CARBONATE WEATHERING AND DIC FLUXES IN CARBONATE-BEARING LANDSCAPES, MICHIGAN AND SLOVENIA

We are investigating surface water geochemistry and mineral weathering fluxes from carbonate-bearing watersheds in the glaciated Midcontinent of the US and in alpine/valley/karst regions of the country of Slovenia. The Michigan watersheds in the study area discharge into the Great Lakes system, while the Sava River in Slovenia is an arm of the Danube River, which eventually drains into the Black Sea. Carbonate saturation state and CO₂ dynamics were determined from aqueous geochemical analyses of river waters and shallow groundwaters, along with discharge data from river gauging stations. Of the minerals comprising surface rock and soils, only calcite and dolomite can maintain equilibrium in the face of rising atmospheric CO₂ from anthropogenic influences on short human timescales. Furthermore, carbonate mineral solubility in physically open weathering environments is proportional to soil zone PCO₂, so carbonate weathering intensities should be affected by land use types and soil thickness. The hydrogeochemical setting of the US Midcontinent (Michigan) and Slovenia differ significantly in terms of their climate, discharge, land use, tectonic history, and age of bedrock carbonate units. Slovenian soil thicknesses range from 0 cm on steep mountainous slopes to greater than 70cm in the valleys. Michigan soils are typically 1-3 meters deep, controlled by geologic age rather than slope. In both Michigan and Slovenia, shallow groundwaters are generally Ca-Mg-HCO₃^- waters and are typically less than twice saturated for calcite and near equilibrium for dolomite at PCO₂ values up to 20 times atmospheric values. Groundwater Mg²⁺/Ca²⁺mole ratios are similar, and are typically between 0.4 –0.7, suggesting that calcite and dolomite dissolution are roughly equal in importance. Surface water is geochemically similar to the ground water, however PCO₂’s are closer to the atm. (10^-3.5) and many waters are supersaturated for calcite. Michigan rivers generally have HCO₃ ^- values from 2-6 meq/l, while the Sava River values are typically greater than 5 meq/l. The Sava drainage has area normalized DIC fluxes nearly 7-15 times higher than the world average of approximately 10 meq HCO₃ ^-/km²-s and 2-5 times higher than the values for Michigan rivers.