DEPLETION OF C-13 IN SEAWATER SIGMA-CO2 ON MODERN CARBONATE PLATFORMS- SIGNIFICANCE FOR THE CARBON ISOTOPIC RECORD OF CARBONATES

Seawater SIGMACO2 from modern carbonate platforms (Bahama Banks and Fl orida) is depleted in C-13 by as much as 4 parts per thousand relative to open-ocean water. Depletion in C-13 is caused by isotopically ligh t CO2 input from respiration of marine and terrestrial organic matter during water-mass residence on the bank. As such, depletion in C-13 is related to changes in water chemistry driven by evaporation, freshwat er discharge, and CaCO3 withdrawal. Deviation of modern platform seawa ter deltaC-13 values away from those of surface seawater suggests that deltaC-13 values of ancient cratonic carbonate must be cautiously int erpreted. Initial depositional environment, as well as alteration duri ng diagenesis, must be considered in extracting valid secular trends f or oceanic deltaC-13 variation.