Â鶹ÒùÔº

Seismometers reveal Earth's longest-runout undersea sediment flows in unprecedented detail

Turbidity currents are an important natural process that often goes unnoticed: these powerful currents beneath the ocean surface carve deep submarine canyons, create huge sediment deposits and can damage submarine cables and pipelines. Although the phenomenon has been known for about 100 years, its high-energy nature has made it almost impossible to measure directly—any instruments placed in its path would be destroyed by its immense force, much like avalanches on land.

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—normally deployed to study earthquakes—the researchers have, for the first time, revealed the internal structure of these massive currents. Their findings are today in the journal Communications Earth & Environment.

Ocean-bottom seismometers detect longest-runout sediment flows ever recorded on Earth

"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."

To address this knowledge gap, the team deployed seismometers in October 2019 in the Congo Canyon and Channel off the west coast of Africa—one of the largest and deepest submarine canyons in the world. 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 (m/s) 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.

The flows also damaged several submarine cables in January and March 2020, disrupting internet and data communications in West Africa during a particularly critical phase of the early COVID-19 pandemic.

Rethinking turbidity current dynamics

"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. 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.

Beyond introducing an innovative remote sensing method for monitoring turbidity currents, this study deepens our understanding of how these powerful canyon-flushing turbidity currents function. By analyzing their internal dynamics in detail, scientists can better predict their impact on seafloor infrastructure and refine models of sediment and carbon transport in the ocean.