The Arctic Ocean was once a freshwater lake

From lake to sea: the Arctic Ocean was a gigantic freshwater lake until around 36 million years ago. Because the Bering Land Bridge and another barrier between Greenland and Scotland sealed off the Arctic Ocean from the Atlantic and Pacific. It was only when these land bridges slowly sank into the sea that a brackish water area developed and then the salty Arctic Ocean, as geologists report in the journal "Nature Communications".

About 3,300 cubic kilometers of fresh water each flow into the Arctic Ocean - mainly from the melting glaciers of the Arctic. This corresponds to around a tenth of the annual water input of all rivers in the world combined. But the steady influx of salt water from the Atlantic and Pacific now ensures that the water mixes and the Arctic Ocean remains salty.

Sealed off from the rest of the seas

But that was not always the way sediment cores from near the North Pole revealed a few years ago: Researchers discovered large amounts of fossil freshwater algae in them. In the Eocene period around 56 to 34 million years ago, the Arctic Ocean must therefore have contained fresh water. Researchers suspect that at that time it looked more like a gigantic freshwater lake.

This was possible because two land bridges separated the Arctic Ocean from the rest of the seas: In the Pacific, the Bering Land Bridge jutted out of the water and connected Asia with North America. In the Atlantic, a land bridge formed by volcanoes stretched from Greenland to Scotland, preventing the exchange of water with the Arctic Ocean.

First brackish water lagoon ...

The Arctic Ocean only became a sea when the first land bridge sank far enough. When and how this took place, Michael Stark from the Alfred Wegener Institute for Polar and Marine Research and his colleagues have now reconstructed with the help of drill core data and a tectonic-climatic computer model.

It showed that the transition from the freshwater lake to the ocean took place in two steps. The first occurred around 36 to 31 million years ago when the land bridge between Greenland and Scotland sank by around 50 meters. This meant that at least a little salt water could flow in and a brackish water lagoon was formed.

... then sea of ​​salt water

Only then, 32 million years ago at the earliest, did the second step follow: The land bridge now sank below the uppermost wind-mixed water layer and thus enabled a stronger mixing of the water masses for the first time. "Only when the ocean ridge is below this wind-mixed layer can the heavier salty North Atlantic water flow relatively undisturbed through the passage into the Arctic," explains Stark.

Only now did the Arctic Ocean become a real, salty sea. "After the opening of the ocean passage between Greenland and Scotland had overcome this critical depth, the ocean with salinity arose from it, as we know it today from the Arctic," says Starkz. With this connection of the Arctic Ocean to the global ocean circulation, today's climate system also established itself.

The former land bridge between Greenland and Scotland still exists today. However, it is now about 500 meters deep. Iceland is the only part of it that is still above the water level. (Nature Communications, 2017; doi: 10.1038 / ncomms15681)

(Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, 06.06.2017 - NPO)

June 6, 2017