Nodules in the deep sea may be a source of ‘dark oxygen’

Strewn across the Pacific Ocean’s seafloor, potato-sized rocks called nodules are a treasure in the deep sea. Not only are they rich in critical minerals needed for clean energy technologies, but the rocks also often serve as the only anchoring point for many delicate deep-sea creatures. Now a new study suggests the nodules are able to produce oxygen about 4,000 m below the ocean’s surface (Nat. Geosci. 2024, DOI: 10.1038/s41561-024-01480-8).

Almost a decade ago, while sampling the seabed in the Clarion-Clipperton Zone—a region in the northeast Pacific Ocean known for its vast manganese nodule beds—Andrew Sweetman, a marine ecologist with the Scottish Association for Marine Science, noted unexpected oxygen readings. With no light at such dark depths, photosynthesis couldn’t be an explanation for what seemed like ongoing oxygen production. Assuming faulty equipment, “we would come home and recalibrate the sensors, but, over the course of 10 years, these strange oxygen readings kept showing up,” Sweetman said in a statement.

In 2021 and 2022, members of his team set up chambers enclosing small areas on the seafloor in which they measured oxygen levels. Sweetman expected to see a drop in dissolved oxygen inside these chambers as microbes would continuously consume the essential gas. Instead, the researchers observed rising oxygen levels. They suspected that nodules may have been playing a part, in that they may have been functioning like batteries and generating electricity that splits seawater and produces oxygen (and hydrogen). The team called it “dark oxygen.”

To figure out if that were true, Sweetman sent two shoeboxes full of nodules to Franz Geiger, an electrochemist at Northwestern University. Geiger dunked them in beakers containing a seawater-mimicking solution, placed electrodes at various locations on the nodule, and measured electrical activity. “We ended up getting 50 millivolts here, 6 millivolts there, 80 millivolts elsewhere,” he says. The highest reading his team obtained was 950 mV, which is lower than the typical 1,500 mV required for splitting seawater.

But when nodules are clustered together and in physical contact, they could act like geobatteries, Geiger suspects. “So if you’ve got 20 millivolts on one nodule and then maybe 50 on the other, then that adds up,” he says.

While such geobatteries exist in nature, “getting enough voltage to be able to split water is very challenging,” says Shelley Minteer, a bioelectrochemist at Missouri University of Science and Technology who wasn’t involved in the study. She suggests conducting a thorough electrochemical evaluation of not just the nodules but also the seabed“to figure out what potentials are being seen in the sediment.”