the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 May 2024 | Subsequently updated
Water masses in the Atlantic Ocean: water mass ages and ventilation
Abstract. The distribution of oceanic water masses and their properties, such as ventilation constitute fundamental parameters, for instance, the thermohaline circulation patterns and biogeochemical processes in the marine systems. The distributions of main water masses in the Atlantic Ocean have been comprehensively documented in a companion study (Liu and Tanhua, 2021), this study presents quantitative assessments of water mass age characteristics and ventilation time scales through multi-tracer analysis incorporating chlorofluorocarbon-12 (CFC-12), sulfur hexafluoride (SF6), and argon-39 (39Ar). Here we use two distinct age concepts: mean-age as an integrative metric of water mass chronology, and mode-age as a proxy for advective time scales. Empirical results demonstrate systematic age progression with increasing pressure and along water mass trajectories. Surface layer central waters exhibit mean ages up to ~100 years and mode ages reaching ~30 years. In the intermediate layer, meridional age gradients characterize the Antarctic Intermediate Water (AAIW) reaching maximum mean-age (~300 years) and mode-age (~80 years) at 30 °N, whereas zonal variations manifest in Mediterranean Water (MW) with peak values (~400 years in mean-age, ~100 years in mode-age) observed in equatorial regions. As the dominant deep water component, North Atlantic Deep Water (NADW) exhibits extreme ages in the Antarctic Circumpolar Current (ACC) region at 50°S, achieving mean age ~600 years and mode age ~100 years. Bottom layer water masses display their oldest signatures: Antarctic Bottom Water (AABW) from the Weddell Sea reaches ~600 years (mean) and ~100 years (mode) at equatorial latitudes, while its extension, Northeast Atlantic Bottom Water (NEABW), attains exceptional values of ~800 years (mean) and ~120 years (mode) at 50°N. The age analysis reveals significant basin-scale asymmetries, with eastern basins exhibiting younger ages compared to western counterparts. Ventilation efficiency modulates these age distributions, as evidenced by lower mode-ages and reduced apparent oxygen utilization (AOU) in better-ventilated western basins. The calculated oxygen utilization rate (OUR) demonstrates spatial concordance with dissolved oxygen (DO) concentrations, corroborating enhanced oxidative processes in high-oxygen regimes. This integrated age framework provides novel insights into water mass ventilation dynamics and their biogeochemical implications through quantitative characterization of temporal-spatial age distributions across multiple oceanographic provinces.
Key words: Water Mass, Atlantic Ocean, Transient Tracer, Mean- and Mode-age, Ventilation, GLODAPv2 data product
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Version 3 | 06 Aug 2025
RC1: 'Comment on egusphere-2024-1362', Anonymous Referee #1, 05 Sep 2025Comment on
“Water masses in the Atlantic Ocean: water mass ages and ventilation”
submitted by Mian Liu and Toste Tanhua to EGUsphere
Overview:
Mian Liu and Toste Tanhua submitted a manuscript about Atlantic water mass distribution and their (different types of) water mass ages, using transient tracers (the older anthropogenic CFC-12, the younger anthropogenic SF6, and the radioactive 39Ar) and the well-established TTD-method.
They introduce the paper with a short review of Atlantic water masses and their relevance in ocean circulation and for biogeochemical processes, followed by introducing the transient tracers and relevant methods. Moreover, they introduce the oxygen utilization and oxygen utilization rate and their importance in biogeochemical cycles.
Next, they introduce in more detail the application of the TTD method to derive water mass ages, they differentiate different types or definitions of ages and other technical aspects to compute them from the transient tracer data, which they obtained from the GLODAP data repository.
In the large results-section they show and discuss their computed water mass ages along two prominent WOCE/GO-SHIP sections and on maps in various density layers incorporating a large amount of GLODAP tracer data. They do that for several relevant Atlantic water masses, starting from the near surface layer to the deepest Atlantic water layers.
Additionally, they use their previously discussed water mass ages to derive the oxygen utilization rates for the most prominent Atlantic water masses. Finally, they compare the CFC and SF6 based water mass ages with ages derived from the radio nuclide 39Ar and discuss the deviations between the two.
Lastly, they summarize and discuss their findings and interpretation in a conclusion-section.
General comment:
The manuscript is very well written, easy to read and good to understand. The strength of the manuscript is the systematic application of the transient tracer-based TTD method on a Atlantic-wide large data set and the comprehensive discussion of the most relevant Atlantic water masses from surface to bottom. Moreover, the authors apply their findings by calculating and discussing oxygen utilization rates Atlantic-wide. Both might be a good data base for further investigations of biogeochemical cycles in the Atlantic Ocean.
I’d like to recommend publication after minor revision.
For minor revision I have listed a few specific comments (or better suggestions):
General comments:
Since I am a non-native speaker; I do not comment on wording, grammar, spelling, etc. I’d like to leave these issues to the editorial board.
In my pdf all figures are of minor, very blurred quality. I guess, that happened during crating the pdf for the reviewers from the submitted manuscript and submitted figures. So, this comment goes more to the editorial board than to the authors.
Specific comments:
Line 22-23: “eastern basin exhibiting younger ages compared to…” If this is meant in general, it should be rather opposite. Or is this specific for NEABW? Please, clarify.
Line 110-112: “The solubility (F)…” Find a better place in the methods’ section.
Line 113ff: You use the term “diffusive”, as widely common in this context. But wouldn’t it be more logic to use “disperse”. Many people automatically think on “molecular diffusion”, but here something different is meant, which might be better named “dispersion”.
