Composite microbialites: Thrombolite, dendrolite, and stromatolite associations in a modern environment, Pozo Bravo lake, Salar de Antofalla, Catamarca Puna, Argentina

Abstract

Pozo Bravo is a high-altitude Andean lake that harbors modern microbialites thriving in hypersaline conditions in the Salar de Antofalla, one of the driest sites on Earth and located in the Puna region of Catamarca, northwest Argentine. Due to the lake physiography, microbialites are restricted to a narrow belt following Pozo Bravo lake variations. Microbialites exhibit a wide range of external morphologies including domal, discoidal, tabular, and horseshoe-like bioherms which vary considerably in size, as well as large biostromal terraces. As documented by other studies on modern microbialites, external morphology appears to be mainly the product of the environmental setting. In Pozo Bravo lake, high evaporation rates and hypersalinity (driven by high temperature and strong winds), water-level fluctuations, and lake-bottom topography are major controlling factors. The distinctive feature of Pozo Bravo microbialites is their internal structure, showing a gradual transition from a thrombolitic core to dendrolitic structures and to a sharply overlying stromatolitic layer within a single microbialite. We suggest that these various microbialite textures represent a gradual change within an environmental gradient based on lake-level variations, and the influence of these environmental factors on biological activity, mainly by cyanobacteria and diatoms. The study of this site is particularly relevant given that it represents an active system where progressive changes in microbialite type (from thrombolites to dendrolites and stromatolites) are recorded, providing an excellent natural laboratory to study these textural changes from a mechanistic perspective, and it may provide insights for better understanding of the microbialite geological record. In addition, given that these systems are threatened by human activities (mining of lithium-rich brines), its study and preservation are necessary.