the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2024
Measuring varve thickness using µCT: a comparison with thin section
Abstract. X-ray micro-computed tomography (μCT) scans were performed on four varved sediment cores collected in Grand Lake (Labrador) and previously studied with thin sections. These scans allowed to investigate the possibility of using µCT as a substitute for thin sections to carry out counts and thickness measurements of varved sediments. Comparing varve counts of these two methods, μCT counts are slightly higher than the ones made with thin sections. The difference in counts suggests that the petrographic study and a SEM analysis of a thin section remain necessary for determining the varve character of the laminae. Yet, µCT allows measurements in multiple directions, improving the robustness of the counts and allowing avoiding the manufacturing of continuous thin sections along sediment sequence.
As to the thickness measurement, the µCT analyses were made in two perpendicular directions. Not surprisingly, measurements made on the same cutting plane as the thin section are quite similar to the ones made on the latter. However, there are significant differences with measurements made on the perpendicular plane. This highlights the need to perform varve thickness measurements in at least two perpendicular directions for better estimates of varved sediment thicknesses. In addition, the study illustrates that µCT is an effective way to select the least deformed zones with parallel varves to carry out the best possible thickness measurements.
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RC1: 'Comment on egusphere-2024-2511', Anonymous Referee #1, 16 Sep 2024
A review of ms “Measuring varve thickness using μCT: a comparison with thin section”
Generally, this is a well-thought-out yet straightforward study fitting the scope of the journal Geochronology. There are clear goals and motivations behind the study. I appreciate the effort to put more work into a well-known and thoroughly investigated field of varved sediments. The Authors provide a new outlook on the crucial problem of varve chronology development and show the drawbacks and potential limitations of established methods. However, there are problems that, in my opinion, must be addressed before this paper is accepted.
General comments
- There are instances of, for lack of better expression, sloppy editing, repetitions, and overall lack of care. I had an impression that the manuscript’s submission was rushed. See some of the specific comments.
- The powerful µCT equipment used in the study is not readily available to numerous laboratories, and I’d appreciate some general perspective on this.
- Conclusions could be improved, at the moment it is a little too expected, consider improving with something really specific to your study. The use of CT is, in principle, well known.
- A methodological paper like this would be extremely beneficial for the wider scientific community if, at some point, the Authors provided a simple chart or another way of following the “best practices.” Introduce it, even if it is in supplementary materials. In other words, condense the methods section into a streamlined workflow with short explanations. This way, your paper will become a cornerstone for future works more quickly.
- A crucial part, that is not discussed, and I was actually expecting at least some indication of it: using the proposed approach one could more confidently measure the thickness of the varves – this is great. What about actual varve chronology development and transfer of these thicknesses onto the age-depth model? I understand that this is not an aim of this paper, but something that is a natural follow up. Authors discuss missing varves/laminations on different planes. Combining counts from one plane is tedious task, but routinely done. What about now?
- Consider adding a discussion paragraph, where you explain whether it would be possible and cost/time effective to scan the core before opening and then finding the best plane for splitting it. The power of µCT is its non-destructive nature, would all of us benefit from knowing how to split the core before deciding on thin sections? Does your setup allow the scanning of non-halves?
- Generally, figures could use some more care and a unified approach. For example,, multiple fonts are used, text is bolded and not, and size varies too much. Finally, there might be too many in the main body. Consider moving some into the appendix.
Specific comments
29: required – past tense? Or is it an ongoing issue?
31: correct the reference.
32: or mix of these.
37–40: there is a strange repetition in this paragraph that offers no new information.
45: this paragraph is disjointed from the one before.
52: refs – indeed.
52, continued: This sentence reads a little awkwardly.
67: while this is a methodological paper focusing on the technique advances this paragraph should be expanded with some more information on the environmental setting. Clastic varves that are study object are direct result of the environmental conditions, and as such this information are a must. Additionally, while it is visible on the map, it still should be stated in the text that the study site is in Canada (and maybe add a province?).
109, 333: beam hardening is never explained; thus, it might carry no load for non-technical readers.
123: this part of the text, in my opinion, suffers from the same issue as lines 37–40. A problem is diagnosed and then rephrased.
139: I’d appreciate more reasoning behind using the shortest segment. Perpendicularity is self-explanatory, as no measurement should be done at a different angle. However, one can argue, especially with the authors’ later remarks on the varve inclinations, that the narrowest sections could be deformed for numerous reasons and be random. If there is a reason for this, it should be stated here clearly.
155: this enumeration of methods should be rewritten for clarity.
220: either correlations or regression, which has a term for it. This sentence could be clarified, too.
