the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Apr 2025
Evaluation of the Particulate Inorganic Carbon Export Efficiency in the Global Ocean
Abstract. The oceanic carbonate pump corresponds to the production and the sinking of particulate inorganic carbon (PIC) thanks to calcified planktonic organisms. In this study, the global PIC production from ocean colour satellite observations were combined with PIC flux observation from short-term sediment traps deployed during the last decades covering the global ocean. Coccolithophores are the main planktonic calcified group in the euphotic zone, with an important phenological blooming pattern and an important latitude dependant seasonal response. The present study highlights that the PIC production in the euphotic zone and the pelagic PIC flux varied among oceanic regions, depth and season. Based on a geographic matchup between the PIC flux from sediment traps and remote sensing climatology observation, correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at global scale, but only for delimited ocean basin such as in the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC/POC ratios in the euphotic zone, temperate and subpolar areas are more efficient to export PIC compared to equatorial and subtropical areas (higher PIC production and PIC/POC ratios in the euphotic zone). The plankton phenology seems to be an important driver of PIC export efficiency (PEeff) and PIC transfer efficiency (Teff). Pelagic PIC dissolution is assumed to be responsible for the decrease in PEeff and Teff. In a context of climate change, any modification of plankton network community as well as modification PIC Teff is expected to have consequences on surface alkalinity balance and CO2 exchange between ocean and atmosphere. This study suggests that the 'packaging factor' corresponding to the vehicle of the biological carbon pump (marine snow aggregates, fecal pellets) and the plankton network (e.g. zooplankton community, microbial loop) may affect the PIC export efficiency and the PIC transfer efficiency.
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RC1: 'Comment on egusphere-2025-1108', Anonymous Referee #1, 05 Jun 2025
General Comments
The article by Toullec couples satellite measurements of PIC production, based on PIC concentration and coccolithophore physiological information, with deep-sea sediment trap PIC fluxes. The linkage, or lack of linkage in some regions, is then explored in terms of ecosystem structure, driving factors, and relationships between NPP and POC fluxes. Exploration of the deep PIC fluxes globally and regionally provide some interesting new insights into the fate of sinking PIC, however linking these fluxes with ocean color measurements should be treated with caution.
Fundamentally the article assumes that the PIC measured by satellites and that collected in the deep ocean are the same – largely ignoring that while satellite PIC may measure coccolithophore dynamics, it does not provide any information on the other known pelagic calcifiers (foraminifera, pteropods) who may contribute to export. What proportion of the deep fluxes originate from coccolithophores, foraminifera and pteropods? While coccolithophore dominance of PIC production may make sense in terms of their short generational times relative to the larger (>100 um) calcifiers, foraminifera and pteropods, these organisms are well known to be present in sediment traps and deep sea sediments. This is a major issue for the current study – this is an incomplete comparison of PIC production and PIC export, meaning that a comparison of export efficiency is not comparing like with like (but noting that transfer efficiency is a valid comparison as it only relies on the deep fluxes). While the author mentions the relative importance of other pelagic calcifiers in terms of PIC production, there is no discussion of their relative importance to export. As this is a fundamental assumption to the paper, it makes interpretation of the other relationships presented in the paper (e.g., to the modelling work of Nowicki et al., 2022) questionable.
There are a high number of editorial mistakes in the article, where spelling or syntax errors cause lines to be unclear and confusing, and several parts of the paper need better explanation. Further, there are several sections of the discussion (4.2, 4.3, 4.4.1, 4.4.2) that contain lots of information and interesting literature, but lack any direct link to the results presented. Other sections have only limited links to the results presented leaving the reader well informed of the subject but with little clear insights into how this relates to that which is presented.
Specific Comments
Ln 19, what is meant by “plankton network community”?
Ln 26, how would atmospheric CO2 concentration be twice as important? Does the author mean twice as high?
