Authigenic clays as precursors to carbonate precipitation in saline lakes of Salar de Llamara, Northern Chile

The Salar de Llamara, Atacama Desert is host to saline lakes known as Puquios, which are possible analogs for lakes on early Earth or Mars. Here we examine associations between microbial communities, Mg-clay minerals, and carbonates in microbial mat samples from the Puquios using scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam nanotomography, and transmission electron microscopy. We observe micrometer-scale aggregates of nanocrystalline Mg-clay around cyanobacterial cell sheaths and throughout alveolar networks of microbial extracellular polymeric substances. High-Mn calcite occupies pore spaces and surfaces within the clay matrix. We propose a sequence whereby extracellular polymeric substance networks serve as surfaces for precipitation of Mg-clay, which permineralizes the original microorganisms. Formation of the clay reduces kinetic barriers and provides a suitable substrate for the formation of carbonate minerals. We therefore suggest that authigenic clays are important in the preservation of microbial communities and the precipitation of carbonates in non-marine settings such as the saline lakes of the Atacama Desert.