In 2013, ocean scientist Andrew Sweetman observed a puzzling phenomenon while on a vessel in a remote part of the Pacific Ocean. His equipment showed that oxygen was being produced on the seabed, 4,000 meters below the surface, where no light can penetrate. Initially, Sweetman thought his sensors were faulty. He and his team encountered the same readings on three subsequent voyages to the Clarion-Clipperton Zone. Despite sending the sensors back for testing, they were always confirmed to be working correctly.
Photosynthetic organisms such as plants and plankton produce oxygen using sunlight, which then cycles into the ocean depths. Previously, it was believed that deep-sea organisms only consumed oxygen, not produced it. Sweetman's findings challenged this long-held assumption. His research, published in the journal Nature Geoscience, shows oxygen being produced without photosynthesis. This discovery underscores how much is still unknown about the ocean depths and highlights the potential impact of deep-sea mining on these ecosystems.
Sweetman first noticed the "dark" oxygen production while assessing marine biodiversity in an area designated for mining polymetallic nodules. These nodules, which contain metals like cobalt, nickel, copper, lithium, and manganese, form over millions of years and are in high demand for green technologies. Critics warn that deep-sea mining could damage the underwater environment and disrupt carbon storage in the ocean, exacerbating the climate crisis.
During the 2013 experiment, Sweetman used a deep-ocean lander to enclose a small area of the seafloor and measure oxygen levels. He expected to see oxygen levels fall as microscopic animals consumed it. Instead, he found oxygen being produced. In 2021, using a backup method, he confirmed the same result. Sweetman realized he had been overlooking a significant discovery for nearly a decade.
The team took samples back to the lab to understand how oxygen was being produced. They ruled out biological processes and focused on the polymetallic nodules themselves. Sweetman hypothesized that the nodules might act like batteries, generating an electric current that splits seawater into oxygen and hydrogen. Collaborating with electrochemist Franz Geiger, they measured voltages from the nodules and found readings of 0.95 volts, suggesting a natural "geobattery" might be at work.
This discovery challenges the traditional understanding of oxygen cycling in the deep sea. Daniel Jones, an ocean biogeosciences professor, called the finding "amazing and unexpected." The research team provided sufficient data to justify their observations, but alternative explanations could exist. The study suggests manganese nodules might have an unappreciated role in deep-sea ecosystems.
