Climate change and mass movements in the NW Argentine Andes

The chronology of multiple landslide deposits and related lake sediments in the eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary: between 35 000 and 25 000 C-14 yr BP and after 5000 C-14 yr BP. The older cluster may correspond to the Minchin wet period (40 000 and 25 000 C-14 yr BP) identified in tropical and subtropical South America, suggesting a causal relation between enhanced landslide activity and climate change. The younger cluster predates the Titicaca wet period that began at about 3900 C-14 yr BP which also affected other regions in the Andes and the Amazon Basin. No landslide and associated lake sediments are documented during the Tauca wet period (between 16 000 and 8000 C-14 yr BP). However, the two clusters correspond to periods where it assumed that the El Nino/Southern Oscillation (ENSO) and tropical Atlantic sea surface temperature dipole (TAD) were active. The analysis of the present-day precipitation patterns in NW Argentina indicates significant spatial and temporal differences between the intra-Andean part of the study area and the Andean foreland. Whereas the TAD seems to consistently increase rainfall, the intensity of precipitation during the El Nino phase of the ENSO is reduced to only 25% of the mean annual average in the intra-Andean basins, whereas the regions east of the Andes receive more than 125%. Similar results, but with an opposite sign, characterize La Nina events. The comparison of this pattern with paleo-precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and, therefore, increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds. (C) 2000 Elsevier Science B.V. All rights reserved.