Two thousand one hundred forty-eight carbon isotope values for terrestrial organic matter (plant fossils, coal, bulk terrestrial organic matter) provide a proxy record for temporal changes in the global carbon cycle. δ13CTOM, δ13Ccarb, and δ13CCO2 allow a full evaluation of carbon transfer in the atmosphere-ocean system as well as between the marine and the terrestrial realm. The temporal evolution of δ13CTOM parallels respective data sets from marine carbonate carbon, thus reflecting the linkage between both reservoirs via atmospheric carbon dioxide. On the basis of a comparison between measured and predicted isotope records for δ13CTOM, changes in the carbon isotopic fractionation associated with plant metabolism are discernible. This contradicts the notion of an invariable carbon isotopic fractionation associated with C3 plant metabolism. In light of results from current plant growth experiments, these changes are viewed as dependent on the atmospheric O2/CO2. Hence δ13CTOM might serve as a proxy signal for atmospheric composition.