The Arctic ocean was once a giant freshwater lake under half a mile of ice

The Arctic Ocean was once a pool of fresh water capped with an ice shelf half as thick as the Grand Canyon is deep

Tens of thousands of years ago, large sections of Northern Europe and North America were covered by ice sheets, but scientists have so far lacked the evidence to prove the existence of ancient ice sheets covering the Arctic Ocean.

Researchers spent years analysing the composition of marine sediment layers to find that evidence but their research found no traces of sea salt in the deposits. Instead, they found that in at least two very cold points in earths history, the Arctic ocean was freshwater.

The explanation for why the normally salty Arctic Ocean could change into a freshwater body lies in the build-up of huge icebergs and ice shelves that blocked the typical exchange of water. Cut off from other oceans and under vast ice sheets, freshwater accumulated, building up an entirely fresh Arctic Ocean.

According to the study published in the journal Nature, this vast body of freshwater existed for thousands of years until it spilled into the North Atlantic over a short period of time. The sudden influx of freshwater into the salty ocean could explain climate fluctuations in that period, such as the 14 to 18 degree increase Greenland experienced within a few years. It took Greenland hundreds of thousands of years to recover from the surge in temperature.

“We see an example here of a past Arctic climate tipping point of the Earth system,” says Walter Geibert, geochemist at the Alfred Wegener Institute and the study’s lead author.

“Now we need to investigate in more detail how these processes were interconnected, and evaluate how this new concept of the Arctic Ocean helps in closing further gaps in our knowledge, in particular in view of the risks of manmade climate change.”

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