Last Maximum Glacial records in the Rio Blanco basin (32º S), Argentinean Central Andes.

MOREIRAS, S..M.

Dublin

Congreso; INQUA; 2019

Throughout the Andes, the glacier history reveals that glaciers had a maximum extension during the Last Glacial Maximum (LGM) and the Late Glacial, with rather consistent ages of ca. 31?18 ka BP and 18?12 ka BP from Alaska to Southern Chile (Clapperton, 2000). Diachronic intervals of glacier advances have been determined for the humid tropical Andes, the Central Andes of Argentina and Chile, and the Patagonian Andes (Zech et al., 2008). However, a regional glacial chronology is not yet fully established for the Central Andes. Late glacial advances were distinguished both in the subtropical Andes (Espizúa, 1999; Espizúa, 2004; Zech et al., 2005; Zech et al., 2007) and Patagonia (Rabassa et al., 2011), where drifts identified between ~36º-39ºS have been interpreted as LGM in age. However, climatic archives show a complex pattern of timing and even antiphase climate responses in the arid Central Andes (Grosjean et al., 2003; Font and Chiesa, 2015). Knowledge about timing is severely limited in this region due to the scarcity of organic material for radiocarbon dating. In the Blanco River Basin (BRB), a number of ambiguous Quaternary glacial stages have been proposed (Corte, 1957; Polanski, 1958, 1965; Wayne, 1981; Wayne and Corte 1983). Wayne (1981) proposed at least four main glaciations along the BRB that would have reached very low altitudes near the confluence of the Mendoza River valley (1400 masl). Assuming temporal correlations with Northern Hemisphere Glaciations and using relative dating techniques (e.g., soil development, loess thickness, block weathering degree, and preserved morphology) a tentative chronology for glacial deposits was suggested (Wayne and Corte, 1983). The Vallecitos Glaciation, evidenced by moraine deposits (Vallecitos I) and moraines related to fossil rock glaciers (Vallecitos II) located above (2600 to 3400 masl) were assigned to the Late Pleistocene. The authors proposed a speculative age of 17?22 ka BP for Vallecitos I and 12?16 ka BP for Vallecitos II. This work presents the first numerical cosmogenic ages of glacial deposits from the eastern slope of the Plata peak (6200 masl) along the BRB (32ºS). Surface boulders on the top of moraines were sampled for Be10. Obtained ages fit very well with local stratigraphy and regional paleoclimate evidence. The Loma de los Morteritos moraine II (3000 masl) was dated to ~25,000 years, while ages of ~8,000 and ~12,000 years were found for the Angostura II deposit (~3300 masl). These ages could be evidencing the Last Glacial Maximum (LGM) period for this region.