During the last Ice Age, the landmasses around the Arctic Ocean were covered in ice kilometers high. The ocean itself had a much larger expanse of ice than the one that is rapidly disappearing today, probably at least twice as much. Yet evidence has now emerged that a small area north of Scandanavia remained ice-free for most of that time, allowing life to survive in what was otherwise a desert.
Areas of open ocean surrounded by sea or land ice are known as polynyas, and are created by warm water rising from the ocean depths, winds breaking up ice flows, or both. Dr Jochen Knies of the Geological Survey of Norway has found evidence that one persisted through the last Ice Age in the south-west Barents Sea, off northern Norway.
“We were looking for evidence of biological life in sediments at the bottom of the ocean. In doing so, we found that between the sea ice-covered oceans and the ice sheets on land, there must have been a small ice-free corridor that extended over hundreds of kilometres into the Arctic,” Knies said in a statement.
Using a sediment core taken from the area, Knies reports in Nature Communications that the polynya stayed open for at least 5,000 years. It eventually froze over around 17,500 years ago, ironically not because the world had become colder still, but because it had started to warm.
The first stirrings of a warmer world, brought on by slow changes to the Earth's orbit and axial tilt, initiated ice cap melting, and the estimated 670 billion tonnes (740 billion tons) a year of cold freshwater rushing into the oceans reduced salinity enough to make the waters freeze.
Just as modern polynyas provide havens for sea life that can't survive under thick ice, something similar occurred while part of the Barents Sea was open. Some of the algae that formed the base of the food chain are trapped in ocean sediments, revealing the polynya's presence to Knies. When the area finally froze and reopened, it took 2,000 years for the local ecosystem to recover.
Polynyas also play an important part in keeping the great conveyor belt of the ocean's currents going, with cold salty water sinking to the bottom of the oceans, drawing warmer waters in to replace it. During the Ice Age, when the whole system slowed down, the Barents polynya was probably particularly important in keeping things going.
The paper proposes the polynya was probably maintained by the strong winds from the neighboring ice cap and relatively warm upwellings from the Atlantic, while salt displaced from ice formation closer to shore acted like antifreeze.