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https://doi.org/10.5194/egusphere-2024-1162
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2024-1162
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
28 May 2024
| 28 May 2024
Status: this preprint is open for discussion.
Technical Note: Comparison of radiometric techniques for estimating recent organic carbon sequestration rates in freshwater mineral soil wetlands
Abstract. For wetlands to serve as natural climate solutions, accurate estimates of organic carbon (OC) sequestration rates in wetland sediments are needed. Dating using cesium-137 (137Cs) and lead-210 (210Pb) radioisotopes is commonly used for measuring OC sequestration rates in wetland sediments. 137Cs radioisotope dating is relatively simple, with calculations based on a single point representing the onset (1954) or peak (1963) of the 137Cs fallout. 210Pb radioisotope dating is more complex as the calculations are based on multiple points. Here, we show that reliable dating of sediment cores collected from wetlands can be achieved using either 137Cs or 210Pb dating, or their combination. However, 137Cs and 210Pb profiles along the depth of sediment cores need to be screened, analyzed, and interpreted carefully to estimate OC sequestration rates with high precision. To this end, we propose a decision framework for screening 137Cs and 210Pb profiles into high- and low-quality sediment profiles, and we compare dating using the 1954 and 1963 time-markers. Our findings suggest that 137Cs- and 210Pb-based OC sequestration rates are comparable, especially when using the 1963 (vs. 1954) time-marker.
How to cite. Mistry, P., Creed, I. F., Trick, C. G., Enanga, E., and Lobb, D. A.: Technical Note: Comparison of radiometric techniques for estimating recent organic carbon sequestration rates in freshwater mineral soil wetlands, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-1162, 2024.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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RC1: 'Comment on egusphere-2024-1162', Anonymous Referee #1, 27 Jun 2024
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The paper "Technical Note: Comparison of radiometric techniques for estimating recent organic carbon sequestration rates in freshwater mineral soil wetlands" by Mistry et al. evaluates the accuracy of an age model based on lead-210 and cesium-137 or the combination of both radionuclides for the reconstruction of organic carbon sequestration rates / stocks in Canada. In their paper, the authors evaluate the quality of the radionuclide profiles using a visual and statistical approach. On the selected profiles (classified as unperturbed), they compare the error associated with using the 1954 and 1963 time markers to establish age model in comparison to lead-210 age model for reconstructing the carbon sequestration rate.
The paper is interesting and well written, especially the discussion. The descriptions of the results were sometimes more complicated to follow. I have identified some points that should be clarified for the revised version.
- This paper is intended to be a 'technical note'. In my opinion, a technical note should be applicable worldwide. In the introduction, in Table 1 or again in the methods section, the authors mainly introduce 137Cs in North America (in the discussion they introduce Fukushima and Chernobyl peaks). This radionuclide has some specificities around the world that need to be introduced for a technical note. It will also be interesting to introduce and discuss other complementary approaches that can help to identify the 137Cs, the advantages, disadvantage of these techniques (e.g. Pu isotopes) or again the 'classic' combination of 137Cs and 210Pb to establish an age model.
- If I understand correctly, the authors compare the efficiency of both 137Cs age models (using 1954 and/or 1963 peaks) with the 210Pb model after the identification of ‘high-quality profiles’. To me, these two approaches are complementary. Most studies using lake sediments, for example, validate their 210Pb age model by comparing the continuous lead chronology with the 137Cs peaks (1954 and 1963). The authors state that they use this combination, but this information is not clearly stated in the text (or I may have misunderstood them). Do they compare the validity of the 210Pb models in the conventional way (I don't think so) or simply by comparing the 137Cs and 210Pb profiles, which they think are of good quality?
- At various points in the manuscript, the authors emphasize the simplicity of measuring 137Cs compared with 210Pb. This statement is true if researchers use alpha spectrometry, but as reported in some recent reviews (cited by the authors), studies using alpha spectrometry are decreasing, while gamma spectrometry is increasingly used. This technique makes it easy to obtain 137Cs, 210Pb and 241Am. At the end of the introduction, the authors state that "this study helps to reduce the uncertainty in studies that rely on 137Cs or 210Pb radioisotope dating". If you have access to both radionuclides with gamma spectrometry, you don't have to choose between the two because you have access to both radionuclides. The authors should justify why it is important to be able to run the lead-210 and 137Cs models separately.
- The authors of this paper have done important field and analytical work that deserves to be published in an open access database. This will allow other researchers to evaluate their work and share these data with the community. I recommend that the authors make this dataset available for the revised version.
I recommend publishing this article after revision.
General comments :
- Line 13: replace measuring with reconstructing ?
- Line 14: You mention 'a single point' when you actually mention two in the rest of the sentence (1954 and 1963). There may be other regional peaks (1986, 2011) or even more local ones.
- Line 19: You compare your 137Cs age models with the 210Pbxs age model? How?
- Line 24: Perhaps a broader view here to introduce the importance of your topic? Area occupied by these wetlands in the world, Canada, etc.? Data about carbon sequestration by these wetlands?
- Line 25: critical -> important?
- Line 40: formed -> produced during?
- Line 41: 1963 in the northern hemisphere. What about the southern hemisphere? If the article is to be a technical note applicable worldwide, it needs to give a more global view of 137Cs fallout (not just the 1963 peak).
