Analyses of air trapped in an ice core from the South Pole indicate that the CO2 concentration may have increased by about 10 ppm and that the 13C/12C ratio decreased slightly in the thirteenth century. These changes, if really of atmospheric origin, must be due to a significant input into the atmosphere of CO2, either of biogenic or of oceanic origin.
18O/16O ratios in CO2 from different ice cores are much lower than those which have been observed in atmospheric carbon dioxide. A possible explanation is that the CO2 has equilibrated isotopically with the ice. We have calculated equilibrium isotope-fractionation factors between ice and carbon dioxide and found that the observed 18O/16O ratios of CO2 are indeed near isotopic equilibrium with the ice. This indicates that an exchange of oxygen atoms probably occurs between ice and included CO2.