Huge “sinkholes” – one of which could engulf an entire city block of six-story buildings – are emerging along the Arctic seafloor as submerged permafrost thaws and disrupts the area, scientists have discovered.
But even if man-made climate change increases the average temperatures in the Arcticthe thawing permafrost that creates them sinkholes seems to have another culprit – heated, slow-moving groundwater systems.
The Arctic permafrost at the bottom of Canada’s Beaufort Sea has been flooded for about 12,000 years, since the end of the last ice age, when meltwater from glaciers covered the region. Until now, the frozen seabed has been hidden from scientists’ eyes. This remote part of the Arctic has only recently become accessible to researchers on ships as climate change causes sea ice to recede, the researchers said.
Mapping of the seabed
With access to the area, study researchers relied on both shipborne sonar and an autonomous underwater vehicle (AUV) to conduct high-resolution bathymetric surveys of Canada’s Beaufort Sea.
“We know that major changes are occurring across the Arctic landscape, but this is the first time we can use technology to see that changes are also occurring offshore,” said Charlie Paull, geologist at Monterey Bay Aquarium Research Institutes (MBARI ), said in a statement. “While the underwater sinkholes we’ve discovered are the result of longer-term glacial-interglacial climate cycles, we know that the Arctic is warming faster than any other region on Earth,” added Paull, who co-led the research with Scott Dallimore of the Geological Survey of Canada and Natural Resources Canada, with an international team of researchers.
Related: The “last ice sheet” in the Arctic may not survive climate change
When researchers first began studying the region’s seafloor in 2010, they focused on the shelf edge and drop-off in Canada’s Beaufort Sea. About 180 kilometers offshore, they discovered a 95-kilometer band of unusually rough terrain along the sea floor. This stretch of seafloor once marked the edge of Pleistocene permafrost during the last Ice Age. The team wondered what was causing the rough texture of the seabed.
To understand how this roughness evolved over time and what might be causing it, the team conducted three more surveys, using AUVs in 2013 and 2017 and ship sonar in 2019. These snapshots of the same areas over time showed the formation of sheer walls and irregularly shaped depressions. The largest sinkhole-like crater is a whopping 738 feet (225 meters) long, 312 feet (95 meters) wide and 92 feet (28 meters) deep, the researchers said.
Here’s how the researchers propose how the circular holes form: As gradual warming thaws the permafrost beneath the Arctic shelf, an area that was once filled with solid (frozen ground) becomes liquid. The surface material then collapses into this fluid-filled cavity; These seafloor collapses occur intermittently over time, the researchers said.
Related: Why are there ice ages?
In some areas where the discharge of this warm groundwater is more limited, the seawater remains so cold at the bottom that rising groundwater freezes again once it reaches near-surface sediments. This frozen sediment expands and rises to form small conical mounds called pingos. These frozen hills, punctuated by the sinkholes, are responsible for the unusual roughness that researchers first discovered in their surveys.
The research also showed that the sinkholes expand over time. “The sustained magnification of some depressions observed in multiple surveys indicates that the development of these depressions is part of an ongoing process,” the researchers wrote in their research article, published online March 14 in the journal Proceedings of the National Academy of Sciences.
As a cause, the researchers said slow climate changes related to the end of the last ice age — which have been happening for thousands of years — are the likely culprits that started the cycle. Once the submerged permafrost begins to melt, the warmed groundwater from this melted permafrost rises along the bottom of the still-frozen permafrost, resulting in further thawing of the overlying sediments. The process continues in this manner to produce many divots.
Originally published on Live Science.
https://www.livescience.com/sinkholes-opening-arctic-seafloor Gaping “sinkholes” open up on the Arctic seabed.