The potential of 14CO in glacial ice as a tracer for past cosmic ray flux and atmospheric hydroxyl radical abundance

Abstract

The amount of 14C-containing carbon monoxide (14CO) in glacial ice is determined by trapping of atmospheric 14CO into air bubbles in the ice and in situ cosmogenic production of 14CO in relatively shallow ice and firn. Earlier studies of 14CO in ice cores showed large disagreements with regard to rates of in situ cosmogenic production as well as with regard to whether 14CO produced in the firn layer is well retained or largely escapes to the atmosphere via the interconnected pore space. We have reviewed previously published work that included 14CO measurements in ice or firn air, and compared with our more recent high-precision measurements on very large ice and firn samples. The available evidence suggests that very little in situ cosmogenic 14CO is retained in the diffusive part of the firn (the upper ≈ 40 – 100m). In situ cosmogenic 14CO production rates below the firn diffusive zone are non-negligible, with production due to deeper-penetrating muons. At sites with low snow accumulation rates, the in situ cosmogenic 14CO component is expected to be larger than the trapped atmospheric component. This potentially allows to use ice core 14CO measurements from such sites to improve our understanding of past cosmic ray flux variations. In contrast, at sites with very high accumulation rates, trapped atmospheric 14CO is expected to be dominant over the in situ cosmogenic component. This potentially allows 14CO records from such sites to be used for reconstructions of past atmospheric hydroxyl radical (OH) variations.