We need to talk more about the impact of ocean currents on the evolution of continental margins. Such currents create both widespread unconformities and large, alongslope oriented deposits known as contourite drifts, and we here present the first detailed study of the Bjørnøyrenna Drift – a deposit that started accumulating when the Gulf Stream first reached into the Arctic Ocean.
Text and figures by Assoc. Prof. Tom Arne Rydningen, UiT The Arctic University of Norway.
The thermohaline circulation presently transports water masses along the Norwegian Continental Margin and this continuous flow of water erode the substratum in some areas, and deposit sediments on the continental slope where the current slows down. This water flow can be compared to an enormous river running along the margin from the British Isles all the way to the Arctic Ocean, eroding along the centerline of the stream, and depositing “river bank sediments” downslope of the most intense flow. These sediments (contourite drifts) thereby record the onset and evolution of currents in the ocean basins.
The northward transport of warm water masses on the northern hemisphere began when the Gulf Stream started passing over the threshold known as the Greenland-Scotland Ridge, which previously acted as a giant barrier for a long time, separating the North Atlantic from the Norwegian-Greenland Sea. Further north, the Fram Strait gateway was initially closed, isolating the Arctic Ocean. The inflow of warm water and the returning, cold water masses from the Arctic Ocean could therefore not flow through this doorway between Svalbard and Greenland until it opened up, thus fully establishing the thermohaline circulation in the area.
This large-scale ocean circulation established sometime in the early to middle Miocene (17 to 12 Ma), close in time to a global temperature highpoint, the Mid Miocene Climatic Optimum (16 to 14 Ma). The global temperatures dropped following this, possibly partly because of the establishment of this low- to high-latitude ocean communication, which brought both heat and moisture northwards, allowing for growth of glaciers on land on the northern hemisphere, as well as cold surface water from the high-Arctic south and into the North Atlantic, which is interpreted to have cooled accordingly. In the Quaternary, this eventually resulted in continental-sized ice sheet in the Barents Sea that extended to the shelf break, and that deposited vast amount of sediments in a submarine fan, burying the drift.
The large-scale currents in the ocean basins act as important agents of sediment transport. The Bjørnøyrenna Drift accumulated at a rate of 2 to 3 cm per 1,000 years, a rate that is comparable to modern deep-sea sediment input from the giant Mississippi and Amazon rivers. Depending on the velocity of the ocean currents, they may transport grain sizes up to the sand fraction. Based on exploration wells in this study, the Bjørnøyrenna Drift is found to mainly comprise muddy sediments, with some sandstone stringers.
The Bjørnøyrenna drift covers an area of 5,000 km2 and the thickness is up to 1400 m. Similar contourite drifts can be found all along the NE part of the North-Atlantic and Norwegian-Greenland Sea. As such, there is more details to unravel on the ocean current onset and variability, as well as how these influenced on the continental margin evolution.
Reference:
Rydningen, T.A.; Høgseth, G.; Lasabuda, A.; Laberg, J.S.; Safronova, P.A.; Forwick, M. (2020): An early Neogene – early Quaternary Contourite Drift System on the SW Barents Sea Continental Margin, Norwegian Arctic. Geochemistry, Geophysics, Geosystems, 21(11). DOI: 10.1029/2020GC009142 [intranet]
