Conclusions

Geological emissions of gaseous hydrocarbons represent the second natural source of CH4 after wetlands and are comparable with or higher than other man-made sources. The atmospheric fossil fraction of CH4, estimated at ~30 per cent, would support a geo-CH4 source strength that is at least 50 per cent of anthropogenic fossil emissions, ~90-100Tg yr1 (Lassey et al, 2007; Etiope et al, 2008).

Emission factors from geological sources are fairly well known. However, uncertainties arise due to incomplete knowledge of the actual area of microseepage from soils and from submarine emissions. However, the uncertainties of the global emission estimates are lower than or comparable with those of other sources traditionally considered in the literature and in IPCC reports. Further studies, based on direct field measurements (especially for diffuse microseepage and underwater sources) are needed in order to reduce the uncertainties.

Neotectonics, seismicity and magmatism may have a profound influence on planetary degassing because they can control the accumulation, the migration and the discharge of subsurface gases (Morner and Etiope, 2002). The activity of mud volcanoes is also related to seismic events (Mellors et al, 2007). Neotectonic faults and fractures cutting Pleistocene sediments have been recognized as migration pathways for fluids rising from diapirs and hydrocarbon accumulations (for example Revil, 2002). Salt tectonics itself is a powerful factor capable of creating crustal weakness zones that are very effective as gas migration routes (Etiope et al, 2006). Basically, the geological CH4 source follows the physical laws of gas migration in rocks and sediments (i.e. the transport equations for gas in fractured media; for example Etiope and Martinelli, 2002). Today it is clear that the atmospheric greenhouse gas budget is not independent of the geophysical processes of the solid earth that lead to lithospheric degassing, the third 'breath' of our planet.

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