German scientists have discovered that the Arctic Ocean during the past ice ages was almost fresh, and also covered with thick ice shelves. The authors substantiate their conclusions, published in the journal Nature, by the absence of thorium-230 isotopes in marine sediments.
The Arctic Ocean was filled with fresh water at least twice: 62-70 thousand years ago and 131-150 thousand years ago. Scientists determined the salinity of the ocean in those epochs by the amount of thorium-230, which is produced during the decay of uranium dissolved in seawater, and as a part of shells sinks to the seabed, falling into sediments.
Researchers have analyzed samples of sedimentary rocks (cores) of the Arctic and northern seas and found unexpected minimums in the number of thorium isotopes over the past 200 thousand years. The mysterious absence of thorium in the sediments indicates that it did not form in the water above the study sites. Usually, the isotope quickly adheres to drowning particles and is removed from the strata before it can be transported over long distances, while the sedimentation rate of thorium-230 is high even in the Arctic, which is relatively poor in particles. If thorium is not formed in the water, it means that there were no dissolved salts in the seawater.
According to calculations, the Arctic Ocean contained nine million cubic kilometers of completely fresh water (about half of the total volume of the ocean). It could be filled with ice shelves that closed the northern and arctic seas from the influx of salt water from the Atlantic. At the same time, melting land glaciers and precipitation have reduced ocean salinity to a minimum.