MINERALIZED MICROBIALITES AS ARCHIVES OF ENVIRONMENTAL EVOLUTION, LAGUNA NEGRA, CATAMARCA PROVINCE, ARGENTINA

BUONGIORNO, JOY; KAH, L.C.; GOMEZ, F.J.; FIKE, D.

GEOBIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2018

1472-4677

Environmental fluctuations are recorded in a variety of sedimentary archives of lacustrine depositional systems. Geochemical signals recovered from bottom sediments in closed basin lakes are amongst 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 (Burne & Moore, 1987)), however, have been largely overlooked as paleoenvironmental repositories. Here, we investigate concentrically laminated mineralized microbialites fromLaguna Negra, a highaltitude (4100 m above sea level) hypersaline, closedbasin lake in northwestern Argentina, and explore the potential for recovery of environmental signals from these unique sedimentary archives. Spatial heterogeneity in hydrological regime helps define zones inside Laguna Negra, each with their own morphologically distinct microbialite type. Most notably, Zone 3A contains platey microbialites precipitated by evaporative concentration processes, while Zone 3C contains discoidal oncolites, which are interpreted result from biologicallymediated precipitation. This spatial heterogeneity is reflected in petrographically distinct carbonate fabrics: micritic, botryoidal and isopachous, with isopachous fabrics being the main constituent of Zone 3A microbialites. Fabric type is interpreted to reflect a combination of physical and biological influences during mineralization, and paired Cisotope measurement of carbonate and organic matter supports ecological differences as a dominant control on the Cisotopic evolution of lake environments. Laminae of Laguna Negra microbialites preserve a range of δ13Ccarb from +5.75? to +18.25? and δ18 Ocarb from 2.04? to +9.28?. Positive carbon and oxygen isotopic compositions suggest that the primary control on isotopic composition is CO2 degassing during evaporation, especially in Zone 3A. Additionally, successive laminae preserve isotopic trends toward lighter isotopic values over time in some samples, suggesting a longterm increasingly positive water balance. Combined, these results indicate that microbialite archives can provide data that aid in interpretation of both lake paleohydrology and paleoenvironmental change.