Ocean methane played role in extinctions, study says

Huge reservoirs of methane trapped beneath the ocean floor rapidly escaped during prehistoric global warming and depleted much of the sea's oxygen, according to new research into why many forms of life suddenly vanished 183 million years ago.

The findings, reported in today's issue of the journal Nature, shed new light not only on the disappearance of as many as 80 percent of some deep-sea species but also a process suspected in other prehistoric mass extinctions.

The study also raised questions about today's sea-floor reservoir of methane hydrate, which the federal government plans to study as a possible energy source.

"One of the important questions that is debated a lot today is the stability of this methane hydrate reservoir and how easy it is to release the methane that is there," said Stephen Hesselbo, an Oxford University researcher and the study's lead author.

Methane hydrate is formed beneath the sea floor when algae from the surface dies and sinks. Normally a gas, the methane is locked in an icelike state but is subject to changes in pressure and temperature.

The Oxford scientists studied fossil wood deposits and identified a signal that they say indicates an unusual level of light carbon in the Earth's atmosphere.

"It's a question of trying to identify what the source of the light carbon would be," Hesselbo said. "The best explanation in this case is that it comes from methane -- methane hydrate from ocean margin sediment."

The researchers believe massive volcanic eruptions during the Jurassic period initiated global warming by spewing carbon dioxide and other greenhouse gases into the atmosphere. Deep-sea currents also were affected.

Methane, freed from its suboceanic cage by warmer water, then merged with the oxygen in the water or atmosphere to form carbon dioxide. In either case, it would have accelerated global warming.

"A number of important fossil groups disappeared at exactly that time," Hesselbo said. "The extinction and the association with the lack of oxygen has been fairly well established, but the association with methane release is something that hasn't been realized before."

Hardest hit were bottom-feeding clamlike organisms known as bivalves: An estimated 80 percent of the species disappeared.

Others affected included ostacods, belemnites and some marine plants.

The researchers believe the event took place over a period of 5,000 years -- a blink in geologic time. The release was estimated to be 20 percent of the present-day 14,000 billion tons of gas hydrate on the sea floor.

On the Net: The journal Nature: www.nature.com

U.S. Geological Survey site on methane hydrate: marine.usgs.gov/fact-sheets/gas-hydrates/title.html