Effect of Manganese Oxide Mineralogy and Surface Mo Coverage on Mo Isotope Fractionation During the Adsorption Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments on minerals-3367012

 

This study deals with molybdenum isotope fractionation using three manganese oxides. The data obtained show fundamentally important phenomena related to molybdenum isotope fractionation by manganese oxides. Therefore, this manuscript is deemed appropriate for this journal, but some additional explanation is required.

 

Figure 3. Explain why the difference in the amount of molybdenum acid ions adsorbed per surface area increases significantly compared to the amount of molybdenum acid ions adsorbed per weight. Does this mean that the density of molybdate adsorption sites per weight of each mineral is different? If so, explain which sites in each manganese oxide can be adsorption sites for molybdate ions.

 

Figure 3b and lines 198 to 201. For birnessite and todorokite, explain how you evaluated that the Langmuir adsorption isotherm does not fit, but the Freundlich adsorption isotherm does.

 

 

Correct some words for subscripts or superscripts. (line 32, Figure 1b, Table 1)

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Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This article investigates the influence of manganese oxide mineralogy and surface molybdenum (Mo) coverage on Mo isotope fractionation during adsorption. This is a significant topic because Mo isotopes are used as paleo-redox indicators in marine sediments, and adsorption onto manganese oxides is a key control on Mo isotope fractionation in seawater. The authors synthesized three manganese oxide minerals (todorokite, birnessite, and δMnO2) and conducted adsorption experiments with varying Mo concentrations to achieve a range of surface coverages. The key finding is a negative correlation between Mo surface coverage and the degree of isotope fractionation, regardless of the Mn oxide mineral type. This suggests that surface coverage is a more important control on fractionation than mineralogy. The observed fractionation in the experiments (up to 2.7‰) was lower than that observed in natural ferromanganese oxides (~3‰). The authors attribute this difference to the lower Mo surface coverage expected in natural samples.

However, I have certain reservations regarding the fractionation models. The isotopic compositions observed in the experiments for todorokite and δ-MnO₂ generally align with the equilibrium fractionation curves, apart from a few outliers (Figure 4). Given the wide range of Δ⁹⁸/⁹⁵Mo values for birnessite, which lack a discernible pattern, these outliers could be considered anomalies. This raises a plausible argument that the Δ⁹⁸/⁹⁵Mo values for todorokite and δ-MnO₂ may be constant and higher than those for birnessite, with mineralogy potentially being the primary factor driving the variation in Δ⁹⁸/⁹⁵Mo values across different minerals. I would like to ask how the authors address this interpretation and what evidence they provide to rule out this possibility. Furthermore, additional evidence is needed to substantiate the claim that the adsorption fractionation for birnessite is smaller than previously reported results.

1) L36-39: The preferential adsorption of lighter Mo isotopes does not appear to be the sole direct cause for their use as redox indicators. Other isotopes, such as Sr, exhibit similar fractionation behaviors but are not employed as proxies for paleo-redox conditions. Please add more details to explain the reason.

 

2) L198-201: Please incorporate the equations for these two models into the manuscript. Additionally, it would be helpful if you could include the degree of fit or the p-value, either within Figure 3b or in the accompanying text.

 

3) Table 1: Have the molybdenum (Mo) concentrations and isotopic compositions been determined for both the solution and the minerals? If these measurements have been conducted, it would be advantageous to compare the isotopic composition of the initial Mo solution with that which is calculated based on the adsorption fraction and the isotopic compositions observed in the solution and minerals. Such a comparison serves as a validation of the experimental reliability.

 

4) Figure 4a: The use of solid and dashed lines to represent the todorokite equilibrium models, particularly when colored orange, could enhance clarity and readability. Additionally, incorporating further details about these models, such as the magnitude of fractionation, would be beneficial. It would also be advantageous to include or highlight the initial solution values on the corresponding plots. It would help to link the solution values to their initial point.

 

5) L258-260: In the case of birnessite, the degree of isotope fractionation has exhibited considerable variation, and the correlation appears to be insignificant. What could account for this discrepancy?

 

6) L280: The surface coverage of molybdenum (Mo) in this study is likely greater than in previous investigations, which would correspond to a reduced degree of isotope fractionation, as inferred. Please note this correction

 

7) L287: Please correct ’Error! Reference source not found’.

 

8) L340-343: It may not be feasible to conduct experiments with lower Mo/Mn ratios. Nonetheless, it would be highly beneficial if the researchers could replicate the experiments with higher Mo/Mn ratios for todorokite or δ-MnO₂ to achieve surface coverages comparable to those of birnessite. The low Δ^98/95Mo values observed in other minerals comparable to those of birnessite under low surface coverage conditions further substantiate the correlation between surface coverage. Additionally, is it possible to lower the Mo/Mn ratio in the experiments by increasing the quantity of minerals used?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I found this version of the paper significantly improved. The authors have addressed my comments and I recommend publication.