River floods can trigger powerful underwater landslides

A record-length turbidity current triggered by river flooding has revealed a new link between the surface and the deep sea.

In January 2020, an underwater sediment avalanche thundered down West Africa’s Congo Canyon. It flowed 1,130 kilometers (702 miles), popping sensors off the seafloor and breaking communications cables as it plunged to the depths of the Atlantic Ocean. Such a flow, called a turbidity current, can erode a landslide’s worth of sediment.

This 2020 event is the longest one ever caught by scientists. “We were either lucky or unlucky,” said sedimentologist Peter Talling, a professor of submarine geohazards at Durham University in the United Kingdom and lead author of a new paper on the event. Talling is part of a team that in 2019 anchored an array of sensors to the bed of Congo Canyon. Attached to buoys, the sensors stay suspended above the seafloor to measure the speed of the flow coursing beneath them. On 14 January 2020, Talling got an email alerting him that one of the chains had broken and the device had surfaced. Over 2 days, he received 10 more notifications as the other sensors were untethered and carried by currents out into the Atlantic

That kicked off a scavenger hunt to retrieve the sensors and the data they held, an arduous process amid pandemic shutdowns. The team enlisted the help of passing ships and chartered vessels to fish out as many of the sensors before their location-transmitting beacons’ batteries died. They managed to recover nine of the 11 devices—the last one was retrieved the day before it stopped transmitting its position. With the sensors’ data, the team gained new insights into what drives turbidity currents and how they can connect rivers to the deep sea.

Canyon Flushers

Turbidity currents are “one of the last great unknowns in terms of sediment transport across our planet,” said Megan Baker, a sedimentologist at Durham University who was part of the research team.

Much of what’s known about these flows has come from studying the sand and rock deposits they’ve left behind, as well as from tank experiments and numerical models. Only over the past decade have researchers started collecting observations of underwater avalanches in places like Monterey Canyon off the coast of California and the fjords of British Columbia. Scientists have yet to unravel how turbidity currents are triggered, or their basic properties and behavior.

For all their mystique, these phenomena may rival rivers in transporting sediment. “One of them, on its own, can carry more sediment than all the rivers in the world added together,” Talling said. “Not just for 1 year but in some cases decades.” In 1929, a single turbidity current in the northwestern Atlantic moved more than 200 cubic kilometers (48 cubic miles) of sediment. The might of these sediment flows was witnessed by Talling and his team.

In 2019, the researchers had mapped parts of Congo Canyon’s floor. After the monster sediment flow, they mapped it again. From the differences in those maps, they calculated that this one underwater avalanche eroded some 2.7 cubic kilometers (0.65 cubic mile) of sediment. That’s roughly a third of the amount of sediment conveyed annually by all of Earth’s rivers combined, and not far from the amount transported by Earth’s largest landslide, which moved 2.8 cubic kilometers (0.67 cubic mile) of sediment at the start of Mount St. Helens’ eruption in 1980.

Along with the record-length event in 2020, the team measured 13 other submarine landslides. In Congo Canyon, turbidity currents occurred 30% of the time, the team reported in Nature Communications. “The more we monitor them, the more we think they might happen more often,” Baker said.

What Sets Sediment Avalanches in Motion?

It took some detective work for the team to figure out what might have set these sediment flows in motion. Earthquakes are known to trigger turbidity currents, but the team didn’t find any association between Congo Canyon flows and seismic activity. And large storms didn’t seem to set them off either. But during the time of the experiment, the Congo River had been experiencing its largest floods since the 1960s, which may have set the stage for the sediment to slide. The turbidity currents happened weeks to months later.

The team found a correlation between turbidity currents and twice-monthly spring tides. The team isn’t sure yet how the relationship works, but they’ve come up with a few ideas. Flooding may pile sand on the canyon lip, which collapses when disturbed by the spring tide. Or tides may resuspend the fine-grained sand supplied by floods, creating a layer of fluid mud that drains into the canyon and sets off an avalanche.

Although there is more to know about these events, the team is finding ties between river flooding and the deep sea. These underwater landslides may be shuttling fresh organic carbon into the deep sea that can then be buried on a geologic timescale. That rivers more or less directly connect to the deep sea is “beautifully demonstrated” in these instances, said Charles Paull, a marine geologist at the Monterey Bay Aquarium Research Institute who wasn’t part of the work. “It’s a simply amazing experiment,” he said.