Can Orbital Forcing Help Explain Reconstructed Monsoon Winds for the Early Jurassic?

Wind-blown sediments provide direct evidence of atmospheric circulation, which is otherwise very difficult to obtain from the geologic record. Eolian sandstones cover large portions of the Colorado Plateau of the southwestern US. These Early Permian through Early Jurassic sandstones reflect dunes that migrated under a distinctive wind regime that apparently varied little in direction through their 100 Myr span. To the north, the dominant winds came from the NE, curving to NW over the southern portion of the outcrops. Basic concepts of climate would suggest that the NE winds are consistent with low latitude trade winds in the northern hemisphere, and the NW winds consistent with cross-equatorial flow induced by a strong summer monsoonal circulation in the southern hemisphere of Pangea. The dunes in the south also reflect a seasonal wind reversal; during much of the year, slightly weaker winds were from the SE. Southernmost outcrops of the sandstones contain abundant trace fossils and slump structures, suggesting they were deposited at the relatively wet, southern edge of an extensive desert. Conventional wisdom, largely based on paleomagnetic evidence, would place the Colorado Plateau along or just north of the equator during the Early Permian, then moving north through the Triassic to lie near 20°N by the Early Jurassic. Yet the constancy of the wind regime indicated by the dip directions of the cross-strata suggests that the dunes and the Plateau stayed within the same climatic zone, despite the hypothesized large northward movement of Pangea. The wind regimes recorded by the dunes are consistent with paleogeographic reconstructions of the late Permian, with the Colorado Plateau straddling the equator, but are very much at odds with a northward movement to about 20° N by the early Jurassic. Previous modeling work, including our own, has produced nearly symmetric monsoon circulations (i.e., monsoons of nearly equal strength in each hemisphere) in part due to the use of present-day orbital parameters. These simulations, therefore, represent an orbital state that does not represent the extremal states that may have affected dune migration. To help resolve the discrepancy between the paleomag and paleowind Pangean positions, climate simulations with the NCAR CCSM3 were made using end members for orbital parameters.