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Authors: Etiope, G.* 
Lassey, K. R.* 
Klusman, R. W.* 
Boschi, E.* 
Title: Reappraisal of the fossil methane budget and related emission from geologic sources
Issue Date: May-2008
Series/Report no.: / 35 (2008)
DOI: 10.1029/2008GL033623
Keywords: methane
lithosphere degassing
Subject Classification03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases 
Abstract: Converging evidence from new top-down and bottomup estimates of fossil "radiocarbon-free" methane emissions indicates that natural geologic sources account for a substantial component of the atmospheric methane budget. Comparing emission estimates based on atmospheric 14CH4 ("radiomethane") with geologic emissions from seepage, including terrestrial macroseeps, microseepage, marine seeps, and geothermal/volcanic emissions from the Earth’s crust, shows that such "geo-CH4" sources can be conservatively estimated at 53 ± 11 Tg yr 1 globally. This makes geo-CH4 second in importance to wetlands as a natural methane source. Such a new appraisal can easily be accommodated within the uncertainty of the global methane budget as recently compiled, and recognizes the importance of geophysical out-gassing of methane generated within the lithosphere. We propose a new coherent contemporary budget in which 30 ± 5% (based on atmospheric radiomethane measurements) of the global source of 582 ± 87 Tg yr 1 has fossil origin, both natural and anthropogenic.
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