Stable Carbon Isotope Composition of Calcite and Organic Matter From Californian Soils: Implications for CO2 Reconstructions.

Pedogenic calcite precipitates in isotopic equilibrium with a mixture of soil-respired CO₂ derived from in situ oxidation of soil organic matter and CO₂ contributed from the overlying troposphere. Yet, the relative contribution of these CO₂ sources to formation of pedogenic calcite is unknown in most soils. We assess the relative contribution of both soil-respired CO₂ and tropospheric CO₂ through stable carbon isotope (δ¹³C) based mass-balance calculations via measurement of δ¹³C values of pedogenic calcite and coexisting soil organic matter in soils distributed across California, U.S.A. Soil profiles analyzed to date (n=13) contain calcite with δ¹³C values ranging from -14.4 to 1.3‰ (n=47), whereas organic matter δ¹³C values range from -24.0 to -27.7‰ (n=23).

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 δ¹³C values, Δ¹³C_cc-om, is smaller for profiles affected by groundwater saturation, as well as for most Vertisols. Smaller Δ¹³C_cc-om values are interpreted as the product of water logging and limited diffusion of CO₂ out of, and into, the soil, resulting in relatively higher concentrations of soil-respired CO₂.

Larger Δ¹³C_cc-om values in soils with gravity-driven flow are consistent with open-system mixing of tropospheric CO₂ and soil-respired CO₂,  with soil PCO₂ values potentially ranging from ~400 to ~20,000 ppmV at the time of calcite crystallization. There is a positive correlation between soil PCO₂ estimates and a value named E_ppt-U (kJm^-2yr^-1) that represents energy flux through the soil during periods of soil moisture utilization and is the product of water mass and temperature in the profile during the growing season. Thus, soils characterized by high water storage and high growing season temperatures may form pedogenic calcite under conditions of high soil CO₂ concentration resulting from high rates of biological productivity and vice versa.