THE PRESENCE OF LIVING STROMATOLITES AT LAGUNA AMARGA, TORRES DEL PAINE NATIONAL PARK, SOUTHERNMOST CHILE

The area is located in the southernmost part of South America, specifically in the chilean Torres del Paine National Park, in the eastern limit of the Southern Ice Fields. On the Patagonia steppe, a great number of lakes was formed product of the continuous retreated of glaciers in the last 10.500 yr. In the area, it is common to observe Late Jurassic, Cretaceous and Miocene outcrops with glacial lineation, witnesses of the ice retreat.

A particularly interesting small lake is Laguna Amarga. It is a shallow lake with a basin area of 3.18 km² . On the coast a salt belt and an outer aureole of dissecation cracks were produced due to the constant spray caused by the wind and a reduction of the lake size. The lake receives water from ferric springs which has a ph of 6.4-7.2. Chemical analysis of the springs water indicates that the concentration of Fe is enriched with respect to the water of the lake. The chemical analysis of the silts of the spring indicates a content of 38.6% Fe₂O₃, 22.4% CaO and 9.93% SiO₂. The water of the lake is hyper-saline, with an alkaline ph of 8.9 and it is enriched in Mg, Na and K respect to the water of the springs. The Laguna Amarga is the habitat for extensive colonies of modern buildings of calcareous algae that are adhered to the gravels and sand of the lake bottom. In the water near the shore calcareous green-blue algae are floating. Mantles of calcareous algae live on the western shore where they are folded by the current caused by the force of the wind. On the eastern shore live stromatolites that have nodular, bulbous, stratiform and domal forms. The domal stromatolites of the eastern coast are composed by a superior crust of carbonates, an alternation of white and black layers and green-blue algae. The black layers are composed by great quantity of organic carbon, with exogenous clastic material and interwoven green-blue algae. The white layers are composed by carbonatic material, some exogenous clastic material and blue-green algae.

Lago Sarmiento, located in the Torres del Paine National Park, is a deep lake and its basin has an area of 88.372 km². Thick carbonatic deposits have been developed in whole extension of the coast and shore. Is not totally understood if these deposits are associated to organisms, but their geometry and structural features suggest a possible association with thrombolites.