Geological emission of radiocarbon-free methane has been recently recognised as an important component of global natural methane source, being on the same level as biological fluxes from the ocean and termites. Accordingly geological flux may be considered as a "missing source" of the atmospheric methane budget. However, the estimates of the strength of various geological sources need to be refined. Mud volcanoes (MV) on land and the seafloor, and submarine gas seepage are the largest geological methane source. Although flux data from direct measurements are still scarce, four preliminary independent estimates of global methane emission into the atmosphere from MVs have been recently published by Dimitrov (2002), Kopf (2002), Etiope and Klusman (2002) and Milkov et al. (2003). Only Etiope's and Milkov's estimates were based on actual direct flux measurements and/or detailed statistical elaboration. In this presentation we refine those estimates on the basis of 1) new experimental flux data (including diffuse microseepage around craters and vents), and 2) a classification of MV sizes (in terms of area, km2). The studied dataset includes ~120 mud volcanoes. The emission results to be conservatively between 5 and 9 Mt/y, that is 3-6 % of the natural methane sources officially considered in the atmospheric methane budget. The total geologic source, including MVs (this work), seepage from seafloor (Kvenvolden et al., 2001), microseepage in hydrocarbon-prone areas and geothermal sources (Etiope and Klusman, 2002), would amount to 35-45 Mt/y. We believe it is time to add this parameter in the IPCC official tables of atmospheric methane budget.

References:

Dimitrov L. (2002). Mud volcanoes - the most important pathway for degassing deeply buried sediments. Earth-Science Rev., 59, 49-76.

Etiope G. and Klusman R.W., (2002). Geologic emissions of methane into the atmosphere. Chemosphere, 49, 777-789.

Kopf A.J. (2002). Significance of mud volcanism. Rev. Geophysics, 40(2), 1005, doi: 10.1029/2000RG000093.

Kvenvolden K.A., Lorenson T.D. and Reeburgh W. (2001). Attention turns to naturally occurring methane seepage. EOS, 82, 457.

Milkov A.V., Sassen R., Apanasovich T.V., Dadashev F.G. (2003). Global gas flux from mud volcanoes: a significant source of fossil methane in the atmosphere and the ocean. Geoph. Res. Lett., 30(2), 1037, doi:10.1029/2002GL016358.