Measurements of 14 C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14 CH 4 and 14 CO production rates

Abstract Carbon-14 ( 14 C) is incorporated into glacial ice by trapping of atmospheric gases as well as direct near-surface in situ cosmogenic production. 14 C of trapped methane ( 14 CH 4 ) is a powerful tracer for past CH 4 emissions from “old” carbon sources such as permafrost and marine CH 4 clathrates. 14 C in trapped carbon dioxide ( 14 CO 2 ) can be used for absolute dating of ice cores. In situ produced cosmogenic 14 C in carbon monoxide ( 14 CO) can potentially be used to reconstruct the past cosmic ray flux and past solar activity. Unfortunately, the trapped atmospheric and in situ cosmogenic components of 14 C in glacial ice are difficult to disentangle and a thorough understanding of the in situ cosmogenic component is needed in order to extract useful information from ice core 14 C. We analyzed very large (≈1000 kg) ice samples in the 2.26–19.53 m depth range from the ablation zone of Taylor Glacier, Antarctica, to study in situ cosmogenic production of 14 CH 4 and 14 CO. All sampled ice is >50 ka in age, allowing for the assumption that most of the measured 14 C originates from recent in situ cosmogenic production as ancient ice is brought to the surface via ablation. Our results place the first constraints on cosmogenic 14 CH 4 production rates and improve on prior estimates of 14 CO production rates in ice. We find a constant 14 CH 4 / 14 CO production ratio (0.0076 ± 0.0003) for samples deeper than 3 m, which allows the use of 14 CO for correcting the 14 CH 4 signals for the in situ cosmogenic component. Our results also provide the first unambiguous confirmation of 14 C production by fast muons in a natural setting (ice or rock) and suggest that the 14 C production rates in ice commonly used in the literature may be too high.