The depths of the sea could hold far more natural resources than we could ever imagine. Scientists have discovered an unexpected phenomenon 4,000 metres below the surface of the Pacific Ocean: oxygen generated without light, inside experimental chambers placed on the ocean floor. These observations raise new questions about the impact of human activity in the abyssal environment.
by Laurie Henry
An unexpected discovery in the abyssal darkness
Between 2021 and 2022, during several expeditions in the Pacific Ocean in the Clarion-Clipperton zone, researchers from the Scottish Association for Marine Science, Boston University, GEOMAR in Germany and Heriot-Watt University used a device consisting of airtight chambers to enclose microenvironments on the ocean floor. Equipped with oxygen sensors calibrated at different temperatures and depths, these chambers were designed to quantify oxygen consumption linked to microbial and chemical activity in the sediments.
Contrary to expectations, the results show not a decrease but a net increase in dissolved oxygen in the trapped water. The variations observed are not isolated, reaching in several cases up to 18 mmol O2/m3/day. The effect is detected regardless of the type of treatment applied to the chambers (addition of dead algae, ammonium, or simply filtered water), ruling out any bias linked to the injection of external substances.
Changes in oxygen concentrations (μmol/l) measured by calibrated optical sensors over time (in hours) in the various benthic chamber experiments. © Andrew K., et al., 2024
To verify that the results were not due to sensor error, the researchers used another method known as Winkler (a classic chemical technique for measuring dissolved oxygen), which also showed an increase in oxygen. Ex situ incubations in the laboratory, on sediments or nodules alone, also reproduced the same behaviour, making the hypothesis of an experimental artefact increasingly unlikely, without however elucidating the mechanisms at work.
An electrochemical hypothesis related to polymetallic nodules
In their study, A.K. Sweetman and his colleagues point to a recurring element in experiments where net oxygen production is observed: the presence of polymetallic nodules. These natural formations, composed mainly of manganese oxides enriched with nickel, cobalt and copper, cover the seabed of the Clarion-Clipperton zone and are known for their conductivity and metallic layered structure.
To test their potential role, the authors measured differences in electrical potential at the surface of 12 nodules at 153 distinct locations. The voltages recorded varied widely, but some reached as high as 0.95 V at 5°C. Although this voltage remains below the standard threshold of 1.23 V required for water electrolysis, the authors suggest that this process could be facilitated by catalytic effects linked to crystal defects and the porosity of the nodules, particularly in the nickel- and copper-rich layers.
Sampling locations. © Andrew K., et al., 2024The study also highlights a statistical link between the average surface area of the nodules and the intensity of oxygen production observed. It suggests that larger nodules offer a greater reaction surface area or increased density of electrochemically active microstructures. Finally, the researchers also mention a potential ‘geobattery’ mechanism, whereby the internal distribution of metals would create micro-current sources capable of locally inducing the dissociation of seawater. If confirmed, this phenomenon would probably be sporadic and dependent on immediate physical conditions, such as the removal of the surface sediment layer during the impact of the sampling equipment.
A major environmental challenge and a wake-up call for the mining industry
The possible existence of an abiotic oxygen production mechanism in the deep ocean reconfigures an essential part of benthic ecology. At a depth of 4,000 metres, oxygen is generally limited by the slow diffusion of cold water masses from the poles. The discovery of a local, autonomous source of oxygen in the Clarion-Clipperton zone, even if intermittent, could change our understanding of the metabolism of microbial and animal communities in these extreme environments. If black oxygen contributes to the respiration of benthic organisms, it is an unknown but potentially vital link in the functioning of these deep-sea ecosystems.
This hypothesis is all the more worrying as it comes at a time of tense international discussions on the possible opening of deep-sea mining. The nodules studied, at the heart of the suspected phenomenon, are the same as those targeted by large-scale extraction projects that would be used to supply the critical metals industry (particularly for batteries).
While the potential impact of such activity is still unknown, the authors point out that the remobilisation of sediments, or the removal of the nodules themselves, could alter the conditions conducive to black oxygen production in the abyss. Faced with this uncertainty, several voices are calling for an immediate freeze on deep-sea mining operations. The Sustainable Ocean Alliance, which brings together young international environmental leaders, emphasises the lack of robust data to assess the long-term consequences. ‘Black oxygen production, if real, adds a critical element to the already poorly understood ecosystem functions of the ocean floor,’ warns Daniel Cáceres Bartra. He calls for regulatory negotiations to be halted until the scientific assessment is complete. For Jaime-Leigh Edghill, also a member of the delegation to the International Seabed Authority, this case illustrates the need to integrate fundamental research into policy decisions. ‘What we don’t know yet could be precisely what we are destroying.’
Doubts, criticism and ongoing scientific debate
Despite the interest generated by this work, the validity of the hypothesis of abiotic oxygen production remains contested. Several researchers outside the study, as well as representatives of companies involved in mining exploration, point to methodological limitations and results that are considered insufficiently robust. The Metals Company, which partially funded the field expeditions, published a technical response questioning the reliability of the observations. Its environmental biologist, Michael Clarke, questions the failure to take into account certain variables, such as the possible intrusion of oxygen linked to the injection of surface water, poor evacuation of air bubbles or electrical leaks within the experimental chambers.
Other criticisms target the electrochemical measurements themselves. While a peak of 0.95 V was recorded at one location, the majority of voltages measured on the nodules remain well below the threshold required to cause water electrolysis. Lars-Kristian Trellevik, from the company Adepth, points out that the results presented do not allow a reliable conclusion to be drawn about electrochemical activity capable of generating oxygen. Furthermore, some of the experiments presented in the study include cases where oxygen increased even though no nodules were present in the chamber, which weakens the main argument that these nodules are the direct cause.
The European project MiningImpact2 adds an important element to the debate. Its own measurements, carried out with comparable technology, showed no signs of oxygen production. According to researcher Matthias Haeckel, these discrepancies could be due to different experimental conditions, in particular the way the chambers are placed on the seabed. Where Sweetman’s team uses a direct descent that could expose reactive surfaces, other, more controlled deployments would have left the nodules covered with sediment.
À ce stade, la production d’oxygène en l’absence de lumière reste une hypothèse intrigante mais encore loin d’être démontrée. Néanmoins, elle souligne un fait central : l’océan profond demeure largement inexploré, et son altération prématurée par des activités extractives pourrait compromettre la connaissance du fonctionnement de certains équilibres.
Source : Andrew K. Sweetman et al, “Evidence of dark oxygen production at the abyssal seafloor”. Nature Geoscience (2024)