New measurements of oil deep in the gulf challenge previous assertions that most of the spill had disappeared.
Scientists have found an enormous plume of hydrocarbons in the deep waters around BP's blown out well.
The finding highlights how little is known about how long oil could linger in the gulf.
Microbial activity appears low, suggesting bacteria have been slow to consume the oil.
An unprecedented survey of the waters around the site of the Deepwater Horizon disaster has found and mapped a massive, submerged plume of hydrocarbons. And it may not be the only one.
Hovering 3,600 feet deep in the Gulf of Mexico, the plume is more than a mile wide, 650 feet thick and at least 35 kilometers (22 miles) long, but probably longer, as the researchers had to break off because of Hurricane Alex. The plume is moving southwest with the water currents, down the continental slope.
Earlier this month, government officials claimed that just a quarter of the oil from the blown out Macondo well remained as "residual" in the gulf. But the new report on the plume, published today in Science Express, is the third finding issued this week that suggests government estimates were wildly off target.
Using both an autonomous robot to dive and zigzag through the plume and old-fashioned ship-based sampling of the waters, scientists mapped the plume to first see if such a plume existed and then to try and explain why it did not come to the surface like the rest of the oil.
"Why did this oil decide to take a hard right turn?," Christopher Reddy, a marine geochemist and oil spill expert at Woods Hole Oceanographic Institution, said. "That's my question."
But even before the question could be asked, there was the controversial matter of whether oil from such a deep leak could get mixed with water and move sideways in the frigid, high pressure depths.
"Many people discounted that such a (plume) could exist," Richard Camilli, the chief scientist of the Woods Hole expedition that identified the plume in late June, said.
Among the things that might have helped create the deep sea plume, Camilli told Discovery News, are the high pressure and low temperatures of the water. Under those conditions some of the lighter, most volatile compounds in the oil – the smelly stuff like benzene, toluene, ethylbenzene and xylenes -- that would seem most likely to buoy to the surface can instead stay liquid and blend into the sea water.
Then there's the sheer depth of water that the oil had to climb through to reach the surface. That gave the hydrocarbons plenty of opportunity to mix with sea water, Camilli said.
The oil spill compounds were mapped out in the plume using a shoe-box sized mass spectrometer at the nose of the Sentry autonomous vehicle. Oxygen levels inside the plume were also mapped; scientists expected them to be low due to microbes busily gobbling up oxygen as they wolfed down the hydrocarbons.
But that's not what they found. Oxygen levels in the plume were normal, meaning that the microbes weren't all that busy and the hydrocarbons may stick around for some time.
"There is not very much known about microbes degrading oil in the deep ocean," Benjamin Van Mooy, another marine geochemist from Woods Hole and a coauthor of the report, said. Based on laboratory experiments, scientists expect the cold water to slow microbes down five to ten times compared to what they do on the surface, he said. But that's about all they can say.
The problem, Camilli said, is that no one has ever studied such a deep spill before, or found such a plume.
"We were first trying to establish that the plume could exist," he said. "So we focused on the one plume that was the strong candidate. That's not to say there are not others."
The expedition had no time to look for others because they were chased back to port by inclement weather.
What this all means for the movement of the oil underwater or how much of the spilled oil remains in the oceans still has to be worked out, Reddy said.
"These budgets are going to change," he said, referring to a recent report by the National Oceanic and Atmospheric Administration that estimated that most of the oil was gone, and to another budget by the Georgia Sea Grant which argued that more oil remained. "They are based on the best available data."
"As exemplified by BP's commitment of $500 million over 10 years to study the impacts of the oil on the Gulf, BP wants to know as much as anyone what is the impact of the oil spill," John Curry, spokesman for BP's Deepwater Horizon Response, said. "We live here, we work here, we play here, we fish here. We are committed to understanding what happened and to making it right."
He cautions, however, that the Woods Hole study is very limited.
"While it is a slice in time and adds another piece of information to the database for our understanding of the issue, the data seems stale and out of date and not relevant to today's activities," Curry said.
Representatives of NOAA did not respond to Discovery News requests to comment on the deep sea plume.