Scientists are preparing a deep-sea expedition to investigate “dark oxygen,” oxygen that has been detected on the Pacific Ocean seafloor in a place where sunlight does not reach. The work will focus on the Clarion-Clipperton Zone, an abyssal region known for polymetallic nodules, after unexpected measurements suggested oxygen production too deep for photosynthesis.
The effort is backed by The Nippon Foundation and will deploy newly built seafloor landers designed to sit on the bottom and take targeted measurements linked to the mystery. The expedition aims to test whether oxygen is being produced without sunlight, and if so, what chemical or biological processes could be driving it.
Where “dark oxygen” was found
The oxygen signals were detected about 4,000 meters below the ocean surface in the Clarion-Clipperton area between Hawaii and Mexico, according to one account. Another account describes dark oxygen originating approximately 13,000 feet below the ocean floor. The early observations came from a team led by seafloor ecologist Andrew Sweetman of the Scottish Association for Marine Science, who was conducting environmental studies in the region rather than searching for a new oxygen source.
In this part of the Pacific, the seabed is scattered with polymetallic nodules—metal-rich concretions that contain manganese and cobalt, and are often discussed in relation to seabed mining. The measurements raised concern because the oxygen pattern could not be explained by ocean currents, and the team considered the signal too consistent to dismiss as an instrument problem.
New landers and new tests
By May, a $5.2 million Nippon Foundation-funded expedition is expected to return to the Clarion-Clipperton Zone aboard the research vessel Nautilus to run follow-up measurements with equipment built for this question. The team plans to use landers that can stay on the seafloor to observe oxygen-related changes over time, rather than relying on shorter visits. The landers discussed publicly include two units named Alisa and Kaia, and they are designed to help determine whether nodules interact with salt water in ways that generate electricity and potentially support oxygen formation.
One planned improvement is adding pH sensors to detect changes in proton concentration in seawater, which researchers say could indicate water-splitting chemistry that produces molecular oxygen. Alongside ocean deployments, the team also plans laboratory experiments in pressure chambers meant to simulate the extreme conditions of the deep sea, described as around 400 atmospheres. The landers themselves are described as engineered to withstand extreme pressure, with one description saying up to 1,200 times the pressure at Earth’s surface.
The project is described as a three-year effort that brings together Sweetman, Boston University geobiologist Jeff Marlow, and Northwestern University chemist Franz Geiger. In addition to looking for oxygen production, the work is framed as a way to better understand how life can persist in deep environments that receive no sunlight.
The leading explanations scientists will test
Researchers have highlighted two main ideas that could explain “dark oxygen,” and they are not presented as mutually exclusive. One hypothesis focuses on electrochemistry: polymetallic nodules could act like catalysts and enable reactions that split water, in a way compared to electrochemical cells. A related framing describes the nodules as “natural batteries” that can separate seawater into oxygen and hydrogen, an interpretation linked to research published in Nature Geoscience in 2024.
The second hypothesis centers on biology, with the possibility that microbial communities associated with the nodules could contribute to the process. Marlow has described electrochemistry and biology as the “main culprits,” and the research plan includes mapping how microbes, minerals, and metabolic activity relate inside or around nodules. Geiger’s work is described as using microscopy and many electrode measurements to check for voltage differences on nodule surfaces in salt water, and he has said lab experiments have already detected substantial voltages using recovered nodules while key questions remain about how the electrical potential and oxygen production work under abyssal pressures.
Mining links and scientific scrutiny
The discovery emerged during research connected to The Metals Company, a Canadian firm interested in mining the seabed, according to one report. After the Nature Geoscience publication, some researchers linked to the company raised doubts and concerns, prompting a post-publication review that Nature Geoscience is said to have confirmed is still ongoing. The same report says a critical opinion article also appeared in Frontiers in Marine Science.
Sweetman has argued that understanding whether dark oxygen is real and how it functions matters for abyssal ecosystems, particularly if mining were to move forward and scientists are asked to advise on ways to reduce harm. The broader initiative has also been described as supported by the Intergovernmental Oceanographic Commission of UNESCO as part of a United Nations Ocean Decade initiative.