Ln 27, Photosynthesis does not ‘uptake CO2 from the atmosphere’ – the source of C is dissolved in seawater.
Ln 33, As phrased here, this sentence makes no sense – the C in CaCO3 (inorganic C) is not equivalent to POC (organic C).
Ln 35, ‘are’ rather than ‘were’.
Ln 37, What does the author mean by ‘period is optimal’?
Lns 47-19, Definitions of export efficiency and transfer efficiency are identical here but are used throughout to mean different things (and are incorrect relative to the literature, e.g., Henson et al., 2012). This makes the article very confusing. Also, does the author mean PEeff only for POC (POC flux/NPP) or for both POC and PIC (PIC flux/PIC production)? The meaning of the terms change in the paper, causing further confusion (see next comment).
Ln 53, Please rewrite and/or explain better ‘CaCO3 incorporation into aggregates and fecal pellets support the idea that high PEeff could be coupled with high CaCO3 flux’. It is not clear what the point that the author is making is here. Does PEeff refer only to POC in this instance or to PIC?
Ln 55, ‘is’ is missing from ‘and hence expected’.
Ln 62, Is ‘the packaging factor’ theory the correct terminology to be used?
Figure 1 – This is not quantitative in any way and does not fully make any sense? The arrows on the far right imply latitudinal variability in Teff and PEeff and sinking POC lability. Also, where is lability of sinking POC discussed in the introduction?
Ln 75, Please change ‘Emiliania huxleyi’ to ‘Gephyrocapsa huxleyi’ after Bendif et al. (2023).
Ln 78, Please explain and cite relevant literature for ‘There remains a gap between the amount of photosynthetically produced organic carbon, and it transferred fraction to the deep.’
Ln 79, Please explain and cite relevant literature to support ‘Nowadays, heterotrophic respiration in sinking aggregates is considered to dissolve CaCO3 particles in the upper ocean’.
Ln 81-82, Please rewrite and explain better ‘Without upper ocean CaCO3 dissolution, the ocean output 20% more CO2 to the atmosphere through low-latitude upwelling regions’.
Ln 85, Please rewrite and explain better ‘which holds the uptake of atmospheric carbon and acidification in surface waters.’
Ln 94, More details are needed on the ‘physiological constant associated with’ (Emiliania) Gephyrocapsa huxleyi.
Ln 122, It would be helpful to avoid confusion and explain how PIC concentrations and PIC production were put into the same units.
Ln 179, Rough sentence which seems to be missing context ’54 tations were out of the RECCAP2 mask and then have been removed from the 6057 PIC flux observations subset.’ Where does this belong? Text above or Figure legend.
Ln 181, ‘ocean colour’ observations rather than ‘ocean colours’.
Ln 243, This is a (very) late point to introduce a new dataset, which then becomes important to the rest of the paper (e.g., Figure 6b) – it’s also introduced in very little detail. To make this part of the article, much more information needs to be introduced throughout.
Ln 255-257, This line (about diatoms and coccolithophores dominating productive areas) appears to contradict with much of the paper and the themes explored. This paradox needs some explanation.
Ln 260, Different (and correct) definition of transfer efficiency to that given earlier in the article.
Ln 271, ‘sinking fecal pellets’ rather than ‘singing’.
Sections 4.2 and 4.3 – how does this section relate to the results presented? No link is made to the article. Also, sections 4.4.1 and 4.4.2 include no links to the present study.
