Reviewer comments – egusphere-2025-4395 – Evaluation of the Particulate Inorganic Carbon Export Efficiency in the Global Ocean

Overview:

The research article by Jordan Toullec, entitled “Evaluation of the Particulate Inorganic Carbon Export Efficiency in the Global Ocean”, explores particulate inorganic carbon export in the global ocean and discusses the role of various planktonic groups, differences between oceanic regions, and correlation with net primary production of particulate organic carbon.

Overall, the manuscript is interesting as it combines available data collected in the field, using sediment traps, for example, and satellite imaging. The amount of work transpires from all the data compiled, which is presented in the “Results” section. The discussion also targets a variety of processes, making it relevant. The “Materials and Methods” and “Results” sections are somewhat rich in information but not overly long, which is good, and the results also report on a variety of data that are well compiled together.

However, the reviewer believes that major revisions are required on the writing itself. The manuscript would benefit significantly from a deep review of the text. In numerous instances, sentence formulations are not appropriate, and missing words or repetitions make the text unnecessarily complicated to read in some parts. Consistency and flow are lacking, especially in the introduction and discussion, where the reader quickly gets lost in the amount of information. Similarly, verbs are missing in some instances, and sentences are not properly formulated. Nevertheless, the reviewer believes that a careful re-reading and re-writing of the above-mentioned sections of the manuscript by the author would decrease these issues significantly. The reviewer will then consider assessing the second version of the manuscript once these comments have been implemented. The reviewer also acknowledges that the author may not be a native speaker, but believes that with care given to the grammar and once all point-by-point comments added below have been implemented, another round of review will lead to a quality paper suitable for publishing.

Comments:

Line 8: I believe this should read “calcifying” rather than “calcified” (same throughout the text).

Line 9: same as above.

Line 24: instead of “due to”, I would advise using “through”.

Line 25: same as first comment.

Line 26: word missing, it should read “referred to as”.

Line 24-27: sentence is a bit overloaded, it would benefit from a rewrite, likely in 2 distinct sentences.

Line 30-31: statement of “to estimate a particle/sinking flux” made twice, consider deleting one.

Line 40: I would slightly edit this sentence, as technically speaking, the calcification process removes alkalinity and releases CO2. Later, once CaCO3 has dissolved, I agree that alkalinity increases, as well as the uptake of CO2. However, given that it is a “loop”, I think stating that it increases alkalinity and CO2 uptake may be misleading. This would be up to the author to edit, but it should be considered.

Line 41: this should read “associated with”.

Lines 45-47: this sentence has no verb.

Line 64: this should read either “with a complex food web” or “with complex food webs”.

Line 78-79: the sentence is confusing; consider rephrasing.

Lines 84-85: the paper from Kwon et al., 2024 states that 20% more CO2 would be emitted to the atmosphere if CaCO3 dissolution in the upper ocean did not occur. This does not necessarily mean that 20% more CO2 is captured. The formulation used here is misleading.

Line 86: while the research article from Renforth and Henderson, 2017, is an important piece of work for carbon sequestration research, the reviewer expresses some concerns as to why it is used here. Aren’t there more suitable references? Please justify the use here.

Line 105: this should read “such as that PIC…”.

Line 114: there is one too many “to”, it should read “integrated to 100 m using…”.

Line 122: the link provided did not work at the time of the review. Please review and make sure that the data are accessible.

Lines 125-128: I do not fully understand the use of semicolons here. Commas would be more appropriate, and there is no need for capitalised words after the colon and semicolons.

Line 130: please review the citation. I believe Biogesociences has specific formats for referencing websites, such as the date of access and further details. Please review.

Line 137: why consider the first 200m for foraminifers and pteropods, while only considering the first 100m for coccolithophores?

Line 158: I do not understand where the value of 388 comes from. Please elaborate.

Line 189: “deployment” should be plural

Line 202: I believe “coverage” should be used instead of “covering”, but this might need to be double checked.

Line 204: “fecal pellet” should be plural.

Line 223: “regions” is missing.

Line 224: in figure 2, could the tiles “a” and “b” be set on the same y-axis scale? The use of a logarithmic scale in “b” makes the direct comparison trickier. Please review whether it makes sense to put it on the same scale as “a” for easier comparison. If not, please provide the comparison in the response to reviewers.

Line 253: this should read “gives us” and consider a full stop after “event” to separate the two trains of thought.

Line 281: the sentence “NPP compared as compared to PIC” is misleading. Please edit accordingly.

Line 285-287: the caption would benefit from some details as to what “n.s.” means (which I believe is “no significant”) and especially what value is considered “n.s.”

Lines 295-296: given that the 6 depths have already been discussed and introduced before, there is no need to add them here again (same as line 300). Only saying “at all depths” for the North Atlantic and “between 500 and 4000m in the North Indian Ocean” would lighten the text from numbers, making it more reader-friendly.

Line 313-314: the start of the sentence is a bit awkward. Please revise.

Line 333: duplicate “in”.

Line 340: the formulation “and follow the pattern than zooplankton” does not make sense. Please revise.

Line 356: “demonstrates” should not take an s here, and please review the sentence as it does not sound correct.

Line 365: this should either read “there is a large uncertainty in the…” or “there are large uncertainties in the…”.

Lines 376, 377, 380: “coccolithophore” should be plural (check throughout the text)

Line 385: “fecal pellet” should be plural (check throughout the text)

Line 392: the sentence is not correct. Please review.

