The presence of toxic metals in the soil, water, and atmosphere poses a threat to human populations and the biosphere as a whole. My interests lie primarily in quantifying the atmospheric and geologic sources of trace metals such as Pb and Hg in the natural environment and evaluating the roles that landscapes, hydrology, and carbon cycling pathways play in redistributing the atmospheric burden of toxic trace metals. The atmosphere is a principal transporter and source of Pb and Hg in terrestrial ecosystems while the biosphere and geosphere are a locus for chemical reactions that may contribute metals to the system, change speciation, isolate metals through sorption or coprecipitation, or allow transport for great distances. The carbon cycle is intimately tied to metals cycling in part because carbon recycling in soils and near surface waters controls geochemical parameters such as pH and redox state. As atmospheric CO2 rises over the next 200 years, increased carbon cycling rates in surface landscapes may alter processing rates of toxic metals in natural systems. My proposed study will draw on the expertise of Dr. Lynn Walter, Dr. Gerald Keeler, and Dr. Donald Zak and will evaluate the cycling of Pb and Hg in two geologically, geochemically and ecologically distinct watershed systems: the Tahquamenon Watershed on the Upper Peninsula and the Cheboygan Watershed in the northern Lower Peninsula of Michigan. The Cheboygan Watershed is underlain by thick carbonate-bearing glacial drift, displays dramatic topography, and hosts carbonate-rich waters, while the Tahquamenon Watershed is underlain by thin sandy drift, displays subdued topography, and is characterized by bogs and organic-rich, slightly acidic waters. Planned data collection (atmospheric fluxes in throughfall, precipitation, and snowpack; surface water and groundwater chemistries and fluxes; soil chemistries) will provide detailed information on the comparative biogeochemistries of the Cheboygan versus Tahquamenon watersheds, including atmospheric deposition of metals, metal concentrations in surface and groundwaters, and organic and inorganic carbon contents of soils and surface waters.