Review of « Revisiting five decades of ²³⁴Th data: a comprehensive global oceanic compilation.” By Ceballos Romero et al.

This paper presents of global database of discrete ²³⁴Th data measurements in the ocean. Following previous databases compiling Th derived POC flux and C:Th ratios, this represents the next step. No need to say that this is a very useful compilation and that such effort fits perfectly well within the scope of ESSD. Having such compilation will provide information not only on downward export rates of particles but also on processes such as remineralisation, fragmentation or even scavenging rates of Fe for instance. The data and the associated metadata are adequately compiled and presented. Pathways to data access and availability is also well described. I particularly enjoyed the historical perspectives provided in section 4. This provides a nice overview of the ²³⁴Th story since the start to nowadays.

There are other biological carbon pump processes apart the downward flux that ²³⁴Th could be used for. For instance, processes such as remineralisation, fragmentation, scavenging rates of TMs or particles sinking velocities could be estimated. I believe that such section could easily be added to the final paragraph without going into much details.

Apart from that, only minor issues should be addressed before publication. These are listed below:

Specific comments:

L129: How was bloom stage assessed?

L141: Could CTD data including fluorescence and PAR be added?

L145: Formatting

L169: µm dpm L^-1 ????

L173: Why not fluorescence?

L208: Larger

L241-244: I would also stress the fact that other empirical methods such as the ¹⁵N new prod (f-ratio) techniques has been proven as unappropriated to estimate C export (Yool et al. 2007) in the late 2000’s.

L245: Section 4

L276: # ?

L295: Most glass fiber filters (so called GF) used for particulate material analysis have a pore size of 0.7 um.

L366: formatting

L435: formatting

L474: 2002 for the BSi (Friedrich and van der Loeff 2002)

L544: Consider adding “….and avoid some tedious filter folding sessions.”

L610: formatting

L619: GEOSECS

L626: cruises

L628: A third one is coming up very soon. I suggest adding it here if the reference is made available on time.

L700: How about alternative export pathways (not necessarily gravitational)? In addition, fragmentation (Briggs et al. 2020) could also play a role in setting the magnitude of flux attenuation . Would Th excess be useful to study such mechanism?

L707: format

L725: not always, see work by (Xie et al. 2020).

L779: formatting

References

Briggs, N., G. Dall’Olmo, and H. Claustre. 2020. Major role of particle fragmentation in regulating biological sequestration of CO2 by the oceans. Science (80-. ). doi:10.1126/science.aay1790

Friedrich, J., and M. M. R. van der Loeff. 2002. A two-tracer (Po-210-Th-234) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current. Deep. Res. I 49: 101–120.

Xie, R. C., F. A. C. Le Moigne, I. Rapp, J. Lüdke, B. Gasser, M. Dengler, V. Liebetrau, and E. P. Achterberg. 2020. Effects of 238U variability and physical transport on water column 234Th downward fluxes in the coastal upwelling system off Peru. Biogeosciences 17. doi:10.5194/bg-17-4919-2020

Yool, A., A. P. Martin, C. Fernandez, and D. R. Clark. 2007. The significance of nitrification for oceanic new production. Nature 447: 999–1002. doi:10.1038/nature05885

Review of  “Revisiting five decades of ²³⁴Th data: a comprehensive global oceanic compilation” by Ceballos-Romero et al.

General comments

This paper provides a global dataset on ²³⁴Th data in the ocean, accompanied by additional parameters used in the application of the ²³⁴Th approach to derive export fluxes, mainly particulate carbon export fluxes. This effort comes after the compilation of ²³⁴Th fluxes by Le Moigne et al. (2013) and the recent compilation of POC:²³⁴Th ratios by Puigcorbé et al. (2020), both published in ESSD, therefore I believe it is well suited within the scope of ESSD. This compilation will be very useful, as the authors mention, to better constrain the contemporary ocean carbon uptake by providing easy access to all the ²³⁴Th data collected over the last 50 years.

This is a major effort that has brought together a large number of datasets. The information provided regarding the global dataset, what parameters are included, how they are organized and how to access them is clear for the reader. Additionally, the authors have added a detailed history of ²³⁴Th and its use as particle tracer from its beginnings until today, and even looking into future programs.

I did enjoy reading the entire manuscript, but I also believe it should be streamlined because there is a bit of repetition between sections and whitin the same section. As a general comment, there were several typos and formatting errors along the manuscript, repeated words and sometimes sentences appeared out of the blue or were unfinished/misplaced. I had the feeling that the manuscript needed an additional proof reading by the authors prior to submission, but I tried to provide detailed comments when I spotted some of these issues.   

Particular to Section 5, this section does not always discuss the gaps in the dataset, but it expands on issues that, although very important, most reader will be aware of them and they are not real gaps of the dataset, at least not in the way they are presented. To be honest, this section feels like it is a section to “advertise” the work of the first author (7 citations out of 8 in page 24 are Ceballos-Romero et al.), which it is actually really good, but I don’t think it belongs to the “gaps of the dataset” section. Maybe move it to the overview of the dataset or to the discussion. If the authors decide to keep it in Section 5, as mentioned in my detailed comments, those particular paragraphs should be more to the point.

The ²³⁴Th timeline part (4. Discussion) would largely benefit from a figure showing a timeline and the key dates/milestones listed in the text. I think the readers and future researchers interested in ²³⁴Th would appreciate having Section 4 summarized in a nice timeline figure.

