(ORDO NEWS) — Giant “karst sinkholes” – one of which could swallow up an entire city block with six-story buildings – are appearing on the Arctic seafloor as underwater permafrost thaws, scientists have found.
But even if anthropogenic climate change is causing average temperatures in the Arctic to rise, the thawing permafrost that creates these sinkholes seems to have another culprit – heated, slow-moving groundwater systems.
Arctic permafrost at the bottom of Canada’s Beaufort Sea has been submerged 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 the eyes of scientists. This remote part of the Arctic has only recently become accessible to explorers by ship as climate change causes sea ice to retreat, the researchers say.
With access to the area, the researchers used both shipborne sonar and an autonomous underwater vehicle (AUV) to conduct high-resolution bathymetric surveys in Canada’s Beaufort Sea.
“We know big changes are happening across the Arctic landscape, but this is the first time we’ve been able to apply technology to see changes happening at sea,” Charlie Poll, a geologist at the Monterey Aquarium Research Institute, said in a statement. Bay (MBARI).
“While the submarine sinkholes we have 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 Poll, who led the study with Scott Dallimore of the Geological Survey of Canada and the Department of Natural Resources Canada, as well as with an international team of researchers.
When researchers first began conducting seafloor surveys in the region in 2010, they focused on the shelf edge and slope in the Canadian Beaufort Sea.
Approximately 110 miles (180 km) from the coast, they spotted a 59 miles (95 km) long strip of unusually uneven seafloor topography. This stretch of seafloor once marked the boundary of the Pleistocene permafrost during the last ice age. The team wondered what caused such an uneven ocean floor.
To understand how this bump has evolved over time and what might be causing it, the team conducted three more studies using AUVs in 2013 and 2017, and then shipboard sonar in 2019.
These images of the same areas over time showed the appearance of irregularly shaped steep-sided depressions. The largest funnel-shaped crater is an impressive 738 feet (225 meters) long, 312 feet (95 meters) wide and 92 feet (28 meters) deep, the researchers say.
Here’s how the researchers surmise the round holes form: As the permafrost beneath the Arctic shelf thaws gradually, an area that was once filled with solid matter (frozen soil) becomes liquid. The surface material then collapses into this fluid-filled void; such seafloor collapses occur periodically over time, the researchers say.
In some areas, where the outflow of warm groundwater is more limited, seawater at the bottom remains cold enough that groundwater seeping upwards freezes when it reaches the near-surface sediments.
Frozen sediments expand, rising up and forming small conical mounds called pingos. These frozen mounds, interrupted by sinkholes, are the cause of the unusual irregularities that the researchers first noticed in the course of their research.
Research has also shown that sinkholes expand over time. “The continued increase in some of the troughs observed over several surveys indicates that the development of these troughs is part of an ongoing process,” the researchers write in their scientific paper, published March 14 in the journal Proceedings of the National Academy of Sciences.
As for the cause, researchers say the likely culprit for starting the cycle is the slow climate change associated with the end of the last ice age, which has been going on for thousands of years.
When submerged permafrost begins to thaw, heated groundwater from that thawed permafrost rises up the bed of the still-frozen permafrost, further thawing the sediments above. So the process continues, as a result of which many failures are formed.
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