What will be the impact on the ocean if humans have to exploit the sea depths? This is a question that is gaining in urgency as interest in marine minerals grows.
The deep seabed of the ocean is dotted with ancient potato-sized rocks called “polymetallic nodules” which contain nickel and cobalt – minerals in high demand for making batteries, such as powering electric vehicles and storing renewable energy, and in response to factors such as increasing urbanization. The deep ocean contains large amounts of mineral-laden nodules, but the impact of mining the ocean floor is both unknown and highly disputed.
Today, ocean scientists at MIT shed some light on the matter, with a new study of the cloud of sediment that a collector vehicle would lift by picking up nodules from the seafloor.
The study, published today in scientific advances, reports the results of a 2021 research cruise to a region of the Pacific Ocean known as the Clarion Clipperton Zone (CCZ), where polymetallic nodules abound. There, researchers outfitted a pre-prototype collector vehicle with instruments to monitor disturbances from the sediment plume as the vehicle maneuvered on the seafloor, 4,500 meters below the ocean surface. Through a sequence of carefully designed maneuvers. MIT scientists used the vehicle to monitor its own 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 dispersed, the plume remained relatively low, remaining less than 2 meters from the seabed, instead of rising immediately higher in the water column as had been postulated.
“It’s quite a 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 will need to consider these processes that we have identified, in order to assess their extent.”
The study’s co-authors include lead author Carlos Muñoz-Royo, Raphael Ouillon, and Souha El Mousadik of MIT; and Matthew Alford of the Scripps Institution of Oceanography.
Maneuvers on the high seas
To collect the polymetallic nodules, some mining companies offer to deploy tractor-sized vehicles to the ocean floor. The vehicles would suck up the nodules as well as sediment along their path. The nodules and sediment would then be separated inside the vehicle, with the nodules being sent by a riser to a surface vessel, while most of the sediment would be evacuated immediately behind the vehicle.
Peacock and his group have previously studied the dynamics of the sediment plume that associated surface ships can blow back into the ocean. In their current study, they focused on the opposite end of the operation, 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 for ways to extract metal-rich nodules. A European-based science team, Mining Impacts 2, also conducted separate studies in parallel. The campaign was the first in over 40 years to test a “pre-prototype” collectible vehicle in the CCZ. The machine, called Patania II, is about 3 meters high, spans 4 meters wide and is about a third the size of a commercial-scale vehicle.
While the contractor tested the vehicle’s nodule-collecting performance, MIT scientists monitored the sediment cloud created in the vehicle’s wake. They did this using two maneuvers the vehicle was programmed for: a “selfie” and a “drive-by”.
Both maneuvers begin in the same way, with the vehicle going in a straight line with all of its vacuum systems activated. The researchers let the vehicle roll for 100 meters, collecting the nodules in its path. Then, in the “selfie” maneuver, they ordered the vehicle to turn off its vacuum systems and turn around 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 minutes after the vehicle stirred it.
A film of the collectible pre-prototype Patania II vehicle entering, traversing and exiting the low-level turbidity current plume as part of a selfie operation. For scale, the instrumentation station attached to the front of the vehicle reaches approximately 3m above the seabed. The film is accelerated by a factor of 20. Credit: Global Sea Mineral Resources
For the drive-by maneuver, the researchers placed a sensor-laden mooring 50 to 100 meters from the vehicle’s intended trajectories. As the vehicle rolled collecting nodules, it created a plume that eventually spread beyond the anchorage after an hour or two. This drive-by maneuver allowed the team to monitor the sediment cloud on a longer time scale of several hours, capturing the evolution of the plume.
Over several 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 the vehicle would roll through clear water, seeing the nodules on the seabed,” Peacock said. “And then, all of a sudden, there’s this very sharp sediment cloud that passes as 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 stirred up a heavy amount of sediment dense enough that, even after some mixing with the surrounding water, it generated a plume which behaved almost like a separate fluid, spreading under its own weight in this called a turbidity current.
“The turbidity current propagates under its own weight for some time, tens of minutes, but as it does, it deposits sediment on the seabed and eventually dies out,” says Peacock. “After that, the ocean currents become stronger than the natural spread, and the sediments are transported by the ocean currents.”
By the time the sediment drifted past the anchorage, the researchers estimate that 92-98% of the sediment had settled or remained within 2 meters of the seabed as a low cloud. There is, however, no guarantee that the sediment will always stay there rather than drifting higher in the water column. Recent and the research team’s future studies are addressing this question, with the aim of solidifying the understanding of deep-sea mining sediment plumes.
“Our study clarifies the reality of what initial sediment disturbance looks like when you have some type of nodule mining,” says Peacock. “The big takeaway is that there are complex processes such as turbidity currents that occur when you do this type of collection. So any effort to model the impact of a deep-sea mining operation will need to capture these processes.
“Sediment plumes produced by seabed mining are a major concern in terms of environmental impact, as they will spread over potentially large areas beyond the actual mining site and affect the life on the high seas,” says Henko de Stigter, a marine geologist at the Royal Netherlands Institute for Marine Research, who was not involved in the research. “The current paper provides essential information on the initial development of these plumes.”
This research was supported, in part, by the National Science Foundation, ARPA-E, the 11th Hour Project, the Benioff Ocean Initiative, and Global Sea Mineral Resources. Funders had no role in any aspect of the research analysis, the research team states.