Study rules out ancient bursts of seafloor methane emissions

Measurements made from the largest Greenland ice sample ever analyzed have confirmed that an unusual rise in atmospheric methane levels about 12,000 years ago was not the result of a catastrophic release of seafloor "hydrate deposits," as some scientists had feared. The findings, to be published Friday in the journal Science, are good news for those who have worried that this unusual mechanism of releasing methane into the atmosphere might provide a serious reinforcement to global warming at some point in the future.

The five-year project was funded by the National Science Foundation, American Chemical Society and other agencies.

It now appears almost certain that the major methane increases that occurred near the end of the last Ice Age were due to the growth of wetlands and the methane releases associated with that, which occurred shortly after some significant warming in the Northern Hemisphere. They did not come from sudden bursts of methane trapped in deep seafloor deposits.

The newest conclusions were made possible by identification of some ancient ice exposed at the edge of a Greenland ice sheet, and samples of it cut with chain saws that totaled thousands of pounds.

"To get enough air trapped in ice to do the types of measurements we needed, it took some of the largest ice samples ever worked on," said Edward Brook, a professor of geosciences at Oregon State University, and international expert on using ice samples to explore ancient climate.

"The test results were unequivocal, but it was a lot of heavy lifting," Brook said. "It was like working in a quarry. We could have used some help from the OSU football team."

Methane, and the possible sources of it, is a significant concern to scientists because it is a potent greenhouse gas. It has natural sources in places like wetlands and permafrost, and its concentration has more than doubled since the Industrial Revolution from human activities such as natural gas exploration, landfills and agriculture. Natural gas used for home heating is composed mostly of methane.

But more hidden, and potentially of much greater concern, are massive deposits of methane buried beneath the sea in solid hydrate deposits, where cold temperatures and pressure supposedly keep it stable and unable to enter the atmosphere in large amounts. There have been concerns that this methane might be released suddenly by warming of ocean waters or other causes. These huge deposits of methane hold more carbon in them than all the known oil and gas fields on Earth.

If only 10 percent of that seafloor methane were to be released in a few years, it could be the equivalent of a 10-fold increase in the level of atmospheric carbon dioxide, the researchers said in their report. And 12,000 years ago, methane levels went up 50 percent in less than 200 years, according to studies by Brook and others. Researchers wanted to know why.

"There are hundreds to thousands of times more methane trapped in seafloor deposits than there is in the atmosphere, and it's important that we know whether it's stable and is going to stay there or not," Brook said. "That's a pretty serious issue."

To test whether the seafloor deposits had been the source of the large methane increase thousands of years ago researchers measured levels of carbon 14, an isotope of carbon, from the Greenland ice samples. The seafloor deposits are old and have very little carbon 14 in them. Based on the results of those measurements, the scientists were able to determine whether the methane increases 12,000 years ago were linked to seafloor deposits or not.

"The data made it pretty clear that seafloor methane hydrates had little to do with the increase in methane thousands of years ago," Brook said. "This largely rules out these deposits either as a cause of the warming then or a feedback mechanism to it, and it indicates the deposits were stable at that point in time. The increased methane must have come from larger or more productive wetlands that occurred when the climate warmed."

Researchers now hope to do similar experiments in Antarctica to verify the results of this study, Brook said.

Source: Oregon State University