Experts say discovery of ‘dark oxygen’ on the ocean floor is ‘fundamentally inconsistent with thermodynamics’ and should be retracted

A 2024 study that claims to have discovered an entirely new source of oxygen in the deep ocean, called “dark oxygen,” is flawed, contradicts previous research, and is “fundamentally inconsistent with thermodynamics,” critics argue in a new opinion piece.

Despite this backlash, the researchers behind the 2024 study recently announced that they would deploy a robot to the ocean floor between Mexico and Hawaii in May to confirm the findings and determine the cause of the phenomenon.

But even without this new investigation, “we have enough evidence to defeat them all.” [critics’] Study lead author Andrew Sweetman, professor and leader of the Seabed Ecology and Biogeochemistry Research Group at the Scottish Marine Science Society, told Live Science via email. Some of this evidence is currently being reviewed for publication in the journal Nature Geoscience, where the original study was published, Sweetman said.

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A 2024 study proposed that potato-sized chunks of metal on the deep ocean floor could split seawater through electrolysis, producing dark oxygen (so called because the suggested reaction involves no light). If the discovery survives rigorous scrutiny, it could fundamentally change our understanding of natural oxygen production, challenge the widespread belief that the deep ocean is a sink for oxygen, and raise important questions about the origins of life on Earth.

But in an opinion piece published in Frontiers in Marine Science in December 2025, critics say the study’s methodology is questionable and the researchers haven’t provided enough evidence to support their unusual claims.

“We downloaded the data and replotted everything,” said Anders Tenberg, co-author of the opinion article. “Everything points to this being true,” Tengberg, a product manager and scientific advisor at water technology company Aanderaa-Xylem and a researcher at the University of Gothenburg in Sweden, told Live Science.

In a joint interview, Tenberg and Per Hall, co-author of the opinion piece and professor emeritus of marine science at the University of Gothenburg, said that the authors of the 2024 study likely did not properly ventilate the measuring device when it landed on the ocean floor. As a result, oxygen trapped inside the device could have distorted gas concentrations measured at the seafloor, an undesirable effect Tenberg, Hall and colleagues warned about in a 2021 study.

Angel Cuesta Sisker, professor of electrochemistry and physical chemistry at the University of Aberdeen in Scotland and co-author of the opinion piece, said that even if Sweetman et al.’s study had measured oxygen concentrations correctly, the mechanism they showed for how oxygen is produced by chunks of metal, also known as polymetallic nodules, is meaningless.

“Explanation of how it forms is simply impossible because it violates the laws of thermodynamics,” Cuesta Sisker told Live Science. “Thermodynamics tells us what is possible and what is not possible if the laws of the universe are as we think they are. So far, in four centuries of science, no one has been able to prove that the laws of thermodynamics are correct.” [do not apply]. ”

“Experimental item”

Sweetman and his colleagues drew their unique conclusions from experiments they conducted in the Clarion-Clipperton Zone (CCZ), a vast abyssal plain 13,000 to 20,000 feet (4,000 to 6,000 meters) deep in the North Pacific Ocean between Mexico and Hawaii. The CCZ is littered with polymetallic nodules, deposits of cobalt, nickel, manganese and other metals that are essential for the production of batteries and electronics, making the area a target for deep-sea mining exploration companies.

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The researchers received funding for their research from Canadian deep-sea mining company The Metals Company and UK Seabed Resources, a subsidiary of Lockheed Martin’s UK division focused on deep-sea mining. But the results, published at what the opinion authors call a “critical juncture in the development of international regulations for deep-sea mining,” suggest that mining polymetallic nodules may have more negative impacts on ecosystems than previously understood.

The study described a steady release of oxygen from the ocean floor, which Sweetman and colleagues attributed to polymetallic nodules. Specifically, the researchers proposed that the potential difference between the metal ions within the nodule could cause a redistribution of electrons, creating a charge that splits seawater into hydrogen and oxygen.

Initially, the results seemed significant, but when Tenberg and his colleagues looked closer, “it became clear that there was no way that was true,” he said. Sweetman used a special chamber to measure oxygen levels on the ocean floor, which must be flushed out with bottom water before monitoring can begin to avoid contamination by air bubbles from the upper layers of the water column. This means that the oxygen measurements in the chamber should be similar at the start of each experiment, but they are “variable all over the place,” Tenberg said.

“Chamber cultures should be started with a bottom water composition equal to, or identical to, the surrounding bottom water outside the chamber,” Hall said, adding that Sweetman’s starting oxygen measurements were consistently higher than the bottom oxygen concentrations typically obtained in CCZs. “This is a clear sign that they are not doing a proper chamber culture and that their oxygen flow is unreliable.”

Traditionally, deep-sea experiments 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 this data, Tenberg said. Notably, previous studies have not found oxygen production from polymetallic nodules on the ocean floor, Tenberg et al. write in an opinion piece.

The 2024 study did not present data from a “negative control experiment,” which would involve culturing in the absence of polymetallic nodules to ensure that there was no oxygen production in the absence of nodules, the critics wrote. But according to this opinion piece and a 2024 preprint paper on the non-peer-reviewed server Earth ArXiv, this data exists and shows oxygen production even in the absence of nodules.

“This strongly suggests that oxygen production is an experimental product,” Hall said. The increase could be due to oxygen bubbles reaching the ocean floor and being left unventilated, becoming trapped in the chamber and gradually dissolving, he added.

seawater electrolysis

Electrochemists on the opinion article team provided additional arguments for why polymetallic nodules are unlikely to be a source of oxygen on the deep sea floor.

First, they argued that seawater electrolysis requires large amounts of energy and cannot proceed naturally. Sweetman and his colleagues also said they were unable to identify a source of energy large enough to generate an electric charge to split seawater.

“The explanation that Sweetman and his collaborators are proposing is tantamount to suggesting that energy is created out of thin air, or that things naturally go uphill rather than downhill, if you like,” Cuesta Siscal said. “We know that the energy in the universe is constant and cannot be created out of thin air.”

The study also did not provide measurements of hydrogen concentration 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 a reaction.

“My guess is that there are just honest mistakes that are not recognized,” Cuesta Siscal said.

In response to the opinion piece’s discussion, Sweetman said he and his team could not provide a meaningful response until the review of additional evidence in Nature Geoscience (NG) was completed. “If the NG rebuttal is rejected, we will naturally file a response to the Frontiers article,” he said.

Researchers are now preparing for a spring expedition to the CCZ, where they will deploy two highly specialized landers to determine exactly how dark oxygen is produced. The project is funded by the Nippon Foundation, a private Japanese organization that promotes humanitarian work, diplomacy, and industrial maritime development.

The search for dark oxygen continues, but many experts doubt it will lead to anything substantial, Hall said. “We don’t believe this,” he said. “I hope Nature Geoscience retracts the paper.”