How the Arctic Ocean turned from a gigantic freshwater lake to a part of the saline North Atlantic

The Arctic Ocean today: Drift ice and pack ice seen from aboard the German research vessel Polarstern, on the way to the North Pole. Photo: Stefan Hendricks / AWIResearchers from German Alfred Wegener Institute (AWI) simulated how the land bridge between Greenland and Scotland submerged

The Arctic Ocean was once a gigantic freshwater lake. Only after the land bridge between Greenland and Scotland had submerged far enough did vast quantities of salt water pour in from the Atlantic. Researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) now have demonstrated how this process took place, allowing us for the first time to understand more accurately how Atlantic circulation as we know it today came about. The results of the study have been published in the journal Nature Communications.

The study allows understanding more accurately the Atlantic circulation as we know it today and how it came about. The formation of ocean passages plays a vital role in global climate history, as it leads to changes in heat transport between the middle and polar latitudes.

Ca. 56 to 34 million years ago, the influx of saline Atlantic and Pacific water, which today finds its way into the Arctic Ocean from the Pacific via the Bering Strait and from the North Atlantic via the Greenland-Scotland Ridge, wasn’t possible – the region that is today completely submerged was above the sea at that time.

The researchers gradually submerged the land bridge to a depth of 200 m. „In reality, this tectonic submersion process lasted several million years,” says Climate Scientist Michael Stärz, first author of the study. „Interestingly, the greatest changes in the circulation patterns and characteristics of the of the Arctic Ocean only occurred when the land bridge had reached a depth of over 50 meters below the surface.”

This threshold depth corresponds to the depth of the surface mixed layer and determines where the relatively light Arctic surface water ends and the underlying layer of inflowing North Atlantic water begins. „Only when the oceanic ridge lies below the surface mixed layer can the heavier saline water of the North Atlantic flow into the Arctic with relatively little hindrance,” explains Stärz. „Once the ocean passage between Greenland and Scotland had reached this critical depth, the saline Arctic Ocean as we know it today was created.”

The theory that the Arctic Basin was once isolated is supported by the discovery of freshwater algae fossils in Eocene deep-sea sediments that have been obtained during international drilling near the North Pole in 2004. What was once a land bridge now lies ca. 500 m under the ocean and consists almost entirely of volcanic basalt. Iceland is the only section remaining above the surface.

Michael Stärz, Wilfried Jokat, Gregor Knorr, Gerrit Lohmann: Threshold in North Atlantic-Arctic Ocean circulation controlled by the subsidence of the Greenland-Scotland Ridge. Nature Communications.
DOI: 10.1038/NCOMMS15681

Source: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI)

(18.07.2017, USA: 07.18.2017)