(CNN) – 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 on the seabed 4,000 meters below the surface, where light cannot penetrate. The same thing happened on three subsequent trips to a region known as the Clarion-Clipperton Zone.
“I basically told my students to just put the sensors back in the box. We will send them back to the manufacturer and have them analyzed because they are just telling us nonsense,” said Sweetman, a professor at the Scottish Marine Science Association and leader of the institution’s biogeochemistry and seabed ecology group. “And each time the manufacturer responded: ‘They work.'” They are calibrated.’”
Photosynthetic organisms such as plants, plankton and algae use sunlight to produce oxygen that circulates into the deep ocean, but previous studies conducted in the deep sea have shown that they only consume oxygen, not produce it, organisms that live there, Sweetman said.
Now, his team’s research challenges this long-held assumption, finding oxygen produced without photosynthesis.
“You have to be 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 deep ocean and underscores what is at stake in the push to mine the ocean floor for rare metals and minerals. Their finding 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.
Sweetman first made the unexpected observation that “dark” oxygen was produced on the seafloor while assessing biodiversity in an area earmarked for the extraction of 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 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, as noise and uplifted sediment columns by mining equipment deteriorate midwater ecosystems, as well as organisms on the seabed that often live in nodules.
It is also possible, these scientists warn, that deep-sea mining could alter the way stores carbon in the ocean, contributing to the climate crisis.
For that 2013 experiment, Sweetman and his colleagues used a deep-ocean lander that sinks to the seafloor to insert a camera, smaller than a shoebox, into the sediment to enclose a small area of bottom. marine and a volume of water above it.
What he hoped the sensor would detect was that oxygen levels would slowly drop over time as the microscopic animals inhaled it. From that data, he planned to calculate something called “sediment community oxygen consumption,” which provides important information about the activity of seafloor 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 on the seafloor and needed to monitor 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 again and again for almost a decade and in several places in the Clarion-Clipperton area, a large area that extends more than 6,400 kilometers and is beyond the jurisdiction of any country.
The team took some of the samples of sediment, seawater and polymetallic nodules to study in the laboratory to try to understand exactly how oxygen was produced.
Through a series of experiments, the researchers ruled out biological processes such as microbes and pointed to the nodules themselves as the origin of the phenomenon. Perhaps, they reasoned, this was oxygen released from the manganese oxide in the nodule. But such release is 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 there saying, ‘That’s a battery on a rock,’” he recalled. “Seeing this, I suddenly thought: could it be electrochemical? These things that they want to extract to make batteries, could they really be batteries?”
Electrical current, even from an AA battery, when put in salt water, can split the water into oxygen and hydrogen, a process known as seawater electrolysis, Sweetman explained. Maybe the nodule did 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 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 nodules clump together.
“It appears we discovered a natural ‘geobattery,’” Geiger, the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences, said in a news release. “These geobatteries are the basis for a possible explanation for the production of dark oxygen in the ocean.”
Challenging the paradigm
The discovery that abyssal or deep-sea nodules produce 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 has worked with Sweetman but not participated directly in the research. “Findings like this demonstrate the value of maritime expeditions to these remote but important areas of the world’s oceans,” he said by email.
The study definitely challenges “the traditional paradigm of oxygen cycling in the deep sea,” 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 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.
“However, as with any new discovery, there may be alternative explanations,” he said by email.
“The regional significance of such (dark oxygen production) cannot really be assessed due to the limited nature of this study, but it does suggest a possible unappreciated ecosystem function of manganese nodules on the deep sea floor,” said Smith, who also did not participate in the study.
Unraveling the origins of life
He United States Geological Survey It is estimated that there are 21.1 billion dry tons of polymetallic nodules in the Clarion-Clipperton zone, containing more key 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 will meet in Jamaica this month to consider new rules allowing companies to extract metals from the ocean floor.
However, several countriesincluding the United Kingdom and France, have expressed caution and supported 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 exploiting deep-sea nodules.
Smith, of the University of Hawaii, said he favored a pause in nodule harvesting, considering the impact it would have on a vulnerable, biodiverse and pristine environment.
Early mining attempts in the area in the 1980s provided a warning, Geiger said.
“In 2016 and 2017, marine biologists visited sites that were mined in the 1980s and found that not even bacteria had been recovered in the mined areas,” Geiger said.
“However, in unmined regions marine life flourished. “It is still unknown why these “dead zones” persist for decades,” he added. “However, this puts a major asterisk on strategies for seafloor mining, as the diversity of ocean floor fauna in nodule-rich areas is greater than in more diverse rainforests.”
Sweetman, whose scientific research has been funded and supported by two companies interested in mining the Clarion-Clipperton area, said it was crucial to have scientific oversight of 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 can also shed light on the origins of life, Sweetman added. A long-standing theory is that life evolved in vents deep sea hydrothermaland the discovery that electrolysis of seawater could form oxygen at depth could inspire new ways of thinking about how life on Earth began.
“I think more science needs to be done, especially around this process and its importance,” Sweetman said. “I hope it’s the start of something amazing.”
