Carbon stable isotope composition of modern calcareous soil profiles in California: Implications for CO₂ reconstructions from calcareous paleosols

Fourteen soil profiles from California were collected in order to measure the δ¹³C 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 δ¹³CPDB (δ¹³C value vs. the calcite standard Peedee Belemnite) values range from -14.4 to 1.3%, whereas organic matter δ¹³CPDB 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 δ¹³CPDB values, Δ¹³C_cc-om, is smaller in profiles affected by groundwater saturation as well as most Vertisols and may be a product of waterlogging. The larger Δ¹³C _cc-om 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 PCO ₂ 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 PCO₂, and vice versa. The results of this research have important implications for reconstructions of paleoclimate from stable carbon isotopes of calcareous paleosol profiles.