Also related to Section 4, I have provided specific comments below, but I believe that, even though this manuscript has a whole section of “storytelling”, without providing additional data (which I enjoyed reading), the division by era’s should provide some useful information for scientific purposes (e.g., period of years that the era lasted, using the name of the main program of the era, or stating what was done during such era). Although it is quite “artistic” to have named them by well-known novels linked to the ocean, those titles don’t serve a clear purpose (they don’t clearly describe the actual era) and changing their names might be more adequate when publishing a research paper. Moreover, in that line, I think the manuscript, since it is a research publication, should provide some actual data, not just referring to the number of publications and number of datapoint, but actual data on ²³⁴Th, similar to what was done in the two previous compilations related to ²³⁴Th (see Fig. 1, 3, 7 and Table 2 from Le Moigne et al. (2013) or Fig. 2, 3 and 5 from Puigcorbé et al. (2002)). This manuscript does not provide data on ²³⁴Th, just on the number of studies/cruises/stations/parameters. The authors could include a table with ²³⁴Th concentrations (tot/part/diss) similar to Table 2 from Le Moigne et al. (2013) or in a figure similar to Fig. 4 from Puigcorbé et al (2020) or something completly different, but they should report actual ²³⁴Th data/values, not just data points. A description of the number of datasets, studies and locations is already provided in the abstract of the dataset in the PANGAEA repository. In this manuscript, the authors should provide something additional to it, that goes beyond explaining the structure of the dataset, the percentage of studies that do one thing or another, or the history of ²³⁴Th.

Despite agreeing on the fact that this dataset is very necessary, and it will be very useful, I have two (relatively major) issues with it:

• Not all the studies reported analyzed POC, some analyzed PIC+POC. There is no reference made regarding this "problem". Either in the manuscript or in the dataset (or both), a comment should be made to indicate if the data reported is truly POC, or if it is not, why it is still considered as POC (e.g., negligible amount of PIC).
• The authors have compiled additional data, when available, on water temperature, salinity, ²³⁸U, and particulate nitrogen, and if they can, they also report the bloom stage. However, the authors have omitted a critical parameter for the application of the ²³⁴Th method. This is particularly surprising since one of the authors has recently (2020) published a PNAS paper entitled "Metrics that matter for assessing the ocean biological carbon pump". It has been shown that the integration depth has an impact on the magnitude of the fluxes (the strength of the biological carbon pump) and therefore, we should be using reference depths related to the euphotic zone (or primary production zone (PPZ); Owens et al. (2015)). I understand that not all the studies, particularly in earlier days, provided this information, but recently, the integration depth is no longer fixed and authors explain what kind of reference depth they use to integrate the fluxes. I believe fluorescence and/or PAR data to define the euphotic zone or the PPZ (or providing those depths directly) should be added in the dataset, the same way that temperature and salinity are. With that addition, in the manuscript it should also discuss the importance of the integration depth and cite the Buesseler et al. (2020b) paper in a more appropiate line than where it is currently cited (L778). If this dataset is expected to be “alive” and authors want to be contacted to update it by adding new data, having this kind of input is crucial for future applications of the ²³⁴Th method. 

Finally, regarding the datasets available in PANGAEA, I found that https://doi.pangaea.de/10.1594/PANGAEA.918125 provides the metadata information and there is a table with parameters shown in that file. However, https://doi.pangaea.de/10.1594/PANGAEA.937414 contains the actual data but there is no table of parameters there. Please consider including such table for clarity (even if it is a long one), since not all the “parameters” might be clear to the reader/user, e.g., “integrated_depth_real_particle (m)”. Notice my comment below for Table 1, where the starting and end date for the sampling period have been reversed in some cases. The dates are also incorrect in the dataset. I noticed some from Table 1 but please double check them all again in the dataset as well. 

Detailed comments:

L11-16:  “However, it was…. the biological pump.” This is a very long sentence.  I don’t think that level of detail is necessary for the abstract. The information detailed here is already discussed in detail in the manuscript. Please, simplify it.

L 20-21: “…that include two size classes;…”. By reading the rest of the paper more por sizes (e.g., <70 μm) are mentioned and reported. Maybe just finish that sentence after “filtration method” in the abstract.

L23: “…included, including…”. Try to find another word to avoid repetition

L38-39: I understand where that is coming from but production rates of anthropogenic radionuclides might not always be that well characterized or can vary over time.

L41: I would even say more than just years (paleo tracers).

L44: “use” instead of “used”

L45: Delete “a” from “a ²³⁴Th”

L53: “when high scavenging by particles”. Correct, there is something missing.

L55-56: Instead of citing that many papers without a clear reason behind their selection, maybe cite Le Moigne et al. 2013, which provides a compilation of studies using the ²³⁴Th approach to derive estimates of POC export.

L57-58: PIC and BSi abbreviations are not needed. PIC is only used one more time, in L 474, where it is also defined, and BSi is not used anywhere else in the manuscript.

L61: “…is in part…” seems to imply that there is another part, but it is never mentioned.

L63-64: Not sure how necessary is to provide the editors’ names for the special issue.

L86: Maybe delete the “only”

L88: Missing Th after ²³⁴

L93: “..and useful metadata to be used”. Change words to avoid repetition of “use…”

 L114: Add the GEOTRACES website

L124: “…spanning all oceanic regions” maybe add “across”: “spanning across all oceanic regions”

L129: Do you mean when reported by the original authors? There could be issues on the way the different authors, over the different years, have decided if the conditions where non-bloom, pre-bloom, bloom or post-bloom. Please discuss it a bit more.

L145: 210 should be a superscript. Same for L435, L537, L610, L665 and L666

L158-159: “…the authors were directly contacted for data is indicated in the localization.” Please rephrase for clarity.

L169: It should be μmol instead of μm. Same for L178

L169-170: “…and include… in which we included”. Try to find synonyms to avoid repetition of words

L174: Correct “The particles’ sampling…”

L180: “Measurement uncertainties in the data points as provided by the data measurer…”. Please rephrase for clarity. 

L200-203: Delete or move to the section “General overview”, rephrase or delete “here” since you refer to “here” for the entire manuscript, for a particular section that you are discussing and for a particular section that you are about to discuss, which makes it confusing. 

L203: “on in” correct it      

L208: “larger” instead of “lager”

L209: “During these 50 years of study, we have….”. Maybe rephrase so it doesn’t sound like you took 50 year to write this manuscript. Something like “During these 50 years of ²³⁴Th data” similar to the title.

