Verification of regional-to-global scale environmental factors in paleosol stable carbon isotope ratios through the lower Permian succession of north-central Texas, U.S.A.

Abstract Upper Paleozoic (~300 Ma) rocks in the Midland Basin of Texas (USA) record a transition from icehouse (glaciated) to greenhouse (non-glaciated) conditions in low latitudes, western Pangea. This work revisits the stratigraphic interval that records stable carbon isotopic compositions of paleosol materials from previous studies and uses newly collected paleosol samples to test the reproducibility of paleosol stable carbon isotopic values, and assesses paleoclimate and paleoatmospheric pCO2 changes across the global climate transition. Thirty upper Wolfcampian (Nacona Formation) to upper Leonardian (Clear Fork Group) paleosol profiles were measured and described in detail. Detailed petrographic analysis of paleosol carbonate nodules was used to identify the best representatives of pristine pedogenic microcrystalline calcite cements and separate them from later diagenetic or altered calcite cements. The best candidates for pedogenic micritic calcite cement were drilled, and organic-rich fractions of acid-treated residues co-existing within carbonate nodules were collected. The δ13C values of 23 paired, pristine pedogenic micritic calcite samples (δ13Ccc) and associated, occluded organic matter from carbonate nodules and calcareous rhizoliths (δ13Com) range from −6.5‰ to −3.3‰ and −25.3‰ to −20.5‰, respectively. The stratigraphic trends in paleopedogenic micritic calcite δ13Ccc values are essentially no different from results published in previous studies, which supports the reproducibility of paleosol carbonate as a reliable proxy for the carbon isotopic composition that reflects the regional-to-global paleoclimatic and paleoenvironmental trends. However, δ13Com values measured here are consistently more negative than previously reported for fossilized plant compressions preserved within strata associated with these paleosol profiles ( Montanez et al., 2007 ). We suggest that the coexisting organic matter from acid-treated residues in paleosol carbonate is a better material to assess original soil δ13Com values that was contributing toward soil CO2 than the fossilized components of above-ground, photosynthesizing vascular plant parts as was utilized in previous studies. The isotopic differences between the paleosol micritic calcite δ13Ccc values and associated organic-rich-residue δ13Com values were used in conjunction with a two-component CO2 mixing model to yield atmospheric pCO2 estimates. The estimates are different and with opposite excursions, ranging from 1.7 to 2.7 times pre-industrial atmospheric levels (PIAL) and from 2.3 to 4.2 times of PIAL, when applying the relationships of modern soil pCO2 (Sz)-mean annual precipitation (MAP) and Sz-pedotype, respectively, to assess the Sz of paleosols. The different approaches result in different assumed Sz of paleosols at the time of calcite crystallization and lead to lower estimates of atmospheric pCO2 than previously reported. These results suggest that the icehouse-to-greenhouse climate transition was associated with much lower pCO2 thresholds than previously thought.