LettersCoseimic massive methane release from a submarine mud volcano

We report here the discovery of two methane-enriched plumes in a water column above the Kumano Knoll ♯5 submarine mud volcano (KK#5) in Nankai Trough during 2004; a slight methane enrichment was observed in this region in 2000 and 2010. The deeper of the two plumes spread horizontally at depths near the top of KK#5, whereas a shallow plume spread at a height of 300 m from the top of KK#5. The end-member δ13C values of methane in both plumes, −46.1±4.8‰ and −51.4±5.8‰ in the deep and shallow plumes, respectively, and methane extracted from sediment in KK#5 (−52±2‰) are statistically indistinguishable. We conclude that the plumes were created by massive release of methane from KK#5 and that this event probably occurred in response to the large earthquakes (Mw 7.5) that occurred shortly before our observation in 2004. The discharge rates from submarine mud volcanoes (SMVs) in subduction zones may be highly variable in response to major earthquakes.

89Highlights► Methane-rich plumes were found above a submarine mud volcano after earthquakes. ► The isotopic ratios of methane suggest the plumes were derived from the mud volcano. ► The heights of the plumes suggest occurrence of unusual event on the mud volcano. ► Discharge rates from submarine mud volcanoes are highly variable.