SILICATE AND CARBONATE MINERAL WEATHERING CONTRIBUTIONS TO SOIL WATERS: A GEOCHEMICAL CASE STUDY FROM SOIL PROFILES DEVELOPED ON CARBONATE-BEARING GLACIAL DRIFT (SOUTHERN MICHIGAN)

We are investigating mineral weathering reactions in soils developed on carbonate-rich glacial drift in two field locations: the agriculturally managed plots at the Long Term Ecological Research site (LTER) at the Kellogg Biological Station (KBS) of Michigan State University and a protected upland forest ecosystem at the University of Michigan E.F. George Reserve. Each site is instrumented with soil water and gas samplers and soils have been mineralogically and geochemically characterized. The parent material is composed of quartz, K-feldspar, plagioclase, calcite and dolomite with minor mica and hornblende. The soils are about 12,000 years old with pronounced zones of accumulation rich in Fe, Al hydroxides. Carbonates have been removed from the upper meter of the soil profile. Organic carbon content in the O horizon is 1-2 wt%, and carbonates comprise up to 15 wt% with calcite and dolomite in nearly equal amounts. Soil waters and gases were sampled for complete geochemical analyses over a seasonal cycle at both sites. Mineral dissolution contributions to soil waters were evaluated for Na⁺, Ca⁺², and Mg⁺². Atmospheric contributions to cation budgets are only significant for Na, which was corrected based on Cl concentration, to arrive at the Na from plagioclase dissolution. Soil water Ca budgets are controlled by plagioclase and calcite/dolomite dissolution while Mg budgets are controlled by mafic mineral and dolomite dissolution. Importantly, Mg/Ca mole ratios in both soil waters and soil acid extractions decrease with depth then remain constant at a value near 0.35 once the carbonate-rich parent material is reached. This requires dissolution of nearly equal amounts of dolomite and calcite, suggesting a mixed thermodynamic/kinetic control on carbonate mass transfer. Contributions from plagioclase and mafic mineral dissolution to Ca and Mg solute budgets were quantified using a mass balance model. These results show that the upper soil zones are dominated by silicate mineral dissolution while the deeper carbonate-rich soil zones have water chemistries dominated by calcite and dolomite dissolution, with little further addition from silicate dissolution. Thus, silicate weathering is largely completed within the relatively shallow, organic carbon rich A/O soil horizon.