Based on their analyses of seafloor core samples around the Macondo well (white star), David Valentine and his colleagues mapped hopane contamination across a nearly 3,200 square kilometer region of seafloor. At least one coral reef (white circle) lies within the region of higher oil contamination. Graphic: G. Burch Fisher

By James Urton, special to
24 November 2014

( – Images from the 2010 Deepwater Horizon disaster endure, from the collapsing platform to oil-fouled coastline. But beneath the surface is a story photographers cannot as easily capture.

Two days after the April 20, 2010 explosion that killed 11 and injured 16, the Deepwater Horizon oil rig sank. During the five months it took to seal the Macondo well 1,500 meters below the surface, nearly 5 million barrels of oil gushed into the ocean. In a paper published online on Oct. 27 in the Proceedings of the National Academy of Sciences (PNAS), researchers reported the first solid evidence that some of this oil settled on the seafloor.

“We have placed one more piece of the jigsaw puzzle,” said geochemist David Valentine, leader of the team of scientists from the University of California, Santa Barbara and the Woods Hole Oceanographic Institution. Valentine and his colleagues analyzed more than 500 seafloor sediment cores collected from sites around the Macondo well by the U.S. National Oceanic and Atmospheric Administration.

The team detected the chemical hopane from cores across an area of nearly 3,200 square kilometers. “Hopane is just one of the many compounds in oil from the Macondo well,” biologist Sarah Bagby, a member of Valentine’s team, told Since it degrades slowly, it is a useful indicator of oil contamination, she added. “This is the first time [scientists] showed that liquid oil or its byproducts settled on the seafloor,” environmental engineer Scott Socolofsky of Texas A&M University, who was not a member of Valentine’s team, told

Hopane levels were highest in sites closest to the Macondo well and west of it, corresponding to ocean current patterns during and after the disaster. The oil in the seafloor sediment cores probably sank without ever rising to the surface, since it did not show the telltale signs of decay from air exposure. Valentine and his team concluded that the seafloor oil most likely came from the 2 million barrels that had become trapped between layers of deep ocean waters, which Socolofsky had previously helped to track. “Oil trapped in the deep ocean from this event fell to the seafloor, like a light mist settling over approximately [3,200 square kilometers],” Valentine told [more]

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