CARBONATE EQUILIBRIA AND MASS TRANSPORT DYNAMICS IN A MID-CONTINENT WATERSHED, MI

Continued anthropogenic emissions will likely produce large increases in atmospheric CO₂ levels and related climate changes (e.g., temperature, hydrology). On this short human time scale, shallow reactive carbon pools in the terrestrial environment will be significantly impacted.  Importantly, these involve strong coupling between inorganic carbon pools (carbonate minerals) and organic carbon reservoirs (biomass and soil carbon). 

Carbonate mineral solubilization in surficial weathering environments generally is proportional to PCO₂. Thus, the flux of fossil fuel-produced CO₂ to fresh water systems depends on mass transport rates of carbonate weathering products. We have investigated carbonate fluxes and saturation state/PCO₂ dynamics in a carbonate-rich, glaciated mid -continent watershed (Huron River, MI) via detailed aqueous geochemical analyses (soil, wetland, stream, and groundwater) along with available watershed discharge measurements.

Groundwaters are dominated by dissolution of carbonate minerals and are at saturation with respect to calcite at elevated CO₂ partial pressures. Surface waters are largely groundwater fed, but can be modified chemically by addition of deicing salts and variable extents of CO₂ degassing, which can lead to extreme supersaturations (IAP/K > 15).

Along flow paths in the watershed, calcium carbonate is back precipitated as marl deposits in wetlands and lakes. The Mg²⁺/HCO₃^- ratio is the most sensitive indicator of precipitation because this ratio is fixed in groundwaters and increases systematically with increasing loss to precipitation. Although losses of calcium carbonate are observed throughout the sampling periods and sites, it is greatest during times of lower stream flow. Data taken from USGS gages in the watershed over a longer time period support this inverse relationship between loss of calcium carbonate and riverine discharge. Overall, however, the net loss of CaCO₃ to surface fens and lacustrine deposits is rather low, comprising less than 20% of the original groundwater source term.