L209 and L211: “14 major ocean programs” vs “as part of one of these 13 programs”

L211: “as reported by the authors”. I’m not sure which authors are you referring to, the ones of that major program? Yourselves? If it is yourselves there is no need to have that sentence. If it is referring to the authors of those datasets, I don’t think it is correct, because they could not tell that their dataset represented 30% of the total datasets.

L212: “reported” instead of “compiled”

L214: delete the - after TSS

L218: “time-series stations” should be TSS as defined in L214.

L222: “least” instead of “last”

L222-223: “re-occupied” should be  “reoccupied”, as in L223

L225-231: Missing the degree (°) symbol for the coordinates of all the time series stations

L237: POC(PON) to ²³⁴Th ratios have been defined previously (e.g., 144) as POC(PON):²³⁴Th.

L247: Correct “datasets”

L253: Correct “expeditions to the”

L258: “is not significant of the number”. Rephrase for clarity.

L263: Correct “experiments”

L264: Delete “started in 1969” or add “which”

L271: it should be “of” instead of “on”       

L276: Delete #

L288: Same as previous comment from L237

L301: it should be “sample” instead of “sampled”

L304: “yet maximize…” add “the number of locations sampled”

L305: “were” instead of “was”

L308: Correct for consistency “POC(PON):²³⁴Th”

L317: “…in situ pumps, bottles and…” remove the “and” and change for a comma.

L324-326: Missing reference to “Not et al (2012). Conservative behavior of uranium vs. salinity in Arctic sea ice and brine. Marine Chemistry 130-131; pp. 33–39. doi:10.1016/j.marchem.2011.12.005

L337: “...in an oceanographic expedition…”, delete “field” is redundant.

L340: I would recommend changing the names of the eras in order to actually provide some information about each era. Naming era 1 “The Old Man and the Sea” doesn’t say much about what happened during this era. I would provide the periods of each era and, if possible, something more such “JGOFS”, “GEOTRACES” or something related to the main advances of each era, e.g., “the beginning of ²³⁴Th as tracer of POC export” or in that line.

L344: Bhat had co-authors in that first ²³⁴Th publication.

L345-347: “Bhat et al. (1969)… part of the total ²³⁴Th.” What’s the purpose of this sentence? It is interesting but it appears out of the blue and it is not discussed any further in a way that it is linked to the following sentences. Maybe rephrase between L345 and L354 to facilitate the flow.

L353: It sounds strange to say that you use “this radionuclide” (referring to ²³⁴Th) to study the scavenging of thorium. All the thorium isotopes are particle-reactive. 

L358: Rephrase “realizing the relevance to the downward ²³⁴Th flux” for clarity.

L367: Correct “increase” instead of “increased”

L371: Correct “prolong” instead of “prologue”

L376: It should be “MnO₂” (the 2 should be a subscript)

L380: Linked to my comment from L340, here the authors refer to JGOFS era. Avoid confusions.

L382-385: Missing the degree (°) symbol for the coordinates of all the time series stations

L388-389: Avoid repeating “introduced”

L391: “…a precipitate that serves for removing Th…”. Change to “…a precipitate that removes Th…”

L393: Maybe refer to it as “For the MnO₂ cartridges technique”, since MnO₂ is the method being used currently, although mainly without the cartridges.

L394: Same comment as L376

L394: Delete “(MnA and MnB)”. There is no graph and, therefore, there is no need to specify the name of the cartridges (which could also be “named” Mn-I and Mn-II or Mn-1 and Mn-2 or many other ways).

L394: Did you mean “and gamma count them”?  Not sure it is needed here since gamma counting is mentioned in L398.

L397: Why the differentiation between MnO₂ and Mn cartridges? We have only been talking about cartridges with fibers impregnated with MnO₂.

L398: Refer to it as MnO₂ cartridges for consistency. Otherwise, modify the text above and refer to Mn cartridges the whole time, after specifying that those are cartridges with fibers impregnated with MnO₂.

L403: The reference to the volumes sampled doesn’t seem to have a purposed. It is not discussed or linked in any way to the rest of the paragraph. It provides information but I don’t think it is relevant as it is presented right now, plus the sample volume reference is not done for the studies cited between L404-410.

L407: Correct “where” instead of “with” or say, “with a predominant particulate size class cut-off of 0.45 μm”. If it is just for one study, can you say it is the predominant size class? Were there others in that same study?

L408-410: “Note that… McKee et al., 1986).” This sentence seems not to match the size class cut-off of 0.45 μm mentioned just before. Is this operational distinction applying to the previous studies cited between L403 and L405? If that’s the case, move it there. If not, remove or integrate better to the paragraph.

L435: Same comment as L145: 210 should be a superscript

L436: Same comment as L340

L437: The authors keep referring to the JGOFS era: “…what we are calling the “JGOFS era” (1989-2003)” but this is not an actual era in their manuscript, not by period of time nor by name. Please avoid confusion by naming the eras appropriately to refer to something that has a meaning linked to the history of ²³⁴Th.

L443-444: It is odd to cite a reference from 2008 in the 2^nd era (1992-2001). Moreover, the following sentence starts with “Accordingly, …”. Please cite the first paper discussing this topic, not one from 2008.

L446 or 447-448: Adapt the format of equation 1 to match the format of the different parameters in the description (e.g, add 234 as a superscript, keep consistency with italics and non-italics, etc.)

L477: Add the units of the ²³⁴Th_flux as done for the ratio.

L452: It should be “Element: ²³⁴Th” instead of “POC:element”. I don’t think the italics are needed.

L453: “its” should be “their”

L453: Delete “dependent” since it already says that they vary with both depth and particle size. Or say that they are “depth and particle size dependent”

L454-456: “…can be calculated from the disequilibrium between the ²³⁸U-²³⁴Th resulted from particle sinking…” This has already been said a couple of times before. Simplify: “²³⁴Th flux can be calculated by evaluating….”

