CONTROLS ON CARBONATE WEATHERING FLUXES AND STREAM GEOCHEMISTRY IN GLACIATED MID-CONTINENT WATERSHEDS: LANDSCAPE LEVEL LINKS TO CO2 CYCLING

Carbonate weathering reactions are very rapid and largely regulate the pH and ionic strength of terrestrial aquatic systems. We are investigating landscape level controls on the carbonate geochemistry of surface water/shallow groundwater systems in Michigan, where watersheds have some of the most intense carbonate mineral weathering rates in the world. This region also is especially vulnerable to impacts of elevated CO₂ due to the close association with forest regrowth, landscape disturbance, and hydrologic linkage to the Great Lakes. Importantly, carbonate solubility is greater at low temperatures and is controlled by PCO₂. Thus, the carbonate carrying capacity of high latitude terrestrial ecosystems is closely linked to carbon processing rates. Dissolved inorganic carbon (DIC) fluxes from terrestrial environments is dominated by northern hemisphere continents. In N. America, 90% of the flux is derived from the recently glaciated mid-continent region where carbonate dissolution occurs in soils established on relatively young glacial drift. Study watersheds span an environmental gradient in temperature, drift lithology, and land cover. In some areas, surface water flow systems are associated with fens and lacustrine carbonates, indicating that reprecipitation of calcite can occur. Accordingly, emphasis was placed on seasonal variation and on feedbacks between stream discharge and carbonate chemistry. Shallow groundwaters are generally near equilibrium with respect to dolomite, but at PCO₂ values for each watershed which vary in a systematic manner from north to south. Surface waters degas CO₂ and are commonly highly supersaturated with respect to calcite (log IAP/K as great as 1.5). Stream HCO₃ concentrations are relatively insensitive to discharge, indicating that weathering is limited only by solubility. Stream Mg/HCO₃ ratios monitor calcite precipitation, and reveal that carbonate precipiation is most significant at low flow conditions. Dissolved organic carbon concentrations are commonly greater than 0.5 mM, which may play a role in inhibiting calcite precipitation. On balance, most of the inorganic carbon flux solubilized by carbonate dissolution is exported to the Great Lakes. Carbon mass transport from these watersheds is thus an important shuttle between the terrestrial carbon cycle and the oceanic C reservoir.