Scientists discover “black oxygen” being produced at the bottom of the ocean

A mysterious phenomenon first observed in 2013 aboard a ship in a remote part of the Pacific Ocean seemed so absurd that it convinced ocean scientist Andrew Sweetman that his monitoring equipment was faulty.

Sensor readings appeared to show that oxygen was being produced deep in the sea 4,000 meters below the surface, where no light can penetrate. The same thing happened on three subsequent trips to a region known as Clarion-Clipperton Zone .

“I basically said to my students, just put the sensors back in the box. Let’s send them back to the manufacturer and test them because they’re just giving us crap,” said Sweetman, a professor at the Scottish Association for Marine Science and leader of the institution’s deep-sea ecology and biogeochemistry group. “And every time the manufacturer would say, ‘They’re working. They’re calibrated.’”

Photosynthetic organisms, such as plants, plankton and algae, use sunlight to produce oxygen that travels to the depths of the ocean, but previous studies in the deep sea have shown that oxygen is only consumed, not produced, by the organisms that live there, Sweetman said.

Now, his team’s research is challenging this long-held assumption, discovering oxygen produced without photosynthesis .

“You become wary when you see something that goes against what should be happening,” he said.

The study, published on Monday (22) in Nature Geoscience magazinedemonstrates how much is still unknown about the ocean’s depths and highlights the stakes of exploring the seafloor for rare metals and minerals. The discovery that there is another source of oxygen on the planet besides photosynthesis also has far-reaching implications that could help unravel the origins of life.

Seabed sampling

Sweetman made the unexpected observation that “black” oxygen was being produced on the seafloor while assessing marine biodiversity at an area targeted for mining potato-sized polymetallic nodules. The nodules form over millions of years through chemical processes that cause metals to precipitate from the water around shell fragments, squid beaks and shark teeth, covering a surprisingly large area of ​​the seafloor.

Metals such as cobalt, nickel, copper, lithium and manganese contained in the nodules are in high demand for use in solar panels, electric car batteries and other green technologies. However, critics say deep-sea mining could irrevocably damage the pristine underwater environment, with noise and sediment plumes kicked up by mining equipment harming mid-water ecosystems as well as deep-sea organisms that often live in the nodules.

It’s also possible, these scientists warn, that deep-sea mining could disrupt the way carbon is stored in the ocean, contributing to the climate crisis.

For that 2013 experiment, Sweetman and his colleagues used a deep-sea lander that sinks to the seafloor to sink a chamber, smaller than a shoebox, into the sediment to enclose a small area of ​​the seafloor and the volume of water above it.

What he hoped the sensor would detect was the oxygen level slowly falling over time as the microscopic animals breathed it in. From this data, he planned to calculate something called “sediment community oxygen consumption,” which provides important information about the activity of deep-sea fauna and microorganisms.

It wasn’t until 2021, when Sweetman used another alternative method to detect oxygen and got the same result, that he accepted that oxygen was being produced deep in the sea and that he needed to understand what was happening.

“I thought, ‘My God, for the last eight or nine years, I’ve been ignoring something profound and huge,’” he said.

Sweetman observed the phenomenon repeatedly over nearly a decade and at multiple locations in the Clarion-Clipperton Zone, a vast area stretching more than 4,000 miles and beyond the jurisdiction of any one country.

The team took some of the sediment, seawater and polymetallic nodule samples back to the lab to try to understand exactly how the oxygen was being produced.

Understanding Black Oxygen

Through a series of experiments, the researchers ruled out biological processes, such as microbes, and focused on the nodules themselves as the source of the phenomenon. Perhaps, they reasoned, it was oxygen being released from manganese oxide in the nodule. But such release was not the cause, Sweetman said.

A documentary about deep-sea mining that Sweetman watched in a hotel bar in São Paulo, Brazil, sparked a discovery. “There was someone saying, ‘That’s a battery in a rock,’” he recalled. “Watching that, I suddenly thought, could it be electrochemical? Could these things they want to mine to make batteries be batteries themselves?”

