LAKE LEVEL CHANGES AND HUMAN IMPACT AS RECORDED IN SEDIMENTS OF CRATER LAKES IN SOUTHERN PATAGONIA (ARGENTINA)

High resolution terrestrial proxy data from high latitudes of the southern hemisphere are scarce. However, they are a key to a better evaluation of teleconnections, inter-hemispheric differences and consequently to our understanding of the global climate system. Within the project SALSA environmental archives in the Pali Aike Volcanic Field (52°S) in southern Patagonia, Argentina, are investigated. Here we present results from a reconnaiscence survey carried out in austral summer 2002 during which we found two lakes with permanent water bodies, Laguna Azul and Laguna Potrok Aike, which show marked differences in their depositional environment. The small (600 m), but deep (55 m) Laguna Azul (AZU) is a young (<6000 years old) freshwater lake. AZU has no tributaries and is characterized by high plankton productivity and a stratified water column. In contrast, Laguna Potrok Aike (PTA) is a larger lake (3500 m) with brackish water and presumably much older than 20,000 years. Seasonally active tributaries supply detritus from the littoral and the catchment to the deep (100 m) lake basin. Several proxies indicate a massive human impact on terrestrial and lacustrine ecosystems between 50 and 65 cm in the AZU record. Starting with fire indicated by charcoal, intensive sheep farming (first frequent occurrence of Rumex) began and caused lake eutrophication and soil erosion (changes in algae- and diatom-assemblages, d¹³C_org- and C/N-values). The exact timing of first European impacts in southern Patagonia is still a matter of debate. It may be associated to the beginning of sheep farming at the end of the 19^th century or to the first European contact in the late 16^th century. Ongoing AMS ¹⁴C dating of AZU and PTA sediments will provide a more precise chronology for this event. We hypothesize that calcite precipitation occurs during periods of evaporative enrichment of the lake water which are equivalent to low lake levels. This interpretation is corroborated by isotopic data indicating relatively high contributions of aquatic macrophytes to the lake center at this time. The combination of TIC and isotopes provides a promising tool for quantitative reconstructions of wet/dry cycles manifested as lake level changes in PTA sediments and in subaquatic and on-shore lake level terraces.