On determining ice accumulation rates in the past 40,000 years using in situ cosmogenic ¹⁴C

Radiocarbon is produced in situ in ice by nuclear spallations of oxygen by cosmic ray neutrons. As the firn accumulates, it acquires a predictable concentration of in situ¹⁴C, inversely proportional to the rate of accumulation. Most of this production occurs when the amount of overlying ice is less than (2–3) Λ, where Λ is the absorption mean free path for cosmic radiation in ice, about 150 g · cm⁻², i.e. within the top 10 m. In most accumulation areas, this is firn. In situ produced ¹⁴C is added to the firn as it accumulates, and is not expected to be lost by diffusion. During the firnice transition, atmospheric CO₂ is trapped, adding ¹⁴CO₂ to the ice. The signature of in situ¹⁴C is however not obliterated since ∼ 60% of in situ¹⁴C is instantly oxidized to ¹⁴CO in the ice. We discuss the results available to date and propose that this in situ¹⁴CO can be used to determine ice accumulation rates back to 40,000 yrs in the past.