References
Bendif et al. (2023) Rapid diversification underlying the global dominance of a cosmopolitan phytoplankton. ISME J 17, doi: https://doi.org/10.1038/s41396-023-01365-5
Citation: https://doi.org/10.5194/egusphere-2025-1108-RC1 - AC1: 'Reply on RC1', Jordan Toullec, 12 Aug 2025
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RC2: 'Comment on egusphere-2025-1108', Anonymous Referee #2, 04 Aug 2025
This study investigates the factors influencing the differences between particulate inorganic carbon (PIC) export efficiency and PIC transfer efficiency across the global ocean. By integrating global PIC production estimates derived from satellite observations with PIC flux measurements from sediment traps, the author assesses spatial and seasonal variability and correlations in these metrics. Notably, the study finds no strong global correlation between PIC production and PIC flux, except in two key regions: the North Atlantic and the Southern Ocean. The author also introduces the concept of a divide between high- and low-latitude regimes, suggesting that temperate and subpolar regions exhibit higher PIC export efficiency compared to equatorial and subtropical areas.
The topic is timely and important, addressing the ocean carbonate pump, a key component of the global carbon cycle that remains poorly quantified. Exploring regional and seasonal variations in PIC dynamics, especially at different depths, offers valuable insights into carbon export processes and their controls.
However, the current manuscript falls short in several areas that must be addressed prior to publication:
- Introduction: The literature review lacks recent references and fails to clearly frame the current study within the context of recent advancements. A more thorough and up-to-date overview of the state of knowledge is needed to establish the relevance and novelty of the research.
- Results: The presentation of the results is at times unclear, making it difficult to follow the core findings. Clearer structure, better use of figures, and more direct links between observations and interpretations would significantly improve readability.
- Discussion: The discussion lacks specificity. It should better connect the results to the broader implications for the carbonate pump and global carbon cycling. Currently, the discussion reads as disconnected and somewhat speculative without robust support from the data.
Overall, the study has the potential to contribute meaningful insights to our understanding of oceanic carbon export mechanisms. However, I strongly recommend substantial revisions to improve clarity, coherence, and scientific rigour before it can be considered for publication.
Broader comments:
- The introduction should better explain the concept of the ballast effect. Currently, L33-34 hint at the effect of the carbonate pump of the soft tissue pump without explaining that PIC is a dense mineral, potentially increasing the sinking velocity of POM and thus carbon storage. Please, also explain that this is still a debated effect with evidence for and against it.
- The introduction needs to use more up-to-date literature and better justify the aim of the study.
- The concepts presented in Figure 1 (and associated text) do not rely on current knowledge. It also does not match with Figure 3b.
- The key results are not clearly highlighted and described with enough details in the results section. The discussion provides an interesting overview of the literature, but with no clear link to the results of the paper, which makes it challenging to see its use.
Specific comments:
L28: Replace “In other hand” with “On the other hand”
L32-34: Misconception that “CaCO3 is composed of carbon” and thus transport carbon to the deep ocean. This is in line with my general comment about the ballast effect. CaCO3 sinking has the effect of transporting alkalinity from the ocean surface to the deep and mediates POC settling via the ballast effect. Please, can you amend accordingly?
L36-37: Add Neukermans et al. (2023) for a reference directly comparing Coccolithophores with other planktonic calcifiers contribution to PIC.
Neukermans, G., Bach, L. T., Butterley, A., Sun, Q., Claustre, H., & Fournier, G. R. (2023). Quantitative and mechanistic understanding of the open ocean carbonate pump-perspectives for remote sensing and autonomous in situ observation. Earth-Science Reviews, 239, 104359.
L38-39: The statement that coccolithophores “uptake more carbon in high-light, stratified, and low-nutrient surface waters (Balch et al., 2011; Krumhardt et al., 2017)”, which implies that coccolithophores preferentially grow in subtropical gyres, does not match common understanding and the used citations here. Both references (and others) highlight high coccolithophore PIC in the northern part of the Southern Ocean (referred to as the “Great Calcite Belt”), which is due to Emiliania huxleyi’s blooming characteristics. Same for the statement “In low latitudes, which are CaCO3-productive regions” (L60).
L41: Please use more recent literature, which you can find in Krumhardt et al. (2019).