- Line 44: 1954 and 1963 - 1954 or 1963. Most publications using the 137Cs age model use both the 1954 and 1963 time points to construct their age models. When using two time points, the sedimentation rate is not always (often) constant.
- Lines 47-48: In Europe the story is more complex and it is sometimes difficult to distinguish the 1963 peak from the 1986 peak. Authors should introduce additional markers (241Am, Pu isotopes) to help distinguish these peaks and avoid errors in age modeling.
- Line 54: I don't agree that the 210Pb models are "complicated", as the authors have already stated in the abstract. Lead models provide complementary information than 137Cs age models. There are many R codes that facilitate the establishment of 210Pb models (e.g. SERAC model, Bruel and Sabatier, 2020).
- Line 64: remove 'for’
- Line 67: or mass accumulation rate?
- Line 70: only cite Appleby and Odfield?
- Line 71: In my opinion, 137Cs provide temporal markers as mentioned by the author, but not 210Pbex, which provides a continuous age model. The 137Cs help to validate the 210Pb model.
- Line 77: If you do gamma spectrometry, you get both210Pbex, which provides a continuous age model. The 137Cs help to validate the 210Pb model.
- Line 77: If you perform gamma spectrometry you obtain both 137Cs and 210Pb (and 241Am) and you don’t have any extra cost, time and specialized equipment.
- Line 85: change intact through undisturbed?
- Lines 86-87: methods?
- Line 94: This figure should be in the main manuscript.
- Line 95: change 'intact’ by undisturbed ?
- Lines 114-119: the authors must discuss what can change the shape of these peaks (e.g. bioturbation, erosion phenomena, deposition of 137Cs-labelled particles, climatic events, ...), see existing bibliographic reviews.
- Line 140: The authors should describe where these reference samples were collected and describe the environment (e.g. undisturbed grassland?). Authors use one reference core per site? This part was not clear for me
- Line 164: Use organic carbon instead of OC in the title.
- Line 169: the authors should explain how they get from g.cm2.year to Mg.ha.year-1 (representativeness of the core to extrapolate the value to the ha scale?)
- Line 171 - 173: why do the authors not use the 241Am to more clearly identify the 1963 peak (when the shape of the peak is not clear)? If they are not sure about the peak, why not use the onset of 137Cs in 1954 instead of remove the profile from their selection (maybe I don’t have understand here)?
- Line 178: if you use density correction to build your age models, you should describe it in this section.
- Line 230: repositioned? what does that mean?
- Line 231: How do the authors explain the high stocks of these cores? Are these cores in an accumulation area?
- Lines 257-259: This part corresponds to the methodology already described in the Materials and Methods section. I will delete it from the results
- Line 310: To the best of my knowledge, you need a complete decay profile to apply the CRS model, it's the same for the CFCS model? Can you give a reference for this statement?
- Line 311: I didn't understand if you corrected your 210Pb age model with density to avoid perturbations/inconsistencies/soil properties? please clarify.
- Line 326: Fukushima and Chernobyl releases are not 'less', just not recorded in North American lakes or wetlands to the best of my knowledge.
- Line 331: continuous age model instead of 'can provide multiple time markers?
- Line 345 : references for this statement?
- Line 361: It was not possible to collect a reference site near each wetland?
- In section 4.1, the authors should discuss alternative methods that may help to identify the peak of 137Cs (e.g. Pu isotopes).
- Lines 383-386: very long sentence...
- Line 416: in the last century?
- Table 1: Why mention only the 1963 peak if your article is a technical note? Why not present the other peaks (sources of 137Cs) that could be found in other regions of the world? The 1963 peak, which is generally not dated to 1963 in the southern hemisphere? Why not include 241Am, which you may have in your gamma measurements and which may be useful in age modeling? These concepts need to be explained somewhere in your manuscript.
- Figures 2; 3: Mass or mass depth?
- Table 3: Most of the disadvantages of the 210Pb listed in this table are related to the alpha spectrometry method, but the 210Pb can also be measured by gamma spectrometry, which is easier to use.
Citation: https://doi.org/10.5194/egusphere-2024-1162-RC1
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Purbasha Mistry
School of Environment and Sustainability, University of Saskatchewan, 117 Science Place, Saskatoon, SK, S7N 5C8, Canada
School of Environment and Sustainability, University of Saskatchewan, 117 Science Place, Saskatoon, SK, S7N 5C8, Canada
Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada
Charles G. Trick
Department of Health and Society, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada
Eric Enanga
Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada
David A. Lobb
Department of Soil Science, University of Manitoba, 13 Freedman Crescent, Winnipeg, MB, R3T 2N2, Canada
Short summary
Precise and accurate estimates of wetland organic carbon sequestration rates are crucial to track progress of climate action goals through effective carbon budgeting. Radioisotope dating methods using cesium-137 (137Cs) and lead-210 (210Pb) are needed to provide temporal references for these estimations. The choice between using 137Cs or 210Pb, or their combination, depends on respective study objectives, with careful consideration of factors such as dating range and estimation complexity.
Precise and accurate estimates of wetland organic carbon sequestration rates are crucial to...