Large 13C enrichment in primary carbonates from Andean Altiplano lakes, northwest Argentina

We report here extreme 13C enrichments up to +13‰ PDB in primary calcite and aragonite precipitates in saline, well oxygenated waters from high-altitude lakes in the southern Andean Altiplano, northwestern Argentina. Biological effects, as well as variations in carbon source inputs, and in the exchange rate with atmospheric CO2, are commonly considered the main controls on the carbon isotope values of authigenic lacustrine carbonate. We present sedimentological and geochemical evidence that favors physical processes — evaporation effects and CO2-degassing — as major controls on 13C enrichment. We propose that large enrichments may result from the non-equilibrium gas-transfer isotope fractionation during CO2-degassing from thermal springs and evaporation effects in arid environments. The dilution effect by large quantities of 14C-free CO2 hinders accurate 14C chronology of these lake records based on lacustrine organic matter and aquatic plants. Our results indicate that geothermal and volcanic CO2 sources in lake basins located in volcanic settings, and physical fractionation may have a greater significance than commonly accepted to explain lacustrine carbon isotope records.