Instruments and methods: a novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon.

• Published in: Journal of Glaciology
• Main Authors: Petrenko, VV, Severinghaus, JP, Brook, EJ, Muhle, J, Headly, M, Harth, CM, Schaefer, H, Reeh, N, Weiss, RF, Lowe, DC, Smith, AM
• Format: Article in Journal/Newspaper
• Language: English
• Published: International Glaciological Society 2008
• Subjects: Air samplers; Methane; Ice; Glaciers; Air; Isotope dating; Greenland; Journal of Glaciology
• Online Access: http://apo.ansto.gov.au/dspace/handle/10238/1992; https://doi.org/10.3189/002214308784886135

We present techniques for obtaining large (similar to 100 L STP) samples of ancient air for analysis of C-14 of methane ((CH4)-C-14) and other trace constituents. Paleoatmospheric (CH4)-C-14 measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ([CH4]) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of delta N-15 of N-2, delta O-18 of O-2, delta O-18(ice) and [CH4] measurements. The [CH4] blank of the melt extractions was <4 ppb. Measurements of delta O-18 and delta N-15 indicated no significant gas isotopic fractionation from handling. Measured Ar/N-2, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar/N-2, Kr/Ar and Xe/Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample [CH4]. Corrected [CH4] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH4 production in ice at this site. © 2008, International Glaciological Society