What would be the impact on the ocean if humans were to mine the deep sea? It is a question that is gaining urgency as interest in marine minerals grows.
The deep ocean floor is littered with ancient potato-sized rocks called “polymetallic nodules” that contain nickel and cobalt — minerals that are much needed to make batteries, such as to power electric vehicles and store renewable energy, and in response to factors such as increased urbanization. The ocean depths contain vast amounts of mineral-laden nodules, but the impact of ocean floor mining is unknown and highly controversial.
Now oceanographers at the Massachusetts Institute of Technology (MIT) have shed some light on the subject, with a new study of the sediment cloud that a collecting vehicle may stir up when it picks up nodules from the sea floor.
The study, which appears in science advances, Reports the results of a research cruise in 2021 to an area of the Pacific known as the Clarion-Clipperton Zone (CCZ), where polymetallic nodules abound. There, the researchers outfitted a prototype assembly craft with instruments to monitor disturbances of sediment plumes as the craft maneuvered across the sea floor, 4,500 meters below the ocean surface. Through a series of carefully designed maneuvers. MIT scientists used the composite to observe its sediment cloud and measure its properties.
Their measurements showed that the craft created a dense plume of sediment in its wake, which spread out under its own weight, in a phenomenon known in fluid dynamics as a “turbidity stream.” As the water column gradually dispersed, the column remained relatively low, remaining two meters from the sea floor, in contrast to the immediate rise in the water column as had been assumed.
“It’s a very different picture of what these plumes look like, compared to some guesses,” says study co-author Thomas Peacock, professor of mechanical engineering at MIT. “Deep-sea mining plume modeling efforts must take into account these processes that we have identified, in order to assess their prevalence.”
Study co-authors are lead author Carlos Muñoz Royo, Rafael Oyon, and Soha El Mosadeq of the Massachusetts Institute of Technology. and Matthew Alford of the Scripps Institution of Oceanography.
deep sea maneuvers
To collect polymetallic nodules, some mining companies are proposing to deploy tractor-sized compounds to the ocean floor. The vehicles empty the nodules with some sediment along their path. The nodules and sediment are then separated inside the vehicle, with the nodules sent through a rising tube to a surface vessel, while most of the sediment is discharged directly behind the vehicle.
Peacock and his team have previously studied the dynamics of the sediment plume that associated surface ship operations may pump back into the ocean. In their current study, they focused on the other end of the process, to measure the sediment cloud 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 exploring the CCZ in search of ways to extract mineral-rich nodules. A European scientific team, Mining Impacts 2, also conducted separate studies in parallel. The cruise was the first in more than 40 years to test a “prototype” assembly vehicle in the Clarion-Clipperton Fracture Zone. The machine, called Patania 2, is about 3 meters high, extends 4 meters wide, and is about a third the size of what a commercial vehicle would be expected to be.
While the contractor tested the car’s nodule assembly performance, MIT scientists monitored the cloud of sediment created in the car’s wake. They did this using two maneuvers that the car was programmed to take: “self-portrait” and “car on the road.”
Both maneuvers started the same way, the car sped off in a straight line, and all of its suction systems were turned on. The researchers allowed the vehicle to walk 100 metres, collecting any nodules in its path. Then, in a “selfie” maneuver, they instructed the vehicle to turn off its suction systems and double its motion to drive through the cloud of sediment it had just created. Sensors installed in the vehicle measured the concentration of sediment during this “selfie” maneuver, allowing scientists to monitor the cloud within minutes of moving the vehicle.
For the “driving” maneuver, the researchers placed a sensor-loaded berth 50 to 100 meters from the vehicle’s planned paths. As the car travels along the collecting nodules, it forms a plume that eventually spreads past the mooring after an hour or two. The “driving” maneuver enabled the team to observe the sediment cloud over a longer timescale of several hours, and capture the evolution of the plume.
ran out of steam
Over the course of several rover rounds, Peacock and his team were able to measure and track the evolution of the plume of sediment generated by the deep-sea mining rover.
“We saw that the car was going in clear water, and you see nodules on the sea floor,” Peacock says. “Then suddenly a very sharp cloud of sediment appears when the car gets into the shaft.”
Through selfies, the team observed a behavior predicted by some previous modeling studies: The vehicle stirred up a large amount of sediment that was sufficiently dense that, even after mixing with the surrounding water, generated a plume that behaves as a separate fluid, spreading under its own weight in what’s known as a turbidity stream.
“The turbidity stream propagates under its own weight for some time, for tens of minutes, but during that, it deposits sediment on the sea floor and eventually runs out of steam,” Peacock says. “After that, ocean currents become stronger than normal diffusion, and the sediment turns into ocean currents.”
By the time the sediments drifted through the anchorage, the researchers estimated that 92 to 98 percent of the sediments had either settled back down or remained within two meters of the sea floor as a low cloud. However, there is no guarantee that sediment will always remain there rather than drift further into the water column. Recent and future studies by the research team investigate this question, with the goal of furthering the understanding of the plumes of deep-sea mining sediments.
“Our study really shows what an initial sediment disturbance looks like when you have a certain type of nodule mining process,” Peacock says. “The big takeaway is that there are complex processes such as turbidity currents that occur when doing this type of gathering. So, any effort to model the impact of deep sea mining has to capture these processes.”
This research was supported in part by the National Science Foundation, ARPA-E, The Eleventh Hour Project, the Benioff Ocean Initiative, and Global Sea Mineral Resources. The research team stated that the funders had no role in any aspect of the research analysis.