IN NOVEMBER 1929, a huge earthquake in the Grand Banks off the south coast of Newfoundland in Canada sent tremors as far as New York. As the sea floor shook, a vast quantity of sand and mud began to stir up and flow down a canyon, gathering momentum as it went, creating a dramatic underwater avalanche. It involved enough material to make two Mount Everests and triggered a tsunami that killed more than 25 people.
This is the biggest known example of an undersea avalanche, but it wasn’t a one-off. Beneath the waves, the largest avalanches in the world regularly occur in Earth’s coasts and oceans, carving out the deepest and longest canyons on our planet. Most of the time, they happen without anyone noticing.
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For hundreds of years, the only witnesses to these events were fish and deep-sea creatures, which might have been carried out to sea or fed by the nutrient-rich sediments that the currents carry with them. More recently, ruptured gas pipelines and broken communication cables were proof that something extreme was going on. Over the past few years, however, things have started to change.
Now, thanks to a series of experiments and a bit of luck, we have captured these Earth-carving events in action. It turns out the mazes of underwater canyons, many of which were long thought to be geologically inactive, are anything but. Armed with new data, researchers have begun to piece together a better picture of what submarine avalanches are like, how they shape Earth and their vital role in locking away the carbon warming our world.
The deepest and longest canyon systems on the planet are similar in scale and shape to the Grand Canyon in Arizona. But unlike their counterparts on land, carved out by the constant scouring action of sand and gravel carried by rivers, underwater canyons are created by erratic avalanches that cascade off the continental shelf and down to the deep ocean (see “Ocean falls”, pictured below). Rivers dump silt onto the continental shelf where it heaps up, eventually becoming unstable – or sometimes topples after being given a shove by an earthquake, storm or flood – tumbling off the shelf and sculpting a canyon system as it goes.
The sediment flows, also known as turbidity currents, transport more material than any other natural process on Earth. They carry sediment rich in organic carbon and sweep up debris as they go, including decaying seaweed, plant material and marine life. As they swoosh onto the abyssal plain – a flat area that covers more than 50 per cent of the sea floor – these flows create a mosaic of specialised habitats, exposing methane-bearing sediments in some regions while smothering other areas to create lobes of oxygen-free muds. This unusual environment supports diverse and unique ecosystems including specialised chemosynthetic communities, such as tubeworms and vesicomyid clams, usually found near hydrothermal vents, sustained by hydrogen sulphide and methane. Sea cucumbers dig decaying morsels out of the freshly deposited mud, while pom-pom anemones are swept along, occasionally landing on a meal.
The avalanches self-accelerate and gain energy, like their snowy equivalent on a mountain. “This means they can travel huge distances into the deep sea and transport vast amounts of material,” says , a geologist at the University of Leeds in the UK.
These huge shifts in sediment play a role in Earth’s carbon cycle, burying carbon contained in organic matter and locking it away at the bottom of the ocean for millions of years. But getting a handle on exactly how much carbon they carry has been challenging. “We’ve not been able to collect information from these massive flows, partly because they are rare and unpredictable,” says Hodgson, “and partly because they trash our equipment.”
Detached ocean canyons
A small proportion of the 9000 or so submarine canyons we know of are still connected to river mouths at the coast. But around three-quarters became detached from their rivers when sea levels rose following the last glacial period. Now, these detached canyons lie far out to sea.
About 300 kilometres south-west of Cornwall, UK, sits the Whittard Canyon, a Grand Canyon-sized network of channels that juts into the North Atlantic Ocean. Since it is the only submarine canyon that enters waters over which the UK has rights, , a marine geohazards researcher at the National Oceanography Centre in Southampton, UK, applied for funding to study it. “To be honest, I wasn’t initially that excited,” he says. “The prevailing view was that there was no obvious way of triggering turbidity currents in a detached canyon, so I thought it was going to be boring.”
Nevertheless, in June 2019, Clare and his team placed two deep-water moorings in the canyon and wired up instruments to monitor sediment movement, in the hope of capturing an underwater avalanche. They also placed a rotating carousel of bottles 10 metres above the sea floor at the first mooring to catch sediment. Then, they waited. To their surprise, within three weeks, they had caught one in action.
