HR: 1340h
AN: A53B-1165    [Abstracts]
TI: Methods for radiocarbon measurements of ultra-small samples of paleoatmospheric methane
AU: * Petrenko, V V
EM: vpetrenko@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Severinghaus, J P
EM: jseveringhaus@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Smith, A M
EM: ams@ansto.gov.au
AF: Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
AU: Brailsford, G
EM: g.brailsford@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, Wellington, 1, New Zealand
AU: Brook, E J
EM: brooke@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Wilkinson Hall, Corvallis, OR 97331, United States
AU: Riedel, K
EM: k.riedel@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, Wellington, 1, New Zealand
AU: Hua, Q
EM: qhx@ansto.gov.au
AF: Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
AU: Lowe, D
EM: d.lowe@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, Wellington, 1, New Zealand
AU: Mühle, J
EM: jens@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Headly, M
EM: mheadly@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Harth, C M
EM: chris@gaslab.ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Schaefer, H
EM: hinrich@lgge.obs.ujf-grenoble.fr
AF: Laboratoire de Glaciologie et Géophysique de l'Environnement, 54 rue Moliere, BP 96, St Martin d'Heres, 38402, France
AU: Reeh, N
EM: nr@oersted.dtu.dk
AF: Danish National Space Center, Technical University of Denmark, Ørsteds plads, Building 348, Kgs. Lyngby, DK-2800, Denmark
AU: Weiss, R F
EM: rfweiss@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Dr, Mail Code 0244, La Jolla, CA 92093, United States
AU: Levchenko, V
EM: vld@ansto.gov.au
AF: Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
AU: Bromley, T
EM: t.bromley@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, Wellington, 1, New Zealand
AU: Moss, R
EM: r.moss@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, Wellington, 1, New Zealand
AB: We present methods developed to perform the first measurements of ¹⁴C of methane (¹⁴CH₄) on air extracted from ancient glacial ice. Air samples containing ~20 μg of CH₄ carbon and dating to the last glacial termination were obtained by field melt-extraction from glacial ice outcropping at the Pakitsoq ablation margin in West Greenland. A low methane blank of < 4 parts per billion (ppb) for the field extraction was achieved by applying a chemical polishing treatment to the extraction vessel. CH₄ was separated from the air by conversion to CO₂ and subsequent cryogenic trapping. The amount of extraneous carbon added in this processing step was reduced to 0.26 ± 0.16 μg through the construction of a new small-volume CH₄ conversion line that utilized platinized quartz wool for CH₄ combustion. The resulting CO₂ was subsequently graphitized and the ¹⁴C was measured by accelerator mass spectrometry. The amount of modern (100 pMC) carbon added during the graphitization step was reduced to 0.03 μg through the use of an ultra high-purity iron catalyst. 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. The overall procedural ¹⁴C blank for all stages of sample handling was 0.75 ± 0.39 pMC for ~20 μg, ¹⁴C-free air samples with CH₄ concentration of 500 ppb, which amounts to ~2.5% of the ¹⁴C content of a typical sample. The resulting overall ¹⁴CH₄ uncertainties for the ancient air samples were ~ 1.0 pMC, or ~ 3% of measured values.
DE: 0394 Instruments and techniques
DE: 0724 Ice cores (4932)
DE: 0793 Biogeochemistry (0412, 0414, 1615, 4805, 4912)
DE: 4930 Greenhouse gases
DE: 4994 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting