In November 1929, a large earthquake interrupted a transatlantic cable linking Europe to SH.BA at 28 different points, but not the shake caused damage. This was the first scientific proof that the large submarine sedimentary avalanches – geologically talking currents – form the ocean floor, as the cable was cut from the sediment and stones detached from the earthquake and traveled for many kilometers.
Although they have been known for about 100 years, the high-energy nature of the turbulent currents has made it almost impossible to measure them directly-any instrument placed on its way would be destroyed by extraordinary strength. After a few hours, turbulent currents can transport more sediments to the deep sea than the influx of global annual mass from all the combined rivers.
Now, an international team led by Geomar Helmholtz Center for Ocean Research Kiel and Durham University (UK) has developed a new method to monitor these flows from a safe distance. Using the seismometers at the bottom of the ocean, researchers, for the first time, have discovered the inner structure of these massive currents.
“Turbulent currents are the predominant mechanism that transports sediments and carbon from coastal areas to the deep sea, just as rivers transport sediment over land,” explains Dr. Pascal Kunat, seismologist in Geomar and main author of the study. “However, unlike rivers, they are among the least understood processes of sediment transport.”
The team set seismometers – normally used to study earthquakes – in October 2019 in the Congo Canyon, one of the largest and deepest submarine canyons in the world, outside the Western coast of Africa. The instruments were placed a few kilometers outside the shaft of the canyon canal, beyond the devastating currents of currents, allowing them to record seismic signals created by the flow turbulence and the transport of the associated sediment.
Using this method, researchers traced two turbulence currents moving at speeds of 5 to 8 meters per second (by comparison, a snow avalanche can exceed 80 meters per second, but moves through a much less dense medium) at a distance of 1100 kilometers-from the mouth of the Congo River through Congo-Sea fans. These are the longest sediment flows ever recorded.
“Our results show that the dense front of these canyon blur currents is not a single continuous flow, but consists of many pulses, each lasting between five and 30 minutes,” says Kunat. This observation suggests that turbulent currents behave much more as a waste flow than an avalanche. Arctic, the fastest pulses occur up to 20 kilometers after the front. These growth eventually transcend the main edge, supplying sediments and the momentum needed to keep the flow at long distances.
This finding challenges previous assumptions that higher speeds occur in the front of the flow. On the contrary, new data suggests that turbulent mixing with seawater or other retardation forces significantly affect the behavior of these flows over long distances.
The turbulent currents not only form modern ocean flooring, but sediments deposited by such currents are of great interest to geologists. They act as reservoir rocks – porous and permeable geological formations that can store and transmit hydrocarbons such as oil and gas.
The study, “the seismometers at the bottom of the ocean reveal the dynamics of growth in the longest stems of the Earth’s sediment,” was published in the newspaper Communications land and environment.
Additional materials and interviews provided by the Helmholtz Association of German Research Centers.