Dark oxygen causes problems for deep-sea mining

For years, proponents of deep-sea mining have had their eye on small lumps of metal that cover large swathes of the seafloor. These polymetallic nodules are rich in manganese, nickel and cobalt, all of which are used in lithium-ion batteries. But new research shows that the nodules could already be a type of battery—one that potentially plays an important role in the deep-sea ecosystem.

A group of scientists has found increasing oxygen levels on the sea floor where the nodules occur. Since there is no sunlight to enable photosynthesis, something else must be producing this “dark oxygen.” The scientists published their findings on July 22 in Natural Geosciences.

Andrew K. Sweetman of the Scottish Association for Marine Science hypothesized that the nodules themselves generated an electrical current that split oxygen from the seawater. The researchers called this the “geobattery” hypothesis and said the findings raise pressing questions about the impact of deep-sea mining on these poorly understood ecosystems.

The research increases concerns about nature conservation Some scientists say this needs to be investigated before mining companies expand the collection of polymetallic nodules. At the same time, demand for the metals is expected to rise as battery-powered electric vehicles become more common. Proponents of nodule collection say the impacts of land-based mining are worse, involving well-documented human rights abuses, habitat destruction and pollution. Moreover, the potential harm caused by removing the nodules must be balanced with the urgency of phasing out fossil fuels to avoid the worst impacts of climate change.

“Is it really wise to move forward so quickly with this large-scale industrial activity in one of the ocean’s most pristine and biodiverse habitats?” —Craig Smith, UH at Manoa

So it may come as a surprise that this latest research was largely funded by The Metals Company, a deep-sea mining company that is about to launch large-scale operations. The company is working in the Clarion-Clipperton Zone, a vast swath of the Pacific Ocean floor. conducted a pilot project of its mining technology in 2022. Earlier this year, the company announced that it produced its first cobalt sulfate from the briquette-sized deposits.

In a statementThe Metals Company questioned the results of the study, claiming that Natural Geosciences is biased against deep-sea mining. The company announced that it would prepare a “comprehensive rebuttal” to correct the scientific findings.

“The methodology and results raise doubts about the validity of their data and conclusions,” the company said.

How researchers recorded oxygen production on the seafloor

Sweetman and his team used test chambers called benthic chambers to isolate the water around a small area of ​​the seafloor and measure oxygen levels. Marine scientists record oxygen levels to assess how active life forms are. Marine animals in a benthic chamber typically consume oxygen, causing it to decrease over time.

“We have consistently found that more O₂ More oxygen accumulated in the chambers than was consumed, resulting in a net O₂ production,” the scientists wrote.

An increase in oxygen levels at the ocean surface has never been described in the scientific literature before. Danielle de Jonge, a co-author of the study, said the team did not expect these results and initially suspected that the experiment had been a mistake.

“We were very surprised,” said de Jonge. “We did a lot to find out what we did wrong.”

However, the results still pointed to the production of oxygen. Ultimately, the nodules themselves were the next logical source for investigation.

Craig Smith, a marine biologist and professor emeritus at the University of Hawaii at Manoa, believes the data is valid, but the issue needs to be studied more thoroughly, he said.

The researchers agree. The highest voltage measured from a single nodule was 0.95 volts. At least 1.5 volts is needed to hydrolyze seawater. The team says several nodules grouped together could potentially generate enough electricity, but this has not yet been confirmed. It is not known how much oxygen the nodules add to the ecosystem or how consistently they produce it across the entire seafloor.

Impacts of deep-sea mining

The Metals Company uses an unmanned underwater vehicle to pull nodules and sediment through a segmented metal tube to a surface vessel. After extracting the nodules, the crew releases the sediment back into the ocean at medium depth.

The resulting cloud of sediment settles back on the sea floor, and scientists are still debating what effect this will have on the ecosystem. Sweetman and his team suspect that the nodules produce less oxygen when they are covered in sediment.

The seabed is home to anemones and sea cucumbers as well as a variety of smaller life forms, including tardigrades, small crustaceans, Bristleworms so-called polychaetes, and sea licewhich are distant relatives of the woodlice.

Life forms are very diverse in this area of ​​more than 4.5 million square kilometers. Scientists cannot rule out that collecting even a small number of tubers could lead to the extinction of species that only occur in this area.

An uncertain future for deep-sea mining

Smith, who has led seven research expeditions to the Clarion-Clipperton Zone, says he supports a moratorium on deep-sea mining.

“Is it really wise to move forward so quickly with this large-scale industrial activity in one of the ocean’s most pristine and biodiverse habitats?” he asked.

Lithium-ion batteries could become obsolete for electric vehicles in the next ten years. Vehicle manufacturers Race to develop solid-state batteries for which the metals of the tubers are not needed.

But until this technology is ready, demand for precious metals will remain high.