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 situ 14 C, 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 14 C is added to the firn as it accumulates, and is not expected to be lost by diffusion. During the firnice transition, atmospheric CO 2 is trapped, adding 14 CO 2 to the ice. The signature of in situ 14 C is however not obliterated since ∼ 60% of in situ 14 C is instantly oxidized to 14 CO in the ice. We discuss the results available to date and propose that this in situ 14 CO can be used to determine ice accumulation rates back to 40,000 yrs in the past.