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Erupting Seas

An awesome mix of fire and water may lie behind mass extinctions

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Stagnant water may have unexpected dangers. We already know that certain volcanic lakes can burst forth in clouds of choking gas. Now someone suggests that the ocean itself could erupt in explosive gas under the right—make that the wrong conditions.

The bursting lakes are Nyos and Monoun, in the central African nation of Cameroon. They occupy the craters of volcanoes, and the magma beneath them steadily leaks carbon dioxide gas into their deep waters. Many lakes in the tropics are stagnant (or meromictic), and over the years the bottom waters of Nyos and Monoun grow supercharged with gas. In 1984 something disturbed Monoun's balance, and like a champagne bottle uncorked, the lake erupted in a thick froth of carbon dioxide. The dense gas flowed over the crater wall and down the mountainside, suffocating 37 people. In 1986 Nyos did the same, killing some 1700.

Imagine such a thing happening to a whole ocean. Gregory Ryskin has done that, and it's worse than you imagine.

Ryskin is a professor at Northwestern University, where he teaches chemical engineering. On his Web site he notes, "I became interested in geology and geophysics a few years ago." Putting together two facts, he built a grim picture.

The first fact is that the seafloor continuously leaks methane, the same thing we burn as natural gas. Some comes from serpentinization and some from the action of microbes. As the bubbles rise, the gas dissolves into the deep seawater, where oxygen and microbes in the water consume it.

The second fact is that at times in Earth history, the ocean currents have failed to stir the deepest waters. That's true today of the entire Black Sea, for instance. At other times, whole ocean basins have stagnated. Conditions there quickly become anoxic as the oxygen in the water disappears.

Ryskin asked what would happen to that gas in a stagnant ocean basin. His answer, in the September 2003 Geology, is an oceanic eruption driven by explosive methane. The scenario is horrifying in several ways.

Enough methane dissolves in deep, cold water (about 0.4 percent by molar volume) if that water were to rise, the gas would come out of solution and create a mist whose volume is seven times greater than pure water. The resulting eruption would quickly spread and release the whole ocean basin's worth of natural gas in great clouds. These would inevitably ignite. The amounts of gas would be enormous, and the worldwide fires and explosions would be catastrophic. Even the formation of fullerene compounds, now considered a sure sign of asteroid impacts, is plausible. Land organisms would suffer mass extinction.

In the ocean, the eruption of sterile, anoxic water would smother sea life, and the stirred-up sediment—and more washing in from the burned-over continents—would kill more. So there would be Strangelove oceans and marine mass extinctions, too. Finally, the atmosphere would fill with soot and carbon dioxide, and a full-fledged climate crisis would result. And the deep sea would start right over again, building up methane for the next burst. Only the slow shift of continents would change the sea's configuration so that deep currents could resume.

This spectacular story will be tested against the geologic record, but Ryskin hints that the human record may be relevant. The kind of methane eruption he describes could happen on a smaller scale in places like the Black Sea or lesser basins. If people witnessed one, they might describe it much like the epic of Gilgamesh, in which the land was flooded, burned, shrouded in darkness and "shattered like a pot." They might also come up with the ominous words of Genesis: "All the fountains of the great deep burst forth."

PS: Lakes Nyos and Monoun are fixed now: large floating pipes tap the gas-charged deep waters, which rise up in a self-sustaining fountain. By venting the gas, the pipes prevent eruptions. See for yourself. Something like them ought to be possible for Ryskin's methane eruptions.

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