Peer review of: The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation (2020)



 

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
I made three major criticisms of this paper. The first and third were linked, and were of the authors’ 
possibly overstated conclusions that Ca concentration was the dominant factor controlling marine P 
burial over geological time. The authors now agree in this revision that Ca concentration is not the 
most important control. So this point is now resolved, but it is resolved by removing the most 
impactful finding of the paper. I question now whether these results are interesting enough for the 
general readership of Nature Communications, but I will leave that decision for the editor. 
 
If the editor still wishes to publish this paper, or when they submit elsewhere, then my second point 
should also be addressed properly. The point I made was that their Phanerozoic global scale model 
does not consider many of the important processes in O2 regulation, and thus was no use for making 
conclusions on what level of O2 variation could be achieved by changing Ca concentration. The model 
they use was designed to explore feedbacks in isolation, not to make Phanerozoic pO2 predictions. I 
suggested instead using a comparison where the relative changes in O2 caused by changing Ca, 
bioturbation etc. could be assessed, and I am glad that they have done this in figure 5. But there are 
problems with this: 
 
1. There is almost no mention of figure 5 in the text. I am left with many questions about this figure: 
Why the different time scales for each experiment? Why is one effect so much stronger when they 
look to be a similar magnitude in figs 2 and 3? Why the odd dynamics for the solid line? what is 
driving this reversal? This is all very important and not explained. 
 
2. I recommended that the Phanerozoic figure be removed, and it has not been. The authors rebuttal 
notes that the qualitative relationships are robust even if the model is too simple. This is true, but my 
point was about the quantitative results, and the lack of data used to force the model, and neither 
have been addressed. I regret that my previous point was not given in enough detail so here are 
specific lines: 
 
142: “We further forced this global carbon cycle model with previously estimated, independently 
derived values for seawater DIC, seawater dissolved Ca and Mg concentrations, seawater pH, and 
bioturbation intensities through the Phanerozoic”. 
- None of this information is plotted or described, so the model is still very opaque. 
 
152: coupled model shows that “Phanerozoic CFA burial fluxes largely follow contemporaneous 
variations in seawater dissolved Ca concentrations” 
- But this is because many other important processes have been left out. What about erosion, 
sedimentation and organic carbon burial for example? What about the weathering flux of P? How does 
anoxia change in the model? 
 
156: “Our coupled global model also suggests that major swings in CFA and P burial result in similarly 
sizeable swings in atmospheric oxygen levels.” 
-The swings are on different orders of magnitude, not the same size. 
 
158: “Variations in marine Ca concentrations could have caused shifts in atmospheric pO2 from less 
than 10% to greater than 25% (v/v), encompassing essentially the full range of values previously 
predicted for the Phanerozoic as a whole”. 
-But if the model contained other important processes (fire feedback, sulfur cycle etc.) these numbers 
would be much less. 
 
163: “Fluid-inclusion data suggest an increase in seawater dissolved Ca concentrations during the 
early Cambrian 11”. -The inclusion data does not show this. There is only one data point pre-

Devonian. 
 
178: “Our results do not preclude the possible importance of the rise of land plants in shaping 
atmospheric pO2. However, they suggest that seawater dissolved Ca concentration may have been an 
equally important factor influencing surface oxygen levels over the duration of the Phanerozoic” 
-As earlier, Ca is very important in this model because not much else is considered. The rise of plants 
is not considered in this model in any detail so the relative importance cannot be assessed. 
 
193: “Within this traditional framework, atmospheric pCO2 and pO2 should be positively correlated 
over major tectonic cycles during Earth’s history” 
- I do not recognise this ‘traditional framework’. This statement is only true if spreading rates change 
while everything else remains fixed, which is not a good representation of the way the system works. 
Biological weathering enhancements should increase O2 and reduce CO2? 
 
194: “This theoretical paradigm, however, clashes with the broadly negative correlation observed 
between pCO2 and pO2 in reconstructions of Phanerozoic atmospheric composition” 
- The latest estimates I can find for Phanerozoic O2, which include some of these authors and are in 
this journal, show a decrease from 200 Ma to present, which does correlate with CO2 decrease. 
 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The authors carefully considered all of the reviewers’ comments in the revision of the manuscript. It is 
now in very good shape and can be published as submitted. 
 
