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A New Method for Analyzing ¹⁴C of Methane in Ancient Air Extracted from Glacial Ice

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https://ir.library.oregonstate.edu/concern/articles/41687n74v

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  • We present a new method developed for measuring radiocarbon of methane (¹⁴CH₄) in ancient air samples extracted from glacial ice and dating 11,000–15,000 calendar years before present. The small size (~20 μg CH₄ carbon), low CH₄ concentrations ([CH₄], 400–800 parts per billion [ppb]), high carbon monoxide concentrations ([CO]), and low ¹⁴C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air samples was quantitatively removed using the Sofnocat reagent. ¹⁴C procedural blanks were greatly reduced through the construction of a new CH₄ conversion line utilizing platinized quartz wool for CH₄ combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and ¹⁴C activity of extraneous carbon added in the new CH₄ conversion line were determined to be 0.23 ± 0.16 μg and 23.57 ± 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 μg. The overall procedural blank for all stages of sample handling was 0.75 ± 0.38 pMC for ~20-μg, ¹⁴C-free air samples with [CH₄] of 500 ppb. Duration of the graphitization reactions for small (<25 μg C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. ¹⁴C corrections for each step of sample handling have been determined. The resulting overall ¹⁴CH₄ uncertainties for the ancient air samples are ~1.0 pMC.
  • This is the publisher’s final pdf. The published article is copyrighted by the University of Arizona and can be found at: https://journals.uair.arizona.edu/index.php/radiocarbon/index.
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  • Petrenko, V. V., Smith, A. M., Brailsford, G., Riedel, K., Hua, Q., Lowe, D., & Brook, E. J. A new method for analyzing ¹⁴C of methane in ancient air extracted from glacial ice [Electronic version]. Radiocarbon, 50(1), 53-79.
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  • 50
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  • 1
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  • This work was supported by NSF grants OPP0221470 (to JPS) and OPP0221410 (to EJB), and by a Packard Fellowship (to JPS).
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