Scientists discover ‘dark’ oxygen being produced more than 13,000 feet below the ocean’s surface

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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 at the bottom of the sea, 4,000 meters (about 13,100 feet) below the surface, where no light can penetrate. The same thing happened on three subsequent trips to a region known as the Clarion-Clipperton Zone.

“Basically I told my students, just put the sensors back in the box. We will send them back to the manufacturer and test them because they are just giving us nonsense information,” said Sweetman, professor at the Scottish Marine Science Association and leader of the institution’s deep-sea ecology and biogeochemistry group. “And every time the manufacturer would come back: ‘They’re working. They are calibrated.’”

Photosynthetic organisms such as plants, plankton and algae use sunlight to produce oxygen that circulates in the deep ocean, but previous studies of 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, finding oxygen produced without photosynthesis.

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

The study, published Monday in the journal Nature Geoscience, demonstrates how much is still unknown about the depths of the oceans and highlights what is at stake in the effort to explore the ocean floor in search of 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 first made the unexpected observation that “dark” oxygen was being produced on the seafloor while assessing marine biodiversity in an area designated for mining potato-sized polymetallic nodules. The nodules form over millions of years through chemical processes that cause metals to precipitate out of the water around shell fragments, squid beaks and shark teeth and cover a surprisingly large area of ​​the sea floor.

Metals such as cobalt, nickel, copper, lithium and manganese contained in the nodules are highly sought after 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 caused by mining equipment that harms midwater ecosystems as well as the deep-sea organisms that often live in the nodules.

It is 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 an ocean lander that sinks to the seafloor to insert 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 oxygen levels slowly dropping 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 backup method to detect oxygen and produced the same result, that he accepted that oxygen was being produced under the sea and needed to understand what was happening.

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

Sweetman observed the phenomenon repeatedly over nearly a decade and in several locations in the Clarion-Clipperton zone, a large area that stretches more than 4,000 miles and is outside the jurisdiction of any country.

The team took some samples of sediment, seawater and polymetallic nodules to study in the laboratory to try to understand exactly how oxygen was being produced.

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Understanding Dark Oxygen

Through a series of experiments, the researchers ruled out biological processes, such as microbes, and identified the nodules themselves as the source of the phenomenon. Perhaps, they reasoned, it was the oxygen released from the manganese oxide in the nodule. But such a 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 breakthrough. “There was someone saying, ‘That’s a battery in a rock,’” he recalled. “Watching this, I suddenly thought, could it be electrochemical? Could these things they want to extract to make batteries be batteries themselves?

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

Sweetman approached Franz Geiger, an electrochemist at Northwestern University in Evanston, Illinois, and together they investigated further. Using a device called a multimeter to measure small voltages and voltage variations, they recorded readings of 0.95 volts on 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 nodules are grouped together.

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

Challenging the paradigm

The discovery that abyssal, or deep-sea, nodules are producing oxygen is “a surprising and unexpected finding,” said Daniel Jones, professor and head of ocean biogeosciences at the National Oceanography Center in Southampton, England, who worked with Sweetman, but was not directly involved in the research. “Discoveries like this demonstrate the value of maritime 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, 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 investigation.

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

“(A)s with any new discovery, however, there may be alternative explanations,” he said in an email.

“The regional importance of such (dark oxygen production) cannot really be assessed with the limited nature of this study, but it suggests a potential unappreciated ecosystem function of deep-sea manganese nodules,” said Smith, who also wasn’t involved with the study.

Uncovering the origins of life

O US Geological Survey estimates that there are 21.1 billion dry tons of polymetallic nodules in the Clarion-Clipperton zone – containing more critical metals than the world’s terrestrial reserves combined.

The International Seabed Authority, within the scope of the United Nations Convention on the Law of the Sea, regulates mining in the region and issued exploration contracts. The group will meet this month in Jamaica to consider new rules that would allow companies to extract metals from the bottom of the ocean.

However, many countriesincluding the UK It is France, expressed caution, supporting a moratorium or ban on deep-sea mining to safeguard 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.

Smith, of the University of Hawaii, said he favors a pause in 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 wake-up call, 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 unmined regions, however, marine life flourished. It is not yet known why such ‘dead zones’ persist for decades,” he added. “However, this puts a big asterisk on deep-sea mining strategies, since the diversity of deep-ocean fauna in nodule-rich areas is greater than in more diverse tropical forests.”

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

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

Understanding how the ocean floor 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 ventsand the discovery that seawater electrolysis could form oxygen at depth could inspire new ways of thinking about how life began on Earth.

“I think there is more science that needs to be done, especially around this process and its importance,” Sweetman said. “I hope this is the start of something amazing.”

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