Climatic Inferences from Paleozoic Loessite

Sedimentologic and geochemical data collected from a number of loessite (lithified loess) deposits across low-latitude, western Pangaea (western U.S.) during the Permo-Pennsylvanian provide paleoclimatic information on both long and short time spans. Deposits of these upper Paleozoic loessites are commonly intercalated with paleosols on a 1-10 m scale, similar to Quaternary loess deposits, but these ancient loess deposits are commonly much thicker (100 to > 700 m). The accumulation of these loessites spans the timing of maximum continental glaciation associated with the late Paleozoic ice-house, and also the onset of monsoonal circulation that initiated over western Pangaea during the late Paleozoic.

Magnetic susceptibility measurements combined with geochemically based weathering indices from numerous loessite-paleosol profiles in several localities suggest these deposits record temporal, high-frequency changes in relative humidity over a large region of the Pangaean tropics. Furthermore, differences in provenance and quartz grain-size between the loessite and pedogenically modified loessite suggest that the intensity and direction of low-latitude atmospheric circulation patterns varied either in response to, or as a cause of, this variation in tropical aridity.

Our data suggest that intervals of loess deposition reflect drier, windier times likely associated with glacial maxima in which (summer) monsoonal circulation predominated in the western Pangaean tropics with attendant westerly winds. Periods in which aggradational paleosols formed are inferred to reflect wetter, less windy times likely associated with glacial minima (but not necessarily interglacials) during which zonal circulation in low-latitudes predominated over monsoonal circulation.

Sedimentology and geochemistry are common tools used for inferring regional atmospheric circulation patterns from Quaternary loess. We suggest these same analyses are possible in very ancient loess and provide the same level of inference for paleo-atmospheric circulation.