I have reviewed the manuscript entitled "The triple oxygen isotope composition of phytoliths, a new proxy of atmospheric relative humidity: controls of soil water isotope composition, temperature, CO2 concentration and relative humidity." submitted by Outrequin et al., for publication in Climate of the Past. This investigation is directed to develop a new proxy for quantitative paleo-humidity reconstructions based on triple oxygen isotopes in biogenic silica from plants. The authors calibrate their proxy using growth chamber and demonstrate that the 17Oexcess parameter in photoliths is sensitive different RH conditions, while is barely affected by temperature or CO2 concentrations, among other parameters. The analytical approach and the experimental set-up seems thoughtful and well-designed. The authors try to explain slight deviations of the analytical data with respect to expected results, for example, by assuming that heterogeneous silicification processes may contribute to an apparent kinetic fractionation between evaporated leaf water and silica. The manuscript is well-written and the science is convincing. In my opinion, the authors should address some analytical details and clarifications before publication:  

1. Lines 75 to 80. The authors should cite here recent studies that used triple oxygen and hydrogen isotopes in hydration water of minerals as a quantitative proxy for paleo-humidity reconstructions, including Evans et al., 2018 and Gázquez et al., 2018. These studies are totally related to the final goals of this manuscript and should be cited as an example of quantitative RH proxy based on triple oxygen isotopes.

2. In lines 108 to 114. I wonder if the author could translate this paragraph into a conceptual figure, explaining the sensitivity the isotope ratios to these parameters. Otherwise, it may be difficult to follow for non-specialized readers.

3. Sections 3.1 and 3.2. Did the air inlet to the chamber atmosphere come from the same cylinder as for the analyzer when doing the calibration with liquid waters? Did the instrument use Air Zero (dry synthetic air)? Did you replace the air in the chambers with the same carrier? I am asking this because, in my personal experience, the use of different carrier gases (i.e. dry atmospheric air vs dry synthetic air) for calibration and for online measurements of water vapor can produce an offset in 17Oexcess. This needs to be clarified in this sections.

4. In section 3.2. Please, can you give the typical H2O concentrations measured with the CRDS analyzer from the chamber atmosphere? Did you consider/apply any linearity correction for the isotopic values? Did you take any measurement to monitor the drift of the instrument between calibrations?

References

Evans, N.P., Bauska, T.K., Gázquez, F., Brenner, M., Curtis, J. H., Hodell, D.A., 2018. Quantification of drought during the collapse of the classic Maya civilization. Science, 361 (6401), 498–501.

Gázquez, F., Morellón, M., Bauska, T., Herwartz, D., Surma, J., Moreno, A., Staubwasser, M., Valero-Garcés, B., Delgado-Huertas, A., Hodell, D.A., 2018. Triple oxygen and hydrogen isotopes of gypsum hydration water for quantitative paleo-humidity reconstruction. Earth Planet. Sci. Lett. 481, 177–188.

The triple oxygen isotope composition of phytoliths, a new proxy of atmospheric relative humidity: controls of soil water isotope composition, temperature, CO2 concentration and relative humidity

Clément Outrequin et al.

This study combines new and previously published phytolith triple oxygen isotope data to develop a new proxy to determine paleo-RH. The approach and chamber studies are interesting and thorough, and the utility of 17O-excess as a proxy for paleo-RH will be of great interest to many in the paleoclimate/paleohydrology communities.  The writing is generally clear and is well supported by the existing literature. Occasionally some of the writing syntax could be cleaned up. With some revisions, this will be a welcome contribution to the growing 17O literature.

