What will be the impact on the ocean if humans dig deep into the sea? This is a question that is gaining urgency as interest in marine minerals has grown.
The ocean floor is littered with ancient potato-sized rocks called “polymetallic nodules” that contain nickel and cobalt, minerals in great demand for battery production, powering electric vehicles and storing renewable energy, and in response to factors such as increased urbanization. The deep ocean contains large amounts of mineral-laden nodules, but the impact of ocean floor mining is unknown and highly contested.
Now ocean scientists at MIT have shed some light on the subject, with a new study of the cloud of sediment that a collection vehicle would elicit as it collects nodules from the sea floor.
The study, which appeared in Scientific advances, reports the results of a 2021 research cruise in a region of the Pacific Ocean known as the Clarion Clipperton Zone (CCZ), where polymetallic nodules abound. There, researchers equipped a pre-prototype collector vehicle with tools to monitor sediment plume disturbances as the vehicle maneuvered across the sea floor, 4,500 meters below the ocean’s surface. Through a carefully studied sequence of maneuvers. MIT scientists used the vehicle to monitor their sediment cloud and measure its properties.
Their measurements showed that the vehicle created a dense plume of sediment in its wake, which spread under its own weight, in a phenomenon known in fluid dynamics as a “turbidity current”. As it gradually dispersed, the plume remained relatively low, staying within 2 meters of the sea floor, instead of immediately rising higher in the water column as had been assumed.
“It’s a pretty different picture of what these plumes look like, compared to some of the conjectures,” says study co-author Thomas Peacock, a professor of mechanical engineering at MIT. “Modeling efforts of deep-sea mining plumes will need to take into account these processes we have identified in order to assess their extent.”
Co-authors of the study include lead author Carlos Muñoz-Royo, Raphael Ouillon, and MIT’s Souha El Mousadik; and Matthew Alford of the Scripps Institution of Oceanography.
To harvest the polymetallic nodules, some mining companies are proposing to deploy tractor-sized vehicles on the ocean floor. The vehicles would suck up the nodules along with some sediment along their path. The nodules and sediments would then be separated inside the vehicle, with the nodules sent through a riser to a surface vessel, while most of the sediment would be discharged immediately behind the vehicle.
Peacock and his team previously studied the dynamics of the sediment plume that ships associated with surface operations can pump back into the ocean. In their current study, they focused on the opposite end of the operation, to measure the cloud of sediment created 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 who is exploring the CCZ for ways to extract metal-rich nodules. A European-based science team, Mining Impacts 2, also conducted separate studies in parallel. The cruise was the first in over 40 years to test a “pre-prototype” collector vehicle in the CCZ. The car, called Patania II, is about 3 meters high, 4 meters wide and about one third the size of a commercial vehicle.
While the contractor tested the vehicle’s nodule-gathering performance, MIT scientists monitored the sediment cloud created in the vehicle’s wake. They did this using two maneuvers that the vehicle was programmed to perform: a “selfie” and a “drive-by”.
Both maneuvers began the same way, with the vehicle starting in a straight line, all of its intake systems turned on. The researchers let the vehicle drive for 100 meters, picking up any nodules in its path. Then, in the “selfie” maneuver, they ordered the vehicle to shut off its intake systems and go back to drive through the cloud of sediment it had just created. Sensors installed on the vehicle measured sediment concentration during this “selfie” maneuver, allowing scientists to monitor the cloud minutes after the vehicle moved.
For the drive-by maneuver, the researchers placed a sensor-laden mooring 50-100 meters from the vehicle’s planned tracks. As the vehicle proceeded to collect nodules, it created a plume that eventually spread beyond the mooring after an hour or two. This drive-by maneuver allowed the team to monitor the sediment cloud over a longer time span of several hours, capturing the evolution of the plume.
Over multiple vehicle runs, Peacock and his team were able to measure and track the evolution of the sediment plume created by the deep-sea mining vehicle.
“We saw that the vehicle was driving in clear water, seeing the nodules on the sea floor,” says Peacock. “And then suddenly there’s this very sharp cloud of sediment that comes when the vehicle enters the plume.”
From the selfie views, the team observed behavior that had been predicted by some of their previous modeling studies: the vehicle lifted a heavy amount of fairly dense sediment that, even after mixing with the surrounding water, generated a plume that behaved almost like a separate fluid, diffusing under its own weight in what is known as a turbidity current.
“The turbidity current spreads under its own weight for some time, tens of minutes, but as it does, it deposits sediment on the sea floor and eventually runs out of steam,” Peacock says. “After that, ocean currents become stronger than natural diffusion and the sediment passes to be carried by ocean currents.”
By the time the sediment drifted past the mooring, the researchers estimate that 92 to 98 percent of the sediment either re-deposited or remained within 2 meters of the sea floor as a low cloud. However, there is no guarantee that the sediment will always stay there rather than drifting higher in the water column. Recent and future studies by the research team are examining this question, with the aim of solidifying the understanding of plumes of deep-sea mineral sediments.
“Our study sheds light on the reality of what the initial sediment disturbance looks like when you have a certain type of lump extraction operation,” says Peacock. “The big result is that there are complex processes like turbidity currents that occur when doing this type of harvest. So any effort to model the impact of a deep-sea mining operation will have to capture these processes.”
What will happen to sediment plumes associated with deep-sea mining?
Carlos Muñoz-Royo et al, An in situ study of abyssal turbidity current sediment plumes generated by a preprotype collector vehicle for deep seabed polymetallic nodule extraction, Science advances (2022). DOI: 10.1126 / sciadv.abn1219. www.science.org/doi/10.1126/sciadv.abn1219
Provided by the Massachusetts Institute of Technology
Citation: Ocean scientists measure sediment plume raised by deepwater mining vehicle (2022, September 21) recovered September 22, 2022 from https://phys.org/news/2022-09-ocean-scientists-sediment-plume-deep – sea-mining.html
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