Glacial–interglacial shifts in atmospheric circulation of western tropical Pangaea

Abstract

Combined sedimentology, whole-rock geochemistry and U–Pb geochronology of Upper Paleozoic loessite–paleosol couplets of the early Permian Abo Formation (Abo–Tubb interval; northeastern New Mexico) record glacial–interglacial shifts in atmospheric circulation within tropical western Pangaea. Trends in whole-rock geochemistry and magnetic susceptibility in loessite compared to paleosols are consistent with a relative increase in humidity and decrease in eolian flux that accompanied the transition from glacial (peak loess influx) to interglacial (reduced loess influx) states. Distinct differences in age spectra of detrital zircons between loessites and paleosols suggest a significant shift in provenance and hence wind directions that accompanied the shift in relative humidity. Zircons within the loessites exhibit strong age modes between 1800 and 1600 Ma (Yavapai–Mazatzal terrane) and a significant fraction of very young zircons (< 300 Ma) both of which reflect westerly sources. Zircons within the paleosols, however, contain smaller and fewer zircons with ages of 1800–1600 Ma, and no very young zircons, but a large number of Neoproterozoic grains (740–570 Ma) and Paleozoic grains (500–300 Ma) reflecting southeast or easterly sources. These differences persist in the two loessite and two paleosol units analyzed. Loessite–paleosol couplets 1–5 m thick make up the majority of the upper Abo–Tubb interval. Thus, during successive loess deposition events, strong seasonal westerly winds were dominant whereas weaker (south) easterly winds predominated during periods of pedogenesis. We interpret these recurring shifts in atmospheric circulation to reflect glacial–interglacial changes in the intensity of northern-hemisphere-summer monsoonal circulation of western Pangaea between times of loess deposition (glacial), and pedogenesis (interglacial).