Krumhardt, K. M., Lovenduski, N. S., Long, M. C., Lévy, M., Lindsay, K., Moore, J. K., & Nissen, C. (2019). Coccolithophore growth and calcification in an acidified ocean: Insights from community Earth system model simulations. Journal of Advances in Modeling Earth Systems, 11(5), 1418-1437.
L43-44: Same, include more recent literature: Kwon et al. (2024)
Kwon, E. Y., Dunne, J. P., & Lee, K. (2024). Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity. Science Advances, 10(13), eadl0779.
L44-45: Refer to Dean et al. (2024)
Dean, C. L., Harvey, E. L., Johnson, M. D., & Subhas, A. V. (2024). Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle. Science Advances, 10(45), eadr5453.
L46: Consider adding a reference to Neukermans et al. (2023) here, too.
L47-50: The idea that low latitudes have low export efficiency and high transfer efficiency is quite debatable since Henson et al. (2012), because satellite estimates have large uncertainties (Henson et al., 2019; Ryan-Keogh et al., 2023; Weber et al., 2016).
Henson, S., Le Moigne, F., & Giering, S. (2019). Drivers of carbon export efficiency in the global ocean. Global biogeochemical cycles, 33(7), 891-903.
Ryan-Keogh, T. J., Thomalla, S. J., Chang, N., & Moalusi, T. (2023). A new global oceanic multi-model net primary productivity data product. Earth System Science Data, 15(11), 4829-4848.
Weber, T., Cram, J. A., Leung, S. W., DeVries, T., & Deutsch, C. (2016). Deep ocean nutrients imply large latitudinal variation in particle transfer efficiency. Proceedings of the National Academy of Sciences, 113(31), 8606-8611.
L51: Please include more recent literature than Henson et al. (2012), Honjo (1976) and Pilskaln and Honjo (1987).
L62-66: Try to link this paragraph with the ballast effect and check for more recent literature on the higher correlation of Teff with carbonate flux. See the comprehensive review in section 3.4 in Neukermans et al. (2023).
L79-80: Please add reference (Kwon et al., 2024) to the statement “heterotrophic respiration … dissolve CaCO3 particles”.
L179: It looks like a half sentence. Can you double check there is no formatting issue here?
L202-203: And also along the Great Calcite Belt in the Southern Ocean and in the North Atlantic, but not in the Northern Indian Ocean.
Figure 3: Not clear what the seasonal bias is and how it is calculated. Same for the residence time. Can you add some text to the methodology?
Figure 5: Can you comment more on the trends presented in this figure in the main text? Currently, there is no description and interpretation of how correlation fluctuates between the regions and why.
L241: Define “SB”.
L242-246 and Figure 6: It is not clear why you are presenting the seasonal bias and the contributions to export from aggregates and faecal pellets. Can you add more explanations?
L262-263: Can you relate more PIC transfer efficiency to POC transfer efficiency here? You present them both but make no link so the last sentence hangs without connection with what it said before.
L268-269 (Section 4.1): I struggle here with the statement that the ballast effect is not related to ecosystem structure. On the contrary, I would argue that the ballast effect results from ecosystem structure as it depends on the synergy of biological processes, such as PIC production by pelagic calcifiers, surface dissolution due to respiration and packaging of CaCO3 with organic matter either due to grazing and fecal pellets or aggregation.
L270: “high CaCO3 productive systems (Subtropics)” is misleading. Francois et al. (2002) refer to the subtropics as CaCO3-dominated systems in the context of opal versus calcium carbonate ballast regimes. This is a subtle difference but key one as again and also underdeveloped the view that coccolithophores current understanding presents them to be higher in the subpolar regions. Also, here, the point made about Le Moigne et al. (2014) is not consistent with Francois et al. (2002), highlighting that subtropics export flux is not associated with mineral ballast. Can you address these points?
L277-280: These look like important results of your study that aren’t detailed in the results section. Can you expand on these in the results section? This also relates to my comments on Figure 5.
