"Dark Oxygen" Muddies the Waters for Deep Sea Mining

For years, proponents of deep sea mining have eyed small metallic lumps that line much of the ocean floor. These polymetallic nodules are rich in manganese, nickel, and cobalt, all used in lithium-ion batteries. But new research shows the nodules might already be a kind of battery—one that might be playing an important role in the deep-sea ecosystem.

A group of scientists have detected increasing levels of oxygen on the seafloor where the nodules are present. Without any sunlight to allow for photosynthesis, something else had to be creating this “dark oxygen.” The scientists published their findings 22 July in Nature Geoscience.

Andrew K. Sweetman of the Scottish Association for Marine Science hypothesized that the nodules themselves were producing an electric current that split oxygen off from ocean water. Calling it the “geo-battery” hypothesis, the researchers said the findings raise urgent questions about the impact of deep-sea mining on these little-understood ecosystems.

The research adds to conservation concerns some scientists say must be investigated before mining companies scale up collection of polymetallic nodules. At the same time, demand for the metals is expected to increase with the wider adoption of battery-powered electric vehicles. Proponents of nodule collection say the impact of land-based mining is worse, with well-documented human rights abuses, habitat destruction, and pollution. Additionally, the potential harm of removing the nodules must be balanced with the urgency of eliminating fossil fuels to avoid the worst effects of climate change.

“Is it really wise to push quickly with this major industrial activity in one of the most pristine and biodiverse habitats in the ocean?” —Craig Smith, UH at Manoa

So it might come as a surprise that this latest research was largely funded by The Metals Company, a deep-sea mining firm poised to start large-scale operations. Working in the Clarion-Clipperton Zone, a vast swath of the Pacific Ocean’s floor, the company conducted a pilot of its mining technology in 2022. Earlier this year, it announced it had produced its first cobalt sulfate from the briquette-sized deposits.

In a statement, The Metals Company questioned the study’s results and claimed that Nature Geoscience is biased against deep sea mining. The company said it will produce a “comprehensive rebuttal” to correct the scientific record.

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

How researchers recorded oxygen production on the seafloor

Sweetman and his team used testing compartments called benthic chambers to isolate the water around a small area of the seafloor and measure oxygen rates. Marine scientists record oxygen levels to evaluate how active life forms are. Ocean fauna would typically consume oxygen in a benthic chamber, leading it to decrease over time.

“We consistently found that more O₂ was accumulating in the chambers than was being consumed, resulting in net O₂ production,” the scientists wrote.

An increase of oxygen at the base of the ocean has not been reported in scientific literature before. Danielle de Jonge, a co-author on the paper, said the team didn’t expect the results and initially suspected there was an error in the experiment.

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

But the results kept indicating the creation of oxygen. Eventually, the nodules themselves became the next logical source to investigate.

Craig Smith, a marine biologist and professor emeritus at the University of Hawaii at Manoa, said he believes the data is valid. However, the subject needs deeper investigation, he said.

The researchers agree. The highest voltage detected from a single nodule was 0.95 volts. Seawater hydrolysis requires a minimum of 1.5 V. The team says multiple nodules grouped together could potentially generate enough current, but haven’t confirmed this. It’s unknown how much oxygen the nodules add to the ecosystem or how consistently they produce it across the ocean floor.

Deep-sea mining impacts

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

The resulting plume of sediment resettles to the ocean floor, and scientists are still debating what impact this will have on the ecosystem. Sweetman and his team hypothesize that the nodules produce less oxygen when covered with sediment.

The sea bed is home to anemones and sea cucumbers, as well as a profusion of smaller life forms including tardigrades, small crustaceans, bristly worms called polychaetes, and marine isopods, which are distant cousins of the pill bug.

The life forms vary across this expanse of more than a 4.5 million square kilometers. Scientists can’t rule out the possibility that collecting even a small number of nodules could lead to the extinction of species that only exist in that area.

An unclear 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 push quickly with this major industrial activity in one of the most pristine and biodiverse habitats in the ocean?” he said.

Lithium-ion batteries could become obsolete for EVs in the next decade. Vehicle-makers are racing to develop solid-state batteries that won’t require the nodules’ metals.

But until that technology’s ready, demand for valuable metals will remain high.

This article appears in the October 2024 print issue as “Deep Sea “Dark Oxygen” Poses Questions for Mining.”