Huge “Bathtub Ring” Left By Oil Spill

NOAA/Woods Hole Oceanographic Institute. A red brittle starfish clings to a dead coral, thought to have been killed by oil from the Deepwater Horizon disaster.

BP's Deepwater Horizon oil spill is turning out to be worse than anyone realized, with an area the size of Rhode Island covered in coagulated oil on the sea floor.

In 2010, the Deepwater Horizon rig exploded. As the name suggests, the rig was drilling in waters 1.5 kilometers beneath the surface, a much greater depth than most offshore oil projects, making it particularly difficult to stem the flood. Eleven people died and 650 million liters of oil gushed into the Gulf of Mexico, destroying fish stocks and killing wildlife. However, BP has claimed that the Gulf has proved resilient.

In the Proceedings of the National Academy of Sciences, a team led by Professor David Valentine of the Univesity of California, Santa Barbara, has bad news to report. “This work identifies a fallout plume of hydrocarbons from the Macondo Well contaminating the ocean floor over an area of 3,200 km2."

As the authors note, “The novelty of this event makes the oil’s subsequent fate in the deep ocean difficult to predict.” Roughly 40% of the oil that escaped has not been accounted for, and the paper suggests it has become trapped at depths of 1,000 - 1,300 meters.

“Oil initially was suspended in deep waters and then settled to the underlying sea floor,” the authors conclude, comparing what they found to "a 'bathtub ring' formed from an oil-rich layer of water impinging laterally on the continental slope.” 

G. Burch Fisher. The study area around the Deepwater Horizon well (stared) showing hopane concentrations.

Sediment collections from over 500 locations indicate that 1.8 tonnes of hopane, a marker of crude oil, coated the area to the southwest of the well. More may be found in patches at greater distances, where sampling has been less thorough. Hopane was tested for because other chemicals in the crude oil mix have degraded over the period since the spill occurred.

“Our findings suggest that these deposits come from Macondo oil that was first suspended in the deep ocean and then settled to the sea floor without ever reaching the ocean surface,” Valentine said. “The pattern is like a shadow of the tiny oil droplets that were initially trapped at ocean depths around 3,500 feet (1,000 meters) and pushed around by the deep currents. Some combination of chemistry, biology and physics ultimately caused those droplets to rain down another 1,000 feet (300 meters) to rest on the sea floor.”

BP has responded that just because the area around the well is covered with hydrocarbons, doesn't mean these have anything to do with them. Hopane is emitted by oil seeps across the Gulf of Mexico, and BP argue that the far greater concentration in surface layers around their well could be coincidental.

G. Burch Fisher. Contamination levels at sampling sites around the well.

The sea floor at these depths hosts an ecosystem based around corals, which in some cases have been badly damaged by oil from the spill.

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