Extreme carbon-isotope enrichments in evaporating brines

Abstract

We report here extreme 13C enrichments in the dissolved inorganic carbon pool (ΣCO2) in evaporating brines. These enrichments may influence the interpretation of the stable carbon isotope record of both inorganic and organic carbon deposited in saline environments. Carbon isotope values of ΣCO2 respond to types of carbonate inputs, rate of exchange with the atmospheric CO2 and to biological effects1,2. δ13C values of up to +14‰, (all values are reported here with respect to the PDB standard) for ΣCO2 from anoxic waters have been reported where methane production by CO2 reducing bacteria occurs3. We document an abiotic effect in evaporating brines which resulted in 13C enrichments of up to +16.5‰) in natural conditions (solar evaporation ponds used for potash production) and up to +34.9‰ in laboratory evaporation experiments, the most enriched δ13C values reported to date in oxic conditions. These extreme enrichments may result from a non-equilibrium gas-transfer isotope fractionation.