The dark ocean floor of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is littered with what looks like chunks of charcoal. These modest metal deposits, called polymetallic nodules, contain metals such as manganese and cobalt, which are used to make batteries, and are a target for deep-sea mining companies.
Now, researchers have discovered that this precious nodule has a surprising function. That is, they produce oxygen and that is done even without sunlight. “This is a completely new and unexpected discovery,” says Lisa Levin, professor emeritus of biological oceanography at the Scripps Institution of Oceanography, who was not involved in the study.
The idea that some of Earth’s oxygen gas may come from nonliving minerals in total darkness, rather than from photosynthetic organisms, suggests that “where the oxygen is “It goes very strongly against what we traditionally think about how things are made.” Marlowe is a co-author of the new study published in Nature Geoscience.
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The story of the discovery dates back to 2013. At the time, deep-sea ecologist Andrew Sweetman was faced with a frustrating problem. His team was trying to measure how much oxygen was consumed by life on the CCZ ocean floor. The researchers sent a lander more than 13,000 feet to the ocean floor, created a sealed chamber on the ocean floor, and tracked how oxygen levels in the water dropped over time.
However, oxygen levels did not drop. In fact, it has increased significantly. Sweetman thought the sensor was broken and sent the device back to the manufacturer. Mr Sweetman, who researches ocean floor ecology and biogeochemistry at the Scottish Marine Science Society, said “this has happened four or five times” in five years. “I literally told my students, ‘Throw your sensors in the trash.’ They just don’t work.”
He then returned to the CCZ in 2021 on a research expedition organized by deep-sea mining company Metals Company. Again, his team used a lander to create a sealed chamber on the ocean floor and monitor oxygen levels. This time they used a different technique to measure oxygen, but they observed the same strange result: oxygen levels increased dramatically. “All of a sudden I realized that I had ignored this very important process, and I just blamed myself,” Sweetman says.
Researchers initially thought that deep-sea microorganisms were producing the oxygen. While the idea may have once seemed far-fetched, scientists recently discovered that some microorganisms can produce “dark oxygen” even in the absence of sunlight. In a laboratory experiment that replicated ocean floor conditions, Sweetman and his colleagues contaminated seawater with mercury chloride to kill microorganisms. Yet oxygen levels still increased.
Scientists reasoned that if this dark oxygen did not result from a biological process, it must have resulted from a geological process. They suggested several possibilities, including that the radioactivity in the nodule might be breaking down seawater molecules to produce oxygen, or that something might be pulling oxygen out of the manganese oxide in the nodule. The hypotheses were tested, but ultimately they were rejected. Then, one day in 2022, Sweetman was watching a video about deep-sea mining when he heard about a blob called a “battery in the rock.” This bit of marketing was just a metaphor, but it led him to wonder if this nodule was somehow acting as a natural geobattery. If they are electrically charged, they can split seawater into hydrogen and oxygen through a process called seawater electrolysis. (A similar effect occurs if the battery is dropped into salt water.)
“Surprisingly, there was almost a volt (of charge) on the surface of these nodules,” Sweetman said. For comparison, an AA battery charges about 1.5 volts. Nodules can become electrically charged as they grow because different metals are deposited randomly over millions of years, creating charge gradients between each layer. Seawater electrolysis is currently the researchers’ leading theory for dark oxygen production, and they plan to test it further.
However, it is unclear whether (or how much) these clumps naturally produce oxygen on the ocean floor. In most experiments, oxygen production stopped after two days. This could indicate that the lander triggered the production of oxygen by disrupting something in the environment. But it’s also possible that a “bottle effect” in the sealed chamber eventually stopped the reaction, Marlowe said. “The products accumulate, the reactants run out, and then the reaction stops. But in an open system…it can be a more consistent process,” he explains.
Bo Barker Jorgensen, a marine biogeochemist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, is skeptical that these nodules would produce oxygen if left on the ocean floor. (Jorgensen was not involved in the study but was one of the reviewers for the Nature Geoscience paper.) Still, the nodules appear to be producing oxygen through electrolysis, he says. added. “That in itself is a very interesting observation. To my knowledge, it has never been observed before.”
Researchers do not yet know whether this oxygen is important for life on the CCZ sea floor. The nodules and surrounding sediments are habitat for deep-sea life, from tiny microorganisms to larger “megafauna” such as fish and starfish. Half of the ecosystem’s megafauna are found only on tubercles. This is “a poorly understood ecosystem,” Levin said. “We haven’t even discovered, let alone studied, most species in the deep sea.”
The proposed deep-sea mining project would extract nodules from a belt of the CCZ ocean floor. The International Seabed Authority (ISA) is still drafting rules and regulations for the mining of nodules and other deep-sea targets. At least 32 member states of the ISA have called for a moratorium, precautionary moratorium, or ban on deep-sea mining.
These discoveries are “another thing you need to consider when deciding, ‘Do I go deep and mine or not?'” Sweetman said. “For me, that decision needs to be based on sound scientific advice and opinion.”
