Ancient Fault System Feeding Methane to Enormous Craters on Arctic Ocean Seafloor Discovered

A 250-million-year-old fault system supplying methane to enormous craters that line the floor of the Arctic Ocean has been discovered.

Hundreds of craters, with some over 3,000 feet wide, were uncovered by scientists in 2017 when they were mapping the floor of the Barents Sea. These craters were formed by huge methane explosions thousands of years earlier, at the end of the last ice age. The region was once covered in an ice sheet, weighing down the land below. As the planet warmed and the ice retreated, methane reservoirs deep beneath the ocean surface became unstable and moved upwards, creating enormous mounds on the seafloor. Eventually, these mounds exploded, leaving a crater behind.

Researchers also found unexploded methane mounds on the seafloor, some over 1,600 feet wide.

While researchers knew methane was seeping from the seafloor in this area, where the methane came from was unknown. In a study published in Scientific Reports, researchers led by Malin Waage, from the Arctic University of Norway, have now found what they believe to be the source of the methane creating these mounds and subsequent craters.

The team used high-resolution 3D seismic data to look at what was going on beneath the craters. Their findings showed that the craters were within a fault zone dating back around 250 million years. "Fault intersections and fault planes typically define the crater perimeters," the team wrote. They say the fault system is responsible for the size and shape of the craters, with the methane rising up through the cracks in the bedrock.

Previously researchers had suggested global warming at the end of the last ice age had caused the instability that led to the methane explosions. However, the latest findings "add several layers to that picture," Waage said in a statement. "We now see that there has been a structural weakness beneath these giant craters, for much longer than the last 20,000 years."

Researchers say similar processes may lead to methane explosions and crater formation in other areas where ice is retreating in the same way it happened over the Barents Sea thousands of years ago. They also say a wider region needs to be mapped to fully understand how far the fault system extends. "Our 3D survey covered approximately 20 percent of the entire crater area," Waage said. If a similar fault system exists across a broader area, it could pose a threat to marine operations, she said.

Professor Tim Minshull, from the University of Southampton, U.K., who has studied similar seabed features elsewhere but was not involved in the latest research, said that the study is significant to understanding how a common type of seafloor feature can be formed, as well as when and how gas can escape from the seafloor.

"Seafloor craters such as those described in the paper have been observed in a variety of locations around the world, and precisely how they form is a bit of a puzzle," he told Newsweek in an email. "There is a general understanding that there is a depression because something has been removed from underneath, but usually we have insufficient information to say much more. At this location, there is added interest because of the proposed relationship between gas escape and the retreat of an ice sheet—a phenomenon that could repeat in the future.

"The work also has some practical significance—if we are going in future to store beneath the seabed some of the carbon dioxide that we generate, to choose the right locations we need to understand where and how it might escape."

Waage said the team is now collecting and analyzing data from a region in the Barents Sea that has similar crater structures, allowing them to produce a larger and more detailed map of the fault. "There is still very much that we don't know about this system," she said.