Seawater ΣCO2 from modern carbonate platforms (Bahama Banks and Florida) is depleted in 13C by as much as 4‰ relative to open-ocean water. Depletion in 13C is caused by isotopically light CO2 input from respiration of marine and terrestrial organic matter during water-mass residence on the bank. As such, depletion in 13C is related to changes in water chemistry driven by evaporation, freshwater discharge, and CaCO3 withdrawal. Deviation of modern platform seawater δ13C values away from those of surface seawater suggests that δ13C values of ancient cratonic carbonate must be cautiously interpreted. Initial depositional environment, as well as alteration during diagenesis, must be considered in extracting valid secular trends for oceanic δ13C variation.