Early Permian Paleosol Morphologies and Paleo-Atmospheric pCO2 Estimates, North-Central Texas

Abstract

Upper Paleozoic (~300 Ma) rocks in the Midland Basin of Texas record a transition from icehouse (glaciated) to greenhouse (non-glaciated) conditions in low-latitude, western Pangea. This work assesses paleoclimate and paleoatmospheric pCO₂ evolution across this global climate transition. Thirty upper Wolfcampian (Nacona Formation) to upper Leonardian (Clear Fork Group) paleosol profiles were measured and described in detail. These paleosols are organized into 10 discrete morphotypes that indicate a long-term climatic drying trend through the stratigraphic section. Samples in which δ13C values (δ13C difference between pedogenic calcite and associated organic matter) were >15‰ are thought to reflect formation under an open pedogenic system with two- component δ13C mixing and are appropriate for paleoatmospheric pCO₂ estimates. The δ13C values of 24 unaltered pedogenic calcite samples and associated, occluded organic matter from carbonate nodules and calcareous rhizoliths yield atmospheric pCO₂ estimates ranging from 600 to 3400 ppm. These atmospheric pCO₂ estimates are similar to those reported in previous studies of the Early Permian, which suggests that δ13C values of paleosol calcite and co-existing organic matter provide a robust record of terrestrial carbon cycling in open systems. In addition, pCO₂ excursions toward higher values correspond to sea level rises recorded in the regional lithostratigraphy. However, pCO₂ estimates do not increase significantly during the Permian icehouse-greenhouse transition, as previously described in these strata. Although the data presented here indicate important climate transitions occurred in conjunction with, or in response to, large changes in atmospheric pCO₂, lithostratigraphic changes previously ascribed to the icehouse-greenhouse transition in western equatorial Pangea do not coincide with substantial changes in atmospheric pCO₂.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015