Even though the scientific paper was published during the summer holiday, it has garnered significant attention from various media outlets and the scientific community.
Evidence of dark oxygen production at the abyssal seafloor was published in Nature Geoscience by an international team of researchers led by Dr. Andrew Sweetman at The Scottish Association for Marine Science. Their research is based on several cruises in the Pacific Ocean’s Clarion-Clipperton Zone (CCZ).
In their research communiqué, the team proposed that polymetallic nodules on the seafloor may create currents strong enough to produce oxygen by splitting water molecules through a process known as electrolysis. The term “dark oxygen” refers to the production of oxygen in an environment where photosynthesis is not possible.
The Metals Company (TMC), a prominent explorer in the CCZ, has criticized the paper, citing possible flaws. The response was hardly unexpected as TMC has vested interests in the mineral resources in the area and aims to commence extraction within a few years.
See also: Nodules may indeed be battery rocks
However, now another exploration company with no association with nodules or the CCZ has entered the debate. A team at Norway-based Adepth Minerals has examined the paper and published a critical review.
Several critical omissions and methodological flaws
Adepth maintains that their analysis, written by Alden Denny (Chief Geoscientist), Werner Svellingen (CTO), and Lars-Kristian Trellevik (CSOO) and published by Medium, is driven purely by scientific curiosity and environmental concerns.
The authors present a list of issues regarding data collection, validation, and interpretation, questioning the very premise of the paper and believe substantial re-examination is warranted.
A few of the issues noted by the Adepth team pertain to oxygen and hydrogen production. The Sweetman et al. paper has shown that oxygen production occurs initially following the lander’s arrival, followed by oxygen consumption. This is not consistent with a significant and steady-state process. The Adepth paper also notes several other issues regarding validation for the lander system.
Additionally, as the hypothesis suggests that the nodules may enable electrolysis of seawater (H20 -> H2 + O2), the Adepth authors remark that any data or discussion on hydrogen observations is lacking and would be critical for validating the proposed process.
The critical review also questions whether the observed nodule voltages are sufficient to enable electrolysis. The highest reported value (0.95 V) is inadequate, and also seen as an outlier not supported by the data graph, where the highest value is 0.24 V. “This discrepancy suggests an error or selective data reporting, both of which are problematic.”
In their conclusion, Denny, Svellingen, and Trellevik firmly state that the paper is fraught with methodological flaws, data omissions, and misinterpretations. They suggest a slight, yet crucial change of the title, replacing “Evidence …” with “Possible Observations …”.
The authors further suggest that to uphold scientific integrity, it would be beneficial for the feedback provided during the peer review process to be published alongside the article. “This transparency would enhance understanding and trust in the scientific community.”
As a side note, the Adepth team comments that the maximum oxygen production proposed in the paper represents magnitudes less O2 production than a similar area of forest. Assuming the process described in the paper is real and occurs over geologic time, it would still be a negligible factor in global oxygen production. This fact undermines the paper’s implied significance of the findings.
A lot to prove
Given the harsh criticism the Sweetman et al. paper has received so far, including the methodology, they have a lot to prove. It might even be necessary for them to complete another cruise, gathering more data and in a manner that satisfies the critics. In the meantime, the authors should respond to the other issues regarding misinterpretation and data omissions.
It may take a long time before any sort of consensus is reached on the topic, but it is vital. As the Adepth team concludes: “Rigorous validation, comprehensive data reporting, and accurate interpretation are imperative for advancing our understanding of seafloor nodule environments and their broader implications.”
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.
Adepth Minerals aims to secure licenses for mineral exploration in the Norwegian Economic Zone. The company focuses on sulfide deposits located on and around the mid-oceanic ridge. The Norwegian authorities plan to award the first licenses in the first half of 2025.
