Beautiful new evidence suggests that the Northern Ocean has been covered by a thick layer of ice at least twice during the last 150,000 years and filled with fresh water. The observation may eventually explain strange and dramatic climatic deviations associated with these ice ages.
With all the meltdown caused by humans these days, it’s hard to imagine our world sinking into ice. But this was the case during recent ice ages, when large parts of North America, Northern Europe, Greenland and parts of the Bering Sea were dominated by massive ice sheets. With much of the earth’s water trapped in this ice, and with sea levels dramatically lower than today, something strange happened for which we have no modern analogues.
New testimony presented in Nature today proposes an incredible explanation for the surprising lack of thorium-230 – an isotope that accumulates on the seabed of salty oceans – in marine precipitation drawn 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, contains a basin of fresh water and is covered with a layer of 900 meters of ice.
Evidence suggeststhis has happened on at least two occasions, once between 150,000 and 130,000 years ago and then again between 70,000 and 50,000 years ago. These salt-free seas are covered by a giant ice and lasted thousands of years, according to newspaper, co-author of Walter Geibert, a marine geochemist from the Helmholtz Center for Polar and Marine Research at the Alfred Wegener Institute.
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The gaps of thorium-230 were detected in sediment cores drawn from the Arctic Ocean, the Fram Sea Strait (the passage between Greenland and Svalbard) and the North Seas. This isotope is a by-product of the dissolution of uranium, which occurs naturally in salt water. Thorium adheres to solid particles, floats to the seabed, is buried in sediment and waits to be discovered by scientists.
Analysis of nuclear samples dating back approximately 200,000 years has revealed at least two periods in which thorium-230 basically does not exist. For Geibert and his colleagues, this pointed to the presence of salt-free water bodies.
“We found that a natural trace of radioactive material, which is always left behind by overlying seawater, was absent for certain periods in a very large region,” Geibert explained in an email.
To explain this, the authors presented a scenario in which massive ice shelves extend into the Nordic Sea, stretching from the Bering Strait to the high Greenland-Scotland Reef (GSR). These ice shelves acted like a giant pond separating the North Pole from the salty Atlantic and Pacific Oceans. Low sea levels, in which the waters were about 130 meters lower than today, contributed to this process. The Arctic basin, now isolated, began filling with fresh meltwater and formed a giant freshwater lake, according to the authors.
“The accumulated fresh water in the Northern Ocean we are talking about would have occupied a volume larger than the Mediterranean, but it was covered by extremely thick layers of ice,” Geibert said.
As the authors further explain, the ice dams will sometimes fail, leading to the sudden supply of heavier salt water to the Arctic.ean. When this happens, the salt water quickly displaces the lighter fresh water and forces it across the shallow boundary to the North Atlantic Ocean. This sudden rush of cold fresh water into the oceans of the world may explain the strange climatic deviations that scientists have previously detected, including sudden temperature rises in Greenland.
‘The consequences – a very sudden warming of the Arctic seas and possibly a temporary cooling of the North Atlantic Ocean – has been described for some time, ”Geibert explained. “What we are adding now is a possible explanation for some of these significant shifts in temperature distribution that have had a satisfactory explanation so far.”
To which he added: ‘We are sure that this concept will provoke a lot of debate and research – we now need models of this physically very complicated situation and link it to other high-quality records, ”he said. “Ironically, some of the clear indicators of very cold periods we have found should now help us identify periods of a warmer Arctic in the past with more confidence.”
The paleo-oceanographer Sharon Hoffmann, writes in an accompanying News & Views article, poured some light cold water on the new findings.
“Arctic sediments are notoriously difficult to date due to the lack of microfossils, and because the sedimentation rate varies. It is therefore uncertain whether the [thorium-deficient] intervals in the nuclei were produced at the same times at all sites across the ocean basins, ”wrote Hoffmann, who is not associated with the new research. Also, “no freshwater fauna [animals] has been identified in the relevant sediments, direct evidence of freshwater intrusion must therefore be found in deep Arctic areas, ”she added.
Hoffman said the new research “provides exciting avenues for future research” and that future “geochemical and fossil analyzes could help support or challenge the claim that the Northern Ocean may have been fresh.”