L456: Why “of the dissolved ²³⁴Th”? It has been done with total ²³⁴Th (see Savoye et al. 2006-Marine Chemistry and references in there).

L458: Add “a”: “only a single ²³⁴Th profile”

L458: Remove the comma after “NSS model”

L458-460: Simplify it. Too many sentences within one sentence. “…when activity gradients can be assessed in time thanks to repeated sampling, ideally over the course of 2 to 4 weeks.”

I think the authors should refer to Resplandy et al. (2012)-DSR-I work (http://dx.doi.org/10.1016/j.dsr.2012.05.015 )

L473: PIC abbreviation is not required since it isn’t used anymore. Also, notice that PIC and POC fluxes are elemental fluxes of the same element, C.

L476: It should be “MnO₂” (the 2 should be a subscript)

L481: Missing the degree (°) symbol for the coordinates of the time series station

L487: “with large volumes (2500-3500 L) passing…” instead of “passed”

L498: Why “we initiated”? Delete

L489: Why “and filtered”? Delete

L492-494: There are 7 studies that sampled 2 size fractions but there are only 4 papers cited, 2 for 53 μm screens and 2 for 70 μm screens. Either cite the 3 remaining studies or add “e.g.,” when citing the other studies.

L494: How many studies used sediment traps? 7 studies did size fractionation, it would be good to mention those that used sediment traps, for consistency.

L503: Another reference to the “JGOFS era”. Maybe consider this as one of the eras then?

L505: “at” instead of “in”

L506: Based the era period described by the authors (1992-2001), the NABE study should not be included here. It’s not part of the overlap, it’s overlapping with era 1, but not with era 2.

L506-520: There is no consistency in the amount and type of information provided for each process study. For example, why the objective of the EqPac study is described but not for the other studies, for which mainly the dates and locations are provided?

L523: “initiatives” should be singular (only OMEX included here).

L525: “carried out” instead of “made”

L532-536: “Time-series…….Murray, (1992))”. Cut the sentence, it’s too long.

L535-536: 10 studies used sediment trap and also ²¹⁰Pb-²¹⁰Po at the same time (same studies for both)? If not, rephrase for clarity

L536: Same comment as L145: 210 should be a superscript.

L538: Same comment as L340

L543-544 and L551-552: Same sentences. Delete one to avoid repetition or change them to say different things.

L556-557: Missing the degree (°) symbol for the coordinates of the PAP station

L557-558: Why there is so much detail provided for the PAP station and not for the others? Why it is not explained what sensors the other TSS have? Provide similar levels of detail for all.

L560-564: I’m not sure the level of detail provided here is necessary. Simplify.

L569-571: “An additional improvement… method.” This is out of place, this should be moved and discussed in the following era since it was published in 2021.

L573: I guess “contraptions” is supposed to be “concentrations”

L573: Delete “and” after “POC:²³⁴Th”

L578: What do you mean by “surveys”? Cruises, programs? It can’t be studies because those 4 “surveys” are differentiated from the 87 studies. Please clarify

L580: Are HOT, BATS and DYFAMED considered programs? In table 2 they are referred as TSS. The program that initiated these time series was JGOFS. Maybe refer to them as TSS?

L584: Missing the degree (°) symbol for the coordinates of the K stations

L586: Correct “JGOFS”

L587: U.S. JGOFS-Japan refers to a collaborative station between US and Japan?

L587: Missing the degree (°) symbol for the coordinates of the KNOT station

L590: Rephrase: “And a pilot cruise was reported under the GEOTRACES program” or similar

L590: Correct “…discussed in the next section”

L592: “Finally, a great number of projects and experiments belong to this era (see Table S3 for a chronological summary).”  Remove, starting the sentence with “finally” creates great expectations for the reader, but there nothing after that.

L593: “Sampling”

L601: “while and” is incorrect. Please choose which one to use.

L608: Change “used” for “use”

L610: 210 should be a superscript

L612: Same comment as for L340

L619: It should be “GEOSECS”

L624: “carried out two years later”. Later from which reference year? Just say the year that it took place.

L626: Correct “cruises”

L629-632: Maybe some of the studies could be cited, including Clevenger et al., (2021) cited in the previous era, or the GEOTRACES intermediate data products from 2014 and 2017 (Mawji et al., (2015) and Schlitzer et al. (2018)) or the trace metal fluxes studies (Black et al., 2018; maybe add some more studies here since you refer to a shift from POC to trace metal fluxes).

L638: “and known” should be “are known”

L643-645: The 8 cruises that sampled in the NH and the SH are included in the 27 that sampled in the SH or are considered a separate group. If so, maybe mention them apart from the NH cruises.

L646: Refer to GEOTRACES instead of “a major ocean program”.

L676: Add “and” in between “2^nd 3^rd”

L646-651: Why are this projects/experiments/cruises not cited in Table 2? What criteria was used to include them or not in that table?

L659: Change to “...whether a SS, a NSS or both models were used”

L660-661: Are those citations referring to what? The use of SS, NSS or both? Why so many citations? Just write “e.g.,” and cite max 3.

L665 and L666: Same comment as L145: 210 should be a superscript.

L666: Correct “compared them”

L668: Same comment as L340

L675: the abbreviation for net primary production is not needed since it is not used anymore

L677-678: “availability for phytoplankton at a key junction of the global ocean circulation”. Rephrase for clarity

L684: BCP has not been defined yet. It is defined in L697. Abbreviation probably not needed, used a few more times later on but there is a large number of abbreviations in this manuscript already.

L685-686: Why APERO and PICCOLO are no included in the previous group of projects? What is the criteria to differentiate them from COMICS, EXPORTS, CUSTARD, etc.

