Abstract. The air occluded in ice sheets and glaciers has, in general, a younger age (defined as the time after its isolation from the atmosphere) than the surrounding ice matrix because snow is first transformed into open porous firn, in which the air can exchange with the atmosphere. Only at a certain depth (tim-ice transition) the pores are pinched off and the air is definitely isolated from the atmosphere. The tim-ice transition depth is at around 70 rn under present climatic conditions at Summit, central Greenland. The air at this depth is roughly 10 years old due to diffusive mixing, whereas the ice is about 220 years old. This results in an age difference between the air and the ice of 210 years. This difference depends on temperature and accumulation rate and did thus not remain constant during the past. We used a dynamic tim densification model to calculate the tim-ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition. Past temperatures and accumulation rates have been deduced from the •5180 record using time independent functions. We present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the Greenland Ice Core Project (GRIP) and the Greenland Ice Sheet Project Two (GISP2) ice cores for the last 100,000 years. During the Holocene, the age difference stayed rather stable

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