Measurements of in situ ¹⁴C 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 ¹⁴C in CO and CO₂ 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 ¹⁴C in ice consists of both in situ-produced cosmogenic ¹⁴C, which can be either in the form of CO or CO₂, and trapped atmospheric ¹⁴CO₂. In agreement with our previous work on studies of ¹⁴C concentrations in Antarctic firn and ice samples and some GISP2 samples, we found that the cosmogenic in situ-produced ^l4C (from spallation of oxygen nuclei) is greater than the trapped atmospheric ¹⁴C component. Also, we can show the total ¹⁴C amounts in ice are consistent with an efficient retention of the in situ variety of ¹⁴C in older ice where we expect lower average temperatures at the time of formation. The firn samples appear to have lost much of the ¹⁴C, probably during storage. The ice accumulation rates are estimated after due corrections for the CO₂ content of trapped air and secondary ¹⁴C 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 ¹⁴C 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 ¹⁴C 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 (unpublished manuscript, 1995).