When hydrocarbons meet water at low temperature and high pressure, their molecules can become trapped inside icelike cages (see the article by Wendy Mao, Carolyn Koh, and Dendy Sloan, Physics Today, October 2007, page 42). The vast majority of such gas hydrates store methane and form in marine sediments on continental shelves. In 1993 geoscientists Anders Solheim and Anders Elverhøi discovered a handful of giant underwater craters, some as wide as a kilometer, at the bottom of the Barents Sea. The pair speculated that the giants might have been created during methane releases when gas hydrates destabilized at the end of the most recent ice age.
When Karin Andreassen of the Centre for Arctic Gas Hydrate, Environment, and Climate (CAGE) revisited the area nearly a quarter century later, she was surprised to find it pitted with more than a hundred of the giants, shown in the figure. Andreassen and her CAGE colleagues have now published an account of the new, larger survey, which bears out the earlier speculation with a unified theory of how the craters formed. The team used high-resolution bathymetry to map the seafloor, sound waves to image methane seeps rising from it, and seismic waves to image the bedrock below. To unravel the craters’ origins, the researchers incorporated their data into a numerical model of the evolution of the hydrates’ stability under changing thermodynamic conditions. Subject to the crushing pressure of an ice sheet 30 000 to 20 000 years ago, free methane migrated upward through an underground network of cracks and fissures, only to accumulate and freeze in place as gas hydrates under the seafloor. As the ice sheet retreated, the seafloor’s slow rebound began a long period of depressurization and warming. During that time, the layer of methane hydrates progressively thinned. Eventually the seafloor, whose underlying veneer of hydrates blocked the escape of deeper free gas, became thin enough to bow upward into large mounds, or pingos. Around 12 000 years ago, the gas-filled pingos blew their tops.
The CAGE researchers propose that those processes were likely widespread across previously glaciated petroleum fields and that they may recur if gas hydrate reservoirs destabilize under contemporary ice sheets. But whether the methane—a potent greenhouse gas—actually reaches the atmosphere after a blowout remains an open question. (K. Andreassen et al., Science 356, 948, 2017.)
