Paleoclimatic significance of lacustrine microbialites: A stable isotope case study of two lakes at Torres del Paine, southern Chile

摘要

Two Patagonian lakes studied here, Lago Sarmiento and Laguna Amarga, are located within the orographic rain shadow formed to the east of the Southern Patagonian Ice Field in the Andes Range. Major thrombolite colonies are present in Lago Sarmiento, whereas widespread stromatolites occur in Laguna Amarga. Based on the characterization of the hydrologic system of these two lakes, together with an estimation of the isotopic balance and an analysis of the equilibrium conditions between the water and biologically induced carbonates, it is concluded that the microbialites of Lago Sarmiento are better suited as paleotemperature indicators than those of Laguna Amarga. Lago Sarmiento thrombolites contain unique carbonate mineral species in which carbonate precipitation occurs close to isotopic equilibrium and where the variation in water temperature controls fractionation of the stable oxygen isotope.The results indicate that at 1215cal yr Bp the level of the lake was at 85m a.s.l with a temperature close to 9.3°C, was at 82m a.s.l. at 600cal yr Bp with a temperature close to 8.5°C. This coincides with the timing of the Northern Hemisphere Medieval Warming Period. At 183cal yr Bp the level of the lake was at 80m a.s.l with a cooler temperature close to 7.7°C, representing a colder period coinciding with the timing of the Little Ice Age (LIA). An interesting outcome of this study is that it reinforces the idea that the δ~(13)C signal in carbonate deposits can be an effective tool in distinguishing between inorganic and biologically induced precipitation.