Thu, 02 Mar 2006 12:00 CST
Around 15 per cent of today's global warming is down to methane, but where does all this gas come from? Some at least could be bubbling up from an unlikely source - deep-sea volcanoes.
Until now, such volcanoes were thought to be a negligible source of atmospheric methane because everyone assumed the gas would oxidise long before it reached the surface. However, research on Håkon Mosby, a mud volcano 1250 metres down in the Norwegian Sea, has overturned this assumption.
Eberhard Sauter of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and his colleagues found a huge column of gas bubbles rising to the surface from Håkon Mosby. They used an echo sounder to get acoustic images of the plume. Meanwhile, a remote-controlled robot dived down and videotaped the bubbles while a probe sampled the water and took temperature and depth readings.
The gas inside the bubbles turned out to be methane, which was protected from oxidation by a tough skin around the bubbles. "We found that a gas hydrate membrane enabled them to rise for around 800 metres," says Sauter. The bubbles eventually dissolved in surface waters, much of it probably ending up in the atmosphere (Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2006.01.041).
"We estimate that several hundred tonnes of methane are being released from this location every year," says Sauter. The atmospheric methane budget is around 600 gigatonnes per year, so this won't make a huge difference. However, if every deep-sea volcano turned out to release a similar amount, it would be a different matter.
No one really knows how many of these volcanoes there are. Estimates vary from thousands to tens of thousands, and it is unlikely that they are all active at the same time. "I'm not sure if it's time to say that deep sea methane is a significant source of atmospheric methane," says Alexei Milkov, a petroleum systems analyst for BP America in Houston, Texas. "The jury is still out."