Geological Sources of Methane

Gaia's breath—global methane exhalations

“…Global flux estimates range from 0.9 to 3.2 Tg CH 4 /yr (Lacroix, 1993) and 2.5-6.3 Tg CH 4 /yr (Etiope and Klusman, 2002) or an average of w3 Tg CH 4 /yr. Finally, the world ocean ridge system emits negligible amounts of CH 4 to the oceans, estimated by Welhan and Craig (1983) to be 0.1 Tg/yr, as reported in Judd (2000). The amount of CH 4 reaching the atmosphere from mid-ocean ridges is also likely negligible.…”

“…Microbial, thermogenic, and abiogenic CH 4 originates on Earth with each category having distinctive ranges of carbon isotopic (d 13 C) compositions which are not absolute but are useful for classification: microbial, mainly between K60 and K70‰, with values ranging to as light as K110‰; thermogenic, generally from K20 to K52‰; and abiogenic, ranging from about K5 to K45‰, reported relative to the PeeDee Belemnite Standard (summarized by Judd, 2000). These initial carbon isotopic signatures are partially fractionated by processes of oxidation as the CH 4 escapes from the Earth's surface.…”

“…Observations of the 'world's most spectacular marine hydrocarbon seeps' offshore from Coal Oil Point, California, by Hornafius et al (1999) were extrapolated to obtain a CH 4 flux to the atmosphere. These and other estimations have been compiled by Judd (2000) and Judd et al (2002). Table 1 summarizes the estimates of CH 4 emissions from natural seeps in the oceans.…”

The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source

“…Lelieveld et al, 1998;Houweling et al, 2000). However, some well-founded estimates are much larger, ∼50 Tg yr −1 (Lacroix, 1993;Judd, 2000;Etiope and Klusman, 2002;Etiope, 2004), making such sources much more significant.…”

Gas flux to the atmosphere from mud volcanoes in eastern Romania

“…Natural methane emission from geological sources has recently been recognized as an important component in the atmospheric methane budget. Some authors have only focused their attention on submarine gas seepage and gas hydrates (Judd, 2000; Kvenvolden et al ., 2001), recent studies have suggested that mud volcanoes (MVs) on land and microseepage in hydrocarbon‐prone areas are also significant geological greenhouse‐gas sources (Etiope and Klusman, 2002; Morner and Etiope, 2002; Etiope et al ., 2003; Milkov et al ., 2003; Etiope and Milkov, 2004). MVs, occurring both on land and on the seafloor, are cone‐shaped structures produced by the advective upwelling of sediments (mud) fluidized by water and gas, generally comprising 90–99% CH 4 , along faults in petroliferous sedimentary basins (at least 900 subaerial MVs are globally known; Etiope and Milkov, 2004).…”