic field. We found that the light isotopes of Hg were preferentially emitted as Hg 0(g) from hydrothermal waters, suggesting that natural Hg0 (g) emissions may display negative δ202Hg values. Next, we used Hg isotopes to explore the role of photochemical reactions in Arctic atmospheric mercury depletion events. We found that a significant percentage of the Hg deposited during these events is subsequently photochemically reduced and lost, producing negative MIF of Hg remaining in the snow. We also found that a portion of the reemitted Hg accumulates on nearby ice crystals. These results suggest that although a large percentage of the Hg deposited during these events is lost from the snow, a portion of the Hg is retained locally and may enter local aquatic ecosystems. Finally, we investigated whether Hg isotopes could be used to trace anthropogenic Hg emissions to deposition sites and ultimately to fish populations. We found that Hg in precipitation collected downwind of a coal-fired power plant in Florida displayed anomalously negative δ 202Hg values. We then explored whether this isotopic signature could be used to trace Hg emitted by the power plant into local sport fish. Instead, we found that Hg in largemouth bass was largely derived from accumulated Hg in lake sediments.;This dissertation demonstrates that measurement of natural Hg isotopes in environmental samples is a powerful tool that can be used to gain greater understanding of complex Hg biogeochemistry. This relatively new technique may allow us to answer difficult questions about Hg atmospheric chemistry and biogeochemistry, to trace natural and anthropogenic Hg emissions, and to better understand pathways of human exposure to this hazardous trace metal.