Mineralized microbialites as archives of environmental evolution of a hypersaline lake basin: Laguna Negra, Catamarca Province, Argentina

Environmental fluctuations related to climate, biological productivity, and evaporation can be recorded by sedimentary archives within lacustrine depositional systems. Sediments within terminal, closed-basin lakes are amongst the most sensitive paleoenvironmental indicators, and have great potential for permitting detailed reconstruction of environmental conditions via a variety of geochemical and isotopic proxies. Microbialites, however, have been largely overlooked as repositories of paleoenvironmental data. Here, we investigate mineralized microbialites within Laguna Negra, a high-altitude (4100 meters above sea level) hypersaline, closed-basin lake in the Argentinian Puna region and explore the potential recovery of environmental signals from these unique sedimentary archives. Mineralized microbialites within Laguna Negra preserve complex layering composed of three distinct carbonate fabrics—isopachous cement phases, botryoidal precipitates, and microbially-associated micrite. These phases are interpreted to reflect differential physical and biological influences on carbonate nucleation and growth. Detailed preservation of successive laminae within these microbialites provides the opportunity for time-wise reconstruction of geochemical signatures.

Geochemical analysis of Laguna Negra microbialites shows overall range of δ¹³[delta-13]C_carb[carbonate carbon] is from +5.75‰ [permil] to +18.25‰ and from -2.04‰ to +7.36‰ for δ¹⁸[delta-18]O_carb[carbonate oxygen]. Long-term signals preserved within successive laminae of lighter δ¹³C_carb and δ¹⁸O_carb through time suggests increased input water contribution since the beginning of microbialite deposition. General hydrological evolution of Laguna Negra is reconstructed via integration of observed oxygen isotopes signatures with published water balance models, and is consistent with regional records from other lacustrine systems. Examination of paired δ¹³C_carb and δ¹³[delta-13]C_org[organic carbon] permits interpretation of the relative effects of evaporation and biotic activity in the evolution of Laguna Negra, and reveals that extrinsic parameters, such as CO₂ [carbon dioxide] degassing and evaporation are the primary control on isotopic evolution.

Geochemical and petrographic differences between microbialites across a spatial gradient, however, suggest that Laguna Negra microbialites also preserve signals that represent both heterogeneous evolution of environments within the lake and influence of biogenic activity. Ultimately, understanding of the relative effects of environmental and biotic parameters on the evolution of lacustrine deposits will enhance our understanding of both paleoenvironmental change and its potential relationship to microbialite mineralization.