L689-691: Please clarify what JETZON does. “…international coordinating umbrella to serve as a focal point…” means that they organize or fund cruises, or meetings? Are any of the recent programs cited before (COMICS, EXPORTS, CUSTARD, OTZ, SOLACE, APERO, PICCOLO) included or linked to JETZON in a formal way?

L697: See my comment for L684

L697: Delete “of”

L700: “time-series site” should be TSS

L707: Use “< 1%” instead of “less than 1%” (if not add a space after “than”)

L718: Correct “for the application”

L726: Change “…since most cruises do not allow repeated measurements…”

L727: Delete “i.e., neglecting changes….”. It has already been explained what a SS model implies.

L718-731: This paragraph seems repetitive with previous sections of the manuscript, when presenting SS and NSS models. This section should describe how many studies applied SS and NSS or both and in which cases the application of one model or the other had a significant impact. Maybe citing the work by Resplandy et al. (2012). http://dx.doi.org/10.1016/j.dsr.2012.05.015 can help to provide the overview on this particular gap.

L732-733: Too broad. Not all the radionuclides present deficits in the water column and not just because of their half-lives, also because of their physicochemical behavior. Please rephrase referring only to ²³⁴Th, since it is the radionuclide of interest for this manuscript.

L736: Add “e.g,” in front of the citation to Ceballos-Romero et al. (2018), there are other studies that have reported similar results.

L736-744: This paragraph is not referring to a gap in the dataset. This is an explanation about the importance of the bloom period and the reliability of the SS and NSS approaches (issues that the readers accessing this dataset are most probably already aware of). I don’t think it is needed, plus it seems a bit repetitive and too extensive considering that an explanation about SS and NSS already stared in L718. Please simplify from L718 to L744 in order to highlight the importance of this considerations and their implications in a much briefly way.  L745-751 are great, they report the actual data and highlighting the importance of reporting bloom conditions.

 L755: Correct “occurs”

L758: Delete “sampled” since it has been stated several times that all the oceans were sampled.

L760-766: Again, this is not a discussion about a gap in the dataset, it is explaining again what data is reported, how important it is to combine ²³⁴Th with other tracers or devices to quantify export, etc. This can be deleted since it has already been presented and discussed along the manuscript.

L761: Correct “provides”, since it is the “combined interpretation” that provides us with the tools.

L763-764: The “Ceballos-Romero et al. 2016” citation is not needed or add other studies, this is not the only one that has combined Th-234 with other tools.

L764-765: “For that reason……. might be missing.” These two sentences are important, but they should be moved to the section describing the dataset, section 2. Data.

L777: Change “us” by “as”

L778: Please rephrase “Together with particle sinking velocities disequilibrium derived”, I do not follow it.

L781: It should be BCP

L783: all the ways to re-evaluate the ²³⁴Th-derived POC fluxes can be done using the dataset of this compilation, but it does not allow to evaluate those fluxes in relation to the particle sinking velocity or the synthesizing modelled particle remineralization, so please rephrase to clarify what can be done with it alone and what could be done if combined with other datasets.

Table 1:

Please check sampling periods. In several instances the dates are placed reversed i.e., first the end of the sampling followed by the starting date. See: Matsumoto, 1975; Minagawa and Tsunogai 1980; Wei and Murray, 1991; Porcelli et al. 2001 (different format used: yyyy/mm/dd); Lepore et al. 2009

This problem is also found in the dataset available in PANGAEA. Please correct it there too. I noticed these ones but please review them all again.

Also, some of the studies have only one sampling date, therefore, that term could be simplified by reporting just that date instead of reporting it twice. See: Moran and Buesseler, 1992; Buesseler et al. 2000; Cai et al 2002; Coppola et al 2002; Guo et al 2002; Hung and Gong 2007; Buesseler et al 2010 and Pabortsava, 2014

It would be good to increase the width of the “Reference” column because some of them take more than one row. No data repository is reported in that column, so the title of that column can be simplified.

Table 2:

As mentioned in my detailed comments, I would like to understand the criteria used to include programs or in this table or not, since some mentioned in the manuscript are not included. On the other hand, OSP- Station PAPA Ocean Weather Station “P” is not mentioned in the manuscript but is included in the table.

Please format the table so that the “Acronym” column as well as the “Activity” column are wide enough to have the entire name in one row. Right now, it is too big and it makes it hard to follow.

Figure 1:

Panel c) LTTS has not been used anywhere in the manuscript, the acronym used is TSS. If for some reason you prefer to use long-term, high-frequency time-series stations, do not use the acronym.

Same for L831, TTS station is the wrong acronym.

Panels a) and c) have already a lot of symbols and colours reporting data. I don’t see the necessity to provide the topography data. A scale of greys for the background might be less shocking and allow to better visualize the data reported in these panels.

Figure 2:

L842: vi) remove the “” from the category Others in the caption.

For panel b) the lines for 2001 and 2010 are very out of place. Please add the legend in this graph again if this panel is place right below panel a) for publication.

For panels a) and c) the doted lines indicating the tipping points are also not centred in relation to the specific year.

Figure 3:

L873: Delete one of the “by the”

Same as per Figure 1, I think a grey background might allow to better visualize the datapoint reported in the different panels.

The caption does not match the symbols/colours of the map:

Panel a) ii) ²³⁸U + total ²³⁴Th the caption should be dark blue squares; iii) no ²³⁸U the caption should be magenta dots

Panels b, c and d) iii) the caption should be no total ²³⁴Th sampled (magenta dots); iv) the caption should be no POC: ²³⁴Th ratios (light blue triangles)

Notice that the legend for panels b), c) and d) the Th in the light blue triangles shouldn’t be a superscript.

