Could sediment plumes from deep-sea mining pollute vast swaths of the ocean?

A new report suggests that deep-sea mining would generate damaging sediment plumes and noise pollution that would negatively affect the midwater column—a critical ocean ecosystem that begins 200 meters below the surface of the sea.

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Squid, fish, shrimp, copepods, medusa, filter feeding jellies and marine worms are among the midwater creatures that could be affected by deep sea mining. Image: weslie.leung, CC BY-SA 2.0

Beyond the sun’s reach is a nutrient-rich, species-abundant part of the ocean called the midwater ecosystem. This zone, which extends from a depth of 200-5,000 meters (660-16,400 feet) to 5 kilometers (3 kilometers) below the sea’s surface, represents 90 per cent of the Earth’s biosphere, contains a biomass of fish 100 times the annual global fish catch, regulates carbon, and supports the pelagic food web through nutrient regeneration. But deep-sea mining, a new activity intended to extract minerals from the seafloor, could seriously threaten the midwater ecosystem, a new paper suggests.

While deep-sea mining has not yet begun, numerous companies are interested in exploring for  minerals near hydrothermal vents, seamounts, and the abyssal plains where polymetallic nodules are found. The International Seabed Authority (ISA), an intergovernmental body set up to oversee and control mining beyond any country’s jurisdiction, is set to finalise a set of rules surrounding deep-sea mining, referred to as the “mining code,” later this year, which would allow mining to begin.

In the meantime, the ISA has issued 30 licenses for mining companies to explore for minerals over approximately 1.5 million square kilometers (580,000 square miles) of the seafloor in international waters. There’s also mining interest within several countries’ territorial waters.

The new paper, published this month in the Proceedings of the National Academy of Sciences (PNAS), says that past research has tended to examine the possible impacts of deep-sea mining on seafloor ecosystems, but has overlooked midwater ecosystems.

Authored by 19 scientists, the opinion piece argues that deep-sea mining “poses significant risks to midwater ecosystems,” and that these risks must be comprehensively evaluated to determine whether or not deep-sea mining should proceed.

It’s very clear now that mining is going to discharge huge volumes of muddy seawater into these mid waters, and we need to begin discussing what those effects will look like.

Jeffrey Drazen, professor in the Department of Oceanography, University of Hawaii 

Each deep-sea mining company will likely use different practices. Most, however, will use a seafloor vehicle to collect minerals and sediment and some kind of vertical system to transport the materials to the surface. Another machine on board a ship will draw out the ore, and discharge unwanted sediment and water back into the ocean.

These activities would generate two types of “environmentally detrimental plumes,” according to the paper: one at the seafloor where minerals are collected, and one in the water column where waste is discharged. Both would disperse sediment and dissolved metals in the immediate environment, and eventually over a greater region of the ocean, spanning tens to hundreds of kilometers, the piece suggests.

These plumes, according to the authors, will introduce heavy metals into the pelagic food chain, which could even contaminate the human seafood supply; clog organisms’ filtration and breathing structures; and introduce visual and mobility barriers that could prevent organisms from properly functioning.

“It’s very clear now that mining is going to discharge huge volumes of muddy seawater into these mid waters, and we need to begin discussing what those effects will look like,” Jeffrey Drazen, the paper’s lead author and a professor in the Department of Oceanography at the University of Hawaii at Manoa, told Mongabay. “And it’s not a pretty picture.”

Natural levels of sediment in the midwater column are between 10 and 20 micrograms per liter, Thomas Peacock, a co-author of the paper and professor of mechanical engineering at MIT, told Mongabay. Right now, it’s not clear what increased levels of sediment will do to the midwater ecosystem, but the paper suggests that there would be a “host of negative consequences,” similar to the effects of air pollution on land.

“Since the background levels are so low, the threshold could be 2 times, 5 times, 10 times, 100 times background levels?” Peacock said in an email. “Determining threshold levels is central to determining the extent of potential impact of a midwater plume (and also a collector plume down at the bottom of the ocean).”

The mining machinery would also generate a lot of underwater noise, which could stress out marine mammals, fish and other marine organisms, and potentially disrupt key activities like foraging, communication and the ability of larvae, which use sound to navigate through the ocean, to settle onto the seafloor,  according to the paper.

