Ancient Arctic Mud Just Shattered a Climate Ice Myth
Evidence of algae growth and climate simulations reveal the Arctic had seasonal ice, not a permanent ice shelf, for much of the past 750,000 years. These insights challenge long-standing theories
For a long time, researchers have debated whether a thick, continuous ice shelf once blanketed the entire Arctic Ocean during the most extreme ice ages.
However, a new study published in Science Advances challenges this theory, finding no supporting evidence for a vast ice shelf approximately one kilometer thick.
Instead, the findings suggest that the Arctic Ocean was covered by seasonal sea ice, which would have allowed for patches of open water and conditions capable of sustaining life, even during the coldest periods of the last 750,000 years.
This discovery provides important clues about how the Arctic has previously responded to climatic changes and offers valuable context for anticipating future changes in the region.
Tiny traces of life in ancient mud
The research, conducted under the European Research Council Synergy Grant project Into the Blue – i2B, focused on sediment cores retrieved from the seafloor in the central Nordic Seas and the Yermak Plateau, located north of Svalbard.
These sediments contain microscopic chemical traces left behind by ancient algae that once inhabited the ocean. Some species of algae thrive only in open water, while others are adapted to live beneath seasonal sea ice that melts and reforms annually.
“Our sediment cores show that marine life was active even during the coldest times,” explained Jochen Knies, the study’s lead author from UiT The Arctic University of Norway and co-leader of the Into the Blue – i2B project. “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 kilometre-thick slab of ice.”
A crucial piece of evidence was the presence of a molecule called IP25, which is produced by algae that live in seasonal sea ice. Its consistent detection in the sediment layers suggests that the Arctic experienced recurring sea ice cycles, rather than being permanently sealed by a solid ice cover.
Simulating ancient Arctic climates
To further investigate the geological evidence, the researchers ran simulations using the AWI Earth System Model, a high-resolution climate model. They focused on two of the coldest periods in recent Earth history: the Last Glacial Maximum around 21,000 years ago and another major glaciation roughly 140,000 years ago, when extensive ice sheets blanketed much of the Arctic.
“The models support what we found in the sediments,” said Knies. “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.”
The simulations also revealed that the ice cover was not constant. Instead, it varied with the seasons, creating ice-free areas where sunlight could penetrate the ocean surface and support marine life.
These findings reshape our understanding of historical Arctic environments and carry important implications for forecasting future climate trends. By learning how sea ice and ocean currents reacted to past climate extremes, scientists can enhance the accuracy of models predicting future changes in a warming world.
“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 this study, based at Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and co-lead of Into The Blue – i2B. “That matters when trying to anticipate how ice sheets and sea ice might behave in the future.”
Re-thinking the giant ice shelf theory
Some scientists have argued that features on the Arctic seafloor suggest that a huge, grounded ice shelf once covered the entire ocean. But this new study offers another explanation.
“There may have been short-lived ice shelves in some parts of the Arctic during especially severe cold phases,” said Knies. “But we don’t see any sign of a single, massive ice shelf that covered everything for thousands of years.”
One possible exception could have occurred about 650,000 years ago, when biological activity in the sediment record dropped sharply. But even then, the evidence points to a temporary event, not a long-lasting frozen lid over the Arctic.
Understanding the Arctic’s future
The study sheds new light on how the Arctic has behaved under extreme conditions in the past. Knowing how sea ice and ocean circulation responded to past climate shifts helps scientists understand what might lie ahead.
“These past patterns help us understand what’s possible in future scenarios,” said Knies. “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.”
Reference
“Seasonal sea ice characterized the glacial Arctic-Atlantic gateway over the past 750,000 years” by Jochen Knies, Lukas Smik, Pengyang Song, Monica Winsborrow, Henning A. Bauch, Gerrit Lohmann and Simon T. Belt, 4 July 2025, Science Advances. DOI: 10.1126/sciadv.adu7681
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Jerry Krause
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Hi PSI Readers,
The crystal structure of ice, which causes ice to float on liquid water, makes ice a fairly good thermal insulator. So, no matter how cold the atmosphere gets, the ice’s natural maximum thickness does not exceed even 10 feet; unless the wind blows an ice sheet over a sheet being held fast by a solid shore line. For water, upon which the ice floats, is being warmed at the water’s base by the geothermal activity in the earth’s interior which produces volcanos. And one doesn’t need to have a college degree to be aware of these natural observations.
Have a good day
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Matt Holl
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The Gulf Stream will also be pushing warmer oceanic water to circulate under the sea ice, along with any geothermal heating.
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Jerry Krause
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Hi PSI Editor and Matt,
I had posted a comment which has disappeared. So I repeat my comment. Matt, I believe you can answer “How were these Northern Hemisphere’s glaciers formed?” .
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Jerry Krause
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Hi Matt,
What causes (drives) the Gulf Stream? I’m discovering we are using words without defining or describing them. For example: Matter is composed of atoms. Describe (define) what a carbon atom is.
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Jerry Krause
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Hi Matt,
Things are not working. So this is a test.
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Jerry Krause
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Hi Matt,
How were glaciers anywhere formed? You should be able to answer this!!!
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Jerry Krause
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HI PSI Editor,
Less than two hours ago I made a comet here and saw that it was posted. Now when I come back to make another comment here there is no previous comment. This is a serious problem.
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Jerry Krause
| #
Hi PSI Editor,
Now after submitting the 2nd comment the first comet appears. But I don’t know what I am going to find when I leave this article to read another article and than come back to this article. Just trying to describe the details. Yesterday my problems were with a different article and I was making many mistakes. But today I have not made any functional mistakes.
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Jerry Krause
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Hi PSI Editor,
Now when I went away and left PSI and then came back the notice is No Comets but when I click it all my previous comments are there.
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Jerry Krause
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Hi PSI Readers,
I have reread this article and I now see the authors of the study seem to conclude that which I concluded for different reasoning: which is the thick northern hemisphere’s glaciers were not formed by freezing the Arctic Ocean. Which poses the question: How were these Northern Hemisphere’s glaciers formed?
Have a good day.
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Jerry Krause
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Hi PSI Readers and PSI Editors,
The question which hasn’t been answered is “How were these Northern Hemisphere’s glaciers formed?” Is anyone interested in proposing a possible answer to this question? I have a possible NEW theory but I don’t want to waste my time typing my theory if no one here is interested.
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Aaron
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it was god silly
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Whokoo
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Godzilla was not even there at the time.
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Jerry Krause
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Hi PSI Readers,
AI seems to not know what Louis Agassiz claimed was his greatest achievement: That he taught some students to SEE.
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Jerry Krause
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Hi PSI Editor,
Now when I went away and left PSI and then came back the notice is 7 Commets but when I click the notice there are only 6 commets..
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