T23 Paleosols and Phanerozoic Climate: Geochemistry to Trace Fossils

Sunday, 27 October 2002
8:00 AM - 12:00 PM

GOETHITE, CALCITE, AND ORGANIC MATTER FROM PERMIAN SOILS: CARBON ISOTOPES AND CO2 CONCENTRATIONS

Paper number: T23-9
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

Pedogenic goethites in each of two Early Permian soils appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The d13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are –13.5‰ for the Lower Leonardian (~283 Ma BP) soil (MCGoeth) and –13.9‰ for the Upper Leonardian (~270 Ma BP) soil (SAP). These goethites contain the most 13C–rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. Scatter in the measured d13C values of organic matter permits (within one standard deviation) calculated Early Permian atmospheric PCO2 values that range from 1xPAL to about 4xPAL (PAL=present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments.

The average d13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth range from –6.5‰ to –4.4‰, with a mean d13C value for all profiles of –5.4‰. Thus, the value of D13C between the pedogenic calcite dataset and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric PCO2 and organic matter d13C values were the same for both soil types. Furthermore, the atmospheric PCO2 calculated for the Early Permian from the carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1xPAL, with a calculated probable maximum of ~4xPAL. This is the first stratigraphically constrained, intrabasinal study which demonstrates that ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite are the same.