In 2013, a team of oceanographers working along the Pacific seafloor observed an unexpected rise in dissolved oxygen at depths where sunlight cannot penetrate. This anomaly hinted at a previously unknown source of oxygen.
Andrew K. Sweetman of the Scottish Association of Marine Science (SAMS) questioned the readings so intensely that he sent the expedition’s sensors back to the manufacturer for recalibration. After the company confirmed the instruments were functioning correctly, Sweetman replicated the measurements in the field and obtained the same results.
The team now argues that polymetallic nodules—potato‑sized clusters of manganese, cobalt, nickel, and other metals—on the ocean floor are the culprit. Under laboratory conditions, the uneven distribution of metals in these nodules generates electrical potentials strong enough to split water molecules into hydrogen and oxygen, even in complete darkness.
These findings were published in July 2024 in the journal Nature Geoscience. If validated, the discovery could upend our understanding of how Earth’s oceans became oxygenated and when aerobic life first emerged.
The observations were made in the Clarion‑Clipperton Zone, a 1.7 million‑square‑mile expanse between Hawaii and Mexico that is rich in polymetallic nodules.
By collecting and examining individual nodules, the researchers found significant voltage differences across their surfaces, caused by the unequal distribution of metal ions. They hypothesize that these potential differences facilitate a form of seawater electrolysis, separating water into hydrogen and oxygen and accounting for the surprising oxygen readings.
Specialized benthic chambers were deployed to isolate sections of the seafloor and monitor oxygen concentrations. In typical conditions, oxygen in such chambers declines as marine organisms consume it. Instead, Sweetman’s team recorded a steady rise, indicating an unrecognized source of oxygen production.
Sweetman’s conclusions have placed the future of the ocean floor under intense scrutiny. The same nodules that may be generating oxygen also contain metals prized for lithium‑ion batteries, attracting the interest of companies like The Metals Company, which sponsored the expedition.
Following the publication, The Metals Company released a sharp rebuttal, labeling the claims as unsubstantiated and suggesting that equipment faults or water movement could explain the oxygen anomalies. Sweetman maintains that the methodology is sound and that proceeding with large‑scale mining without a full understanding of these ecosystems would be premature.
The stakes are high: confirming dark oxygen would reshape theories about Earth’s early biochemistry, while the prospect of commercially viable deep‑sea mining hinges on whether these nodules can be harvested without disrupting potential oxygen production.
