PHYSICAL AND CLIMATIC PARAMETERS WHICH INFLUENCE THE AIR CONTENT IN POLAR ICE

Under present-day climatic conditions, the air content of polar ice (V) generally shows a high sensitivity to the atmospheric pressure and hence to the surface elevation of the ice sheet where the ice is formed. The results presented here are from sixteen different sites (fourteen in Antarctica, one in Greenland and one in the Yukon Territories, Canada), and they allow a better understanding of the parameters which influence the air content in ice. It is demonstrated that V can be expressed very simply as a function of the atmospheric pressure (P(i)), the temperature (T(i)) and the porous volume of ice (V(i)) at which the air in the firn becomes isolated in terms of pressure from the atmosphere during the process of pore close-off. Our results confirm a V(i) increase with temperature and show no clear V(i) dependence on snow accumulation rate. The possible non-linearity of the V(i)-T(i) relation we obtain could be due to a wind influence on V(i) and/or to a second-order effect of the accumulation rate. Our V(i) results are compared to measurements of the volume of closed pores versus depth in the firn. To close, we discuss the parameterization that we can obtain to interpret the air content variations observed in deep ice cores (over periods covering glacial-interglacial transitions) in terms of paleoclimatic conditions prevailing at the surface of the ice sheet.