Thursday, 4 September 2003
Rossella Lorenzi
Discovery News
Did a massive methane bubble from deep beneath the ocean trigger a mass extinction?
A massive explosion of colourless, odourless natural gas erupting from the ocean depths may have caused the worst mass extinction in the Earth's history some 251 million years ago, according to U.S. geologists.
In the September issue of Geology, Associate Professor Gregory Ryskin of Northwestern University in Illinois contends that an extremely fast, explosive release of dissolved methane gas could have killed 95% of Earth's marine species, and some 70% of land animal and plants at the end of the Permian era - long before dinosaurs lived and died.
According to Ryskin, methane from bacterial decay or from frozen methane hydrates, continuously produced beneath the ocean floor, accumulated to high concentrations in the stagnant and deep prehistoric waters.
Just one disturbance - a small meteorite impact, an earthquake or a seafloor volcano - could have triggered gas-saturated water closer to the surface, where the gas would have bubbled out. The result would have been a catastrophic eruption.
"The erupting region would have 'boiled over', ejecting a large amount of methane and other gases into the atmosphere, and flooding large areas of land," Ryskin said.
He added that the explosive mechanism is the same as Cameroon's Lake Nyos disaster of 1986. That lake erupted, creating a gas-water fountain almost 120 m high and releasing a lethal cloud of carbon dioxide. A water surge washed up the shore to a height of more than 24 m.
More energy than nuclear weapons
The situation would have been much worse in the Permian era, he said. Ryskin calculated that prehistoric oceans could easily have contained enough methane to liberate an energy about 10,000 times greater than the world's entire nuclear weapons stockpile going off at once.
Scientists have long wondered what caused the massive extinction, proposing various possibilities such as an asteroid colliding with Earth, volcanic eruptions in Siberia or an ancient greenhouse effect.
Ryskin's hypothesis lists seven very different pieces of observational evidence, including: the extreme rapidity of marine and terrestrial extinctions, a sharp peak in the atmospheric concentration of carbon dioxide, a drastic negative sulphur and carbon isotope excursion, and the presence in some locations of metallic and glassy superheated debris particles called microspherules.
"There is no final proof for what I am suggesting, but this is the only hypothesis that explains all those pieces of evidence at once," Ryskin said.
Similar, smaller-scale eruptions of methane over time could account for other events and climatic changes, including the Biblical flood, Ryskin added. But most of all, other sluggish seas might still be accumulating methane at their depths, representing a future risk.
"I do think that such eruptions will occur in the future, though perhaps not in the immediate [future], and not on the same scale. I cannot predict the exact time or location. It is very important to start research in oceanography to identify methane deposits," Ryskin said.
Methane has already been identified as a cause for catastrophic disasters in prehistoric times, according to Professor Gerald Dickens of Rice University in Houston.
"But Ryskin's model is different because he invokes methane stored in deep water, rather than from terrestrial systems such as peat deposits or gas hydrates in sediment," Dickens said. "I have not made the mass balance calculations to evaluate his model, but my gut feeling is that there are some problems in the amount of methane that can be stored in deep water."
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