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Historically, turbidity currents have been described as fast currents that descend into submarine canyons, carrying sand and mud in deep water. But a new document in Nature Communications shows that, rather than simply being composed of seawater laden with sediment on the seafloor, turbidity currents also involve large-scale movements of the seabed itself. This dramatic discovery, the result of a multi-institutional study of Monterey Canyon that lasted 18 months, could help oceanographers avoid damage to pipelines, communication cables and other bottom structures. oceanic.
Geologists have been aware of turbidity currents since at least 1929, when a major earthquake unleashed a violent current that traveled several hundred kilometers and damaged 12 transatlantic communication cables. Turbidity currents are still a threat as people place more and more cables, pipelines and other structures on the sea floor. Turbidity currents are also important for petroleum geologists as they leave behind them layers of sediment that make up some of the largest oil reserves in the world.
Despite nearly a century of research, geologists have struggled to develop a conceptual model describing in detail the formation and evolution of turbidity currents. The coordinated canyon experiment was designed, in part, to resolve this debate. During this 18-month long study, researchers from the Monterey Bay Aquarium Research Institute (MBARI), the US Geological Survey, the University of Hull , the National Center for Oceanography, the University of Southampton, the University of Durham and the Ocean University of China have combined their expertise and equipment to monitor a stretch of Monterey Canyon 50 km long (31 miles) with unprecedented details.
During the experiment, researchers placed more than 50 different instruments at seven different locations in the canyon and performed detailed measurements on 15 different turbidity flows. Nearly all flows started near the head of the canyon in waters less than about 300 meters (1000 feet) deep. Once launched, the flows have traveled at least several kilometers in the canyon. The three largest flows traveled more than 50 kilometers and swept through the deepest canyon monitoring station at a depth of 1850 meters (6000 feet).
This extensive research program has shown that turbidity currents in Monterey Canyon involve both sediment movements saturated with water and sediment-laden water. As described in the recent Nature Communications In the paper, the most important part of the process is a dense layer of sediment saturated with water that moves rapidly on the bottom and remobilises the few upper meters of the pre-existing seabed.
This is very different from previous conceptual models of turbidity currents, which focused on turbid and sediment-laden water flows moving over the seabed. The authors of the recent paper have observed plumes of water sediments in turbidities, but suggest that they are secondary features that occur when the impulse of saturated sediments mix with the overlying seawater.
"This whole experiment was about understanding what was happening at the bottom of the canyon," said Charlie Paull, MBARI Marine Geologist and first author of the recent article. "For years, we've seen the instruments at the bottom move unexpectedly and we suspected that the seabed might be moving in. We now have real data showing when, where and how that happens." Among the instruments used in the experiment were current meters mounted on seven moorings along the bottom of the canyon. By analyzing the data from these instruments and measuring the time required for the flows to move between the moorings, the researchers were surprised to find that the flows appeared to descend into the canyon at speeds higher than the actual currents measured.
Although the tilt and other movements of the current meters may explain some of these observations, the scientists eventually concluded that their instruments were not simply displaced by turbid water currents flowing over the seabed.
The researchers also placed beach ball size sensors, called benthic event detectors (BEDs), in the seabed. BEDs were designed to be transported by turbidity flows while carrying instruments that recorded depth, horizontal and vertical movement and rotation. Other motion sensors were mounted on large steel frames weighing up to 800 kilograms. These were designed to remain stationary while the flow was passing around them.
However, BEDs and heavy chassis were washed away into the canyon during high turbidity. In fact, the heavy mounts of clumsy instruments often move as fast as relatively light and contoured BEDs.
The researchers also noticed large waves of sand up to two meters high on the canyon floor. Repeated bottom surveys have shown that these sand waves are moving dramatically during turbidity events, restoring the upper two or three meters of the seabed. But the researchers still did not know exactly how this remodeling had occurred.
BED data provided an important index. During many events, BEDs did not just go down the canyon in deeper waters, but traveled as fast or faster than the overlying waters. They also went up and down in the current up to three meters at regular intervals.
The researchers concluded that, rather than being "dragged" along the bottom by a strong current, their instruments were "driven" by a dense and seductive layer of sediment saturated with water. They hypothesized that BED movements occurred when the instruments traveled on individual sand waves. As Paull noted, "the BEDs provided a vital core of new data that allowed us to understand the movement of the seabed for the first time".
"Manuals and modeling efforts have traditionally focused on dilute flows of sediment-laden water to the bottom," added Paull. "But we now know that diluted runoff is only part of the equation.It turns out that they are the end of the process, which actually begins at the bottom of the sea. "
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