Extreme ¹³C depletions in seawater-derived brines and their implications for the past geochemical carbon cycle

Extreme depletions in the ¹³C content of the total dissolved inorganic carbon (C_T) were found in brines overlying microbial mat communities. Total alkalinity (A_T) and C_T in the brines suggest that intense photosynthetic activity of the microbial mat communities depletes the C_T from the brine. We suggest that this depletion drives a large, kinetic, negative fractionation of carbon isotopes similar to that observed in highly alkaline solutions. In brines of extreme salinity where microbial mat communities no longer exist, the ¹³C content of the C_T increases, probably because photosynthesis no longer dominates the gas-exchange processes. This mechanism explains light carbon-isotope compositions of carbonate rocks from evaporitic sections and bears on the interpretation of δ¹³C values in bedded stromatolitic limestones that are ca. 3.5 b.y. old.