Ages and composition of gas trapped in Allan Hills and Byrd core ice

Abstract

Gas is extracted from large (6-31 kg) Antarctic ice samples to obtain sufficient CO2 for 14C measurements with small low-level proportional counters. The 14C ages of Byrd core ice are in accord with glaciological estimates ranging from (2.2-1.1+1.4)×103 yr at 271 m depth to more than 8 × 103 yr at 1071 m depth. The CO2 abundances in gas extracted from Byrd core ice range from 0.0216 to 0.051%, with below present-day atmosphere CO2 abundances for ice from 1068 and 1469 m depths. The CO2 abundance in gas from Allan Hills surface ice samples ranges between four and six times the atmospheric value and the CO2 had a specific activity three times that of contemporary carbon. A possible explanation for the anomalously high specific activity is surface melting with the incorporation into CO2 of 14C produced by cosmic ray spallation of oxygen in ice. The CO2 abundance in gas extracted from subsurface Allan Hills ice ranged from 0.030 to 0.065%, and the specific activities are below contemporary carbon, indicating ages greater than ∼5×103 yr. The 18O/16O ratio of oxygen in the trapped gas is the same as that of atmospheric oxygen and differs markedly from the 18O/16O ratio in the ice. The O2, N2, and Ar abundances and isotopic compositions are similar to those in contemporary air, except for positive 15N/14N ratios in a few samples. © 1982.