The effects of water temperature, stratification, and biological activity on the stable isotopic composition and timing of carbonate precipitation in a hypersaline lake

The δ ¹⁸0 of carbonate minerals in a seasonally stratified hypersaline lake are > +2.3% PDB and are inconsistent with calculated δ ¹⁸O values for carbonate in equilibrium with the lake water during the spring and winter. When the lake is stratified, the calculated δ ¹⁸O of carbonate that would precipitate from the water column is less than that measured in the sediment because of freshwater dilution in the mixolimnion and high temperatures (up to 46°C) in the monimolimnion. Thermodynamic calculations indicate that the lake is supersaturated with respect to carbonate phases throughout the year. However, the δ ¹⁸O data indicate that carbonate is precipitated during the late summer and autumn when the lake is unstratified, biologic activity is high, and the water temperature is moderate. The δ ¹³C values of the calcium carbonate minerals precipitated in the lake are heavier than the dissolved inorganic carbon values (δ ¹³C _DIC) of the lake water and cannot be explained by kinetic fractionation, indicating a biologic control on primary carbonate precipitation. The δ ¹³C of dolomite found in the lake sediments are in equilibrium with porewater δ ¹³C _DIC, suggesting a high degree of porewater interaction in dolomitization. Both the δ ¹⁸O and δ ¹³C compositions of the dolomite suggest that it formed in equilibrium with current porewater but not in equilibrium with the coexisting calcium carbonate minerals.