The age of the air in the firn and the ice at Summit, Greenland

Abstract

In the framework of the Eurocore drilling project 1989 at Summit, Central Greenland, air samples were collected from a drill hole in the firn at several depth levels. The samples have been analyzed for 85 Kr, CO 2, CH 4, CFC's (F‐11 and F‐12) contents and for the isotopic composition of nitrogen and oxygen. The measured data are compared with the results of a diffusion model. CO 2, for example, takes 12 years (with a standard deviation of 7.5 years) to diffuse from the surface to the firn‐ice transition under the present climatic conditions at Summit. The difference between the age of the ice and the mean age of the occluded air is approximately 210 years. An enrichment of the heavier isotopes due to separation by gravity has been observed. Molecular diffusion alone suffices to explain the observed concentration trends in the firn column. We can, however, not exclude the existence of a convective zone below the surface. We have also measured the density and closed porosity along the firn core that has been retrieved when drilling the hole. The porosity data reveal that the air is occluded into bubbles mainly between 65 and 80 m, corresponding to densities of 790 and 830 kg/m 3, respectively.