Line 129-130: “Based on the TTD determination…” This sentence stays a little bit alone at this place. Skip or find better location.
Line 140: “the TTD is only a spike.” Moreover, in such a case the Δ becomes infinite small and the IG (Eq. in line 122) formally becomes a δ-function and can be computed the same way, so it is just a special case of the same formalism and not something completely different. Eventually worth to note here.
Line 141: “…underestimated by the tracer-age.” Eventually worth to refer to Sonnerup, 2001, who diels with different “tracer-ages” and the systematic deviation between tracer-ages from old and young or low and high concentrations.
Line 151-152: “The atmospheric concentration…” The sentence is important, bit at this place it stands a bit out of context. Please, find better location in section 3.
Line 152-175: The entire Δ/Γ-section could be shortened or focused. 1. The trouble is not enough tracers to compute Δ and Γ and there is almost no way out. Many authors previously discussed this issue and using Δ/Γ=1 is widely common and accepted. 2. The entire discussion here, if Δ/Γ= 0.6, 0.8, 1.0, 1.2 etc. does not lead to a final conclusion. Moreover, why not 0.1, 0.5, 1, 2, 5? 3. In principle and in theory the authors HAVE TWO tracers (or even three with the 39Ar) and COULD compute both Δ and Γ. But I guess, that might not help, since at least CFC-12 and SF6 do not provide enough independent information. So, keep it short. All tracer people know, why, and all others don’t care.
After line 175:
One would like to know, HOW the Γ is computed from the tracers. I guess a least square fit or something similar. But here is my most urgent question, if the authors do that by using CFC-12 and SF6 individually or at once. It is stated somewhere further below in the manuscript, but it should already be mentioned in the methods.
Which atmospheric data do the authors use? Pleas add a reference.
Line 177-178: In this study … vertical layers…” Could find a better place or, since independent from the following, mage an own paragraph of this sentence.
Line 188ff: As mentioned above, move this to methods-section?
Line 197-200: Move this paragraph to a better location, e.g. together with the sentence in lines 177-178.
Figure 6: Could you add “mean age” and “mode age” into the figure? (Holds for all such figures.)
Figure 7: Limit from 0 to 1000 dbar?
Figure 9: Limits from 0 to 3000 dbar?
Line 290: “over THE ridge”?
Line 359-366: The discussion why to favor the mode age instead the mean age does not convince me. In my understanding, the mode age and the mean age are almost one order of magnitude apart, hence, the OUR would also be almost 1 order of magnitude apart. Thus, an argument is needed, why the one and not the other. As I understand the authors, the mean age is subject to mixing with adjacent water masses. True. But the mode-age is, too. So, this argument does not count. I have also no clue, what is the right argument for favoring the mode-age. I guess it is related to either the “right” order of magnitude OR it has something to do with the integrative effect of the mean-age (the mean age sums all the damn history) and the more advective character of the mode-age.
Line 428-429: If the authors decide to put the technical aspect of how to compute Γ into the method-section, this could also go there. As I understand right, it is just the equation from line 122 with an additional decay factor e-λ/t ? Might be worth to note clearly.
Line 442: “(Fig. 18)” does not exist.
Line 455-465: What I would really enjoy would be a Figure 18 as a property-property plot as CFC/SF6 mean-age versus 39Ar mean-age and CFC/SF6 mode-age versus 39Ar mode-age and a short related discussion of such a figure. I’m sure one could learn something from it.
Line 467: I would put this sentence to the end of this paragraph.
Line 468: Add 39Ar to the bracket.
Line 470: Replace “investigated” by “discussed”?
Line 472: Replace “different parts” by “different fractions” or “different pathways”?
Line 498-505: Here I was puzzled. I can’t put this into the context of the manuscript. I can’t see climate change in the manuscript, since no repeat sections or stations are compared. Numerical simulation methods were not applied here. Please, either skip or clarify in the manuscript’s context.
Citation: https://doi.org/10.5194/egusphere-2024-1362-RC1 -
AC1:
'Reply on RC1', Mian Liu, 06 Feb 2026
Dear Reviewer,
We sincerely thank you for your valuable feedback and meticulous review of our manuscript, "Water masses in the Atlantic Ocean: water mass ages and ventilation." We were very pleased to read your overall positive assessment, particularly your comments that the manuscript is "very well written, easy to read and good to understand," and your recognition of the systematic application of the transient tracer-based TTD method across the Atlantic and the comprehensive discussion of major water masses from surface to bottom. These remarks were greatly encouraging.
We have carefully read and thoroughly considered every specific suggestion you provided and have addressed each point accordingly in the revised manuscript. The revisions and responses to each suggestion have been uploaded in the attachment (the black font represents the original text by the reviewer, and the blue font represents the responses from the authors). Your comments have been instrumental in enhancing the clarity and rigor of our work. The main revisions and considerations include:
Structural and Logical Improvements: Following your suggestions, we have relocated certain passages (e.g., the description of solubility, the mention of atmospheric histories, the paragraph introducing the sections) to improve the logical flow and coherence between the Methods and Results sections.
Clarification and Refinement of Methodology: We have rewritten and condensed the section on the choice of the mixing ratio (Δ/Γ), providing a clearer rationale for adopting the standard value Δ/Γ = 1. Furthermore, we now specify earlier in the Methods section how the mean age (Γ) is calculated, the strategy for using the tracers, and the source of atmospheric histories, thereby increasing the transparency and reproducibility of our methods.
