Inorganic carbon isotope systematics in soil profiles undergoing silicate and carbonate weathering (Southern Michigan, USA)

The upper Midwest USA features glacial-derived till materials enriched in carbonate minerals, but with the uppermost soil layer progressively leached of carbonates in the interval since glaciation. Groundwaters and groundwater-fed surface waters are profoundly influenced by carbonate mineral dissolution. Stable carbon isotope compositions of soil waters and groundwaters in two southern Michigan watersheds (Huron and Kalamazoo) were studied as a function of pH, δ13CCO2, types of weathering reactions (silicate vs. carbonate), and degree of isotope equilibration. This comprehensive study of carbon isotope biogeochemistry in the vadose zone, including soil gas, soil water/groundwater, and soils (organic matter/carbonate phases), elucidates relations between the chemical weathering rates and CO2 fluxes in the soil zone. Such information is important to evaluate responses of terrestrial ecosystems to global climate change.