Kategorie: News
The history of the Arctic Ocean
Climate historians actually know quite well what the European continent looked like during the Ice Age. In contrast to deposits on land, where erratic boulders, moraines and glacial valleys are the obvious landmarks of glaciers, only few traces of vast ice shelves had been found so far in the Arctic Ocean - and Meltwater leaves no marks.
Researchers from the Alfred Wegener Institute and MARUM have now published new findings about the past of the Arctic Ocean: Since the last 150,000 years, it is believed to have been covered at least twice by ice shelves more than 900 meters thick and to have been completely filled with fresh water during the last ice age. The long geoscientific investigation, with these findings, can be read in the current issue of the scientific journal Nature.
Walter Geibert, a geochemist at the Alfred Wegener Institute in Bremerhaven, Germany, and his team conducted research that corroborated the thesis of a freshwater-filled northern ocean.
"Connections to the North Atlantic as well as the Pacific admittedly exist today. And thus also a constant exchange of water. At that time, the sea level was about 130 meters lower, the so-called 'Mediterranean of the North' was completely isolated and apparently filled with fresh water." explains Walter Geibert
Something crucial is missing
Geological analyses of ten sediment cores from different areas of the Arctic Ocean and the European Arctic Ocean have shown that the layered sediment deposits represent the Arctic climate history of the past ice ages. However, when the cores were examined layer by layer, something crucial was missing.
"In salty seawater, the decay of natural uranium always produces the isotope thorium-230, which is deposited on the seafloor and can be detected there for a very long time because of its half-life of 75,000 years," Walter Geibert explains.
The isotope is formed in salty seawater by the decay of uranium and is deposited in sediments. In science, it is used as a timekeeper because of its half-life of 75,000 years. "To our knowledge, the only plausible explanation for the isotope's absence is that the Arctic Ocean was filled only with fresh water twice in its recent history - in liquid and frozen form," explains co-author and AWI micropaleontologist Dr. Jutta Wollenburg.
This means that there was no salt water in the Arctic Ocean at that time, because the isotope only forms in it - in contrast to fresh water.
"Such a scenario is conceivable 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 on the Arctic Ocean slowed down the exchange of water masses," explains co-author and AWI geologist Prof. Dr. Rüdiger Stein, who also works at the MARUM Center for Marine Environmental Sciences at the University of Bremen.
While summer ice melt and northward-flowing rivers contributed at least 1200 cubic kilometers of freshwater per year to the Arctic Ocean, shallow straits such as the Bering Strait or Sunde in the Canadian Archipelago, which were drying up at the time, prevented the influence. In the European Arctic Ocean, icebergs or glacial tongues probably blocked the outflow.
Freshwater pulses from the Arctic Ocean may also explain climate fluctuations during the last ice age. The temperature over Greenland changed 8 to 10 degrees Celsius several times in a few years. "We see here that there were also crucial tipping points of the Earth system around the Arctic in recent Earth history. Our task now is to investigate these connections in more detail and to check whether our new idea of the Arctic Ocean helps to close further gaps in our knowledge, especially with regard to the risks of man-made climate change," says Walter Geibert.