Imagine a world so alien and lightless that it rivals the vacuum of space. More than 13,000 feet below the Pacific Ocean’s surface, in the Clarion-Clipperton Zone (CCZ), sunlight does not exist. Temperatures hover near freezing. Life survives under crushing pressure on a vast abyssal plain.
Scientists long assumed this environment contained very little oxygen. Sparse microbes on the seafloor were thought to consume what little remained. But a study in Nature Geoscience overturned that assumption. Researchers discovered oxygen forming in complete darkness without plants, photosynthesis, or sunlight.
This “dark oxygen” challenges centuries-old ideas about how Earth produces its life-sustaining gas. It may even reshape theories about how life first emerged on our planet.
The Surprising Discovery
Andrew K. Sweetman and his team from the Scottish Association for Marine Science led expeditions to the CCZ between 2021 and 2022. They focused on polymetallic nodules potato-sized mineral lumps scattered across the seabed like marbles. These nodules form over millions of years as metals such as manganese, iron, cobalt, and nickel slowly precipitate from seawater.
They blanket enormous areas of the CCZ, a region roughly the size of the continental United States.
To investigate seafloor chemistry, the team deployed benthic chambers. These sealed enclosures trap water just above the seabed. The researchers then monitored oxygen levels inside the chambers over time.
They expected oxygen levels to fall as organisms consumed it. Instead, oxygen concentrations rose sharply. In some cases, levels tripled within 47 hours.
Measured rates of dark oxygen production (DOP) ranged from 1.7 to 18 millimoles per square meter per day. By contrast, seafloor communities typically consume only about 0.7 millimoles per square meter per day. Oxygen production far exceeded oxygen demand.
The team then tested the phenomenon under controlled shipboard conditions. They incubated nodules in cold, dark seawater that mimicked abyssal conditions. Once again, oxygen levels increased.
Production strongly correlated with nodule surface area (Spearman’s ρ = 0.664, p = 0.031). Larger surface areas generated more oxygen.
To rule out biological activity, the researchers added mercury chloride to kill microbes. Oxygen production continued. This result eliminated known biological pathways, including those used by extremophiles in unusual environments.
Something non-biological was driving oxygen formation in total darkness.
These nodules aren’t just inert rocks; they’re densely packed at about 1,170 per square meter in the study area, acting like a vast, natural laboratory.
How Does It Work?
Nodules as Geo-Batteries
The mystery likely begins with the nodules’ electrical properties. When researchers measured their surfaces, they detected voltage potentials as high as 0.95 volts. That is enough to behave like tiny natural batteries.
The leading hypothesis centers on seawater electrolysis. Electrical currents split H₂O molecules into hydrogen gas and oxygen. Scientists routinely use this process in laboratories to generate hydrogen fuel. In the deep sea, however, natural geochemistry may power the reaction.
The nodules contain uneven distributions of metals such as manganese, iron, cobalt, and nickel. These differences create electron gradients. Under extreme pressure and near-freezing temperatures, these gradients may drive redox reactions without sunlight.
“If confirmed, polymetallic nodules may function as natural geo-batteries on the abyssal seafloor.”
Researchers also tested alternative explanations. Radiolysis from radioactive decay could contribute oxygen, but calculations show it accounts for less than 0.5% of observed production. Chemical reductions of manganese oxides contribute negligible amounts.
The team reanalyzed older datasets from other CCZ expeditions. They found similar unexplained oxygen spikes. The pattern suggests this may not be an isolated anomaly.
Implications for Life’s Origins and Beyond
If dark oxygen production occurs widely, it challenges long-standing biological assumptions.
Scientists traditionally credit Earth’s oxygen-rich atmosphere to photosynthetic cyanobacteria. These organisms triggered the Great Oxidation Event about 2.4 billion years ago. Their activity reshaped planetary chemistry and enabled complex life.
However, abiotic oxygen production in the deep sea would complicate that timeline. Oxygen-dependent organisms may have evolved in darkness before surface photosynthesis dominated.
“If oxygen formed abiotically in the abyss, geology not biology may have set the stage for complex life.”
This possibility carries major astrobiological implications. Icy moons such as Europa and Enceladus harbor subsurface oceans. If geological processes can generate oxygen without sunlight, similar mechanisms could support life beyond Earth.
The discovery also raises immediate environmental concerns. Companies target the CCZ for deep-sea mining. They want to extract polymetallic nodules for battery metals and electronics.
Removing these nodules could disrupt dark oxygen production. Abyssal ecosystems may depend on that oxygen for respiration. Mining plumes could also bury remaining nodules and alter their electrical gradients.
Controversies and Critiques: Is It Too Good to Be True?
Not all researchers accept the findings.
A critique published in Frontiers in Marine Science, led by Patrick Downes and colleagues, argues that experimental artifacts likely explain the results. The authors highlight thermodynamic constraints. Standard seawater electrolysis requires at least 1.23 volts. Many reported nodule voltages fall below 0.24 volts.
Under deep-sea conditions, electrolysis might also generate chlorine gas instead of oxygen. That possibility complicates the interpretation.
“The reported voltages appear insufficient to sustain seawater electrolysis under abyssal conditions,” the critics argue.
The critique also identifies methodological concerns. Initial oxygen levels inside the chambers exceeded ambient seafloor concentrations. That discrepancy suggests incomplete flushing or trapped air bubbles. Control chambers without nodules still showed oxygen increases. Chamber materials themselves may have influenced the readings.
Researchers also failed to detect hydrogen gas, the expected byproduct of electrolysis. Moreover, decades of prior studies in similar nodule fields consistently showed oxygen consumption, not production.
The authors warn that dramatic claims could influence policy prematurely. They call for independent replication before regulators adjust deep-sea mining frameworks.
Sweetman and colleagues acknowledge limitations. They report nonlinear oxygen production and emphasize the need for mechanistic validation. However, they maintain that their controls rule out simple artifacts.
“Extraordinary claims require extraordinary evidence and independent replication.”
What Happens Next?
This debate highlights how little we understand about the deep ocean. It remains one of Earth’s least explored frontiers.
Future expeditions will test for hydrogen production and refine voltage measurements. Sweetman’s team plans a 2025 follow-up campaign. Researchers aim to deploy improved instrumentation and stricter controls.
If independent teams confirm dark oxygen production, the finding could reshape theories of planetary habitability. If replication fails, the episode will still illustrate the difficulty of measuring chemistry under extreme conditions.
Either outcome advances science.
The abyss continues to challenge assumptions. Whether dark oxygen proves revolutionary or illusory, it forces researchers to reexamine what they thought they knew about life, geology, and the limits of our planet.
References
- Sweetman, A. K., et al. (2024). Evidence of dark oxygen production at the abyssal seafloor. Nature Geoscience, 17, 740–746.
- Downes, P., et al. (2025). Extraordinary claims require extraordinary evidence: evaluating nodule-associated dark oxygen production. Frontiers in Marine Science, 12:1721853.
