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Natural gas dominates Gulf plumes: microbes are degrading easily digested hydrocarbons first.

The plumes that formed in the Gulf of Mexico's depths this spring and summer in the aftermath of the BP oil well blowout were actually only about one-third oil, scientists estimate, with the remainder consisting of natural gas.

In June, marine microbes were primarily feeding on propane and ethane gases in the plumes, researchers report online in Science September 16.

"We estimate that there's about two times as much gas sitting in those subsurface plumes as there is oil--and there's about a million barrels of oil in them," says David Valentine of the University of California, Santa Barbara, speaking by phone from a National Oceanic and Atmospheric Administration research vessel in the Gulf.

Chemists had been trying to estimate how much oxygen might disappear as microbes degraded BP's spilled oil. It turns out that the oil is only part of the issue. "Probably 66 to 75 percent of the oxygen loss--maybe even a bit more--will ultimately come from bacterial metabolism of the gases," Valentine projects.

Microbes were not consuming much methane, a gas molecule that is biochemically much harder to crack than propane and ethane. That suggests that the most digestible components of the plumes were the first to be attacked by microbes.


The new research "is quite solid and something people will be taking seriously," says Benjamin Van Mooy, a chemical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts. He was part of a team that recently reported finding substantial undegraded oil in the Gulf's deep-sea plumes.

Van Mooy expressed surprise at the amount of gas that the new report measured in the plumes. But then again, he adds, "It's amazing how little we still know about what's down there," especially regarding the gases contained in the spill plume. "This is the first paper that really takes that issue head-on."

Another team has used a new technique to provide what may be a more accurate estimate of the total release of oil from the damaged well. By following the movement of billows in the plume visible in two snippets of seafloor video, Timothy Crone and Maya Tolstoy of Columbia University gauged the minute-by-minute flow of oil. Their projected total--5.2 million barrels by the well's shutdown on July 15-appears online in Science September 23. The figure slightly exceeds an earlier federal estimate.

One big concern since the initial discovery of deep-sea hydrocarbon plumes has been what will happen to oxygen concentrations near the seabed. Some scientists have questioned whether fish-suffocating dead zones might develop. But a September 7 federal study looked for evidence of such oxygen deprivation in zones affected by plumes and found none.

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Title Annotation:Environment
Author:Raloff, Janet
Publication:Science News
Geographic Code:1USA
Date:Oct 9, 2010
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