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 14C, inversely proportional to the rate of accumulation. Most of this production occurs when the amount of overlying ice is less than (2--3) &Lgr;, where &Lgr; 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 accumulaiton 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 firn-ic transition, atmospheric CO2 is trapped, adding 14CO2 to the ice. The signature of in situ 14C is however not obliterated since ~60% of in situ 14C is instantly oxidized to 14CO in the ice. We discuss the results available to date and propose that this in situ 14CO can be used to determine ice accumulation rates back to 40,000 yrs in the past. ¿ American Geophysical Union 1990