Recent work suggests an inverse relationship between the stable carbon isotope composition of marine organic matter (d13Corg) and the molecular CO2 concentration, [CO2(aq)], under which it was formed. Based on theoretical considerations, such a link is anticipated when organic matter is photosynthetically formed from CO2(aq) that passively enters autotrophic cells via diffusion. Based on this relation-ship, past CO2 concentrations in the surface ocean can be reconstructed from d13Corg measurements of sedimentary organic matter. Possible interferences from changes in the d13Corg due to degradation of primary produced organic carbon, transfer to higher trophic levels, and admixture of terrigenous organic matter can largely be overcome by using biomarkers ('chemical fossils') which are diagnostic of their biological origin. Combining [CO2(aq)] reconstructions with
estimates of paleotemperature (e.g. via 18O or UK37) provides a potential tool to determine paleoceanic partial pressure of carbon dioxide (PCO2) via Henry's law.
Here we propose a quantitative model for the carbon isotope fractionation of marine phytoplankton. This model assumes diffusive transport of molecular CO2 and accounts for isotope fractionation associated with carbon transport through a boundary layer and into the cell and fixation by the carboxylating enzymes. Close agreement is found between model predictions and experimental results on carbon isotope fractionation of marine phytoplankton. Model predictions are compared with sedimentary d13Corg measurements.