The δ18O values of pedogenic calcites, phyllosilicates, and iron (oxyhydr)oxides from fossil soils throughout the southwestern United States show systematic paleolatitudinal and temporal trends that indicate a significant change in soil moisture conditions and atmospheric circulation patterns over southwestern Euramerica throughout latest Pennsylvanian and Early Permian time. A progressive depletion of as much as 6‰ in mineral 18O values with increasing distance northward (to 10°) of the paleoequator indicates a weakening or disruption of zonal easterly flow over the study area by latest Pennsylvanian time. Furthermore, elevated δ18O values suggest a proximal source of paleoprecipitation over the study area, perhaps due to initiation of reversed equatorial flow over tropical southwestern Euramerica. Coupling of the geographic and temporal trends defined by pedogenic mineral δ18O values with those defined by climatically sensitive paleosol pedotypes in the study area, along with overall elevated mineral δ18O values, provides some of the strongest evidence that Northern Hemisphere monsoonal circulation was well established over Pangea by Early Permian time.

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