The findings suggest that the ecological effects of oil spills could last longer than previously thought. The paper comes on the heels of the most recent spill, detected May 12. About 88,200 gallons of oil were released from an underwater pipeline operated by Shell about 90 miles off the coast of Louisiana, according to news reports. Much of the oil has been recovered, and there are as yet no reported impacts on wildlife. But scientists are just beginning to assess the effects.
"We knew oil pollutants can be carried downward by marine snow, but we didn't expect the pollutants to stay in the water for such a long time," said Beizhan Yan of the Lamont-Doherty Earth Observatory, an environmental chemist who is lead author of the study.
Some researchers have contended that contaminants found on the seafloor could be coming from natural oil seeps. But Yan and colleagues used various "fingerprinting" techniques to demonstrate that the hydrocarbons in the water were derived from crude oil of the kind leaking from the Deepwater Horizon site. The presence of barium and the distribution of olefin compounds, two key components in drilling mud, confirmed the contaminants were associated with the spill.
"It's kind of like a smoking gun for the source of the contaminants," Yan said.
The study also sheds light on why these contaminants can stay so long—five months—in the water column. "The deposition of hydrocarbons was largely controlled by the particle sources, which are available sporadically," Yan said. "Hydrocarbons, especially high molecular weight ones, were adsorbed tightly to fine particles. These fine particles can linger in the water column for weeks." But a bloom of diatoms, microscopic marine plants, acted as a "dust bunny" to accumulate the particles and carry them below after the diatoms died, he said.
"Normally we don't think of oil as sinking," said co-author Uta Passow, a biological oceanographer at the Marine Science Institute at the University of California Santa Barbara. "People in the past have not really ever considered oil coming to the seafloor, especially very, very deep. We now know how the oil gets down there in large amounts and affects the communities that live there."
Though it's tough to measure exactly how much of the spilled oil winds up on the seafloor, Passow said it could be substantial. "I would argue it's probably more than 10 percent, probably even more than 15 percent," she said. That could add up to millions of gallons.
Other studies have documented how the oil and other contaminants dispersed, and have established that petroleum hydrocarbons from the spill have accumulated on the seafloor. Scientists also have known that phytoplankton, microscopic marine plants, play a role in delivering the oil to the seafloor. In the new study, the researchers describe how that happens.