MICROBIALITES IN A HIGH-ALTITUDE ANDEAN LAKE AS A NATURAL ANALOGUE FOR PROTEROZOIC STROMATOLITE FABRICS

Stromatolites preserve a spectrum of fabrics that reflect complex physical, chemical, and biological interactions. Interpretation of stromatolite fabrics is therefore critical to our understanding of Earth’s environmental and biological evolution. Today, however, stromatolites are relatively rare in the shallow marine environments that are analogous to their ancient settings. By contrast, most modern forms, especially those with textural similarity to ancient stromatolites, occur in extreme environments that are poor analogues for the neutral pH, high alkalinity, and prokaryote-dominated environments inferred for Proterozoic carbonate platforms.

Here we report the occurrence of mineralized microbial mats within Laguna Negra, a high-altitude, hypersaline Andean lake in Argentina. Mineralized microbialites in Laguna Negra preserve a range of carbonate fabrics, including micritic laminae, botryoidal cement fans, and isopachous cement laminae that are strikingly similar to those observed in Proterozoic stromatolites. These characteristic fabrics are arrayed spatially along environmental gradients that permit evaluation of both environmental and biological effects on carbonate fabric formation, and thus provide insight into potential mechanisms of mineralization in Proterozoic stromatolites.

Geochemical modeling of lake and inlet water compositions suggests that, in shoreline environments, precipitation reflects a combination of sustained evaporation within the primary water body and episodic mixing with low-salinity fluids, which results in both elevated carbonate saturation and enhanced carbonate ion activity. Within the zone of mineralization, however, carbonate precipitation also reflects a range of biological influences, including enhanced local saturation state via photosynthetic drawdown of CO₂ and the production of favorable nucleation sites during decomposition of microbial biofilms and EPS. Similarities between Laguna Negra microbialites and numerous Proterozoic stromatolite fabrics suggest that zones of fluid mixing may have been similarly important in ancient settings and, by extension, are potentially important zones for extraterrestrial exploration.