The Arctic Ocean remained open to life during the Ice Ages

Madrid, July 4 (EFE).- In the past, the Arctic was covered by a layer of seasonal sea ice that supported life even during the coldest periods of the last 750,000 years. The discovery will help us understand how this ocean responded to climate change in the past and how it might do so in the future.

For years, scientists have debated whether a giant, thick ice shelf once covered the entire Arctic Ocean during the coldest ice ages, but the new study challenges this idea, as its authors found no evidence of the presence of a massive ice shelf roughly a kilometer across.

The study, published this Friday in the journal Science Advances, was led by researcher Jochen Knies of the Arctic University of Norway.

As part of the Into the Blue - i2B project (funded by the European Research Council), the research team studied sediment cores collected from the seafloor of the central Nordic Seas and the Yermak Plateau, north of Svalbard.

These cores contain tiny chemical traces of algae that lived in the ocean long ago. Some of these algae only grow in open water, while others thrive under the seasonal sea ice that forms and melts each year.

"Our sediment cores show that marine life was active even during the coldest periods," says Knies, lead author of the study and co-director of the Into The Blue (i2B) project.

"That tells us there should be light and open water at the surface. You wouldn't see that if the entire Arctic were covered in a kilometer-thick layer of ice."

One of the key indicators the team looked for was a molecule called IP25, produced by algae that live on seasonal sea ice and whose regular appearance in sediments shows that sea ice appeared and disappeared with the seasons, rather than remaining frozen year-round.

Simulation of ancient Arctic climates

To test their findings based on the geological record, the team used a high-resolution computer model to simulate Arctic conditions during two particularly cold periods: the Last Glacial Maximum, about 21,000 years ago, and a deeper glaciation, about 140,000 years ago, when vast ice sheets covered much of the Arctic.

"The models support what we found in the sediments," Knies emphasizes. "Even during these extreme ice ages, warm Atlantic waters continued to flow into the Arctic inlet. This helped prevent some parts of the ocean from freezing completely."

The models also showed that the ice was not static and changed with the seasons, creating openings in the ice where light could reach the water and where life could continue to thrive.

This research not only redefines our view of past Arctic climates, but also has implications for climate predictions.

Understanding how sea ice and ocean circulation responded to past climate extremes can improve models that project future changes in a warming world.

"These reconstructions help us understand what is possible—and what isn't—with regard to ice sheet and ocean dynamics," says Gerrit Lohmann, co-author of this study and a researcher at the Helmholtz Centre for Polar and Marine Research (AWI).

"This is important when trying to anticipate how ice sheets and sea ice might behave in the future."

A gigantic ice shelf

Some scientists have argued that the characteristics of the Arctic seafloor suggest that a massive land-based ice shelf once covered the entire ocean. But this new study offers another explanation.

"It's possible that there were short-lived ice shelves in some parts of the Arctic during particularly severe cold spells, but we don't see any evidence of a single, massive ice shelf covering the entire Arctic for thousands of years," Knies emphasizes.

A possible exception could have occurred around 650,000 years ago, when biological activity in the sediment record declined dramatically, but even then, the evidence points to a temporary event, not a lasting ice sheet over the Arctic.