Undersea pits tens of meters deep or striking hills with an ice core: permafrost also has a strong influence on the topography under water. American and Canadian researchers describe in PNAS how, during years of bottom observations in Canada’s Beaufort Sea, they mapped permafrost-related landscapes that emerged at the end of the last ice age some 12,000 years ago. This knowledge is important for estimating future undersea permafrost thaws.

Climate change is currently causing rapid, large-scale thaws of permafrost in the Arctic. The disappearance of this year-round frozen subsoil has consequences for both the landscape and the atmosphere: large amounts of greenhouse gas are released from the thawing soils. A lot of research is done on permafrost on land, but little is known about underwater permafrost. It is known from drilling in the Beaufort Sea, north of Alaska and Canada, that undersea permafrost can occur to depths of more than 600 meters.

A large depression in the bottom of the Beaufort Sea, created by melting permafrost.
Photo Eve Lundsten/MBAR

The authors of the PNASarticle became interested in the undersea permafrost during a 2010 research expedition that mapped the underwater topography of the Beaufort Sea with a multibeam sonar, which emits sound waves and can create a 3D map of the ocean floor. Between 120 and 200 meters of water depth, they saw a remarkably relief-rich topography, which they studied further during subsequent expeditions. The deep pits and high hills appeared to be the result of permafrost thaw and accretion, respectively

When the sea level rose about 12,000 years ago due to the melting of glaciers, this also had consequences for the temperature in the seabed: it increased. As a result, part of the undersea permafrost thawed, which led to the release of groundwater. This spread upward through the bottom and caused further thawing there. That thawed soil then partially collapsed, creating deep, circular depressions.

ice lens

However, the rising, warm groundwater did not thaw everywhere. The researchers also discovered underwater mounds ten meters high. Hills with an ice core: pingos. Remarkably enough, these also arise under the influence of rising groundwater. “It is only at higher concentrations that groundwater will thaw the permafrost layer, as in the depressions,” said lead author Charles Paull, a marine geologist at the California research institute MBARI, in an email. “In lower concentrations, such as with the pingos, the groundwater freezes during the rise and an ice lens is formed that pushes the bottom up.” The water around the pingos was cold enough, -1.4 degrees Celsius, to keep the permafrost layer intact.

Even today, the remaining undersea permafrost deep in the Beaufort Sea is protected by a ‘blanket’ of water at -1.4 degrees Celsius. “As a result, there is no accelerated thaw for the time being,” says Paull. “But that could change if the average annual soil water temperature rises.”

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