Dark oxygen in deep sea battery raises questions on mining

Scientists have discovered a source of “dark oxygen” on the abyssal seafloor, a place where no sunlight can penetrate.

The oxygen, which appears to be made from metals forming a natural battery, might play a role in the ecosystem of the deep ocean – or even the origins of life on Earth.

The researchers say that the discovery has serious implications for deep-sea mining, since removing these metals might interfere with ocean ecosystems.

According to the international team’s study, which is published in Nature Geoscience, it’s not yet clear how widespread this deep-sea oxygen production is.

“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,” says lead author Professor Andrew Sweetman, a researcher at the Scottish Association for Marine Science, UK.

“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 [oxygen] 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.”

Sweetman and colleagues were assessing the seabed of the Clarion-Clipperton Zone, in the middle of the north Pacific Ocean, to see what effects potential deep-sea mining might have in the area.

This zone has drawn a lot of interest from deep-sea miners, like the Metals Company which funded this study, because of its mineral resources.

The researchers placed chambers on the sea floor, about 4.2 kilometres deep, at locations across more than 4,000km of the zone.

They found that oxygen concentration increased steadily over two-day periods in these chambers.

They then went looking for the mechanism for this oxygen production, since there was no sunlight for photosynthesis.

Lab experiments showed them that “polymetallic nodules”, chunks of resource-rich rock which cover the sea floor in this area, could be the source.

These nodules can provide an electric charge when two different metals are placed near each other.

A small amount of charge – 1.5V, or an AA battery – is enough to make seawater undergo electrolysis, a reaction where water (H₂O) is split into hydrogen (H₂) and oxygen (O₂).

The researchers found that individual nodules had a potential of 0.95V, and multiple nodules together could easily provide the zap needed to make oxygen out of seawater.

“It appears that we discovered a natural ‘geobattery’,” says co-author Professor Franz Geiger, a chemist at Northwestern University, US.

“These geobatteries are the basis for a possible explanation of the ocean’s dark oxygen production.”

The researchers say that this oxygen source might not only play a role in the ocean’s current ecosystems, but the origins of organisms which metabolise oxygen.

“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,” says Sweetman.

“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?”

It also raises questions about the viability of deep-sea mining in these areas.

“The polymetallic nodules that produce this oxygen contain metals such as cobalt, nickel, copper, lithium and manganese – which are all critical elements used in batteries,” says Geiger.

“Several large-scale mining companies now aim to extract these precious elements from the seafloor at depths of 10,000 to 20,000 feet [3,000–6,000 metres] below the surface.

“We need to rethink how to mine these materials, so that we do not deplete the oxygen source for deep-sea life.”

Geiger says that nodule-rich areas of the ocean floor support more biodiversity than tropical rainforests.

“In 2016 and 2017, marine biologists visited sites that were mined in the 1980s and found not even bacteria had recovered in mined areas,” says Geiger.

“In unmined regions, however, marine life flourished. Why such ‘dead zones’ persist for decades is still unknown. However, this puts a major asterisk onto strategies for sea-floor mining.”