Sections 4.2, 4.3, 4.4.1, 4.4.2, 4.4.3.1 (L362-380), 4.4.3.2 do not even mention the results, so it is hard to relate to it. Can you spell the links more clearly?
L314-315: Again, this seems like an important result “PIC export flux and deeper flux … are globally lower in these regions (despite higher PIC production)”. Can you expand on this in your results section, especially in contrasting PIC Teff with PIC production?
L320-328: Please introduce here what you are trying to show: an explanation for the decoupling between PIC Teff and PIC production. Not easy to guess.
L400: Not clear what “This study” refers to.
Figure 7 shows an interesting pattern, but needs to be much better integrated with the results of the study.
Citation: https://doi.org/10.5194/egusphere-2025-1108-RC2 - AC2: 'Reply on RC2', Jordan Toullec, 12 Aug 2025
Status: closed
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RC1: 'Comment on egusphere-2025-1108', Anonymous Referee #1, 05 Jun 2025
General Comments
The article by Toullec couples satellite measurements of PIC production, based on PIC concentration and coccolithophore physiological information, with deep-sea sediment trap PIC fluxes. The linkage, or lack of linkage in some regions, is then explored in terms of ecosystem structure, driving factors, and relationships between NPP and POC fluxes. Exploration of the deep PIC fluxes globally and regionally provide some interesting new insights into the fate of sinking PIC, however linking these fluxes with ocean color measurements should be treated with caution.
Fundamentally the article assumes that the PIC measured by satellites and that collected in the deep ocean are the same – largely ignoring that while satellite PIC may measure coccolithophore dynamics, it does not provide any information on the other known pelagic calcifiers (foraminifera, pteropods) who may contribute to export. What proportion of the deep fluxes originate from coccolithophores, foraminifera and pteropods? While coccolithophore dominance of PIC production may make sense in terms of their short generational times relative to the larger (>100 um) calcifiers, foraminifera and pteropods, these organisms are well known to be present in sediment traps and deep sea sediments. This is a major issue for the current study – this is an incomplete comparison of PIC production and PIC export, meaning that a comparison of export efficiency is not comparing like with like (but noting that transfer efficiency is a valid comparison as it only relies on the deep fluxes). While the author mentions the relative importance of other pelagic calcifiers in terms of PIC production, there is no discussion of their relative importance to export. As this is a fundamental assumption to the paper, it makes interpretation of the other relationships presented in the paper (e.g., to the modelling work of Nowicki et al., 2022) questionable.
There are a high number of editorial mistakes in the article, where spelling or syntax errors cause lines to be unclear and confusing, and several parts of the paper need better explanation. Further, there are several sections of the discussion (4.2, 4.3, 4.4.1, 4.4.2) that contain lots of information and interesting literature, but lack any direct link to the results presented. Other sections have only limited links to the results presented leaving the reader well informed of the subject but with little clear insights into how this relates to that which is presented.
Specific Comments
Ln 19, what is meant by “plankton network community”?
Ln 26, how would atmospheric CO2 concentration be twice as important? Does the author mean twice as high?
Ln 27, Photosynthesis does not ‘uptake CO2 from the atmosphere’ – the source of C is dissolved in seawater.
Ln 33, As phrased here, this sentence makes no sense – the C in CaCO3 (inorganic C) is not equivalent to POC (organic C).
Ln 35, ‘are’ rather than ‘were’.
Ln 37, What does the author mean by ‘period is optimal’?
Lns 47-19, Definitions of export efficiency and transfer efficiency are identical here but are used throughout to mean different things (and are incorrect relative to the literature, e.g., Henson et al., 2012). This makes the article very confusing. Also, does the author mean PEeff only for POC (POC flux/NPP) or for both POC and PIC (PIC flux/PIC production)? The meaning of the terms change in the paper, causing further confusion (see next comment).
