All the methane that erupted in the Deepwater Horizon disaster has disappeared. What happened to it? One theory says it was eaten.
On April 20, 2010, a bubble of methane raced up the drill column of the Deepwater Horizon oil rig, bursting through the seals and barriers in its way. By the time it exploded on the platform's surface, it had grown to 164 times its original size. The rig, severed by the explosion, caught fire and sank two days later, allowing oil and gas to spew into the Gulf of Mexico for 83 long days.
This chaotic methane bubble was just a vanguard. With the well unsealed, substantial amounts of the gas were released into the gulf. This plume of dissolved methane should have lurked in the water for years, hanging around like a massive planetary fart. But by August, it had disappeared. On three separate trips through the gulf, John Kessler from Texas A&M University couldn't find any traces of the gas above background levels. He thinks he knows why – the methane was eaten by bacteria.
The gas pouring out of the broken well spurred the growth of bacteria called methanotrophs, which can break down methane as their only source of energy. They made short work of the gas. By the time that Kessler reached the gulf, just four months after the initial blowout, he found plenty of bacteria and precious little methane.
"Kessler's paper is very nice work," says Terry Hazen, who has studied how bacteria reacted to the Deepwater Horizon disaster. "I also suspected that methanotrophs would be active very quickly with this release of methane. We also have supporting data that we have not yet published… wish we had gotten ours out faster."
Kessler's discovery goes beyond last year's disaster. It also tells us about what happens to methane bubbles that naturally rise from the deep ocean, all over the world. Within the ocean floor, methane lies trapped within cages of ice called methane clathrates (or methane hydrates). Methane naturally escapes from these deposits, as well as from undersea vents and natural oil or gas seeps. If the gas reaches the atmosphere, it could have serious effects on the planet's climate. After all, methane is a potent greenhouse gas and undersea clathrates contain about twice as much carbon as all the fossil fuels in the world.
But first, the gas has to reach the atmosphere, and Kessler thinks that this is unlikely. While scientists can hardly release large bursts of methane to see what happens, they can rely on "natural experiments" like the unplanned surge that followed the Deepwater Horizon well. If that gas failed to make it past the gauntlet of underwater bacteria, then natural seeps would probably meet the same fate.
Some scientists have suggested that methane freed from clathrates could have contributed to the climate upheavals behind some of Earth's greatest extinctions events, including the day when life nearly died – the Permian-Triassic extinction. *
Greg Retallack from the University of Oregon initially backed this idea, but his mind has since changed. "I love this paper as it signals the final break with my long love affair with methane clathrate release as a cause of [the Permian-Triassic] mass extinction," he says. "It seems that marine methane cannot even make it out of the ocean because it's rapidly consumed there." **
Richard Camilli, who has studied the Deepwater Horizon oil plume, agrees. He says that Kessler's conclusions probably apply to natural methane leakages, as well as to other oil spills too. "[It] is likely to become a classic reference," he says.
In June, when David Valentine first described the plume of oil in the Gulf, he found that most of the methane was hanging in a layer between 800 and 1200 metres down. When Kessler arrived at the same area in late August, aboard the NOAA Ship Pisces, he found no traces of it. He did, however, find the degraded remnants of oil chemical and a suspiciously low amount of oxygen.
Many methane-eating bacteria use up oxygen to break down the gas, so Kessler reasoned that the microbes had done away with the methane. He even found the bacteria in question. In September, Kessler recovered several species of methane-eating bacteria from seven different sites. In some areas, these specialists made up a third of the local bacteria.
Back in June, the methane-eaters were nowhere to be found. Instead, Valentine and Hazen detected several other groups of bacteria that were breaking down other oil hydrocarbons, such as ethane and propane. They were first on the scene. Valentine predicted that other species would follow and mop up the methane, in "boom and bust cycles of bacterial succession". He was right. By September, the bacteria that dominated the gulf in June had all but vanished and the methane-eaters had taken their place. Even they were no longer active – they were just the remnants of a population that had bloomed in July and August.
Hazen adds that the methane-eaters "have the ability to degrade over 300 other compounds," and may have helped the clean-up efforts in the Gulf, beyond just breaking down the methane. He has been working on ways of turning this bacterial appetite to our advantage. In 1995, Hazen patented a process for seeding polluted groundwater with methane, to stimulate the growth of bacteria that would break down the other contaminants. Many companies around the world now use this process, enticing bacterial janitors with a methane menu.
* This scenario was the basis of one of the worst pieces about the Deepwater Horizon disaster – a frenzied article claiming that BP's drilling operation "may have triggered an irreversible, cascading geological Apocalypse that will culminate with the first mass extinction of life on Earth in many millions of years." It was ably debunked.
** It's possible that the Permian-Triassic event might have involved methane in such large amounts that it "would have overwhelmed the methanotrophs capable of handling the small Gulf spill." But Retallack can't find enough methane clathrates in the ocean to account for such a large plume. He still thinks that methane was still involved in the Permian extinction but now he suspects it came from disturbed coal seams.
Reference: Science http://dx.doi.org/10.1126/scienc…