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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Petrenko, Vasilii V., Severinghaus, Jeffrey P., Schaefer, Hinrich, Smith, Andrew M., Kuhl, Tanner, Baggenstos, Daniel, Hua, Quan, Brook, Edward J., Rose, Paul, Kulin, Robb, Bauska, Thomas, Harth, Christina, Buizert, Christo, Orsi, Anais, Emanuele, Guy, Lee, James E., Brailsford, Gordon, Keeling, Ralph, Weiss, Ray F.
• Language: unknown
• Published: 2023
• Subjects: 58 GEOSCIENCES; Taylor Glacier; Antarc*; Antarctica; Ice; ice core; permafrost
• Online Access: http://www.osti.gov/servlets/purl/1737573; https://www.osti.gov/biblio/1737573; https://doi.org/10.1016/j.gca.2016.01.004

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. Here, 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.