Ln 53, Please rewrite and/or explain better ‘CaCO3 incorporation into aggregates and fecal pellets support the idea that high PEeff could be coupled with high CaCO3 flux’. It is not clear what the point that the author is making is here. Does PEeff refer only to POC in this instance or to PIC?
Ln 55, ‘is’ is missing from ‘and hence expected’.
Ln 62, Is ‘the packaging factor’ theory the correct terminology to be used?
Figure 1 – This is not quantitative in any way and does not fully make any sense? The arrows on the far right imply latitudinal variability in Teff and PEeff and sinking POC lability. Also, where is lability of sinking POC discussed in the introduction?
Ln 75, Please change ‘Emiliania huxleyi’ to ‘Gephyrocapsa huxleyi’ after Bendif et al. (2023).
Ln 78, Please explain and cite relevant literature for ‘There remains a gap between the amount of photosynthetically produced organic carbon, and it transferred fraction to the deep.’
Ln 79, Please explain and cite relevant literature to support ‘Nowadays, heterotrophic respiration in sinking aggregates is considered to dissolve CaCO3 particles in the upper ocean’.
Ln 81-82, Please rewrite and explain better ‘Without upper ocean CaCO3 dissolution, the ocean output 20% more CO2 to the atmosphere through low-latitude upwelling regions’.
Ln 85, Please rewrite and explain better ‘which holds the uptake of atmospheric carbon and acidification in surface waters.’
Ln 94, More details are needed on the ‘physiological constant associated with’ (Emiliania) Gephyrocapsa huxleyi.
Ln 122, It would be helpful to avoid confusion and explain how PIC concentrations and PIC production were put into the same units.
Ln 179, Rough sentence which seems to be missing context ’54 tations were out of the RECCAP2 mask and then have been removed from the 6057 PIC flux observations subset.’ Where does this belong? Text above or Figure legend.
Ln 181, ‘ocean colour’ observations rather than ‘ocean colours’.
Ln 243, This is a (very) late point to introduce a new dataset, which then becomes important to the rest of the paper (e.g., Figure 6b) – it’s also introduced in very little detail. To make this part of the article, much more information needs to be introduced throughout.
Ln 255-257, This line (about diatoms and coccolithophores dominating productive areas) appears to contradict with much of the paper and the themes explored. This paradox needs some explanation.
Ln 260, Different (and correct) definition of transfer efficiency to that given earlier in the article.
Ln 271, ‘sinking fecal pellets’ rather than ‘singing’.
Sections 4.2 and 4.3 – how does this section relate to the results presented? No link is made to the article. Also, sections 4.4.1 and 4.4.2 include no links to the present study.
References
Bendif et al. (2023) Rapid diversification underlying the global dominance of a cosmopolitan phytoplankton. ISME J 17, doi: https://doi.org/10.1038/s41396-023-01365-5
Citation: https://doi.org/10.5194/egusphere-2025-1108-RC1 - AC1: 'Reply on RC1', Jordan Toullec, 12 Aug 2025
-
RC2: 'Comment on egusphere-2025-1108', Anonymous Referee #2, 04 Aug 2025
This study investigates the factors influencing the differences between particulate inorganic carbon (PIC) export efficiency and PIC transfer efficiency across the global ocean. By integrating global PIC production estimates derived from satellite observations with PIC flux measurements from sediment traps, the author assesses spatial and seasonal variability and correlations in these metrics. Notably, the study finds no strong global correlation between PIC production and PIC flux, except in two key regions: the North Atlantic and the Southern Ocean. The author also introduces the concept of a divide between high- and low-latitude regimes, suggesting that temperate and subpolar regions exhibit higher PIC export efficiency compared to equatorial and subtropical areas.
The topic is timely and important, addressing the ocean carbonate pump, a key component of the global carbon cycle that remains poorly quantified. Exploring regional and seasonal variations in PIC dynamics, especially at different depths, offers valuable insights into carbon export processes and their controls.