This study presents a compilation of oceanic ²³⁴Th measurements made at global scale over the past 50 years (1967-2018). The dataset is composed of several parameters including total, dissolved, and particulate ²³⁴Th activity, POC:²³⁴Th and PON:²³⁴Th ratios, along with ²³⁸U activity, POC and PON concentrations when available. This set of parameters constitutes the basis for the use of ²³⁴Th as a proxy of carbon and nitrogen export fluxes from the oceanic upper water column. The data were obtained from several sources, the vast majority from peer-reviewed articles (214) and to a lesser extent from PhD manuscripts (4), public data repositories (8), and unpublished datasets (9). The database is composed of 223 excel spreadsheets along with a compilation file hosted in PANGAEA repository. For each data set, relevant metadata (geographic location, sampling date, project, sampling and processing methodology, bloom stage, etc.) have been systematically included. The associated paper introduces the dataset, presents a global overview, and then discusses the timeline of ²³⁴Th measurements according to four periods covering the last 50 years. Finally, the authors discuss gaps in the dataset and present some perspectives on its future uses.

General comments

First, I would like to acknowledge the extensive work that has gone into this very comprehensive and clear compilation of over 50 years of research on the short-lived radionuclide ²³⁴Th. Such a compilation was lacking until now and represents a new important step in the use of ²³⁴Th as a proxy for the export of C and other elements (N, BSi, CaCO₃, and trace elements) from the upper water column. The database is well structured and clearly described with detailed metadata of significant importance. It also appears very exhaustive and I could only identify a few minor omissions or errors (see detailed comments below).

My first general concern is with the form, as the manuscript contains a significant number of typographical and editorial errors that would have benefited from careful review before its submission. This concerns in particular the list of bibliographic references which contains a significant number of errors (authors list, authors name, type of reference such PhD manuscript, book chapter, research article). Still on the form, there is a surprising confusion between concentration and activity made throughout the manuscript. The ²³⁴Th and ²³⁸U data you report are activities not concentrations.

On the content, I was pleased to read the timeline of the Th studies which summarizes the major stages that contributed to the development of the method. For clarity to the reader, I would recommend to indicate for each period the corresponding years both in the manuscript in the subsection headings and in the figures.

About the different eras, I might have subdivided era 1 and 2 a little differently by considering the JGOFS program as the beginning of era 2. In fact, it seems more logical to me to take into account the sampling periods corresponding to different programs rather than the date of publication of the resulting studies. This is illustrated further in Table 2, where most of the studies belonging to era 2 were performed in the framework of JGOFS.

Regarding the first era, you may think to introduce the GEOSECS program earlier than L365 after the description of the Coale and Bruland papers. I think it could be relevant to mention that it was in the framework of GEOSECS that the concept of scavenging by particles has really emerged, especially for the open ocean. You may cite the seminal work of Turekian (1977) who introduced the concept of the “great particle conspiracy”. Furthermore, it might be interesting to mention that during GEOSECS little attention was given to ²³⁴Th in comparison to ²²⁸Th and ²³⁰Th for studying scavenging processes in the open ocean (Broecker and Peng, 1982). It was only later, with the papers of Coale and Bruland, that the role of ²³⁴Th as a tracer of short-term particle dynamics was really highlighted.

Regarding era 4 related to the GEOTRACES program, I would set the starting year to 2008 or even 2007, which corresponds to the International Polar Year (2008) and the start of the GEOTRACES sampling program. Regarding this program, it would be relevant to include in table 2 all cruises that have been sampled for ²³⁴Th so far. For consistency, I would recommend using the name of the section or process study considered as defined by GEOTRACES, you can then indicate which country has been involved in the sampling (US/UK/Netherland/Germany/India/France). This remark is also valid for the Th_database file, the projects performed within the framework of GEOTRACES should be named in a more consistent way such as for instance GEOTRACES (section or process study number, country, and eventually project acronym).

Still on the GEOTRACES program, I think it is important to mention that not all sections analyzed for ²³⁴Th are available in the 2014 and 2017 Intermediate Data Products (L367). Even if data obtained as part of GEOTRACES are published in peer-reviewed journals, their inclusion in the IDP requires some additional steps (submission and acceptance by the GDAC). Also, you may indicate that the last IDP (2021) was released very recently.

I have also some concern regarding the section 5. Significant gaps in the global dataset. It is not clear to me what the gaps you want to discuss are. Reading the first paragraph (L696-701), it seems the gaps you want to consider are related to the current understanding of the Biological Carbon Pump. On this topic, I would recommend to include some more recent reviews (Henson et al., 2019; Boyd et al., 2019; Siegel et al., 2016), which detail some of the processes that require further consideration. Reading the following, I notice you discuss two main points, the first one is related to the spatio-temporal distribution of 234Th data and the second one, too long considered from L711 until the end of the section L768,  is related to the modeling approach (SS vs NSS) used for estimating the export fluxes of ²³⁴Th. It is surprising to note that this entire section is mostly discussed using only two references and written by the first author of this review. In my view, this section needs to be reconsidered, first by giving more attention to the existing literature on the SS/NSS approach and the different oceanic contexts to which it has been applied (not only the North Atlantic), and second to the other numerous issues related to the ²³⁴Th method. Among these, it is important to point out the role of physics (lateral and vertical advection and diffusion) (Buesseler et al., 1995; Savoye et al., 2006; Resplandy, 2012; Le Gland et al., 2019; Roca-Marti et al., 2017), the importance of the depth of integration (Buesseler et al, 2020), and finally all the other issues related to the conversion to carbon fluxes using the POC to Th ratio of sinking particles (the choice of the relevant size fraction, the interpolation methods, etc.). By following these guidelines and considering that export fluxes have not been calculated or compiled in this review, you may be able to give recommendations to future users of this database.

Finally, and still about the gaps in this dataset, there is one point that could be considered that concerns the quality of available data. I understand that it is difficult to answer this point but if we take into account the successive evolutions of the methods used for the determination of ²³⁴Th, all the data are probably not of the same quality. I think this should be at least mentioned or even taken into account in the form of a quality flag assigned to each dataset.

