Scientists are preparing a new deep-sea expedition to find out whether “dark oxygen” is really being produced on the ocean floor in the Clarion-Clipperton Zone (CCZ), a vast region between Hawaii and Mexico. The team’s earlier measurements suggested oxygen levels could rise inside sealed chambers placed on the seabed—an unusual result in total darkness where photosynthesis cannot occur.
The idea has sparked debate because it could challenge long-held assumptions about how oxygen forms on Earth and because the same seafloor is targeted for potential deep-sea mining of polymetallic nodules. Researchers say the next round of tests will use new, purpose-built landers and tighter controls to determine whether the oxygen signals come from the nodules, the instruments, or something else entirely.
Oxygen found where sunlight can’t reach
The oxygen mystery centers on deep Pacific seafloor sites around 4,000 meters down, where darkness is constant and pressure is extreme. The anomalous oxygen readings were reported from the CCZ, an abyssal plain known for polymetallic nodules—metal-rich rocks that form over millions of years and sit on top of seafloor mud.
Earth.com reports that earlier deployments sealed patches of mud and nodules inside chambers, and oxygen sometimes increased instead of falling as animals and microbes consumed it. In one paper described by Earth.com, oxygen rose for nearly two days, reaching about triple background levels, while nodule surfaces reached 0.95 volts.
Researchers later used the term “dark oxygen” for oxygen that would be produced without sunlight or photosynthesis, and they have said the earlier findings raised as many questions as they answered. “Our discovery of ‘Dark Oxygen’ was a paradigm shift in our understanding of the deep sea and potentially life on Earth, but it threw up more questions than answers,” marine ecologist Andrew Sweetman said, according to Earth.com.
What could be producing “dark oxygen”?
One possible explanation is electrolysis—electricity splitting water into hydrogen and oxygen—driven by electrical activity associated with the nodules. Earth.com says voltage differences across a nodule’s metal layers might create an electric current that drives reactions at the surface, potentially generating oxygen right beside the rocks.
Another possibility involves biology, including microbial communities on or around the nodules that could influence seafloor chemistry. Ultima Bozza reports that the team is considering electrochemistry and biology as leading suspects, and quotes Boston University geobiologist Jeff Marlow saying, “Our main culprits are electrochemistry and biology. Maybe they work separately, maybe they work in tandem.”
The nodules also matter as habitat: Earth.com notes that animals such as anemones and sponges can anchor to hard nodule surfaces, with scavengers moving between rocks, and microbes coating the surfaces as well. Earth.com adds that removing nodules could strip away shelter and alter local chemistry, even if the surrounding seafloor remained otherwise untouched.
New landers and a May expedition
To test the claim under more controlled conditions, Sweetman and colleagues have announced new deep-sea landers designed specifically to study the oxygen signals near the seafloor. ScienceAlert says the team unveiled two landers capable of diving to 11 kilometers (7 miles), with sensors designed to measure “seafloor respiration,” and that the equipment can withstand about 1,200 times the pressure at Earth’s surface.
Earth.com describes a new lander named Alisa that is being tested to work at depths approaching 11 kilometers and says it carries chambers and sensors for oxygen, hydrogen, pressure, conductivity, and turbidity. Earth.com also says tracking oxygen and hydrogen together is important because true electrolysis should produce both gases, not oxygen alone.
On timing and funding, ScienceAlert reports the new research is funded by the Japanese Nippon Foundation and that Sweetman’s team plans to spend May on a research ship in the CCZ. Ultima Bozza describes a $5.2 million Nippon Foundation-funded expedition returning to the CCZ aboard the research vessel Nautilus and says the plan includes landers designed to measure “dark oxygen” production.
Ultima Bozza also reports that the updated landers include pH sensors intended to detect proton concentrations in seawater, with an increase potentially indicating water splitting and oxygen formation, and says the first instruments were not built to measure that. ScienceAlert quotes Sweetman saying the team expects to confirm whether dark oxygen is being produced within 24 to 48 hours after the landers are recovered, but that broader results may not be known until the ship returns in June and follow-up work on land could take months.
Scrutiny, skepticism, and deep-sea mining pressure
The “dark oxygen” idea sits in the middle of a wider argument about deep-sea mining, because polymetallic nodules are a key target for companies seeking metals used in technologies such as electric car batteries. Earth.com notes that regulators at the International Seabed Authority are still debating how to manage mining beyond national waters, and it reports a study found test tracks from 1979 remained visible, with long-lasting sediment damage and fewer large animals.
At the same time, scientists disagree on what the oxygen readings mean and whether they reflect a real seafloor process or a measurement artifact. ScienceAlert reports that some researchers have suggested the oxygen signals could come from air bubbles trapped in instruments, while Sweetman said his team tested for that possibility and argued it was unlikely based on their experience with the equipment.
ScienceAlert also reports that Matthias Haeckel, a biogeochemist at Germany’s GEOMAR Helmholtz Centre for Ocean Research, said his own research did “not show any hint towards oxygen production” from the nodules, while adding he and Sweetman plan to compare methods during a cruise later in the year. Ultima Bozza reports that the original discovery occurred during work commissioned by The Metals Company, and says some researchers linked to the company raised concerns after publication, prompting a post-publication review that Nature Geoscience confirmed is ongoing.
Why the answer could matter
If dark oxygen is confirmed, Earth.com says it would mean oxygen can form in some settings without sunlight, which could change how scientists treat oxygen as a “biosignature” in the search for life. Earth.com quotes Sweetman saying he is already in conversations with NASA experts who believe dark oxygen could reshape thinking about how life might be sustained on other planets without direct sunlight.
The upcoming tests aim to separate real chemistry from noise by repeating measurements in areas with fewer nodules and by checking whether oxygen and hydrogen rise together under tighter controls. As ScienceAlert describes it, the work is designed to confirm whether dark oxygen exists and, if it does, to understand how metallic nodules on the seafloor could be involved.