Klaus Wallmann 
 
 
Reviewer #3 (Remarks to the Author): 
 
Enclosed is my second review of Zhao et al. 
 
Zhao et al. have addressed some of my comments. I still find the paper unclear in many parts with too 
much of what was actually done present in the SI or methods. However, I appreciate the authors 
moving more of the paper to the methods sections as this makes much of the paper more 
transparent. 
 
As I stated in my original review, the paper is interesting and the overall basic message is clear. 
However, I still find myself having trouble understanding what precisely the authors did and more 
importantly their assumption. This is because a large number of models and assumptions are being 
made that, when reading the main text, are not clear. I appreciate the author’s response to this — 
that I can just explore the issues with the model myself (as stated in the response). However, as far 
as I could tell, the code would not be published alongside the paper, but rather would only be 
available upon request and so I cannot at this time do this. 
 
In summary, I personally think this paper would do better and have more impact in a longer-format 
journal. The authors disagree, and I think this is a fair disagreement. I appreciate the authors 
comments that more people will read the paper if published in Nat. Com and thus that their central 
message will get across to a larger body of people. The authors have tried to clarify things and have 
responded in earnest to the reviews. As such, I do not believe I need to see it again as my main 
reservation is not one of whether or not what they have done is correct or interesting, but rather the 
great reliance on the SI . If the editors think it is sufficiently broad and accessible, then that is good 

enough for me. 
 
Main comments: 
 
(1) This paper is a modeling paper with multiple models presented. However, the code used for the 
calculation is not provided such that a reader cannot inspect what they have done or attempt similar 
sensitivity tests. It is stated that the code can be requested from the authors. However, I do not know 
the state of the code and if, at this moment, someone not involved in the work could actually use it — 
i.e. is there clear documentation associated with the code? At this point, I believe the authors should 
include all code used as part of this study for all models discussed (including that coupled to the global 
carbon cycle model) with appropriate documentation. I believe these should be provided as part of the 
SI. 
 
(2) There remains a deeply unclear aspect of the work. The authors, in their response, acknowledge 
that total phosphorus input equals total phosphorus output. Their models, through changes in Ca2+ 
affect the dissolved phosphorus content of the oceans, which they then take to limit productivity (and 
thus corg burial). As total phosphorus burial remains constant for any geologically relevant time 
frame, the parameters that matter for our understanding of how Ca2+ affects biogeochemical cycling 
is the overall C/P ratio of sediments following diagenesis. Put another way, what we need to know is 
the percent of P recycled form sediments back to the water column vs. the amount of organic carbon 
deposited and respired. And we need to know how these change together as a function of Ca2+ 
concentrations. This is presumably built into the model (C/P ratios), but such numbers would be useful 
to present for their given scenarios as this is a commonly measured parameter (C/P ratio). 
Regardless, there are numerous statements about important changes to P burial by Ca2+. This is a 
only sort of correct as far as I understand it as net phosphorus burial is a constant (unless weathering 
changes). The authors are well aware of all of this, but it would be helpful if they were careful in their 
language on the matter and provided C/P ratios to help guide the reader and provide testable 
numbers. I’ll note in my last review, I recommend the authors include C/P ratios, but they did not 
address this. 
 
Specific comments: 
 
Line 48 — Stoichiometry: Does the precise stoichiometry matter? If more or less Co3 is put in does it 
have an effect? I do not see a reference for this chosen stoichiometry. 
 
Line 51 — I would substitute ‘should’ for ‘could’. I’ll note this sort of statement is common and I 
advise the authors to think carefully about what they have inferred vs. demonstrated. 
 
Line 60 — What is an ‘intrinsic rate law’. Perhaps I am ignorant, but I have never heard of such a 
thing, nor do I see a definition. I did a quick google scholar search for this term, and came up with 22 
references total when searched in quotes. Regardless, I don’t know what the authors are talking 
about. It seems like rate law is just proportional to the degree of undersaturation vs. supersaturation, 
which is a common model form. Are you sure the rate law is order 1 vs. some other order as is 
common for calcite dissolution? How well does your model fit the experimental data? 
 
