Shock discovery reveals deep sea nodules are a source of oxygen

Metallic nodules scattered across the seabed in the Indian and Pacific oceans are a source of oxygen for nearby marine life – a finding that could upend our understanding of the deep ocean.

Abyssal plains in some regions are scattered with potato-sized nodules packed with valuable cobalt, manganese and nickel, a target for deep-sea mining activity.

Andrew Sweetman at the Scottish Association for Marine Science in Oban, UK, first noticed something strange about these ocean areas back in 2013, while conducting research in the Clarion-Clipperton Zone, a nodule-rich part of the Pacific Ocean.

Sweetman and his colleagues were sending machines down to the ocean floor to seal off a 22 centimetre square patch of seabed and measure its oxygen flux. Instead of oxygen content decreasing in the monitored sections, the data suggested it was increasing.

But without any noticeable plant life, that didn’t make sense, says Sweetman. “I’ve been taught from a very young age that oxygenated ecosystems are only possible through photosynthesis,” he says. His conclusion was that the machinery he was using was faulty. “I literally ignored the data,” he says.

Then, in 2021, Sweetman was on another research cruise in the Pacific and machines returned the same finding – increasing oxygen levels on the seabed. Using a different measurement approach yielded the same result.

“We were seeing the same oxygen production in these two different datasets,” says Sweetman. “And suddenly I realised that for the last eight or nine years I had been ignoring this hugely groundbreaking process.”

He and his colleagues deduced that the metallic nodules must be playing a role in raising the oxygen levels in the deep sea. Lab testing, which involved poisoning the sediment and nodules, ruled out the presence of oxygen-producing microbes.

Instead, Sweetman says the materials in the nodules are acting as a “geobattery”, generating an electric current that splits seawater into hydrogen and oxygen. “These nodules are being mined because there’s everything there that you need to make an electric car battery,” he says. “What if they are acting as natural geobatteries, by themselves?”

Each nodule can produce up to 1 volt of electric potential, the team discovered by probing the rocks. If the rocks were clustered together to join forces, there would be enough voltage to split seawater into hydrogen and oxygen via electrolysis, explaining the elevated oxygen levels.

“Potentially we have discovered a new natural source of oxygen,” says Sweetman. “How pervasive that is in time and space, I don’t know. But it’s something that’s very, very interesting.”

There are many outstanding questions. For example, the source of energy generating the electric current remains a mystery. It is also unclear whether the reaction happens continuously, under what conditions and the contribution this “dark” oxygen plays in sustaining surrounding ecosystems. “We don’t have all of the information yet, but we know it’s happening,” says Sweetman.

In deep-sea environments devoid of sunlight and plants, some life forms get their energy from chemicals that erupt from the sea floor at hydrothermal vents. Some scientists think life on Earth first appeared at these vents, but these early organisms would have needed a source of oxygen to make food from inorganic compounds. The findings raise the possibility that nodules could have been the source of that oxygen to help life get started, says Sweetman.

That interpretation might be a stretch, says Donald Canfield at the University of Southern Denmark. Oxygen is needed to produce the manganese oxide contained in the nodules, he points out. “Oxygenic photosynthesis is a prerequisite for their production,” he says. “For this reason, oxygen production by the nodules does not represent an alternative type of oxygen production to be equated with oxygenic photosynthesis. It is very unlikely that they have played a role in the oxygenation of the planet.”

However, Ruth Blake at Yale University says the idea of oxygen production in the deep ocean is still “exciting” and warrants more research into the phenomenon and its potential impact on deep-ocean ecosystems.

Sweetman’s study was part-funded by The Metals Company (TMC), a deep-sea mining company looking to harvest metallic nodules in the Clarion Clipperton Zone. In response to the study, Patrick Downes at TMC said he has “serious reservations” about the findings, adding that its own analysis suggests Sweetman’s results are due to oxygen contamination from external sources. “We will be writing a rebuttal article,” Downes said in a statement to New Scientist.

However, the findings are likely to strengthen calls for a ban on deep-sea mining, a position backed by many oceanographers who say our understanding of such areas is still developing. Paul Dando at the UK’s Marine Biological Association says the paper reinforces the view of deep-sea scientists “that no mining should take place until we understand the ecology of these nodule fields”.

Sweetman says his findings aren’t necessarily “the nail in the coffin” for deep-sea mining, but it might restrict extraction to locations where oxygen production is low. More research is also needed to investigate the impact of sediment stirred up by the mining process on the oxygen production, he says.

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