Dark Oxygen Production (DOP) was a hypothesis published by Sweetman et al. (2024) to explain anomalous data from experiments in the Clarion-Clipperton Zone, Pacific ocean. These experiments recorded rising oxygen for the first few hours, when previous experiments of the same type had only ever recorded reducing oxygen due to consumption by benthic organisms. In order to explain this phenomenon, Sweetman et al. claimed that manganese nodules, which were present in many of the experiments, must be responsible for the oxygen production. It was thus given the name "dark oxygen production", borrowing the term "dark oxygen" that had previously been used for oxygen production by bacteria without sunlight. Sweetman et al. claimed that in this case the oxygen was being produced by electrolysis, using measured voltages from the surface of the nodules as evidence. The article, published in nature geoscience, was widely reported in mainstream media and social media, and contributed to the ongoing debate around deep sea mining. However, it has since emerged that there are many problems with the theory, and may even be an example of scientific misconduct since contrary results and pertinent information were omitted from the article. The omitted data showed that manganese nodules could not have been responsible for the phenomenon. A collaborative rebuttal published by leading scientists and industry experts instead claims that the rising oxygen levels are just experimental artefact, and not due to any natural phenomenon. Sweetman et al. have publicly stepped back from the two main conclusions in the article; that manganese nodules produce oxygen, and that they do this by electrolysis. Retraction of the paper has been called for by independent scientists.
Benthic chamber landers are a form of benthic lander where a box, or chamber, is lowered to the seafloor and partially penetrates the sediment, creating a sealed volume of water which is then separate from the surrounding water column. In this chamber, various sensors and sampling devices are used, including oxygen sensors which measure the uptake of dissolved oxygen from the seawater by organisms within the chamber. However, Sweetman et al. published data from benthic chamber landers that appeared to show oxygen rising in many of their chambers during the first few hours of the experiment. Subsequently, further experiments were done by taking samples of the seafloor and incubating them in the lab, and these appeared to confirm rising oxygen in the presence of manganese nodules. This led to the hypothesis that manganese nodules were somehow causing or producing the rising oxygen. The mechanism was not explained until voltages were measured on the surface of the nodules, and a reported voltage of 0.95 V was used as evidence that the manganese nodules were splitting water via electrolysis, producing oxygen and, presumably, hydrogen (although no data on hydrogen was released). And so the theory was formed, that manganese nodules can split water by electrolysis and produce oxygen, and this was published in Nature Geoscience in 2024. The article made bold claims about how this could influence our understanding of the oxygenation of the Earth, the emergence of life and even life on other planets. It quickly attracted a lot of attention, and the article was widely reported, becoming one of Nature Geoscience's most read articles.
The Dark Oxygen hypothesis quickly attracted doubt from industry leaders and independent scientists, stating numerous issues with the research. Some of the initial issues included that no energy source for electrolysis had been identified, and no other studies had observed the same phenomenon despite being conducted in similar places and in the presence of manganese nodules. Shortly, pre-print rebuttals emerged which brought solid claims against the findings, including that crucial data was omitted from the article in order to support the dark oxygen hypothesis. Direct rebuttals to the article are summarised here.
Subsequently, many of the authors involved in the rebuttals collaborated to produce a comprehensive and peer reviewed critique of the original article, published in Frontiers in Marine Science in 2025.
Retraction of the paper has been called for. Nature Geoscience, who published the article, are a member of the Committee on Publication Ethics (COPE). They say that retraction of a paper may be warranted if "the work contains such seriously flawed or erroneous content or data that its findings and conclusions cannot be relied upon". The rebuttals indicate that several severe methodological flaws are present in paper, including that oxygen rose in chambers without nodules, and that oxygen concentration measurements were unreliable and indicate contamination. These lead to the conclusion that the findings of the paper cannot be relied upon, since they show that manganese nodules cannot be responsible for the rising oxygen, and that the rising oxygen is just experimental artefact.
Another reason given by COPE for retraction is for any form of misrepresentation, including data falsification, such as "omitting data or results so the research is not accurately represented in the record". The rebuttals indicate that at least two important pieces of information were omitted - that oxygen rose in experiments without manganese nodules present, and that oxygen rose in a control experiment conducted without nodules. These two pieces of omitted information invalidate the main claim of the article, that manganese nodules are responsible for the oxygen concentration increases.
This is a list of some of many issues raised with the research.