Precision in Terminology and Wording: We have corrected an erroneous statement regarding the east-west basin age comparison, clarified the theoretical relationship of the purely advective case as a specific limit of the TTD model, and added key references (e.g., Sonnerup et al., 2001) where appropriate to support our discussion.
Improvements to Figures and Details: We have added labels for "mean-age" and "mode-age" to the relevant figures as requested, adjusted the depth ranges in some figure panels, and corrected the figure number citations in the text.
Strengthening of Core Arguments: In response to your crucial query regarding our choice of mode age over mean age for calculating the oxygen utilization rate (OUR), we have revisited and rewritten the relevant discussion. We now more clearly explain the differences between the two age concepts in terms of their integrative effect and representation of advective timescales, and how this influences the OUR estimation.
Focusing the Conclusions and Outlook: We have removed the original, inaccurately phrased paragraph that attempted a broader outlook involving direct climate change comparisons and numerical modeling, which was not supported by our study. The conclusion now more tightly focuses on the contribution of our work in providing a crucial baseline dataset for future studies and model validation.
Regarding your valuable suggestion to create a property-property plot comparing CFC/SF₆-based ages with ³⁹Ar-based ages, we were unable to construct a complete cross-section for direct comparison due to the limited spatial coverage of the ³⁹Ar data. However, we have addressed this concern by adding a comparative table and providing a more detailed discussion in the text regarding the applicable temporal ranges and limitations of the different tracers.
We believe the manuscript has been substantially improved through these revisions. We are truly grateful for the time you dedicated and for your highly constructive and expert comments, which were essential for refining this study.
Thank you once again for your support and guidance.
Sincerely,
Mian Liu & Toste Tanhua
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AC1:
'Reply on RC1', Mian Liu, 06 Feb 2026
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RC2:
'Comment on egusphere-2024-1362', Anonymous Referee #2, 17 Nov 2025
Review of “Water masses in the Atlantic Ocean: water mass ages and ventilation” by Mian Liu and Toste Tanhua.
General comments
The study is of the GLODAP data product, in particular transient tracers and dissolved oxygen data are used to estimate water mass ages and ventilation. The transient tracers of CFC-12, SF6 and 39Ar are examined with the assumption of an Inverse Gaussian function in order to find the mean- and mode-age of different water masses. The resulting calculations of age and ventilation of different water masses is outlined in great detail. Further inferences regarding the oxygen utilization rate are outlined with implications for biogeochemistry.
I think that this study could be worthy of publication in EGU Ocean Sciences, once my below comments have been addressed. As further outlined below, the Data and methods section was confusing and needs a strong revision. The figures were not of a high production quality (often too small, low resolution) but could be improved with some effort. Finally, the authors should take more care in stating the limitations of the results and when the conclusions are more speculative. I have tried to note specific examples in the below comments.
On a positive note, I will say that much of the paper was clear and easy to follow (e.g. Introduction and Results sections). While the paper reads a little like a report at times, with results described in detail one after another, I think that this is probably unavoidable due to the nature of the study. Overall, I think it would be beneficial to the community to have the work published, as work on water mass age and ventilation is important to the community.
Major comments
- Data and methods section: I am not an expert in age tracer methods, so I found that this section was very confusing to follow. I suggest to strongly revise this section, keeping in mind to explain the overall methods and reasoning to the reader at each step. For example, the equations (Lines 116-122) were introduced abruptly with little explanation. I was left with basic questions e.g. is width defined as \Delta or \Delta^2? What are the units on the mixing ratio? What is 2-IG? These are just some examples. Some information later in the manuscript (e.g. Lines 352-366) would have been helpful earlier in Section 2.
- Figure quality was low: figures were quite low resolution (at least in the version I had). They were often too small (e.g. Figure 6) and required zooming in to see the details. I also suggest that different colors could be used for the different parameters in Figure 14, in order to more easily distinguish between the parameters in the panels. Some captions could also do with more clarity, and I have tried to note these points in more detail in the comments below. The actual data on the figures was interesting, and I think it would be a relatively straightforward fix to improve the figure quality across the entire paper.
- Being more careful outlining limitations of results and speculation: Some of the content in Section 2.1 and 2.3 outlined some of the main limitations of the results, such as assuming an Inverse Gaussian profile and assuming a mixing ratio of 1. I understand that these assumptions are required in order to get useful results out of the data, but I think that the results and conclusions section could help with re-iterating these limitations. There were also a few times where the wording was a bit strong on speculative statements, for example in the conclusion “The results show that with global warming, the age of the Atlantic water mass shows a certain degree of change.” But my take on the paper was that it was showing the water age and ventilation, rather than changes with climate? So this seems a little speculative and I have tried to note these parts in the comments below.
Specific comments
- Abstract: “The distribution of oceanic water masses and their properties, such as ventilation constitute fundamental parameters, for instance, the thermohaline circulation patterns and biogeochemical processes in the marine systems.” This sentence does not make sense and needs revision.
- Line 19-22: Can remove the brackets around “mean” and “mode” in this sentence.
- Paragraph from Line 45-55: There has been more work recently using ocean models to discuss the age tracer, please consider citing some of these (e.g. Li, Q., England, M. H., Hogg, A. M., Rintoul, S. R., & Morrison, A. K. (2023). Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature, 615, (7954), 841-847).
- End of Introduction: Consider adding a paragraph with a brief outline of the rest of the paper to orientate the reader.
