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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 (firn‐ice transition) the pores are pinched off and the air is definitely isolated from the atmosphere. The firn‐ice transition depth is at around 70 m 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 firn densification model to calculate the firn‐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 δ 18 O 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 around 200 years, while it reached values up to 1400 years during the last glaciation for the colder scenario. The model results are compared with age differences obtained independently by matching corresponding climate events in the methane and δ 18 O records assuming a very small phase lag between variations in the Greenland surface temperature and the atmospheric methane. The past firn‐ice transition depths are compared with diffusive column heights obtained from δ 15 N of N 2 measurements. The results of this study corroborate the large temperature change of 20 to 25 K from the coldest glacial to Holocene climate found by evaluating borehole temperature profiles.
Schwander et al. (Fri,) studied this question.