Scientists monitoring levels of methane gas in the Gulf of Mexico following last year's Deepwater Horizon oil rig disaster say levels of the toxic biochemical compound have returned to normal. The methane was gobbled up by bacteria living in the Gulf waters in record time.
When BP's Deepwater Horizon offshore oil platform blew up last April in the Gulf of Mexico, it triggered a massive leakage of almost five million barrels of oil and the release of huge quantities of methane, a highly toxic hydrocarbon that's a constitutent of natural gas and is typically found packed together with oil and other hydrocarbons at drill sites.
But scientists say bacteria living in the warm Gulf waters consumed the methane, reducing it to normal, background levels in about four months, a record time that surprised researchers including John Kessler, a professor of chemical oceanography at Texas A&M University in College Station.
Kessler led a team of scientists conducting a study of methane levels in the Gulf of Mexico.
"Of what was emitted, natural gas was approximately 30 percent by weight of the material that was emitted," said Kessler. "And what we can say pretty conclusively at this point was that all of that natural gas, which includes methane, has been consumed at this point by microorganisms that live in the waters."
Kessler says that's not what scientists predicted last June, two months after the disaster, when their measurements showed large concentrations of methane molecules suspended in deep Gulf waters near the massive, flowing oil plume. Scientists figured it would probably take years for the methane to disappear.
Kessler says researchers found something unexpected on a second expedition in September to measure levels of the suspended hydrocarbon,
"Lo and behold we didn't get a time point we got an endpoint; that the methane had decomposed to near background levels," he said.
Four months after the spill, Kessler says the methane had dispersed over an area of just over one-million square kilometers.
Researchers sampling water from more than 200 locations found near normal levels of methane, a leading greenhouse gas. But tests conducted by scientists indicated little of it had been released into the atmosphere.
Instead, low oxygen levels in the water samples told the tale. Kessler explains that when some organisms eat, their oxygen levels go down.
"If methane just transitted to a different location, there would be a lack of methane but there would be normal levels of oxygen," said Kessler. "In fact, there was both a concurrent lack of methane and a reduction in oxygen. And when we summed up all of the oxygen lost from all of these 207 different stations, it came out to a number that was so large it could only be explained by the complete respiration of the emitted [spilled] methane."
Kessler says marine bacteria, which had been in fairly low concentrations within the first two months after the Deepwater Horizon disaster, suddenly bloomed, feeding on the methane.
Scientists say methane molecules are released in large volumes all of the time in nature, particularly along the sea floor through hydrocarbon seeps, hydrothermal vents and the decomposition of solid methane crystal deposits. But Kessler says the methane mop-up by microorganisms in the Gulf of Mexico is the fastest scientists have ever seen.
An article highlighting the role of bacteria in cleaning up the Gulf following the BP oil disaster is published this week in the journal Science.