Polar ice ablation rates measured using in situ cosmogenic ¹⁴C

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.