Abstract Fourteen soil profiles from California were collected in order to measure the δ 13 C of coexisting soil calcite and organic matter. Thirteen of the profiles contained a measurable amount of calcite ranging from 0.04 to 54.6 wt %. Soil calcite δ 13 C PDB (δ 13 C value vs. the calcite standard Peedee Belemnite) values range from −14.4 to 1.3‰, whereas organic matter δ 13 C PDB values range from −24.0 to −27.7‰. The hydrology of these profiles is divided into two broad groups: (1) soils characterized by gravity-driven, piston-type vertical flow through the profile and (2) soils affected by groundwater within the profile at depths where calcite is present. The difference between soil calcite and organic matter δ 13 C PDB values, Δ 13 C cc _ om , is smaller in profiles affected by groundwater saturation as well as most Vertisols and may be a product of waterlogging. The larger Δ 13 C cc-0 m values in soils with gravity-driven flow are consistent with open-system mixing of tropospheric CO2 and CO2 derived from in situ oxidation of soil organic matter with mean soil PCO2 values potentially in excess of ~20,000 ppmV at the time of calcite crystallization. There is a correlation between estimates of soil PCO2 and a value termed “E ppT.U ” (kJm 2 /yr) among the soil profiles characterized by gravity-driven flow. E ppT.U is the energy flux through the soil during periods of soil moisture utilization, and it is the product of water mass and temperature in the profile during the growing season. Thus, soils with high water-holding capacity/storage and/or low/high growing season temperature may form soil calcite in the presence of high soil PCO 2 , and vice versa. The results of this research have important implications for reconstructions of paleoclimate from stable carbon isotopes of calcareous paleosol profiles.