Oceanographers measure a plume of sediment kicked up by a deep-sea mining vehicle

What impact will it have on the ocean if humans mine the deep sea? This question is gaining urgency with the growing interest in sea minerals.

The deep ocean floor is littered with ancient, potato-sized rocks called “polymetallic nodules” that contain nickel and cobalt — minerals that are in high demand for making batteries such as powering electric vehicles and storing renewable energy in response to factors such as increasing urbanization. The deep sea contains vast amounts of mineral-bearing nodules, but the effects of seafloor degradation are both unknown and highly controversial.

Now, MIT marine scientists have shed some light on the issue with a new study of the plume of sediment that a collection vehicle would throw up when it picks up nodules from the seafloor.

The study, published 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 are abundant. There, researchers outfitted a pre-prototype collector vehicle with instruments to monitor plume disturbances as the vehicle maneuvered across the seabed 4,500 meters below sea level. Through a series of carefully considered maneuvers. MIT scientists used the vehicle to monitor its own sediment plume and measure its properties.

Their measurements showed that the craft produced a dense plume of sediment in its wake that spread under its own weight, in a phenomenon known in fluid dynamics as “turbidity current”. As it gradually dissipated, the cloud stayed relatively low, remaining within 2 meters of the sea floor, rather than immediately projecting higher into the water column as had been postulated.

“It’s a very different picture of what these springs look like compared to some guesswork,” says study co-author Thomas Peacock, a professor of mechanical engineering at MIT. “Efforts to model deep-sea mining plumes must consider these processes we have identified to assess their magnitude.”

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

deep sea maneuvers

To collect polymetallic nodules, some mining companies suggest using tractor-sized vehicles on the ocean floor. The vehicles would suck up the nodules along with some sediment along the way. The nodules and sediment would then be separated inside the craft, with the nodules being conveyed through a riser to a surface vessel, while most of the sediment would be discharged immediately aft of the craft.

Peacock and his group have previously studied the dynamics of the plume of sediment that associated surface operations vessels may be pumping back into the ocean. In their current study, they focused on the other end of the operation to measure the sediment plume generated by the collectors themselves.

In April 2021, the team joined an expedition led by Global Sea Mineral Resources NV (GSR), a Belgian shipbuilder exploring the CCZ for opportunities to extract metal-rich nodules. A Europe-based science team, Mining Impacts 2, conducted separate studies in parallel. The cruise was the first in over 40 years that a “pre-prototype” collector vehicle was tested at the CCZ. The machine, named Patania II, is about 3 meters high, has a wingspan of 4 meters and is about a third the size of a commercial vehicle.

While the contractor tested the vehicle’s nodule collection performance, MIT scientists monitored the plume of sediment formed in the vehicle’s wake. They did this with two maneuvers that the vehicle was programmed to perform: a “selfie” and a “drive-by.”

Both maneuvers began the same way, with the vehicle traveling in a straight line with all suction systems engaged. The researchers let the vehicle drive 100 meters and collected all the nodules on its way. Then, in the “selfie” maneuver, they instructed the vehicle to turn off its suction systems and reverse to drive through the cloud of sediment it had just created. The vehicle’s installed sensors measured the sediment concentration during this “selfie” maneuver, allowing scientists to monitor the plume within minutes of being kicked up by the vehicle.

For the “drive-by” manoeuvre, the researchers placed a berth equipped with sensors 50 to 100 meters from the planned tracks of the vehicle. As the vehicle drove along and collected nodules, it created a plume that eventually spread beyond the berth after an hour or two. This “drive-by” maneuver allowed the team to monitor the sediment cloud over a longer period of several hours and record plume development.

Out of breath

Over several vehicle trips, Peacock and his team were able to measure and track the evolution of the sediment plume produced by the deep-sea mining vehicle.

“We saw that the vehicle would be going in clear water and saw the nodules on the seabed,” says Peacock. “And then suddenly this very sharp cloud of sediment comes through as the vehicle enters the cloud.”

From the selfie views, the team observed behavior predicted by some of their previous model studies: the craft kicked up a large amount of sediment dense enough that even after some mixing with the surrounding water, it created a plume that that was behaving almost like a separate liquid, spreading out under its own weight in a so-called turbidity current.

“The turbidity current propagates 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. “After that, ocean currents become stronger than natural spread, and the sediments are carried by the ocean currents.”

As the sediment drifted past the mooring, the researchers estimated that 92 to 98 percent of the sediment had either re-settled or remained as a low-lying cloud within 2 meters of the seafloor. However, there is no guarantee that the sediment will always stay there instead of drifting further up into the water column. Recent and future studies by the research team are investigating this question with the aim of consolidating the understanding of sediment plumes in deep-sea mining.

“Our study encapsulates the reality of what the initial sediment disruption looks like when you have a certain type of nodule mining operation,” says Peacock. “The big advantage is that there are complex processes like turbidity currents that take place when you do this type of collection. Therefore, any effort to model the effects of deep-sea mining must capture these processes.”