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Science And Nature

Ocean scientists measure sediment plume stirred up by deep-sea-mining vehicle

Exactly what will function as impact to the ocean if humans are to mine the deep sea? It is a question that’s gaining urgency as fascination with marine minerals is continuing to grow.

The ocean’s deep-sea bed is scattered with ancient, potato-sized rocks called “polymetallic nodules” which contain nickel and cobalt — minerals which are in popular for the manufacturing of batteries, such as for example for powering electric vehicles and storing renewable energy, and in reaction to factors such as for example increasing urbanization. The deep ocean contains vast levels of mineral-laden nodules, however the impact of mining the ocean floor is both unknown and highly contested.

Now MIT ocean scientists have shed some light on this issue, with a fresh study on the cloud of sediment a collector vehicle would stir up since it accumulates nodules from the seafloor.

The analysis, appearing in Science Advances, reports the outcomes of a 2021 research cruise to an area of the Pacific Ocean referred to as the Clarion Clipperton Zone (CCZ), where polymetallic nodules abound. There, researchers equipped a pre-prototype collector vehicle with instruments to monitor sediment plume disturbances because the vehicle maneuvered over the seafloor, 4,500 meters below the ocean’s surface. By way of a sequence of carefully conceived maneuvers. the MIT scientists used the automobile to monitor its sediment cloud and measure its properties.

Their measurements showed that the automobile created a dense plume of sediment in its wake, which spread under its weight, in a phenomenon known in fluid dynamics as a “turbidity current.” Since it gradually dispersed, the plume remained relatively low, staying within 2 meters of the seafloor, instead of immediately lofting higher in to the water column as have been postulated.

“It’s a significant different picture of what these plumes appear to be, compared to a few of the conjecture,” says study co-author Thomas Peacock, professor of mechanical engineering at MIT. “Modeling efforts of deep-sea mining plumes will need to take into account these procedures that people identified, to be able to assess their extent.”

The study’s co-authors include lead author Carlos Muoz-Royo, Raphael Ouillon, and Souha El Mousadik of MIT; and Matthew Alford of the Scripps Institution of Oceanography.

Deep-sea maneuvers

To get polymetallic nodules, some mining companies are proposing to deploy tractor-sized vehicles to underneath of the ocean. The vehicles would vacuum up the nodules alongside some sediment along their path. The nodules and sediment would then be separated within the vehicle, with the nodules sent up by way of a riser pipe to a surface vessel, some of the sediment will be discharged immediately behind the automobile.

Peacock and his group have previously studied the dynamics of the sediment plume that associated surface operation vessels may pump back to the ocean. Within their current study, they centered on the contrary end of the operation, to gauge the sediment cloud developed by the collectors themselves.

In April 2021, the team joined an expedition led by Global Sea Mineral Resources NV (GSR), a Belgian marine engineering contractor that’s exploring the CCZ for methods to extract metal-rich nodules. A European-based science team, Mining Impacts 2, also conducted separate studies in parallel. The cruise was the initial in over 40 years to check a “pre-prototype” collector vehicle in the CCZ. The device, called Patania II, stands about 3 meters high, spans 4 meters wide, and is approximately one-third how big is just what a commercial-scale vehicle is likely to be.

As the contractor tested the vehicle’s nodule-collecting performance, the MIT scientists monitored the sediment cloud created in the vehicle’s wake. They did so using two maneuvers that the automobile was programmed to take: a “selfie,” and a “drive-by.”

Both maneuvers began just as, with the automobile aiming in a straight line, all its suction systems fired up. The researchers allow vehicle drive along for 100 meters, collecting any nodules in its path. Then, in the “selfie” maneuver, they directed the automobile to show off its suction systems and double back around to operate a vehicle through the cloud of sediment it had just created. The vehicle’s installed sensors measured the concentration of sediment in this “selfie” maneuver, allowing the scientists to monitor the cloud within a few minutes of the automobile stirring it up.

For the “drive-by” maneuver, the researchers placed a sensor-laden mooring 50 to 100 meters from the vehicle’s planned tracks. Because the vehicle drove along collecting nodules, it created a plume that eventually spread at night mooring after a couple of hours. This “drive-by” maneuver enabled the team to monitor the sediment cloud over an extended timescale of a long time, capturing the plume evolution.

Out of steam

Over multiple vehicle runs, Peacock and his team could actually measure and track the evolution of the sediment plume developed by the deep-sea-mining vehicle.

“We saw that the automobile will be driving in pure water, seeing the nodules on the seabed,” Peacock says. “And suddenly there’s this very sharp sediment cloud coming through once the vehicle enters the plume.”

From the selfie views, the team observed a behavior that has been predicted by a few of their previous modeling studies: The automobile stirred up much level of sediment that has been dense enough that, even with some mixing with the encompassing water, it generated a plume that behaved almost as another fluid, spreading under its weight in what’s referred to as a turbidity current.

“The turbidity current spreads under its weight for quite a while, tens of minutes, but since it does so, it’s depositing sediment on the seabed and finally running out of steam,” Peacock says. “From then on, the ocean currents get more powerful than the natural spreading, and the sediment transitions to being carried by the ocean currents.”

By enough time the sediment drifted at night mooring, the researchers estimate that 92 to 98 percent of the sediment either settled back off or remained within 2 meters of the seafloor as a low-lying cloud. There’s, however, no guarantee that the sediment always stays there instead of drifting further up in the water column. Recent and future tests by the study team want into this question, with the purpose of consolidating understanding for deep-sea mining sediment plumes.

“Our study clarifies the truth of what the original sediment disturbance appears like if you have a particular kind of nodule mining operation,” Peacock says. “The big takeaway is that we now have complex processes like turbidity currents that happen once you do this type of collection. So, any effort to model a deep-sea-mining operation’s impact will need to capture these procedures.”

This research was supported, partly, by the National Science Foundation, ARPA-E, the 11th Hour Project, the Benioff Ocean Initiative, and Global Sea Mineral Resources. The funders had no role in virtually any aspects of the study analysis, the study team states.

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