MID-HOLOCENE AND LATE-HOLOCENE LIMNOGEOLOGY OF LAGUNA-DEL-NEGRO-FRANCISCO, NORTHERN CHILE, AND ITS PALEOCLIMATIC IMPLICATIONS

Lacustrine sediments from the high elevation, endorheic, saline lake N egro Francisco (27 degrees 28'S/69 degrees 14'W, 4125 m provide a deta iled mid- and late-Holocene environmental history of the Southern Chil ean Altiplano. Mineralogy, chemical composition and sedimentary facies analyses of the deposits confirm the regional significance of mid-Hol ocene aridity between 6000 and 3800 BP. Precipitation rates were signi ficantly lower than the c. 250 mm yr(-1) of today. Fully arid conditio ns were interrupted by short-term moist spells with an average return period of about 200 years. Effective moisture increased after 3800 BP and peaked between 3000 and 2600 BP and between 2200 and 1800 BP in tw o phases with conditions more humid than today. Comparison with region al palaeodata suggest that these two late-Holocene humid spells were m ost likely related to Pacific moisture sources. Salinity levels increa sed again in Laguna del Negro Francisco after 1800 BP. The onset of mo dern climatic conditions was a stepwise, nonlinear process. However, t he functioning of modern climate in this remote part of the world is n ot understood well enough to explain fully possible mechanisms of clim atic changes in the past.