Critics Shot Down A Major 'Dark Oxygen' Discovery. Experts Are Trying to Prove Them Wrong.

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29-01-2025

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In the summer of 2024, an international team of scientists reported the presence of dark oxygen along the ocean floor and theorized that polymetallic nodules may have created this gas via seawater electrolysis.
A flurry of criticism followed the article's publication, including from the deep-sea mining company that funded the study, and challenged the scientists' conclusions.
Now, the researchers will once again return to the deep sea specifically to study dark oxygen production and hopefully settle this scientific disagreement once and for all.
Most oxygen on Earth is produced by photosynthesis, as plants and plankton gobble sunlight for food and produce that oh-so-wonderful, life-giving O2. However, that's not the only way oxygen can be produced. Enter 'dark oxygen,' a process that isn't as nefarious as it sounds. Simply put, it refers to oxygen produced without sunlight.
Although a couple examples of dark oxygen exist, a new potential source—discovered at the dark depths of the Pacific Ocean's Clarion-Clipperton Zone (CCZ), nestled between Hawaii and the western coast of Mexico—has created a firestorm of interest. It's also sparked significant controversy, as the source of this new dark oxygen could be polymetallic nodules, which are mineral-rich objects that are the prize of deep sea mining companies like The Metals Company.
In July of 2024, an international team of scientists published a paper in the journal Nature Geoscience claiming that these polymetallic nodules (also known as manganese nodules) could be producing oxygen via electrolysis. As millions of years of metallic layers built up in the nodules—similar to a voltaic pile—the differing electric potential could theoretically produce enough energy to split water into oxygen and hydrogen. Almost immediately upon publication, The Metals Company (who helped fund the research) disagreed with the results, and other doubts in the academic community (none of which have been peer reviewed) began to surface.
Andrew Sweetman—lead author of the study and a deep-sea ecologist at the U.K.'s Scottish Association for Marine Science (SAMS)—and his colleagues have stood by the results.
'What if you're getting millions of these little tiny batteries on manganese oxide particles in the nodules […] and there's electrons flowing through the internal layer structure of the manganese oxide?,' Sweetman told Popular Mechanics back in September of 2024. 'I was the worst critic of this paper for 10 years. I said 'no, this is not possible' until I couldn't say no anymore.'
Now, Sweetman—backed by funding from a Japanese charity called The Nippon Foundation—will venture once more to the CCZ to specifically test nodules from the sea floor and hopefully put these criticisms to rest. According to New Scientist, the team will use custom-built landers that can descend up to 12,000 meters below sea level, which is twice the required depth needed to study the nodules. The researchers will also search for the presence of hydrogen, which was not specifically tested in previous research, to further confirm that electrolysis is taking place. The first of three research expeditions will set sail in January of 2026 from San Diego, California, and Sweetman remains confident of the outcome.
This research comes at a sensitive time, as the International Seabed Authority (the global governing body that presides over the CCZ) is still determining the rules and regulations of mining these resource-rich nodules. If these hunks of metal are producing trace amounts of oxygen vis seawater electrolysis, the damage to this already sensitive area could be severe—and conservation groups have taken notice.
'This is an excellent example of what it means to have the deep ocean as a frontier, a relatively unexplored part of our planet,' Oceanographer Lisa Levin told the Deep Sea Conservation Coalition at the time of the initial paper's publication. 'There are still new processes to discover that challenge what we know about life in our ocean.'
This will be the first research expedition undertaken with the express purpose of searching for dark oxygen. Previous research on this discovery was part of a more general research expedition investigating ocean respiration. Although Sweetman has faced mounting criticism—which he admits has taken a personal toll—he remains confident of this future expedition's outcome, as he's seen evidence of this inexplicable oxygen show up in his work for more than a decade.
'I know [dark oxygen production] is happening. We have found this now in six places,' Sweetman told New Scientist. 'I know we are going to find it.'
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