Measurements of cosmic-ray-produced ¹⁴C in firn and ice from antarctica

Abstract

We have studied the levels of ¹⁴C in firn and ice from two regions accumulating ice in Antarctica, Dome C, and Newall Glacier. The observed concentrations indicate variable amounts of in-situ ¹⁴C from cosmic-ray spallation of oxygen. ¹⁴C appears to be produced as both CO and CO₂ in variable amounts, but our results suggest rapid conversion of ¹⁴CO to ¹⁴CO₂. Much of the in-situ ¹⁴C is retained which results in a significant modification of the ¹⁴C/¹²C 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 correction for the in-situ ¹⁴C. In Newall ice, we see some evidence for an enhancement in in-situ ¹⁴C at or near the time of the Maunder Minimum. Results for Dome C firn and ice indicate higher levels of in-situ ¹⁴C in older ice. This is due either to a reduced accumulation rate in the past, or possible variations in cosmogenic ¹⁴C production. The levels of in-situ ¹⁴C in firn and ice, and its impact on ¹⁴C dates on trapped CO₂ in ice are discussed.