Abstract

The study focuses on the deposition of freshwater lacustrine tufa in recently deglaciated areas, specifically in Lago Sarmiento. Lago Sarmiento is a large (water volume of ca. 9 km3 and a maximum depth of ca. 310 m), alkaline and oligotrophic lake situated on folded, mudstone-rich turbiditic deposits from the Upper Cretaceous. The lake margin features a semicontinuous tufa section, reaching up to approximately 10 meters in thickness. This section comprises variably amalgamated tufa mounds that can merge into terraces or be arranged as isolated mounds, domes, and V-shaped build-ups of metric sizes. The deposits are irregular, displaying a clotted to slightly dendritic fabric, and high porosity. Vugs within them are filled with authigenic materials (peloids and gastropods) and terrigenous grains (quartz + plagioclase ± Fe-Mg silicates). Microbialites records are discernible at the microscale within the tufa framework. These records consist of variably micritized and/or eroded shrubs composed of fascicular Mg-calcite (mostly 6 - 8.5 mol% MgCO3) encrusting filamentous structures interpreted as formed after the calcification of EPS around radially organized cyanobacteria of the genus Rivularia, as deduced from 16S rRNA analysis in a microbial mat sample. Scant framboidal pyrite is observed in SEM images, suggesting minimal contributions of sulfate-reducing bacteria to carbonate precipitation. Stable isotope analysis of the tufa (δ13C and δ18O) and lake waters (δ2H, δ18O, and δ13C-DIC) indicates that the positive δ13C tufa composition results from variable amounts of CO2 degassing and microbial photosynthesis over a lake DIC pool that remained near isotopic equilibrium with atmospheric CO2. The slightly negative δ18O tufa composition is interpreted as precipitation during warm/dry periods in the Holocene. Lake water chemistry is characterized by relatively low Mg/Ca molar ratios (0.51 - 1.8), intermediate alkalinity (6 - 11.6 meq/L), and low Ca/Alk ratios (0.1 - 0.59 meq/L). In lakes with similar water chemistry, the supply of Ca+2 and Mg+2 is required to achieve carbonate saturation, e.g., through groundwater discharge. A distinctive record of tufa filling fractures in the mudstones that compose the bedrock is identified in the uppermost portions of the lacustrine tufa section, resembling a seepage system that fed the lake. The putative "seepage carbonates" include thicker infills (up to 20 cm) with microfabrics similar to lacustrine tufa, as well as thin (> 2 cm) and laminated cements with syntaxial crystal growth, arranged as interconnected vein-like structures. The "seepage carbonates" exhibit higher aragonite content, Sr/Ca ratio, and lower content of Mg in calcite than the lacustrine tufa. However, the similar δ13C and δ18O composition among both groups suggest the precipitation of the former in sub-lacustrine conditions. In-situ 87Sr/86Sr analysis of lacustrine tufa formed in opposed margins of the lake reveals strong heterogeneity in the source of Sr, with 87Sr/86Sr compositions spanning between those of the bedrock (~0.7075) and Holocene volcanism in southern Patagonia (~0.7050). These findings indicate complex interactions between intrabasinal and extrabasinal sources, climate, and microbial mediation that influenced the multi-episodic growth of the tufa deposits after the local retreat of the glaciers approximately 12.5 ka BP.