EOLIAN DEPOSITIONAL EVENTS DURING THE PAST 55 KA AT A SUBTROPICAL INLAND PALEO-DUNE FIELD AND INFERRED CLIMATE VARIABILITY, WESTERN PAMPAS, ARGENTINA

Presently stabilized and active dune fields occur from the Argentinian Pampas to the Andean piedmont. These presently stabilized eolian landscapes grade texturally from loess and loess-like deposits in the east, to sand in the central, western and southern areas. This large eolian system, referred as the “Pampean Sand Sea” has a diversity of landforms and deposits spanning the late Quaternary. These eolian systems, including loess deposition, were active during the last and prior glacial cycles, and recent studies have documented that many eolian systems in western Argentina were active during the Holocene, concomitant with loess deposition in the eastern Pampas.

Eolian depositional events on the eastern Pampas were previously identified which span the past 150 ka, though the chronology is nascent. The oldest evidence of eolian activity is the presence of an eolian sand deposit beneath the lowermost loess unit, with an inferred age of ~60 ka. However, OSL dating of the lowermost loess at the same site yielded a minimum limiting age of the Pampean Sand Sea of ~145.6 ka and overlying loessic deposits gave OSL ages at ~63 to 69 ka. This contribution presents new geomorphic, stratigraphic, sedimentological, and chronologic data for eighteen 2+m–thick eolian sequences for the San Luis paleo–dune field, western Pampas. This study, combined with prior observations yield a database for identifying five eolian depositional events interposed with paleosols and/or bounded by erosive surfaces to better constrain the depositional history of the Pampean Sand Sea. Age control is from over 50 OSL ages of small aliquots of quartz grains from eolian stratigraphic units. These inferred eolian events are ca. 200–50 yr, 12–1 ka, 21–14 ka, 30–25 ka and 39–32 ka; representing periods of eolian sedimentation and separated by intervals of pedogenesis and/or hiatuses in deposition. Bounding OSL ages place the periods of pedogenesis at 12 to 17 ka, 24 to 26, and 28 to 32 ka, which broadly overlap with high stands of central Andean pluvial lakes and coincident with Northern Hemisphere Heinrich events. This study indicates that the added precipitation associated with expansion of the Southern Hemisphere monsoon with Heinrich events penetrated to at least 40° S, with some moisture derived from the strengthened low level jet along the Andean Slope.