Reconstruction of Paleoceanic PCO2 Levels from Carbon Isotopic Compositions of Sedimentary Biogenic Components

The principles and practice of reconstruction of concentrations of dissolved CO₂ from carbon isotopic compositions of contemporaneous, coeval marine organic matter and carbonates are summarized here. Techniques for determination of the isotopic fractionation accompanying photosynthetic fixation of carbon are exemplified by three oceanic case histories. The northern Gulf of Mexico during the late Quaternary represents an environment with near-equilibrium distribution of CO₂ between the atmosphere and ocean surface, whereas non-equilibrium conditions prevail in the central equatorial Pacific. The Black Sea, with surface waters buffered strongly by high concentrations of dissolved CO₂ at depth, represents an exotic PCO₂ environment. Consideration of a preliminary water-column model focussed on the loci of production of biogenic phases used for reconstruction of paleoceanic PCO₂ illuminates some of the major issues to be considered as this technique is refined.