In the past 150,000 years, the Arctic Ocean contained pure fresh water twice when it was largely separated from the other seas by ice sheets. Researchers come to this surprising result on the basis of the geochemical investigation of sediments. As they explain, at the end of these periods the water masses presumably flowed gushingly into the North Atlantic. These sudden freshwater inflows could shed light on abrupt climatic fluctuations, the causes of which are so far unclear.
On land in many places boulders, glacial valleys and the terminal moraines of the glaciers testify to the existence of enormous ice sheets that have spread in the north of the earth during the cold phases of the last 150,000 years. But what was the situation in the sea areas of the far north at these times? In contrast to land, there are no clear traces of the Arctic Ocean’s cover with floating ice shelves. In order to gain information, the researchers led by Walter Geibert from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) have now examined sediment drill cores from various areas of the Arctic Ocean as well as from the Fram Strait and the European Arctic Ocean. As they explain, the layered sediment deposits depict the Arctic climatic history of the past ice ages, and geochemical analyzes can be used to draw conclusions about the former conditions at the sample locations.
No trace of salt water
As scientists report, their results show that the floating foothills of the northern ice sheets covered the entire Arctic Ocean during the Vistula Ice Age 70,000 to 60,000 years ago and a second time during the so-called Saale Ice Age 150,000 to 130,000 years ago. The particularly surprising result, however, was that in both periods under the more than 900 meters thick ice masses, so much fresh water evidently accumulated that it completely shaped the Arctic Ocean and the Northern European Sea. “With these results, we are turning the previously valid idea of the history of the Arctic Ocean in the Ice Age climate on its head,” says Geibert.
The basis of this conclusion was the lack of an isotope in the layers of the two periods: “In salty sea water, the isotope thorium-230 is always produced by the decay of natural uranium. It is deposited on the ocean floor and is there for a very long time due to its half-life of 75,000 years, ”explains Geibert. Geologists therefore use the thorium isotope as a timer. “This time, however, its repeated and, above all, widespread absence gives us a decisive clue: the only plausible explanation for this, as far as we know, is that the Arctic Ocean was only filled with freshwater twice in its recent history – in liquid and in frozen form”, says co-author Jutta Wollenburg from AWI.
But how could the huge ocean basin, which is linked to the North Atlantic and the Pacific Ocean by several broad connections? According to the researchers, this can be explained by the then lower sea level and the barrier function of the ice. “Such a scenario seems plausible if we assume that the global sea level during the Ice Ages was up to 130 meters lower than today and that the ice shelves in the Arctic Ocean slowed the exchange of water masses,” explains co-author Rüdiger Stein from the MARUM Center for marine environmental sciences from the University of Bremen.
Isolated by land and ice
Specifically, shallow straits such as in the area of the Bering Strait or the Sunde in the Canadian archipelago had dried up at that time and were thus excluded as inflow and outflow. In the European Arctic Ocean, glacier tongues or icebergs lying on the sea floor probably blocked a large-scale outflow of freshwater, the researchers explain. According to them, the huge freshwater reservoir in the Arctic Ocean basin was fed by the summer ice melt and the rivers running north. Some of these water masses probably flowed through the few connections in the Greenland-Scotland ridge over the European Arctic Ocean into the North Atlantic, thus preventing the influx of salt water. So the Arctic Ocean could gradually fill with fresh water.
“When the ice barrier mechanism failed, however, the heavy salt water was finally able to penetrate the Arctic Ocean again,” says Geibert. “We believe that it then quickly displaced the lighter fresh water upwards when it flowed in, so that at a certain point the stored amounts of fresh water spilled over the shallowest edge of the European North Sea – the Greenland-Scotland Ridge – into the North Atlantic,” he said Scientist.
As he and his colleagues explain, their results and explanatory approaches could now shed light on the causes of previously unclear climate phenomena and sea level levels in the past. Because the sudden releases of fresh water could have caused abrupt fluctuations. “We can see here that there have been decisive tipping points in the Earth system around the Arctic in recent Earth history. Our task now is to examine these relationships more closely and to check whether our new conception of the Arctic Ocean helps to close further gaps in knowledge, especially with regard to the risks of man-made climate change, ”concludes Geibert.
Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Nature, doi: 10.1038 / s41586-021-03186-y