Rock batteries: How oxygen is produced in the deep sea without photosynthesis

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A few days ago, news broke that researchers had detected an unexpected form of oxygen production in the Clarion-Clipperton Zone (CCZ), an area around 4,000 meters deep in the Pacific Ocean. In their study published in Nature Geoscience, they call this form "Dark Oxygen" because it is produced in an environment where photosynthesis cannot take place due to complete darkness.

The international research team, led by Andrew K. Sweetman from the Scottish Association for Marine Science (SAMS), carried out experiments in the CCZ on the deep sea floor, measuring the oxygen content on the sea floor. Surprisingly, the scientists found that the oxygen content rose within just under two days. This discovery challenges previous assumptions about the processes in the deep sea and could have far-reaching consequences for understanding the origins of aerobic life in marine ecosystems.

"For aerobic life to emerge on Earth, there had to be oxygen, and our understanding is that the Earth's oxygen supply began with photosynthesizing organisms," says Professor Dr. Sweetman in a statement from the GEOMAR Helmholtz Centre in Kiel. "But we now know that oxygen is produced in the deep sea, where there is no light. So I think we need to revisit questions like the origins of aerobic life."

"For me, this is really something completely unexpected," Matthias Haeckel, expert for processes on the deep sea floor at GEOMAR, commented on the results to SWR. "Normally, we have oxygen consumption in the deep sea by microorganisms that convert organic matter through the respiration of higher organisms. And this contradicts everything that is written in the textbooks."

The Clarion-Clipperton Zone (CCZ)

The CCZ is an approximately 7000 kilometer long fracture zone in the oceanic crust in the Central Pacific. It extends between Hawaii and Mexico at a depth of around 4000 to 5000 meters. The zone is of particular economic interest, as large deposits of manganese nodules can be found there. These contain valuable raw materials such as nickel, cobalt and manganese, which are needed for modern technologies. Several countries, including Germany, hold exploration licenses for this area. The CCZ is at the center of the debate about deep-sea mining and its potential ecological impact.

Water electrolysis thanks to electrochemical potentials

One possible explanation for the oxygen production could be the abundance of manganese nodules in the CCZ. These metal-rich nodules could generate electrical voltages that lead to the splitting of water into hydrogen and oxygen - a process known as "water electrolysis". The nodules studied exhibited high electrochemical potentials of up to 0.95 volts on their surface and effectively act as rock batteries. However, to carry out electrolysis using seawater, a voltage of approximately 1.5 volts – is required, corresponding to a conventional AA battery. More precisely, according to the researchers, the reaction requires an input voltage of 1.23 volts and an overvoltage of around 0.37 volts to split seawater into _H2 and _O2 at an average pH value of 7.41 in the deep sea. The researchers assume that this value could be reduced by several hundred millivolts if the reaction took place via a mechanism mediated by lattice oxygen.

The complex structures of the nodules consist of several layers of different metal oxides, which leads to potential differences. Metals such as manganese, nickel and copper contribute to the development of electrochemical voltages, with their concentrations and distributions playing an important role.

Mechanism still needs to be researched

The physical structure of the nodules, including their porosity and surface properties, therefore optimizes the adsorption of reactants and increases conductivity, the researchers write in their study. In particular, the manganese oxides in the nodules, enriched with transition metals such as nickel, act as effective catalysts for electrochemical reactions.

Larger tubers often contain more nickel- and copper-rich layers, which can increase their electrochemical activity. These nodules tend to have higher voltages, presumably due to a larger number of active sites.

The measured voltages could also result from an internal redistribution of electrons between different metal ions. In addition, the interface between the nodules and the seawater plays an important role in electrochemical reactions and voltages.

The exact mechanism of the surface tensions is not yet fully understood. The complex structure and composition of manganese nodules leads to a variety of possible electrochemical processes. Further research is needed to quantify the specific contributions of each factor and to fully decipher the mechanism. These findings could expand our understanding of deep-sea ecology and have implications for future technologies and resource utilization.

Previous measurements dismissed as measurement errors

Interestingly, the discovery of "dark oxygen" is not entirely new. Similar observations were made several years ago, but were classified as measurement errors. Sweetman himself says: "I just ignored it because I was taught that you only get oxygen through photosynthesis". To ensure that this is not the case, Sweetman worked with GEOMAR. Co-author of the current study and PhD student at GEOMAR, Tobias Hahn, explains in the institute's press release: "Through different setups, we were able to try to find different explanations and finally came to the conclusion that it is indeed a phenomenon that cannot be explained by a systematic error."

"Through our exchange, we have ensured a data quality that goes beyond the typical measurement uncertainties and includes the individual drift and pressure behavior of the sensors," Hahn adds. He is amazed at the results and at the same time relieved to be able to reconfirm the quality and validity of the oxygen sensor data obtained during the first field campaigns.

CCZ in the focus of deep-sea mining

The investigation of the CCZ is of particular importance as this area is the focus of planned deep-sea mining. The manganese nodules found there contain valuable raw materials such as nickel, cobalt and manganese, which are required for the production of wind turbines, solar panels and batteries for electric vehicles. The newly discovered oxygen production now raises additional questions about the potential impact of deep-sea mining on the marine ecosystem.

Haeckel emphasizes the need for further research to SWR: "The point now is simply to really understand whether this process is important. And if it is relevant, what role it actually plays down there in the deep-sea ecosystem. If it is important, then it must be considered through regulations that the International Seabed Authority is now prescribing for deep-sea mining and these systems must then be protected accordingly."

Prof. Dr. Sweetman agrees: "This discovery has raised many unanswered questions. I think we need to think a lot about how we mine these nodules that are actually batteries in the rock."

The discovery of oxygen production in the deep sea illustrates how little is still known about this important habitat. It also shows how important it is to carefully study the effects of human activities such as deep-sea mining before irreversible damage is done to sensitive ecosystems.

(vza)