HR: 1340h
AN: B43A-0896    [Abstracts]
TI: The 14CH4 Measurement Record, Geologic Sources of Methane, and a new Reconciliation of the Global Methane Budget
AU: * Lassey, K R
EM: k.lassey@niwa.co.nz
AF: NIWA, PO Box 14-901, Wellington, 6241, New Zealand
AU: Etiope, G
EM: etiope@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, via Vigna Murata, 605, Rome, 00143, Italy
AU: Klusman, R W
EM: rklusman@mines.edu
AF: Dept Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, United States
AB: An enduring consensus of the global methane budget, as expressed in recent IPCC Assessment Reports, is that about 20%\ of emissions have geologic origin (ie, are free of ¹⁴CH₄). Based on atmospheric ¹⁴CH₄ measurements and confounded by ill-quantified ¹⁴CH₄ emissions from the nuclear- power industry, this "fossil fraction" estimate is quite uncertain, typically 18± 9%. A very recent re-determination of 30± 5%\ (± 2σ) for the fossil fraction which does not require that the nuclear-power emission be specified may still be compatible with the earlier assessment despite an upward revision by 50%. Anthropogenic emissions via identified pathways of fossil-fuel exploitation are assessed in the Fourth IPCC Assessment Report at 90± 16 Tg~yr^-1. Natural emissions extrapolated conservatively from new and widespread measurements of terrestrial seeps are 50± 10 Tg~yr^-1, at least three times larger than previously accepted. The aggregate fossil emission of 140± 19 Tg~yr^-1 is conservative yet still accounts for 24± 5%\ of the global source of 582± 87 Tg~yr^-1, a value that is compatible with both ¹⁴CH₄-based estimates of the fossil fraction. Nevertheless, while the fossil contribution to the methane source inventory remains incompletely determined the currently accepted estimate of ~20%\ for the fossil fraction may need to be revised upward, with substantial consequences for pre-industrial and contemporary source inventories.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0400 BIOGEOSCIENCES
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0490 Trace gases
SC: Biogeosciences [B]
MN: 2007 Fall Meeting