please see pdf

This manuscript is another contribution in a series of rigorous papers from this group on quantifying the rates of petrogenic organic carbon oxidation. The rate of oxidation of petrogenic organic matter and sulfide minerals is highly uncertain in global carbon budgets and this work both reports contemporary rates of oxidation and provides an accessible and novel framework for making additional measurements. The manuscript is clear and well written, with careful attention to methodological details allowing reproducibility and extension of this technique to other studies. Beyond the methodological contributions, novel observations were presented wherein two different sites independently showed similar dependence of bedrock co2 fluxes on environmental variables (temperature and moisture) and similar drawdown of O2, despite differences in mineralogy.

I offer some minor comments on areas that could use further clarification below, but I kept returning to the following questions as I read the paper and would encourage the authors to address these questions (or discuss why they could not be addressed) as I suspect other readers will share these questions:

1)         Is the calculated rate of oxidation consistent with the loss of OCpetro from the rock (and erosion rate)? Has it been established that the rock from near the surface shows sufficient depletion of OCpetro (relative to parent bedrock) such that the observed CO2 fluxes could be explained reasonably well by the observed uplift rate, bulk density, and solid phase loss? This woud be a critical ‘check’ on the reliability of the rates measured with this new technique.

2)         What evidence is there that the CO2 accumulation in the chambers is exclusively driven by mineral weathering and OCpetro oxidation? The authors take great care to identify sampling locations where rock is exposed and therefore CO2 is likely to be sourced from respiration of OCpetro, however, the photos show plants nearby at a lengthscale that could conceivably influence CO2 fluxes. I fear that even radiocarbon and d13C measurements of CO2 may not be sufficient to rule out (relatively) recently fixed C as a source of CO2, as typically even soils and saprolites show accumulation of radiocarbon dead pools of C.

Other comments:

Figure 4 is well done and very instructive. 

You may want to check the paper for issues with significant figures (e.g. L556).

Are the ranges of values for tC/km2/year that appear in the abstract associated with the values in line 458? If so, please add the range of values to the text and help readers connect Table 5 to this range.

I commend the authors for leveraging well studied sites. I found it a bit unconventional to cite a list of publications for relatively straight forward site attributes (e.g. L117, L120). This makes it harder to identify the source of the information cited. I also wonder whether there are stream/surface water fluxes of DIC or major cations that could be used to comment on the feasibility of the dissolution and transport of CO2 (i.e. what is proposed in L8).

L51: This could be written more clearly to support the motivation for the work presented here. Tune et al. use the gradient method as well as water chemistry to demonstrate that deep CO2 is associated with deep roots in bedrock. They find that the solid phase is depleted in OCpetro but that the CO2 that is transiting upwards from the rock is not derived from oxidation of OCpetro (via C14.) Note also that Tune et al., 2023 perform rock incubation experiments and report lab-based oxidation rates. These could be compared to the fluxes reported here.

L85: Is this in reference to solid phase measurements of OCPetro? Could clarify.

L112- Could you clarify what is meant by “more comparable between the catchments?”

L365-375: This section was a bit challenging to parse and could be clarified to separate hypotheses from observations. As is written, it is well established that diffusive fluxes would decrease under reduced air filled porosity and increased tortuosity associated with moisture increase. It would be helpful to spell out other contributions to changes in observed fluxes more clearly. For example, later in the paper we learn about solifluction and solid phase changes which could be described here instead.

L384- remove likely? Under what circumstances might this not be the case? Also, consider revisiting Sanchez Canete et al., 2018 and citing here.

While I don’t propose a major overhaul on the structure of the paper, I did find it a bit unconventional and a bit of a challenge to work through all of the calculations in the discussion. The paper would perhaps be more accessible and easier to read if the methods and calculations were all spelled out in the methods section for easier reference. This is ultimately at the author’s discretion.

Figure 6 and associated text: Were there indications of which events led to infiltration to corroborate the 5 mm for 3 day threshold? Did surveyors observe other evidence of subsurface wetting that aided in the classification of data between wet and dry?

L512: 30% air filled porosity seems very high for a fractured rock. I was not able to find the porosity and saturation measurements that this was based on in the papers cited. Can you present the evidence to corroborate these very high air-filled porosity values, or if not, could you justify them and provide clarification on how they affect the results?

L530: Are these rocks fractured?

L536-541: I was not able to follow how the cited papers support the Zrock calculation. This section seems a bit contradictory with the site description- could you clarify the text to address this?

L555-558: Why aren’t rock masses reported separately for the two catchments given that CO2 fluxes are reported separately? 

Sánchez-Cañete, E.P., Barron-Gafford, G.A. and Chorover, J., 2018. A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere. Scientific Reports, 8(1), p.13518.