ANN ARBOR—Methane trapped in undersea sediments could drive potentially dangerous eruptions in the ocean, similar to those that occurred in two African lakes in the 1980s, a University of Michigan scientist’s research suggests. Methane released to the atmosphere in this way could also contribute to global warming, said Youxue Zhang, associate professor of geological sciences. “There is a huge amount of methane stored in marine sediment in the form of methane gas, methane hydrate (a methane-rich, ice-like substance) and methane dissolved in pore water,” said Zhang, whose research is scheduled to be published this month in Geophysical Research Letters. “Methane hydrate is stable at low temperature and high pressure, but certain changes in the ocean environment may cause it to be released.” For example, a drop in sea level would result in lower pressure. That would cause methane hydrate to become unstable and dissociate to form methane gas. An increase in ocean bottom temperature could have the same effect. Marine landslides and faulting might also cause methane hydrate grains and methane gas bubbles to be released into seawater. A pocket of seawater that contains a large number of gas bubbles will rise, because the bubbly water has a lower density than the surrounding water, Zhang explained. As it rises, pressure on the bubbly pocket of water decreases, allowing the bubbles to expand and more bubbles to form. With more and bigger bubbles, the pocket of water becomes even less dense, so it’s able to shoot toward the surface and erupt explosively, just as champagne spews from a bottle when you pop the cork. The two known cases of lake eruptions—both of which occurred in the African nation of Cameroon—were driven by carbon dioxide, but methane-driven ocean eruptions would behave similarly, said Zhang. However, because the mass of gas involved for an ocean eruption can be 1000 times bigger than that for a lake eruption, the event might be much more violent. “Methane-driven ocean eruptions could pose a threat to ocean-going vessels and planes that fly no more than a few thousand meters high,” Zhang said. “Such eruptions not only represent a yet unrecognized geohazard, but they also provide a pathway for methane to rapidly enter the atmosphere as a greenhouse gas.” Though such events have never been observed in the ocean, they may have occurred in the past and certainly could occur in the future, says Zhang. “Just because something has not been observed does not mean it has never occurred. The lakes in Cameroon probably erupted many times, but only in the 1980s was the phenomenon recognized and investigated.” Calculations and observations now show that each of the two lakes could erupt again in 20 to 50 years, and the lakes are being degassed to prevent that from happening. “Realizing that big geological processes such as lake eruptions have only recently been recognized, one wonders how many new phenomena are waiting to be discovered,” Zhang said.

Related links:

Department of Geological Sciences Youxue Zhang

The degassing of Lake Nyos

E-mail: rossflan@umich.edu