A 2024 study that claimed to have discovered an entirely new source of oxygen in the deep ocean — dubbed “dark oxygen” — was flawed, inconsistent with previous research and “fundamentally at odds with thermodynamics,” critics argue in a new opinion piece.
Despite this setback, the researchers behind the 2024 study recently announced that they will deploy robots to the ocean floor between Mexico and Hawaii in May to confirm the findings and determine what is causing the phenomenon.
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The 2024 study suggested that potato-sized lumps of metal on the deep ocean floor could split seawater through electrolysis to make dark oxygenso called because there is no light involved in the proposed reaction. If the discovery stands up to scrutiny, it will radically change our understanding of natural oxygen production, challenge the widely held idea that the deep ocean floor is an oxygen sink, and raise key questions about the origins of life on Earth.
But in the opinion article, published in December 2025 in the journal Frontiers in Marine Sciencecritics say the study’s methods were questionable and that the researchers did not provide enough evidence to support their extraordinary claims.
“We downloaded the data and replotted everything,” said Anders Tengbergco-author of the opinion piece. “Everything just points against this being correct,” Tengberg, a product manager and scientific advisor at the water technology company Aanderaa-Xylem and a researcher at the University of Gothenburg in Sweden, told LiveScience.
It appears that the authors of the 2024 study did not properly ventilate the measuring equipment when it landed on the seabed, Tengberg and Per Hallco-author of the opinion article and professor emeritus of marine science at the University of Gothenburg, said in a joint interview. As a result, oxygen trapped inside the equipment may have skewed the gas concentrations measured on the seafloor—an undesirable effect that Tengberg, Hall, and colleagues warned against in a study from 2021.
Although Sweetman and his colleagues had measured oxygen concentrations correctly in their study, the mechanism they provided for how oxygen was produced by the metal clumps, also known as polymetallic nodules, does not make sense, said Angel Cuesta Ciscarprofessor of electrochemistry and physical chemistry at the University of Aberdeen in Scotland and co-author of the opinion article.
“This explanation of how it is formed is simply impossible, because it violates the laws of thermodynamics,” Cuesta Ciscar told Live Science. “Thermodynamics tells you what is possible and what is not possible if the laws of the universe are what we think they are. Until now, no one in four centuries of science has been able to show that the laws of thermodynamics (do not apply).”
“Experimental Artifact”
Sweetman and his colleagues drew their original conclusions from experiments they conducted in the Clarion-Clipperton Zone (CCZ), a giant abyssal plain 13,000 to 20,000 feet (4,000 to 6,000 meters) deep in the North Pacific between Mexico and Hawaii. The CCZ is littered with polymetallic nodules, which are accretions of cobalt, nickel, manganese and other metals critical to producing batteries and electronics, making the area a target for deep-sea mining exploration companies.
The researchers received funding for the study from The Metals Company, a Canadian deep-sea mining company, and UK Seabed Resources, a subsidiary of the British arm of Lockheed Martin that focuses on deep-sea mining. Still, the results, published at what the authors of the opinion piece called “a critical juncture in the development of international regulations for deep-sea mining,” suggested that mining polymetallic nodules may have a worse impact on the ecosystem than previously understood.
The study described steady emissions of oxygen from the ocean floor that Sweetman and his colleagues attributed to polymetallic nodules. In particular, the researchers proposed that the difference in electrical potential between metal ions in the knots could lead to a redistribution of electrons, triggering a charge that could split seawater into hydrogen and oxygen.
The result initially seemed significant, but when Tengberg and his colleagues looked more closely, “it became clear that it couldn’t have been true,” he said. Sweetman used special chambers to measure oxygen concentrations at the seabed that must be flushed with bottom water before monitoring begins to avoid contamination by gas bubbles from higher up in the water column. This means that oxygen readings inside the chambers should be similar at the start of each experiment, but they are “all over the place,” Tengberg said.
“You need to start your chamber incubations with bottom water composition similar — identical — to the ambient bottom water outside the chambers,” Hall said, adding that Sweetman’s starting oxygen readings were consistently higher than bottom oxygen concentrations typically obtained in the CCZ. “It’s a clear sign that they didn’t do good chamber incubations and that their oxygen flows … can’t be trusted.”
Traditionally, experiments in the deep ocean using chamber incubations also measure other gases to get a clear picture of the environment and its chemistry, but Sweetman and his colleagues did not provide that data, Tengberg said. In particular, no previous study has found oxygen production from polymetallic nodules on the seabed, Tengberg and his colleagues wrote in the opinion article.
The 2024 study did not present data from “negative control experiments,” which in this case would have been incubations without polymetallic nodules to confirm the absence of oxygen production when nodules are not present, the critics wrote. But according to the opinion piece and a 2024 preprint paper on the server Earth ArXiv which has not been peer reviewed, this data exists – and it shows oxygen production even in the absence of nodules.
“This strongly suggests that the oxygen production is an experimental artifact,” Hall said. The increase may have been the result of oxygen bubbles being trapped and gradually dissolved inside the chambers after they reached the sea floor and sat there unventilated, he added.
Seawater electrolysis
Electrochemists on the opinion article team provided further arguments for why polymetallic nodules are unlikely to be a source of oxygen on the deep ocean floor.
First, seawater electrolysis requires a significant amount of energy and cannot proceed spontaneously, they argued. And Sweetman and his colleagues did not identify an energy source large enough to generate an electrical charge and split seawater, they said.
“The explanation proposed by Sweetman and his collaborators amounts to suggesting that energy is created out of nothing, or, if you will, that things go uphill spontaneously, instead of going downhill,” Cuesta Ciscar said. “We know that the energy in the universe is constant, and it is not created out of nothing.”
The study also provided no hydrogen concentration measurements to support the idea of seawater electrolysis. For every oxygen molecule produced by water electrolysis, two hydrogen molecules are also formed, so the presence of hydrogen is a clear sign of the reaction.
“It’s just, I would expect, an honest mistake that hasn’t been recognized,” Cuesta Ciscar said.
In response to the arguments in the opinion piece, Sweetman said he and his team cannot respond meaningfully until the review of their additional evidence is completed at Nature Geoscience (NG). “If the rebuttal at NG is rejected, we will of course submit a response to the Frontiers piece,” he said.
The researchers are now preparing for a spring expedition to the CCZ, where they will deploy two highly specialized landers to identify exactly how dark oxygen can be produced. The project is funded by the Nippon Foundation, a private organization in Japan that promotes humanitarian work, diplomacy and industrial maritime development.
The hunt for dark oxygen continues, but many experts doubt it will lead to anything significant, Hall said. “We don’t believe this,” he said. “I hope that Nature Geoscience retracts the paper.”
Downes, P., Cuesta, A., Denny, A., Tengberg, A., Hall, P.O.J., Trellevik, L., Svellingen, W., Jaspars, M., Webber, A.P., De Freitas, F.S., Bento, J.P., Marsh, L., & Clarke, M. (2025). Extraordinary claims require extraordinary evidence: evaluation of nodule-associated dark oxygen production. Frontiers in Marine Science, 12. https://doi.org/10.3389/fmars.2025.1721853
