An international team of researchers, including a Northwestern University chemist, has discovered that metallic minerals on the deep ocean floor produce oxygen – 13,000 meters below the surface.
The surprising discovery challenges long-held assumptions that only photosynthetic organisms, such as plants and algae, generate Earth’s oxygen. But the new discovery suggests there may be another way. It appears that oxygen can also be produced at the bottom of the sea – where light cannot penetrate – to support oxygen-breathing (aerobic) marine life that lives in complete darkness.
The study will be published Monday (July 22) in the journal Nature Geoscience.
Andrew Sweetman, of the Scottish Society for Marine Science (SAMS), made the discovery of “dark oxygen” while conducting ship-based fieldwork in the Pacific Ocean. Franz Geiger of Northwestern led the electrochemistry experiments, which potentially explain the finding.
“For aerobic life to begin on the planet, there had to be oxygen, and our understanding has been that Earth’s oxygen supply began with photosynthetic organisms,” said Sweetman, who directs the Seafloor Ecology and Biogeochemistry research group at SAMS. “But we now know that oxygen is produced in the deep sea, where there is no light. So I think we need to revisit questions like: Where might aerobic life have started?”
Polymetallic nodules – natural mineral deposits that form on the ocean floor – lie at the heart of the discovery. A mixture of different minerals, nodules measure anywhere between tiny particles and the size of an average potato.
“The polymetallic nodes that produce this oxygen contain metals such as cobalt, nickel, copper, lithium and manganese—all of which are critical elements used in batteries,” said Geiger, who co-authored the study. “Some large-scale mining companies now aim to extract these precious elements from the seabed at depths of 10,000 to 20,000 meters below the surface. We need to rethink how to extract these materials so that we don’t deplete it source of oxygen for marine life.”
Geiger is the Charles E. and Emma H. ​​Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences at Northwestern and a member of the International Institute for Nanotechnology and the Paula M. Trienens Institute for Energy and Sustainability.
‘Something innovative and thoughtless’
Sweetman made the discovery while sampling the seafloor of the Clarion-Clipperton Zone, a submarine mountain ridge along the seafloor that stretches nearly 4,500 miles across the northeast quadrant of the Pacific Ocean. When his team first discovered the oxygen, he assumed the equipment must be broken.
“When we first got this data, we thought the sensors were wrong because every study ever done in the deep sea has only looked at oxygen consumption instead of production,” Sweetman said. “We would go home and recalibrate the sensors, but over the course of 10 years, these weird oxygen readings kept coming up.
“We decided to get a backup method that worked differently than the optode sensors we were using. When both methods came back with the same result, we knew we were onto something innovative and thought-provoking.”
‘Geobatteries’ hidden in the game
In the summer of 2023, Sweetman contacted Geiger to discuss possible explanations for the source of the oxygen. In his earlier work, Geiger discovered that rust, when combined with salt water, could generate electricity. The researchers wondered if deep ocean polymetallic nodules generated enough electricity to produce oxygen. This chemical reaction is part of a process called electrolysis of seawater, which pulls electrons from the water’s oxygen atom.
To investigate this hypothesis, Sweetman sent several kilograms of polymetallic nodules, which were collected from the bottom of the ocean, to Geiger’s laboratory at Northwestern. Sweetman also visited Northwestern last December, spending a week in Geiger’s lab.
Just 1.5 volts – the same voltage as a typical AA battery – is enough to split seawater. Amazingly, the team recorded voltages as low as 0.95 volts on the surface of single nodes. And when multiple nodes are stacked together, the voltage can be much more important, like when batteries are connected in series.
“It appears we have discovered a natural ‘geobattery,'” Geiger said. “These geobatteries are the basis for a possible explanation of dark ocean oxygen production.”
A new consideration for miners
Researchers agree that the mining industry should consider this discovery before planning deep-sea mining activities. According to Geiger, the total mass of polymetallic nodules in the Clarion-Clipperton Zone alone is enough to meet global energy demand for decades. But Geiger sees mining efforts in the 1980s as a cautionary tale.
“In 2016 and 2017, marine biologists visited sites that had been mined in the 1980s and found that the bacteria had not recovered from the mined areas either,” Geiger said. “However, in unmined regions, marine life flourished. Why such ‘dead zones’ persist for decades is still unknown. However, this places a star in seafloor mining strategies, as the faunal diversity on the floor of the ocean in knot-rich areas is higher than in more diverse tropical rainforests.”
The study, “Evidence of dark oxygen production on the seafloor,” was supported by Nauru Ocean Resources Inc., a subsidiary of The Metals Company Inc.
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