It’s one of the earliest things you learn in elementary school science class—Earth’s life-sustaining oxygen is produced by plants and algae during photosynthesis using a combination of carbon dioxide and sunlight. But the recent discovery of what researchers call “dark oxygen” may upend conventional notions of how the critical element can be created–and what that might mean for the origins of life.
According to a study published in Nature Geoscience on July 22, natural mineral deposits known as polymetallic nodules located at the bottom of the ocean appear capable of generating oxygen without any source of light. These nodules are found as far as 20,000 feet below the ocean surface and range in size from particles to nodules as large as a human hand. Because they contain combinations of cobalt, copper, lithium, and manganese, they have long been eyed by large-scale mining companies as a potential untapped source of coveted metals needed to produce batteries and other electronics. But as lucrative as they may be for industrial uses, they now seem far more vital to life within ocean ecosystems.
The first indications that something strange was occuring within polymetallic nodules arrived over 10 years ago in a northeastern region of the Pacific Ocean. While on a sampling expedition in the area’s mountainous submarine ridge known as the Clarion-Clipperton Zone, Andrew Sweetman of the Scottish Association for Marine Science (SAMS) noticed odd readings on his equipment.
“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 says in an accompanying statement. “We would come home and recalibrate the sensors, but, over the course of 10 years, these strange oxygen readings kept showing up.” After double checking the findings using a different sensor array, Sweetman and his team knew they were “onto something groundbreaking and unthought-of.”
In 2023, Sweetman contacted Northwestern University electrochemistry expert Franz Geiger about the strange evidence and sent him multiple pounds of polymetallic nodules. Electrolysis, the process of splitting a target into its separate elements, needs only 1.5 volts to initiate in seawater—and after attaching sensors to a single nodule, Sweetman and Geiger detected voltages as high as 0.95 volts. This power increased even more when they placed the formations close together, much like stacking batteries.
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“It appears that we discovered a natural ‘geobattery,’” Geiger says in a statement. “These geobatteries are the basis for a possible explanation of the ocean’s dark oxygen production.”
The existence and possible source of this dark oxygen may eventually rewrite the narrative of how life originated on Earth. As Sweetman explains, experts have long theorized that the planet’s aerobic life began due to oxygen created by photosynthetic organisms like early plants and algae. Now that they know oxygen can be produced even in the ocean’s lightless depths, these theories may need updating.
“I think we… need to revisit questions like: Where could aerobic life have begun?” says Sweetman.
But polymetallic nodules may not have just helped start life on Earth—they may also continue to keep it going near the ocean floor. And this poses a major issue for viewing them as a potential natural mining resource. Geiger explains in Monday’s announcement that 2016 and 2017 examinations by marine biologist examinations of deep sea areas mined during the 1980s revealed total dead zones that lacked even the presence of bacteria.
“Why such ‘dead zones’ persist for decades is still unknown,” Geiger says. “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.”
Unfortunately, all that deep ocean biological diversity may mean little to the corporations that view polymetallic nodules as potential profits. Geiger notes that the total mass of all the formations within the 4,500 miles that compose the Clarion-Clipperton Zone is likely enough to supply global energy demands for decades. But as countless examples already show, the destruction of one seemingly distant ecosystem can initiate deadly and dangerous ripple effects elsewhere.
“We need to rethink how to mine these materials, so that we do not deplete the oxygen source for deep-sea life,” Geiger warns.