However, the current manuscript falls short in several areas that must be addressed prior to publication:
- Introduction: The literature review lacks recent references and fails to clearly frame the current study within the context of recent advancements. A more thorough and up-to-date overview of the state of knowledge is needed to establish the relevance and novelty of the research.
- Results: The presentation of the results is at times unclear, making it difficult to follow the core findings. Clearer structure, better use of figures, and more direct links between observations and interpretations would significantly improve readability.
- Discussion: The discussion lacks specificity. It should better connect the results to the broader implications for the carbonate pump and global carbon cycling. Currently, the discussion reads as disconnected and somewhat speculative without robust support from the data.
Overall, the study has the potential to contribute meaningful insights to our understanding of oceanic carbon export mechanisms. However, I strongly recommend substantial revisions to improve clarity, coherence, and scientific rigour before it can be considered for publication.
Broader comments:
- The introduction should better explain the concept of the ballast effect. Currently, L33-34 hint at the effect of the carbonate pump of the soft tissue pump without explaining that PIC is a dense mineral, potentially increasing the sinking velocity of POM and thus carbon storage. Please, also explain that this is still a debated effect with evidence for and against it.
- The introduction needs to use more up-to-date literature and better justify the aim of the study.
- The concepts presented in Figure 1 (and associated text) do not rely on current knowledge. It also does not match with Figure 3b.
- The key results are not clearly highlighted and described with enough details in the results section. The discussion provides an interesting overview of the literature, but with no clear link to the results of the paper, which makes it challenging to see its use.
Specific comments:
L28: Replace “In other hand” with “On the other hand”
L32-34: Misconception that “CaCO3 is composed of carbon” and thus transport carbon to the deep ocean. This is in line with my general comment about the ballast effect. CaCO3 sinking has the effect of transporting alkalinity from the ocean surface to the deep and mediates POC settling via the ballast effect. Please, can you amend accordingly?
L36-37: Add Neukermans et al. (2023) for a reference directly comparing Coccolithophores with other planktonic calcifiers contribution to PIC.
Neukermans, G., Bach, L. T., Butterley, A., Sun, Q., Claustre, H., & Fournier, G. R. (2023). Quantitative and mechanistic understanding of the open ocean carbonate pump-perspectives for remote sensing and autonomous in situ observation. Earth-Science Reviews, 239, 104359.
L38-39: The statement that coccolithophores “uptake more carbon in high-light, stratified, and low-nutrient surface waters (Balch et al., 2011; Krumhardt et al., 2017)”, which implies that coccolithophores preferentially grow in subtropical gyres, does not match common understanding and the used citations here. Both references (and others) highlight high coccolithophore PIC in the northern part of the Southern Ocean (referred to as the “Great Calcite Belt”), which is due to Emiliania huxleyi’s blooming characteristics. Same for the statement “In low latitudes, which are CaCO3-productive regions” (L60).
L41: Please use more recent literature, which you can find in Krumhardt et al. (2019).
Krumhardt, K. M., Lovenduski, N. S., Long, M. C., Lévy, M., Lindsay, K., Moore, J. K., & Nissen, C. (2019). Coccolithophore growth and calcification in an acidified ocean: Insights from community Earth system model simulations. Journal of Advances in Modeling Earth Systems, 11(5), 1418-1437.
L43-44: Same, include more recent literature: Kwon et al. (2024)
Kwon, E. Y., Dunne, J. P., & Lee, K. (2024). Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity. Science Advances, 10(13), eadl0779.
L44-45: Refer to Dean et al. (2024)
Dean, C. L., Harvey, E. L., Johnson, M. D., & Subhas, A. V. (2024). Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle. Science Advances, 10(45), eadr5453.
L46: Consider adding a reference to Neukermans et al. (2023) here, too.