Line 66 — What are the key simplifying assumptions you have made for you basic model exercise. Is 
there anything the author should worry about here that was a big assumption you made? Does the 
rate of organic carbon oxidation matter for example? I.e., if you speed up the rates of remineralization 
so more happens at the surface, do you not make CFA at all? 
 
Lines 73-77 — what are calibrating? This implies you do not know certain terms and are doing a fitting 
exercise to find them. Is FOAM really the best place to do it? If you take your parameters and then 
use them in another site where things are well understood, do they give appropriate answers? 
 

Line 114 — does bottom water temperature play a role? Are you assuming a temperature dependence 
both for the saturation state as well as the kinetics of dissolution and reprecipitation? This is important 
given the large changes in bottom water temperatures over the Cenozoic. 
 
Line 121 : Is the data controlled for depth, bottom water temperature, and all other variables. Are all 
sites from the different ocean basins present for all time? This is an important dataset for the paper, 
but it is largely unexplored and explained. If you look at the Pacific vs. Atlantic, do they have the 
same behavior over time? Why does this only go back to 80 million years? 
 
Lines 121: If you are going to state something ‘correlates strikingly’, you should provide the R2 of the 
correlation and actually show the cross plot. 
 
Line 129: I don’t think this is a ‘test’ as opposed to exploring with the model 
 
Line 140: Is it DIP that really matters for GPP? Or rather is it the shallow/deep water mixing flux. DIP 
is very low in the surface ocean where it is limiting. But there can still be high GPP if the flux from 
below is high enough driving by extensive recycling. This raises an important point — you appear to 
be modeling marine primary productivity levels including both GPP and NPP. Please provide those 
values so we can see if they are reasonable. Also, what happens to deep-ocean O2 concentrations as 
you lower GPP and NPP at low DIP? Will we lose our anoxic zones? Or do you just have pO2 change to 
compensate — i.e., you are keeping dissolved ocean O2 the same. 
 
Complex model — provide the estimated C/P ratios for the model run and compare to data? This 
would be quite helpful. 
 
Figures 
 
Figure 1: Do we have an external data on what a reasonable P regeneration percent should be? If we 
look at the modern calculated values, they are all predicted in the model to be about 50%. So I should 
expected, based on this, that typical C/P ratios of sediments (assuming they have chosen typical 
values) should be around 200-250. This seems correct to me. What about for the anoxic endmember? 
Is this consistent with data from those basins? If you look at C/P of 100 million year old sediments, 
are they different given you large predicted change in the regeneration rate? 
 
Figure 2: Is burial efficiency here just one minus the regeneration rate from figure 1? If so, perhaps 
use the same units. I would like to see the predicted C/P ratios for these curves as these can be more 
easily related to actual data to check whether what we are seeing is reasonable. Such a ratio is 
inherently calculated by the model. 
 
Figure 3: Again I would provide all data in terms of a C/P ratio where possible. Additionally, something 
I find confusing is that total burial of P is a constant (it is just the weathering flux). So when the 
amount of CFA in sediments grow, what happens to the other sinks? I.e., where does the additional P 
come from or go to in the sediments as CFA concentrations change. 
 
Figure 4: The model is built on the FOAM site, which is not a deep sea sediment. Is this an appropriate 
comparison? Why not use deep-sea sediments for the parameterization. Please break down the 
PCFA/Preactive based on the cores they came from and basins. It would be nice to make sure there is 
not an aliasing in the record. Finally, please show these a C/P ratios. I would like to make sure those 
change as would be predicted. 
 
Figure 5 Is the pO2 actual percent (i.e. taking into account 0.8 bars of N2) or is it a pressure. 
 
Figure 6: Again, it would be useful to see a C/P ratio as this is something that is measurable and 
presumably predicted by the model given absolute corg burial fluxes are being used to determine pO2.