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Gas Blasts

Methane once frozen under the seafloor may help heat up the climate
aging



J. W. Stewart
ice
Image: GEOMAR

METHANE TRAPPED IN ICE feeds the reddish flame that is consuming this hydrate sample.

Colossal belches of methane--a greenhouse gas with nearly 30 times the heat-trapping power of carbon dioxide--may bubble up from the seafloor and exacerbate global warming, some scientists now say. Indeed, a slew of new research results presented at the fall gathering of the American Geophysical Union last week revealed clues as to how this might happen.

Interest in hydrates has skyrocketed in recent years because global deposits are thought to harbor more fuel energy than all the world's coal, oil and natural gas reserves combined. Even so, these methane hydrates also pose a climate threat. They are extremely fragile: the slightest temperature increase or release of pressure can destabilize them. When this happens, some of the methane then makes its way to the ocean surface and into the atmosphere--where it doesn't last, but it does react with oxygen to form carbon dioxide, which lingers for centuries or even millennia.

Researchers have been studying this process in a concentrated effort 100 kilometers off the coast of Oregon, along a dumbbell-shaped promontory called Hydrate Ridge for the icy deposits that virtually pave the seafloor there. Methane, the carbon-hydrogen compound that is the main component of natural gas and cow flatulence, gets trapped inside crystalline cages of frozen water in the muddy ocean bottom.

Most often, the gas leaks slowly from hydrate layers into the ocean water above. But Erwin Suess of the Research Center for Marine Geosciences (GEOMAR) in Kiel, Germany, and his colleagues have found a shortcut by which methane zooms from the seafloor to the sky.

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Image: GEOMAR

SONNE, a German research vessel, has spearheaded many explorations of Hydrate Ridge off the coast of Oregon.

In 1996 Suess and his colleagues were the first team to recover methane hydrate from the ocean floor, with the help of the research vessel, Sonne. Then last summer at Hydrate Ridge they discovered something that they had never seen before: Fizzing chunks of hydrate, some the size of refrigerators, broke off the seafloor a kilometer deep and floated to the surface before disintegrating.

Suess now attributes this buoyancy, not a typical hydrate characteristic, to large bubbles of gas that accumulate in the top layers of the seafloor ooze before freezing. Evidence of this process exists in the "bubble fabric" of hydrates retrieved from the seafloor in the vicinity of the floating hydrates, Suess says.

Methane escapes from hydrate deposits even when the pieces don't float to the surface intact. At the December AGU meeting, Robert Collier of Oregon State University presented a glimpse of the methane concentrations in the water above Hydrate Ridge. Several of his colleagues compiled their measurements, taken at more than 40 stations during a series of research cruises last summer.

So far, these leaking hydrate deposits don't seem to be a climate danger. "You can see that right over the ridge are the areas of highest [methane] concentration," Collier says. "But we can make a pretty good argument that not much of this makes it into the atmosphere." His colleague, Marie de Angelis of Humboldt State University in California, reported evidence that most of the methane oxidizes to carbon dioxide before it reaches the surface.

Because methane hydrates are so sensitive to temperature, Michael Hutnak of the University of Washington and his colleagues tracked water temperature above the southern stretch of the ridge. They discovered temperature oscillations that seem to vary with the cycle of ocean tides and most likely influence 40 to 60 million cubic meters of sediment. Most of the hydrate should remain stable within the recorded temperature changes, Hutnak says, but if the hydrates are disrupted in another way, "it might be enough to push it them over the edge."

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Image: GEOMAR

FIZZING CHUNKS of methane hydrate, some the size of refrigerators, occasionally tear away from the seafloor and float to the surface before releasing the greenhouse gas trapped inside.

Almost certain to push the hydrates "over the edge" would be the occurrence one of the magnitude 9 earthquakes that hits the area every 650 years or so, says Chris Goldfinger of Oregon State University. Hydrate Ridge is a pile of sediments that has been scraped off and piled up as the tectonic plate carrying the northwest Pacific Ocean slides under North America. As strain builds during this subduction process, the ridge compresses like a spring. (The strongest methane vents occur where the sediment breaks open as it bends.)

Thin carbonate rock encrusts the bent part of Hydrate Ridge like a chocolate-dipped ice cream cone, Goldfinger says. Using sonar reflections to map the shallow seafloor, he and his colleagues have seen that this thin shell has cracked in certain places and is now sliding down the ridge's steep sides. "My guess is that when these magnitude 9 earthquakes go off, the game board changes completely in just a few seconds," he says. "That's when you might get a catastrophic release of methane all at once."

As it turns out, Hydrate Ridge is scarred with old landslides. Looking deeper into the seafloor sediment than did Goldfinger, Anne Trehu of Oregon State University and her colleagues mapped the history of the ridge's response to the squeezing and folding that it endures. Under southern Hydrate Ridge, they also saw a complicated subsurface "plumbing" system that feeds and drains the hydrates along a network of faults.


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"There are a lot of pieces of the puzzle here," Trehu says of the conference, but she adds that Hydrate Ridge still holds many undiscovered clues about the way that methane from the seafloor might add to the greenhouse-gas problem--a problem that we humans are already creating.

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