1. I think a more nuanced discussion about the role and variability of vapor 17O-excess is warranted. There is very little published vapor 17O-excess data, so I don’t think it is yet quite reasonable (as in line 62) to expect that vapor 17O-excess will vary little from place to place. In thinking about this, it might be useful to draw comparisons between observations of vapor d-excess and expectations of vapor 17O-excess. This study is limited to the tropics, but are there other regions with a limited range of vapor 17O-excess where a similar paleo-RH proxy might be worth exploring?
2. Is the ∆’ (or ∆', note the difference between the apostrophe and the prime notation and please be consistent throughout the manuscript) defined in Equation 3 necessary? This may be confusing for beginning readers because some recent triple oxygen isotope studies (e.g., Aron et al., 2021, Sharp et al., 2018, the 2021 RiMG book) have used the ∆' notation rather than 17O-excess. If possible, I think it would be good to avoid the ∆ symbol in this instance to minimize confusion.
3. Section 3.2, paragraph 1: Did the authors account for potential memory effects in the vapor measurements when switching between ports on the manifolds? Were any measurements dropped or ignored just after switching to vapor measurements from a new chamber? Previous vapor isotope studies have shown that memory effects on vapor d¹⁸O and d-excess need to be accounted for when a this type of manifold setup is used (e.g., Simonin et al., 2013). I imagine that the sensitivity of 17O-excess to mixing makes this consideration important in this case as well.
4. Figure 1 provides a very useful schematic to understand the experimental setup. However, I have a few questions about the isotopic values reported. First, if evaporation is prevented from the soil, why are the soil d18O and soil 17O-excess values not identical (within analytical precision) to those of the irrigation water? Second, why is the 17O-excess of the final vapor (-6 per meg) so low? Is this a product of vapor mixing within the chamber? I encourage the authors to add d-excess data when possible (I assume this is accessible from the Picarro measurements) to explore the hydrologic processes that are going on in the chambers.

References:

 Aron, P. G., Levin, N. E., Beverly, E. J., Huth, T. E., Passey, B. H., Pelletier, E. M., et al. (2021). Triple oxygen isotopes in the water cycle. Chemical Geology, 565, 120026. https://doi.org/10.1016/j.chemgeo.2020.120026

Sharp, Z. D., Wostbrock, J. A. G., & Pack, A. (2018). Mass-dependent triple oxygen isotope variations in terrestrial materials. Geochemical Perspectives Letters, 7, 27–31. https://doi.org/10.7185/geochemlet.1815

Simonin, K. A., Roddy, A. B., Link, P., Apodaca, R., Tu, K. P., Hu, J., et al. (2013). Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables. Plant, Cell and Environment, 36(12), 2190–2206. https://doi.org/10.1111/pce.12129

Authors Outrequin et al. submitted a manuscript about recent experiments investigating the controls over the triple oxygen isotope composition of phytoliths and the feasibility of using phytoliths as a paleo-aridity proxy. The authors detail a well thought out plant growth chamber experiment where temperature, carbon dioxide concentration, and humidity are each controlled. The authors conclude that relative humidity has the largest influence on the triple oxygen isotope value of the phytolith. The authors provide a new dataset from West Africa and examine the range in triple oxygen isotope values. They compare their new results to previously published plant growth experiments and data from West Africa grasslands. It would be interesting to see values from different regions. However, the authors note in the conclusions that doing so is beyond the scope of the study. The only major critique of the paper is that the data from West Africa are not really described in terms of how it can be used to reconstruct relative humidity. The manuscript only notes that it follows closer to the 2018 growth experiment calculation due to the differences in the δ¹⁸O value of the initial water. It would be interesting to use Eq. 12 to predict the relative humidity in the modern analog (knowing the initial δ¹⁸O value of the precipitation water). Overall, this manuscript details a very time intensive and difficulty study and does a good job of distinguishing the main driver of the oxygen isotope composition of phytoliths. This manuscript is fitting for the journal and suitable for publication, pending addressing the major (optional) comment above and the small (and optional) comments below.

Line 97: The denominator should be 18, not 17

Figure 4: Are there any open red or blue circles? (Phyto predicted?) There are dotted lines but in the legend it says there are open red and open blue circles. May be worthwhile to add error bars on the phytolith measurements. Why not add Eq. 12 and predict relative humidity of the natural phytolith samples?