Line 396: “layer” should be plural.

Line 397: ”regardless of the respective depths of calcite and aragonite saturation” does not make sense. While I understand the idea, the sentence is not correct. Please edit.

Line 400: change “in there” to “therein”.

line 420: here “suggest” should take an s.

line 424: here it should read either “microzooplankton vacuoles induce PIC dissolution” or “microzooplankton vacuole induces PIC dissolution”.

Lines 429-431: the sentence is misleading, consider separating into 2 distinct sentences.

Line 443-445: I am not fully sure I agree that blooming is an avoidance mechanism. Please argue your thoughts, but consider editing the sentence.

Line 445-447: this sentence is not correct. It should read “could produce so much biomass that…”, “won’t” should be edited to “will not” ... Please review.

Line 448: the use of “regarding” does not make much sense here. Similarly, the sentence is confusing. Please edit.

Line 453: this should read “a mesocosm” study. Otherwise, “the mesocosm study from XXX”.

Line 455-456: why is “E. huxleyi” used here, while throughout the author used G. huxleyi? Please edit for consistency.

Line 461: the end of the sentence does not make sense. Consider changing “more” to “higher” or equivalent. Please edit.

Line 462: I am not sure how phytoplankton phenology is a time-dependent concept, why saying “During […] phytoplankton bloom phenology”? Please edit.

Line 477: here, “that” should be added after “showed”. Please edit.

Line 489: duplicate of “In subtropical areas” from the previous sentence. Consider deleting.

Line 507: please edit to maintain consistency in abbreviation, especially regarding PIC Eeff and PIC Teff.

Line 512: duplicate of “effect”. Please edit.

Line 520: this should read “result in”. Please review.

The manuscript by Toullec provides a comprehensive analysis supported by global ocean sediment traps data, ocean color satellite measurements and models to understand PIC fluxes, its export and transfer efficiency in the global ocean, and the role of plankton phenology in determining the PIC fluxes and standing stock. The analysis is strong and well structured and demonstrates a high level of knowledge also supported by relevant literature in the field.

I found the study an important and timely contribution in the understanding of global ocean carbon fluxes, the biological carbon pump and the carbonate pump, under fast-changing global ocean scenarios due to anthropogenic CO2 rise. The study is also very relevant in ocean-atmosphere interactions processes and CO2 balance. 

While highlighting important processes in a very complete and comprehensive way, with appropriate methodology and statistical tools, I found the overall manuscript hard to read , encountering difficulties in following the argumentations for the way the manuscript is structured, for the long sentences, at times truncated (verbs missing), and for the variety of concepts and arguments introduced several times at different points. Therefore I suggest a thorough revision of the language and of the sections for clarity and smoothness of the text and a re-organization of the discussion sections with less sub-headings and with more highlights on novel concepts and differences to what has been known so far in the field (that I found hard to identify in the overall text). In particular, I suggest organizing the introduction and discussion section with clear parts on how the present study addresses the discrepancies in the field and how it advances the knowledge or proposes new pathways and hypotheses. I honestly find it difficult to follow PIC/POC standing stock, Teff, PEeff argumentations and the taxa or processes responsible behind these estimates as these concepts are introduced several times but rather in an unstructured way. I suggest major revisions for this reason, but the underlying concepts in my opinion are there. 

Specific comments (with line numbers):

45-48: incomplete sentence

48-57: unclear text. I suggest revisiting. Why the BCP is boosted?

58-60: greater lability of organic matter is associated with higher degradation rates, therefore I expect that this labile fraction is not exported to the deep ocean. 

62: Are there more recent studies with respect to Lima et al 2014? I would expect a lower Teff for higher turnover at low latitudes for a variety of other biological processes also temperature mediated. Are there more in-situ observations? Or do you imply that in CaCO3 productive regions despite a fast turnover and high lability or organic matter, this organic matter is fast removed by the ballast effect?

72: PCB = BCP?

90: the depth of reference? I think 100-200 m (as in the methods section?) but I would clarify it here too.

95: how deep do ocean color sensors go?

Eq. 1: can acidification and rising temperature effects be included in the growth rate calculation for coccolitophores?

210: Can you specify the difference in estimation? How many months were used in the model?

210-215: Can you add why NPP and PIC production are not concentrated in the same regions? Is it entirely due to plankton phenology?

220: Can you add why the difference in residence time between POC and PIC?

250-255: unclear sentence/statement, suggest to revise.

265-270: calcified taxa PIC stock does not correlate with PIC export flux on a global scale. This concept is repeated twice. However, here it is stated that coccolitophores dominate at high latitudes, previously it is stated that coccolitophores dominate the PIC standing stock, but it is also stated that PIC standing stock or production is higher at lower latitudes. So if not coccolitophores, who is dominating PIC standing stock at low latitudes?

279-283: unclear statement with some repetitions, can you rephrase?

Discussion Section: as already indicated, I strongly suggest to revise the structure of the whole section.

409-410: I find this sentence contrasting, to me very slight losses would agree with well-preserved coccoliths in fecal pellets.

Figure 8: what are the highlights and major findings of the present study, and its novelty? I think the results are not clearly evidenced as they are kind of hidden between different argumentations and references to previous works. I think it would be important to clearly describe Figure 8 that resumes the present study results and advancements in a sub-paragraph per-se.