- Data and methods section: See Major Comment 1 (above). Unfortunately, by the end of Section 2 I still did not really understand why mean- and mode-ages are used for different applications. Please strongly revise Section 2, as it is by far the weakest section.
- When G(t) first introduced: Explain physically G(t) and the assumptions going in to G(t). My understanding is that you are assuming the shape of the profile as G(t) and then further assuming the mixing ratio, and either the width of the mixing or \Lambda? Then you have the profile such as in Figure 1b. But it is still not clear to me at this stage what measurement is required for Figure 1b. How does CFC-12=300ppt play into G(t)?
- Line 144-147: “The mean-age is obtained by the average value of all the different aliquots in one water sample by considering the transport time of different pathways (Fig. 1, b). This concept of water mass age is often useful for biogeochemical studies. In other cases, such as when discussing the transport times of water masses from the formation area, the mode-age concept is useful.” This seems directly in contrast with Lines 471-473: “The mean-age, which shows the average value of all the different parts in one water sample, is used to show the static distribution of water masses. The mode age, which shows the age of the dominant water mass in the sample, is used to trace the biogeochemical phenomena.”
- Figure 1: How does the description of (a) in the caption match with the figure in panel (a)? Text on the figure is very small also.
- Figure 2: Very small text and the vertical axis line was not present on panel (a).
- Figure 3: Low resolution and tricky to make out the results as the panels are small. What are the green lines? Please also state the units of age either on the figures or in the caption. Also, it would help to use the same mixing increments as in Figure 2(b) i.e. 0.6, 1.0 and 1.4. This would connect Figures 2 and 3 better.
- Line 156: “ration” -> “ratio”
- End of Section 2: The justification of mixing ratio of 1 is not particularly strong, as it does seem like a value that could change a lot regionally depending on physical processes in the ocean. I agree that the authors are limited here so a mixing ratio of 1 is probably the right choice, but it would help to highlight that this is a limitation of the study.
- Table 1 caption: Consider simplifying the text to something like: “Summary of the selected hydrographic cruises to the Atlantic Ocean chosen for the oceanographic sections in this study”.
- Figure 4: What are the green contour lines on the figures?
- Section 3.1: This sub-section starts with a long paragraph. Consider splitting it into two paragraphs for ease of reading.
- Figure 6: plots are small. Please make them bigger.
- Line 230: “… section through in the distribution …” Please remove “in”
- Line 235-236: “In principle, the AAIW is supposed to get higher ages towards the north.” Consider explaining why this is expected.
- Line 239-240: “The reason for the above result is the intervention from surrounding water masses.” I find this sentence strongly worded for this more speculative result. Other reasons for the result could be that the IG function is not a good assumption, or that mixing ratio really does change across the ocean.
- Line 240-241: “The maximum distance of AAIW to the north can reach 30 °N, but between 241 20 °N and 30 °N, this water mass mixes with the ENACW and upper NADW…” Consider saying “we speculate that this water mass mixes…” or “this water mass likely mixes…”
- Paragraph of Lines 253-262: Again, I find much of this discussion on mixing as speculative. Please consider mentioning this or weakening the statements a little.
- Figures 8, 10, 14: Panels are small, please make them bigger.
- Figure 14: AOU top panel. What does it mean for this value to go negative? And how does this influence the OUR result?
- Figure 15: Caption does not appear to line up with shown panels.
- Figure 16, 17: “pentagrams” symbols would more commonly be called “stars”.
- Figure 16,17 caption: “The solid isolines show the 50% fractions of water masses and the dashed lines show the 20% fractions.” Is this consistent with the lines shown in Figure 15?
- Line 442: Refers to Figure 18. Is this figure missing?
- Line 467-468: “As the continuous work…” perhaps replace with “In the present work…” ?
- Conclusions: “Three different definitions of water mass ages are investigated. The tracer-age assumes the ocean as a totally advective situation without diffusion and underestimates the actual age in the realistic ocean.” Tracer-age is briefly discussed in the manuscript, but no figures or results are shown of it, so I would disagree that it has been investigated here. I suggest to just be careful of the wording here.
- Final two paragraphs of the conclusions: Talk about effects of climate change, but (as far as I can see) this is not really touched on in the actual results of this paper. So perhaps it is a bit speculative? It would also help to remind the reader of the limitations of the study in the conclusions.
Citation: https://doi.org/10.5194/egusphere-2024-1362-RC2 -
AC2:
'Reply on RC2', Mian Liu, 06 Feb 2026
Dear Reviewer RC2,
We sincerely thank you for your thorough and constructive review of our manuscript, "Water masses in the Atlantic Ocean: water mass ages and ventilation," and for your positive assessment of its potential contribution to the field. We greatly appreciate the time and effort you dedicated to providing detailed feedback, which has been instrumental in improving the quality of our work.
We have carefully read and considered each of your suggestions. In response, we have undertaken a comprehensive revision of the manuscript. The key modifications, which directly address your major and specific comments, are summarized below:
Complete Revision of the Data and Methods Section (Section 2): We have entirely restructured and rewritten this section to enhance clarity and logical flow. This includes a clearer introduction of concepts and equations, explicit definitions of key parameters, and the addition of a new dedicated subsection (2.2) that explains the definitions and distinct applications of tracer-age, mean-age, and mode-age early in the methodology.
Significant Improvement of Figure Quality: All figures have been revised to ensure high resolution, readability, and improved labeling. We have increased panel sizes, optimized color schemes (e.g., in Figure 14), enhanced captions, and replaced low-resolution elements with vector graphics where applicable.
