Scientists are partly baffled and excited over the discovery of an unknown source of oxygen on the Pacific Ocean floor, where no sunlight reaches and photosynthesis just doesn’t work. Called “dark oxygen,” this phenomenon was discovered in the Clarion-Clipperton Zone, an area the size between Hawaii and Mexico, and may turn our perceptions of how life on Earth began upside down.
The find was made by a group of scientists headed by Andrew Sweetman, a specialist in sea-floor ecology at the Scottish Association for Marine Science in Oban, UK. His team was investigating the impact of mining polymetallic nodules, lump-shaped deposits the size of plums that have lain on the seafloor for millions of years, on the deep-sea ecosystem. Such nodules, rich in metals including cobalt, nickel, and lithium, could potentially produce oxygen because they can catalyze a process called the splitting of water molecules.
“We have another source of oxygen on the planet, other than photosynthesis,” Sweetman said. How that is produced, no one knows. The research was published in Nature Geoscience and could have important implications for the understanding of the emergence of life and the deep-sea mining that might affect it.
The initial discovery dates back to 2013 when Sweetman and colleagues recorded unusual oxygen levels during fieldwork in the CCZ. Using a module that sinks to the seafloor and carries out automated experiments in confined seawater chambers, they observed an increase in oxygen concentrations over time, something contrary to expectations. This continued into follow-up trips in 2021 and 2022, at which point Sweetman realized he had been missing a potentially groundbreaking process for years.
A fascinating observation that is frustrating in the number of questions it raises, says Donald Canfield, a biogeochemist from the University of Southern Denmark. Sweetman and his team hypothesized that the polymetallic nodules act as some form of catalyst to split water and form molecular oxygen. They wired up voltage differences across the surface of these nodules, measuring voltages up to 0.95 volts, close to the 1.5 volts needed to split a water molecule.
According to Franz Geiger, a chemist at Northwestern University, understanding the process could have practical applications, such as in the development of improved catalysts. Biogeochemist Eva Stüeken from the University of St Andrews suggested that the findings may also impact the hunt for life elsewhere in the universe. It might have a bearing on interpreting signs of oxygen with more caution.
Sweetman said mapping areas where this oxygen production is happening should happen before any deep-sea mining takes place. “If there’s oxygen being produced in large amounts, it’s possibly going to be important for the animals that are living there,” he said.
In the process, this “dark oxygen” discovery does much to turn conventional thinking upside down about how Earth gets its oxygen and opens new questions about the origins of life. In regard to the start of aerobic life on the planet, there has to be oxygen, and the understanding has been that Earth’s oxygen supply began with photosynthetic organisms,” said Sweetman. “But we now know that there is oxygen produced deep in the ocean, where there is no light. I think we therefore need to open up questions like: where could aerobic life have begun?”
Hence, this finding underlines the need for further investigation and serious consideration of the environmental impacts of deep-sea mining in light of such polymetallic nodules that might be important in sustaining deep-sea ecosystems.