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A mysterious phenomenon first observed in 2013 on a boat in a remote area of the Pacific Ocean seemed so strange that ocean scientist Andrew Sweetman was convinced that his monitoring equipment was at fault.
Sensor readings appear to indicate that oxygen is created at the bottom of the ocean 4,000 meters (about 13,100 feet) below the surface, where no light can penetrate. The same thing happened on the next three voyages to the area known as the Clarion-Clipperton Zone.
“I tell the students, just put the sensor in the box. We will send it back to the manufacturer and test it because it just gives us nonsense,” said Sweetman, a professor at the Scottish Marine Science Association and leader of the institution’s seafloor ecology and biogeochemistry group. “And every manufacturer comes back: ‘They work. They’re calibrated.'”
Photosynthetic organisms such as plants, plankton and algae use sunlight to produce oxygen that cycles into the deep ocean, but previous studies conducted in the deep ocean show 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 that oxygen is produced without photosynthesis.
“You watch out if you see something that goes against what should happen,” he said.
The study, published Monday in the journal Nature Geoscience, shows that much is still unknown about the depths of the ocean and underscores what is at stake in efforts to exploit the seabed for rare metals and minerals. The discovery that there are other sources of oxygen on the planet besides photosynthesis also has implications that could help unravel the origins of life.
Seabed sampling
Sweetman first made the unexpected observation that “dark” oxygen was being produced at the bottom of the ocean while assessing marine biodiversity in an area earmarked for mining potato-sized polymetallic nodules. The nodules were formed over millions of years through a chemical process that caused metal deposits to come out of the water around shell fragments, squid beaks and shark teeth and cover an impressive area of the sea floor.
Metals such as cobalt, nickel, copper, lithium and manganese found in these nodules are in high demand for use in solar panels, electric car batteries and other green technologies. However, critics say that deep-sea mining can irreversibly damage the underwater environment, with noise and sediment released by mining equipment destroying mid-water ecosystems as well as organisms on the seabed that often live in nodules.
It is possible, the scientists warn, that deep-sea mining could disrupt the way carbon is stored in the oceans, causing a climate crisis.
For that 2013 experiment, Sweetman and his colleagues used a deep-sea lander that sank to the sea floor to drive a chamber, smaller than a shoebox, into the sediment to cover a small area of the ocean and the volume of water above it.
What the sensor hopes to detect is the slow drop in oxygen levels as the microscopic animals inhale. From these data, they plan to calculate what they call “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 similar results, that he accepted that oxygen was being produced on the seabed and that he needed to know what was going on.
“I thought, ‘Oh my God for the last eight or nine years, I’ve just been ignoring something important and big,'” he said.
Sweetman has repeatedly observed the phenomenon for nearly a decade and at several locations in the Clarion-Clipperton Zone, a vast area that stretches more than 4,000 miles (6,400 kilometers) and extends beyond state jurisdiction.
The team took several samples of sediment, sea water and polymetallic nodules back to study in the Lab to try to understand exactly how oxygen is produced.
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Know dark oxygen
Through many experiments, the researchers ruled out biological processes such as microbes and zones in the nodule as the origin of the phenomenon. Perhaps, they reason, it is oxygen that is released from manganese oxide in the nodule. But the release wasn’t the cause, Sweetman said.
A documentary about deep-sea mining that Sweetman watched in a hotel bar in São Paulo, Brazil, sparked a breakthrough. “Someone said, ‘That’s a battery in a rock,'” he recalled. “Looking at this, I suddenly thought, could it be electrochemical? These things that are going to be mined to make batteries, could they be batteries themselves?”
An electric current, even from an AA battery, when placed in salt water, can split the water into oxygen and hydrogen — a process known as seawater electrolysis, Sweetman said. Perhaps, the node was doing the same thing, he thought.
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, he recorded a reading of 0.95 volts from the surface of the nodule.
These readings are less than the 1.5 voltage required for seawater electrolysis but suggest that significant voltages may occur when nodules are collected.
“It appears that we have 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 fundamental to the explanation of the production of dark oxygen in the ocean.”
Challenge the paradigm
The discovery that abyssal nodules, or deep seas, produce oxygen is a “surprising and unexpected discovery,” said Daniel Jones, professor and head of ocean biogeosciences at the National Oceanographic Center in Southampton, England, who has worked with Sweetman but was not directly involved in the research. “Finds like this show the value of marine expeditions to remote but important areas of the world’s oceans,” he said via email.
The research certainly challenges “the traditional paradigm of oxygen cycling in the deep ocean,” 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 valid signal,” said Orcutt, who was not involved in the research.
Craig Smith, professor emeritus of oceanography at the University of Hawaii at Mānoa, cites the geobattery hypothesis as a reasonable explanation for the production of dark oxygen.
“(A) with the new findings, however, there may be alternative explanations,” he said via email.
“The regional importance (production of dark oxygen) cannot be assessed with the limited nature of this study, but it shows the ecosystem function of manganese nodules that cannot be appreciated at the bottom of the sea,” said Smith, who also ‘ t participated in the study.
Listening to the origin 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 reserves combined.
The International Seabed Authority, under the UN Convention on the Law of the Sea, regulates mining in the area 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 sea.
However, some countries, including the UK and France, have been cautious, supporting moratoriums or bans on deep-sea mining to protect marine ecosystems and preserve biodiversity. Earlier this month, Hawaii banned deep-sea mining in state waters.
Sweetman and Geiger say the mining industry should consider the implications of this new discovery before potentially exploiting the sea nodule.
Smith of the University of Hawaii said he would like to take a break from mining these nodules, because of the impact on a vulnerable, biodiverse and pristine environment.
Early attempts at mining efforts in the zone in the 1980s provided the tale, Geiger said.
“In 2016 and 2017, marine biologists visited sites mined in the 1980s and found no recoverable bacteria in the mine area,” Geiger said.
“But in areas that are not mined, marine life thrives. Why such a ‘dead zone’ exists for decades is still unknown,” he said. “However, this puts a major asterisk in the strategy for mining on the sea floor because the diversity of fauna on the ocean floor 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 it is critical to have scientific oversight of deep-sea mining.
Many questions remain unanswered about how dark oxygen is produced and what role it plays in deep-sea ecosystems.
Understanding how the ocean floor produces oxygen could also shed light on the origins of life, Sweetman added. One long-standing theory is that life evolved in deep-sea hydrothermal vents, and the discovery that electrolysis of seawater can form deep oxygen could inspire a new way of thinking about how life on Earth works.
“I think there’s a lot more science to be done, especially about this process and its importance,” Sweetman said. “I hope this is an amazing start.”
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