Also on Tuesday, the National Oceanic and Atmospheric Administration acknowledged the existence of swathes of underwater oil many miles from the wellhead, which has been spewing oil since April 20. For weeks, federal scientists at the agency had discounted reports of subsurface plumes.

The agency's chief administrator, Jane Lubchenco, said Tuesday, "NOAA is confirming the presence of very low concentrations of subsurface oil."

The Gulf Oil Spill

See graphics covering how the spill happened, what's being done to stop it, and the impact on the region.

 

Samantha Joye, a senior marine scientist at the University of Georgia who just completed a two-week research expedition through the spill zone, said Tuesday that her instrument readings revealed levels of methane gas dissolved in deep seawater that were between 100 times and 10,000 times higher than normally found in the Gulf waters. Such unusually high levels of methane may be spurring the growth of microbes that, in turn, deplete the oxygen on which fish and other marine organisms depend, she said.

"I've never seen concentrations of methane this high anywhere," said Dr. Joye, who analyzed samples from a submerged oil plume that she said was 15 miles long, five miles wide and 300 feet thick. "The whole water column has less oxygen than it normally does."

Researchers at the University of South Florida in St. Petersburg reported Tuesday that their analysis of samples collected at the end of May revealed traces of an undersea oil cloud about 1,200 feet below the surface about 45 miles northeast of the well site. Their sonar surveys suggested the patch might be up to 22 miles long and about 100 feet thick.

Their water sampling also revealed an oil plume 142 miles southeast of the damaged well, but their laboratory analysis showed the oil was different chemically from that coming from the well. They didn't indicate what the source could be. The entire Gulf basin is a natural source of seeping oil and venting methane gas.

Scientists have been checking water chemistry in and around the spill zone during the past three weeks. Efforts to gauge the scale and impact of petroleum compounds below the surface have been hindered by a lack of data, even though NOAA and independent researchers are working on a flotilla of academic and federal research vessels analyzing the spill with sonar, undersea robots and automated buoys.

Generally, the scientists found petroleum compounds in very low concentrations, measuring less than half a part per million. Fish eggs and larva can be affected by concentrations as small as one part per million.

"We know for certain that there is some oil at depth and it appears to be in layers," said University of South Florida oceanographer Ernst Peebles. "It was more like a cloud than a continuous stream of oil."

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