80 KA RECORD OF EOLIAN DEPOSITION OF THE PAMPEAN SAND SEA, WESTERN ARGENTINA AND POTENTIAL LINKAGES BETWEEN NORTHERN AND SOUTHERN HEMISPHERE CLIMATE VARIABILITY

The Pampean Sand Sea, which occurs from the Argentinian Pampas to the eastern Andean piedmont, hosts presently stabilized dune fields spanning the late Quaternary. This study integrates previous results and presents new geomorphic, stratigraphic, sedimentological, and chronologic data for twenty, 2 to 10 m thick eolian successions for the San Luis paleo-dune field, western Pampas, to better constrain the depositional history and associated paleoenvironments. At least six eolian depositional phases are identified spanning the past 80 ka, many with interposed paleosols and/or bounded by erosive surfaces. Age control for eolian stratigraphic units was derived from 70 OSL ages of small aliquots of quartz grains by the Single Aliquot Regeneration protocols. The inferred timing of eolian phases, from youngest to oldest, are at ca. 70 ± 10 yr, 190 ± 20 yr, 12 to 1 ka, 22 to 17 ka, 29 to 24 ka, and 40 to 32 ka. There is also limited stratigraphic and chronologic data for possibly two older eolian depositional events at ca. 50 ka and 75 ka ago. A maximum span for periods of pedogenesis at ca. 12 to 17 ka, 22 to 24 ka, and 29 to 32 ka was provided by bounding OSL ages, which broadly overlap with high stands of pluvial lakes and glacier advances in the central Andes. We infer that the added precipitation may reflect expansion of the Southern Hemisphere austral monsoon, associated with cooling of the Equatorial Atlantic from Northern Hemisphere Heinrich events, sourced from continental ice sheets and thus leading to episodes of significantly wetter conditions (>350 mm, mean annual precipitation) to at least to 35° S. Most of the Holocene (12 ka to 0.8 ka) was characterized by sand sheet deposition under drier than present conditions (100-450 mm MAP), associated with Monte-type vegetation (shrub steppe). The latest two eolian depositional phases, occurred at ca. 190 and 70 yr ago (~ AD 1790 and 1930), during the historic period with European settlement and are related to anthropogenic landscape disturbance, such as cereal agriculture and cattle grazing, though the youngest phase was concomitant with 1930s drought that struck Argentina. Wet conditions dominated since ca. AD 1970 with new lakes and rivers forming across this eolian terrain; an incongruous environmental response in reference to drier conditions for most of the Holocene.