MEASUREMENTS OF COSMIC-RAY-PRODUCED C-14 IN FIRN AND ICE FROM ANTARCTICA

We have studied the levels of C-14 in firn and ice from two regions ac cumulating ice in Antarctica, Dome C, and Newall Glacier. The observed concentrations indicate variable amounts of in-situ C-14 from cosmic- ray spallation of oxygen. C-14 appears to be produced as both CO and C O2 in variable amounts, but our results suggest rapid conversion of (C O)-C-14 to (CO2)-C-14. Much of the in-situ C-14 is retained which resu lts in a significant modification of the C-14/C-12 in ice derived from trapping of air at and below the firn-ice transition zone. This means that direct dating of accumulating ice cannot be used without correct ion for the in-situ C-14. In Newall ice, we see some evidence for an e nhancement in in-situ C-14 at or near the time of the Maunder Minimum. Results for Dome C firn and ice indicate higher levels of in-situ C-1 4 in older ice. This is due either to a reduced accumulation rate in t he past, or possible variations in cosmogenic C-14 production. The lev els of in-situ C-14 in firn and ice, and its impact on C-14 dates on t rapped CO2 in ice are discussed.