Peer Review Report For: Anomalous oxygen concentration increases in benthic experiments from the Clarion Clipperton Zone are not related to polymetallic nodules [version 1; peer review: 1 approved, 1 not approved]

* Background Polymetallic nodules in the deep sea of the Clarion Clipperton zone (CCZ) have been implicated in the abiotic production of oxygen during benthic chamber respirometer experiments. The hypothesis, termed “dark oxygen”, was supported with a correlation between average nodule surface area and oxygen production rates, given as a Spearman’s correlation coefficient of ⍴ = 0.664, p = 0.031, n = 11, suggesting that larger polymetallic nodules cause more oxygen production. However, this correlation and the dark oxygen hypothesis appears to be incompatible with the observation that experiments in the western CCZ and a control experiment recorded increasing oxygen with no nodules present. Methods To investigate the dark oxygen hypothesis, we repeated the surface area analysis using image segmentation, and expanded the dataset with three additional chamber images, the weight of the nodules recovered from the chambers and a further three experiments for which nodule weight data are available. Results We find no significant correlations between total nodule area, average area, nodule weight or average nodule weight and rising oxygen concentrations, indicating that there is no link between the abundance or size of polymetallic nodules and rising oxygen concentrations. The inclusion of the three additional chamber images in the analysis reduces the significance of the previously stated correlation to ⍴ = 0.433, p = 0.122, with confidence intervals that cross zero (−0.13, 0.78). Furthermore, we note that a link between polymetallic nodules and rising oxygen concentrations is directly contradicted by the experiments from the western CCZ and the control experiment that recorded rising oxygen when no polymetallic nodules were present. Conclusion Since all these experiments recorded increasing oxygen concentration regardless of the size, abundance, or presence of polymetallic nodules, we find that increasing oxygen concentrations in these experiments is unrelated to polymetallic nodules.