On January 14, 2020, a vast underwater avalanche swept down the Congo Canyon, off Africa's Atlantic Coast. Material traveled an astonishing 1,100 kilmeters (660 miles) and reached a depth of 4,500 meters (15,000 feet) having started not far below sea level. The transfer of organic material to the depths in this way is a neglected part of the Earth's carbon cycle.
Measuring underwater avalanches was once considered impossible, and the nay-sayers appeared to have been right when monitoring equipment was lost. However, oceanographers succeeded in retrieving it and have reported the information gained in Nature Communications.
Professor Peter Talling of Durham University and co-authors attribute the event to severe flooding in the Congo River – the world's second largest by outflow even in normal times – and unusually large spring tides. Together, these triggered movements in the sand and mud in the Canyon's upper reaches.
From an initial speed of 5.2 meters per second (11.5 miles per hour), the sediment accelerated as it swept up material that had been undisturbed for 30-91 years, reaching 8 m/s (18 mph) as it exited the Canyon. This self-acceleration or “ignition” has been proposed before, but never previously documented underwater.
Observing this event required two remarkable strokes of luck.
In 2019 researchers dotted measuring devices down the length of the canyon. Just five months later they were rewarded with what appeared to have been the Congo Canyon's largest avalanche in almost a century. Unfortunately, the event was so immense it swept 11 of the instruments away, taking their data with them.
Floats and beacons with three months' power had been attached in case of just such an event, but Talling said in a statement, “the odds of retrieving football-sized sensors were tiny, as they drifted in different directions, dragged by currents across hundreds of kilometres of ocean. Rescuing those buoys seemed entirely improbable.”
Nevertheless, a passing boat happened on one of the floats and agreed to seek others. Collaboration between several international research institutes, with assistance from several vessels that joined the search, led to what Talling called “one of the most remarkable bits of field science in the ocean I’m ever likely to see.”
The sensors were not the only human technology affected. Two seabed telecommunications cables were cut, slowing Internet speeds across much of Africa as data needed to find an alternative route.
The legacy of underwater earthquakes has been seen on the ocean floor near continental shelves since we have had submarines capable of exploring these depths. However, these have previously been attributed to earthquakes. This may indeed be the main cause away from large river mouths, but the paper shows floods can also produce what the authors call “canyon flushing flows”.
The January avalanche wasn't the only one the team observed in the Congo Canyon. The paper notes that “in one year, these turbidity currents eroded 1,338-2,675 million tonnes of sediment from one submarine canyon, equivalent to 19–37 percent of annual suspended sediment flux from present-day rivers.” Globally, that means “the sediment-mass carried by these flows rivals that of any other process on Earth, including rivers, or glaciers, or settling from the surface ocean.”
The deep sea burial of organic carbon has always helped keep atmospheric carbon levels in balance and is needed now more than ever. It seems we have been ignoring a major part of that process.
Meanwhile, the telecommunications cables broken in this avalanche won't be the last to be damaged in this way. Indeed, the same West African cables have been broken by turbidity currents three times since, despite not having broken for the previous 18 years. With 99 percent of intercontinental data moved this way, predicting major avalanches might help us avoid such events by better siting the cables.
By creating a model reconstructing the Congo Canyon avalanche, the authors believe they've taken a big step towards that, although major questions remain, such as why some underwater avalanches speed up as they fall, while others slow down.