Gas in the Gulf
The first killer in the Gulf this spring was not oil. It was a bubble of methane gas that escaped from the Deepwater Horizon well, shot straight up the rig, and then ignited and exploded on the platform above. About half of the hydrocarbons that gushed from the seafloor over the next three months came in the form of natural gas -- the principal component of which is methane.
Much of this gas remained in the deep ocean, under high pressure, dissolved in giant plumes or crystallized in water molecules. What’s happened to all that natural gas, and what impact it could have on both the deep ocean and the atmosphere, remains one of the largest open questions about the BP disaster.
“Everything is about oily birds and oily sea turtles, and oily beaches and marshes,” says chemical oceanographer John Kessler of Texas A&M University. “All of those things are awful, and I don’t mean to downplay them at all, but methane was by far the most abundant molecule emitted from the wellhead -- and our understanding of where it’s going and what it will do to the environment is rather rudimentary.”
Marine biologist Lisa Suatoni agrees. A senior scientist at NRDC, Suatoni worries that many environmental studies have focused exclusively on oil and ignored natural gas -- leaving out half the picture. “Our understanding is being revised every day,” she says. “One of my biggest fears about the oil spill is that the science isn’t adequately funded, and this process is going to be stunted.”
Kessler co-authored a new study published today by the journal Science that offers a first glimpse of the fate of the methane and other natural gases unleashed into the Gulf. It appears that many are being ravenously consumed by bacteria, causing up to 70 percent of the oxygen losses observed in the early deepwater plumes. Propane and ethane appear to have been consumed first. “There are thousands of different compounds that comprise oil, and different bugs that have preferences for them,” explains biogeochemist David Valentine of the University of California, San Diego, the lead author of the study.
But once the most palatable gases are gone, the authors expect oxygen levels to rise again as fresh water is introduced by moving sea currents. That could at least potentially alleviate concerns that the methane would cause large, long-term oxygen losses in the deep ocean.
Methane is the simplest hydrocarbon, with the least complex molecular structure. That means it’s also chemically the most stable and difficult to crack. Methane offers microbes no long molecular tail to grab onto, and no available bonds to exploit. It requires a high level of energy -- and oxygen -- to degrade. “Methane is notoriously slowly consumed,” says Kessler. “Most of the other hydrocarbons are going to be gone, but methane is going to stay a long time.”
Indeed, over the eleven-day period of their study, researchers found that methane was not being significantly consumed by microbes. It was only after other gases were degraded that the activity of methane-eaters began to pick up. It’s unclear whether they were inhibited by the other gases or simply slow to get going.
“It is a distinctive kind of organism that consumes methane,” explains Valentine. “Because of their lifestyle and the amount of energy they have to put into growing, I figured they would be slow -- I didn’t know how slow.”
The handful of scientists studying natural gas in the Gulf were initially afraid that the methane-eating microbes would deplete significant amounts of oxygen. Biogeochemist Samantha Joye of the University of Georgia, for one, anticipated that the “mind-boggling” quantities of methane she was measuring would take hundreds or thousands of days to degrade.
Although Valentine and his colleagues still expect a substantial loss of oxygen, they also anticipate that the ocean will help repair the damage, as underwater currents continually mix trillions of gallons of seawater, breaking up the methane plumes and bringing oxygen back to polluted areas.
“We observed plumes in four different places,” Valentine points out. “This pretty clearly shows there was a complex current pattern that was moving different things in different directions at different times.”
Aboard the RV Oceanus in the Gulf, Joye now agrees. “Physical mixing seems to be a lot faster than anybody anticipated that it would be,” she says. A fellow scientist aboard the boat cited oxygen measurements reported by NOAA that show a generalized, though modest, reduction in oxygen levels throughout the water column -- which may be evidence that methane is being more broadly distributed than anticipated.
“I think it’s probably being eaten up or actually coming up to the surface and being released,” says microbial ecologist Terry Hazen of the Lawrence Berkeley National Laboratory. “We’re going to have to do a lot of studies and try to figure that out.” (Though methane is a potent greenhouse gas, scientists do not believe the amount released into the Gulf is enough to impact climate.)
Some 50 miles away from Joye’s vessel, Valentine and Kessler are back to sampling the deepwater and looking for methane, this time aboard the NOAA ship Pisces. Valentine says it’s still unclear whether the methane will stick around for years or perhaps could already be gone. “Those are both still on the table,” he says. “It’s not so easy to predict what will happen... We’re really looking really hard, and it’s a big ocean out here.”
For now, methane may remain the biggest untold story of the Gulf -- and no one is sure how it will end.
