Researchers find oxygen production 13,000 feet below the ocean’s surface, challenging the idea that only photosynthetic organisms make Earth’s oxygen. Minerals at the deep seafloor may act like batteries, creating oxygen without sunlight, enough to support sea life in total darkness. This discovery could impact seafloor mining strategies.
EVANSTON, Ill. — An international team, including a Northwestern University chemist, discovered that metallic minerals on the deep-ocean floor produce oxygen 13,000 feet below the surface. This finding suggests oxygen can be made in darkness to support sea life.
The study was published in Nature Geoscience. Andrew Sweetman of SAMS found this “dark oxygen” during Pacific Ocean fieldwork. Northwestern’s Franz Geiger led the electrochemistry experiments explaining the process.
“For aerobic life to start on Earth, there had to be oxygen, and we thought it began with photosynthetic organisms,” said Sweetman, head of the Seafloor Ecology and Biogeochemistry group at SAMS. “But now we know oxygen is also made in the deep sea without light. We need to reconsider where aerobic life could have started.”
Polymetallic nodules, natural mineral deposits on the ocean floor, are key to this discovery. These nodules contain metals like cobalt, nickel, copper, lithium, and manganese used in batteries.
Geiger, a study co-author and professor at Northwestern, said, “Mining companies plan to extract these metals from the seafloor. We must rethink mining to avoid depleting the oxygen source for deep-sea life.”
Sweetman discovered something unexpected while sampling the seabed in the Clarion-Clipperton Zone, a submarine ridge in the Pacific Ocean. When his team first detected oxygen, they thought the equipment was faulty.
Sweetman said, “We initially believed the sensors were broken because all past studies showed oxygen was only consumed in the deep sea. After recalibrating for ten years, the unusual readings persisted. We used a different method to confirm our findings. When both methods showed the same results, we realized we had found something ground-breaking and unthought-of.”
In the summer of 2023, Sweetman contacted Geiger to explain the oxygen source. Geiger’s past work showed that rust and saltwater can generate electricity. They wondered if polymetallic nodules on the deep-ocean floor could create enough electricity to produce oxygen through seawater electrolysis.
To test this, Sweetman sent several pounds of these nodules to Geiger’s lab and visited Northwestern last December. They found that just 1.5 volts, the voltage of an AA battery, can split seawater. The nodules recorded voltages up to 0.95 volts, and when clustered, the voltage increased.
Geiger said, “We discovered a natural ‘geobattery,’ which may explain the ocean’s dark oxygen production.”
The researchers believe the mining industry should consider this discovery before starting deep-sea mining. Geiger noted that the Clarion-Clipperton Zone has had enough polymetallic nodules for decades to meet global energy demand. However, mining in the 1980s left areas with no recovery, not even bacteria.
Geiger said, “In 2016 and 2017, biologists found that marine life in mined areas hadn’t recovered, while unmined areas flourished. This suggests mining can create long-lasting ‘dead zones’ and affects seafloor biodiversity, which is higher than in tropical rainforests.”
Journal reference:
- Sweetman, A.K., Smith, A.J., de Jonge, D.S.W. et al. Evidence of dark oxygen production at the abyssal seafloor. Nature Geoscience. DOI:10.1038/s41561-024-01480-8.