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.
In shallow soil zones where only silicate weathering was occurring, respiratory CO2 was the major source of
soil water DIC with little addition from the atmospheric CO2. Isotopic equilibration between δ13CDIC and
δ13CCO2 occurred in an open system with respect to soil CO2. In the deeper soil horizons carbonate dissolution
dominated soil water chemistry and saturation with respect to calcite and dolomite was attained rapidly.
Mass balance calculation showed that large amounts of soil CO2 were consumed by carbonate dissolution,
such that the deeper soil zone may not have been an open system with respect to CO2. Constant δ13CDIC
values (∼−11‰) were observed in these deep soil waters and also in shallow groundwaters of the Huron
watershed. Thus, isotopic equilibrium might not be reached between DIC and CO2, possibly due to a rapid
kinetics of carbonate dissolution and limited gas–water exchange in the soils. If so, DIC was equally
contributed by carbonate minerals (δ13CCaCO3=0‰) in reaction with soil CO2 (δ13CCO2=−22‰). Soils
beneath an agricultural site with a wheat/corn/soybean rotation (the Kalamazoo watershed) displayed a
wide range in δ13CCO2 values (−22 to −12‰), and the δ13CDIC of deeper soil waters in contact with carbonate
minerals was controlled by seasonal variations of δ13CCO2 as well as by strong acids produced by nitrification
and to a lesser degree by pyrite oxidation, both of which could react to dissolve carbonate minerals, in
addition to carbonic acid dissolution. Keywords: δ13C ; Soil water ; Chemical weathering ; pCO2 ; Global C-cycle
