Towards 14C-dating of gases in ice cores – constraining the in situ cosmogenic 14C production rates by muons

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Date
2021-11-17
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Australian Nuclear Science and Technology Organisation
Abstract
Radiocarbon dating of glacial ice has been a longstanding goal in ice core science. In glacial ice, ¹⁴ C is incorporated mainly through trapping of ¹⁴ C-containing atmospheric gases (¹⁴ CO₂ , ¹⁴ CO, and ¹⁴ CH₄ ). However, ¹⁴ C in ice is also produced in situ, directly in the ice lattice from reactions with secondary cosmic rays. In situ ¹⁴ C in ice mostly accumulates after bubble close-off (generally at firn depths between 50-120 m) because almost all of the in situ produced ¹⁴ C in the firn column is lost to the atmosphere via diffusion. The in situ ¹⁴ C at corresponding close-off depths of most ice core sites is generally dominated by production from deep penetrating muons. Understanding the muogenic ¹⁴ C production rates is thus important to deconvolve the in situ cosmogenic and atmospheric ¹⁴ C signals in ice cores. In this study, we use measurements of ¹⁴ C in ancient ice (>50 kilo-annum before present, ka BP) from the Taylor Glacier ablation site, Antarctica to calibrate the muogenic ¹⁴ C production rates. We find that literature values are overestimated 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. Furthermore, the partitioning between the in situ ¹⁴ C species appears to be constant (¹⁴ CO:¹⁴ CO₂ ratio of 1:2, with small <0.2% contributions from ¹⁴ CH₄ ). Our results allow for future ice core ¹⁴ C studies to be potentially used for several applications, including absolute dating of gases and improving the ¹⁴ C calibration curve in periods where high-resolution tree ring data are not available.
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Keywords
Carbon 14, Gases, Ice, Drill cores, Glaciers, Tree rings, Muons
Citation
Dyonisius, M., Petrenko, V., Smith, A., Hmiel, B., Neff, P., Yang, B., Hua, Q., Place, P., Menking, J., Shackleton, S., Beaudette, R., Harth, C., Kalk, M., Roop, H., Bereiter, B., Armanetti, C., Buizert, C., Schmitt, J., Brook, E., Severinghaus, J., Weiss, R., & McConnell, J. (2021). Towards 14C-dating of gases in ice cores – constraining the in situ cosmogenic 14C production rates by muons. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 57). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf