Amazing new evidence suggests that the Arctic Ocean was covered with a thick layer of ice and filled with fresh water at least twice in the past 150,000 years. The observation could finally explain the strange and dramatic climate anomalies associated with these ice ages.
With all the man-made melting going on today, it’s hard to imagine our world smeared in ice. But that was the case during recent Ice Ages, when significant parts of North America, Northern Europe, Greenland, and parts of the Bering Sea were dominated by massive ice sheets. With much of Earth’s water tied up in this ice, and with the sea levels dramatically lower than they are today, something very strange happened, for which we have no modern analogues.
New proof Presented today in Nature presents an incredible explanation for the surprising lack of thorium-230 – an isotope that accumulates on the seafloor of saline oceans – in marine deposits pulled from the sedimentary layers of our northernmost oceans. This is a possible sign that the Arctic Ocean, cut off from the rest of the planet’s oceans, contained a basin filled with fresh water and covered with an ice sheet of 900 meters.
Evidence suggestsThis happened at least twice, once between 150,000 and 130,000 years ago and then again between 70,000 and 50,000 years ago. These salt-free seas were covered with a gigantic layer of ice and took thousands of years, according to paper, co-authored by Walter Geibert, a marine geochemist at the Helmholtz Center for Polar and Marine Research at the Alfred Wegener Institute.
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Thorium-230 gaps were detected in sediment cores originating from the Arctic Ocean, the Fram Strait (the passage between Greenland and Spitsbergen) and the Nordic Seas. This isotope is a byproduct of uranium dissolving, which occurs naturally in salty seawater. Thorium clings to solid particles, drifts to the sea floor, is buried in sediment, and awaits discovery by scientists.
Analysis of core samples some 200,000 years old revealed at least two time periods when thorium-230 was essentially non-existent. For Geibert and his colleagues, this indicated the presence of salt-free water bodies.
“We found that a naturally occurring trace of radioactive material, which is always left behind by overlying seawater, was absent in a very large area for periods of time,” explains Geibert in an email.
To explain this, the authors presented a scenario in which huge ice shelves stretched far into the Nordic Seas, stretching from the Bering Strait to the high Greenland-Scotland Ridge (GSR). These ice shelves acted as a gigantic dam separating the Arctic from the salty Atlantic and Pacific Oceans. The low sea level, in which the water was about 130 meters lower than it is now, contributed to this process. The polar basin, now isolated, began to fill with fresh meltwater, forming a giant underground freshwater sea, the authors speculate.
“The accumulated fresh water in the Arctic Ocean that we are talking about would have taken up a volume greater than the Mediterranean, but it was covered with extremely thick layers of ice,” Geibert said.
As the authors explain, the ice dams sometimes failed, resulting in the sudden influx of heavier salt water into the Arctic Ocean.ean. When this happened, the salt water quickly displaced the lighter fresh water and forced it across the shallow boundary into the North Atlantic Ocean. This sudden stream of icy fresh water in the world’s oceans could explain strange climate anomalies previously discovered by scientists, including sudden temperature spikes in Greenland.
“The effects – a very sudden warming of the Arctic seas and possibly a temporary cooling of the North Atlantic Ocean – have been described for some time, ”explained Geibert. “What we’re adding now is a possible explanation for some of these substantial shifts in the temperature distribution for which there has been no satisfactory explanation so far.”
To which he added: ‘We are sure that this concept will lead to much discussion and research – we now need models of this physically very complex situation, and we can link them to otherquality records, ”he said. “Ironically, some of the clear indicators of very cold spells we found should now help us identify periods of a warmer Arctic in the past with more confidence.”
Paleoceanographer Sharon Hoffmann, write in an accompanying News & Views article, poured some slightly cold water over the new findings.
“Arctic sediments have been notoriously difficult to date so far because of the lack of microfossils and because sedimentation rates vary. It is therefore uncertain whether the [thorium-deficient] intervals in the nuclei were produced at exactly the same times at all locations in the ocean basins, ”wrote Hoffmann, who is not affiliated with the new study. Also “no freshwater fauna [animals] have been identified in the affected sediments, so direct evidence of freshwater intrusion into deep arctic basins has yet to be found, ”she added.
That said, Hoffman said the new research offers “exciting avenues for future research” and that future “geochemical and fossil analyzes may help support or challenge the claim that the Arctic Ocean could have been fresh.”