Fig 9 and other similar (and corresponding parts of the text): any comment on the different slope?
225, 235: these are a little bit short paragraphs.
240: rewrite this sentence.
270: this opens a much broader discussion and all-time problem of sedimentary environment interpretation. If using one core, we can see that we’re missing something on the thin section plane; what guarantees that one 60–90 mm wide core is not missing even more?
293: spelling a number then putting it in a parenthesis is a little too much given the simplicity of number “1”.
298: Isn’t it stating the obvious? Flesh it out in some way if you want to carry out with this message, otherwise it is a given.
303: The way it is written now it is a study conclusion rather than discussion.
325: “Our paper…” this is not the right place for this.
341: this is hardly surprising, other methods such as μXRF mapping or HSI prove that 40 µm is good enough with this varve thickness.
Conclusions: the first conclusion is a little too obvious, given the large body of work on CT.
Fig. 5. No figure caption should start with “the fig shows…” – applies to A and B.
Fig 8. This is a really big figure, I don’t have an adequate suggestion, but consider splitting it and putting closer to the corresponding text.
Fig 14 (and corresponding parts of the text) is an example of a deeper problem. Does the mere presence of lamination/horizontal feature make it a varve? This should be discussed somehow, so it is not as definitive.
Fig 15 (and corresponding parts of the text): This is a great example of µCT use! We often find deformed sediments in thin sections that prevent us from confident counting. Put it more into the front!
Fig 18 (and corresponding parts of the text): I’d argue that an experienced investigator would easily count on approx. 100 µm imagery.
Citation: https://doi.org/10.5194/egusphere-2024-2511-RC1 -
AC1: 'Reply on RC1', Marie-eugénie Jamba, 23 Sep 2024
Preprint “Measuring varve thickness using µCT: a comparison with thin section.”
Response to Reviewer
Dear reviewer,
We thank you for your feedback and appreciate the time and effort you have put into providing feedback on our manuscript and are grateful for the insightful comments and valuable improvements you suggest to our article. We will incorporate most of the suggestions you made for the final submission. Please see below for a point-by-point response to the general comments and your concerns.
Reviewer' Comments to the Authors:
Reviewer comment 1:
There are instances of, for lack of better expression, sloppy editing, repetitions, and overall lack of care. I had an impression that the manuscript’s submission was rushed. See some of the specific comments.
Author response: Thank you for your frankness. While the manuscript submission was not rushed, we acknowledge it was not internally reviewed by an English native speaker. That will be done for the final submission.
Reviewer comment 2:
The powerful µCT equipment used in the study is not readily available to numerous laboratories, and I’d appreciate some general perspective on this.
Author response: Nowadays, this type of micro-CT is starting to be available in many laboratories today. For instance, the Excite network (https://excite-network.eu/) provides access to XCT facilities. We will add some information about this, and also refer to the university's website and on the merchant's website to improve this part of the paper.
Reviewer comment 3:
Conclusions could be improved, at the moment it is a little too expected, consider improving with something really specific to your study. The use of CT is, in principle, well known.
Author response: this is a good suggestion; we will add something specific to our study in the revised version.
Reviewer comment 4:
A methodological paper like this would be extremely beneficial for the wider scientific community if, at some point, the Authors provided a simple chart or another way of following the “best practices.” Introduce it, even if it is in supplementary materials. In other words, condense the methods section into a streamlined workflow with short explanations. This way, your paper will become a cornerstone for future works more quickly.
Author response: We think this is an excellent suggestion.
Reviewer comment 5:
A crucial part, that is not discussed, and I was actually expecting at least some indication of it: using the proposed approach one could more confidently measure the thickness of the varves – this is great. What about actual varve chronology development and transfer of these thicknesses onto the age-depth model? I understand that this is not an aim of this paper, but something that is a natural follow up. Authors discuss missing varves/laminations on different planes. Combining counts from one plane is tedious task, but routinely done. What about now?
Author response: This is indeed not the purpose of our article, but we will add a short discussion about the impact of this approach on varve chronologies.
Reviewer comment 6:
consider adding a discussion paragraph, where you explain whether it would be possible and cost/time effective to scan the core before opening and then finding the best plane for splitting it. The power of µCT is its non-destructive nature, would all of us benefit from knowing how to split the core before deciding on thin sections? Does your setup allow the scanning of non-halves?
Author response: We agree, this is a section that will be included in the article.
Reviewer comment 7:
Generally, figures could use some more care and a unified approach. For example, multiple fonts are used, text is bolded and not, and size varies too much. Finally, there might be too many in the main body. Consider moving some into the appendix.
Author response: We will work to improve the figures for the final submission and will consider to move some into the appendix.
