‘Dark Oxygen’ Discovery in Deep Sea Faces Scrutiny & New Investigation

A recent study claiming the discovery of “dark oxygen” – oxygen produced in the deep sea without sunlight – is facing significant criticism from the scientific community. Experts argue the findings are inconsistent with established thermodynamic principles and question the methodology used in the original research. The debate centers on whether metallic lumps on the ocean floor can, in fact, generate oxygen, a process that would fundamentally alter our understanding of oxygen production and potentially the origins of life itself.

The Discovery and Initial Claims

Published in 2024 in the journal Frontiers in Marine Science, the study proposed that polymetallic nodules – potato-shaped mineral deposits found in the Clarion-Clipperton Zone (CCZ) – were producing oxygen through a process akin to electrolysis. The research, led by Professor Andrew Sweetman of the Scottish Association for Marine Science, suggested these nodules, rich in metals like cobalt, nickel, copper, and manganese, could split seawater molecules, releasing oxygen in the process. The CCZ, a vast abyssal plain between Hawaii and Mexico, is of particular interest not only for its unique ecosystem but also since it’s a target for deep-sea mining operations. These nodules contain valuable metals crucial for batteries and other technologies.

Methodological Concerns and Thermodynamic Impossibilities

However, a new opinion article published in December 2025 challenges these claims. Co-authored by Anders Tengberg of Aanderaa-Xylem and Per Hall of the University of Gothenburg, the critique argues the study’s methods were flawed and the evidence presented insufficient. Tengberg and Hall contend that the researchers failed to properly ventilate their measuring equipment after deployment on the seafloor, potentially leading to skewed oxygen readings due to trapped gas. This issue echoes concerns raised in a 2021 study cautioning against similar experimental pitfalls.

Beyond the methodological concerns, the core claim of oxygen production violates fundamental principles of thermodynamics, according to Angel Cuesta Ciscar, a professor of electrochemistry at the University of Aberdeen and another co-author of the critical analysis. “That explanation of how it’s formed is simply impossible, because it violates the laws of thermodynamics,” Cuesta Ciscar told Live Science. Thermodynamics, a cornerstone of physics, dictates that energy cannot be created from nothing, and spontaneous processes always move towards disorder, not the creation of complex molecules like oxygen without an external energy source.

What Does This Mean for Understanding Oxygen Production?

The implications of the original study, if validated, would have been profound. It would have challenged the long-held belief that oxygen production is solely reliant on photosynthesis – a process requiring sunlight. It also suggested the deep seafloor, traditionally considered an oxygen sink, could be a source of oxygen. It raised intriguing questions about the origins of life on Earth, potentially suggesting alternative pathways for oxygen accumulation in the early oceans. However, the current criticisms cast significant doubt on these possibilities.

The lack of negative control experiments – incubations without nodules to establish a baseline – further weakens the original study’s conclusions. The researchers did not provide data on hydrogen concentrations, which would be expected if seawater electrolysis were occurring. The absence of this data, coupled with the inconsistent starting oxygen measurements, fuels skepticism about the validity of the findings.

Funding and the Deep-Sea Mining Context

It’s worth noting that the original study received funding from The Metals Company and UK Seabed Resources, both companies involved in deep-sea mining exploration in the CCZ. This funding raises questions about potential bias, although Sweetman maintains the integrity of the research. The timing of the study’s publication, coinciding with critical discussions about international regulations for deep-sea mining, also adds a layer of complexity to the debate. The initial results suggested that mining polymetallic nodules could have more significant environmental impacts than previously understood, potentially influencing regulatory decisions.

Ongoing Investigation and Future Steps

Despite the criticisms, Professor Sweetman and his team are proceeding with further investigation. They plan to deploy robots to the seafloor in May to confirm their findings and pinpoint the mechanism behind the observed oxygen production. Sweetman asserts they have additional evidence supporting their claims, currently under review at Nature Geoscience. “Even without this new investigation, we have more than enough evidence to quash all their [critics’] statements,” he stated in an email to Live Science.

However, many experts remain unconvinced. Hall expressed hope that Nature Geoscience will retract the original paper. The scientific community will be closely watching the outcome of the ongoing investigation and the peer-review process at Nature Geoscience. The debate highlights the importance of rigorous methodology, adherence to fundamental scientific principles, and transparency in research, particularly when dealing with potentially impactful discoveries and commercially sensitive areas like deep-sea mining. For now, the claim of “dark oxygen” production remains highly contested, and further evidence is needed to resolve the controversy.

The process of scientific validation is ongoing. The team’s additional evidence will be scrutinized, and independent replication of the results will be crucial. The international community will continue to monitor developments in the CCZ, balancing the potential benefits of deep-sea mining with the need to protect this unique and largely unexplored ecosystem. Further research will be essential to determine the true nature of oxygen dynamics in the deep ocean and their implications for our understanding of life on Earth and beyond.