OPTICAL DATING OF LATE QUATERNARY EOLIAN DEPOSITIONAL RECORDS FROM WESTERN ARGENTINA

Stabilized dune fields are common in western Argentina and reflect potentially drought variability in the late Quaternary. Geochronologic studies focused on the San Luis dune field on the western Pampas, with a mean annual precipitation of ~ 700 mm. Compound and digitate parabolic dunes and sand sheet deposits are common. Exposures reveal a complex eolian history with multiple buried soils and intercalated eolian sand. Optical dating focused on the 100-150 micron quartz grains using single aliquot regeneration (SAR) protocols. These eolian sands are immature mineralogically with quartz percentages between 40 and 50%. Unlike other Andean-sourced quartz Pampean sediments show a clear dominance of fast luminescence component with medium and slow components near background levels. This quartz exhibits relatively small sensitivity changes (<10%) through the SAR protocols. Repeat beta dose (~1-10 Gy) to the quartz yield near identical luminescence response, after sensitivity correction. Further, a known dose given in the laboratory was recovered by the SAR protocols. Geochronometric sensitivity of Pampean quartz reflects full solar resetting, the dominance of fast luminescence components and high light output, possibly reflecting multiple dose and solar reset cycles for this quartz. Optical ages on quartz sand indicate periods of eolian deposition ca. during the Last Glacial Maximum at ca. 25 and 18 ka, early Holocene 10-9 ka and multiple episodes in the late Holocene, ca. 5.0-3.5 ka and at 1.6 ka. Dune reactivation in semiarid western Argentina during the Holocene is indicative of a “megadrought” and is associated with a weakened South American Monsoon, surpassing climate variability since the 19^th century.