SOIL AND PALEOSOL CALCITE AND ORGANIC MATTER δ13C VALUES AS PROXIES OF PRODUCTIVITY AND PRECIPITATION

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=18) 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 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 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; It is especially sensitive to variations in rainfall. 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.

Middle and Upper Permian paleosol profiles among China, France, Italy, Morocco, Niger, South Africa, Tanzania, U.S.A, and Zambia record a large range of Δ¹³C_cc-om values; the largest of which occur in Niger suggesting this region was among the least productive and driest paleolandscapes of the Permian.