Follow-up cruises in the Gulf of Mexico after the Deepwater Horizon oil spill was plugged found that microorganisms consumed all of the excess methane, reducing levels to background within about four months of the start of the spill, according to findings published today.
"It was remarkable," said John Kessler of Texas A&M University in College Station, lead author of the study, in Science. "We had gone out there assuming that there would be plenty of methane still there and the fact was that it was all gone."
The team had seen a different picture when they sampled the area in June 2010, before the July 15 sealing of the well. Methane levels were high, dissolved in plumes about two-thirds of the way to the sea floor, and decomposition rates were low.
"It seemed that methane would be there for a much longer time period, possibly several years," Kessler said.
But when the team returned in three cruises between Aug. 18 and Oct. 4, expecting to track the slow degradation of methane, they found that it all was gone. Concentrations had returned to background levels.
Methane-consuming microbes comprised the highest proportion researchers had ever seen in the overall microbial community, and the oxygen levels the team measured could only be explained by complete consumption of the methane. This evidence supported the idea that it was microbes that made the methane disappear, Kessler said.
"For the oil spill, it was a rather good story," he said, especially since methane was the largest component of the Deepwater Horizon spill.
The findings also provide insight into what might happen in a future natural methane release from the sea floor.
There are massive amounts of undersea methane. One type alone may hold double the carbon of all of the terrestrial fossil fuel, such as coal, oil and natural gas, including stores that could never be recovered. Methane is a powerful greenhouse gas, contributing 25 times more toward warming than carbon dioxide, kilogram for kilogram over a century.
Underwater methane releases have occurred in geologic history, sometimes as big burps of gas and sometimes as seeps lasting hundreds or thousands of years, Kessler said. Also, climate scientists have suggested that releases of vast stores of one type of water-associated methane -- an ice-like form called methane clathrate -- could happen as warming oceans allow some of these to melt.
While the team's findings suggest that methane-chomping microorganisms have the potential to consume large quantities of released methane, "We can't generalize these findings to the greater planet," he said.
For one thing, the Gulf of Mexico has a large number of natural gas and oil seeps occurring naturally. That means that there is a natural population of microorganisms ready to respond to the presence of methane. Such an ideal population may or may not be present at a future release site.
Also, "if you're at a more shallow site, that gas has the potential to bubble straight through to the atmosphere," Kessler said.
Indeed, another paper published today in Science finds that the amount of methane being released from freshwater sources around the globe may be much greater than was previously estimated for exactly this reason.
David Bastviken of Linköping University in Sweden and colleagues found that the amount of methane bubbling directly to the atmosphere from freshwater sediments may be playing a large, previously unidentified role.
"While the vegetation on land is taking up greenhouse gases, some of them are being released back from freshwater," Bastviken said. Indeed, his estimates of the total amount of methane released from freshwater offset the amount of carbon sequestered by land sources by 25 percent.
This suggests, for example, that forested land with freshwater should not be assumed to absorb as much net carbon as previously estimated.
"We should be very careful with local environments that act as sinks. They are precious," Bastviken said. But also, "we should be careful if we try to focus on preserving natural sinks instead of reducing fossil fuel emissions. We have to work on both fronts simultaneously."
"It must be very clear that this is not a new environmental threat. Lakes have always emitted methane. It's a natural process. We just haven't paid attention to it before," he added.
Methane is formed in freshwater by microorganisms degrading organic matter in the sediments in the absence of oxygen.
This finding does not increase the total estimated amount of methane in the atmosphere, Bastviken said. Rather it helps explain where some of the unaccounted-for methane is coming from.
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