Critical Review of the Article: “Evidence of Dark Oxygen Production at the Abyssal Seafloor” by Sweetman et al. in Nat. Geosci. 1–3 (2024)
Authors ordered alphabetically as first co-authors:
Alden Denny* (Chief Geoscientist), Werner Svellingen* (CTO), Lars-Kristian Trellevik* (CSOO)
*Adepth Minerals, Bergen, Norway
Contact: *Corresponding author email: ad@adepth.no
Statement: This is a Non peer-reviewed document.
DOI:
1 Adepth Minerals, Edvard Griegs Vei 1, 5059 Bergen, Norway
Arising from:
Sweetman, A.K., Smith, A.J., de Jonge, D.S.W. et al. Evidence of dark oxygen production at the abyssal seafloor. Nat. Geosci. (2024). https://doi.org/10.1038/s41561-024-01480-8
Abstract
This review examines the findings and methodologies presented in Sweetman et al (2024)1 (hereafter referred to as ‘the paper’). The paper presents findings contrasting those of all previous comparable work1–4 and has stirred international debate pertaining to deep-sea minerals. We identify significant issues in data collection, validation, and interpretation including unvalidated data collection methods, the omission of crucial observations relevant for electrolysis processes, and unsupported voltage measurements which undermine the study’s conclusions. These issues, coupled with unfounded hypotheses about early Earth oxygenation, call into question the authors’ interpretation of the observations and warrant re-examining the validity of this work. Our analysis is driven by scientific curiosity of mineral related processes, as our company has no vested interest in nodule production.
Introduction
The paper describes the observation of abiotic oxygen production allegedly via electrolysis induced by nodules located within the Clarion-Clipperton Zone. The authors then tie this observation both to implications for potential mining activities and understanding of early Earth oxygen production. However, our review identifies several critical omissions and methodological flaws that call into question the study’s reliability.
Methodological Review
Oxygen Production and Hydrogen Observations
· The oxygen optode observations appear to be robust and supported by Winkler titration measurements. However, the oxygen production trends fit best to a model comprised of an initial oxygen production (24 hours or less) followed by an oxygen consumption typical of deep-sea sediment processes4. The papers’ steady state oxygen production values (given as 1.7–18 mmol O2 m−2 d−1)1 are directly contradicted by this data trend.
· The dissociation of water by electrolysis5 by the reaction of
produces hydrogen in addition to oxygen, which differentiates oxygen production via electrolysis from other means (such as reduction of nodule oxides). Therefore we suggest that a critical validation of the proposed electrochemical process would be the measurement of hydrogen in the benthic chamber, which has been standard practice on similar benthic landers for over 20 years6. Hydrogen measurements of sufficient sensitivity and accuracy is well within the range of off-the-shelf sensors7 commercially deployed on equivalent benthic lander systems8. The omission of such data is a significant procedural shortcoming that raises concerns regarding the thoroughness of this research.
Electrolysis Observations
· The corrected (used in the paper in Figure 2 and reproduced here in Figure 1) voltage measurements do not support seawater electrolysis as the highest given voltage value is 0.24V which eliminates electrolysis as an observed mechanism for oxygen production. While the authors of the paper admit the voltage is insufficient for electrolysis, their insistence on dark oxygen production from electrolysis is not in line with their evidence.
· Excluding anomalous data from the figures but discussing it multiple times in the text without context misleads readers and misrepresents findings.
· The reported nodule-surface voltage value of 0.95V used repeatedly in the text and found in ‘Extended Data Table 4’ of the paper is both insufficient to initiate seawater electrolysis and not found in the corrected data used in the paper Figure 2 (see Table 1 below for comparison). This difference in values seen in the uncorrected “Extended Data Table 4” vs the background corrected “Source Data Fig. 2” suggests that all the high voltage numbers seen in “Extended Data Table 4” were in fact artefacts and not actual observations.
Lack of Validation for the Lander System
· The paper fails to validate the lander device used for data collection, either via presentation of baseline deployments of the system, or with bench-top experiments run on the benthic chamber system. Validation is crucial for ensuring the accuracy and reliability of the measurements.
· Potential issues such as stirring pumps introducing external energy inputs (e.g., induced current, voltage leaking) are not addressed and are a potential source of electrolysis via magnetohydrodynamics effect on the electrolytic process9.
· The lander in question is designed in a way that increases the risk of energy transfer from the lander to the benthic chambers; particularly as stirring motors are mounted directly through sample chamber via a penetrator, which differs from other landers with an isolated motor design (Figure 2)10.
· The absence of data from other sensors, with only O2 sensor readings and samples discussed, raises concerns about the comprehensiveness of the observations.
· Background deployments of the lander in non-nodule areas are not mentioned. However “extended data Fig 3” of the paper uses previously published lander deployments4 in non-nodule areas showing apparent oxygen production. The lack of discussion of this and potentially other ‘background’ stations would be necessary to prove or disprove any connection between oxygen observations and the presence of nodules.
Data Interpretation and Conclusions
· The final statement regarding early Earth oxygen production is fundamentally flawed. The formation of oxides (seafloor nodules) in anoxic water is chemically implausible, indicating either a misunderstanding or intentional misrepresentation of the geological and chemical context12–14. Such a conclusion requires either significant revision or retraction due to its inaccuracy.
Conclusion
The papers’ observation of oxygen production in seafloor nodule environments is of significant scientific interest but is marred by critical methodological flaws, data omissions, and misinterpretations. This work reports unique observations bereft of a plausible formation mechanism. The lack of lander validation, inconsistent nodule voltage data, omission of crucial hydrogen observations, and erroneous conclusions about early Earth oxygen production necessitate a thorough revision or potential retraction of the paper.
The level of care necessary to justify extraordinary claims with such broad implications is absent from the paper. Rigorous validation, comprehensive data reporting, and accurate interpretation are imperative for advancing our understanding of seafloor nodule environments and their broader implications.
Recommendations
The title of the article is misleading, suggesting broader and more definitive findings than the data supports. Accurate representation of the study’s scope and limitations is essential for scientific transparency and credibility. To argue that this study presents “evidence”, considering the questionable data, methodology and analysis presented, is an overreach. We recommend the title of the article be more accurately renamed to “Observations of Possible Dark Oxygen Production at the Abyssal Seafloor” to reflect the tentative nature of the findings.
To uphold scientific integrity, it would be beneficial for the feedback provided during the peer review process to be published alongside the paper. This transparency would enhance understanding and trust in the scientific community.
Given the outlined issues we suggest that this article should be considered for retraction and possible re-submission after addressing the issues raised in this review.
The authors declare the following competing interests
All the authors are employed by Adepth Minerals AS. Adepth Minerals holds no licenses or intent to apply for mineral licenses in regions addressed by this paper. The authors are not associated with any research initiatives associated with deep-sea nodules.
Bibliography
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