Amid a gold rush to mine the seafloor for valuable minerals, scientists from Scripps Institution of Oceanography at UC San Diego, Massachusetts Institute of Technology (MIT), and colleagues conducted an experiment to study turbulent sediment plumes that mining vessels would potentially release back into the ocean.
Based on their observations, the team developed a model that makes realistic predictions of how such a plume generated would be transported through the ocean. Other researchers could use that model to understand the ecological damage that seafloor mining operations could cause and possibly create guidelines to make mining as benign as possible.
“The road to weaning ourselves from carbon is fraught with tricky multi-trillion-dollar questions such as whether we should mine the deep sea in order to get minerals needed for electrification,” said Scripps oceanographer Matthew Alford, a co-leader of the study, which appears in the journal Nature Communications. “The issue is complicated because the impacts of deep-sea mining are unknown. This study, which used several technologies developed at Scripps including the epsilometer and the Phased Array Doppler sonar (PADS), was the first field study of sediment plumes that would be generated at mid depth in the ocean by mining operations.”
On certain parts of the deep ocean seafloor lie baseball-sized rocks layered with minerals accumulated over millions of years. The researchers surveyed one such region in the central Pacific Ocean called the Clarion Clipperton Fracture Zone (CCFZ). The area is believed to contain vast reserves of these rocks, known as “polymetallic nodules,” that are rich in nickel and cobalt—minerals that are commonly mined on land for the production of lithium-ion batteries in electric vehicles, laptops, and mobile phones, among others.
As demand for these batteries rises, plans are underway to mine the ocean for these mineral-rich nodules. Such deep-sea-mining schemes propose sending down tractor-sized vehicles to vacuum up nodules and send them to the surface, where a ship would clean them and discharge any unwanted sediment back into the ocean. But the impacts of deep-sea mining such as the effect of that turbulent plume of the dumped sediments on marine ecosystems are currently unknown.
“There is a lot of speculation about [deep-sea-mining’s] environmental impact,” said Thomas Peacock, a professor of mechanical engineering at MIT and study co-leader. “Our study is the first of its kind on these mid-depth plumes, and can be a major contributor to the international discussion and the development of regulations over the next two years.”
The study was led by researcher Carlos Muñoz Royo of MIT. Researchers from the U.S. Geological Survey, and researchers in Belgium and South Korea also contributed to the study.
Current deep-sea-mining proposals are expected to generate two types of sediment plumes in the ocean: “collector plumes” that vehicles generate on the seafloor as they drive around collecting nodules as much as 4,500 meters (14,800 feet) below the surface; and “mid-depth plumes” that are discharged through pipes that descend 1,000 meters (3,280 feet) or more into the ocean’s aphotic zone, where sunlight rarely penetrates.
In their new study, Alford, Peacock and their colleagues focused on mid-depth plumes and how the sediment would disperse once discharged from a pipe.
“The science of the plume dynamics for this scenario is well-founded, and our goal was to clearly establish the dynamic regime for such plumes to properly inform discussions,” said Peacock, who is the director of MIT’s Environmental Dynamics Laboratory.
In 2018, the researchers boarded Scripps Oceanography Research Vessel Sally Ride and set sail 50 kilometers (31 miles) off the coast of Southern California. They brought with them equipment designed to discharge sediment 60 meters (200 feet) below the ocean’s surface.
“Using foundational scientific principles from fluid dynamics, we designed the system so that it fully reproduced a commercial-scale plume, without having to go down to 1,000 meters or sail out several days to the middle of the CCFZ,” Peacock said.
Over one week the team ran a total of six plume experiments using novel sensor systems such as PADS and the epsilometer developed by scientists with the Multiscale Ocean Dynamics (MOD) group at Scripps Oceanography to monitor where the plumes traveled and how they evolved in shape and concentration. The collected data revealed that the sediment, when initially pumped out of a pipe, was a highly turbulent cloud of suspended particles that mixed rapidly with the surrounding ocean water.
“There was speculation that this sediment would form large aggregates in the plume that would settle relatively quickly to the deep ocean,” Peacock says. “But we found the discharge to be so turbulent that it breaks the sediment up into its finest constituent pieces, and thereafter it becomes dilute so quickly that the sediment then doesn’t have a chance to stick together.”
The team had previously developed a model to predict the dynamics of a plume that would be discharged into the ocean. When they fed the experiment’s initial conditions into the model, it produced the same behavior that the team observed at sea, proving the model could accurately predict plume dynamics within the vicinity of the discharge.
The researchers used these results to provide the correct input for simulations of ocean dynamics to see how far currents would carry the initially released plume.
“In a commercial operation, the ship is always discharging new sediment. But at the same time the background turbulence of the ocean is always mixing things. So you reach a balance. There’s a natural dilution process that occurs in the ocean that sets the scale of these plumes,” Peacock said. “What is key to determining the extent of the plumes is the strength of the ocean turbulence, the amount of sediment that gets discharged, and the environmental threshold level at which there is impact.”
Based on their findings, the researchers have developed formulae to calculate the scale of a plume depending on a given environmental threshold. For instance, if regulators determine that a certain concentration of sediments could be detrimental to local sea life, the formula can be used to calculate how far a plume above that concentration would extend, and what volume of ocean water would be impacted over the course of a 20-year nodule mining operation.
“At the heart of the environmental question surrounding deep-sea mining is the extent of sediment plumes,” Peacock says. “It’s a multiscale problem, from micron-scale sediments, to turbulent flows, to ocean currents over thousands of kilometers. It’s a big jigsaw puzzle, and we are uniquely equipped to work on that problem and provide answers founded in science and data.”
The team is now working on collector plumes, having recently returned from several weeks at sea to perform the first environmental monitoring of a nodule collector vehicle in the deep ocean in over 40 years.
This research was supported in part by the MIT Environmental Solutions Initiative, the UC Ship Funds program, the MIT Policy Lab, the 11th Hour Project of the Schmidt Family Foundation, the Benioff Ocean Initiative, and Fundación Bancaria “la Caixa.”
Adapted from MIT
About Scripps Oceanography
Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.
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