Led by the ERC Synergy project Into the Blue – i2B, the team analysed sediment cores from the seafloor of the central Arctic Ocean and the Yermak Plateau north of Svalbard. These cores contain microscopic chemical fingerprints of algae that once lived in the ocean. Some of these algae only grow in open water; others thrive under seasonal sea ice that forms and melts each year. “Our sediment cores show that marine life was active even in the coldest times,” says Jochen Knies, lead author of the study and co-leader of Into the Blue – i2B at UiT The Arctic University of Norway. “That tells us there must have been open water at the surface. We wouldn’t see that if the entire Arctic was sealed off beneath a kilometre of ice.”

A key indicator the researchers looked for is a molecule called IP25, produced by algae living within seasonal sea ice. Its consistent presence in the sediments indicates that sea ice came and went with the seasons—instead of remaining frozen all year round. “There may have been short-lived ice shelves in some parts of the Arctic during extreme cold phases,” says Knies. “But we see no signs of a single, continuous ice shelf that blanketed the region for thousands of years.” One possible exception may have occurred around 650,000 years ago, when biological activity in the sediments drops significantly. Even then, the evidence points to a temporary event rather than a long-lasting Arctic ice lid.

To test the geological findings, the team used the AWI Earth system model to simulate Arctic conditions during two particularly cold periods—around 21,000 and 140,000 years ago—when large ice sheets covered much of Scandinavia and North America. “The models support what we see in the sediment record,” says Knies. “Even under extreme glacial conditions, warm Atlantic waters continued to flow into the Arctic, preventing a complete freeze-over.”

The simulations also show that the ice was not static. It shifted with the seasons, creating cracks and leads that let sunlight through—enabling life to survive. This research not only reshapes our understanding of past Arctic climate states but also has consequences for future climate projections. By studying how sea ice and ocean circulation responded to extreme climate changes in the past, scientists can better refine models that forecast future changes in a warming world.