Formation of landslide-dammed lakes during a wet period between 40,000 and 25,000 yr B.P. in northwestern Argentina

New AMS radiocarbon data from the deposits of landslide-dammed lakes in the Santa Maria Basin (26°S, 66°W) and the Quebrada del Toro (25°S, 66°W) in the eastern Argentine Cordillera show that important 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 analysis of annual-layered lacustrine strata suggests enhanced inter- and intra-annual fluctuations in precipitation as compared to the present. Variations in sediment characteristics and diatom assemblages with mean periodicities of 2 to 5 and 10 to 14 years, respectively, suggest an influence of the El Niño/Southern Oscillation (ENSO) and the Atlantic sea-surface temperature dipole. The Atlantic dipole slightly increased rainfall every 10–14 years as at present. In contrast, the weak modern ENSO influence in this area differs from the past situation where rainfall increased significantly with ENSO-type periodicities. Both increased humidity and seasonality are believed to have reduced thresholds for the generation of catastrophic mass movements in northwestern Argentina.