Clarification of Limitations and Speculative Statements: We have tempered overly strong or speculative language throughout, particularly regarding mixing processes. A dedicated discussion of methodological limitations (e.g., the fixed mixing ratio, steady-state assumption) has been added to the Conclusions (Section 6). Speculative statements about climate change impacts, which were beyond the scope of our present study, have been removed.
Furthermore, we have addressed all your specific comments point-by-point, including corrections to terminology, figure captions, in-text references, and structural suggestions (e.g., splitting long paragraphs, adding an outline to the Introduction).
A detailed, point-by-point response to each comment is provided in the attached document. We believe the revised manuscript is now clearer, more rigorous, and better aligned with the standards of the journal.
Thank you again for your valuable insights and guidance. We hope the revised version meets with your approval.
Sincerely,
Mian Liu & Toste Tanhua
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Version 2 | 10 Jul 2025
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Version 1 | 23 May 2024
Peer review completion
Interactive discussion
Status: closed
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Version 3 | 06 Aug 2025
RC1: 'Comment on egusphere-2024-1362', Anonymous Referee #1, 05 Sep 2025Comment on
“Water masses in the Atlantic Ocean: water mass ages and ventilation”
submitted by Mian Liu and Toste Tanhua to EGUsphere
Overview:
Mian Liu and Toste Tanhua submitted a manuscript about Atlantic water mass distribution and their (different types of) water mass ages, using transient tracers (the older anthropogenic CFC-12, the younger anthropogenic SF6, and the radioactive 39Ar) and the well-established TTD-method.
They introduce the paper with a short review of Atlantic water masses and their relevance in ocean circulation and for biogeochemical processes, followed by introducing the transient tracers and relevant methods. Moreover, they introduce the oxygen utilization and oxygen utilization rate and their importance in biogeochemical cycles.
Next, they introduce in more detail the application of the TTD method to derive water mass ages, they differentiate different types or definitions of ages and other technical aspects to compute them from the transient tracer data, which they obtained from the GLODAP data repository.
In the large results-section they show and discuss their computed water mass ages along two prominent WOCE/GO-SHIP sections and on maps in various density layers incorporating a large amount of GLODAP tracer data. They do that for several relevant Atlantic water masses, starting from the near surface layer to the deepest Atlantic water layers.
Additionally, they use their previously discussed water mass ages to derive the oxygen utilization rates for the most prominent Atlantic water masses. Finally, they compare the CFC and SF6 based water mass ages with ages derived from the radio nuclide 39Ar and discuss the deviations between the two.
Lastly, they summarize and discuss their findings and interpretation in a conclusion-section.
General comment:
The manuscript is very well written, easy to read and good to understand. The strength of the manuscript is the systematic application of the transient tracer-based TTD method on a Atlantic-wide large data set and the comprehensive discussion of the most relevant Atlantic water masses from surface to bottom. Moreover, the authors apply their findings by calculating and discussing oxygen utilization rates Atlantic-wide. Both might be a good data base for further investigations of biogeochemical cycles in the Atlantic Ocean.
I’d like to recommend publication after minor revision.
For minor revision I have listed a few specific comments (or better suggestions):
General comments:
Since I am a non-native speaker; I do not comment on wording, grammar, spelling, etc. I’d like to leave these issues to the editorial board.
In my pdf all figures are of minor, very blurred quality. I guess, that happened during crating the pdf for the reviewers from the submitted manuscript and submitted figures. So, this comment goes more to the editorial board than to the authors.
Specific comments:
Line 22-23: “eastern basin exhibiting younger ages compared to…” If this is meant in general, it should be rather opposite. Or is this specific for NEABW? Please, clarify.
Line 110-112: “The solubility (F)…” Find a better place in the methods’ section.
Line 113ff: You use the term “diffusive”, as widely common in this context. But wouldn’t it be more logic to use “disperse”. Many people automatically think on “molecular diffusion”, but here something different is meant, which might be better named “dispersion”.
Line 129-130: “Based on the TTD determination…” This sentence stays a little bit alone at this place. Skip or find better location.
Line 140: “the TTD is only a spike.” Moreover, in such a case the Δ becomes infinite small and the IG (Eq. in line 122) formally becomes a δ-function and can be computed the same way, so it is just a special case of the same formalism and not something completely different. Eventually worth to note here.
Line 141: “…underestimated by the tracer-age.” Eventually worth to refer to Sonnerup, 2001, who diels with different “tracer-ages” and the systematic deviation between tracer-ages from old and young or low and high concentrations.
Line 151-152: “The atmospheric concentration…” The sentence is important, bit at this place it stands a bit out of context. Please, find better location in section 3.
Line 152-175: The entire Δ/Γ-section could be shortened or focused. 1. The trouble is not enough tracers to compute Δ and Γ and there is almost no way out. Many authors previously discussed this issue and using Δ/Γ=1 is widely common and accepted. 2. The entire discussion here, if Δ/Γ= 0.6, 0.8, 1.0, 1.2 etc. does not lead to a final conclusion. Moreover, why not 0.1, 0.5, 1, 2, 5? 3. In principle and in theory the authors HAVE TWO tracers (or even three with the 39Ar) and COULD compute both Δ and Γ. But I guess, that might not help, since at least CFC-12 and SF6 do not provide enough independent information. So, keep it short. All tracer people know, why, and all others don’t care.
