STABLE ISOTOPE COMPOSITIONS OF AUTHIGENIC PHYLLOSILICATES, IRON-OXIDES, AND CARBONATE: INSIGHTS TO LATE PALEOZOIC ATMOSPHERIC CIRCULATION

Permo-Pennsylvanian atmospheric circulation remains unresolved. Given that hydrogen and oxygen isotopic compositions of meteoric water generally yield a more depleted value with increasing distance from a moisture source, the isotopic composition of Late Paleozoic authigenic minerals should provide insight to atmospheric circulation patterns over the Pangean supercontinent. We have analyzed d¹⁸O and dD compositions of paleopedogenic phyllosilicates, Fe-oxides and carbonates from Pennsylvanian (Virgilian) and Early Permian (Wolfcampian and Leonardian) strata widely distributed across the western United States (western low-latitude Pangea) that indicate monsoonal circulation was active by Early Permian time. Furthermore, this data set provides evidence of a western source of precipitation at equatorial paleo-latitudes as well a progressive trend toward drier conditions though Early Permian time. dD and d¹⁸O compositions of these minerals plot in an array parallel with the meteoric water line, implying a Permo-Pennsylvanian soil water signature. The most enriched isotopic compositions are found near equatorial paleo-latitudes, with a robust pattern of more depleted values with increasing distance from the paleo-equator. In addition, there is a general stratigraphic trend toward more enriched isotopic values at all sites from Pennsylvanian through Early Permian time.

The paleo-equatorial samples exhibit isotopic values similar to modern tropical coastal soils, suggesting a proximal source of precipitation, rather than precipitation associated with megamonsoonal atmospheric circulation originating over Tethys. Furthermore, a progressive northward isotopic depletion is consistent with a greater contribution of distally derived moisture sources that may be related to monsoonal precipitation originating over Tethys. The stratigraphic shift toward enriched isotopic values over the entire region through the Early Permian indicates a shift toward more arid climate in conjunction with a greater contribution of precipitation from proximal western moisture sources, such as the Midland and Delaware Basins or Panthalassa.