Over the following year, they recorded , each lasting several hours. “We thought this canyon would be dead, so it was a real surprise to see this activity,” says Clare. More than 1000 of the world’s submarine canyons have a very similar setting to the Whittard Canyon. “We’re having to reassess our views on how these canyons work and how active they can be.”
One puzzle was the timing. “If there was going to be any activity, we’d expected it to be in autumn and winter, when storms stirred up the stock of glacial sediment,” says Clare. But the findings show that most turbidity flows occur during spring and summer.
A carbon superhighway
A hint as to why came from the sediment caught in the bottles, which was full of fresh marine-based organic carbon. “Our suspicion is that the turbidity currents are being fed by blooms of algae at the head of the canyon,” says Clare. Many of the world’s canyons provide a focal point for upwelling, where surface ocean currents draw up cool, nutrient-rich water from the deep and create a productive region teeming with life. “It’s why you get whales near the heads of canyons,” says Clare. Regions like this draw a lot of carbon dioxide out of the atmosphere and into organic matter, but for that carbon to be locked away for good, it needs to reach the deep ocean quickly, before it has a chance to be oxidised back into CO2. Canyons like Whittard might provide a previously unrecognised superhighway for marine carbon to get to the ocean floor.
In October 2019, at Durham University in the UK and his team set out to capture another underwater avalanche. This time, their target was still connected to a river. The researchers anchored 12 moorings fitted with transmitters along the floor of the Congo submarine canyon. They began near the head of the canyon, which lies within the estuary of the Congo river on the coast of West Africa, and distributed the rest down the canyon system. The last mooring was placed around 1200km offshore at a depth of nearly 5000m.
The moorings were meant to stay put for a year, carrying instruments that monitor the water column and sediment flows beneath them. But, just a few months later, in January 2020, something curious happened. The researchers received alerts that the moorings had popped up on the surface. “At first, we thought maybe one or two of the moorings had been disturbed by fishing boats,” says at Durham University, who was part of the project. But, one by one, in a regular fashion, each mooring sent an automated email to say it had surfaced. “We started to think something major had happened,” says Baker.
The Congo Canyon avalanche
They were right. A sediment avalanche had started at the mouth of the Congo river. As it moved, it gathered speed – reaching about 30km/h – and turned into an underwater flow that travelled for more than 1130km, making it the longest sediment flow ever measured. It carried a huge amount of sediment and dumped it on the South Atlantic abyssal plain at a depth of more than 5000m. Two major seabed telecommunication cables were sliced by the flow, causing the internet to slow significantly across much of Africa, from Nigeria to South Africa.
The fact that the moorings had been caught in such a huge current was exciting, but there was a problem. Their anchors had been broken by the powerful flow and they had to be recovered before researchers could read their data. Each was just three times the size of a football and was lost in the Atlantic as the covid-19 pandemic was erupting. To make matters worse, the batteries in the moorings would only survive for three months. “I never expected to get any of them back,” says Talling.
Luckily, there were clues. “The automated email told us the time that each mooring arrived at the ocean surface, which gave us a rough idea of where they might be,” says Talling. Boats were directed to the places where the moorings had first popped up. The vessel sent to repair the internet cables was one of the first to reach the scene and it joined in the hunt, recovering five of the sensors before they strayed too far or ran out of battery. Nine of the moorings were eventually recovered.
In July 2022, the team’s results were published, revealing even more astounding details. We know earthquakes can trigger canyon-flushing avalanches, like the Grand Banks event. But the Congo turbidity current showed that river floods can also instigate them. Three weeks before the Congo avalanche, the Congo river had its largest flood in more than 50 years. “The flood will have washed a lot of sediment into the upper part of the canyon, in the Congo river mouth, but it didn’t instantly trigger a flow,” says Talling. “Instead, the sediment stacked up for a few weeks before toppling – that was a surprise.” Exactly what made the sediment finally topple isn’t yet clear, but the flows are more likely to start at low tide, when gas bubbles trapped in the sediment are able to expand, weakening the sediment structure.
