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Geochemical Controls on Carbonate Equilibria and Mass Transport in Glaciated Mid-Continent Watersheds

The continuing emission of CO2 from fossil-fuel burning in the 21st century is, volumetrically, earth's most significant environmental pollutant. The dissolution of calcite and dolomite is very rapid and solubility is strongly CO2-dependent, which forges a close linkage between carbonate weathering and organic carbon respiration rates. This will be especially important in near surface groundwaters and streams, because these systems will be the first to respond to climatic and atmospheric changes due to the short residence time of water in these reservoirs. Thus, factors limiting dissolved inorganic carbon fluxes from watersheds will be very important in predicting the response of earth's surface reservoirs to anticipated changes in biomass and carbon budgets in the next few centuries, the timescale of most pressing human and environmental concern.<br/>For North America, nearly 90% of the HCO3- flux is derived from the mid-continent region, which has a high proportion of sedimentary bedrock mantled by carbonate-bearing glacial drift. Importantly, the low temperature of these temperate weathering environments maximizes carbonate mineral solubility. Indeed, Michigan watersheds have some of the highest area-normalized carbon fluxes in the world. This region thus provides an ideal field laboratory in which to assess controls on the HCO3- content of streams and the shallow groundwaters interacting with them on human timescales. Our proposed research program involves field study of the hydrogeochemistry of 6 endmember watersheds in the Great Lakes region, spanning the upper and lower peninsulas of Michigan. The specific field sites were selected on the basis of water chemistry, bedrock lithology, drainage basin type, land use and ecosystem type, mean annual temperature, and availability of real-time USGS gauging stations. <br/>Our goal is to produce a high quality analytic and theoretical characterization of the dissolved inorganic and organic carbon systematics of surface waters and groundwaters in the glaciated mid-continent region. Our initial studies demonstrate that surface and groundwater Mg concentrations are excellent conservative tracers of carbonate dissolution. As such, the Mg/HCO3 ratio provides a sensitive indicator of mass balances of carbonate dissolution versus precipitation in the streams, wetlands and lakes present within the watershed boundaries. The new data set we produce will integrate physical factors (type of river basin drainage, discharge, annual cycles) and with measured inorganic and organic carbon fluxes. Identifying equilibrium versus kinetic controls on carbonate mass transfer will permit more accurate predictions to be made of how earth's landscapes and surface hydrogeochemical systems will respond to increasing rates of carbon metabolism and fluxes in an elevated CO2 world.

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