Living Stromatolites from Northeastern Patagonia (Argentina): In situ and experimental geomicrobiological investigations

EYMARD, INES; ALVAREZ MARÍA DEL PILAR; BILMES, ANDRÉS; GONZALEZ DOBRA, MARIANO; SUAREZ, FERNANDO; VASCONCELOS, C.; ARIZTEGUI DANIEL

Encuentro; SwissSed Meeting; 2016

Fossil and living stromatolites have been identified in the lacustrine environment of theMaquinchao basin in northeastern Patagonia (Argentina) providing an ideal opportunity tofulfill the lack of microbialite studies in the region. Furthermore, it allows monitoring the environmental factors controlling microbialite formation and to use them as proxy forpaleoenvironmental conditions.Stromatolites are laminated benthic microbial deposits. They result from the interactionbetween environmental and microbial factors. One of the main processes leading to thelamination that often characterized these buildups is the trapping and binding of sediment grains along with mineral precipitation. Most of the modern living stromatolites have been described in shallow marine and saline lacustrine setups whereas studies in freshwater environments are scarce. Moreover, in southernmost South America there is a clear paucity of information concerning the development of microbialites.Presently, the Maquinchao basin contains two separate lakes, Cari Laufquen Grande and CariLaufquen Chica, but there is evidence that this close lacustrine system has had major water level fluctuations during recent times. Today, both lakes are linked through the Maquinchao River and are located at more than 700m above sea level. Fossil stromatolites outcrop along paleoshorelines showing different shapes while living stromatolites are only found in the Maquinchao River.Two field campaign have been lead respectively in Austral summer 2011 and Austral spring2015. Living stromatolites recovered from the first campaign were set in an aquarium in the laboratory under similar temperature and light conditions as in the natural environment. A substantial growth of the biofilm has been observed in the lab as well as the development of a smooth greenish/transparent layer covering the biofilms previously developed around the rock nucleus. Preliminary observation under SEM shows the presence of a matrix of extracellular polymeric substances (EPS) of diverse thickness. Filamentous bacteria and streptobacilli morphotype are observed in the thickest EPS whereas the areas covered with thinner EPS contain more cococoides bacteria, diatoms, low‐Mg calcite crystals and partially dissolved ostracode shells. The last field campaign (November/December 2015) allowed collecting new physicochemical data as well as sampling both living and fossil stromatolites.Ongoing investigations in the living stromatolites include the identification of microbial communities through DNA sequencing as well as SEM observations. They aim to understand the role of microbes in carbonate precipitation (organomineralisation) using different microscopic, geochemical and microbiological tools that will be compared with detailed petrographic observations and geochemical analyses of the fossils counterparts.