The criticisms of the study has led to the authors of the article retreating from the two main findings. The first claim was that manganese nodules were responsible for rising oxygen. However, as early as September 2024, Andrew Sweetman conceded that some of the landers did not contain nodules, but only small particles of manganese oxide. Later, in a 2025 AGU conference abstract about the dark oxygen hypothesis, manganese nodules are not mentioned, and instead only "manganese oxides" and "polymetallic oxides" are referred to. In an interview in 2026, the lead author said "It's true that there were no nodules in the chambers. They're saying it's just about the nodules" [translated from Norwegian]. This change in messaging has likely been forced due to the revelation that the lander experiments conducted in 2018 did not contain manganese nodules, and only sometimes contained manganese oxide grains. This is a hugely significant concession as it now implicates manganese oxide found anywhere in the ocean, not just manganese nodules, which are found in limited areas. Manganese oxide is a common constituent of oxic deep sea sediments, which suggests that the findings of Sweetman et al., if they were real, should be applied to deep ocean sediments in general, not just in areas of polymetallic nodules.
The second concession is that electrolysis is not the mechanism that explains the rising oxygen, despite it being fundamental to the original paper. The 2025 AGU abstract states that "It is presently unclear what the mechanism behind DOP is", and electrolysis is not mentioned.
In the paper and elsewhere in the media, Sweetman et al. made many claims, sometimes contrary, about the process and how important it might be to the benthic ecosystem, to the oxygenation of the Earth, and even to the emergence of life. However, there is a clear problem with these claims since the presence of oxygen is a pre-requisite for the formation of manganese oxides - and thus, they could not exist without the Earth first being oxygenated by other means. Currently, the oxygenation of the Earth, known as the Great Oxidation Event, is thought to have been caused by photosynthetic bacteria, and therefore life was already present at that time. Therefore, the theory is "unlikely at best".
In terms of the importance to the benthic ecosystem, the original article by Sweetman et al. simultaneously suggests its importance whilst also saying that the results can not be extrapolated, and that the equipment used may have somehow caused or triggered the phenomenon, and so "we shouldn't think these nodules are bubbling oxygen all the time". There is currently no evidence that dark oxygen production is important to the benthic ecosystem.
Sweetman et al. state that "Our findings contrast with all published deep-sea benthic O2 flux studies", and states that more work is needed to study the phenomenon. It is therefore important to examine the body of work that has already been done. Many previous studies have measured oxygen consumption rates in the deep sea, including in areas . None of these studies have reported rising oxygen levels, or any evidence of oxygen production. Some of these studies include:
Anomalous results in the context of so much previous work is a strong indication of experimental artefact - an example of Twyman's law, which states that "the more unusual or interesting the data, the more likely they are to have been the result of an error of one kind or another". It is established that when a result deviates significantly from a body of previous work, the default interpretation should be that there is an artefact, not that a new phenomenon has been discovered. However, although Sweetman et al. note that their results are anomalous, and they attempt to address some possible sources of errors that might have occurred, they do not discuss or attempt to explain why their results differ so greatly from all previous work. In general, the scientific method requires that extraordinary results be repeated in order to be accepted by the scientific community, to build scientific consensus, but in this case the experiments have already been repeated multiple times by multiple groups of researchers, without seeing the same phenomenon. Furthermore, the study published by Sweetman et al. itself repeats the experiments multiple times, and yet still only sees the phenomenon in some of the experiments. There is no explanation given as to why the phenomenon occurs in some experiments but not others. The most likely explanation is that sometimes the equipment functioned correctly, and oxygen production was not observed.
On the basis of these findings, Andrew Sweetman received a grant from the Nippon Foundation for ã2M over three years for a project that "seeks to understand the actual conditions surrounding dark oxygen [and] will involve surveying 11,000 meters below the ocean surface using newly developed, specialized experimental equipment, and in addition to identifying the source of dark oxygen, will attempt to clarify its role in deep-sea ecosystems." The collaborative rebuttal by Downes et al. states "key omissions, including data from control experiments showing that the DOP signal originates from the benthic chamber lander itselfâÂÂnot the nodulesâÂÂstrongly indicates that the reported oxygen increases are experimental artefact rather than a genuine geochemical process" and that the hypothesis "is incompatible with established knowledge and inconsistent with experimental evidence", and so this funding may have been "potentially misallocated on the basis of speculative assertions".
manganese nodule deep sea mining dark oxygen scientific misconduct