Gulf of Mexico seabed

Seafloor animals congregate near the site of the Deepwater Horizon spill in 2012. (Photo: Woods Hole Oceanographic Institution/NOAA)

It has been nearly five years since BP spilled 205 million gallons of oil into the Gulf of Mexico, and we may finally be solving one of the disaster's most vexing mysteries. While scientists have long known where much of the oil went, several million gallons have remained missing — until now. Two recent studies suggest the oil sank to the bottom, creating a huge, possibly dangerous stain on the seafloor.

"This is going to affect the Gulf for years to come," says Florida State University oceanographer Jeff Chanton, lead author of the newest study. "Fish will likely ingest contaminants because worms ingest the sediment, and fish eat the worms. It's a conduit for contamination into the food web."

But why would it sink? Doesn't oil normally float on water? Yes, Chanton says, and lots of oil from the 2010 BP spill did float at first. But some of it probably got caught in clumps of clay and slime, causing it to quietly slip down to the seafloor while scientists looked for it in the water column.

"Bacteria in the water produce a mucus when they're exposed to oil," Chanton says. "These clumps of mucus aggregate, and pick up clay particles because the Mississippi River is nearby. Clay provides ballast, and the larger these particles become, the faster they sink."

BP's 2010 oil spill was by far the largest in U.S. history, and only a quarter of it was cleaned at the surface or captured by deep-sea containment systems. Another quarter of the oil naturally dissolved or evaporated, according to a U.S. government report, and about 24 percent was dispersed, either naturally or due to the controversial use of chemical dispersants. (Those dispersants may have helped oil sink, Chanton says, but that's still an area of active research.) It's unclear exactly how much of the rest ended up on the seabed, but the new study estimates it's between 6 million and 10 million gallons.

2010 Gulf oil spill

NASA's Terra satellite captures the Deepwater Horizon oil spill from orbit on May 24, 2010. (Photo: NASA)

The researchers found this missing oil by using the radioactive isotope carbon-14 as an "inverse tracer." Oil doesn't contain carbon-14, so patches of sediment without the isotope immediately stand out as places where oil settled. "A lot of times you'll add a tracer to something if you want to follow it through the environment," Chanton explains. "This is sort of like the opposite of that."

A 2014 study used different methods to reach a similar conclusion, mapping hydrocarbons on the seabed to identify a "bathtub ring" of oil stretching 12,000 square miles (3,200 square kilometers) around the Macondo oil well. Chanton says he wouldn't use the same description, but his research did find comparable amounts of oil across 9,200 square miles (2,400 square km). Both studies build upon previous research that suggested at least some of the oil eventually sank to the seabed.

"I don't know about the bathtub ring analogy as much. It's more of a layer," he says. "It's all within a 1-centimeter layer, so it's confined to the upper centimeter of sediment. It's relatively surficial right now. But over time, more sediments will continue to accumulate and bury it more deeply."

Natural oil seeps are common in the Gulf of Mexico, providing a trickle of energy for small populations of bacteria that have evolved to eat petroleum. Those microbes initially played a key role in cleaning up the spill, devouring about 200,000 tons of oil by September 2010. But now that all this oil has sunk to the seabed, lower oxygen levels in the deep ocean may help preserve the oil, Chanton says, by impeding the ability of bacteria to eat it. That means this oil could pose an indelible danger to local sea life, passing from worms, tilefish and other bottom feeders up through the food web.

oil-eating microbes

Oil-eating microbes like these played an important role in the 2010 spill. (Photo: Lawrence Berkeley National Laboratory)

"Sediments may serve as long-term storage for hydrocarbons for as yet unknown periods of time," the researchers write in the new study, published Jan. 20 in the journal Environmental Science & Technology. "With that storage, there is potential for re-exchange with the water column due to either chemical or physical processes that occur in surface sediments."

The next step is to figure out how long these oily sediments might linger. Chanton is now studying the site of the Ixtoc I oil spill, which released about 126 million gallons off Mexico's Bay of Campeche in 1979. "I want to see how much of this stuff is left years later," he says. "That's what we're doing at Ixtoc."

The new study was funded by money BP allocated for research on the 2010 spill, but the company has criticized its methods as "flawed," noting the study can't definitively prove the oil came from its Macondo well. BP has already spent billions of dollars on fines, cleanup costs and other expenses related to the spill, and still faces billions more in an ongoing trial over Clean Water Act violations.

Although scientists are still trying to chemically identify the source of this oil, Chanton says he has no doubt it came from the 2010 BP spill. Not only did he and his colleagues avoid areas with known oil seeps, but the carbon-14 signature of the oil they found doesn't match natural seepage. On top of that, the shape and placement of this oil resemble the huge oil plume that mysteriously vanished in 2010.

"The areas where we saw the most oil, those only had 1 centimeter of radiocarbon depletion," Chanton says. "The natural seeps don't look like that at all – in a natural seep, radiocarbon is depleted all the way down. So it's a layer of radiocarbon-depleted sediments over sediments that have more radiocarbon in them. And it's a footprint that looks like the plume on the seafloor. If you couple that with observations from the time about this plume underwater, I think it's pretty much a slam dunk."

Yet despite the spill's lingering legacy, it hasn't triggered a sea change in Washington. Congress has passed no new laws to address offshore-drilling safety since 2010, and last month the Obama administration proposed allowing oil rigs in parts of the Atlantic and Arctic oceans. Those plans are far from finalized, but critics say they suggest key lessons from 2010 remain unlearned five years later.

"This takes us in exactly the wrong direction," Natural Resources Defense Council director Peter Lehner said in a recent statement about the proposal. "It would expose the Eastern Seaboard, much of the Atlantic and most of the Arctic to the hazards of offshore drilling. It ignores the lessons of the disastrous BP blowout, the growing dangers of climate change and the promise of a clean energy future."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.