Detailed comments

L15: the 234Th-238U pair is primarily used for assessing export fluxes, to look at export efficiency you need to compare with the net primary production, which is actually not included in the dataset. Please clarify

L20: replace concentrations by activities and at all other occurrences in the manuscript for both 234Th and 238U.

L29: the term uptake is a bit ambiguous and not directly related to the 234Th method and to export fluxes. Uptake can be used either for air-sea exchanges or biological assimilation, please clarify.

L35: correct the reference (Cochran and Masqué, 2003)

L45: remove “a” in front of 234Th

L50: mean life time

L52: check the end of sentence “high scavenging”

L55: correct reference for Cochran, 1993 (Kirch is the first name)

L57-60: you may cite also the works of Lemaitre et al. (2016) for N, BSi, Fe export in the Kerguelen area (KEOPS2 project) and Lemaitre et al. (2020) for BSi, CaCO3, lithogenic material and trace elements along the GA01 section (GEOVIDE) in the North Atlantic.

L63-64: remove the editors list of the special issue

L88: add Th after 234

L104: 238U activity

L105: you mean compiled and not “complied”

L114-115: as mentioned earlier, data obtained in the frame of GEOTRACES are not necessarily in the IDP

L124: not necessary to define supplementary material with an acronym (figure or table number is sufficient).

L129: could you provide more details on how the bloom stages were distinguished?

L145: please clarify what you mean with CHN data and how this differ from POC and PON concentrations reported in the dataset (as mentioned L104). Furthermore, CHN is not the only analytical method that can be used to determine POC and PON, EA-IRMS is also widely used for this purpose.

L151: Check sentence “Where data…”

L168: remove concentration for 234Th

L169: correct units µmol dpm^-1 for POC and PON to 234Th ratios

L172: correct >53 µm and not <53µm

L173: was responsible instead of is

L178: check units µmol L^-1

L181: correct one sigma

L190: remove the _ after authors

L193: check sentence and correct “used for”?

L198: the previous compilation by Le Moigne et al. (2013) include 234Th fluxes, POC:Th ratios, POC fluxes and NPP estimates.

L200: correct aspects

L201-202: check the end of the sentence

L205: remove SM

L208: correct larger

L214: put the acronym in brackets

L222: correct “on at least..”

L235: repeated occupation?

L242: Correct “the way ocean is currently…”

L275: you may include the KEOPS project (sampling in 2005) with the study of Savoye et al. (2008) and also the great Calcite Belt expedition with the study of Rosengard et al. (2015) carried out both in the Atlantic and Indian sectors of the SO.

L276: remove the # and correct “in terms”

L283: correct “field”

L284-285: what do you mean with “chemical scavenging”

L290: correct “through years”

L292: check the beginning of the sentence

L305: correct “were”

L329-333: the end of the paragraph is not related to 238U measurements but to additional metadata information (model assumption and bloom stages)

L357-358: check sentence

L364-366: I would consider the GEOSECS program earlier (see my general comment)

L387: Clarify the beginning of the sentence

L397: correct “do not”

L404: remove concentration with activity

L409: Nucleopore

L420-423: I would also mention that physical terms were not considered for solving the 234Th activity balance.

L423: remove of or rephrase

L433: Clarify what kind of models were used (one or two boxes) and the corresponding fluxes (scavenging fluxes J and export fluxes P). Also check the end of the sentence.

L437: JGOFS started in 1987 according to Table 2

L441-442: correct relative to its parent nuclide or 238U

L443: clarify what you mean with “response time” and also to what corresponds the second equation

L455: correct “with time”

L456: here you keep focusing on the SS vs NSS approach. You need to better account for the physical terms (according to Buesseler et al., 1995 in specific ocean settings such as upwelling regions). Furthermore, vertical diffusion can be quantified from a single 234Th profile if the diffusivity coefficient is known or assumed.

L458: check sentence, you may change to “when temporal fluctuations in 234Th activity can be assessed”. In addition, and as mentioned by Savoye et al. (2006), NSS approach requires the same water mass to be sampled. This is another difficulty of the NSS approach that can be difficult to meet in dynamical settings.

L466: correct “0.001<colloids…”

L471: There is a confusion, the 3-D model was not built to estimate the gradients of 234Th activities but to estimate the physical components to the flux (V terms). Without these components, the sinking flux would have been largely underestimated in the equatorial upwelling region.

L482: I don’t think the SEATS time-series was operated in the frame of the French JGOFS program, most probably by Taïwan.

L489: Clarify the end of the sentence “filtered”

L494: sediment traps collect sinking particles not bulk.

L505: correct “at the BATS…”

L506: replace “boxes” with areas

L508: check coordinates for the NABE experiment

L518: correct “field work”

L520: Australian

L523: correct “other major initiatives…”

L534: remove “during sampling”

L537: clarify what are the CHN data

L569: the reference Clevenger et al. (2021) is apparently missing in the reference list.

L573: clarify “234Th contraptions”

L626: Correct “Portland (USA, Oregon)” and “Numerous cruises”

L638: correct “are known…”

L664: remove “of the studies”

L666-667: check the sentence

L680: correct ”emphasizes”

In the section, you may include the MOBYDICK project (Marine Ecosystem Biodiversity and Dynamics of Carbon around Kerguelen: an integrated view, 2018-2022) during which the 234Th method has been implemented.

For the APERO project, it will start in 2022 to 2026 and the cruise is planned for 2023 in the western North Atlantic.

L706: correct “gaps”

L722: correct “with time”

L777-778: correct “such as” and clarify the whole sentence

Table 1. Check start and end date (sometimes inverted). The design needs to be improved as it is very difficult to identify which parameters has been measured for each studies.

