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Experts spar over Gulf methane’s fate

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Wearing protective masks, Texas A&M’s John Kessler and David Valentine of the University of California at Santa Barbara stand in front of ground zero of the Deepwater Horizon Gulf oil spill in June 2010. Their research, published in the journal Science early this year, is the subject of renewed debate this week.

What happened to all the methane that was released into the ocean during last year’s Gulf of Mexico oil spill? Scientists are revisiting that question this week in dueling research papers.

The two papers appearing in this week’s issue of Science follow up on a study that was published in the same journal in January. Back then, a team of scientists reported that bacteria gobbled up the methane released during the Deepwater Horizon spill at a surprising rate. Experts estimate that 200,000 tons of methane were released during the spill, in addition to the more than 200 million gallons of oil. The researchers concluded that “nearly all” of the methane was consumed in the ocean before it reached the atmosphere. A rival group, led by University of Georgia marine ecologist Samantha Joye, takes issue with that conclusion in this week’s issue. Joye and her colleagues say too many uncertainties surround the observations. “I believe there is still a lot to learn about the environmental factors that regulate methane consumption in the Gulf’s waters and elsewhere,” Joye said in a news release. The researchers behind the earlier study defended their results in a response that was published alongside their critics’ comments. “The case is actually in pretty good shape right now,” said David Valentine, a geochemist at the University of California at Santa Barbara who was behind the earlier research as well as today’s response to Joye’s group. “They’re not countering the data, they’re not countering any sort of technical issues. They’re just really trying to argue interpretation without offering any alternative.” Why it matters Although the methane from the Gulf spill is pretty much gone by now, one way or the other, this isn’t an empty debate. Scientists say the fate of the Gulf of Mexico methane could hint at how Earth’s ecosystems might respond to future methane releases — for example, a meltdown of frozen deep-sea methane reserves caused by climate change. Methane is a greenhouse gas even more potent than carbon dioxide, and some fear that a massive atmospheric release could create a climate catastrophe. If methane-munching microbes could handle a meltdown, that might put that nightmare to rest.

Valentine and his colleagues concluded that the microbes did the trick by tracking ocean methane levels as well as reductions in oxygen levels. Such oxygen anomalies might indicate that the microbes were oxidizing the methane gas in order to digest it. But Joye and her colleagues suggested that the lower oxygen levels might have been caused by other factors, such as the “dead zone” phenomenon observed in the Gulf. Joye also pointed to studies indicating that considerable amounts of methane released from natural deep-sea vents are not consumed by microbes. “A range of data exists that shows a significant release of methane seeping out at the seafloor to the atmosphere, indicating that the microbial biofilter is not as effective,” she said in the news release. Valentine, however, said there was “absolutely no basis” for the dead-zone argument, because that kind of oxygen depletion occurs in shallow waters, not in the depths that he and his colleagues studied. They insist in their Science paper that “natural emission does not mimic” the Deepwater Horizon release of methane. Oily, gassy debateThis scientific back-and-forth is likely to go on for a while, just like the similar back-and-forth over what happened to the oil spilled in the Gulf. Joye happens to be involved in that debate as well, which similarly centers on how efficiently microbes removed the hydrocarbons that leaked into the sea. She contends there’s a lot more oil left behind than other researchers have claimed. Valentine said one of the factors behind the disagreement is a study that Joye and her colleagues published in Nature Geoscience in February. That study included an estimate of the hydrocarbon leakage that turned out to be too high, he said. “That sets a false stage for the argument and makes it seem as if there’s a much greater discrepancy than there is,” he told me.

Geochemist David Valentine discusses the Deepwater Horizon oil spill at the American Society for Microbiology’s 2011 annual meeting.

So how well can Mother Nature clean up after our oily, gassy messes? The answer to that question is still being contested, but eventually, both Joye and Valentine would like to get this debate resolved. “For me, it’s important to get this right because I’m trying to understand how nature works,” Valentine told me. “Undestanding how the ocean deals with these inputs is an important thing, in order to understand what happened in the past — and what will happen in the future.”

– Alan Boyle



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