Credit: NOAA

While testing equipment off the Californian coast last month, a newly refitted research vessel stumbled across plumes of methane gas rising 1,400 metres from the sea floor.

The Okeanos Explorer, commissioned last year by the US National Oceanic and Atmospheric Administration (NOAA) after a US$20-million refit, was testing a new multi-beam sonar system in the Mendocino fracture zone (see sonar image, above). On 15 July the ship returned to the site to capture plume material for analysis in the coming weeks.

The plumes, which measure up to 1 kilometre across, typically dissipate about 600 metres below the surface. Cruise scientist Stephen Hammond of the NOAA office in Newport, Oregon, suspects this is because ice with methane gas trapped in its crystal structure melts at the combination of pressure and temperature at that depth. Similar methane plumes have been discovered from the Oregon coast to the Black Sea, but not this large or numerous.