3D image of the Monterey Canyon system derived from approximately 4,000,000 soundings. (Credit: NOAA)
Just like avalanches on the surface, large slides can occur under the ocean’s wake. These underwater avalanches can carry huge amounts of sediment, organic material and even pollutants down into submarine canyons and further into the deep sea.
Despite the destructive nature of these events, geologists know very little about how sediment moves as they play out. To learn more, researchers from multiple countries are placing dozens of instruments in Monterey Canyon, an underwater gorge that would look similar to the Grand Canyon if it weren’t submerged.
The international effort, called the Coordinated Canyon Experiment (CCE), promises to give scientists a uniquely detailed and comprehensive view of sediment movement within the canyon. It is being led by scientists at the Monterey Bay Aquarium Research Institute in partnership with others at the U.S. Geological Survey, the Ocean University of China, the National Oceanography Centre in Southampton, England, and the University of Hull, England.
During the canyon experiment, researchers hope to gather information on what triggers underwater sediment flows, how fast they move and how far they travel. They will also try to measure how much coarse sediment is carried along near the seafloor and how much fine sediment is carried tens of meters above the seafloor. Finally, they will study how the sediment flows change the contours of the seafloor and the shape of the canyon over time.
To make these measurements, the researchers will be pooling both their expertise and a wide variety of instruments. Robotic vehicles will be used to place instruments on the seafloor, provide communications links to shore, and make detailed maps of the canyon floor.
Ten arrays of instruments will be installed on the seafloor to monitor movements of water and sediment at depths of 200 to 1,900 meters and distances of 4 to 40 kilometers from shore. These instruments will measure currents, sediment concentrations, and the physical properties of the seawater. The instruments will be left in place for up to 18 months, documenting any flows that take place during that time.
In addition to the fixed instrument arrays, four beach-ball-sized “benthic event detectors” (BEDs) will be buried in the seafloor sediment. When sediment 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 are tumbled by the flow.
Unusual instruments such as the BEDs are necessary because the floor of Monterey Canyon is such an active place. If you could see Monterey Canyon from the sea surface, it would look somewhat like the Grand Canyon, with steep-sided walls and a narrow, winding canyon floor. Both canyons have central channels filled with sand.
Sand in the Grand Canyon is moved down-canyon by the Colorado River. After studying the “river of sand” in Monterey Canyon for 15 years, scientists have so far concluded that sediment moves down Monterey Canyon in a variety of ways. Some sediment flows consist of strong water currents carrying just a small amount of sediment. Other flows, known as turbidity currents, consist of 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 canyon events could involve all three of these processes.
During the Coordinated Canyon Experiment, researchers hope to study all of these different types of sediment flows by gathering extremely detailed data from a wide variety of instruments over a large area. Scientists say that they 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.
Placing all of the instruments in the canyon involved a concerted effort throughout the month of October 2015. But that’s just the beginning. The CCE is scheduled to continue until spring 2017. In fact, because it is so hard to communicate with instruments on the seafloor, the researchers won’t get detailed information from the instruments until the middle of 2016 when underwater robots will be used to service the instruments and retrieve data.
Top image: 3D image of the Monterey Canyon system derived from approximately 4,000,000 soundings. (Credit: NOAA)