2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 251-4
Presentation Time: 1:35 PM

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


TABOR, Neil J.1, MYERS, Timothy S.1, SIDOR, Christian A.2 and SMITH, Roger M.H.3, (1)Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275-0395, (2)Biology, University of Washington, Seattle, WA 98195, (3)Karoo Palaeontology, Iziko South African Museum, Cape Town, 8000, South Africa

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, Δ13Ccc-om, is smaller for profiles affected by groundwater saturation, as well as for most Vertisols. Smaller Δ13Ccc-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 Δ13Ccc-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 Eppt-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 Δ13Ccc-om values; the largest of which occur in Niger suggesting this region was among the least productive and driest paleolandscapes of the Permian.