I noted a few mistakes for the studies that I know (I didn’t check all entries in this table)

The study of Stewart et al. (2007) reports POC:Th ratio from in-situ pumps and sediment traps but not PON:Th ratios (check the crosses)

Thomalla studies in the Atlantic ocean was published in 2008

For Jacquet et al. (2011), PON:Th ratio were not provided but POC:Th ratio from sediments traps yes

For the Rutgers van der Loeff et al. (2011) study, underway sampling were performed and reported

Planchon et al. (2015) study does not report PN:Th ratio but C:Th ratio from sediment traps. Also dissolved Th was plotted in Figure 2. The PN:Th ratios for this cruise (KEOPS2) can be found in Lemaitre et al. (2016).

For the Lemaitre et al. (2018) study, please correct the reference as follow:

Lemaitre, N., Planchon, F., Planquette, H., Dehairs, F., Fonseca-Batista, D., Roukaerts, A., Deman, F., Tang, Y., Mariez, C., and Sarthou, G.: High variability of particulate organic carbon export along the North Atlantic GEOTRACES section GA01 as deduced from 234Th fluxes, Biogeosciences, 15, 6417–6437, https://doi.org/10.5194/bg-15-6417-2018, 2018.

The study of Maïti et al. (2016) reports total Th profiles

Table 2.

As for Table 1, the design needs some improvements. Column headings needs to be clarified and further details on the other programs than JGOFS could be included. This could be especially the case for GEOTRACES as well as EXPORTS. Furthermore, I do not see the reason why a given program is considered a major program. For instance, the HiLATS program is indicated only for 2001

Check the start date for DYFAMED

Figure 1: use the same acronym as in the text for long-term time series (TTS)

Figure 3: check the legend “by the” written twice

Regarding the dataset, it would be worth checking if the data of Stuckel et al. (2015) and Ducklow et al. (2018) have been included.

References:

Boyd, P.W., Claustre, H., Levy, M., Siegel, D.A., Weber, T., 2019. Multi-faceted particle pumps drive carbon sequestration in the ocean. Nature 568, 327–335. https://doi.org/10.1038/s41586-019-1098-2.

Buesseler, K.O., Andrews, J.A., Hartman, M.C., Belastock, R., Chai, F., 1995. Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EqPac program. Deep Sea Research Part II: Topical Studies in Oceanography 42, 777–804. https://doi.org/10.1016/0967-0645(95)00043-P.

Buesseler, K.O., Boyd, P.W., Black, E.E., Siegel, D.A., 2020. Metrics that matter for assessing the ocean biological carbon pump. Proc Natl Acad Sci USA 117, 9679. https://doi.org/10.1073/pnas.1918114117.

Ducklow, H.W., Stukel, M.R., Eveleth, R., Doney, S.C., Jickells, T., Schofield, O., Baker, A.R., Brindle, J., Chance, R., Cassar, N., 2018. Spring–summer net community production, new production, particle export and related water column biogeochemical processes in the marginal sea ice zone of the Western Antarctic Peninsula 2012–2014. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 376, 20170177. https://doi.org/10.1098/rsta.2017.0177.

Henson, S., Le Moigne, F., Giering, S., 2019. Drivers of Carbon Export Efficiency in the Global Ocean. Global Biogeochemical Cycles 33, 891–903. https://doi.org/10.1029/2018gb006158.

Le Gland, G., Aumont, O., Mémery, L., 2019. An Estimate of Thorium 234 Partition Coefficients Through Global Inverse Modeling. Journal of Geophysical Research: Oceans 124, 3575–3606. https://doi.org/10.1029/2018JC014668.

Lemaitre, N., Planquette, H., Dehairs, F., Planchon, F., Sarthou, G., Gallinari, M., Roig, S., Jeandel, C., Castrillejo, M., 2020. Particulate Trace Element Export in the North Atlantic (GEOTRACES GA01 Transect, GEOVIDE Cruise). ACS Earth Space Chem. 4, 2185–2204. https://doi.org/10.1021/acsearthspacechem.0c00045.

Lemaitre, N., Planquette, H., Dehairs, F., van der Merwe, P., Bowie, A.R., Trull, T.W., Laurenceau-Cornec, E.C., Davies, D., Bollinger, C., Le Goff, M., Grossteffan, E., Planchon, F., 2016. Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes. Deep Sea Research Part I: Oceanographic Research Papers 117, 11–27. https://doi.org/10.1016/j.dsr.2016.09.002.

Resplandy, L., Martin, A.P., Le Moigne, F., Martin, P., Aquilina, A., Mémery, L., Lévy, M., Sanders, R., 2012. How does dynamical spatial variability impact 234Th-derived estimates of organic export? Deep Sea Research Part I: Oceanographic Research Papers 68, 24–45. https://doi.org/10.1016/j.dsr.2012.05.015.

Roca-Martí, M., Puigcorbé, V., Iversen, M.H., van der Loeff, M.R., Klaas, C., Cheah, W., Bracher, A., Masqué, P., 2017. High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean. Deep Sea Research Part II: Topical Studies in Oceanography 138, 102–115. https://doi.org/10.1016/j.dsr2.2015.12.007.

Savoye, N., Benitez-Nelson, C., Burd, A.B., Cochran, J.K., Charette, M., Buesseler, K.O., Jackson, G.A., Roy-Barman, M., Schmidt, S., Elskens, M., 2006. 234Th sorption and export models in the water column: A review. Marine Chemistry 100, 234–249.

Siegel, D.A., Buesseler, K.O., Behrenfeld, M.J., Benitez-Nelson, C.R., Boss, E., Brzezinski, M.A., Burd, A., Carlson, C.A., D’Asaro, E.A., Doney, S.C., 2016. Prediction of the export and fate of global ocean net primary production: the EXPORTS science plan. Frontiers in Marine Science 3, 22.

Stukel, M.R., Asher, E., Couto, N., Schofield, O., Strebel, S., Tortell, P., Ducklow, H.W., 2015. The imbalance of new and export production in the western Antarctic Peninsula, a potentially “leaky” ecosystem. Global Biogeochemical Cycles 29, 1400–1420. https://doi.org/10.1002/2015GB005211.