Surprisingly frequent canyon floods
Huge canyon-flushing events could be far more frequent than previously expected, in the Congo Canyon at least. Major earthquakes might trigger a canyon-flushing event every 100 to 300 years, but the Congo river experiences a significant flood every 20 to 50 years. And smaller flows, carrying sediment around 200km, also seem to be fairly frequent. It has been shown that the upper reaches of the canyon are active around one-third of the time.
This might sound worrying, especially for cable breaks, but there is a potential upside. In future, with flooding predicted to increase due to climate change, there could be more underwater flows, burying more carbon. “We used to think most organic carbon washing off the land was dumped on the continental shelf,” says Talling. This would mean much of it was oxidised and returned to the atmosphere as CO2. “But these results show that over geological timescales – thousands of years – turbidity currents could be an important mechanism for locking away carbon that we’d previously almost ignored,” he says.
Right now, geologists are still at the stage of trying to quantify how much carbon is transported this way, but if turbidity currents are a significant carbon pump, as the Congo and Whittard measurements suggest, then we will need to alter our models of the carbon cycle.
Sadly, carbon isn’t the only thing turbidity currents are transporting to the sea floor. “Pesticides, pharmaceuticals and plastics will also be ending up in large quantities in the deep sea via these flows,” says Hodgson. “That’s worrying because they are entering the base of the food chain and we don’t know what kind of impact they will have.”
Previous research has shown that 99 per cent of the 8 million tons of plastic entering the ocean each year is unaccounted for. The suspicion is that much of this ends up in the deep ocean, but how it gets there has – until now – been a bit of a mystery. “It’s starting to look like turbidity currents are an important mechanism,” says Hodgson.
The Congo Canyon study made geologists rethink just how monumental these flows can be. “Our estimates suggest that the equivalent of one-third of the sediment eroded by all the rivers in the world in one year was flushed down this canyon in one single event lasting a couple of days,” says Talling. In recent years, research is suggesting that underwater landslides, another kind of Earth-shaping marine event, are also happening much more than we thought (see “Tsunami starters”). All in all, there seems to be a lot more going on than we had ever realised.
Now, researchers are keen to venture further afield and investigate other detached canyons. Top of the list is the Amazon Canyon, one of the biggest submarine channels in the world, which starts about 200km off the coast of Brazil and extends more than 1000km into the South Atlantic.
Vast quantities of organic carbon are leached from the Amazon rainforest into the Amazon river, then washed out onto the continental shelf. “Given what we are seeing in both the Whittard and Congo submarine canyons,” says Clare, “I think we need to be asking more questions about the fate of that organic material.”
Tsunami starters
Some 8000 years ago, a massive chunk of Norway’s continental shelf collapsed. Known as the Storegga Slide, around 3200 cubic kilometres of sediment plummeted to the ocean floor. The resulting tsunami travelled as far as Greenland and Canada, with waves of more than 20 metres high crashing over the Shetland Islands in the UK. Some propose the event may have drowned Doggerland, an area once above the waves, but now submerged by the North Sea, cutting the UK off from Europe.
As opposed to the flow of a turbidity current (see main story), submarine landslides shift a coherent lump of sediment in one single, dramatic movement. But like turbidity currents, submarine landslides hadn’t been measured until recently.
, a seismologist at the Scripps Institution of Oceanography in California, wasn’t looking for landslides when he started analysing seismic readings from the Gulf of Mexico. “I was looking for earthquakes,” he says. “But then my attention was drawn to lots of odd seismic signals lighting up the Gulf of Mexico, which was a surprise because this isn’t an active tectonic area.”
Fan and his colleagues identified 85 previously unknown submarine landslides in the gulf between 2008 and 2015, 75 of which were triggered by the arrival of waves emanating from earthquakes as far away as 1500km and with magnitudes as low as 5. “We’ve never been able to link these events over such a long distance before,” says Fan. It isn’t yet clear whether the Gulf of Mexico – which has jelly-like sediments – is a special case or whether remote earthquakes are triggering submarine landslides in other parts of the world, too.
Kate Ravilious is a freelance journalist based in York, UK
