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The environmental impact of deep-sea mining

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Mining clumps of metal at the bottom of the ocean could address critical shortages, but the damage to the seabed may deter attempts to make this process mainstream.

Last April, engineers on a ship in the middle of the Pacific Ocean held their breath as a 25-tonne mining tractor lay stranded on the silty ocean floor, more than 4km below the surface. On the 13th and final trial dive of a world-first experiment, a robot had broken free of its tether and lay in some of the darkest waters on Earth. “I’m not superstitious,” Kris De Bruyne, engineer and project manager with Belgium’s Global Sea Mineral Resources (GSR), told a science conference examining the impact of deep-sea mining. “But I will never name a dive ‘thirteen’ again.” A remote underwater vehicle helped engineers regain control within days as independent scientists watched on. But the incident galvanised public resistance to mining the deep seas.

Having lain undisturbed for millennia, the murky depths of the Pacific will be busy in the coming years, as prospectors put their deep-sea mining machines – which some have compared to giant vacuum cleaners – through their paces. GSR, a subsidiary of the DEME Group, plans to test a new, larger prototype in 2024, and Canadian firm The Metals Company plans a pilot test of its entire mining system later this year.

In late autumn, another independent research ship will travel the four days from San Diego, California, to the north-east Pacific – the Clarion Clipperton Zone (CCZ) – to learn more about the biology and geology of the deep, where most species are new to science. Since 2015, some €30m of research funding has gone towards discovering the potential impact of deep-sea mining, with two major studies in the bag.

Campaigners from Ocean Rebellion protest in Rotterdam against the exploitation of the seabed in February 2022

Campaigners from Ocean Rebellion protest

Image credit: Gettyimages

Can we mine the deep seas – where a treasure trove of rare earth elements has accumulated over millions of years – without further trashing the ocean? And critically, how can such remote operations be monitored successfully?

Seabeds in the CCZ, a vast area stretching from Hawaii to Mexico, are pristine but tempting – an estimated 30 billion tonnes of metallic nodules lie on the ocean floor. Rare earth elements are also found around hydrothermal vents and in cobalt crusts and other nodule fields in other seas, but mining the CCZ at scale makes most economic sense. Nodules can be sucked up, whereas crusts and mineral vents must be cut up by hefty mechanical tools.

We’ve known riches lay at the bottom of the sea since a British expedition in the late 19th century dredged up nodules from the Atlantic, Pacific and Indian Oceans. Since the 1970s, industry has known it was feasible to mine the deep. But costs and complex legislation have hamstrung commercial ambitions to date.

Seabed polymetallic nodules contain the manganese, cobalt, nickel, and copper required for electric vehicles, smartphone batteries and wind and solar power. They can be mined on land, but – as The Metals Company’s pitch goes – this takes too long, is messy and involves complicated geopolitics and in some cases child labour. Why not meet soaring demand by plucking them from the seabed in what is essentially common land? To be commercially viable, economists say, deep-sea mining must begin in the next few years while prices are high.

But a 2020 plea by Sir David Attenborough to leave the high seas alone, in the wake of an environmental warning, raised the stakes. In September 2021, thousands of conservationists, scientists and diplomats called for a global moratorium until more is known about life and ecosystems in the deep ocean. BMW, Samsung, Google and Volvo Group also backed the call.

All mining causes environmental damage, and scientists believe deep-sea mining will cause a permanent loss of biodiversity. Restoration could help repair environmental impacts, but scientists say they don’t yet know enough about how this could be done. Recent research has found there is far more life at the bottom of the sea than ever suspected – an estimated 90 per cent of deep-sea species are unknown to science.

Noise and light pollution from vehicles and pumps, a mobile blanket of sediment squirted behind the vehicle and caterpillar tracks crushing the seabed will upset delicate ecosystems before we’ve even had a chance to understand them, say campaigners. At these depths some species use light to hunt or attract prey and are extremely vulnerable to artificial lights of submersibles, yet there are no international limits for light or noise pollution in the deep seas. Noise and vibrations can disturb natural behaviour of sea life. Mining could also spread metallic ore about, contaminating surrounding areas.

Before the pandemic, companies hoped a set of standards – an international mining code to cover the common heritage of the high seas – would be adopted in 2020. This must become law before mining beyond national waters can begin. A UN ocean treaty requires prevention of serious harm and protection of the marine environment.