L47-50: The idea that low latitudes have low export efficiency and high transfer efficiency is quite debatable since Henson et al. (2012), because satellite estimates have large uncertainties (Henson et al., 2019; Ryan-Keogh et al., 2023; Weber et al., 2016).
Henson, S., Le Moigne, F., & Giering, S. (2019). Drivers of carbon export efficiency in the global ocean. Global biogeochemical cycles, 33(7), 891-903.
Ryan-Keogh, T. J., Thomalla, S. J., Chang, N., & Moalusi, T. (2023). A new global oceanic multi-model net primary productivity data product. Earth System Science Data, 15(11), 4829-4848.
Weber, T., Cram, J. A., Leung, S. W., DeVries, T., & Deutsch, C. (2016). Deep ocean nutrients imply large latitudinal variation in particle transfer efficiency. Proceedings of the National Academy of Sciences, 113(31), 8606-8611.
L51: Please include more recent literature than Henson et al. (2012), Honjo (1976) and Pilskaln and Honjo (1987).
L62-66: Try to link this paragraph with the ballast effect and check for more recent literature on the higher correlation of Teff with carbonate flux. See the comprehensive review in section 3.4 in Neukermans et al. (2023).
L79-80: Please add reference (Kwon et al., 2024) to the statement “heterotrophic respiration … dissolve CaCO3 particles”.
L179: It looks like a half sentence. Can you double check there is no formatting issue here?
L202-203: And also along the Great Calcite Belt in the Southern Ocean and in the North Atlantic, but not in the Northern Indian Ocean.
Figure 3: Not clear what the seasonal bias is and how it is calculated. Same for the residence time. Can you add some text to the methodology?
Figure 5: Can you comment more on the trends presented in this figure in the main text? Currently, there is no description and interpretation of how correlation fluctuates between the regions and why.
L241: Define “SB”.
L242-246 and Figure 6: It is not clear why you are presenting the seasonal bias and the contributions to export from aggregates and faecal pellets. Can you add more explanations?
L262-263: Can you relate more PIC transfer efficiency to POC transfer efficiency here? You present them both but make no link so the last sentence hangs without connection with what it said before.
L268-269 (Section 4.1): I struggle here with the statement that the ballast effect is not related to ecosystem structure. On the contrary, I would argue that the ballast effect results from ecosystem structure as it depends on the synergy of biological processes, such as PIC production by pelagic calcifiers, surface dissolution due to respiration and packaging of CaCO3 with organic matter either due to grazing and fecal pellets or aggregation.
L270: “high CaCO3 productive systems (Subtropics)” is misleading. Francois et al. (2002) refer to the subtropics as CaCO3-dominated systems in the context of opal versus calcium carbonate ballast regimes. This is a subtle difference but key one as again and also underdeveloped the view that coccolithophores current understanding presents them to be higher in the subpolar regions. Also, here, the point made about Le Moigne et al. (2014) is not consistent with Francois et al. (2002), highlighting that subtropics export flux is not associated with mineral ballast. Can you address these points?
L277-280: These look like important results of your study that aren’t detailed in the results section. Can you expand on these in the results section? This also relates to my comments on Figure 5.
Sections 4.2, 4.3, 4.4.1, 4.4.2, 4.4.3.1 (L362-380), 4.4.3.2 do not even mention the results, so it is hard to relate to it. Can you spell the links more clearly?
L314-315: Again, this seems like an important result “PIC export flux and deeper flux … are globally lower in these regions (despite higher PIC production)”. Can you expand on this in your results section, especially in contrasting PIC Teff with PIC production?
L320-328: Please introduce here what you are trying to show: an explanation for the decoupling between PIC Teff and PIC production. Not easy to guess.
L400: Not clear what “This study” refers to.
Figure 7 shows an interesting pattern, but needs to be much better integrated with the results of the study.
Citation: https://doi.org/10.5194/egusphere-2025-1108-RC2 - AC2: 'Reply on RC2', Jordan Toullec, 12 Aug 2025
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