Thousands of metres below the Pacific’s sunlit surface, oxygen should generally be used up as organisms and chemical processes consume it. Yet in experiments on the abyssal seafloor, researchers recorded oxygen levels rising inside sealed chambers. The finding, published in 2024, prompted an unusual question: could oxygen be forming in a place where photosynthesis is impossible?
The researchers called the phenomenon “dark oxygen.” Their study points to potato-sized lumps of metal-rich material, known as polymetallic nodules, as a possible source. But the observation and its proposed explanation are not the same thing. The study reported an unexpected oxygen signal; it did not settle how the oxygen formed or show that nodules are a major oxygen source for the ocean.
A surprising signal in the deep
The research took place in the Clarion-Clipperton Zone, a vast area of the Pacific seabed between Hawaii and Mexico. The region lies in the abyssal zone, around four kilometres below the surface in the study area. Sunlight does not reach the bottom, so photosynthesis cannot account for oxygen being made there.
Researchers used benthic chambers—equipment placed on the seabed to enclose a patch of sediment and measure changes in the water within it. In some experiments, oxygen levels rose instead of falling. That was unexpected because deep-sea organisms and chemical reactions normally consume oxygen. The team reported that oxygen accumulated during observations lasting as long as 47 days.
The result matters in part because it challenges a simple assumption: that oxygen in the deep ocean is only a resource delivered from elsewhere and then consumed. It suggests that, at least in the conditions measured, there may be an additional process affecting oxygen levels near the seafloor.
Could metal nodules be involved?
Polymetallic nodules grow slowly around small nuclei on the seabed. They contain metals including manganese, nickel, copper and cobalt. Their potential value has drawn interest from companies and governments exploring deep-sea mining, which would collect nodules from the ocean floor.
The study’s proposed explanation is that the nodules may help split water molecules into hydrogen and oxygen through electrolysis. In familiar technology, electrolysis requires an electrical voltage. The researchers measured electrical potential associated with nodules and suggested that, in an arrangement of many nodules, electrical activity might help drive the process.
That is a hypothesis, not a demonstrated mechanism. The oxygen increase was the central observation; the study did not directly establish every step from nodule electricity to water splitting and oxygen release. Confirming the explanation would require additional measurements and experiments that can rule out other ways oxygen might have entered or been recorded in the chambers.
This distinction is important. A surprising signal can be real even when its cause remains uncertain. The finding also does not mean that deep-sea nodules produce oxygen on a scale comparable to photosynthesis in the ocean’s sunlit waters, or that the seabed could replace the planet’s better-understood oxygen-producing systems.
Why the result matters for mining debates
Interest in nodules is not purely scientific. Deep-sea mining could disturb seabed habitats and sediment, and the long-term effects are difficult to assess because much of the deep ocean remains poorly studied. If nodules participate in local oxygen chemistry, that would add another process scientists may need to understand before evaluating what removing them could change.
But the study is not, by itself, a verdict on whether mining should proceed. It does not quantify the global contribution of the reported process, establish how widespread it is, or determine what would happen to oxygen levels if nodules were removed. Those are separate questions requiring broader evidence about the seabed, its organisms and its chemistry.
Deep-sea mining discussions already involve questions about biodiversity, sediment plumes and the difficulty of restoring habitats after disturbance. The possibility of an unrecognized oxygen-producing process makes the case for careful baseline research stronger, while leaving the scale and ecological consequences uncertain.
What researchers need to find out next
Follow-up work can test whether the oxygen pattern appears repeatedly at different sites and under different experimental conditions. Researchers can also examine whether oxygen production occurs in the absence of nodules, measure the proposed electrochemical reactions directly, and check whether the chamber equipment or changes in water chemistry could affect the signal.
For now, “dark oxygen” is best understood as a striking research result with an open explanation. It does not overturn the central role of photosynthesis in generating Earth’s oxygen. Instead, it draws attention to how much remains to be learned about the deep seafloor—and to the need to distinguish a promising hypothesis from a proven process before using it to make claims about ocean ecology or mining.
