Tracking avalanches – under the sea
Underwater avalanches and turbidity currents carry huge amounts of sediment, organic material and pollutants down submarine canyons and into the deep sea. Yet geologists know very little about how sediment moves during these events. This month, in what may be the most ambitious submarine-canyon study ever attempted, marine geologists from several countries are placing dozens of sophisticated instruments in Monterey Canyon. The Coordinated Canyon Experiment (CCE) promises to give scientists a uniquely detailed and comprehensive view of sediment movement within the canyon.
The project is being led by geologist Charlie Paull of the Monterey Bay Aquarium Research Institute (MBARI), in collaboration with researchers from the United States Geological Survey, Ocean University of China and two British instiutions: the National Oceanography Centre in Southampton and the University of Hull.
Paull and his fellow researchers hope to learn what triggers underwater sediment flows, how fast they move and how far they travel. They’ll also study how sediment flows scour the seafloor and reshape the canyon over time.
Beach-ball sized BEDs
In addition to deploying fixed instrument arrays, four beach-ball-sized “benthic event detectors” (BEDs) will be buried in the sediment. When flows occur, the BEDs will be carried along with the sediment. Sensors inside each BED will record how fast they move, how far they go and how they’re tumbled by the flow.
Unusual instruments such as the BEDs are necessary because the floor of Monterey Canyon is such a dynamic place. The profile of the undersea Monterey Canyon resembles Arizona’s Grand Canyon, with steep-sided walls and a narrow, winding canyon floor. Both have central channels filled with sand.
Sand in the Grand Canyon is moved down-canyon by the Colorado River. After studying the oceanic “river of sand” in Monterey Canyon for 15 years, Paull and his colleagues have concluded that sediment moves down Monterey Canyon in a variety of ways.
Some is moved by strong currents that carry just a small volume of sediment. More dramatic flows, known as turbidity currents, are dense, fast-moving slurries of sand and water that can travel kilometers down the canyon in a matter of minutes. Still other flows occur when sediment on the floor of the canyon becomes unstable and slumps down-canyon. To make matters even murkier, some events could involve all three of these processes.
Unraveling the mechanics of sediment flows
During the Coordinated Canyon Experiment, researchers hope to study all of these different types of sediment flows by gathering detailed data from a variety of instruments over a large area. As Paull put it, “We want to determine not just when sediment moves in the axis of the canyon, but how it moves, how long it moves and where it moves. The point is to cover as much of the canyon as possible, from the head down to a couple of thousand meters depth.”
“We usually measure just a snapshot in time and space, so we’re like the blind men and the elephant – just seeing part of the picture,” he added. “By putting as many instruments in the canyon as we can, all at one time, we’ll get a better view of what’s going on down there.”
The CCE is scheduled to continue until spring 2017. Because it’s so hard to communicate with instruments on the seafloor, the researchers won’t get their first detailed information until April 2016. That’s when MBARI’s underwater robots service the instruments and retrieve the data – following what’s expected to be a strong El Niño winter. If the Central California coast experiences powerful storms and unusually large waves, they could sweep huge volumes of sand from the beaches of Monterey Bay and into the head of Monterey Canyon, potentially triggering underwater avalanches.
Paull is optimistic that the team will collect plenty of interesting data – even if not all of the equipment survives the onslaught.
“There is always a risk in putting instruments in the canyon,” Paull said. “But if we don’t take risks, we’re never going to be able to figure out what’s going on down there.”