It has been found that the frozen ocean of the North (for how long could it be still named so?) has its origins in ... a lake. 20 million years ago, what is now the Arctic Ocean was just a very large lake, whose fresh water flew southwards through a narrow strait into the Atlantic.

But 18.2 million years ago, the tectonic plates started to move and the strait started to get larger. Slowly, during probably 750,000 years, salt water from the Atlantic entered the Arctic transforming it into the current ocean. A Swedish team led by Martin Jakobsson at Stockholm University in Sweden, analyzing the first core of sediments from the bottom of the Arctic Ocean extending for the past 53 million years, extracted in 2004, reached this conclusion.

The 5.6 m (19 ft) long segment dated 18.2 to 17.5 million years ago was different: it was located over a layer of gray silt containing freshwater fossils (pointing to a lake), and within it, the mud had three very distinct bands of color: one made of thin lines of black and gray silt, one of gray sediment and a brownish layer.

The thin black silt lines are rich in an organic matter which is not decomposed, from a period when the bottom of the lake was poorly oxygenated and the organic matter did not degrade.

"As the Fram Strait, wedged between Greenland to the west and the Norwegian archipelago of Svalbard to the east, began opening up 18.2 million years ago, the large Arctic lake went through an estuarine phase. We suspect that there was a flow very similar to the Black Sea, with fresh water flowing out of the Arctic on the surface, and heavier salt water flowing in underneath," said Jakobsson.

The heavier salt water lacked oxygen, settling on the bottom of the ancient lake, killing all organisms and leaving the layers of black anoxic silt. Still, this did not occur in a smooth way.

"This is why you have the stripes. It goes back and forth between lake stage and estuarine stage. It must have been driven by sea-level change. And then what happens is that as the strait gets wider, eventually it gets wide enough for the Earth's rotation to influence the flow, and you can get water on one side of the strait flowing in, and on the other side going out. That is when you get a much bigger flush into the Arctic Ocean." said Jakobsson.

This circular flow, fueled by the Coriolis effect, impeded the Atlantic waters to further slide under the Arctic waters: they entered the surface in the strait and oxygenated before getting down to bottom. This turned the sediments gray, due to oxygenation and the first marine fossils emerged.

The last, brownish silt is abundant in iron and manganese oxides, denoting high oxygen amounts and full of marine fossils.