Modern Microbialites of Pozo Bravo Lake: a new deposit in the Argentine Puna (Catamarca province)

LENCINA AGUSTINA INÉS; SORIA, MARIANA NOELIA; GOMEZ, FERNANDO JAVIER; FARÍAS, MARÍA EUGENIA

Dijon

Conferencia; Conference and Workshop. Microbialites: formation, evolution, diagenesis.; 2019

Universite Bourgogne, Total

Pozo Bravo lake is located in the western margin of the Salar de Antofalla. It is a small(552.60 x 69 m), shallow (< 3 m), slightly alkaline (pH 8.4), hypersaline lake (165.5 mS/cm),placed at 3300 masl in the region of Catamarca Puna (northwest of Argentina). Microbialitesare distributed around the entire perimeter of the lake and are restricted to a deph of 0.50 m.They show biostromal megastructures, domal, discoidal and tabular macrostructures withsizes range between 0.40 to 0.05 m high and 0.07 to 1.20 m in diameter. There are alsoextensive carbonates pavements with crinkled laminae mesostructures and unlithifiedmicrobial mats. Analysis under transmitted light microscopy and scanning electronmicroscopy revealed diatom-rich subspherical aggregates and bacterial-diatoms shrubs. X-raydiffraction showed that all the microbialites morphotypes consists of calcite.As described for many modern and ancient microbialites and microbial carbonates, thecharacteristics observed at different scales (megastructures, macrostructures, mesostructuresand microstructures) in Pozo Bravo microbialites, seems to be the results from the synergybetween environmental conditions and biological activity. The hydrological mixing zones inthe north and south of the lake, owed to fresh water inputs from wetlands (named as ?vegasandinas?), and the photosynthetic activity by microorganism (bacteria, cyanobacteria anddiatoms) are suggested to play a central role in controlling carbonate saturation. Therefore, itsproposed here that microbialites are greatly affected by mixing between groundwater and lakewater, photosynthetic activity and subaerial exposure, all these factors are related to the lakelevelchanges. In Pozo Bravo lake, features like domal macrostructures are associated with therise of lake-level, meanwhile the laterally expansion forming biostromal microbialites andcrinkled microbial laminites are commonly related with the lake-level decrease. Mesoscopicand microscopic bacterial shrubs associated with diatoms are suggesting biological influenceon carbonate precipitation.Microbialites from Pozo Bravo could provide a window to understand the synergy betweenenvironmental conditions, microbe-mineral interactions and diatoms influence onmicrobialites formation in modern environments and in the geological record.