MICROBIALITES AS ARCHIVES OF ENVIRONMENTAL EVOLUTION, LAGUNA NEGRA, CATAMARCA PROVINCE, ARGENTINA
IPGP - Îlot Cuvier
Dr. Fernando J. Gomez
CICTERRA-CONICET-Universidad Nacional de Cordoba (Argentina)
Environmental fluctuations are recorded in a variety of sedimentary archives of lacustrine and marine depositional systems. Geochemical signals recovered from bottom sediments in closed-basin lakes are among the most sensitive paleoenvironmental indicators, and are commonly used in reconstructing lake evolution. Microbialites (i.e. organosedimentary deposits accreted through microbial trapping and binding of detrital sediment or in situ mineral precipitation on organics), however, have been largely overlooked as paleoenvironmental repositories. Here, we investigate concentrically laminated oncoidal microbialites from Laguna Negra, a high-altitude (4100 m above sea level) hypersaline, closed-basin lake in northwestern Argentina, and explore the potential for recovery of environmental signals from these unique sedimentary archives. Microbialites and related abiogenic carbonates of the Laguna Negra are highly enriched in both 13C and 18O. These carbonates precipitate in the zone of recharge into the lake where more dilute groundwater is input. Large enrichments of 13C in dissolved inorganic carbon and 18O in water occur as the input waters evolve. These enrichments and isotopic variability can be explained through abiotic processes like water-equilibration and mixing, evaporation, degassing, and carbonate precipitation. The successive laminae of the Laguna Negra microbialites preserve isotopic trends over time, suggesting a long-term change in the water balance. Our results indicate that in evaporative settings, microbialites can have isotopic compositions that are unrelated to the biological processes controlling microbialite formation. Therefore, similarly enriched isotopic compositions from microbialites and associated facies in the rock record should not necessarily be attributed to biological processes solely based on their association with microbialites. Constraints of the depositional environment in which a microbialite formed and analysis of several associated facies including abiogenic carbonates are necessary to accurately interpret the meaning of isotopic values found in microbialites. Our results indicate that microbialite archives can provide data that aid in interpretation of both lake paleohydrology as well as paleoenvironmental change.