Researchers from a global collaboration have unveiled groundbreaking evidence of dark oxygen production (DOP) at the abyssal seafloor, challenging existing understanding of deep-sea oxygen dynamics. The study, published in Nature Geoscience, reveals that polymetallic nodules on the abyssal Pacific Ocean floor may contribute to unexpected oxygen production in the deep sea.

Polymetallic Nodules reveal new role

A team led by Prof. Andrew Sweetman of the Scottish Association for Marine Science (SAMS) in Oban, a partner of UHI, made the ‘dark oxygen’ discovery while on ship-based fieldwork in the Pacific Ocean. During in situ benthic chamber experiments where equipment was deployed in the eastern Clarion Clipperton Zone (CCZ) of the Pacific Ocean, oxygen levels increased by over three times the background concentration within 47 hours, a phenomenon previously unreported in deep-sea studies. The experiments suggest that polymetallic nodules, which cover vast areas of the seafloor in the CCZ, are producing oxygen benefitting nearby marine life despite the lack of sunlight.

Prof. Sweetman said: “For aerobic life to begin on the planet, there had to be oxygen and our understanding has been that Earth’s oxygen supply began with photosynthetic organisms. But we now know that there is oxygen produced in the deep sea, where there is no light. I think we therefore need to revisit questions like: where could aerobic life have begun?”

In addition, the nodules were found to carry a higher electric charge compared to their surroundings, which could lead to the splitting of seawater into hydrogen and oxygen in the process called seawater electrolysis. The nodule voltage potentials were as high as 0.95V in experiments, a voltage high enough to facilitate the electrolysis.

Scientific implications

This discovery indicates that DOP could provide an additional source of oxygen for deep-sea benthic respiration, altering our understanding of oxygen cycles in abyssal environments. The research team calls for more extensive studies to elucidate the exact mechanisms behind DOP and its implications for deep-sea ecosystems.Understanding these processes could shed light on broader relationships between metal-oxide deposition, biological evolution, and the oxygenation of Earth’s oceans.

In addition, the findings have potential impact of deep-sea mining as polymetallic nodules can contain metals such as nickel, manganese and cobalt which are required to produce lithium-ion batteries.

When reflecting on previous studies, Prof. Sweetman said: “When we first got this data, we thought the sensors were faulty, because every study ever done in the deep sea has only seen oxygen being consumed rather than produced. We would come home and recalibrate the sensors but over the course of 10 years, these strange oxygen readings kept showing up.

“We decided to take a back-up method that worked differently to the optode sensors we were using and when both methods came back with the same result we knew we were onto something ground-breaking and unthought-of.”

Open Access article: 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). https://www.nature.com/articles/s41561-024-01480-8

Read more from the SAMS press release here