Using ice core measurements from Taylor Glacier, Antarctica, to calibrate in situ cosmogenic 14C production rates by muons

Cosmic rays entering the Earth's atmosphere produce showers of secondary particles such as protons, neutrons, and muons. The interaction of these particles with oxygen-16 (O-16) in minerals such as ice and quartz can produce carbon-14 (C-14). In glacial ice, C-14 is also incorporated through trapping of C-14-containing atmospheric gases ((CO2)-C-14,(CO)-C- 14, and (CH4)-C-14). Understanding the production rates of in situ cosmogenic C-14 is important to deconvolve the in situ cosmogenic and atmospheric( 14)C signals in ice, both of which contain valuable paleoenvironmental information. Unfortunately, the in situ C-14 production rates by muons (which are the dominant production mechanism at depths of > 6 m solid ice equivalent) are uncertain. In this study, we use measurements of in situ C-14 in ancient ice (> 50 ka) from the Taylor Glacier, an ablation site in Antarctica, in combination with a 2D ice flow model to better constrain the compound-specific rates of C-14 production by muons and the partitioning of in situ( 14)C between CO2, CO, and CH4. Our measurements show that 33.7 % (+/- 11.4%; 95 % confidence interval) of the produced cosmogenic C-14 forms (CO)-C-14 and 66.1 % (+/- 11.5%; 95 % confidence interval) of the produced cosmogenic C-14 forms (CO2)-C-14. (CH4)-C-14 represents a very small fraction (< 0.3%) of the total. Assuming that the majority of in situ muogenic 14C in ice forms (CO2)-C-14, (CO)-C-14, and (CH4)-C-14, we also calculated muogenic( 14)C production rates that are lower by factors of 5.7 (3.6-13.9; 95 % confidence interval) and 3.7 (2.0-11.9; 95 % confidence interval) for negative muon capture and fast muon interactions, respectively, when compared to values determined in quartz from laboratory studies (Heisinger et al., 2002a, b) and in a natural setting (Lupker et al., 2015). This apparent discrepancy in muogenic C-14 production rates in ice and quartz currently lacks a good explanation and requires further investigation.