This finding challenges the traditional belief that only photosynthetic organisms, such as plants and algae, generate Earth's oxygen. The discovery suggests oxygen can also be produced at the seafloor, supporting aerobic sea life in complete darkness.

The study will be published on Monday, July 22, in the journal Nature Geoscience.

Andrew Sweetman from the Scottish Association for Marine Science (SAMS) discovered this "dark oxygen" during ship-based fieldwork in the Pacific Ocean. Franz Geiger from Northwestern University led the electrochemistry experiments that potentially explain this phenomenon.

"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, head of the Seafloor Ecology and Biogeochemistry research group at SAMS. "But we now know that there is oxygen produced in the deep sea, where there is no light. I think we, therefore, need to revisit questions like: Where could aerobic life have begun?"

At the heart of the discovery are polymetallic nodules, natural mineral deposits found on the ocean floor. These nodules, ranging in size from tiny particles to the size of an average potato, are composed of various minerals.

"The polymetallic nodules that produce this oxygen contain metals such as cobalt, nickel, copper, lithium, and manganese - which are all critical elements used in batteries," said Geiger, co-author of the study. "Several large-scale mining companies now aim to extract these precious elements from the seafloor at depths of 10,000 to 20,000 feet below the surface. We need to rethink how to mine these materials, so that we do not deplete the oxygen source for deep-sea life."

Geiger holds the Charles E. and Emma H. Morrison Professorship of Chemistry at Northwestern's Weinberg College of Arts and Sciences and is a member of the International Institute for Nanotechnology and the Paula M. Trienens Institute for Energy and Sustainability.

Unexpected Findings in the Pacific Ocean
Sweetman discovered the oxygen production while sampling the seabed of the Clarion-Clipperton Zone, a vast underwater mountain ridge extending nearly 4,500 miles in the northeast Pacific Ocean. Initial findings were met with skepticism.

"When we first got this data, we thought the sensors were faulty because every study ever done in the deep sea has only seen oxygen being consumed rather than produced," Sweetman said. "We would come home and recalibrate the sensors, but, over the course of 10 years, these strange oxygen readings kept showing up.

"We decided to take a back-up method that worked differently to the optode sensors we were using. When both methods came back with the same result, we knew we were onto something ground-breaking and unthought-of."

The Role of "Geobatteries"
In summer 2023, Sweetman reached out to Geiger for potential explanations. Geiger had previously discovered that rust combined with saltwater can generate electricity. The researchers hypothesized that the polymetallic nodules on the ocean floor could generate sufficient electricity to produce oxygen through seawater electrolysis.

Sweetman shipped several pounds of these nodules to Geiger's lab at Northwestern for further testing. Sweetman also visited the lab in December to collaborate on the experiments.

The team found that just 1.5 volts - the same as a typical AA battery - is enough to split seawater. They recorded up to 0.95 volts on the surface of single nodules, with higher voltages possible when multiple nodules are clustered together, similar to batteries in series.

"It appears that we discovered a natural 'geobattery,'" Geiger said. "These geobatteries are the basis for a possible explanation of the ocean's dark oxygen production."

Implications for Deep-Sea Mining
The researchers caution that this discovery should inform future deep-sea mining efforts. According to Geiger, the polymetallic nodules in the Clarion-Clipperton Zone alone could meet global energy demands for decades. However, he warns against the ecological impacts seen in previous mining activities.

"In 2016 and 2017, marine biologists visited sites that were mined in the 1980s and found not even bacteria had recovered in mined areas," Geiger said. "In unmined regions, however, marine life flourished. Why such 'dead zones' persist for decades is still unknown. However, this puts a major asterisk onto strategies for sea-floor mining as ocean-floor faunal diversity in nodule-rich areas is higher than in the most diverse tropical rainforests."

Research Report:Evidence of dark oxygen production at the abyssal seafloor

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