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Arctic Ocean Once Covered by Ice and Filled With Fresh Water

Scientists’ discovery that fresh water, not salt water, once filled ice-covered sections of the Arctic could dramatically reshape our understanding of both ancient sea levels and climate change.

(CN) — An analysis of sediment in the Arctic Ocean and Nordic seas found that during at least two glacial periods over the last 150,000 years the bodies of water were covered by a thick layer of ice and filled mostly with fresh water, not salt water, according to a study released Wednesday.

Tens of thousands of years ago, large sections of Northern Europe and North America were covered by ice sheets. Vast ice shelves stretched across huge swaths of what is now Canada and dominated the expanse between Ireland and the Eastern rim of the Kara Sea in what is now Europe. Sections of the Bering Sea coastline and all of Greenland were also covered by glaciers.

For the Arctic and Nordic regions, separate studies have proposed that the area was once covered by a large ice shelf. But scientists lacked evidence proving the existence of ancient ice sheets covering most of the Arctic Ocean. 

Researchers from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) and MARUM Center for Marine Environmental Sciences at the University of Bremen spent years analyzing the composition of marine sediment layers in the region to find that very evidence.

Their research found no traces of sea salt in the deposits.

Instead, they found that during two glaciations — periods marked by glacial movement and frigid temperatures — freshwater accumulated under the vast ice sheets, building up an entirely fresh Arctic Ocean, according to the study published in the journal Nature

According to the study, the periods where freshwater was stored occurred once between 70,000 and 62,000 years ago and again between 150,000 and 131,000 years ago.

The study’s lead author Walter Geibert, geochemist at the Alfred Wegener Institute, said in a statement the findings throw out prior reconstructions of sea levels from the frigid era.

"These results mean a real change to our understanding of the Arctic Ocean in glacial climates,” Geibert said. “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 body of freshwater existed for thousands of years until it spilled into the North Atlantic over a short period of time.

According to the study, 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 and return to its original glacial conditions.

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

In 10 sediment cores examined by scientists, analyses showed that thorium — a slightly radioactive metal and indicator of the presence of saline water — was absent in the same two periods.

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

Study co-author Jutta Wollenburg of the Wegener Institute said in a statement the thorium isotope analysis helps build a timeline of events.

"Here, its repeated and widespread absence is the giveaway that reveals to us what happened,” Wollenburg said. “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 explanation for why the normally salty Arctic Ocean could change into a freshwater body lies in the buildup of huge icebergs and ice shelves that blocked the typical exchange of water.

"Such a scenario is perceivable if we realize that in glacial periods, global sea levels were up to 130 meters (427 feet) lower than today, and ice masses in the Arctic may have restricted ocean circulation even further," Ruediger Stein, geologist at the AWI and the MARUM, said in a statement.

Rivers and runoff from melting glaciers also delivered huge amounts of freshwater to the Arctic Ocean and Nordic water system, the study found.

"Once the mechanism of ice barriers failed, heavier saline water could fill the Arctic Ocean again," Geibert said in the statement. "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."

Researchers did not immediately respond to an emailed request for further comment on the study.

Categories / Environment, Science

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