After line 175:
One would like to know, HOW the Γ is computed from the tracers. I guess a least square fit or something similar. But here is my most urgent question, if the authors do that by using CFC-12 and SF6 individually or at once. It is stated somewhere further below in the manuscript, but it should already be mentioned in the methods.
Which atmospheric data do the authors use? Pleas add a reference.
Line 177-178: In this study … vertical layers…” Could find a better place or, since independent from the following, mage an own paragraph of this sentence.
Line 188ff: As mentioned above, move this to methods-section?
Line 197-200: Move this paragraph to a better location, e.g. together with the sentence in lines 177-178.
Figure 6: Could you add “mean age” and “mode age” into the figure? (Holds for all such figures.)
Figure 7: Limit from 0 to 1000 dbar?
Figure 9: Limits from 0 to 3000 dbar?
Line 290: “over THE ridge”?
Line 359-366: The discussion why to favor the mode age instead the mean age does not convince me. In my understanding, the mode age and the mean age are almost one order of magnitude apart, hence, the OUR would also be almost 1 order of magnitude apart. Thus, an argument is needed, why the one and not the other. As I understand the authors, the mean age is subject to mixing with adjacent water masses. True. But the mode-age is, too. So, this argument does not count. I have also no clue, what is the right argument for favoring the mode-age. I guess it is related to either the “right” order of magnitude OR it has something to do with the integrative effect of the mean-age (the mean age sums all the damn history) and the more advective character of the mode-age.
Line 428-429: If the authors decide to put the technical aspect of how to compute Γ into the method-section, this could also go there. As I understand right, it is just the equation from line 122 with an additional decay factor e-λ/t ? Might be worth to note clearly.
Line 442: “(Fig. 18)” does not exist.
Line 455-465: What I would really enjoy would be a Figure 18 as a property-property plot as CFC/SF6 mean-age versus 39Ar mean-age and CFC/SF6 mode-age versus 39Ar mode-age and a short related discussion of such a figure. I’m sure one could learn something from it.
Line 467: I would put this sentence to the end of this paragraph.
Line 468: Add 39Ar to the bracket.
Line 470: Replace “investigated” by “discussed”?
Line 472: Replace “different parts” by “different fractions” or “different pathways”?
Line 498-505: Here I was puzzled. I can’t put this into the context of the manuscript. I can’t see climate change in the manuscript, since no repeat sections or stations are compared. Numerical simulation methods were not applied here. Please, either skip or clarify in the manuscript’s context.
Citation: https://doi.org/10.5194/egusphere-2024-1362-RC1 -
AC1:
'Reply on RC1', Mian Liu, 06 Feb 2026
Dear Reviewer,
We sincerely thank you for your valuable feedback and meticulous review of our manuscript, "Water masses in the Atlantic Ocean: water mass ages and ventilation." We were very pleased to read your overall positive assessment, particularly your comments that the manuscript is "very well written, easy to read and good to understand," and your recognition of the systematic application of the transient tracer-based TTD method across the Atlantic and the comprehensive discussion of major water masses from surface to bottom. These remarks were greatly encouraging.
We have carefully read and thoroughly considered every specific suggestion you provided and have addressed each point accordingly in the revised manuscript. The revisions and responses to each suggestion have been uploaded in the attachment (the black font represents the original text by the reviewer, and the blue font represents the responses from the authors). Your comments have been instrumental in enhancing the clarity and rigor of our work. The main revisions and considerations include:
Structural and Logical Improvements: Following your suggestions, we have relocated certain passages (e.g., the description of solubility, the mention of atmospheric histories, the paragraph introducing the sections) to improve the logical flow and coherence between the Methods and Results sections.
Clarification and Refinement of Methodology: We have rewritten and condensed the section on the choice of the mixing ratio (Δ/Γ), providing a clearer rationale for adopting the standard value Δ/Γ = 1. Furthermore, we now specify earlier in the Methods section how the mean age (Γ) is calculated, the strategy for using the tracers, and the source of atmospheric histories, thereby increasing the transparency and reproducibility of our methods.
Precision in Terminology and Wording: We have corrected an erroneous statement regarding the east-west basin age comparison, clarified the theoretical relationship of the purely advective case as a specific limit of the TTD model, and added key references (e.g., Sonnerup et al., 2001) where appropriate to support our discussion.
Improvements to Figures and Details: We have added labels for "mean-age" and "mode-age" to the relevant figures as requested, adjusted the depth ranges in some figure panels, and corrected the figure number citations in the text.
Strengthening of Core Arguments: In response to your crucial query regarding our choice of mode age over mean age for calculating the oxygen utilization rate (OUR), we have revisited and rewritten the relevant discussion. We now more clearly explain the differences between the two age concepts in terms of their integrative effect and representation of advective timescales, and how this influences the OUR estimation.
Focusing the Conclusions and Outlook: We have removed the original, inaccurately phrased paragraph that attempted a broader outlook involving direct climate change comparisons and numerical modeling, which was not supported by our study. The conclusion now more tightly focuses on the contribution of our work in providing a crucial baseline dataset for future studies and model validation.
Regarding your valuable suggestion to create a property-property plot comparing CFC/SF₆-based ages with ³⁹Ar-based ages, we were unable to construct a complete cross-section for direct comparison due to the limited spatial coverage of the ³⁹Ar data. However, we have addressed this concern by adding a comparative table and providing a more detailed discussion in the text regarding the applicable temporal ranges and limitations of the different tracers.
We believe the manuscript has been substantially improved through these revisions. We are truly grateful for the time you dedicated and for your highly constructive and expert comments, which were essential for refining this study.
