Why Did ‘Shocking' Amounts of BP Oil Fall to the Seafloor?

A newly discovered process that makes oil drop like stones may inspire better cleanup strategies, experts say.

Editor's Note: This is the third of four stories marking the fifth anniversary of the Deepwater Horizon oil spill in the Gulf of Mexico.

In April 2010, as the biggest oil spill in U.S. history stained beaches and plastered birds with thick goo, something totally unexpected was happening under the surface of the Gulf of Mexico (map).

Out of the public eye, a mass of ocean plants, creatures, and other detritus bigger than the state of Connecticut was speeding oil to the seafloor in "shockingly large" amounts—likely millions of gallons, according to David Hollander, a chemical oceanographer at the University of South Florida in St. Petersburg. (See pictures from the oil spill in National Geographic magazine.)

The phenomenon known as marine snow usually works like this: Phytoplankton that is stressed by oil or chemicals pump out a sticky mucus nicknamed sea snot. This substance glues together algae, feces, and other random bits into clumps that resemble falling snowflakes. (Related: "'Sea Snot' Explosion Caused by Gulf Oil Spill?")

But scientists say something different happened in 2010 after the blowout of BP’s Macondo well. Clay particles from the Mississippi River and dispersants used in cleanup efforts joined the mix, forming a big "dirty blizzard," Hollander says. Then heavy, oil-rich particles plummeted to the bottom of the Gulf like stones—"a perfect storm of events."

This mechanism for how oil reaches the seafloor, dubbed MOSSFA or marine oil snow sedimentation and flocculent accumulation, is new to science. But it's apparently not new to history. Scientists now think it may be commonplace.

The same process likely helped oil sink to the bottom of the Bay of Campiche (map) in the southern Gulf of Mexico during the 1979-1980 Ixtoc spill, according to new research. (Also see "Gulf Oil Spill 'Not Over': Dolphins, Turtles Dying in Record Numbers.") Sediment cores, taken between 1,600 and 6,500 feet (500 and 2,000 meters) below the bay's surface, still contain oil from the spill 35 years later.

Hollander says the decades that the oil has lingered in the Bay of Campiche suggests that the Gulf region also may have a long time to wait until the 2010 spilled oil is gone. That's a concern for tiny creatures on the seafloor, some of which were nearly wiped out.


Mysterious Seafloor Oil

In the aftermath of the 2010 spill, Hollander and colleagues sampling sediment were initially puzzled after the spill to find so much oil at the bottom of the Gulf.

They reported that up to 10 percent of the total spilled oil—which was officially estimated at 100 million gallons (380 million liters)—had settled onto the seafloor.

It usually takes weeks to months to reach the bottom. So "in some unknown way, oil was being packaged into marine snow and transported to the seafloor" within days of the disaster, says Uta Passow, a biological oceanographer at the University of California, Santa Barbara.

By recreating the recipe of sea snot, marine snow, oil, and clay in lab experiments, the team simulated the process on a smaller scale—and solved the mystery, Passow says.

Three published scientific papers have detailed how the marine snowstorm ferried oil to the seafloor. Bottom sediments contained byproducts of oil linked to the BP well and algae that live only at the Gulf's surface.

BP disputes those findings.

"While marine snow is a known phenomenon, there is no evidence that it has contributed to the deposition of 'pools of oil' on the seabed floor," said Geoff Morrell, BP's senior vice president for U.S. Communications and External Affairs.

"Extensive sampling and rigorous chemical analysis has shown that the only significant concentrations of Macondo oil that were found in sediments in the offshore environment were found in the vicinity of the wellhead," Morrell says. (See "First Gulf Oil Spill Anniversary: Resilience Amid Unknowns.")

Hollander disagrees, saying they used an "incredibly accurate" technology to date the oil in the sediments. "It's not our fantasy—we can see it, others have seen it," he says.

As for BP's claim that oil stayed near the well, that has "been fairly well dismissed," he adds. Last year, a team published evidence that a "plume" of hydrocarbons from the Macondo well contaminated about 1,200 square miles (3,200 square kilometers) of ocean floor.

Bad News for Bottom Dwellers

Once the oil settled onto the bottom of the northern Gulf of Mexico, some scientists say it nearly killed off a tiny bottom dweller.

Eight months after the spill, chemical oceanographer Patrick Schwing and colleagues compared single-cell creatures known as foraminifera in surface sediment collected at three sites in the Gulf of Mexico. Foraminifera, which make up the base of the Gulf’s food web, are often eaten by larger bottom dwellers such as bristle worms, amphipods, and small fish. (See an interactive of Gulf life.)

The density of foraminifera was 80 to 93 percent lower at the two oiled sites than at an unoiled site, according to the study. The decline was likely due to compounds called PAHs that are known to be toxic to foraminifera, and it has persisted for years, the scientists wrote.

But BP's Morrell says surveying two sites was insufficient to reach any conclusions about the organisms. In addition, he says the study provides no evidence that the oil was responsible for their decline or that the sediment was toxic.

"The researchers acknowledge they could not directly determine the cause of the density decline at the two sites,” he says.


Cleanup Made It Worse?

An influx of clay entered the Gulf when the response team diverted the Mississippi River to flush oil-clogged marshes. The team discovered that the clay particles likely contributed the most to sinking oil. That's because clay readily sticks to oil, creating dense particles that sink faster to the bottom. (See satellite pictures of the Gulf oil spill's evolution.)

The cleanup efforts may have unwittingly contributed to the process in other ways, too. Burning oil released PAHs that adhere easily to sea snot and enhanced the dirty blizzard. Dispersants also broke the oil into tiny parts that more easily lumped together. (Also see "Gulf Oil Cleanup Crews Trample Nesting Birds.")

"What you are trying to do as a responder is to make the least bad thing happen," says Nancy Kinner, director of the Coastal Research Response Center at the University of New Hampshire. That may mean choosing between damage to beaches, birds, and bottom dwellers.

Now that responders know about the new sedimentation process, they could control how much clay flows into the Gulf when flushing marshes.

"There's a lot of woulda, coulda, shoulda," Kinner says. “The question really for me is, 'Were something like that to happen again, would we have the capability to respond better?' I think the answer is yes."

Follow Christine Dell'Amore on Twitter and Google+.

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