STABLE ISOTOPE PALEOCLIMATE RECONSTRUCTION FROM PERMO-PENNSYLVANIAN PALEOSOLS OF THE SOUTHWESTERN UNITED STATES

We present d¹⁸O and dD compositions of paleopedogenic phyllosilicates, Fe-oxides, and carbonates from Permo-Pennsylvanian strata (Virgilian, Wolfcampian, and Leonardian) across the southwestern United States to elucidate the status of paleoclimate over this region and make clear how climate changed during this period. These strata were chosen for several reasons: (1) the Permo-Pennsylvanian is a time characterized by possible reorganization of atmospheric circulation patterns into the Pangean Megamonsoon; resulting in continental aridity. (2) General circulation and energy balance models indicate this reorganization was most pronounced in the western equatorial region of Pangea (e.g. U.S. southwest). An isotopic study in this region should provide evidence for this environmental change. (3) This region represents the most complete Permo-Pennsylvanian terrestrial record.

Significant findings emerge from this data set. dD and d¹⁸O compositions of paleopedogenic minerals analyzed thus far show good correlation with the meteoric water line. Both dD and d¹⁸O compositions of samples from equatorial paleolatitudes are consistent with modern soil signatures in equilibrium with coastal tropical precipitation. The dD and d¹⁸O of contemporaneous samples exhibit a robust pattern of progressively more depleted isotopic composition with increasing distance from the paleoequator. Lastly, there is a consistent temporal pattern over the region toward generally more enriched isotopic values from Pennsylvanian through Early Permian time.

Correlation of Permo-Pennsylvanian samples with the meteoric water line probably reflects Permo-Pennsylvanian soil water compositions, rather than detrital or diagenetic signatures. The paleo-equatorial samples exhibit isotopic values similar to modern tropical coastal soils, suggesting this region was not effected by megamonsoonal atmospheric circulation originating over the Tethys. Rather, it is likely that climate was effected by local circulation patterns with moisture derived from proximal inland seas to the west of the region. Furthermore, a northward isotopic enrichment is consistent with a trend toward relatively more arid climates. The temporal trend toward enriched isotopic values over the entire region indicates a long-term shift toward progressively more arid climate through the Early Permian.