The Potential of In Situ Cosmogenic Carbon-14 in Polar Ice for Reconstructing Past Variations in Cosmic Ray Flux

Carbon-14 (¹⁴C) is produced in situ in ice grains by secondary cosmic ray neutrons and muons. Recent measurements of ¹⁴C in ice cores from Taylor Glacier, Antarctica provide improved constraints on in situ ¹⁴C production rates and show that at sites with low snow accumulation rates, in situ cosmogenic ¹⁴C in the carbon monoxide phase (¹⁴CO) is expected to be strongly dominant over trapped atmospheric ¹⁴C. This should allow ¹⁴CO measured in ice cores at these sites to act as a record of changes in the historical flux of galactic cosmic rays. Further, our new results from Summit, Greenland show that in situ cosmogenic ¹⁴CO in ice below close-off is almost entirely due to production by muons that have energies of 15 GeV or greater at the surface. Primary cosmic radiation capable of generating such muons would be insensitive to modulation by heliospheric and geomagnetic fields. ¹⁴CO in ice would therefore only record variations in the galactic cosmic ray flux itself. Further, ¹⁴CO in ice would provide an opportunity to examine for the first time long-term variations in the higher-energy component of the flux. Preliminary analysis in a model of in situ cosmogenic ¹⁴CO production indicates that ice core samples from Dome C, Antarctica (a low-accumulation site), would be able to identify long-term variations in the galactic cosmic ray flux of ≈20% or better.