Thank you once again for your support and guidance.
Sincerely,
Mian Liu & Toste Tanhua
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AC1:
'Reply on RC1', Mian Liu, 06 Feb 2026
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RC2:
'Comment on egusphere-2024-1362', Anonymous Referee #2, 17 Nov 2025
Review of “Water masses in the Atlantic Ocean: water mass ages and ventilation” by Mian Liu and Toste Tanhua.
General comments
The study is of the GLODAP data product, in particular transient tracers and dissolved oxygen data are used to estimate water mass ages and ventilation. The transient tracers of CFC-12, SF6 and 39Ar are examined with the assumption of an Inverse Gaussian function in order to find the mean- and mode-age of different water masses. The resulting calculations of age and ventilation of different water masses is outlined in great detail. Further inferences regarding the oxygen utilization rate are outlined with implications for biogeochemistry.
I think that this study could be worthy of publication in EGU Ocean Sciences, once my below comments have been addressed. As further outlined below, the Data and methods section was confusing and needs a strong revision. The figures were not of a high production quality (often too small, low resolution) but could be improved with some effort. Finally, the authors should take more care in stating the limitations of the results and when the conclusions are more speculative. I have tried to note specific examples in the below comments.
On a positive note, I will say that much of the paper was clear and easy to follow (e.g. Introduction and Results sections). While the paper reads a little like a report at times, with results described in detail one after another, I think that this is probably unavoidable due to the nature of the study. Overall, I think it would be beneficial to the community to have the work published, as work on water mass age and ventilation is important to the community.
Major comments
- Data and methods section: I am not an expert in age tracer methods, so I found that this section was very confusing to follow. I suggest to strongly revise this section, keeping in mind to explain the overall methods and reasoning to the reader at each step. For example, the equations (Lines 116-122) were introduced abruptly with little explanation. I was left with basic questions e.g. is width defined as \Delta or \Delta^2? What are the units on the mixing ratio? What is 2-IG? These are just some examples. Some information later in the manuscript (e.g. Lines 352-366) would have been helpful earlier in Section 2.
- Figure quality was low: figures were quite low resolution (at least in the version I had). They were often too small (e.g. Figure 6) and required zooming in to see the details. I also suggest that different colors could be used for the different parameters in Figure 14, in order to more easily distinguish between the parameters in the panels. Some captions could also do with more clarity, and I have tried to note these points in more detail in the comments below. The actual data on the figures was interesting, and I think it would be a relatively straightforward fix to improve the figure quality across the entire paper.
- Being more careful outlining limitations of results and speculation: Some of the content in Section 2.1 and 2.3 outlined some of the main limitations of the results, such as assuming an Inverse Gaussian profile and assuming a mixing ratio of 1. I understand that these assumptions are required in order to get useful results out of the data, but I think that the results and conclusions section could help with re-iterating these limitations. There were also a few times where the wording was a bit strong on speculative statements, for example in the conclusion “The results show that with global warming, the age of the Atlantic water mass shows a certain degree of change.” But my take on the paper was that it was showing the water age and ventilation, rather than changes with climate? So this seems a little speculative and I have tried to note these parts in the comments below.
Specific comments
- Abstract: “The distribution of oceanic water masses and their properties, such as ventilation constitute fundamental parameters, for instance, the thermohaline circulation patterns and biogeochemical processes in the marine systems.” This sentence does not make sense and needs revision.
- Line 19-22: Can remove the brackets around “mean” and “mode” in this sentence.
- Paragraph from Line 45-55: There has been more work recently using ocean models to discuss the age tracer, please consider citing some of these (e.g. Li, Q., England, M. H., Hogg, A. M., Rintoul, S. R., & Morrison, A. K. (2023). Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature, 615, (7954), 841-847).
- End of Introduction: Consider adding a paragraph with a brief outline of the rest of the paper to orientate the reader.
- Data and methods section: See Major Comment 1 (above). Unfortunately, by the end of Section 2 I still did not really understand why mean- and mode-ages are used for different applications. Please strongly revise Section 2, as it is by far the weakest section.
- When G(t) first introduced: Explain physically G(t) and the assumptions going in to G(t). My understanding is that you are assuming the shape of the profile as G(t) and then further assuming the mixing ratio, and either the width of the mixing or \Lambda? Then you have the profile such as in Figure 1b. But it is still not clear to me at this stage what measurement is required for Figure 1b. How does CFC-12=300ppt play into G(t)?
- Line 144-147: “The mean-age is obtained by the average value of all the different aliquots in one water sample by considering the transport time of different pathways (Fig. 1, b). This concept of water mass age is often useful for biogeochemical studies. In other cases, such as when discussing the transport times of water masses from the formation area, the mode-age concept is useful.” This seems directly in contrast with Lines 471-473: “The mean-age, which shows the average value of all the different parts in one water sample, is used to show the static distribution of water masses. The mode age, which shows the age of the dominant water mass in the sample, is used to trace the biogeochemical phenomena.”
- Figure 1: How does the description of (a) in the caption match with the figure in panel (a)? Text on the figure is very small also.
- Figure 2: Very small text and the vertical axis line was not present on panel (a).
- Figure 3: Low resolution and tricky to make out the results as the panels are small. What are the green lines? Please also state the units of age either on the figures or in the caption. Also, it would help to use the same mixing increments as in Figure 2(b) i.e. 0.6, 1.0 and 1.4. This would connect Figures 2 and 3 better.