Citation: https://doi.org/10.5194/egusphere-2024-2511-AC1
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RC2: 'Comment on egusphere-2024-2511', Anonymous Referee #2, 06 Oct 2024
Review of “Measuring varve thickness using µCT: a comparison with thin section”
The paper provides some much needed information on counting and measuring clastic varves by 3D analysis with X-ray μCT compared to the conventional thin-section method. The authors formulate clear aims and motivations and have developed an appropriate methodology. The results provide important insights on the variations in varve counting and thickness measurement between the two methods and show their advantages and limitations. However, some parts still needs to be elaborated and improvements in scientific English and formatting need to be made before the paper can be accepted. I would like to make the following comments:
- The writing is not always clear and precise and needs to be improved. There are many grammatical errors, incorrect expressions and repetitions.
- Not all figures are formatted correctly and not all are required in the body text. Consider moving some to the appendix.
- Line 50-52: The limitations associated with the production of thin sections, i.e. sediment modification, should be explained and references added.
- In “2.1 Sample selection” information on the environmental setting is missing and the figure of the study area needs to be improved.
- In “2.3 Experimental setting: µCT acquisition and reconstruction”: Why were the halfcores scanned in STAMINA mode? Helical scan trajectories (available in TESCAN CoreTOM) would avoid blurred parts in your samples.
- Line 139: Why is the shortest line segment separating two parallel varve borders measured, as these could potentially be disturbed? What is the reason for this?
- As this is a methodological paper, I would recommend proposing a standardised workflow in the discussion section to simplify its application for the scientific community and to enable future comparisons between studies.
Citation: https://doi.org/10.5194/egusphere-2024-2511-RC2 -
AC2: 'Reply on RC2', Marie-eugénie Jamba, 21 Oct 2024
Preprint “Measuring varve thickness using µCT: a comparison with thin section.”
Response to Reviewer
Dear reviewer,
Thank you very much for your valuable suggestions and comments on our manuscript. We appreciate the time and effort that you have dedicated to providing your valuable feedback on my manuscript. We will incorporate most of the suggestions you made for the final submission. Here is a point-by-point response to your comments and concerns.
Reviewer' Comments to the Authors:
Reviewer comment 1: The writing is not always clear and precise and needs to be improved. There are many grammatical errors, incorrect expressions, and repetitions.
Author response: we acknowledge it was not internally reviewed by an English native speaker. That will be done for the final submission.
Reviewer comment 2: Not all figures are formatted correctly and not all are required in the body text. Consider moving some to the appendix.
Author response: We will work to improve the figures for the final submission and will consider to move some into the appendix.
Reviewer comment 3: Line 50-52: The limitations associated with the production of thin sections, i.e. sediment modification, should be explained and references added.
Author response: Indeed, we will provide more explanation.
Reviewer comment 4: In “2.1 Sample selection” information on the environmental setting is missing and the figure of the study area needs to be improved.
Author response: We will work to improve this part for the final submission. Could you give us more details on the changes you think should be made to the figure? Could it be the color? The font size?
Reviewer comment 5: In “2.3 Experimental setting: µCT acquisition and reconstruction”: Why were the halfcores scanned in STAMINA mode? Helical scan trajectories (available in TESCAN CoreTOM) would avoid blurred parts in your samples.
Author response: Indeed, the helical mode is now accessible in TESCAN CoreTOM.
However, the half-core scans were done well before the helical mode was integrated into the μCT. Also, the results of the tests done on sediment cores and wood cores with the helical mode presented several problems. First, the size and duration of the reconstruction were far too large, requiring much more powerful computers than those present in our laboratory. Also, even if the problem of the cone beam artifact was solved with the helical mode, there was the presence of new artifacts (which we could not explain) that degraded the quality of the images.
Reviewer comment 6: Line 139: Why is the shortest line segment separating two parallel varve borders measured, as these could potentially be disturbed? What is the reason for this?
Author response: Since the varves are assumed to be parallel at the time of deposition, we use the smaller distance between two parallel varves to try to get as close as possible to the initial thickness of the varve before deformations.
Yes, indeed varves can be deformed as shown in Figure 16b-c, and these deformations can occur during the coring process. Mainly for poorly consolidated sediments, the layers are deformed at the edges as the core liner gradually sinks into the sediment. We could include some explanations in the article.
Reviewer comment 7: As this is a methodological paper, I would recommend proposing a standardised workflow in the discussion section to simplify its application for the scientific community and to enable future comparisons between studies.
Author response: We agree with you, it's a great idea to propose a standardized workflow.
Citation: https://doi.org/10.5194/egusphere-2024-2511-AC2
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