Arctic Ocean was covered by kilometre-thick ice shelf and filled entirely with freshwater, ‘surprising’ new evidence reveals

For thousands of years the Arctic Ocean remained covered by an enormous ice sheet almost a kilometre thick, effectively cutting off routes to other bodies of water and resulting in the ocean filling entirely with freshwater, analysis by scientists in Germany has revealed.

Today, the Arctic Ocean is a key focus of the rapid heating defining the global climate crisis, with alarming declines in sea ice extent recorded with each passing decade.

But new research has revealed the staggering amounts of ice which accumulated during previous periods of glaciation.

Using sediment records the scientists demonstrated that this build up of ice and freshwater happened twice in the last 150,000 years: once about 60,000-70,000 years ago and also about 130,000-150,000 years ago.

“These results mean a real change to our understanding of the Arctic Ocean in glacial climates,” said Dr Walter Geibert, a geochemist at the Alfred Wegener Institute in Bremerhaven, and first author of the study.

“To our knowledge, this is the first time that a complete freshening of the Arctic Ocean and the Nordic Seas has been considered – happening not just once, but twice.”

The scientists said the huge amount of freshwater which could be quickly released into the North Atlantic when the ice receded could explain some rapid climate oscillations for which “no satisfying explanation has previously been found”.

It had not previously been known exactly how sea ice had behaved during ice ages. On land, evidence of glaciation is relatively easy to find. Erratic boulders, moraines and glacial valleys are the obvious landmarks of glaciers, but only few traces of vast ice shelves had been found so far in the Arctic Ocean.

But the ice sheets which extended across Europe and America help explain how the Arctic Ocean and the Nordic Seas could become an enormous freshwater pool.

Around 60,000 to 70,000 years ago, during a particularly cold part of the last glacial period, swathes of Northern Europe and North America were covered by ice sheets.

The European ice sheet spanned a distance of more than 5,000 kilometres (3,100 miles), from Ireland and Scotland via Scandinavia to the Eastern rim of the Kara Sea (Arctic Ocean).

In North America, large parts of Canada were buried under two large ice sheets. Greenland and parts of the Bering Sea coastline were glaciated too.

Across the Arctic Ocean it had not been known the extent to which the ice covering the continents continued to stretch northwards across the open water, but the new research indicates “large parts” of the ocean were covered by a “thick ice shield”, which could reach 900 metres in depth.

The research is based on the analysis of seabed cores, which showed a surprising lack of the uranium isotope Thorium.

Thorium is not found in the sediments from the two periods of glaciation, so salt water must have been absent.

"In saline sea water, the decay of naturally occurring uranium always results in the production of the isotope thorium-230,” said Dr Geibert.

“This substance accumulates at the sea floor, where it remains detectable for a very long time due to its half-life of 75,000 years.”

The team analysed 10 sediment cores from different parts of the Arctic Ocean, Fram Strait and the Nordic Seas and found the same pattern showing the missing Thorium.

“Here, its repeated and wide-spread absence is the giveaway that reveals to us what happened,” said co-author and micropalaeontologist Dr Jutta Wollenburg, also from the AWI.

“According to our knowledge, the only reasonable explanation for this pattern is that the Arctic Ocean was filled with freshwater twice in its younger history – in frozen and liquid form.”

The findings meant the scientists have examined how an ocean basin the size of the Arctic, which is connected by several straits to the North Atlantic and Pacific Ocean, can turn entirely fresh.

Professor Ruediger Stein, a geologist at AWI and at MARUM – the Centre for Marine Environmental Sciences at the University of Bremen, said: “Such a scenario is perceivable if we realise that in glacial periods, global sea levels were up to 130m lower than today, and ice masses in the Arctic may have restricted ocean circulation even further.”

This low sea level meant shallow connections like the Bering Strait or the sounds of the Canadian Archipelago were above sea level at the time, cutting off the connection with the Pacific Ocean entirely.

In the Nordic Seas, large icebergs or ice sheets extending onto the sea floor from the glaciated continent restricted the exchange of water masses.

And meanwhile the flow of glaciers, ice melt in summer, and rivers draining into the Arctic Ocean kept delivering huge amounts of fresh water to the system, which the scientists estimated could have been “at least 1200 cubic kilometres per year”.

A seabed formation which still exists and influences ocean currents today, known as the Greenland-Scotland Ridge, would have also played a key role in keeping the saline water in the Atlantic Ocean at bay.

As glaciers and ice melt fed into the Arctic Ocean, the flow through the narrow connections in the formation would have been from north to south, with the freshwater pushing the saltwater back, and keeping the Arctic salt-free.

“Once the mechanism of ice barriers failed, heavier saline water could fill the Arctic Ocean again,” Dr Geibert says.

“We believe that it could then quickly displace the lighter freshwater, resulting in a sudden discharge of the accumulated amount of freshwater over the shallow southern boundary of the Nordic Seas, the Greenland-Scotland-Ridge, into the North Atlantic.”

The idea that enormous amounts of freshwater were stored in the Arctic Ocean and could be rapidly released would help understanding the connection between a range of past climate fluctuations, the scientists said.

It would also offer an explanation for some apparent discrepancies between different ways of reconstructing past sea levels.

“The remains of coral reefs have pointed to a somewhat higher sea level in certain cold periods than reconstructions from Antarctic ice cores, or reconstructions from the calcareous shells of small marine organisms, would suggest,” said Dr Geibert.

“If we now accept that freshwater may not only have been stored in solid form on land, but some of it also in liquid form in the ocean, the different sea level reconstructions agree better and we can reconcile the location of the coral reefs with calculations of the freshwater budget.”

Freshwater release from the Arctic Ocean might also serve as an explanation for some abrupt climate change events during the last glacial period.

During such events, temperatures in Greenland could rise by up to 10C within a few years, and then only returning to the original cold glacial temperatures over the course of hundreds or thousands of years.

“We see an example here of a past Arctic climate tipping point of the Earth system. 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,” said Dr Geibert.

The research is published in the journal Nature.