Bo Barker Jørgensen, a marine biogeochemistry expert who was not involved in the research but peer-reviewed the study, said in an interview that it was a “very unusual finding.”

The findings could have implications for the deep-sea mining industry, whose players have sought to be allowed to explore the depths of the ocean and retrieve minerals like those that make up polymetallic nodules. Such minerals are seen as crucial for the green energy transition. Environmental activists and many scientists believe deep-sea mining is dangerous because it can destabilize ecosystems in unpredictable ways and could affect the ocean’s ability to help contain climate change. The study received funding from companies active in seabed mining exploration.

When Andrew Sweetman, the lead author of the study, first recorded unusual oxygen readings coming from the bottom of the Pacific Ocean in 2013, he thought his research equipment had malfunctioned.

“I basically told my students, just put the sensors back in the box. We’ll ship them back to the manufacturer and get them tested because they’re just giving us gibberish,” Sweetman, head of the seafloor ecology and biogeochemistry research group at the Scottish Association for Marine Science, told CNN. “And every single time the manufacturer came back: ‘They’re working. They’re calibrated.’”

In 2021 and 2022, Sweetman and his team returned to the Clarion-Clipperton Zone, an area under the central Pacific known for having large quantities of polymetallic nodules. Confident that their sensors worked, they lowered a device more than 13,000 feet below the surface that placed small boxes into the sediment. The boxes stayed in place for 47 hours, conducting experiments and measuring levels of oxygen consumed by the microorganisms that live there.

Instead of oxygen levels going down, they went up — suggesting that more oxygen was being produced than consumed.

The researchers hypothesized that the electrochemical activity of the different metals that make up polymetallic nodules were responsible for the oxygen production measured by the sensors — like a battery in which electrons flow from one electrode to another, creating an electric current, said Tobias Hahn, one of the co-authors of the study, in an interview.

This hypothesis would add a layer to our understanding of how organisms came to exist under the sea, said Hahn, who focused specifically on the sensors used in the study experiments. “We thought that life began on Earth when photosynthesis kicked in, as oxygen was brought to Earth through photosynthesis. It could be that actually, this process of electrochemically dividing water into oxygen and hydrogen supplied oxygen to the ocean,” he said.

“This could be a kind of game changer in the story about how life started,” he added.

A news release about the study said its findings challenge “long-held assumptions that only photosynthetic organisms, such as plants and algae, generate Earth’s oxygen.”

But if that finding is borne out, “we need to rethink how to mine” materials like cobalt, nickel, copper, lithium and manganese underwater, “so that we do not deplete the oxygen source for deep-sea life,” said Franz Geiger, a professor of chemistry at Northwestern University and one of the co-authors of the study, in the release.

Mining conducted under the sea in the 1980s serves as a cautionary tale, Geiger said. When marine biologists visited such sites decades later, they “found not even bacteria had recovered.” But in areas that were not mined, “marine life flourished.”

“Why such ‘dead zones’ persist for decades is still unknown,” he said. But the fact that they do suggests that mining the seafloor in areas with plenty of polymetallic nodules could be especially harmful, because those areas tend to have more faunal diversity than “the most diverse tropical rainforests,” he said.

Though the study has pointed to an interesting new pathway for sustaining life deep under the ocean, many questions remain, Hahn said. “We just don’t know” how much “dark oxygen” can be created through this process, how it affects the polymetallic nodules or what quantities of nodules are needed to enable oxygen production, he said.

While the study methodology is solid, “what is lacking is an understanding of what is going on, what kind of process this is,” said Barker Jørgensen.