Their Game Is Mud

By Carl Zimmer|Thursday, May 01, 1997
RELATED TAGS: ALTERNATIVE ENERGY, OCEAN
Last summer Jerry Dickens and his fellow geologists were hauling mud-filled pipes up from the seafloor onto the deck of the research vessel Resolution when one of the mud-core samples exploded. Just as we were pulling it up, it blew, and mud shot 100 feet like a cannon, says Dickens. The geologists weren’t entirely surprised. They had lugged up the mud--It looks like green Play-Doh, says Dickens--in sampling tubes after drilling about 1,400 feet into ocean sediments. Each 30-foot-long tube has a one- inch hole where a little extra sediment sometimes squeezes out if the material is under high pressure. Some of their earlier sample tubes had come up empty, leading the crew to wonder if an entire 30-foot-long mud sample could have blown out through the quarter-size hole. We thought, ‘That’s crazy,’ says Dickens. And then we had one blow up on deck. Fortunately, no one was hurt.

What gave the mud its kick was a huge load of methane. Geologists have long suspected that the seafloor is dotted with deposits of a slushy mix of methane and water, but the Resolution’s crew was the first to measure how much is actually there. The amount is stunning. Along Blake Ridge alone, 250 miles off South Carolina, are 35 billion tons of buried methane, equivalent to the United States’ natural-gas consumption over 105 years. And geologists believe it is only one of at least 50 such deposits worldwide. Because the methane is too costly to extract, energy companies aren’t much interested in the discovery, but people who study past and future climates are. Methane is a powerful greenhouse gas, and it doesn’t always need a crew of geologists to come roaring out of the seafloor.

In the laboratory, under suitably high pressure and low temperatures, water molecules form cages around methane, creating a solid substance called methane hydrate. It packs in methane so efficiently that 164 cubic feet of free gas can be squeezed into a single cubic foot of hydrate. In the 1970s geologists began to suspect that methane hydrates might exist outside chemistry labs. They theorized that methane-excreting bacteria in deep sediments might cause a buildup of the gas. And if the sediments were cold enough and at a high enough pressure, they conjectured, hydrates might form. Such hydrates, they realized, would be found only in a narrow band of sediments, however. Below a certain depth, Earth’s internal heat would make it impossible for the reaction to take place.

Geologists soon gave support to these predictions when sonar surveys suggested the existence of hydrate layers about 3,000 feet below the seafloor in a number of spots around the globe. Researchers have since drilled down into these layers and brought up fizzy grayish green gobs of methane hydrate at about ten different ocean sites. Yet it has been hard to gauge exactly how much the ocean holds, because as hydrates are hoisted onto a ship, the drop in pressure lets much of the methane escape. Guessing how much was lost on the way up, researchers offered estimates that differed by as much as a thousandfold. Recently, however, Dickens, a paleo- oceanographer at James Cook University in Queensland, Australia, succeeded with a new drilling method. After the crew had burrowed into the sediment with their drill, they sent down a tool that could grab a 30-foot-long cylinder of sediment and seal it in a pressurized container. When the container was returned to the ship, the pressurized sediment still contained all of its methane.

Dickens was surprised not only by the abundance of the methane but by the form it took. As much was floating free in bubbles as was caged in hydrates. (It was this free methane that created the mud cannon.) No one is sure how the bubbles got there, but Dickens suggests that as new sediment piles onto the ocean floor, the zone where hydrates can form rises. The hydrates left behind melt and release their methane, but the overlying seal of hydrates traps the bubbles.

Hydrates will not create a new Exxon any time soon--natural gas can be cheaply extracted on land. But, then, Dickens did not get interested in hydrates to heat his house. Rather, he’s fascinated by the sometimes violent role they may have played in Earth’s history. About 55 million years ago, for example, the climate warmed abruptly, and mammals made swift migrations from Asia to North America over newly opened Arctic land bridges; oxygen levels plummeted in the deep sea and organisms there went extinct in huge numbers.

The key to these changes, according to Dickens, is the vast amount of carbon that flooded the oceans and atmosphere in a matter of several thousand years at most--a rate the world would not see again until humans began burning fossil fuels. The usual suspects like volcanoes don’t put out nearly enough carbon-laden gas.

Look, says Dickens, there’s no way to explain this with the conventional carbon cycle. It’s impossible; it doesn’t make sense. There must be one form of carbon that can be released rapidly in the oceans. And we do have a reservoir like that. That reservoir could be contained in places like Blake Ridge, says Dickens. Because no two hydrate deposits are alike, it’s hard to extrapolate from the 850-foot-thick hydrate layer at Blake Ridge. But it is conceivable that methane hydrates worldwide contain twice the organic carbon contained in all the known deposits of coal, oil, and natural gas.

The geologic record shows that 55 million years ago the deep ocean warmed by at least 4 degrees, probably as a result of a change in ocean circulation. Warmer ocean temperatures could have melted some buried hydrates, resulting in the explosive release of trapped methane. Dickens speculates that this release could have been catastrophic; other researchers found enormous craters on Blake Ridge, where melting hydrates apparently made the seafloor collapse.

Their role in the past may make hydrates a wild card in our future climate. Some computer simulations suggest that the global warming induced by burning fossil fuels could alter ocean temperatures. If the deep ocean suddenly warmed again, then the vast fields of hydrates that Dickens and his co-workers have discovered could be unleashed before our eyes.
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