Measurements of in situ 14 C concentrations in Greenland Ice Sheet Project 2 ice covering a 17‐kyr time span: Implications to ice flow dynamics

We present results of measurements of 14 C in CO and CO 2 phases for selected Greenland Ice Sheet Project 2 firn and ice samples up to a depth of 1838 m (16.6 kyr B.P.). The results show that 14 C in ice consists of both in situ‐produced cosmogenic 14 C, which can be either in the form of CO or CO 2 , and trapped atmospheric 14 CO 2 . In agreement with our previous work on studies of 14 C concentrations in Antarctic firn and ice samples and some GISP2 samples, we found that the cosmogenic in situ‐produced 14 C (from spallation of oxygen nuclei) is greater than the trapped atmospheric 14 C component. Also, we can show the total 14 C amounts in ice are consistent with an efficient retention of the in situ variety of 14 C in older ice where we expect lower average temperatures at the time of formation. The firn samples appear to have lost much of the 14 C, probably during storage. The ice accumulation rates are estimated after due corrections for the CO 2 content of trapped air and “secondary” 14 C production in ice during storage at altitude. These values are in good accord with the recent model estimates of Cutler et al. [1995], except for two samples, at depths 1518.5 m (9740 years B.P.) and 1698.5 m (12,360 years B.P.). In the first case, the in situ 14 C concentration is significantly higher than that predicted by either of the models, by a factor of about 2. In the second case, the estimate of Cutler et al. [1995] is a ∼ 50% lower accumulation rate, but the 14 C result is consistent with adjacent samples. A possible reason for the first discrepancy may be the decreased atmospheric pressure at that time at the site, as suggested by D. Raynaud et al. (unpublished manuscript, 1995).