In search of in-situ radiocarbon in Law Dome ice and firn

Results of AMS radiocarbon measurements on CO and CO2 separated from firn a ir directly pumped from the ice sheet, and on CO2 separated from air extrac ted from ice cores by a dry grating technique, are presented. The fun air s amples and ice cores used in this study were collected from the region of L aw Dome, Antarctica. No evidence of in-situ (CO2)-C-14 was found in the fir n air samples or the ice core air samples from one site although a slight e nhancement of (CO)-C-14 above expected polar atmospheric concentrations was observed for some firn air samples, A clear in-situ (CO2)-C-14 Signal for ice pre-dating the radiocarbon bomb pulse was found, however, in air sample s extracted from an ice core from a second site. We compare these results a nd propose an hypothesis to explain this apparent contradiction. The degree to which in-situ C-14 is released from the ice crystals during trapping an d bubble formation is considered and discussed. The selectivity of the dry grating technique for the extraction of trapped atmospheric gases from ice cores is also discussed and compared with other methods. (C) 2000 Elsevier Science B.V. All rights reserved.