Unlocking the Formation Dynamics of Modern Microbialites: A Geomicrobiological Study in the Maquinchao Basin, Argentina

Judith McKenzie's scientific pursuit focused on comprehending the processes that lead to observable phenomena in the geological record. She consistently emphasized to students and colleagues the fundamental importance of this understanding before delving into any attempt to use proxies. In the late nineties, Judy joined us for a field trip in Patagonia, where we conducted fieldwork in the Maquinchao Basin, Argentina. This location hosts both living and fossil microbialites, presenting a unique opportunity to investigate their formation processes—a goal aligned with Judy's scientific philosophy. Fossil microbialites, distinguished by their globular and cauliflower shapes, populate a continuous palaeoshoreline of a former lake at an altitude of 830 m. Meanwhile, their living counterparts thrive exclusively in the tranquil waters of sheltered or meandering sections of the Maquinchao River. To unravel the intricate interplay between environmental and biological factors governing their development, we sampled and studied living microbialites and their host waters. Contemporary microbialites appear exclusively in freshwater environments with elevated Ca2+ levels. Microscopic inspection reveals heightened photosynthetic organisms in the upper green layer, associated with crypto/microcrystalline calcite (nanoglobules), compared to the lower beige-white biofilm. The latter contains more low-Mg calcite (rhombohedra) precipitates, forming millimeter-sized aggregates in the underlying anoxic layer. While sulphate-reducing bacteria are present throughout the mat, their abundance is more notable in the lower beige-white layer, always associated with Mg calcite. Distinct conditions, such as low salinity and low-turbidity water, coupled with microbial (photosynthetic and heterotrophic) activity, emerge as pivotal factors promoting low-Mg calcite precipitation in the Maquinchao Basin. Notably, these conditions sharply contrast with those proposed for recently described lacustrine microbialites at high altitudes in the subtropical and tropical Andes, as well as in Chilean Patagonia. These observations underscore the significance of geomicrobiological studies in discerning proxies for the hydrological conditions prevailing during the formation of freshwater microbialites.