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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Status: open (until 06 Oct 2024)
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RC1: 'Comment on egusphere-2024-2511', Anonymous Referee #1, 16 Sep 2024
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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
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