One pathway by which methane is released from the underground into the ocean is through submarine mud volcanism. Mud volcanoes, whether they occur onshore or offshore, are major locations of mud and fluid transfer from deep geological layers to the earth’s surface. To date, more than 1 700 mud volcanoes have been recorded, of which about 800 are located offshore. It is estimated that more than 10 000 exist in deep marine waters (Dimitrov, 2002; Fleischer et al., 2001; Ivanov et al., 1996; Kopf, 2002; Milkov, 2000; Milkov et al., 2003; Pimenov et al., 2000). The formation of MVs is often linked to a tectonic compression, dehydration of clay minerals at depth, or rapid deposition of mass flows as slumps or turbidites (e.g. Kopf, 2002; Huguen et al., 2004; Kohl and Roberts, 1994; Vogt et al., 1999). They frequently form mud domes with diameters of up to a few kilometres and heights of several tens of meters above adjacent seafloor.

Worldwide, 10³-10⁵ submarine mud volcanoes (SMVs) are believed to store 10¹⁰-10¹² m³ of methane, i.e. 54-5400 Mt (mega tons) of carbon in associated gas hydrates. During expeditions with R/Vs Polarstern, L’Atalante, and Pourquoi Pas?, we have investigated Haakon Mosby Mud Volcano in respect to geochemical conditions of methane release and the methanotrophic habitats relying upon this peculiar geochemical milieu. Microbes occurring as consortia of archaea and bacteria as well as in symbiosis with host animals such as tubeworms were found to degrade approximately half of the methane ascending from below.

The other fraction, about several hundred tons of methane per year, is released by diffusive and canalized outflow and, to an important part as gaseous methane.

It was assumed earlier that most of this methane is immediately dissolved upon release and rapidly oxidized by particle-bound microbes in the water column. However, several pieces of evidence (see below) suggest, that at a significant portion of the methane released at 1250 m depth makes it into the upper water column and, most likely, to the atmosphere:

• The a large bubble plume was observed by hydro acoustic means reaching up to 800 m high above the sea floor.
• Instead of being rapidly dissolved in the bottom water (at 1250 m depth), bubbles are protected against dissolution by the formation of a gas hydrate skin under the ambient pressure and temperature conditions.
• The rising bubbles induce an upwelling of the ambient water that takes along methane which was dissolved from the bubble plume under way.
• Bubbles were found to be present only as long as they stay within the gas hydrate stability zone (GHSZ). Above this depth, they are dissolved rapidly. However, the water upwelling further continues beyond this depth due to the conservation of momentum, reaching the ocean surface mixed layer.
• Due to isotope fractionation during microbial methane oxidation, the isotopically heavier 13-C methane is enriched in the remaining fraction of sea water-dissolved methane.  However, we have found that the light isotope values in the water column above the Haakon Mosby Mud Volcano, similar to those at the seafloor seeps, suggest that only a minor portion of the rising methane is directly degraded.
• Thus, it seems likely that a significant amount of methane from this deep-sea source  is released to the atmosphere.