The flat, pitch-black seabed of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is littered with what looks like hunks of charcoal. These unassuming mineral deposits, called polymetallic nodules, host a unique deep-sea ecosystem, much of which scientists have yet to catalog. And the deposits are also a key target for companies that are looking to mine the deep sea because they contain metals, such as manganese and cobalt, that are used to make batteries.
Now researchers have discovered that these 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 research. The oxygen gas on planet Earth is typically understood to come from living organisms that convert sunlight, carbon dioxide and water into oxygen and sugar. The idea that some of the gas may come from these inanimate minerals and be produced in total darkness “really strongly goes against what we traditionally think of as where oxygen is made and how it's made,” says Jeffrey Marlow, a microbiologist at Boston University and a co-author of the study, which was published on Monday in Nature Geoscience.
The story of discovery goes back to 2013, when deep-sea ecologist Andrew Sweetman was facing a frustrating problem. He was part of a research team that had been trying to measure how much oxygen organisms on the CCZ seafloor consumed. The researchers sent landers down more than 13,000 feet to create enclosed chambers on the seabed that would track how oxygen levels in the water fell over time.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
But oxygen levels did not fall. Instead they rose significantly. Thinking the sensors were broken, Sweetman sent the instruments back to the manufacturer to be recalibrated. “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 was able to go back to the CCZ on an environmental survey expedition sponsored by a deep-sea mining firm called the Metals Company. Again, his team used deep-sea landers to make enclosed chambers on the seafloor. The chambers enclosed encased sediment, nodules, living organisms and seawater and monitored oxygen levels. Sweetman and his team used a different technique to measure oxygen this time, but they 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. “My mindset completely changed [to] focus on what is causing this.”
“My first thought was microbiology, and that’s because I’m a microbiologist,” Marlow says. It wasn’t a far-fetched idea: scientists had recently uncovered some ways that microbes such as bacteria and archaea could generate “dark oxygen” in the absence of sunlight. In lab tests that reproduced conditions on the seafloor in the new study, the researchers poisoned the seawater with mercury chloride to kill off microbes. Yet the 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 researchers reasoned. They tested and ruled out a few possible hypotheses—such as that radioactivity in the nodules was separating oxygen out of the seawater or that some other environmental factor was separating oxygen gas out of the manganese oxide in the nodules.
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”—a phrase favored by Gerard Barron, the Metals Company’s CEO. That led Sweetman to wonder, “The metals that are in these nodules, could they somehow be acting as natural geobatteries?” If so, they could potentially split seawater into hydrogen and oxygen through a process called seawater electrolysis. (You can try this at home by dropping a small battery into salted water and watching the hydrogen and oxygen gas bubble up.)
“Batteries in a rock” was just a metaphor, as far as the scientists knew—the fact that the nodules contain metals used to make batteries does not mean that they are electrically charged themselves. To create a charge, positive and negative ions would have to be separated to some degree within a nodule, creating a difference in electrical potential. To see whether that was occurring, Sweetman flew to Illinois to test the nodules’ electric charge with Franz Geiger, a physical chemist at Northwestern University.
“Amazingly, there was almost a volt on the surface of these nodules,” Sweetman says—for comparison, a AA battery carries about 1.5 volts. The researchers’ leading theory is that this charge is splitting seawater to create oxygen, though they have not yet tested whether disabling the nodules’ electric charge halts oxygen production. The scientists plan to test this in future studies.
Geiger theorizes that the polymetallic nodules become charged as they grow, with different metals depositing irregularly over time. Nodules form around a small object, such as a shark’s tooth. If you cut one open, “they look like cross sections of tree rings” or like layers of an onion, Geiger says. These metal layers grow only millimeters every million years, and the types of metals being deposited change over time, potentially creating a gradient in charge between each layer that results in electrical potential. That doesn’t explain why there are differences in charge on the surfaces of the nodules, but Geiger theorizes that the nodules are porous enough to leave some of their inner layers exposed.
Rocks are not known to carry charge like this, Geiger says. This “is one of the most fascinating things [I and my lab] have ever worked on,” he adds.
It still isn’t clear whether (or to what extent) these nodules create oxygen naturally on the seabed. In most experiments, oxygen concentrations in the chambers plateaued after two days. That might indicate that the lander changed something about the environment—for example, by kicking up sediment—which then instigated the oxygen production. It’s also possible that oxygen production 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, says the findings are “very odd” and raise many questions. (Jørgensen was not involved in the research but was one of the paper’s peer reviewers for Nature Geoscience.)He is skeptical that these nodules produce oxygen when they are left undisturbed on the seabed. Still, he adds, “it seems to be some electrolytic reaction on the manganese nodule surface that does indeed produce oxygen. And that in itself is a very interesting observation that has not been observed before, to my knowledge.”
The researchers still have no idea what role this nodule-produced oxygen may play in the seabed ecosystems of the CCZ. Environmental surveys have shown that the nodules and surrounding sediment are a habitat for deep-sea life: everything from single-celled microbes to “megafauna,” animals that can be seen with the naked eye, such as fish, sea stars and worms. Approximately half of the megafauna cataloged during the 2013 environmental survey were found only on the nodules.
Like most of the deep ocean, the seafloor of the CCZ is a “poorly understood ecosystem,” Levin says. “We haven’t even discovered most of the species in the deep sea, let alone studied them.”
Deep-sea mining projects proposed across the CCZ would extract nodules from swaths of the seafloor. The International Seabed Authority (ISA), which governs the seafloor in international waters, is currently discussing rules and regulations for mining the nodules and other deep-sea targets. Twenty-seven nations, including 26 member states of the ISA, have called for a moratorium, precautionary pause or ban on deep-sea mining.
“I don’t think [this research is] a ‘nail in the coffin’ for deep-sea mining—that has never been the intention,” Sweetman says. “It’s just 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?’ To me, that decision needs to be based on sound scientific advice and input.”