A recent study published in Nature Geoscience has revealed a fascinating and until now little-understood phenomenon on the ocean floor: dark oxygen production (DOP). The study was conducted by an international team of scientists, led by Andrew K. Sweetman, and took place in the NORI-D license area of the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean. This location is known for its abundance of polymetallic nodules, which cover vast areas of the seafloor.
Polymetallic nodules are mineral concretions that form from the precipitation of dissolved metals in seawater and are of both scientific and commercial interest due to their richness in metals such as manganese, nickel, copper, and cobalt. The researchers conducted multiple experiments using in-situ benthic chambers, which were designed to measure the sediment community oxygen consumption (SCOC) on the seafloor. These experiments included different experimental treatments involving the addition of dead algal biomass, dissolved inorganic carbon, ammonium, and cold-filtered seawater. Controls without injection were also conducted for comparison.
What they found was surprising: instead of observing only oxygen consumption, as expected, they noticed a net increase in oxygen concentration in the chambers. Oxygen concentrations began at approximately 185.2 μmol/l and reached between 201 and 819 μmol/l in 47 hours, indicating a net production of oxygen that had not been previously documented in deep benthic oxygen flux studies.
The researchers proposed that dark oxygen production might be linked to the polymetallic nodules. These nodules exhibit electrical potentials of up to 0.95 V on their surfaces, which could facilitate the electrolysis of seawater and, thus, contribute to oxygen production in the absence of light.

One of the study’s most intriguing hypotheses is the “geo-battery” theory. This theory suggests that the difference in electrical potential between metal ions within the nodule layers could generate an internal redistribution of electrons, leading to oxygen production through seawater electrolysis. The nodules, enriched with transition metal oxides like nickel and characterized by areas of large tunnels and abundant defect sites, could optimize reactant adsorption and improve catalytic performance, facilitating oxygen production.
This process could be related to the exposure of electrochemically active sites in the nodules, which are affected by sediment coverage and removal during deep-sea mining operations.
Dark oxygen production could have significant implications for deep benthic ecosystems, providing an additional oxygen source that could support benthic respiration in the absence of oxygen from the water column. Furthermore, the discovery raises questions about how sediment removal and redistribution processes during deep-sea mining could influence this oxygen production, potentially affecting the biogeography and ecology of these habitats.
The study concludes that dark oxygen production is a real and measurable phenomenon on the deep seafloor, influenced by the presence and characteristics of polymetallic nodules. Although the rates of dark oxygen production observed were greater than the sediment community oxygen consumption, the researchers caution against extrapolating these results to longer time scales or broader areas without further studies.
The study also suggests that additional research is necessary to better understand the mechanisms behind dark oxygen production, its temporal nature, and its spatial distribution in abyssal ecosystems. This will allow for a better understanding of its role in marine biogeochemical cycles and how it may be affected by human activities, such as deep-sea mining.
This discovery not only has scientific implications but also potential consequences for the industry, particularly in the context of deep-sea mining. Mining companies and regulatory agencies will need to consider how the disturbance of polymetallic nodules could influence dark oxygen production and the overall health of deep marine ecosystems.
Additionally, the study opens new lines of inquiry to explore how geological processes can influence the evolution of life and the oxygenation of the planet, providing a unique perspective on the relationship between mineral deposits, biological evolution, and oxygen cycles on Earth.
SOURCES
Sweetman, A.K., Smith, A.J., de Jonge, D.S.W. et al. Evidence of dark oxygen production at the abyssal seafloor. Nat. Geosci. (2024). doi.org/10.1038/s41561-024-01480-8
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