Sensor readings seemed to show that oxygen was being made on the seabed 4,000 meters (about 13,100 feet) below the surface, where no light can penetrate. The same thing happened on three subsequent voyages to a region known as the Clarion-Clipperton Zone.
"I basically told my students, just put the sensors back in the box. We'll ship them back to the manufacturer and get them tested because they're just giving us gibberish," said Sweetman, a professor at the Scottish Association for Marine Science and lead of the institution's seafloor ecology and biogeochemistry group. "And every single time the manufacturer came back: 'They're working. They're calibrated.'"
Photosynthetic organisms such as plants, plankton and algae use sunlight to produce oxygen that cycles into the ocean depths, but previous studies conducted 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, finding oxygen produced without photosynthesis.
"You're 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 ocean depths and underscores what's at stake in the push to exploit the ocean floor for rare metals and minerals. Its finding that there's another source of oxygen on the planet other than photosynthesis also has far-reaching implications that could help unravel the origins of life.
Sampling the seafloor
Sweetman first made the unexpected observation that "dark" oxygen was being produced on the seafloor while assessing marine biodiversity in an area that's earmarked for mining potato-size polymetallic nodules. The nodules form over the course of millions of years through chemical processes that cause metals to precipitate out of 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 technology. However, critics say deep-sea mining could irrevocably damage the pristine underwater environment, with noise and sediment plumes kicked up by mining equipment harming midwater ecosystems as well as organisms on the seabed that often live on 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-ocean lander that sinks to the seafloor to drive a chamber, smaller than a shoebox, into the sediment to enclose a small area of seafloor and volume of water above it.
What he expected the sensor to detect was oxygen levels falling slowly over time as microscopic animals breathed it in. From that data, he planned to calculate something called "sediment community oxygen consumption," which provides important information about the activity of seabed fauna and microorganisms.
It wasn't until 2021, when Sweetman used another, backup method to detect oxygen and it produced the same result, did he accept that oxygen was being produced on the seafloor and he needed to get a handle on what was going on.
"I thought, 'My God for the last eight or nine years, I've just been ignoring something profound and huge,'" he said.
Sweetman has observed the phenomenon time and time again over almost a decade and at several locations in the Clarion-Clipperton Zone, a large area that extends more than 4,000 miles (6,400 kilometers) and is beyond the jurisdiction of any one country.
The team took some of the samples of sediment, seawater and polymetallic nodules back to study in the lab to try to understand exactly how oxygen was being produced.
Understanding dark oxygen
Through a series of experiments, the researchers ruled out biological processes such as microbes and zoned in on the nodules themselves as the phenomenon's origin. Perhaps, they reasoned, it was oxygen being released from manganese oxide in the nodule. But such a release wasn't the cause, Sweetman said.
A documentary about deep-sea mining that Sweetman watched in a hotel bar in São Paulo, Brazil, unleashed a breakthrough. "There was someone on it saying, 'That's a battery in a rock,'" he recalled. "Watching this, I suddenly thought, could it be electrochemical? These things they want to mine to make batteries, could they actually be batteries themselves?"
Electric current, even from an AA battery, when placed into 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 approached 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 less than the voltage of 1.5 required for seawater electrolysis but suggested that significant voltages could occur when nodules are clustered together.
"It appears that we discovered a natural 'geobattery,'" said Geiger, the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern's Weinberg College of Arts and Sciences, in a news release. "These geobatteries are the basis for a possible explanation of the ocean's dark oxygen production."
Challenging the paradigm
The discovery that abyssal, or deep-sea, nodules are producing oxygen is "an amazing and unexpected finding," 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. "Findings like this demonstrate the value of seagoing expeditions to these remote but important areas of the world's oceans," he said via 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 "sufficient 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 Mnoa, called the geobattery hypothesis a reasonable explanation for the production of dark oxygen.
"(A)s with any new discovery, however, there may be alternative explanations," he said via email.
"The regional significance of such (dark oxygen production) cannot really be assessed with the limited nature of this study, but it does suggest a potential unappreciated ecosystem function of manganese nodules at the deep-sea floor," said Smith, who also wasn't involved with the study.
Unraveling the origins of life
The US Geological Survey estimates that 21.1 billion dry tons of polymetallic nodules exist in the Clarion-Clipperton Zone - containing more critical metals than the world's land-based reserves combined.
The International Seabed Authority, under the UN 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 to 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 safeguard marine ecosystems and conserve biodiversity. Earlier this month, Hawaii banned deep-sea mining in its state waters.
Sweetman and Geiger said that the mining industry should consider the implications of this new discovery before potentially exploiting the deep-sea nodules.
The University of Hawaii's Smith said he favored a pause on mining the nodules, 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 not even bacteria had recovered in mined areas," Geiger said.
"In unmined regions, however, marine life flourished. Why such 'dead zones' persist for decades is still unknown," he added. "However, this puts a major asterisk onto strategies for sea-floor mining as ocean-floor faunal diversity in nodule-rich areas is higher than in the most diverse tropical rainforests."
Sweetman, whose scientific research has been funded and supported by two companies interested in mining the Clarion-Clipperton Zone, said that 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 may also shed light on the origins of life, Sweetman added. One long-standing theory is that life evolved on deep-sea hydrothermal vents, and the discovery that seawater electrolysis could form oxygen in the deep could inspire fresh ways to think about how life began on Earth.
"I think that there's more science that needs to be done, especially around this process and the importance of it," Sweetman said. "I hope it's the start of something amazing."
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