Controls of extreme isotopic enrichment in modern microbialites and associated abiogenic carbonates

Abstract

Microbialites and abiogenic carbonates of the closed-basin hypersaline lake Laguna Negra (Catamarca Province, Argentina) are highly enriched in both 13C and 18O. These carbonates precipitate in the zone of recharge where lake water mixes with groundwater. We examined the processes controlling the isotopic evolution of input waters in order to better interpret the isotopic compositions of the carbonates. Large enrichments of 13C in dissolved inorganic carbon and 18O in water occur as input groundwater chemically evolves. These enrichments can be explained through the abiotic processes of water-equilibration, evaporation, degassing, and carbonate precipitation. The 13C and 18O contents of lake carbonates are consistent with equilibrium precipitation from lake water indicating that the currently ongoing processes can explain the isotopic patterns observed in carbonates. Isotopic compositions of these microbialites are largely unrelated to the biological processes controlling microbialite formation – a pattern that may be generalizable to other settings. However, the isotopic compositions of these microbialites record information about their depositional environment.