Glacial-Interglacial period in Lago Sarmiento (Chilean Patagonia) using high resolution seismic

Southern South America is the only large continental landmass situated in the core of the southern westerly wind field. Precipitation in the region is generally produced by frontal systems associated with surface depressions. Stronger westerlies at interannual and longer time scales promote extratropical storms, leading to an increase in precipitation. Orographic effects amplify this process, and the resulting precipitation is an important source of moisture along the Pacific margin of the Andes. Eastward to the Patagonian Andes there is a drying effect, and stronger westerlies produce a decrease in precipitation. Lago Sarmiento is a deep 25 km long and 6 km wide, E-W oriented closed-basin lake located at 51°S 72.5°W and about 70 meters above sea level. The lake is supplied by small ephemeral streams and does not directly receive glacial meltwater. Water level and composition depends on the evaporation-rainfall ratio. The abundant carbonate in the lake sediments precipitate in equilibrium with lake waters. High resolution seismic stratigraphic analyses, obtained using a Chirp 3.5 kHz in Lago Sarmiento, are the basis for an interpretation of the sedimentological changes in the lake. Different acoustic facies show an evolution from glacial sediment facies when ice occupied the drainage basin during the Last Glacial Maximum (LGM). From the Glacial/Glacio-lacustrine acoustic facies we observe an upward trend to a high laminated facies indicating an ice retreat and a domination of lacustrine processes. All shallow water areas of the lake support modern Rivularia (cyanobacteria) bioherms. Individual structures are as much as 5 m in height and 6 m in diameter. Emergent "fossil" bioherms occur along all shorelines and define previous lake highstands. With the exception of autochthonous carbonate deposits along the shoreline of Sarmiento, there are no significant carbonate lithologies present within the drainage basin. In addition to the acoustic facies, a number of geomorphological features indicate a displacement of the ice northward. Despite the fact that the exact timing of late glacial ice advance and extent is not known yet, this new study in Lago Sarmiento will accurately date the ice retreat evolution in the eastern part of Torres del Paine National Park (Chile).