Understanding the production and retention of in situ cosmogenic 14C in polar firn

Radiocarbon in CO 2 , CO and CH4 trapped in polar ice is of interest for dating of ice cores, studies of past solar activity and cosmic ray flux, as well as studies of the paleoatmospheric CH4 budget. The major difficulty with interpreting 14C measurements in ice cores stems from the fact that the measured 14C represents a combination of trapped paleoatmospheric 14C and 14C that is produced within the firn and ice lattice by secondary cosmic ray particles. This in situ cosmogenic 14C component in ice is at present poorly understood. Prior ice core 14C studies show conflicting results with regard to the retention of cosmogenic 14C in polar firn and partitioning of this 14C among CO 2 , CO and CH4. Our new study aims to comprehensively characterize the 14C of CO 2 , CO, and CH4 in both the air and the ice matrix throughout the firn column at Summit, Greenland. We will present measurements of 14C in Summit firn air (the first phase of this study) and discuss the implications for in situ cosmogenic 14C production and retention from initial modeling studies. Preliminary results from firn air indicate a 14CO increase with depth in the lock-in zone resulting from in situ production by muons, as well as a lock-in zone 14CO 2 bomb peak originating from nuclear testing in the late 1950s and early 1960s. A decrease in 14CH4 with depth is observed in the lock-in zone that is in agreement with observations of increasing atmospheric 14CH4 over the past several decades