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% CH4, along faults in petroliferous sedimentary basins (at least 900 subaerial MVs are globally known; Etiope and Milkov, 2004). Although the genesis, geology and geochemistry of MVs have already been described in detail (e.g. Dimitrov, 2002; Kopf, 2002), methane flux to the atmosphere has been the subject of detailed studies only since 2001, when the first measurements, carried out on MVs in southern Italy (Etiope et al., 2002), indicated a CH4 output of 400 t yr over an area of about 1.5 km, suggesting a specific flux of the order of 10)10 t km yr. Preliminary estimates suggest that MVs provide an annual global supply of the order of 5–10 million tonnes of CH4 (Etiope and Klusman, 2002; Etiope and Milkov, 2004), i.e. 3–6% of the natural methane sources officially considered in the atmospheric methane budget. Here we present early data concerning the methane output from the largest MVs in Europe, located in eastern Romania, in the southern part of the Eastern Carpathians Foredeep. Measurements have included localized emissions (vents and bubbling pools, t yr) and diffuse soil degassing (mg m day), in a period with no paroxysmal activity in four areas with MVs and everlasting fires. Vent gases were also analysed to assess the helium (He), nitrogen (N2) and carbon dioxide (CO2) concentration and He ⁄ He isotopic ratio as a tracer of gas origin.