Abstract

Lago Sarmiento is an alkaline (pH 8.3 to 8.8), freshwater, endorheic lake situated in a windy, semi-arid region with a cold steppe climate on the eastern slope of the Patagonian Andes (Solari et al., 2010). This area was deglaciated approximately 12.5 ka ago, and during the Holocene, the climate has experienced fluctuations between cold/wet and warm/dry periods, with the latter prevailing since AD 1890 (Moreno et al., 2018). A 10-meter-thick section of lacustrine tufa is currently exposed along the lake margins due to a drop in lake level caused by higher evaporation rates than inflow. The deposits consist of variably amalgamated mounds, domes, and V-shaped build-ups of metric sizes. The tufa exhibits a thrombolytic to slightly dendritic mesoscale fabric. At the microscale, it contains shrubs composed of fascicular Mg-calcite encrusting filamentous microbial remains indicating a combination of biotic and abiotic factors in their formation. In this study, we integrate isotopic analyses of the lake waters (δD-δ18Olake and δ13C-DIC) and the tufa deposits (δ13Ctufa, δ18Otufa) intending to understand the processes involved during carbonate precipitation and its paleo-environmental context. The stable isotope composition of the lacustrine tufa deposits is positive for δ13Ctufa and slightly negative for δ18Otufa and does not exhibit a positive covariance like other evaporative endorheic lake systems. We integrate new and published data to calculate a Local Evaporation Line (LEL) for the study area. Comparison of the δD-δ18Olake data with the LEL indicates that the modern composition of the Lago Sarmiento waters results from extensive evaporation of waters with compositions like those of the open lakes and rivers in the area, that are fed by melting of snow/ice. This supports a glacial origin of Lago Sarmiento. Considering typical fractionation factors between calcite-water, we calculated the composition of the lake's paleo-waters over different temperatures. The mean values of δ18Otufa are reproduced at low temperatures (4°C) if a composition similar to the modern δ18Olake is considered. Assuming warmer temperatures, like those during summer and spring (7° to 13°C), to reproduce the δ18Otufa values, precipitation would need to have occurred from paleo-waters with δ18Olake compositions 0.5 to 2.6‰ more positive than the modern mean δ18Olake. The modern δ13C-DIC composition of Lago Sarmiento is positive and close to the theoretical isotopic equilibrium between dissolved bicarbonate and atmospheric CO2 (Mook et al., 1974), and similar conditions are inferred to have prevailed during the Holocene. This suggests that the positive δ13Ctufa values in Lago Sarmiento result from variable 13C enrichment in the lake DIC due to microbial photosynthesis and CO2 degassing over a DIC pool that remained near equilibrium with pre-industrial atmospheric CO2. Considering the inverse relation between carbonate solubility and temperature and the positive correlation between carbonate supersaturation and evaporation, it is more likely that tufa precipitation occurred during warm/dry periods rather than cold/wet ones. Since the δ13C isotopic composition of dissolved bicarbonate in isotopic equilibrium with CO2(g) with a fixed δ13C composition is inversely correlated with temperature (Mook et al., 1974), at warmer temperatures, the δ13C-DIC of Lago Sarmiento paleo-waters would have been lower. It is then hypothesized that this mechanism could explain the lack of a positive correlation between δ13Ctufa and δ18Otufa in Lago Sarmiento and could aid in enhancing the interpretation of the depositional context of ancient carbonate systems exhibiting similar C-O isotope trends. Mook, W. G., Bommerson J. C., and Staverman W. H. 1974. EPSL. 22(2), 169-176. Moreno, P.I., Vilanova, I., Villa-Martínez, R., et al. 2018. Sci. Rep. 8, 1–9. Solari, M.A., Hervé, F., Le Roux, J., et al. 2010. Palaeogeogr. Palaeoclimatol. Palaeoecol. 297, 70–82.