“If you’re mining sulphide deposits or crusts on sea mounts, you have to actually grind that hard substrate off of the bottom or out of the sea floor,” Drazen said. “That’s going to be incredibly noisy … [and] because of the temperature and salinity profiles of the ocean sound is conducted for very long distances. So this is bad news.”

The paper indicates that discharge plumes and sediment disturbance, as well as noise pollution, would impact a range of species in the midwater column, including fish, shrimp, copepods, medusas, filter-feeding jellies, and marine worms. For instance, there’s a creature called a larvacean (Bathochordaeus spp.) that uses its gelatinous “house,” which consists of two mucus filters, to help regulate nutrients and carbon in the seawater, but deep-sea mining would immobilise its ability to function, Drazen says.

“These animals are really important because they filter all of the little, sinking organic particles that come from surface waters, and they package it into these houses and into their own bodies, and they make all of that food available to other trophic levels,” he says. “Their filter houses are going to get totally clogged with mud [from mining plumes].”

Drazen says he is also concerned about the dissolved metals that will be released into the ocean during the mining process.

“These metals don’t go away very quickly, they are probably going to build up over time,” Drazen said. “And we don’t know exactly where they’ll be distributed, or how they would move through the oceans … [It]’s possible that these toxic chemicals could build up over time as operators begin mining, and that has the potential to affect pretty vast areas of the ocean.”

The ISA is required to ensure that the marine environment is protected during any mining activities, but the authors of the paper say the impacts on the midwater column need more consideration. “The ISA does include midwater sampling in its baseline study recommendations to contractors,” the paper says, “but the data collected by contractors to date appear to be very limited.”

The ISA did not respond to Mongabay’s request for comment. Mongabay also contacted several organizations currently licensed to explore the seabed for minerals, including Lockheed Martin, the parent company of UK Seabed Resources; Dredging, Environmental and Marine Engineering NV (DEME), the parent company of Global Sea Mineral Resources NV; Japan Oil, Gas and Metals National Corporation; and the Federal Institute for Geosciences and Natural Resources of Germany. However, none responded to our request for an interview.

DeepGreen Metals, a private company based in Vancouver, Canada, with a 15-year license to explore minerals in the Clarion Clipperton Zone (CCZ) in the Pacific Ocean, did respond to our request for comment. Gregory Stone, chief scientist and a director of DeepGreen, told Mongabay that the company’s mining activities probably wouldn’t have a big impact on the midwater column or surrounding ocean.

“Our major strategy is to isolate our activities from the water column as much as we can,” Stone said.

Nevertheless, Stone said that the company will be spending the next three years researching the effects of deep-sea mining on the midwater ecosystem. “The way we approach this is we want to remove every impact we can,” Stone said. “And then we want to minimise every impact we cannot remove.”

“If we do discover something, and it’s really bad, we won’t do it,” he added.

Stone also said that deep-sea mining could provide the metals and other resources urgently needed for renewable energy sources, and that deep-sea mining was actually far less destructive than terrestrial mining.

However, according to a recent report by MiningWatch Canada and The Ocean Foundation, resources for renewable energy could be obtained from existing terrestrial mineral stocks, electronic waste and other alternative sources.

“The metals can be mined on land … and we don’t need to extend the harm that we do with mining on land into the deep sea environment,” Catherine Coumans, the Asia-Pacific program coordinator for MiningWatch Canada and editor of the report, told Mongabay in June.

DeepGreen has hired Drazen and three of his coauthors to be part of a team conducting baseline pelagic ecosystem studies for the company scheduled for next February in the CCZ.

“It is so very important that scientists, such as Drazen and his co-authors, are stating unequivocally that deep-sea mining poses significant risks to the web of life in our oceans from the deep sea to the surface, that marine biodiversity and livelihoods are on the line, and even that the carbon sequestration function of the oceans — so critical to slow down climate change — is jeopardised,” Coumans told Mongabay in a statement last week. “This article supports a growing number of voices, calling for a moratorium on the development of regulations for deep-sea mining until the likely impacts are better understood.”

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