2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 9
Presentation Time: 10:35 AM

Exploring the Sensitivity of Paleo-PCO2 Models Based on δ13C of Pedogenic Gibbsite to Changing Soil Variables with Depth Using Numerical Methods

AUSTIN, Jason C.1, SCHROEDER, Paul2 and DOWD, John2, (1)Geology, University of Georgia, 45 Lamar Lane, Hoschton, GA 30548, (2)Department of Geology, University of Georgia, 210 Field Street, Athens, GA 30602, jaycaustin@live.com

Soil respiration on geologic time scales and resultant depth-dependent δ13C soil gas signatures preserved in the rock record have been predominantly based on models using analytical solutions to the one dimensional Fickian diffusion equation. Numerical models and Monte Carlo simulations allow for sensitivity testing of δ13C affecting variables for which analytical solutions are not possible. A computer program has been developed, based on the finite difference method and is used to explore variability in depth dependent factors such as bulk density, rates of CO2 production, and boundary conditions. Comparison of the analytical and numerical model solutions with literature data for soil CO2 compositions reveals discordance between the solution soil property parameters and measured physical properties of the soil. Variables used to calculate the δ13C of soil CO2 with the analytical solution indicate that the δ13C of the soil biomass and the rate of CO2 production are the most sensitive parameters. The analytical solution indicates that the majority of the variability occurring within the first 20 cm of depth is related to soil CO2 production rate and bulk density. Specifically, the shape of the δ13C profile and depth of maximum inflection is controlled by the rate of CO2 diffusion, i.e. the CO2 production rate or bulk density. The δ13C of the soil biomass and the PCO2 of the atmosphere are responsible for variability at depths greater than 20 cm. The numerical model solutions allow for differences in bulk density, δ13C of biomass, and soil CO2 production rate with depth. This model was also used to perform a Monte Carlo analysis of the interdependence of model solutions on these variables.

Ultimately, these models will have application for paleobarometry studies, which use stable carbon isotopes occluded in pedogenic gibbsite and goethite and assumed climatic conditions at the time pedogenesis.