WESTERLY WINDS OVER THE SOUTHERN PUNA PLATEAU DURING DEPOSITION OF UPPER PLEISTOCENE LOESSIC SEDIMENTS IN THE SUBTROPICAL ANDEAN FOOTHILLS, SOUTH AMERICA

The provenance of loessic sediments can be used to reconstruct ancient wind patterns and provide a useful tool for understanding spatial and temporal variations in surface aridity and dust production. The Tafí del Valle depression at ~27° S in the Andean foothills of western Argentina contains a record of late Pleistocene dust deposition in the subtropics of South America. Large-n U-Pb geochronology dates were measured from detrital zircon aliquots collected from upper Pleistocene loess-paleosol deposits in Tafí del Valle. The major zircon age modes observed are consistent throughout the samples and fall into the major age modes of 0–23 Ma, 450–650 Ma, and 950–1200 Ma. When compared to regional data, the age spectra from the Tafí del Valle samples are most similar to the southern Puna Plateau supporting derivation of these sediments largely from the west counter to the hypotheses that the subtropical pre-Andean loess was derived from the low elevation plains to the east or extra-Andean Patagonia. The detrital zircon U-Pb ages indicate minimal direct volcanogenic input which contradicts previous interpretations of the precordillera Andean loess deposits. Although sampled from interbedded loess and paleosol across a 1.1 Ma interval, the lack of detrital zircon provenance variability implies consistent sourcing of the silt- and larger size sediment fraction. Additionally, these data show no significant change across the Mid-Pleistocene Transition or between drier or more humid intervals. Mapping of n= 866 linear bedrock wind erosion features (e.g., yardangs) on the Puna Plateau yield a mean orientation of 125.7° (1 s.d.= 12.4°). Together, these data are consistent with a sustained westerly dominated wind system over the southern Puna Plateau during periods of high dust accumulation in the Andean foothills. This would equate to at least an ~8° equatorward difference from the mean present-day position of the westerlies.