Researchers reveal the dynamic processes that sculpt the Arctic seafloor

MBARI researchers, working alongside a team of international collaborators, have discovered large underwater ice formations at the edge of Canada’s Beaufort Sea, located in a remote region of the Arctic. This discovery reveals an unexpected mechanism for the continued formation of underwater permafrost ice. The results are published in the Geophysical Research Journal: Earth Surface.

In a previous MBARI study, researchers observed huge craters on the seafloor in this area, attributed to the thawing of ancient permafrost submerged underwater.

While exploring the sides of these craters during a subsequent expedition, MBARI researchers and collaborators from the Korea Polar Research Institute (KOPRI), the Korea Institute of Geosciences and Mineral Resources, the Geological Survey of Canada and the United States Naval Research Laboratory observed exposed layers. of underwater permafrost ice.

The newly discovered ice layers are not the same as ancient permafrost formed during the last ice age, but were instead created under current conditions. This ice is produced when deeper layers of ancient underwater permafrost melt, creating brackish groundwater that rises and refreezes as it approaches the seafloor, where the ambient temperature is around -1 .4 degrees Celsius (29.5 degrees Fahrenheit).

The complex morphology of the seafloor in this region of the Arctic tells a story that involves both the melting of ancient permafrost that was submerged beneath the sea long ago and the disfigurement of the modern seafloor that occurs when the released water refreezes again.

After the last ice age, sea levels rose and covered the ancient permafrost of the Arctic shelf. The base of this ancient permafrost slowly warms and thaws due to heat escaping from the Earth. Much older, slower climate changes are contributing to the melting of this Arctic underwater permafrost, not human-caused climate change.

When this water migrates to the colder seabed, it freezes. Frozen ice lifts ridges and mounds. Seawater seeps into the blistered surface of the seabed, melting layers of ice and leaving behind huge sinkholes. The dynamic interaction between large changes in salinity and small changes in temperature near the seafloor drives this process.

“Our work shows that permafrost ice actively forms and breaks down near the seafloor over large areas, creating a dynamic underwater landscape with massive sinkholes and large ice mounds covered in sediment,” said Charlie Paull , geologist at MBARI and lead author. of the study.

“These dramatic and ongoing seafloor changes have enormous implications for policymakers making decisions about underwater infrastructure in the Arctic.”

Since 2003, MBARI has been part of an international collaboration aimed at studying the seabed at the edge of the Canadian Arctic shelf. This isolated area has only recently become accessible to scientists as warmer temperatures have caused sea ice to retreat.

A mapping study by Canadian researchers in 2010 discovered for the first time the region’s particularly rugged marine terrain. In 2013, MBARI researchers and their collaborators conducted the first high-resolution mapping studies in this region. Using an MBARI autonomous underwater vehicle (AUV), the research team documented the seafloor terrain in detail.

Five mapping surveys — two conducted by Canadian research vessels and three with MBARI’s advanced underwater technology — in this area over a 12-year period revealed 65 newly formed craters on the seafloor. The largest crater was the size of a city block made up of six-story buildings.

In 2022, the team returned to the Arctic aboard KOPRI’s icebreaking research vessel Araon. They first used MBARI’s two seafloor-mapping AUVs to identify recently formed craters. Then, they conducted visual surveys in these specific craters with MBARI’s MiniROV. This portable remote-controlled vehicle developed by MBARI engineers can be configured for various scientific missions.

Equipped with cameras and sampling equipment, it is an integral part of the study of the Arctic seabed. While exploring the seafloor with the MiniROV, the researchers observed ice formations inside two large, recently formed seafloor craters.

Isotopic analysis of these formations and samples of the surrounding seafloor sediments confirmed that the ice originated from brackish groundwater, created in part by the melting of ancient permafrost rising up through the seafloor. Rising groundwater refreezes near the seafloor, forming extensive layers of ice beneath the seafloor that blister the seafloor, producing ice mounds.

Minor variations in temperature and salinity cause time lags between the freezing of rising brackish groundwater and the melting of ice layers near the seafloor. These ongoing processes work in tandem to create a spectacular underwater landscape consisting of numerous depressions and ice-filled mounds of varying ages.

“These results upend our assumptions about underwater permafrost,” Paull said. “We previously thought that all underwater permafrost was a remnant of the last ice age, but we learned that ice from underwater permafrost also actively forms and decays on the modern seafloor.”

The process that creates these ice formations beneath the seabed has not been previously considered and can occur where bottom water temperatures are below zero degrees Celsius.

“This finding means that the techniques we have previously used to locate underwater permafrost do not work for the types of near-seafloor ice that we have recently discovered exist in the Arctic. We now need to re-examine where the permafrost could exist beneath the Arctic shelf,” Paull said.