Characterization of in situ cosmogenic ¹⁴CO production, retention and loss in firn and shallow ice at Summit, Greenland

Abstract. Measurements of carbon-14-containing carbon monoxide (¹⁴CO) in glacial ice are useful for studies of the past oxidative capacity of the atmosphere as well as for reconstructing the past cosmic ray flux. ¹⁴CO abundance in glacial ice represents the combination of trapped atmospheric ¹⁴CO and in situ cosmogenic ¹⁴CO. The systematics of in situ cosmogenic ¹⁴CO production and retention in ice are not fully quantified, posing an obstacle to interpretation of ice core ¹⁴CO measurements. Here we provide the first comprehensive characterization of ¹⁴CO at an ice accumulation site (Summit, Greenland), including measurements in the ice grains of the firn matrix, firn air and bubbly ice below the firn zone. The results are interpreted with the aid of a firn gas transport model into which we implemented in situ cosmogenic ¹⁴C. We find that almost all (≈ 99.5 %) of in situ ¹⁴CO that is produced in the ice grains in firn is very rapidly (in < 1 year) lost to the open porosity and from there mostly vented to the atmosphere. The time scale of this rapid loss is consistent with what is expected from gas diffusion through ice. The small fraction of in situ ¹⁴CO that initially stays in the ice grains continues to slowly leak out to the open porosity at a rate of ≈ 0.6 % per year. Below the firn zone we observe an increase in ¹⁴CO content with depth that is due to in situ ¹⁴CO production by deep-penetrating muons, confirming recent estimates of production rates in ice via the muon mechanisms and allowing for narrowing constraints on these production rates.