Peer review of: Deglacial water-table decline in Southern California recorded by noble gas isotopes (2019)


RR2
REVIEWERS' COMMENTS:  
Reviewer #1 (Remarks to the Author): 
 
The authors have adequately and carefully responded to the points raised by both reviewers. I 
have the impression that the manuscript is now in extraordinarily good shape and ready for 
publication as it stands, except for final technical corrections. I think I found two minor typos in 
the text, as detailed below. 
 
Line 96: injection of excess air arising (not: airing) from the dissolution of entrapped soil air 
bubbles 
 
Line 204: we find (delete: a) mean regional WTDs 

 
Reviewer #2 (Remarks to the Author): 
 
The authors have thoroughly revised their manuscript and - barring two not-note-worthy 
exceptions - have seriously and sufficiently addressed all points raised in the first round of review. 
 
Next to the four minor points listed below, there are about ten further very minor corrections to be 
recommended - mainly typing mistakes or missing units. They have been highlighted and 
commented in the attached pdf files. But all in all and assuming these minor points are addressed, 
there is nothing that would warrant another round of review and the authors should be 
congratulated for their excellent work! 
 
Minor points: 
 
1) Main text, Figure 1: 
 
I'm sorry, I'm not sure if this figure still needs improving more or not. 
The idea of duplicating the abscissa at the top of the figure is a good one! 
Originally, my main reservation was that one of the two arrows indicating fractionation due to 
water-vapour flux and thermal diffusion is positioned incorrectly (green and bordeaux coloured 
arrows, respectively): I was under the impression that the origins of these arrows should be the 
same at depth = 0 m, and then the arrows should diverge with increasing depth instead of 
converging to the same point at 50 m depth as they currently do. The whole figure gives a bit of a 
"cheap" impression and I (admittedly) jumped to the conclusion that the two arrows were poorly 
aligned graphical objects - possibly because a "snap to grid" feature of the drawing software was 
activated and caused a misplacement of one of the arrows. 
However, the authors have ignored this issue in their revision, and reading deeper into 
Severinghouse et al. GCA 1996 , it seems as if the positioning of the arrows could actually be 
intentional, because of a near-surface effect that off-sets the vapour-flux fractionation before it 
assumes a roughly constant gradient with increasing depth. The same could apply for fractionation 
due to thermal diffusion. If so, then the figure would indeed be correct, but maybe the non-expert 
reader should be alerted to this subtlety. 
Also, based on the numbers presented in Severinghouse et al. 1996, which for noble gases do not 
relate to isotopic fractionation, one could expect the water-vapour flux to have a larger 
fractionation effect at depth than thermal diffusion. Currently, and if correct, Figure 1 suggests the 
opposite. It would be good if the authors checked this. 
Either way, such excellent work as presented here deserves a proper and correct visualization of 
the underlying principles in the form of a figure. 
 
2) Equation S5 in Supplementary Materials: 
 
Error calculations are a lot clearer now, thank you! 
But formally this equation still is not quite correct, because a mixed quadratic sum of relative and 
absolute errors does not make any mathematical sense. The relative errors need to be converted 
back into absolute errors first. The correct formula (under the assumption of independent errors) 
would be as follows: 
SEtot^2=SEext^2 + SEcs^2 + d*sqrt((spld/(sqrt(N)*d'smp))^2 + (SEatm/d'atm)^2) 
where d is the Greek delta value defined in equation S4, d'smp is from equation S2, and d'atm is 
from equation S3. 
Taking the absolute values of d'smp and d'atm is not necessary because of the subsequent 
squaring of the value. 
 
Presumably, this is only an error in the presentation of this equation S5, and the recalculation of 
all of the data performed by the authors during revision was correct (by some means). Had 
equation S5 been used as it stands here, then by the definitions in equations S2 and S3 all SEtot 
would have been increased by a factor of 100 - 1000 or more due to the small values of d'smp and 
d'atm. 
 
3) Equations S6 and S7 in Supplementary Materials 
 
Thank you for correcting these equations. 
In principle, the unit "amu" should also be included every time an exact mass difference is used in 
the equation. 
Given that the d*Kr and d*Xe values in the Data Table file have not changed compared to the 
previous version of this manuscript: was the original data calculated with the current equations S6 
and S7? I'm just asking the authors to confirm this - or make corrections, if necessary. Even if a 
recalculation were necessary, I would be surprised if any of the conclusions changed. 
 
 
4) Supplementary data set / Sheet "Raw measurement details" / Cell D2: 
 
The authors write: "*PI refers to pressure imbalance given in per mil as PI = Vsa/Vref-1, where 
Vsa and Vref are mean sample and reference gas voltages during analysis". 
Please clarify what you mean by "sample and reference gas voltages". To the non-expert it is not 
clear that (presumably) reference is being made to the intensity of the ion beam currents during 
analysis, the unit of which is not the volt! Also, different disciplines in mass spectrometry follow 
different conventions when reporting ion beam currents: some always scale to the voltage 
equivalent over a 1e11 Ohm resistor, whereas others don't. In my experience, reporting ion beam 
currents in units of volts is a source of confusion and should be avoided where possible.