The pitch-black seabed of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is littered with what looks like chunks of coal. These unassuming mineral deposits, called polymetallic nodules, host a unique deep-sea ecosystem, much of which scientists have yet to document. These deposits are also a key target for companies seeking to mine from the deep sea because they contain minerals, such as manganese and cobalt, that are used to make batteries.

Now researchers have discovered that these precious nodules do something remarkable: they produce oxygen, and they do it without sunlight. “This is a completely new and unexpected finding,” says Lisa Levin, emeritus professor of biological oceanography at the Scripps Institution of Oceanography, who was not involved in the research. Oxygen gas on planet Earth is usually thought to come from living organisms that convert sunlight, carbon dioxide and water into oxygen and sugar. The idea that some of the gas might come from these lifeless minerals and be produced in total darkness “really flies in the face of what we traditionally think of as where and how oxygen is produced,” says Jeffrey Marlow, a microbiologist at the University of Boston and co-author of the study, which was published Monday in Nature Geoscience.

The story of the discovery goes back to 2013, when deep-sea ecologist Andrew Sweetman was facing a “problem”. He was part of a research team trying to measure how much oxygen was consumed by organisms at the bottom of the sea. Researchers sent special vehicles down more than 13,000 feet to create sealed chambers on the sea floor that would monitor how oxygen levels in the water fell over time.

But oxygen levels did not drop. On the contrary, they increased significantly. Thinking the sensors were broken, Sweetman sent the instruments back to the manufacturer to be recalibrated. “That happened four or five times” over the course of five years, Sweetman says. “I literally told my students, ‘Throw the sensors in the bin.’ They just don’t work.”

Then, in 2021, he was able to return on an environmental survey mission sponsored by a deep-sea mining company called the Metals Company. Again, his team used deep-sea vessels to build sealed chambers on the sea floor. The chambers contained trapped sediments, nodules, living organisms and seawater and monitored oxygen levels. Sweetman and his team used a different technique to measure oxygen this time, but they noticed the same strange results: oxygen levels rose dramatically.

“My mindset completely changed to focus on what’s causing this.”

“My first thought was microbiology, and that’s because I’m a microbiologist,” says Marlow. It wasn’t a far-fetched idea: scientists had recently discovered ways in which microbes such as bacteria and archaeans could produce “dark oxygen” in the absence of sunlight. In laboratory tests that replicated the seafloor conditions in the new study, the researchers poisoned the seawater with mercuric chloride to kill the microbes. However, oxygen levels were still rising.

If this dark oxygen did not come from a biological process, then it must have come from a geological process, the researchers reasoned. They tested and ruled out a few possible hypotheses—such as that radioactivity in the nodules separates oxygen from seawater, or that some other environmental factor separates oxygen gas from 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 CEO of the Metals Company. This led Sweetman to wonder: “Could the metals found in these nodules somehow function as natural geobatteries?” If so, they could potentially break down seawater into hydrogen and oxygen through a process called seawater electrolysis.

“Batteries in a rock” was just a metaphor, as far as the scientists knew—just because the nodules contain metals used to make batteries doesn’t mean they’re electrically charged themselves. To create a charge, the positive and negative ions must separate to some extent within a node, creating a difference in electrical potential. To see if that was the case, Sweetman flew to Illinois to test the electric charge of the nodules with Franz Geiger, a physical chemist at Northwestern University.

“Strangely, there was almost a volt on the surface of these nodules,” says Sweetman. The researchers’ main theory is that this charge breaks down seawater to create oxygen, although they have yet to test whether turning off the nodules’ electrical charge stops oxygen production. The scientists plan to test this in future studies.

Geiger theorizes that polymetallic nodules become charged as they grow, with different metals deposited 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 the layers of an onion, Geiger says. These metal layers grow only millimeters every million years, and the types of metals deposited change over time, potentially creating a charge gradient between each layer that leads to an electrical potential. This doesn’t explain why there are differences in charge on the surfaces of the nodules, but Geiger thinks the nodules are porous enough to leave some of their inner layers exposed.

Rocks are not known to carry such a charge, Geiger says. This “is one of the most exciting things that [εγώ και το εργαστήριό μου] we have worked,” he adds.

It is not yet clear whether (or to what extent) these nodules generate oxygen naturally on the sea floor. In most experiments, oxygen concentrations in the chambers increased after two days. This may suggest that the rover changed something about the environment—for example, spewing sediment—which then triggered the production of oxygen. It’s also possible that oxygen production eventually stops due to a “bottleneck effect” inside the closed chamber, Marlow says. “The products accumulate, the reactants are removed, and then the reaction 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 strange” 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 left undisturbed on the sea floor. However, he adds, “it appears to be some electrolytic reaction on the surface of the manganese nodule that actually produces oxygen. And that in itself is a very interesting observation that has not been observed before, to my knowledge.”

Researchers still have no idea what role this oxygen-producing nodule might play in the bottom ecosystems of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ). Environmental research has shown that the nodules and surrounding sediments are a habitat for life in the deep: everything from single-celled microbes to “megafauna,” animals visible to the naked eye such as fish, starfish, and worms. About half of the megafauna recorded in the 2013 environmental survey were found in the nodules alone.

Like most of the deep ocean, the bottom of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is a “poorly understood ecosystem,” says Levin. “We haven’t even discovered most species in the deep sea, let alone studied them.”

Deepwater mining projects proposed throughout the Clarion-Clipperton Zone (CCZ) of the Pacific Ocean will extract nodules from seafloor surfaces. The International Seabed Authority (ISA), which governs the seabed in international waters, is currently debating rules and regulations for mining the nodules and other deep-sea targets. Twenty-seven states, including 26 ISA member states, have called for a moratorium, preemptive halt or ban on deepwater mining.

“I do not think that [αυτή η έρευνα είναι] a ‘nail in the coffin’ for deepwater mining – that was never the intention,” says Sweetman. “It’s just another thing that we now have to consider when we’re deciding, ‘Are we going to mine the ocean or not? To me, that decision has to be based on sound scientific advice and evidence.’

With information from Scientific American