Stable-Isotope Ratios and Concentration of CO₂ in Air from Polar Ice Cores

Abstract

Analyses of air trapped in an ice core from the South Pole indicate that the CO 2 concentration may have increased by about 10 ppm and that the 13 C/ 12 C 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 CO 2 , either of biogenic or of oceanic origin. 18 O/ 16 O ratios in CO 2 from different ice cores are much lower than those which have been observed in atmospheric carbon dioxide. A possible explanation is that the CO 2 has equilibrated isotopically with the ice. We have calculated equilibrium isotope-fractionation factors between ice and carbon dioxide and found that the observed 18 O/ 16 O ratios of CO 2 are indeed near isotopic equilibrium with the ice. This indicates that an exchange of oxygen atoms probably occurs between ice and included CO 2 .