RECONSTRUCTION OF THE PENNSYLVANIAN-PERMIAN CLIMATE TRANSITION IN THE ABO AND BURSUM FORMATIONS IN NEW MEXICO AND THE IMPLICATION FOR PCO2

Recent Pennsylvanian-Permian paleoclimate reconstructions have focused on the rise of atmospheric pCO₂ and concomitant continental glacial variability toward an eventual Permian-Triassic greenhouse world. Less research has focused on reconstructing precipitation and understanding what, if any, role such variability may have in the resulting proxy estimates for pCO₂. To that end, we present field-based descriptions of paleosols from the Permo-Pennsylvanian Bursum and Abo formations. The Permo-Pennsylvanian Bursum is dominated by Calcisols whereas the Permian Abo Formation is dominated by clay-rich calcic Vertisol profiles. This stratigraphic trend in paleosol morphology is interpreted to represent a paleoenvironmental shift from stable dry conditions during deposition of the Bursum Formation to highly seasonal and dry conditions in the Abo Formation.

A robust stable carbon isotope dataset taken from coexisting carbonate and occluded organic material sampled from these paleosols ranges from -4.5 to -7.8‰ to -23.6 to 26.0‰, respectively, corresponding to δ¹³C_cc-om values between 15.8 to 20.8 ‰ (n=35). δ¹³C_cc-om values in modern soils exhibit variability in a way that shows a strong parametric correlation with mean annual rainfall amongst a range of climates that include calcite-bearing soils such as Permo-Carboniferous paleosol profiles described in this work. Therefore, Δ¹³C values in the Bursum Formation correspond to a broad range of MAP values from 16 to 31 cm, whereas MAP values for the Permian Abo Formation are tightly clustered between 22 and 25 cm. Significantly these stratigraphic patterns for rainfall estimates from Δ¹³C values are consistent with field observations of increased seasonality of precipitation. Within the Bursum Formation there is a clear trend towards less rainfall up-section, which may be coincident with the Permo-Carboniferous transition, and similar in nature to climate shifts reported within the boundary from other tropical Pangean sites. The changing MAP values reported here have potentially important implications for selecting an appropriate soil-respired CO₂ concentration (Sz) that is necessary for calculated atmospheric pCO₂ estimates from these same sorts of data. Given that moisture availability drives soil-CO₂ production, results presented here suggest that perceived changes in atmospheric carbon dioxide estimates may be conflated with changes in Sz associated with variable soil moisture availability through time that may be independent of global climate change.