『Abstract
　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 CO2 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』

1. Introduction
2. Study areas
3. Methods
　3.1. Sample collection
　3.2. Analyses of soils and soil gases
　3.3. Analyses of water samples
4. Results
　4.1. Soil carbon content and isotopic composition
　4.2. Soil gas CO2 concentrations and δCCO2
　4.3. Soil water/groundwater chemistry and δCDIC values
　4.4. Time scales for C isotope equilibrium: sampling protocols
5. Discussion
　5.1. Soil CO2 fluxes and δ¹³CCO2 variations at the Huron watershed sites
　5.2. Speciation of soil water DIC
　5.3. Is carbonate dissolution in soil profiles always an open system with respect to CO2?
　5.4. Kinetic vs. equilibrium controls on δ¹³CDIC in the Huron watershed
　5.5. δ¹³CDIC in agricultural ecosystems of the Kalamazoo watershed
6. Conclusions and implications
Acknowledgements
References