the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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RC1: 'Comment on egusphere-2024-1162', Anonymous Referee #1, 27 Jun 2024
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 -
RC2: 'Comment on egusphere-2024-1162', Anonymous Referee #2, 15 Jul 2024
This manuscript compares two radiometric techniques as complementary methods for estimating carbon sequestration in wetland soils. The methods for dating using cesium-137 and lead-210 radioisotopes were well explained and the results on the radioisotope profiles were also presented and interpreted well. Given the increasing interest in applying radiometry to the soil carbon sequestration, I think this paper can contribute to disseminating well-documented radiometric methods among the researchers in the various biogeoscience fields. However, I had some concerns about the focus of the current version and a couple of critical methodological details. To be qualified as a technical note, the authors may need to strengthen the “technical” part of the manuscript, and above all technical recommendations readers would expect for this article type. I would also thank the authors if clarify core selection criteria and the limitation of LOI as a measure of soil organic carbon as detailed below.
<Major comments>
- The objective of the technical notes paper
The stated objective (“This research paper compares the use of 137Cs- and 210Pb to estimate recent OC sequestration rates in intact (i.e., not directly 85 impaired by human activities) freshwater mineral soil wetlands located on agricultural landscapes”. “This study helps reduce uncertainty in studies that rely on 137Cs or 210Pb radioisotope dating”) seems more relevant for regular research articles. Given the article type (technical notes), providing more concrete goals would help readers recognize the main contribution of this paper (e.g., providing specific recommendations for selecting better methods or procedures depending on wetland types). In this context, the last sentence of the abstract, along with some concluding part of the Discussion section (lines 400-413), needs also to be more articulated in providing technical suggestions based on the compared results. In terms of providing technical suggestions, the authors can be more straightforward in specifying which methods are more relevant in which types of samples.
- Criteria for selecting suitable cores
Different criteria were used for selecting suitable profiles of Cs and Pb (Line 121-124). First, more detailed descriptions would help readers understand the rationales of the employed criteria. Second, as the triplicate samples were analyzed for each site, please clarify if the selection criteria were applied to each single profile or an averaged result from three replicates per site. Third, it would provide more quantitative evaluation of the selection if it were clearly stated which proportion of the three profiles (e.g., at least two or all three) fulfill selection requirements.
- The term organic C
The authors used organic C to refer to the organic matter content measured by LOI (“OC content was calculated from OC concentration in % measured by loss-on-ignition method”). LOI is a measure of organic matter, but cannot represent organic carbon quantitatively. The limitation of LOI as a measure of organic matter is well known, such as the ignition of non-organic particles at high temperatures). In my opinion, the results of LOI measurements should be reported as LOI (%). Equating LOI with OC would be unacceptable for many soil scientists. I would recommend measuring and reporting OC. If this is not possible, LOI (%) or OM (%) should be used with a prior definition.
<Minor comments>
- Title: if C accumulation occurs mainly in the wetland soils, the title should end like “freshwater wetland soils”.
- Line (L) 23: Without citing any relevant papers, this sentence assumes that wetlands function as C sinks, although wetlands can also function as C sources under degrading conditions or when CH4 emissions are taken into consideration. Please refine the text based these considerations and provide at least a few relevant papers.
- L 56 “210Pb is a naturally occurring radionuclide of 238U”: Do you mean “…radionuclide deriving from 238U?”
- L 66: It would be more reader-friendly if you explain “unsupported” and “supported” by adding a few more words.
- L 76: Please remove “according to” and instead put the references in parentheses.
- L 87: Parentheses here appear unnecessary.
- L 108-108 “the high-purity germanium detectors, Broad Energy Germanium detectors (BE6530) and high-resolution Small Anode Germanium well detectors (GSW275L) (Mirion Technologies, Inc., Atlanta, GA, USA)”: It is not clear whether BE6530 and GSW275L are high-purity and high-resolution detectors, respectively.
- L 112: This would be a good place to describe any QA/QC measures that were employed to guarantee the analytical accuracy.
- L 232-238: Don’t these exceptions demand a refinement of the criteria? For instance, additional criteria can be prescribed considering these exceptions. It would be too arbitrary if we have to accept the failed profiles based on subjective visual inspections.
- L 306: Please include a phrase mentioning OC data like “combined with OC measurements” following “both radioisotopes dating”.
- L 256-261: Please combine these sentences into a single combed paragraph. The following 2-3 sentences throughout the Results section also seem untidy, requiring some editorial refinement.
- L 383-390: Given the article type (technical note) and the goal of suggesting good practices, it would be helpful if you provide some recommendations about the highly disturbed cases.
- L 394: Please cite the relevant figure.
- L 395-398: This assessment is not fully in line with the results shown in Table 2 (particularly the four mean values of C sequestration rates) and the main conclusion on the compatibility of both methods. Don’t you need to mention at least the results (Table 2) here?
Citation: https://doi.org/10.5194/egusphere-2024-1162-RC2
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R script for "Technical Note: Comparison of radiometric techniques for estimating recent organic carbon sequestration rates in freshwater mineral soil wetlands" Purbasha Mistry, Irena F. Creed, Charles G. Trick, Eric Enanga, and David A. Lobb https://doi.org/10.5281/zenodo.10951658
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