A HOLOCENE RECORD OF ENSO CHANGES IN LACUSTRINE SEDIMENTS FROM MERIDIONAL CHILE (38-40°S)

The "El Niño Southern Oscillation" (ENSO) is one important process affecting the present-day climate of the Earth. A better understanding of the ENSO phenomenon requires detailed studies on past records in several contrasted areas. At present, the effects are not well-known for southern South America, although there are indications that El Niño induces precipitation anomalies in Meridional Chile. The aim of the Belgian ENSO-Chile team is to reconstruct the ENSO influence recorded in lacustrine sediments, and this through the Holocene. Two lakes from the Chilean Lake District (Icalma, Puyehue) were cored after seismic reconnaissance surveys. The sediments are analysed through a multi-disciplinary approach including physical properties, magnetic susceptibility, sedimentology, mineralogy, tephrochronology, palynology and micropaleontology. The age model is based on natural radioactive isotopes measurements (210Pb, 137Cs, 14C), calibrated by varve counting and tephrostratigraphy. Our hypothesis is that precipitation intensities affect the erosion rate in the watershed and, consequently the terrigenous supplies delivered to the lake. The temporal changes of the terrigenous supplies could be potentially recorded by different proxies (e.g., grain-size, magnetic susceptibility, detrital vs biological sedimentary content) that have yet to be tested. The present supplies delivered to the lakes have been calibrated using geological, geomorphological and botanical fieldworks in the watersheds (winter 2001-2002). Analyses are still in progress. We have now to convert the sediment cores into a continuous time record, taking out any punctual event like volcanic or seismic-induced layers that are particularly abundant in Icalma record. At present it seems that the sediments from lake Puyehue constitute the best (at least less disturbed) key-record for our paleoclimate purpose.