When Seafloor Meets Ocean, the Chemistry Is Amazing: In More and More Places, Scientists Are Finding Vast Amounts of Natural Gas on the Ocean Bottom

By Whelan, Jean K. | Oceanus, Fall-Winter 2004 | Go to article overview
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When Seafloor Meets Ocean, the Chemistry Is Amazing: In More and More Places, Scientists Are Finding Vast Amounts of Natural Gas on the Ocean Bottom


Whelan, Jean K., Oceanus


Far more natural gas is sequestered on the seafloor--or leaking from it--than can be drilled from all the existing wells on Earth. The ocean floor is teeming with methane, the same gas that fuels our homes and our economy.

In more and more locations through out the world's oceans, scientists are finding methane percolating through the seafloor, bubbling into the water column, collecting in pockets beneath seafloor sediments, or solidifying in a peculiar icelike substance, called methane hydrate, in the cold, pressurized depths of the ocean,

Massive deposits of methane hydrates could prove to be abundant reservoirs of fuel. But in the past, these massive storehouses of methane also may have "thawed" suddenly and catastrophically, releasing great quantities of climate-altering greenhouse gas back into the atmosphere.

In some places, seeping methane sustains thriving communities of exotic organisms that harness the gas as an energy source in their sunless environment. Below the seafloor, an unknown but potentially vast biosphere of microbes may be making the methane that percolates upward. (See "Is Life Thriving Deep Beneath the Seafloor?' page 72.)

Other places on the seafloor show evidence that pockets of gas trapped beneath sediments have exploded to form "mud volcanoes" or may have triggered seafloor avalanches and tsunami waves.

An underestimated phenomenon

Until recently, scientists have largely overlooked seafloor methane and its potentially dramatic impacts. The problem is that methane commonly vents out of isolated cracks in the seafloor--some so small that they are easily missed by oceanic surveillance systems. Once out into the ocean, the methane usually is diluted rapidly by seawater, or it dissolves in seawater and is consumed by microorganisms that convert it metabolically into carbon dioxide. Unless you happen to be looking in the right place at the right time, you'll miss the show.

But evidence has steadily accumulated that natural seepage of methane from the seafloor is a large, continuous, and ubiquitous phenomenon. When oceanographers happen upon these vents (often called "cold seeps"), the scene is often spectacular.

Several researchers have documented large craters or pockmarks on the seafloor, while others have described huge carbonate mounds (formed by organisms that ingest methane and produce carbonate). Both are often relics of past seafloor gas venting. Sometimes gas simply seeps from the ocean floor and sustains communities of unusual tubeworms, mussels, and other creatures like those found at hydrothermal vents. (See "The Evolutionary Puzzle of Seafloor Life," page 78.)

Gas frozen solid at the seafloor

The deep ocean floor around gas seep sites is often covered by methane hydrates. These are solid crystals of methane encapsulated in ice, which form under the low temperatures and high pressures typical of ocean depths greater than about 1,500 feet.

These hydrates look like seafloor carbonate, but when chunks are broken off, methane hydrates float upward (carbonates sink). As those hydrates rise into higher temperatures and lower pressures, they decompose, releasing methane gas into the ocean--a process akin to releasing the pressure on a bottle of soda.

Energy companies have been eyeing methane hydrates as a potentially tremendous new source of natural gas. Since the 1930s, the use of natural gas has increased fivefold to account for more than 25 percent of the world's energy consumption. With existing technology, the world gas supply is estimated to be 5,300 trillion cubic feet (tcf), Robert Kleinberg of Schlumberger and Peter Brewer of Monterey Bay Aquarium Research Institute reported in American Scientist. At the current rate of global consumption (about 85 tcf per year), a 60-year supply remains.

But the amount of gas at various locations around the world varies widely.

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