Mystery oxygen source discovered on the sea floor — bewildering scientists

Something is pumping out large amounts of oxygen at the bottom of the Pacific Ocean, at depths where a total lack of sunlight makes photosynthesis impossible.

The phenomenon was discovered in a region strewn with ancient, plum-sized formations called polymetallic nodules, which could play a part in the oxygen production by catalysing the splitting of water molecules, researchers suspect. The findings are published in Nature Geoscience¹.

“We have another source of oxygen on the planet, other than photosynthesis,” says study co-author Andrew Sweetman, a sea-floor ecologist at the Scottish Association for Marine Science in Oban, UK — although the mechanism behind this oxygen production remains a mystery. The findings could also have implications for understanding how life began, he says, as well as for the possible impact of deep-sea mining in the region.

The observation is “fascinating”, says Donald Canfield, a biogeochemist at the University of Southern Denmark in Odense. “But I find it frustrating, because it raises a lot of questions and not very many answers.”

Sweetman and his collaborators first noticed something amiss during field work in 2013. The researchers were studying sea-floor ecosystems in the Clarion–Clipperton Zone, an area between Hawaii and Mexico that is larger than India and a potential target for the mining of metal-rich nodules. During such expeditions, the team releases a module that sinks to the sea floor to perform automated experiments. Once there, the module drives cylindrical chambers down to close off small sections of the sea floor — together with some seawater — and create “an enclosed microcosm of the seafloor”, the authors write. The lander then measures how the concentration of oxygen in the confined seawater changes over periods of up to several days.

Oxygen currents

Without any photosynthetic organisms releasing oxygen into the water, and with any other organisms consuming the gas, oxygen concentrations inside the chambers should slowly fall. Sweetman has seen that happen in studies he has conducted in areas of the Southern, Arctic and Indian oceans, and in the Atlantic. Around the world, sea-floor ecosystems owe their existence to oxygen carried by currents from the surface, and would quickly die if cut off. (Most of that oxygen originates in the North Atlantic and is carried to deep oceans around the world by a ‘global conveyor belt’.)

But in the Clarion–Clipperton Zone, the instruments showed that the sequestered water became richer, not poorer, in oxygen. At first, Sweetman attributed the readings to a sensor malfunction. But the phenomenon kept occurring during subsequent trips in 2021 and 2022, and was confirmed by measurements with an alternative technique. “I suddenly realized that for eight years I’d been ignoring this potentially amazing new process, 4,000 metres down on the ocean floor,” says Sweetman.

The amounts of oxygen produced are not small: the gas in the chambers reaches concentrations higher than those seen in algae-rich surface waters, Sweetman says. None of the other regions Sweetman has surveyed contained polymetallic nodules, suggesting that these rocks have an important role in the production of this ‘dark oxygen’.

As a first test of this hypothesis, the team recreated the conditions found on the sea floor in a laboratory on their ship. They monitored samples collected from the sea floor — which included polymetallic nodules — and saw that the oxygen concentration increased, at least for a while. “They start producing oxygen, up to a point. Then they stop,” says Sweetman — presumably because the energy that drives the splitting of water molecules gets depleted. This leaves the question of where that energy is coming from. If the nodules themselves were acting as batteries — producing energy from a chemical reaction — they would have become depleted long ago.

Electric potential

But the nodules could serve as catalysts, enabling the splitting of water and the formation of molecular oxygen. The researchers measured voltages across the surface of nodules, and found voltage differences of up to 0.95 volts. This is not quite the 1.5 volts needed to split a water molecule, but, in principle, higher voltages could be produced in the same way that battery voltages can be doubled by connecting two batteries in series, Sweetman says.

Co-author Franz Geiger, a chemist at Northwestern University in Evanston, Illinois, says that it is unclear yet whether the reaction also produces molecular hydrogen — which happens in industrial electrolyser reactions thanks to a catalyst — or releases protons in the water while shuffling the leftover electrons somewhere else. But understanding it could end up having useful applications, he says. “Perhaps there’s some blueprint there at the bottom of the ocean that could help us make better catalysts.”

Eva Stüeken, a biogeochemist at the University of St Andrews, UK, says that the results could also have implications for proposals to look for the signature of possible life in the light spectrum of extrasolar planets. “The presence of O₂ gas on other planets would perhaps have to be interpreted with additional caution,” she says.

Sweetman says that before deep-sea mining starts, researchers should map the areas where oxygen production is occurring. Otherwise, ecosystems that have become dependent on that oxygen could collapse if the nodules are removed. “If there’s oxygen being produced in large amounts, it’s possibly going to be important for the animals that are living there.”