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

EYMARD, INES; MARÍA DEL PILAR ALVAREZ; ANDRÉS BILMES; MARIANO GONZÁLEZ DOBRA; FERNANDO SUAREZ; CRISOGONO VASCONCELOS; DANIEL ARIZTEGUI

Fribourg

Congreso; Twenty-fourth Meeting of Swiss Sedimentologists; 2016

Université de Fribourg

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 theenvironmental 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 sedimentgrains along with mineral precipitation. Most of the modern living stromatolites have beendescribed in shallow marine and saline lacustrine setups whereas studies in freshwaterenvironments are scarce. Moreover, in southernmost South America there is a clear paucityof 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 waterlevel fluctuations during recent times. Today, both lakes are linked through the MaquinchaoRiver and are located at more than 700m above sea level. Fossil stromatolites outcrop alongpaleoshorelines showing different shapes while living stromatolites are only found in theMaquinchao 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 thelaboratory under similar temperature and light conditions as in the natural environment. Asubstantial growth of the biofilm has been observed in the lab as well as the development ofa smooth greenish/transparent layer covering the biofilms previously developed around therock nucleus. Preliminary observation under SEM shows the presence of a matrix ofextracellular polymeric substances (EPS) of diverse thickness. Filamentous bacteria andstreptobacilli morphotype are observed in the thickest EPS whereas the areas covered withthinner EPS contain more cococoides bacteria, diatoms, low‐Mg calcite crystals and partiallydissolved ostracode shells. The last field campaign (November/December 2015) allowedcollecting new physicochemical data as well as sampling both living and fossil stromatolites.