Processes affecting the CO sub(2) concentrations measured in Greenland ice

Detailed CO sub(2) measurements on ice cores from Greenland and Antarctica show different mean CO sub(2) concentrations for samples at the same gas age. The deviation between Antarctic and Greenland CO sub(2) records raises up to 20 ppmv during the last millennium. Based on the present knowledge of the global carbon cycle we can exclude such a high mean interhemispheric difference of the CO sub(2) concentration between high northern and southern latitudes. Diffusive mixing of the air in the firn smoothes out short term variations of the atmospheric CO sub(2) concentration. Nevertheless, we observe short term CO sub(2) variations in Greenland ice in the range of 10-20 ppmv, which cannot represent atmospheric CO sub(2) variations. Due to the low temperature at Summit, meltlayers can be excluded for most of the ice and they cannot account for the frequent anomalous short term CO sub(2) variations and the elevated mean CO sub(2) concentration in the Greenland ice. In this work we give some clues, that in situ production of CO sub(2) in Greenland ice could build up excess CO sub(2) after pore close off. Possible chemical reactions are the oxidation of organic carbon and the reaction between acidity and carbonate. We conclude that the carbonate-acidity reaction is the most probable process to explain the excess CO sub(2) in the bubbles. The reaction could take place in very small liquid-like veins in cold ice, where the mobility of impurities is higher than in the ice lattice. At present, there exists no technique to measure the carbonate concentration in the ice directly. However, a comparison of CO sub(2) analyses performed with a dry- and a wet-extraction technique allows to estimate the carbonate content of the ice. This estimate indicates a carbonate concentration in Greenland ice of about 0.4 plus or minus 0.2 mu mol/l and a much lower concentration in Antarctic ice.