Electric current, even from a single AA battery, when placed in saltwater, can split the water into oxygen and hydrogen — a process known as seawater electrolysis, Sweetman said. Perhaps the nodule was doing something similar, he reasoned.

Sweetman contacted Franz Geiger, an electrochemist at Northwestern University in Evanston, Illinois, and together they investigated further. Using a device called a multimeter to measure tiny voltages and variations in voltages, they recorded readings of 0.95 volts from the surface of the nodules.

These readings were lower than the 1.5 voltage required for seawater electrolysis, but suggested that significant voltages could occur when the nodules are clustered together.

“We appear to have discovered a natural ‘geobattery,’” Geiger, the Charles E. and Emma H. ​​Morrison Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences, said in a press release. “These geobatteries are the basis for a possible explanation of dark oxygen production in the oceans.”

Challenging the paradigm

The discovery that abyssal, or deep-sea, nodules are producing oxygen is “an incredible and unexpected discovery,” said Daniel Jones, a professor and head of ocean biogeosciences at the National Oceanography Centre in Southampton, England, who has worked with Sweetman but was not directly involved in the research. “Discoveries like this demonstrate the value of marine expeditions to these remote but important areas of the world’s oceans,” he said in an email.

The study definitely challenges “the traditional paradigm of the deep-sea oxygen cycle,” according to Beth Orcutt, a senior research scientist at the Bigelow Laboratory for Ocean Sciences in Maine. But the team provided “enough supporting data to justify the observation as a true signal,” said Orcutt, who was not involved in the research.

Craig Smith, professor emeritus of oceanography at the University of Hawaii at Mānoa, called the geobattery hypothesis a reasonable explanation for the production of dark oxygen.

“As with any new discovery, however, there may be alternative explanations,” he said in an email.

“The regional importance of such (black oxygen production) cannot really be assessed with the limited nature of this study, but it suggests a potential unappreciated function of manganese nodules on the deep seafloor,” said Smith, who was also not involved in the study.

Unraveling the origins of life

The U.S. Geological Survey estimates that there are 21.1 billion dry tons of polymetallic nodules in the Clarion-Clipperton Zone — containing more critical metals than all the world’s terrestrial reserves combined.

The International Seabed Authority, under the United Nations Convention on the Law of the Sea, regulates mining in the region and has issued exploration contracts. The group is meeting in Jamaica this month to consider new rules that would allow companies to extract metals from the ocean floor.

However, several countries, including the United Kingdom and France, have expressed caution, supporting a moratorium or ban on deep-sea mining to protect marine ecosystems and conserve biodiversity. Earlier this month, Hawaii banned deep-sea mining in its state waters.

Sweetman and Geiger said the mining industry should consider the implications of this new discovery before potentially exploring deep-sea nodules.

Craig Smith of the University of Hawaii said he was in favor of pausing nodule mining, considering the impact it would have on a vulnerable, biodiverse and pristine environment.

Early attempts at mining efforts in the zone in the 1980s provided a cautionary tale, Geiger said.

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

“In the unmined regions, however, marine life flourished. Why these ‘dead zones’ persist for decades is still unknown,” he added. “However, this puts a big asterisk on deep-sea mining strategies, as the faunal diversity of the ocean floor in nodule-rich areas is higher than in the most diverse tropical forests.”

Sweetman, whose scientific research has been funded and supported by two companies interested in mining the Clarion-Clipperton Zone, said it is crucial to have scientific oversight over deep-sea mining.

Many unanswered questions remain about how black oxygen is produced and what role it plays in the deep-sea ecosystem.

Understanding how the deep ocean produces oxygen could also shed light on the origins of life, Sweetman added. A long-standing theory is that life evolved in deep-sea hydrothermal vents, and the discovery that electrolysis of seawater could form oxygen at depth could inspire new ways of thinking about how life began on Earth.

“I think there’s more science to be done, especially around this process and why it’s important,” Sweetman said. “I hope this is the start of something amazing.”

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