Scientists Discover ‘Dark’ Oxygen Production 13,000 Feet Below Ocean Surface

In 2013, ocean scientist Andrew Sweetman encountered an enigmatic phenomenon while monitoring a vessel in a remote Pacific region that initially led him to believe his equipment was malfunctioning. The sensors indicated that oxygen was being generated on the seabed at a depth of 4,000 meters (about 13,100 feet), where no sunlight can reach. This unusual result was consistent across three subsequent research expeditions to the Clarion-Clipperton Zone.

Sweetman, a professor at the Scottish Association for Marine Science and head of its seafloor ecology and biogeochemistry group, initially doubted the validity of the readings. “I told my students to put the sensors away, thinking they were defective,” Sweetman recalled. “But every time the manufacturer confirmed that the equipment was functioning correctly.”

Traditionally, deep-sea studies have shown that oxygen is consumed by organisms living in these depths, rather than produced, as photosynthetic organisms such as plants and algae generate oxygen through sunlight in shallower waters. Sweetman’s research now challenges this conventional view by revealing oxygen production without photosynthesis.

The study, published in *Nature Geoscience*, highlights the limited understanding of ocean depths and raises concerns about the impact of deep-sea mining. The discovery of an alternative source of oxygen could also provide insights into the origins of life.

Sweetman’s unexpected observation occurred while assessing marine biodiversity in areas designated for mining polymetallic nodules. These nodules, which form over millions of years, are composed of metals like cobalt, nickel, copper, lithium, and manganese. They are crucial for technologies such as solar panels and electric car batteries. However, deep-sea mining poses risks to the pristine underwater environment, potentially disrupting ecosystems and carbon storage.

During the 2013 experiment, Sweetman’s team used a deep-ocean lander to push a small chamber into the sediment, aiming to measure how oxygen levels changed over time. The goal was to assess “sediment community oxygen consumption,” which provides insights into seabed fauna and microorganisms. It wasn’t until 2021, when a backup method confirmed the same results, that Sweetman accepted that oxygen production was indeed occurring on the seafloor.

Sweetman has since repeatedly observed this phenomenon across multiple locations in the Clarion-Clipperton Zone. The team analyzed sediment, seawater, and nodules in the lab to understand the oxygen production mechanism. They initially considered biological processes but later focused on the nodules themselves. The breakthrough came when Sweetman watched a documentary suggesting that the nodules might function as natural batteries, generating electricity in a manner similar to seawater electrolysis.

Collaborating with Franz Geiger, an electrochemist at Northwestern University, Sweetman discovered that the nodules produced a voltage of 0.95 volts, close to the 1.5 volts needed for seawater electrolysis. This finding suggested that the nodules act as natural “geobatteries,” potentially explaining the observed oxygen production.

Experts have praised the discovery, with Daniel Jones from the National Oceanography Centre noting its significance and the value of expeditions to remote ocean regions. Beth Orcutt from the Bigelow Laboratory for Ocean Sciences acknowledged that the study challenges traditional views on oxygen cycling in deep-sea environments, while Craig Smith from the University of Hawaii considered the geobattery hypothesis plausible but noted that further research is needed.

The Clarion-Clipperton Zone, estimated to contain 21.1 billion dry tons of polymetallic nodules, is subject to international regulation by the International Seabed Authority. While some countries advocate for a moratorium on deep-sea mining to protect marine ecosystems, others continue to explore the potential of these resources.

Sweetman and Geiger emphasize the need for careful consideration of the implications of their discovery before proceeding with deep-sea mining. The study also raises questions about the potential role of dark oxygen in the deep-sea ecosystem and its implications for understanding the origins of life on Earth.

“This discovery could be the beginning of something groundbreaking,” Sweetman said. “Further research is essential to fully grasp the significance of this process.”