THE first suggestion that appreciable 14C might be produced in situ in polar ice was made by Fireman and Norris1, who studied 14C in CO2 extracted from both accumulation and ablation samples. In some ablation samples they observed 14C activities between four and six times higher than those expected due to trapped atmospheric CO2. Here we report the detection of an unambiguous signal of in situ cosmogenic 14C in ice samples from two ablation sites in the Antarctic. The 14C is produced mainly by nuclear spallations of oxygen in ice. The observed concentration of 14C in ablation ice samples is 1-3 x 103 atom per g ice-three orders of magnitude higher than expected from the amount of trapped atmospheric CO2 in this ice. The in situ 14C has a unique signature: about 60% exists as 14CO and the remainder as 14CO2. This result is consistent with that expected from studies of artificially produced 11C in solid targets. The 14C concentration is found to decrease with depth as expected for in situ production. The calculated model ablation rates are found to be 5.8±0.7 and 7.6±0.8 cm yr-1 at two sites from the Allan Hills main ice field, in agreement with rates determined by the stake method. Our work indicates that the 14C age of accumulation ice based on trapped (atmospheric) CO2 would be an underestimate of the true age, if a correction is not made for in situ produced 14CO2. This can be done easily because the 14C activities of both the CO and CO2 phases, as well as the trapped CO2 concentration, can be measured.
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