Formation of landslide-dammed lakes during a wet period between 40,000 and25,000 yr BP in northwestern Argentina

New AMS radiocarbon data from the deposits of landslide-dammed lakes in the Santa Maria Basin (26 degrees S, 66 degrees W) and the Quebrada del Toro ( 25 degrees S, 66 degrees W) in the eastern Argentine Cordillera show that i mportant mass movements occurred during a wet period between 40,000 and 25, 000 yr B.P. In addition to increased humidity during this time, detailed an alysis of annual-layered lacustrine strata suggests enhanced inter- and int ra-annuaI fluctuations in precipitation as compared to the present. Variati ons in sediment characteristics and diatom assemblages with mean periodicit ies of 2 to 5 and 10 to 14 years, respectively, suggest an influence of the El Nino/Southern Oscillation (ENSO) and the Atlantic sea-surface temperatu re dipole. The Atlantic dipole slightly increased rainfall every 10-14 year s as at present. In contrast, the weak modem ENSO influence in this area di ffers from the past situation where rainfall increased significantly with E NSO-type periodicities. Both increased humidity and seasonality are believe d to have reduced thresholds for the generation of catastrophic mass moveme nts in northwestern Argentina. (C) 1999 Elsevier Science B.V. All rights re served.