The dark seabed of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is littered with what look like hunks of charcoal. These unassuming metal deposits, called polymetallic nodules, contain metals such as manganese and cobalt used to produce batteries, marking them as targets for deep-sea mining companies.
Now researchers have discovered that the valuable nodules do something remarkable: they produce oxygen and do so without sunlight. “This is a totally new and unexpected finding,” says Lisa Levin, an emeritus professor of biological oceanography at the Scripps Institution of Oceanography, who was not involved in the current research.
According to Boston University microbiologist Jeffrey Marlow, the idea that some of Earth’s oxygen gas may come not from photosynthesizing organisms but from inanimate minerals in total darkness “really strongly goes against what we traditionally think of as where oxygen is made and how it’s made.” Marlow is a co-author of the new study, which was published in Nature Geoscience.
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The story of discovery goes back to 2013, when deep-sea ecologist Andrew Sweetman was facing a frustrating problem. His team had been trying to measure how much oxygen organisms on the CCZ seafloor consumed. The researchers sent landers down more than 13,000 feet and created enclosed chambers on the seabed to track how oxygen levels in the water fell over time.
But oxygen levels did not fall. Instead they rose significantly. Thinking the sensors were broken, Sweetman sent the instruments back to the manufacturer. “This happened four or five times” over the course of five years, says Sweetman, who studies seafloor ecology and biogeochemistry at the Scottish Association for Marine Science. “I literally told my students, ‘Throw the sensors in the bin. They just do not work.’”
Then, in 2021, he returned to the CCZ on a survey expedition sponsored by the Metals Company, a deep-sea mining firm. Again, his team used landers to make enclosed chambers on the seafloor and monitor oxygen levels. They used a different technique to measure oxygen this time but observed the same strange results: oxygen levels increased dramatically. “Suddenly, I realized that I’d been ignoring this hugely significant process, and I just kicked myself,” Sweetman says.
The researchers initially thought deep-sea microbes were producing the oxygen. That idea once might have seemed far-fetched, but scientists had recently discovered that some microbes can generate “dark oxygen” in the absence of sunlight. In laboratory tests that reproduced conditions on the seafloor, Sweetman and his colleagues poisoned seawater with mercury chloride to kill off the microbes. Yet oxygen levels still increased.
If this dark oxygen didn’t come from a biological process, then it must have come from a geological one, the scientists reasoned. They tested a few possible hypotheses—such as that radioactivity in the nodules was decomposing seawater molecules to make oxygen or that something was pulling oxygen from the nodules’ manganese oxide—but ultimately ruled them out. Then, one day in 2022, Sweetman was watching a video about deep-sea mining when he heard the nodules referred to as “a battery in a rock.” That bit of marketing was only a metaphor, but it led him to wonder whether the nodules could somehow be acting as natural geobatteries. If they were electrically charged, they could potentially split seawater into hydrogen and oxygen through a process called seawater electrolysis. (A battery dropped in salt water produces a similar effect.)
“Amazingly, there was almost a volt [of electric charge] on the surface of these nodules,” Sweetman says; for comparison, an AA battery carries about 1.5 volts. The nodules may become charged as they grow, as different metals are deposited irregularly over the course of millions of years and a gradient of charge develops between each layer. Seawater electrolysis is currently the researchers’ leading theory for dark oxygen production, and they plan to test it further.
It isn’t clear, however, whether (or to what extent) these nodules create oxygen naturally on the seabed. In most experiments, oxygen production ceased after two days, which may indicate that the lander caused it by disrupting something about the environment. But it’s also possible that the reaction eventually stopped because of a “bottle effect” within the enclosed chamber, Marlow says. “The products build up, the reactants go away, and then the reaction sort of stops. But in an open system ... it could be a more consistent process,” he explains.
Bo Barker Jørgensen, a marine biogeochemist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, is skeptical that these nodules produce oxygen when they are left undisturbed on the seabed. (Jørgensen was not involved in the research but was one of the paper’s peer reviewers for Nature Geoscience.) Still, it does seem that the nodules are producing oxygen through electrolysis, he adds, “and that in itself is a very interesting observation that has not been observed before, to my knowledge.”
The researchers don’t yet know whether this oxygen would be important to life on the CCZ seabed. The nodules and surrounding sediment are a habitat for deep-sea life, from tiny microbes to larger “megafauna,” such as fish and sea stars. Half of the ecosystem’s megafauna is found only on the nodules. This is a “poorly understood ecosystem,” Levin says. “We haven’t even discovered most of the species in the deep sea, let alone studied them.”
Proposed deep-sea mining projects would extract nodules from swaths of the CCZ seafloor. The International Seabed Authority (ISA) is still drafting rules and regulations for mining the nodules and other deep-sea targets. At least 32 member states of the ISA have called for a moratorium, precautionary pause or ban on deep-sea mining.
These findings are “another thing that we now need to take into account when it comes to deciding, ‘Do we go and mine the deep ocean, or don’t we?’” Sweetman says. “To me, that decision needs to be based on sound scientific advice and input.”