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

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.