The Clarion‑Clipperton Zone contains vast fields of polymetallic nodules that concentrate battery metals. A study in Nature Geoscience reports these nodules produce “dark oxygen” about 4,000 metres below the surface through an apparent electrochemical process. Oxygen rose even after samples were sterilised and nodules showed a natural surface voltage of roughly 0.95 volts. The finding could reshape ideas about the origin of aerobic life and complicates debates over deep‑sea mining and conservation.
‘Dark Oxygen’ Found 4,000 Metres Deep — Seafloor Nodules May Split Seawater
This “Battery In a Rock” Changes the Energy GameIryna Veklich - Getty Images
Scattered across a vast abyssal plain of the Pacific Ocean known as the Clarion‑Clipperton Zone (CCZ) are countless potato‑sized polymetallic nodules prized for their concentrations of nickel, manganese, copper, zinc and cobalt. A new study published in Nature Geoscience reports that these nodules appear to produce oxygen roughly 4,000 metres below the surface — in complete darkness — through a likely electrochemical process.
What the Researchers Found
Lead author Andrew Sweetman, a deep‑sea ecologist at the Scottish Association for Marine Science, and his team detected unexpected increases in oxygen concentrations during long‑term monitoring of the CCZ. After reproducing seafloor conditions in the laboratory and eliminating biological activity with mercuric chloride, the team still observed oxygen accumulation. Measurements also recorded a natural surface voltage on nodules of about 0.95 volts. Together these observations point to a non‑biological, electrochemical mechanism — sometimes described as a natural “geobattery” — that can split seawater and release oxygen even where sunlight cannot reach.
How It Might Work
Polymetallic nodules grow by gradual and irregular deposition of minerals; this heterogeneous growth can generate electrical potentials across nodule surfaces. The study suggests those potentials are large enough to drive redox reactions that separate hydrogen and oxygen from seawater, producing what researchers call “dark oxygen.” Previous studies have shown some microbes can also produce oxygen in the dark, but Sweetman’s experiments indicate an abiotic pathway can operate independently of living organisms.
Scientific and Astrobiological Implications
If confirmed and widespread, dark oxygen production on the seafloor could alter our understanding of early Earth environments and the possible pathways by which aerobic life first emerged. It also expands astrobiological thinking about where oxygen—and therefore aerobic life—might arise beyond Earth, for example on icy moons such as Europa or Enceladus, where sunlight is absent but electrochemical gradients may exist.
Policy, Conservation and Industry
The finding adds urgency to the debate over commercial deep‑sea mining. Companies such as The Metals Company promote nodules as a source of metals for batteries and renewable energy, while delegations from at least 25 countries and many scientists urge the International Seabed Authority (ISA) to impose a moratorium or precautionary pause on mining to allow more research. Scripps Institution of Oceanography biologist Lisa Levin, not involved in the study, called oxygen production by nodules “a new ecosystem function” that must be considered in environmental impact assessments.
What Comes Next
Researchers emphasize that additional independent studies are needed to map how widespread dark oxygen production is, quantify its rates, and understand ecological consequences. As the ISA continues to negotiate regulations, this discovery underscores both how much we still don’t know about deep oceans and how new science can reshape policy choices about resource extraction and conservation.
Source: Sweetman et al., Nature Geoscience; reporting from Scientific American; comments from Lisa Levin (Scripps Institution of Oceanography).