But the clock is ticking. In June 2021, the Pacific island of Nauru – in partnership with The Metals Company – triggered a ‘two-year rule’, meaning the UN’s International Seabed Authority (ISA) that regulates the high seas must produce a mining code by July 2023. Companies sponsored by nations could then begin commercial mining operations – prevailing wisdom suggests it might happen this decade.

Based in Jamaica, ISA listens to scientists as well as commerce, but pressure groups accuse it of being opaque and pro-mining. In May this year, scientists funded through JPI-Oceans, a mostly European intergovernmental research programme, hope to deliver their latest findings to the ISA from a large-scale project to understand the potential impact of deep-sea mining and how the seas could best recover.

Deep-sea biology is expensive. Not only do observation instruments have to cope with remote communication, corrosive seawater, extreme pressures and no light, but creatures on the sea floor can’t be taken to the surface without killing them. The deeper you go, the more it costs.

But the latest scientific mission had more equipment than ever before, says Dr Jens Greinert, a marine geologist at German marine research institute Geomar, who last year watched GSR’s trial from a separate ship. “We had all the monitoring equipment you could think of,” he says. Remote-operated vehicles lowered some 25 tripods onto the seabed equipped with acoustic and optical sensors, 50 in total, to track the density of the sediment plume emitted by GSR’s machine, and spot how far it travelled.


GSR also monitored its own progress with cameras and sensors. An underwater drone travelled across the mining zone taking hundreds of thousands of photos of the sea floor, which will be scoured by AI and analysed by biologists. Latest data revealed that the plume stayed closer to the seabed than expected – good news, according to researchers – but that a concentrated cloud of sediment travelled beyond the research zone. There are fears this will stifle life in a far wider area – some 3cm of sediment settles near the path of the mining machine, latest research shows.

During this year’s research trip, the drone will revisit affected areas to understand the fate of sea life there and to remap the topography. “We want to see what happened to the sponges, sea urchins, brittle stars and more (covered by sediment) a year and a half later,” says Greinert. Scientists have left trays of artificial nodules on the seabed to see whether they might eventually host the sponges and microbes found on metallic nodules.

But poorly understood ecosystems require many years to study accurately, and campaigners say we also need a thorough biological and geological inventory before mining commences. Scientists want a better baseline understanding of noise and light pollution, geochemistry, sea life. How quickly will life crushed by caterpillar tracks recover? Most life is found in surface sediment – and it’s the top 7-8cm that are disturbed by mining. How will the proposed discharge of sediment from water return much closer to the surface block light and affect ocean life? “Sediment discharge both in the water column and behind the vehicle will create a very large off-site impact footprint that would not be acceptable in terrestrial mining,” says Catherine Coumans of Mining Watch Canada.

And The Metals Company acknowledged gaps in our understanding. In filings to the US Securities and Exchange Commission, the company (then DeepGreen Metals) declared: “Impacts on CCZ biodiversity may never be completely and definitively known.” But debate has become too parochial, the company insists; campaigners aren’t considering the problem in the round – our urgent need to tackle climate change.

According to The Metals Company, metals in its designated zone alone could supply batteries for 280 million electric vehicles,and deep-sea mining could generate some 75 per cent less CO2 than its land-based equivalent.

An environmental impact study required of The Metals Company (published via subsidiary Nauru Ocean Resources Incorporated, or NORI) lacks detail, say some campaigners and scientists. “The study lacks an understanding of what the environmental baseline – and variability – in their test area actually is,” says Geomar marine scientist Dr Matthias Haeckel, who led last year’s Mining Impact project. “I am also missing a concrete description of what their monitoring plan is, beyond a list of tools and variables... in my view it’s not a proper environmental impact statement.”

A converted oil and gas drill ship will leave Mexico or San Diego to travel more than 1,200 nautical miles south for the CCZ later this year as part of the first full pilot test by The Metals Company and engineering partner Allseas. On board will be a prototype deep-​water 80-tonne robotic vehicle, which will be gingerly lowered to the seabed, where it will weigh significantly less. At 12m long and equipped with sensors, it will remain connected to the surface by a 10cm-thick cable enclosing fibre-optic, power, and control lines. Caterpillar tracks and more are borrowed from subsea oil and gas exploration technology. Water jets will scoop the nodules into the vessel, where they are graded. “We’ve put in effort to maximising efficiency of pickup and minimising disturbance on the seabed,” says UK-based Jon Machin, head of offshore engineering at The Metals Company. Nodules will be pumped to the surface via a riser – a 4,300m, mostly steel, tube. By 2024 TMC hopes to begin small-scale production, regulation permitting, and aims for full commercial mining in 2025, with a target of collecting 10 million tonnes a year.

