A groundbreaking study reveals that metallic nodules on the deep Pacific seafloor may produce oxygen through electrochemical reactions — a discovery that could rewrite how we understand Earth’s atmospheric history and reshape debates over deep-sea mining.
For decades, science taught us that oxygen on Earth came from photosynthesis — plants, algae, and cyanobacteria splitting water using sunlight to release oxygen into the atmosphere. That foundational idea was upended in July when marine ecologist Andrew Sweetman and colleagues at the Scottish Association for Marine Science published findings suggesting oxygen might be forming in complete darkness.
The research, detailed in Nature Geoscience, focused on the Clarion–Clipperton Zone — a vast region of the Pacific Ocean between Mexico and Hawaii, lying four kilometers below the surface. This area is scattered with polymetallic nodules — potato-sized metallic rocks rich in manganese, nickel, and cobalt — valuable for electric vehicle batteries and other low-carbon technologies. These nodules are also central to growing debates over deep-sea mining.
Sweetman’s team reported that these nodules may generate small electrical currents capable of splitting seawater into hydrogen and oxygen through a process known as electrolysis. If confirmed, this would challenge long-standing theories about how oxygen first appeared on Earth around 2.7 billion years ago.
Why the Deep Seafloor Should Not Make Oxygen
When you picture the deep seafloor, you likely imagine a quiet place where oxygen slowly fades away. That picture is usually accurate — oxygen reaching deep sediments is typically consumed by microbes or chemical reactions called sediment community oxygen consumption (SCOC).
SCOC measurements help scientists estimate how elements like carbon and nitrogen cycle through oceans. They also set a clear expectation — oxygen should decline in sealed chambers placed on the seabed.
That expectation shaped experiments carried out in the Nauru Ocean Resources Inc. license area of the Clarion–Clipperton Zone. Researchers lowered benthic chambers to the seafloor and monitored oxygen levels using sensitive sensors called optodes. Instead of falling, oxygen levels rose.
Across 25 experiments, oxygen increased regardless of treatment — chambers received dead algae, dissolved carbon and ammonium, filtered seawater, or nothing at all. In every case, oxygen climbed. The pattern pointed to what researchers called “dark oxygen production.”
Oxygen concentrations started near 185 micromoles per liter. Within 47 hours, they climbed as high as 819 micromoles per liter. The resulting oxygen production rates ranged from 1.7 to 18 millimoles per square meter per day — far exceeding normal oxygen consumption.
To check the sensors, the team used Winkler titration, a standard chemical method for measuring oxygen. The results matched the optode readings. Statistical tests showed no meaningful difference between treatments or equipment types. Even older datasets from other cruises in the region showed the same trend.
One link stood out — oxygen production increased with the surface area of nodules inside the chambers. Larger nodules were associated with more oxygen. That relationship suggested the nodules themselves mattered.
Could It Be a Measurement Error?
Before proposing new physics or chemistry, the team tried to rule out simpler explanations. Trapped air bubbles were unlikely — at depths of 4,000 meters, bubbles dissolve almost instantly. Leaks or contamination also failed to explain the results. All chambers were built from the same materials and followed identical procedures.
The researchers ran additional tests in sealed containers outside the ocean and again observed oxygen production. Pore-water measurements showed that sediment consumed oxygen rather than released it. These checks pointed away from equipment problems.
Biology was another candidate. Some microbes can produce oxygen through rare chemical pathways. To test this, researchers added mercury chloride, which kills many microbes and blocks known oxygen-producing reactions. Oxygen still appeared. The presence of oxygen in chambers containing only nodules further weakened a biological explanation.
Scientific Pushback and Open Questions
The findings drew sharp criticism. Since July, at least five papers challenging the results have been submitted for review. Many scientists argue the evidence is incomplete.
“He did not present clear proof for his observations and hypothesis,” said Matthias Haeckel, a biogeochemist at GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany. “Many questions remain after the publication. So, now the scientific community needs to conduct similar experiments etc, and either prove or disprove it.”
Olivier Rouxel, a geochemist at Ifremer, France’s national ocean science institute, also expressed doubts. Deep-sea sampling is difficult, he said, and oxygen readings could reflect trapped air in instruments. He questioned how nodules tens of millions of years old could still generate electrical current.
“How is it possible to maintain the capacity to generate electrical current in a nodule that is itself extremely slow to form?” Rouxel asked.
Sweetman said the debate is expected. “These types of back and forth are very common with scientific articles and it moves the subject matter forward,” he told AFP.
A Natural Geo-Battery on the Seafloor
To explore how nodules might produce oxygen, the team measured electrical voltage across nodules collected from several parts of the Clarion–Clipperton Zone. Using platinum electrodes, they recorded voltages at more than 150 points.
Some readings reached 0.95 volts — below the 1.23 volts normally required to split water. Certain metal oxides can lower the energy needed. Polymetallic nodules contain layers rich in manganese, nickel, and copper — metals that can act as catalysts and improve conductivity.
The researchers propose that nodules function as tiny electrochemical systems — sometimes described as geo-batteries. Internal electron movement between metal layers could drive partial electrolysis of seawater. Initial oxygen spikes may occur when nodules are exposed during lander placement. Production may then slow as reactive sites degrade.
What This Could Mean for the Deep Ocean
If dark oxygen production occurs naturally, it could supply oxygen to deep-sea ecosystems in ways scientists never considered. The process may be uneven, depending on nodule size, density, and disturbance.
That uncertainty matters as interest in deep-sea mining grows. Environmental groups argue the discovery highlights how little is known about deep ecosystems. “This incredible discovery underlines the urgency of that call,” Greenpeace said, urging a halt to mining in the Pacific.
Mining companies disagree. The Metals Company, which partly funded the research, criticized the study. Michael Clarke, the company’s environmental manager, said the findings were “more logically attributable to poor scientific technique and shoddy science than a never before observed phenomenon.”
Research findings are available online in the journal Nature Geoscience.
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