For years, the scientific community has engaged in a spirited debate over whether a massive ice shelf once blanketed the Arctic Ocean during the coldest ice ages. A recent study published in Science Advances is now challenging this long-held belief, suggesting instead that the Arctic Ocean remained partially open and life-sustaining, even during the harshest glacial periods of the last 750,000 years.
The research, led by the European Research Council’s Synergy Grant project Into the Blue—i2B, found no evidence of a colossal ~1km ice shelf. Instead, it suggests the presence of seasonal sea ice, allowing for open water and the continuation of marine life. This revelation offers critical insights into the historical climate resilience of the Arctic and its potential future responses to climate change.
Tiny Traces of Life in Ancient Sediments
The research team, based at UiT The Arctic University of Norway, analyzed sediment cores from the central Nordic Seas and Yermak Plateau, north of Svalbard. These cores contain chemical fingerprints from ancient algae, some of which only thrive in open water, while others prefer seasonal sea ice conditions.
“Our sediment cores show that marine life was active even during the coldest times,” said Jochen Knies, lead author of the study. “That tells us there must have been light and open water at the surface. You wouldn’t see that if the entire Arctic was locked under a kilometer-thick slab of ice.”
“The presence of the molecule IP 25, produced by algae living in seasonal sea ice, indicates that sea ice came and went with the seasons, rather than remaining frozen solid year-round.”
Simulating Ancient Arctic Climates
To corroborate their findings, the researchers used the AWI Earth System Model to simulate Arctic conditions during two notably cold periods: the Last Glacial Maximum around 21,000 years ago and a deeper freeze about 140,000 years ago. These models supported the sediment data, showing that warm Atlantic waters continued to flow into the Arctic, preventing a complete freeze.
“The models support what we found in the sediments,” Knies explained. “Even during these extreme glaciations, warm Atlantic water still flowed into the Arctic gateway. This helped keep some parts of the ocean from freezing over completely.”
“These reconstructions help us understand what’s possible—and what’s not—when it comes to ice cover and ocean dynamics,” said Gerrit Lohmann, co-author of the study.
Re-thinking the Giant Ice Shelf Theory
The study challenges the idea that a massive, grounded ice shelf covered the entire Arctic Ocean. While some scientists have suggested that features on the Arctic seafloor point to such a phenomenon, the new research offers an alternative explanation.
“There may have been short-lived ice shelves in some parts of the Arctic during especially severe cold phases,” Knies noted. “But we don’t see any sign of a single, massive ice shelf that covered everything for thousands of years.”
One potential exception occurred about 650,000 years ago when biological activity in the sediment record sharply declined. However, evidence suggests this was a temporary event rather than a prolonged frozen state.
Implications for Future Climate Predictions
This research not only reshapes our understanding of past Arctic climates but also holds significant implications for future climate predictions. By understanding how sea ice and ocean circulation responded to past climate extremes, scientists can improve models that project future changes in a warming world.
“These past patterns help us understand what’s possible in future scenarios,” Knies emphasized. “We need to know how the Arctic behaves under stress—and what tipping points to watch for—as the Arctic responds to a warming world.”
As the Arctic continues to change rapidly, insights from studies like this are crucial for anticipating future developments and preparing for potential impacts on global climate systems.
