As the last Ice Age ended, methane on the floor of the Barents Sea was released in a series of huge explosions. These may have resembled the releases that have recently produced craters in Siberia, but were prodigiously larger. The gasses released contributed to bringing the planet to its interglacial temperatures, and the explosions left an important legacy for today.
The Arctic Sea Floor contains enormous quantities of methane trapped in hydrates, enough to thoroughly cook the planet. Warming induced by either human activity or past cycles in the Earth's orbit, thaws some of these hydrates, releasing methane.
This can happen in relatively slow bubbling to the surface. According to Professor Karin Andreassen of the Arctic University of Norway, however, when 2-kilometers-thick (1.2 miles) ice sheets melted around 15,000 years ago, it was sometimes far more dramatic.
During the last Ice Age, a region of the polar North Atlantic, north of Scandanavia and 310-370 meters (1,000-1,200 feet) below sea level, was covered in thick ice, much like modern-day West Antarctica. “As climate warmed, and the ice sheet collapsed, enormous amounts of methane were abruptly released," Andreassen said in a statement. "This created massive craters that are still actively seeping methane."
The size of the current seepage, and whether it is accelerating, is one of the biggest questions in climate science.
Where the Siberian craters are 50-90 meters across (170-300 feet), those on the sea bed are oval shaped and up to 30 times as wide, meaning they have almost 1,000 times the area. Despite their huge size, underlying features and the passage of time make them difficult to make out on the sea floor.
New technology has made them easier to detect, however, revealing far more than were previously suspected. Within a 440 square kilometer (170 square mile) study zone, Andreassen explained, "We have focused on craters that are 300 meters to 1 kilometer wide, and have mapped approximately 100 craters of this size in the area. But there are also many hundred smaller ones, less than 300 meters wide that is."
When the ice sheets retreated, the methane, instead of distributing evenly on the seabed, became concentrated in mounds. Some of these mounds still survive, but in others, pressure built up, and where there was no mechanism for gradual release the only alternative was explosions large enough to create giant craters.
Although such events are rare, Andreassen thinks they may account for a large proportion of total hydrate methane release. Moreover, methane released at depth by slow degassing is usually captured or converted in the water column without reaching the atmosphere. Sudden outbursts, however, overwhelm the ocean's capacity to oxidize it, instead hitting the air like a giant fart – one that could make the whole world a less pleasant place to be.