Discovery of Deep Sea ‘Dark Oxygen’ Bewilders Scientists

Four kilometres below the Pacific Ocean, scientists have made the shock discovery that ‘dark oxygen’ is being produced by polymetallic nodules in the deep sea – without sunlight! These new findings defy our known understanding of how oxygen is created and cast yet another shadow over the practice of deep sea mining.

Polymetallic nodules – unassuming rock-like mineral deposits on the sea floor – contain metals like cobalt, nickel, copper and manganese, making them a target for companies looking to mine the deep sea. However, scientists, governments, and businesses alike are calling for a moratorium (or official pause) on the practice – which is so far unregulated – as the scale of its impact is still unknown.

So why is ‘dark oxygen’ such an exciting discovery?

Well, until now, it was thought that only living things like plants and algae were capable of producing oxygen via photosynthesis – which requires sunlight. The surprise finding could even require rethinking how life first began on Earth, researchers behind the study suggested.

“I first saw this in 2013 - an enormous amount of oxygen being produced at the seafloor in complete darkness,” explained lead researcher Prof. Andrew Sweetman from the Scottish Association for Marine Science.

They’d sent landers (or autonomous research platforms) to the seabed to track how oxygen levels in the water fell over time – except they didn’t fall. Instead, the levels rose significantly. He assumed these measurements were due to faulty sensors, and had them recalibrated by the manufacturers. This happened multiple times over five years until, in 2021, he went back to the Clarion-Clipperton Zone (CCZ) on an environmental survey expedition sponsored by deep sea mining firm The Metals Company (TMC).

Using a different technique, Sweetman and his team found the same bizarre results – oxygen levels increased dramatically.

“Suddenly I realised that … I’d been ignoring this hugely significant process, and I just kicked myself,” Sweetman said.

They tested multiple hypotheses to discover the cause, but Sweetman’s “aha” moment came when he heard TMC refer to polymetallic nodules as a “battery in a rock.” The metaphor was meant to convey that the nodules contain metals that could be used in batteries, but Sweetman wondered if they could be electrically charged themselves.

And they are. The nodules give off almost as much electricity as an AA battery (around 1 volt and 1.5 volts respectively).

*A polymetallic nodule found on the sea floor, which researchers think could be involved in the oxygen production. Image credit: Camille Bridgewater*

Although they aren't certain how the oxygen is created, the team’s leading theory is that the nodules’ electric current is splitting seawater into oxygen and hydrogen, a process called seawater electrolysis.

The researchers have no idea what role this nodule-produced oxygen may play in the seabed ecosystems of the CCZ, but an estimated 30% to 40% of species present in the area live on these nodules, such as tiny corals, sponges and worms. And some larger animals depend on the nodules to incubate their young, including an adorable ghostly white octopus nicknamed Casper that lays its eggs on the stalks of dead sponges attached to the rocks. This discovery suggests that the nodules themselves - targeted by deep sea mining companies - could be providing oxygen to support life there.

And while the deep sea was once thought to be relatively devoid of life, we now know that it is home to a dazzling array of life. Tens of thousands of species have been discovered so far, with potentially millions more as yet undescribed and unnamed.

The ‘dark oxygen’ study was released as the International Seabed Authority (ISA) holds meetings of both its Council (15-26 July 2024) and Assembly (29 July - 2 August 2024) to continue discussions around allowing Deep Sea Mining to begin. Mining projects proposed across the CCZ would extract nodules from swaths of the seafloor.

Why are experts calling for a moratorium on Deep Sea Mining?

Scientists have warned that deep seabed mining will cause biodiversity loss, both by destroying seabed life where mining would take place (with little prospect of recovery) and by generating sediment plumes, light, toxins and noise that could impact both benthic and mesopelagic marine life far beyond actual mining sites.

Prof. Murray Roberts, a marine biologist from the University of Edinburgh is one of the scientists who signed the call for a moratorium. “There’s already overwhelming evidence that strip mining deep-sea nodule fields will destroy ecosystems we barely understand,” he told BBC News.

“Millions of species, from tiny microbes to anemones with eight-foot long tentacles, call polyemtallic nodules home. Deep-sea mining, on the scale and using the methods planned, will irreversibly impact these ecosystems, potentially affecting the ocean's functioning, including its ability to provide food and regulate the global climate. This is not something we should be rushing into,” Marine Biologist and National Geographic Explorer Dr Diva Amon said.

*Deep-sea biologist Diva Amon looks out from a submersible vehicle during an expedition to explore the ocean’s depths. Image credit: Novus Select bioGraphic*

What about claims that deep sea mining is necessary for the renewable energy transition?

Leading organisations suggest we don’t need to mine our planet’s final frontier if we prioritise the “5 Rs”:

Reduce the amount of critical minerals and products needed
Reuse and create second-life applications for products
Recycle metals at end of life, employ ‘urban mining,’ and recover materials from industrial/mining waste.
Reimagine products to last longer & use fewer critical minerals, limiting disposable electronics.
Repair things when they break, ensure a “right to repair” with products designed for repairability.

Take this graphic from Environment Virginia, comparing projected annual volumes of critical minerals from deep sea mining with the amount disposed of as e-waste in one year, as an example:

The Energy Transitions Commission found in 2023 that circular economy strategies could fully close projected supply gaps for copper and nickel, and significantly narrow them for lithium, cobalt and neodymium by 2030. Meanwhile, advances in electric vehicle (EV) battery technology are leading to the replacement of EV batteries dependent on cobalt, nickel, and manganese.

So where does THIS DISCOVERY leave us?

A sea cucumber known commonly as a "gummy squirrel," imaged in the Clarion-Clipperton Zone (CCZ).
*Image credit: ROV Isis, SMARTEX Project, Natural Environment Research Council, U.K., smartexccz.org*

The discovery of oxygen being produced in the deep sea, Sweetman says, is “just another thing that we now need to take into account when it comes to deciding, ‘Do we go and mine the deep ocean, or don’t we?’ To me, that decision needs to be based on sound scientific advice and input.”

Deep sea mining remains under-researched and under-regulated - a wild west for those seeking to exploit the planet’s final frontier - and growing momentum for a moratorium shows that the global community lacks confidence in the International Seabed Authority (ISA) as an effective regulator and custodian of the deep seabed and its resources, which is recognised as the “common heritage of mankind.”

Pew Charitable Trusts published a study examining the status of draft mining regulations from the ISA and identified more than 30 major outstanding issues, such as how environmental impacts will be monitored and assessed. This analysis—together with the major gaps in scientific information on deep-sea ecosystems—shows that the ISA is a long way from being able to ensure that mining activities will not cause significant and irreversible harm to marine life.