- Line 156: “ration” -> “ratio”
- End of Section 2: The justification of mixing ratio of 1 is not particularly strong, as it does seem like a value that could change a lot regionally depending on physical processes in the ocean. I agree that the authors are limited here so a mixing ratio of 1 is probably the right choice, but it would help to highlight that this is a limitation of the study.
- Table 1 caption: Consider simplifying the text to something like: “Summary of the selected hydrographic cruises to the Atlantic Ocean chosen for the oceanographic sections in this study”.
- Figure 4: What are the green contour lines on the figures?
- Section 3.1: This sub-section starts with a long paragraph. Consider splitting it into two paragraphs for ease of reading.
- Figure 6: plots are small. Please make them bigger.
- Line 230: “… section through in the distribution …” Please remove “in”
- Line 235-236: “In principle, the AAIW is supposed to get higher ages towards the north.” Consider explaining why this is expected.
- Line 239-240: “The reason for the above result is the intervention from surrounding water masses.” I find this sentence strongly worded for this more speculative result. Other reasons for the result could be that the IG function is not a good assumption, or that mixing ratio really does change across the ocean.
- Line 240-241: “The maximum distance of AAIW to the north can reach 30 °N, but between 241 20 °N and 30 °N, this water mass mixes with the ENACW and upper NADW…” Consider saying “we speculate that this water mass mixes…” or “this water mass likely mixes…”
- Paragraph of Lines 253-262: Again, I find much of this discussion on mixing as speculative. Please consider mentioning this or weakening the statements a little.
- Figures 8, 10, 14: Panels are small, please make them bigger.
- Figure 14: AOU top panel. What does it mean for this value to go negative? And how does this influence the OUR result?
- Figure 15: Caption does not appear to line up with shown panels.
- Figure 16, 17: “pentagrams” symbols would more commonly be called “stars”.
- Figure 16,17 caption: “The solid isolines show the 50% fractions of water masses and the dashed lines show the 20% fractions.” Is this consistent with the lines shown in Figure 15?
- Line 442: Refers to Figure 18. Is this figure missing?
- Line 467-468: “As the continuous work…” perhaps replace with “In the present work…” ?
- Conclusions: “Three different definitions of water mass ages are investigated. The tracer-age assumes the ocean as a totally advective situation without diffusion and underestimates the actual age in the realistic ocean.” Tracer-age is briefly discussed in the manuscript, but no figures or results are shown of it, so I would disagree that it has been investigated here. I suggest to just be careful of the wording here.
- Final two paragraphs of the conclusions: Talk about effects of climate change, but (as far as I can see) this is not really touched on in the actual results of this paper. So perhaps it is a bit speculative? It would also help to remind the reader of the limitations of the study in the conclusions.
Citation: https://doi.org/10.5194/egusphere-2024-1362-RC2 -
AC2:
'Reply on RC2', Mian Liu, 06 Feb 2026
Dear Reviewer RC2,
We sincerely thank you for your thorough and constructive review of our manuscript, "Water masses in the Atlantic Ocean: water mass ages and ventilation," and for your positive assessment of its potential contribution to the field. We greatly appreciate the time and effort you dedicated to providing detailed feedback, which has been instrumental in improving the quality of our work.
We have carefully read and considered each of your suggestions. In response, we have undertaken a comprehensive revision of the manuscript. The key modifications, which directly address your major and specific comments, are summarized below:
Complete Revision of the Data and Methods Section (Section 2): We have entirely restructured and rewritten this section to enhance clarity and logical flow. This includes a clearer introduction of concepts and equations, explicit definitions of key parameters, and the addition of a new dedicated subsection (2.2) that explains the definitions and distinct applications of tracer-age, mean-age, and mode-age early in the methodology.
Significant Improvement of Figure Quality: All figures have been revised to ensure high resolution, readability, and improved labeling. We have increased panel sizes, optimized color schemes (e.g., in Figure 14), enhanced captions, and replaced low-resolution elements with vector graphics where applicable.
Clarification of Limitations and Speculative Statements: We have tempered overly strong or speculative language throughout, particularly regarding mixing processes. A dedicated discussion of methodological limitations (e.g., the fixed mixing ratio, steady-state assumption) has been added to the Conclusions (Section 6). Speculative statements about climate change impacts, which were beyond the scope of our present study, have been removed.
Furthermore, we have addressed all your specific comments point-by-point, including corrections to terminology, figure captions, in-text references, and structural suggestions (e.g., splitting long paragraphs, adding an outline to the Introduction).
A detailed, point-by-point response to each comment is provided in the attached document. We believe the revised manuscript is now clearer, more rigorous, and better aligned with the standards of the journal.
Thank you again for your valuable insights and guidance. We hope the revised version meets with your approval.
Sincerely,
Mian Liu & Toste Tanhua
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Version 2 | 10 Jul 2025
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Version 1 | 23 May 2024
Peer review completion
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Cited
2 citations as recorded by crossref.
- An Updated Inventory of Fluorspar CaF<sub>2</sub> Production, Industrial Use, and Emissions of Trifluoroacetic Acid (TFA) from 1930, Including the Period from 2000 to 2020 A. Lindley https://doi.org/10.4236/gep.2025.1311008
- Mesozoic and Cenozoic oceanic gateway evolution and global tectonics: palaeoceanographic and sedimentary implications F. Hernández-Molina & C. Scotese https://doi.org/10.1144/gslspecpub2025-10
Mian Liu
Toste Tanhua
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