It’s not that hard to understand the changes that have controlled the levels of carbon dioxide in the atmosphere during past climate changes. But methane (a potent greenhouse gas itself) does some mysterious things. A lot of it is secreted away in seafloor mud in the form of an odd substance called "methane hydrate"—methane gas in molecular cages of water ice that only survive at low temperature and high pressure.
The thawing of these methane hydrates would release the gas into the ocean and atmosphere. For several rapid climate change events in Earth’s history, this has been proposed as a possible source for sudden increase in greenhouse gasses.
Many questions remain unanswered about methane hydrates, including a rather big one: could our current climate change thaw some hydrates, contributing to further warming in the near future? After all, there are places where plumes of methane bubbles can be seen rising from the seafloor today. Some of those plumes have been active for thousands of years, and (counterintuitively) they might even have the net effect of removing greenhouse gases. But other areas have inspired concern about the risk of accelerated warming.
Svalbard sea floor
While methane hydrates can form on the seafloor, the underside of thick glaciers can also be suitable. The Arctic seafloor of the Barents Sea, between Scandinavia and Svalbard, hosted an ice sheet during the last “ice age,” when sea level was considerably lower. A pair of recent studies examined odd bumps and scars at the bottom of the Barents Sea to work out a story of glaciers—and methane—come and gone.
First up is a study of a site near the Svalbard coast led by Pavel Serov of the University of Tromsø. There, researchers found a cluster of mounds, each a few hundred meters across and 10 to 30 meters tall, with a stream of methane bubbles emanating from the center of each mound. Sediment cores showed that the mounds contain layers of methane hydrates and precipitated calcium carbonate, which is produced by the bacteria that break down methane. Measurements of the carbon isotopes in the methane show that it ultimately came from natural gas reservoirs associated with oil further beneath the seafloor.
The mounds bear a striking resemblance to something called a “pingo” that can form in permafrost areas. Pingos involve groundwater expanding as it freezes and pushing up the ground surface (although these seafloor mounds contain no water ice).
We know that this spot near the Svalbard coast was beneath nearly a kilometer of glacial ice around 20,000 years ago. To work out a timeline, the researchers used a model simulation.
There are a few factors to keep track of here. First, the presence of the glacial ice can create the pressure and temperature conditions for natural gas seeping up along faults to get trapped and accumulate as methane hydrate. Second, the weight of the ice sheet actually pushed the Earth’s surface downward considerably, and the elevation has been slowly rebounding since the ice retreated. And third, once the ice was gone, the area was subject to several different ocean currents of varying temperatures.
Hydrate and ice
In the model simulation, methane hydrate could happily accumulate beneath the glacier between about 35,000 and 20,000 years ago. When the ice sheet retreated and sea level rose, the seafloor was still depressed for a time. This increased the water depth and helped keep those hydrates happy. Along with the slow rebound of the seafloor, a couple climate events around 15,000 and 10,000 years ago shifted warmer ocean currents into the area. During those times, the hydrate should have thawed and bubbled off.
The researchers think the mounds formed around then. The freezing and thawing of methane hydrate around the points where methane was bubbling could have provided the expanding force to push up the mounds. (And we’ll come back to this in a minute... )
Along with a cold-water interval between those warm-water events, cooler water in the last few thousand years has restored a thin layer of methane hydrate in this area. So over the last 30,000 years, there have been cycles of methane accumulation and release. These mounds seem to be part of the story of the changes since the last ice age rather than the changes caused by the recent human-dominated climate.
The second study, led by Karin Andreassen of the University of Tromsø (and involving some of the same researchers), is actually very similar. This study focused on a spot a couple hundred kilometers east of the last one, though still between Svalbard and Norway. Here, instead of a smallish cluster of mounds, researchers mapped more than a hundred craters between 300 and 1,000 meters across and up to 30 meters deep. Many of the craters were pimpled with small mounds around their edges, and methane bubbled up from about 600 different points.
The researchers applied the same sorts of sampling and modeling here, producing a similar timeline. Methane hydrate accumulated beneath the ice sheet, but it thawed as the seafloor bounced upward and warmer waters arrived. But as the zone where methane hydrates could remain stable gradually thinned, the researchers say it probably became more and more concentrated just below the seabed. So while mounds were pushed up as in the other location, the process could have gone further. The mounds could have swelled until fractures in the sediment triggered a rapid, fizzy release of much of the methane hydrate—in essence, blowing out a crater where the mound once stood.
The timing is especially clear here, because the craters had to form after the glacier had retreated from the spot, but long scars left by the keels of huge icebergs scrape some of them. That means the craters formed before the last of the ice sheet that once covered the region had totally disappeared—meaning they formed between 15,000 and 11,000 years ago.
Beyond an explanation for a weird-looking patch of seafloor, the interesting thing here is that the methane from these rapid blowouts would probably have reached the atmosphere. The methane we see gradually bubbling today dissolves into the water and is generally broken down by bacteria before it gets up to the surface (although there are exceptions). These blowouts would be more like a firehose, releasing far more methane than the ecosystem can digest. These rare but violent events could potentially have played an outsized role in the rise of atmospheric methane after the end of the last ice age.
Turning to the future, there are still areas of Greenland and Antarctica where accumulated methane hydrate could be stored beneath the existing ice sheet. So as those ice sheets shrink, we need to understand what might get burped up.
Proceedings of the National Academy of Sciences, 2017. DOI: 10.1073/pnas.1619288114
Science, 2017. DOI: 10.1126/science.aal4500 (About DOIs).
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