LOCUS OF CARBONATE AND SILICATE WEATHERING IN CARBONATE-BEARING GLACIAL DRIFT DEPOSITS: SIGNIFICANCE OF SOIL ZONE PROCESSES AS A CONTROL ON SHALLOW GROUNDWATER GEOCHEMISTRY

Soils in Southern Michigan are developed on thick carbonate-rich Pleistocene glacial deposits with high porosity and permeability allowing a close hydrogeochemical connection between soil zones and unconfined aquifers in this region. The parent drift is mineralogically complex, composed of quartz, K-feldspar, plagioclase, calcite and dolomite with minor mica and hornblende. This setting offers the opportunity to examine thermodynamic versus kinetic controls on carbonate and silicate weathering and to compare soil and ground water geochemistry.

We characterized mineral weathering at two study sites; in the agriculturally managed plots at the Long Term Ecological Research site (LTER) at the Kellogg Biological Station (KBS) of Michigan State University and in a protected upland forest ecosystem at the University of Michigan E.F. George Reserve (GR). Soil water and gas were sampled for complete geochemical analyses over an annual cycle at KBS and GR. Water discharge out of the shallow soils was calculated from the difference of precipitation and evapotranspiration and the fluxes of major cations and the reaction rates of calcite, dolomite and albite were calculated.

The divalent cations, Ca²⁺ and Mg²⁺, are largely contributed by calcite and dolomite dissolution and show a sinusoidal trend with concentration maxima occurring in summer and minima in winter. The deep soil solutions are at equilibrium with respect to calcite and 3-10 times undersaturated with respect to dolomite. As saturation indexes were invariant throughout the year the intensity of carbonate mineral dissolution appears controlled by thermodynamic constraints and by soil gas pCO2, which is a key factor in controlling carbonate solubility in these systems. In contrast to carbonate weathering, plagioclase dissolution is controlled by various environmental and hydrologic factors, such as temperature, pCO₂ and the residence time of fluid in the soils. Soil waters undergo relatively little chemical evolution after leaving the soil zone; soil water and shallow ground waters within study sites had very similar geochemical compositions. This suggests that mineral dissolution primarily occurs within the high pCO₂, biologically active soil zone before reaching the water table.