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Potato-sized metallic nodules littering parts of the deep seabed appear to be producing oxygen
 

Roughly half the oxygen we breathe comes from the ocean, and conventional wisdom has it that this oxygen is a result of marine plants photosynthesizing. But now scientists have discovered that oxygen is also produced by potato-sized metallic lumps on the ocean’s abyssal plains—flat regions of the seafloor at depths of between 10,000 and 20,000 feet (3,000 to 6,000 meters), where sunlight does not reach. These so-called polymetallic nodules produce this oxygen—dubbed “dark oxygen”—through electrolysis, where seawater (H₂O) is split into oxygen and hydrogen in the presence of an electric charge. According to the research, just published in the journal Nature Geoscience, this charge may come from the difference in electric potential that exists between metal ions within the nodules.

The scientific team that conducted the work, led by Professor Andrew Sweetman, leader of the seafloor ecology and biogeochemistry research group at the Scottish Association for Marine Science (SAMS), had originally set out to study the impacts of mining polymetallic nodules on the seafloor ecosystem in the Clarion-Clipperton Zone (CCZ), an abyssal plain spanning 1.7 million square miles (4.5 million square kilometers) between Hawaii and Mexico. Measuring the changes in oxygen concentrations in various locations, the team expected oxygen levels to decline the deeper they went, but their data showed substantial emissions from the seabed.

“When we first got this data [in 2013], we thought the sensors were faulty, because every study ever done in the deep sea has only seen oxygen being consumed rather than produced. We would come home and recalibrate the sensors but over the course of 10 years, these strange oxygen readings kept showing up,” said Prof. Sweetman in a statement. Subsequent measurements using other methods confirmed the results, and Prof. Sweetman finally realized that for years he had been ignoring a potentially huge discovery. “For aerobic life to begin on the planet, there has to be oxygen and our understanding has been that Earth’s oxygen supply began with photosynthetic organisms,” he said. “But we now know that there is oxygen produced in the deep sea, where there is no light. I think we therefore need to revisit questions like: Where could aerobic life have begun?”

The nodules formed over millions of years as fragments of shell or other debris collected the dissolved metals in seawater. Oxides of iron and manganese make up most of these metals, but the nodules also contain cobalt, nickel and lithium, as well as rare earth elements such as cerium—essential components of batteries and electronics. These precious minerals are the reason why numerous mining companies are turning to the deep sea, and why environmental organizations and bodies such as the US National Oceanographic and Atmospheric Administration (NOAA) are raising alarm bells about the devastating impact deep-sea mining could have on seafloor ecosystems. A petition highlighting the environmental risks and calling for a pause on mining activity has already been signed by more than 800 marine scientists from 44 countries.

One of the scientists who signed the petition, Prof Murray Roberts, a marine biologist from the Univerisity of Edinburgh, told BBC News: “There’s already overwhelming evidence that strip mining deep-sea nodule fields will destroy ecosystems we barely understand. Because these fields cover such huge areas of our planet it would be crazy to press ahead with deep-sea mining knowing they may be a significant source of oxygen production.”