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Marine scientists measure sediment plumes hoisted up by deep-sea mining vehicles. MIT News

What impact will human mining of the deep sea have on the ocean? It is a question of increasing urgency as interest in marine minerals grows.

Beneath the deep ocean floor are ancient potato-sized rocks called “polymetallic nodules” that contain nickel and cobalt, minerals in high demand for making batteries, such as powering electric vehicles and storing renewable energy. Dotted. Respond to factors such as increasing urbanization. The deep sea contains a huge amount of mineral-bearing nodules, but the effects of seafloor mining are unknown and highly contested.

Marine scientists at MIT are now shedding light on this topic with a new study of sediment clouds that are whipped up by collection vehicles as they pick up nodules from the ocean floor.

This research is today scientific progress, We report the results of a 2021 research cruise to a region of the Pacific Ocean known as the Clarion Clipperton Zone (CCZ), which is rich in polymetallic nodules. So the researchers equipped a prototype collection vehicle with instruments to monitor sediment plume disturbances as the vehicle traveled across the seafloor at 4,500 meters below sea level. Through a series of carefully thought out operations. Scientists at MIT used this vehicle to monitor their own sediment cloud and measure its properties.

According to their measurements, the vehicle created a dense plume of sediment in its wake that spread under its own weight, a phenomenon known in hydrodynamics as a “turbidity current.” As it gradually dissipated, the plume remained relatively low, staying within two meters of the seafloor.

“Compared to some speculation, it’s a completely different picture of what these plumes might look like,” said study co-author Thomas Peacock, a professor of mechanical engineering at MIT. “Deep-sea mining plume modeling work should account for these identified processes in order to assess their extent.”

Co-authors on this study include MIT lead authors Carlos Muñoz-Royo, Raphael Ouillon, and Souha El Mousadik. Matthew Alford of the Scripps Institution of Oceanography.

deep sea operation

To collect the polymetallic nodules, some mining companies have proposed deploying tractor-sized vehicles on the ocean floor. The vehicle will vacuum the nodule along with some deposits along its path. The nodules and sediment are then separated in the vehicle, the nodules are sent through riser pipes to the surface craft, and most of the sediment is discharged directly behind the vehicle.

Peacock and his group have previously studied the dynamics of sediment plumes that associated surface warfare vessels can send back to sea. Their current study focused on the other side of the operation to measure sediment clouds created by the collectors themselves.

In April 2021, the team participated in a survey led by Belgian marine engineering contractor Global Sea Mineral Resources NV (GSR). GSR is investigating ways to extract metal-rich nodules in CCZ. A European-based scientific team, Mining Impacts 2, also conducted another parallel study. The cruise was his first in over 40 years to test a “pre-production” collection vehicle at CCZ. Called the Patania II, the machine is about three meters high and four meters wide, about one-third the size of a commercial vehicle.

While the contractor tested the vehicle’s nodule collection performance, MIT scientists monitored the sediment clouds created in the vehicle’s wake. They did so using two maneuvers the vehicle was programmed to perform: “selfie” and “drive-by.”

Both maneuvers were started in the same manner, with the vehicle going in a straight line and all suction systems on. The researchers drove his vehicle 100 meters and collected nodules along the way. Then, in a “selfie” maneuver, they instructed the vehicle to turn off its suction system, doubled back, and made their way through the cloud of sediment they had just created. Sensors attached to the vehicle measured the sediment concentration during this “selfie” maneuver, allowing scientists to monitor the cloud within minutes of the vehicle stirring it up.

play video

Footage of the Patania II Pre-Prototype Collector Vehicle entering, passing through, and exiting a lowland turbid plume as part of a selfie maneuver. In terms of scale, the instrument column attached to the front of the vehicle reaches about 3m from the seabed. Videos are 20x faster. Credit: Global Sea Mineral Resources

For “drive-by” operations, the researchers placed sensor-equipped moorings 50 to 100 meters from the vehicle’s planned trajectory. As the vehicle traveled collecting nodules, a plume of smoke developed and spread beyond the anchorage after an hour or two. This ‘drive-by’ operation allowed the team to monitor the sediment cloud and capture the evolution of the plume over longer timescales of hours.

ran out of steam

Multiple vehicle runs allowed Peacock and his team to measure and track the evolution of the sediment plume produced by the deep-sea mining vehicle.

“We saw vehicles driving through clear water and looking at the nodules on the seafloor,” says Peacock. “And when the vehicle enters the plume, all of a sudden there’s this very sharp sediment cloud.”

From the selfie view, the team observed behavior predicted by some previous users. modeling studies: The vehicle stirred up a large amount of sediment that was sufficiently dense to produce a plume that behaved more or less like another liquid, even after some mixing with the surrounding water, and spread under its own weight, known as a turbidity stream. .

“The turbidity current spreads under its own weight for tens of minutes, but as it does so, it deposits sediment on the seafloor and eventually runs out of steam,” says Peacock. “Then the ocean currents become stronger than they naturally spread, and the sediments migrate to be carried by the currents.”

By the time the sediment passes the moorings, researchers estimate that 92-98% of the sediment has subsided or remains within two meters of the seafloor as low-lying clouds. However, there is no guarantee that the sediment will always stay there rather than drift further up the water column. Recently Future work by the research team will explore this question with the goal of synthesizing our understanding of deep-sea mining sediment plumes.

“Our study reveals the reality of what initial sediment disturbances look like when you’re doing certain types of nodule mining operations,” says Peacock. “The key point is that there are complex processes like turbidity flow that occur when doing this type of collection. must be captured.”

“The sediment plumes produced by deep-sea mining are of great concern in terms of environmental impacts, as they potentially extend over large areas beyond the actual mining site and affect deep-sea organisms,” says Marine Geology. Scholar Henko de Stigter says: The Royal Dutch Institute of Oceanography was not involved in the study. “The current paper provides important insights into the early development of these plumes.”

This work was supported in part by the National Science Foundation, ARPA-E, the 11 Hour Project, the Benioff Ocean Initiative, and the Global Sea Mineral Resource. The research team says the funders have no role in any aspect of the research analysis.

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