Rival operator GSR’s new prototype will build on knowledge gleaned from last year’s sortie, where it left nodules neatly piled on the sea floor between caterpillar tracks as the vehicle ploughed up and down. GSR’s new model, Patania III, could be between 12-16m wide and will begin trials in 2024.



Image credit: gsr

Beside the zones of the CCZ allotted to different nations, some 1.4 million km2 within the CCZ are protected and closed to mining as areas of scientific interest, now with a new protected area bang in the middle of zones earmarked for future exploitation. But are these large enough, and far enough away from proposed mining, to avoid fallout from the sediment plumes, scientists want to know? Damage to the seabed can be partially mitigated if these are created and managed correctly.

As things stand, the ISA must fast-track a mining code by 23 July 2023. In the meantime, scientists have a long to-do list that includes investigating the wider and longer-term impacts of large-scale mining once prospectors begin operations, with front runners forecast to be The Metals Company, GSR, possibly followed by Lockheed Martin subsidiary UK Seabed Resources. China is also expected to lead the field. Estimates suggest an operation needs to cover an area of 100 to 200 football fields a day to be economically viable.

Any mining code should be adaptable, and supported by independent monitoring before, during and after operations, say marine scientists. This will provide trusted data to predict and learn how best to protect deep-sea ecosystems and prevent and mitigate negative impacts. Scientists are currently focusing on a call by the EU to develop and test transparent methods and technologies for monitoring and supervising activities in the deep sea.

Few are happy with the idea of companies marking their own homework. “Independent monitoring is possible,” says Haeckel. “It’s a matter of funding in the end... We have much better data now and are getting a better understanding of what the direct impacts will be.”

A moratorium would risk cutting off research funding, Haeckel fears. If commercial interests die, then the need to learn more about the deep seas will wane. “You learn more about a system when you start to disturb it.”

What’s missing, says Haeckel, is a holistic appreciation of long-term, large-scale impacts. “I make the comparison with elephant routes between waterholes in the Serengeti. What happens if you take out three waterholes in the middle? We haven’t put everything together yet or understood the long-term impact on marine life more widely. We know there is genetic exchange and connectivity across the Pacific. If you destroy large areas in between, what will this mean?”


Is there a better way to mine the deep seas?

Swarms of autonomous robots might eventually offer a less invasive method of collecting deep-sea metals. A few start-ups are raising the possibility of next-generation technology that avoids stirring sediment or crushing life on the ocean floor.

Potential systems involve drones or remote-operated vehicles, propelled in some models by slowly undulating fins, that could hover over the seabed and avoid disturbing sediment. Yet-to-be-designed robotic claws could pluck nodules to place them in baskets ready to float to the surface, lifted by pockets of gas.

Some of this technology hasn’t left the drawing board for want of major investment, and the robotics required to pick up nodules are fiendishly complex. “But I’m encouraging people to consider this as one direction in which we could take robotics technology – somebody has to find the vision,” says Pietro Filardo, founder of New York-based Pliant Energy Systems.

But entry costs to deep-sea mining are high and frontrunners are almost poised to go, potentially locked into existing approaches. “The problem is the proposed excavation technology hasn’t moved on much since the 1980s,” says Lucas Wissmann, co-founder of UK-based subsea robotics firm HonuWorx. “We need to do a lot more investigation around how to mitigate potential environmental impacts. Underwater robotics will play a key role in these site evaluation and monitoring phases, and the sustainable harvesting of minerals.”

Pliant Energy Systems has developed a couple of prototype marine robots equipped with fins and novel mechanisms for harnessing energy from currents. Potential applications include crop irrigation or pumping water through a filtration system or scouting shallow waters for the military.

A distributed system promises fewer vulnerabilities and lower energy use. Collection baskets avoid the need for conventional riser pipes and noisy pumps. However, in Pliant’s vision an optical-fibre cable would run down from a crewed surface ship to a base station, where robots could recharge. Chemical reactions could be used at depth to create gas that would lift nodules in a bag to the surface – a method already used in salvage operations – although keeping track of them as they float the few kilometres to the surface is a challenge.  

“Novel robotic techniques could be developed to hover above the seafloor and selectively pick up nodules using machine vision,” says Wissmann. HonuWorx is building systems he says will offer real-time distributed controls and be accessible from anywhere in the world. “Ultimately, deep-sea mining will need a range of enabling technologies, given its remoteness. Once the sector sorts out the environmental issues, we could move very quickly because we have an enabling platform.”


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