Nodules were originally termed “a battery in a rock” by The Metals Company (TMC) to convey to the public the need for deep sea minerals for the green shift. Polymetallic nodules contain mainly four essential battery metals: copper, nickel, cobalt, and manganese.
What they – or anyone else – didn’t consider, was that the potato-sized metallic lumps found in vast swathes in certain parts of the deep sea may act as batteries in their natural state.
In a research communiqué published in Nature Geoscience, an international team of researchers propose that polymetallic nodules produce oxygen by splitting water molecules through a process known as electrolysis.
Lead researcher professor Andrew Sweetman from the Scottish Association for Marine Science tells BBC that he first observed an enormous amount of oxygen being produced on the seafloor in complete darkness in 2013, challenging the idea that oxygen is only produced through photosynthesis.
During several cruises in the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean in 2021 and 2022, Sweetman and his team gathered data using benthic chambers on the seafloor. The researchers created an enclosed environment by pushing the chambers into the sediments on the seafloor.
The team found that O2 consistently accumulated in the chambers, indicating an excess of oxygen production. Several lines of inquiry were pursued to explain the net “dark O2 production” (DOP). They eventually realized that DOP was only observed when nodules were present in the experiments, suggesting that the phenomenon was linked to their presence.
According to the research note, an input voltage of a bit more than 1 V is required to split seawater into hydrogen and oxygen molecules at the CCZ seafloor. This is comparable to the voltage in a household AA battery. Furthermore, lab experiments have shown that nodules may reach surface voltages high enough to facilitate electrolysis.
Although the discovery opens the possibility that deep-sea nodules may represent a significant source of oxygen production, Sweetman tells BBC that he does not believe his study will put an end to the plans for nodule harvesting. Still, he argues that the hypothesis needs to be explored in greater detail and that the information and data we gather going forward need to be used if we are going to do mining in the most environmentally friendly way possible in the deep ocean.
“Serious concerns” regarding validity
It remains to be seen whether the dark oxygen hypothesis stands up to scrutiny. The Metals Company (TMC), one of the most prominent explorers in the CCZ, was however quick to release a statement on the Nature Geoscience communiqué.
TMC says that they were “surprised to see such a flawed paper” published and that the methodology and findings raise serious concerns about the validity of both their data and their conclusions.
For one, TMC argues that the data were not collected under conditions representative of the environment in the area. They further point out that although the paper was accepted in Nature, it had previously been rejected by several other journals. TMC also notes that Nature has taken a strong view against deep-sea mineral sourcing.
The Metals Company also mentions the existence of contradictory studies that show net oxygen consumption, not net production, at the seafloor, using a state-of-the-art methodology. Also, one paper using the Sweetman et al. methodology, measured only O2 consumption in the Korean license area of the CCZ.
Ending the statement, TMC informs that their team is preparing a comprehensive rebuttal.
The CCZ is located in international waters between Mexico and Hawaii. It is considered the largest nodule field in the world. The area is managed by the International Seabed Authority (ISA), and a handful of states are sponsoring exploration activities while awaiting the ISA to complete and adopt necessary regulations that may enable future mining. ISA has communicated that this may happen in 2025.
