EARLY PERMIAN ATMOSPHERIC pCO2 ESTIMATES AND REPRODUCIBILITY OF PALEOSOLS AS ISOTOPIC PROXIES FOR PALEOCLIMATE, EASTERN SHELF OF THE MIDLAND BASIN, NORTH-CENTRAL TEXAS

Late Paleozoic sedimentary rocks of the Eastern Shelf of the Midland Basin, Texas record the last icehouse to greenhouse transition in low latitude, western Pangea. Atmospheric pCO₂ changes are suspected to have played an important role in this climate transition. Assessing paleoatmospheric pCO₂ during this global climate transition could provide geochemistry evidence to correlate the equatorial terrestrial record to the transgression-to-high stand marine record associated with the far-field deglaciation in the Gondwana. Additionally, the reproducibility of the paleosol carbonate paleobarometer has not been validated in contemporaneous stratigraphic successions, despite that paleosol carbonate nodules and the co-existing organic matter are the main source of pre-Cenozoic atmospheric pCO2 estimates. In this study, 30 paleosol profiles from the upper Wolfcampian and Leonardian series were measured and described. The pristine pedogenic micritic calcite cement and the occluded organic-rich residues were collected to determine their d¹³C values. The d¹³C values of 23 pristine pedogenic micritic samples and those of the associated, occluded organic matter from carbonate nodules and rhizoliths range from -6.5‰ to -3.3‰ and -25.3‰ to -20.5‰, respectively. Early Permian atmospheric pCO₂ estimates range from approximately equivalent to preindustrial atmospheric levels (PAL) to slightly more than 3×PAL. While the d¹³C values of pedogenic micritic calcite are similar to the results published previously, the d¹³C values of co-existing organic matter measured here are consistently more negative than those previously reported based on plant fragments within the adjacent strata. This, in conjunction with different assumed concentrations of soil CO₂ at the time of calcite crystallization results in substantially lower estimates of atmospheric pCO₂ than previously reported in Montanez and others (2007). These results suggest that the icehouse-to-greenhouse climate transition was associated with much lower pCO₂ thresholds than previously thought. Nevertheless, the consistency between the two separate datasets suggests carbon isotope composition of pedogenic calcite and co-existing organic matter records large-scale processes which transcend local processes and therefore offer a reliable source of regional to global geochemical trends.