‘Dirty blizzard’ carried Deepwater oil to the seafloor

A new study offers a clearer picture of the hidden effects of the 2010 Deepwater Horizon disaster—the largest marine oil spill in US history.

While oil was undetectable in surface waters a few weeks after the well was capped, contaminants lingered for months in subsurface water before sinking to the seafloor, researchers have discovered.

They say remnants of the oil, black carbon from burning oil slicks, and drilling contaminants combined with microscopic algae and other marine debris to descend in a “dirty blizzard” to the seafloor. The findings appear in the Proceedings of the National Academy of Sciences.

controlled fire in the Gulf of Mexico
Oil burns during a controlled fire in the Gulf of Mexico on May 6, 2010. The resulting black carbon joined other contaminants that eventually settled on the seafloor. (Credit: US Navy)

“This sedimentation is primarily due to marine snow—so-called dust bunnies of the ocean—composed of tiny phytoplankton, zooplankton feces, mucus, and other debris that sinks and carries with it suspended or dissolved substances,” says Uta Passow of UC Santa Barbara’s Marine Science Institute and co-lead author of the study.

The researchers used fine sediment traps deployed 4.5 miles from the capped well to collect marine snow consisting of diatoms—a common type of algae—and other matter slowly sinking through the water.

“It’s kind of like a smoking gun that pins down the source.”

They found contaminants clinging to the tiny particles, including black carbon left over from burning oil slicks, as well as barium and olefin, compounds found in the drilling mud used in efforts to contain the leaking well. Finding barium and black carbon was a surprise, as scientists previously thought these quickly sank to the bottom after being released into the ocean.

“The traps collected this material months after the leak was over,” Passow says. “The material stays in the water much longer than people think.”

And because drilling mud and oil are present whenever drilling is going on, contaminants could be winding up on the bottom in other situations as well, she notes.

Some researchers have contended that contaminants found on the seafloor come from natural oil seeps. But Passow and colleagues used various “fingerprinting” techniques to confirm the hydrocarbons in the sediment traps were derived from the spilled oil. 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 that pins down the source,” says co-lead author Beizhan Yan, an environmental chemist at Columbia University’s Lamont-Doherty Earth Observatory.

Sinking marine snow

Using data collected between August 2010 and October 2011, the team constructed a time series of petrochemical sedimentation following the spill. They found that black carbon continued to sink for two months after the fires were extinguished, and other contaminants such as bioactive barium from the drilling fluid sank for some five months. (Many of the fires were deliberately set to remove the oil.)

Sinking of these contaminants is negligible, until they are captured by marine snow, which provides the vehicle for downward transport. An exceptionally large bloom of diatoms that occurred in August and September 2010 provided such marine snow and in turn allowed the movement of contaminants from the water column to the seafloor.

Two million barrels of oil from Deepwater are still missing

The formation of sinking marine snow also was enhanced by the presence of the oil products, but it remains unclear whether the oil itself played a role in precipitating the bloom.

The work helps explain how contaminants from the spill that were dispersed in the water were concentrated at the seafloor, where they entered the food web, affecting fish and corals in deep waters.

“People in the past have not really ever considered oil reaching the seafloor, especially in very, very deep areas,” Passow says. “We now know how the oil gets down there in large amounts and affects the communities that live there.”

These results may prove helpful in planning future responses to spills, measuring their impact and containing damage to the environment and associated food systems, thereby ensuring food safety. Through a consortium of scientists and research institutions called the Ecosystem Impacts of Oil and Gas Inputs to the Gulf project, the team is currently studying what happens to the oil seeping naturally in the gulf.

Source: UC Santa Barbara

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