A new study published in Science Advances is upending decades of scientific belief about the history of the Arctic. Contrary to the long-held theory that a massive, kilometer-thick ice shelf blanketed the Arctic Ocean during the coldest ice ages, new evidence suggests that the Arctic was never fully frozen. By analyzing ancient ocean mud and conducting advanced climate simulations, researchers found that the region was instead covered by seasonal sea ice that melted and reformed annually—leaving pockets of open water and enabling marine life to thrive even during extreme glacial periods. This discovery challenges long-held assumptions in polar science and may fundamentally reshape how we interpret past and future climate dynamics.
Evidence Buried in Ancient Mud
The findings are based on sediment cores taken from the central Nordic Seas and the Yermak Plateau, north of Svalbard. These seafloor samples contain molecular traces left by ancient algae, particularly a molecule called IP25. This compound is produced only by algae that live in seasonal sea ice, not beneath a permanent, thick ice sheet. Its consistent presence throughout the layers suggests that the Arctic saw repeated freeze-thaw cycles over the past 750,000 years.
As marine geologist Jochen Knies, the study’s lead author from UiT The Arctic University of Norway, explained:
“Our sediment cores show that marine life was active even during the coldest times. 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.”
This evidence directly challenges the theory that the entire Arctic Ocean was sealed off from sunlight and atmosphere by a giant, continuous ice lid for tens or hundreds of thousands of years. Instead, even at the height of glacial periods, sunlight reached the ocean surface—allowing photosynthesis and marine ecosystems to persist.
Climate Simulations Validate the Biological Evidence
To strengthen their case, the research team used the AWI Earth System Model, a high-resolution climate model capable of recreating conditions during some of Earth’s coldest moments. They focused on two particularly extreme periods: the Last Glacial Maximum about 21,000 years ago and another major glaciation around 140,000 years ago.
In both simulations, the Arctic Ocean was not completely sealed off. Instead, the inflow of relatively warm Atlantic waters into the Arctic gateway helped prevent full-scale freezing. This allowed seasonal variation in the sea ice and supported the idea that pockets of open water existed even during deep glaciation.
Gerrit Lohmann, co-author of the study from the Alfred Wegener Institute, emphasized the importance of these findings:
“These reconstructions help us understand what’s possible—and what’s not—when it comes to ice cover and ocean dynamics. That matters when trying to anticipate how ice sheets and sea ice might behave in the future.”
The simulations backed up what the mud samples already showed: a polar ocean with dynamic, shifting ice—not a single, unbroken sheet of ancient ice.
Questioning the Mega Ice Shelf Theory
For years, some scientists argued that the Arctic seafloor showed signs of a massive ice shelf once grounded across the entire ocean. But the new data presents a much more nuanced picture. According to the researchers, while short-lived ice shelves may have existed in localized regions during exceptionally cold periods, there’s no sign of a permanent, continent-sized sheet.
“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.”
The only exception might have occurred around 650,000 years ago, when there’s a sharp drop in biological activity recorded in the sediment layers. Even then, the researchers believe this was a temporary freeze—not a permanent shift. The absence of long-lasting ice shelf signatures in both biological and climate data provides a strong case for a more dynamic and open Arctic Ocean.
What This Means for the Future of the Arctic
This study does more than rewrite ancient history. It gives scientists a better framework to understand how the Arctic behaves under extreme stress—and what that might mean as today’s Arctic warms at unprecedented rates. If the region maintained open waters and ecosystem activity during the harshest glaciations, it shows a level of resilience that could influence how we model future changes in the region.
“These past patterns help us understand what’s possible in future scenarios,” Knies added. “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 warms twice as fast as the rest of the planet, these insights could help researchers predict when sea ice might collapse or when ocean currents might shift—events with far-reaching implications for the global climate system. The evidence not only reshapes our view of the past but also enhances our tools for forecasting what lies ahead.
The picture emerging from ancient mud and high-powered models is not one of a frozen wasteland, but of an Arctic that, even in its coldest moments, pulsed with seasonal life.
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