Methane seeps and mud volcanoes in Italy: Gas origin, fractionation and emission to the atmosphere

Molecular composition, CH4 isotopes and gas flux of all main terrestrial mud volcanoes and other methane seeps in Italy are being assessed for the first time. Whereas 74% of the Italian gas reservoirs are biogenic, about 80% of the seeps release thermogenic gas. Dry- seep gas generally maintains the reservoir C-1/( C-2 + C-3) " Bernard'' ratio while mud volcanoes show molecular fractionation likely occurring during advective migration. Accordingly, a simple and direct use of the " Bernard'' parameter might be misleading when applied to mud volcanoes as it could not always reflect the reservoir composition. Methane flux into the atmosphere from macro-seep areas is in the order of 10(2) - 10(6) t km (-2) y (-1). Microseepage is widespread throughout large areas and, on a regional scale, it provides the main methane output. A first emission estimate for the total hydrocarbon- prone area of Italy suggests levels of 10(5) t y (-1), comparable to national sources from fossil fuel industry.