Lower Jurassic mud volcanoes and methane, Kilve, Somerset, UK

A series of mud volcano structures from Kilve, west Somerset, UK are described. They occur within the Arietites bucklandi ammonite zone of the Lower Jurassic (Hettangian-Sinemurian) Lias Group. The flanks of each of the three mud volcanoes described consist of micritic limestone and alternating shale beds, whilst tufa-like deposits cap the top of each structure. Two of the mud volcanoes contain a mud breccia, which includes dark grey lithoclasts, red mudstone clasts and abundant crinoidal fragments, together with an abundance of the benthic foraminifera Involutina liassica. Evidence for methane associated with the mud volcanoes is derived from the isotopic analysis of the tufa deposits. All three structures yielded very negative δ13C values ( −11.53‰ to −32.35‰), considerably lighter than those of the associated sediments. Such negative δ13C values typically occur when authigenic carbonates incorporate variable proportions of carbon derived from the anaerobic oxidation of methane. Compared to modern mud volcanoes the structures present in Somerset are small, which may indicate limited activity and limited volume extruded. Much methane released from submarine mud volcanoes is commonly oxidized, possibly evidenced by the tufa caps at Kilve. Hence, in conjunction with low activity levels, this may suggest that only a modest amount of methane may have reached the atmosphere during the Jurassic from this source.