In A First, Geoscientists Track Earth’s Largest Sediment Avalanches

In November 1929, a large earthquake severed a transatlantic cable connecting Europe with the U.S. in 28 different points, but not the shaking caused the damage. This was the first scientific proof that large submarine sediment avalanches — geologically speaking turbidity currents — shape the ocean floor, as the cable was cut by sediment and boulders dislodged by the quake and traveling for many kilometers.

Although they have been known for about 100 years, the high-energy nature of turbidity currents has made it almost impossible to measure them directly — any instruments placed in its path would be destroyed by the immense force. In a few hours, turbidity currents can transport more sediments to the deep sea than the global annual mass flux from all rivers combined.

Now, an international team led by GEOMAR Helmholtz Center for Ocean Research Kiel and Durham University (U.K.) has developed a new method to monitor these flows from a safe distance. Using ocean-bottom seismometers the researchers have, for the first time, revealed the internal structure of these massive currents.

“Turbidity currents are the dominant mechanism transporting sediment and organic carbon from coastal areas into the deep sea, just as rivers transport sediment over land,” explains Dr. Pascal Kunath, seismologist at GEOMAR and lead author of the study. “However, unlike rivers, they are among the least understood processes of sediment transport.”

The team deployed seismometers — normally used to study earthquakes — in October 2019 in the Congo Canyon, one of the largest and deepest submarine canyons in the world, off the west coast of Africa. The instruments were placed several kilometers outside the canyon-channel axis, beyond the destructive reach of the currents, allowing them to record the seismic signals generated by flow turbulence and associated sediment transport.

Using this method, the researchers tracked two turbidity currents moving at speeds of 5 to 8 meters per second (for comparison, a snow avalanche can exceed speeds of 80 meters per second but moves through a much less dense medium) over a distance of 1,100 kilometers — from the mouth of the Congo River through the Congo deep-sea fan and canyon system. These are the longest-runout sediment flows ever recorded.

“Our results show that the dense front of these canyon-flushing turbidity currents is not a single continuous flow, but consists of many pulses, each lasting between five and 30 minutes,” says Kunath. This observation suggests that turbidity currents behave much more like a debris flow than an avalanche. Remarkably, the fastest pulses occur up to 20 kilometers behind the front. These surges eventually overtake the leading edge, suppling sediments and the momentum needed to sustain the flow over long distances.

This finding challenges previous assumptions that the highest velocities occur at the flow front. Instead, the new data suggest that turbulent mixing with seawater or other retarding forces significantly influence the behavior of these flows over long distances.

Turbidity currents not only shape the modern ocean floor, but the 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 like oil and gas.

The study, “Ocean-bottom seismometers reveal surge dynamics in Earth’s longest-runout sediment flows,” was published in the journal Communications Earth & Environment.

Additional material and interviews provided by Helmholtz Association of German Research Centres.