Stunning new evidence suggests the Arctic Ocean was covered by a thick layer of ice and filled with fresh water on at least two occasions during the past 150,000 years. The observation could finally explain strange and dramatic climate anomalies associated with these glacial periods.
With all the human-induced melting that’s going on these days, it’s hard to imagine our world slathered in ice. But such was the case during recent glacial periods, when significant portions 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 sea levels dramatically lower than they are today, something very strange happened, for which we have no modern analogues.
New evidence presented today in Nature proposes an incredible explanation for the surprising lack of thorium-230—an isotope that accumulates on the seafloor of salty oceans—in marine deposits pulled from the sedimentary layers of our northernmost oceans. This is a potential sign that the Arctic Ocean, cut off from the rest of the planet’s oceans, featured a basin filled with fresh water and capped with a 3,000-foot (900-meter) layer of ice.
Evidence suggests this 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 were covered by a gigantic sheet of ice and lasted for thousands of years, according to paper, co-authored by Walter Geibert, a marine geochemist from the Helmholtz Centre for Polar and Marine Research at the Alfred Wegener Institute.
The gaps of thorium-230 were detected in sediment cores pulled from the Arctic Ocean, the Fram Strait (the passage between Greenland and Svalbard), and the Nordic Seas. This isotope is a byproduct of dissolving uranium, which naturally occurs in salty seawater. Thorium clings to solid particles, drifts to the seafloor, gets buried in sediment, and waits to be discovered by scientists.
Analysis of core samples dating back some 200,000 years revealed at least two time periods in which thorium-230 was basically non-existent. For Geibert and his colleagues, this pointed to the presence of salt-free water bodies.
“We found that a naturally occurring trace of radioactive material, which is always left behind by overlying sea water, was absent across a very large region for certain time periods,” explained Geibert in an email.
To explain this, the authors presented a scenario in which massive ice shelves extended far into the Nordic Seas, stretching from the Bering Strait all the way to the tall Greenland-Scotland Ridge (GSR). These ice shelves acted like a gigantic dam, separating the Arctic from the salty Atlantic and Pacific oceans. Low sea levels, in which waters were around 430 feet (130 meters) lower than they are today, contributed to this process. The Arctic basin, now isolated, began to fill with fresh melt water, forming a gigantic subsurface freshwater sea, the authors speculate.
“The accumulated freshwater in the Arctic Ocean we are talking about would have occupied a volume larger than the Mediterranean Sea, but it was covered by extremely thick layers of ice,” said Geibert.
As the authors go on to explain, the ice dams would sometimes fail, resulting in the sudden influx of heavier salt water into the Arctic Ocean. When this happened, the saline water quickly displaced the lighter freshwater, forcing it over the shallow boundary into the North Atlantic. This sudden rush of freezing cold fresh water into the world’s oceans could explain strange climate anomalies previously detected 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—have been described for a while,” Geibert explained. “What we add now is a possible explanation for some of these substantial shifts in temperature distribution that were lacking a satisfying explanation so far.”
To which he added: “We’re sure that this concept will spark a lot of debate and research—we now need models of this physically very complex situation, and link them to other high-quality records,” he said. “Ironically, some of the clear indicators from very cold periods that we found should help us now to identify periods of a warmer Arctic in the past with more confidence.”
However, paleoceanographer Sharon Hoffmann, writing in an accompanying News & Views article, poured some mildly cold water on the new findings.
“Arctic sediments are notoriously hard to date owing to the lack of microfossils, and because sedimentation rates varied. It is therefore uncertain whether the [thorium-deficient] intervals in the cores were produced at exactly the same times at all sites across the ocean basins,” wrote Hoffmann, who’s not affiliated with the new research. Also, “no freshwater fauna [animals] have been identified in the sediments concerned, so direct evidence of freshwater intrusion into deep Arctic basins remains to be found,” she added.
That said, Hoffman said the new research presents “exciting avenues for future research” and that future “geochemical and fossil analyses might help to support or challenge the assertion that the Arctic Ocean could have been fresh.”