Mid- and late-Holocene limnogeology of Laguna del Negro Francisco, northern Chile, and its palaeoclimatic implications

Lacustrine sediments from the high elevation, endorheic, saline lake Negro Francisco (27 degrees 28'S/69 degrees 14'W, 4125 m provide a detailed mid- and late-Holocene environmental history of the Southern Chilean Altiplano. Mineralogy, chemical composition and sedimentary facies analyses of the deposits confirm the regional significance of mid-Holocene aridity between 6000 and 3800 BP. Precipitation rates were significantly lower than the c. 250 mm yr(-1) of today. Fully arid conditions 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 two phases with conditions more humid than today. Comparison with regional palaeodata suggest that these two late-Holocene humid spells were most likely related to Pacific moisture sources. Salinity levels increased again in Laguna del Negro Francisco after 1800 BP. The onset of modern climatic conditions was a stepwise, nonlinear process. However, the functioning of modern climate in this remote part of the world is not understood well enough to explain fully possible mechanisms of climatic changes in the past.