Scientists investigate ‘dark oxygen’ in deep-sea mining zone

Scientists will lower instruments to the seafloor to figure out how metallic nodules are generating oxygen in the depths of the Pacific Ocean, an unexpected phenomenon that has fuelled controversy over deep-sea mining.

Researchers, to their surprise, found in 2024 that the potato-sized nodules in the darkness of the Pacific and Indian oceans, including the Pacific’s Clarion-Clipperton Zone, were a source of oxygen, even though it was thought that sunlight and photosynthesis were needed to produce this element on a large scale.

This so-called dark oxygen could be supporting life in the darkness at depths of thousands of metres, including microbes, sea cucumbers and carnivorous anemones. Its discovery raised questions about proposals by deep-sea mining companies to vacuum up the nodules from the seafloor and smelt them for cobalt, nickel and manganese. The finding has been disputed by deep-sea mining companies, and other scientists have also called for more evidence.

Now the researchers who discovered dark oxygen are going back to the Clarion-Clipperton Zone, the most promising area for deep-sea mining, to try to confirm the existence of this dark oxygen and understand how it is being produced.

“Where’s the oxygen coming from for these diverse animal communities to thrive?” Andrew Sweetman at the Scottish Association for Marine Science, who is leading the expedition, said at a press briefing. “This may be a pretty significant process, and that’s what we’re trying to figure out.”

The team has hypothesised that the layers of metals in the nodules are generating an electric current that can break down seawater into hydrogen and oxygen. They have measured up to 0.95 volts of electricity on the surface of nodules, slightly less than an AA battery.

That’s less than the 1.23 volts that would typically be required for this electrolysis, but the researchers think some individual nodules or several nodules grouped together could generate higher voltages.

The team will deploy landers – essentially metal frames with instruments inside – to depths of up to 10,000 metres to measure oxygen fluxes as well as pH, since electrolysis would cast out protons that increase the acidity of the water.

The landers will also obtain sediment cores and nodules to analyse later in the lab, since microscopic organisms may also play a role. Each nodule contains up to 100 million microbes, and the researchers will try to identify microbes through DNA and RNA sequencing and fluorescence microscopy.

“The vast diversity of the microbes remains a moving target. We’re always discovering new species,” said team member Jeff Marlow at Boston University. “Are they active? Are they shaping their environment in interesting and important ways?”

They will also recreate the conditions of the deep sea in a high-pressure reactor and run the electrolysis reaction in it, since electrolysis hasn’t typically been seen at the intense pressures found on the seafloor.

“Four hundred atmospheres of pressure, that is the pressure at which the Titan submersible imploded,” said Franz Geiger at Northwestern University in Illinois, another team member. “We’re interested to see how water splitting may or may not be effective at high pressure.”

The ultimate goal is to try to run the electrochemical reaction under an electron microscope with microbes and bacteria present, all without killing the microscopic organisms, he added.

While the United Nations’ International Seabed Authority hasn’t made a decision on whether deep-sea mining should be allowed in international waters, US President Donald Trump has pushed for extraction to start. The Canadian firm The Metals Company has applied to the US government for a permit to begin deep-sea mining.

A paper published by scientists from The Metals Company argued that Sweetman and his colleagues did not find enough energy to power seawater electrolysis in 2024 and that the oxygen they observed was likely carried from the surface by the landers they deployed.

Sweetman says any bubbles of surface air are washed out as the landers descend, and they haven’t measured oxygen when deployed in other areas of the ocean, such as the seabed in the Arctic, at 4000 metres. Of 65 experiments done in the Clarion-Clipperton Zone, 10 per cent have found oxygen consumption and the rest have found oxygen production, according to Sweetman.

He and his colleagues have also found that the water oxidation part of the electrolysis process can take place at the lower voltage found on the nodules. A rebuttal including this data has been submitted to Nature Geosciences and is currently going through peer review.

“In terms of the commercial interest, there’s definitely an interest to try to silence this area of work,” Sweetman said in response to The Metals Company’s objections to his findings.

“Regardless of the source and motivation of the comments, they need to be addressed,” Marlow said. “That’s what we’re in [the] process of doing.”