the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2024
Locally Produced Sedimentary Biomarkers in High-Altitude Catchments Outweigh Upstream River Transport in Sedimentary Archives
Abstract. Sedimentary records of lipid biomarkers such as leaf wax n-alkanes are not only influenced by ecosystem turnover and physiological changes in plants, they are also influenced by earth surface processes integrating these signals. The integration of biomarkers into the sedimentary record and the effects of integration processes on recorded environmental signals are complex and not fully understood. To determine the depositional constraints on biomarker records in a high-altitude small catchment system, we collected both soil and stream sediments along a 1000 m altitude transect (1500 – 2500 masl) in the Areguni Mountains, a subrange of the Lesser Caucasus Mountains in Armenia. We utilize the existence of a treeline at ~ 2000 masl, which separates alpine meadow above from deciduous forest below, to assess the relative contribution of upstream biomarker transport to local vegetation input in the stream. We find that average chain length (ACL), hydrogen isotope (δD) and carbon isotope (δ13C) values of n-alkanes are significantly different in soils collected above and below the treeline. However, samples collected from the stream sediments do not integrate these signals quantitively. As the stream drops below the treeline, the ACL, δD and δ13C values of n-alkanes preserved in streambed sediments reflect a bias toward n-alkanes sourced from trees. This suggests that there is either 1) minimal transportation of organic matter from the more open vegetation in higher elevations, or 2) greater production of target biomarkers by trees and shurbs found at lower elevations results in overprinting of stream signals by local vegetation. Though this latter observation may preclude using n-alkanes to measure past treeline movement in these mountains, δD values of biomarkers in fluvial deposits in these settings are more likely to record local hydrological changes rather than changes in upstream fractionation differences associated with vegetation turnover.
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RC1: 'Comment on egusphere-2024-724', Joseph B. Novak, 21 Apr 2024
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Review of Brittingham et al.
“Locally Produced Sedimentary Biomarkers in High-Altitude Catchments Outweigh Upstream River Transport in Sedimentary Archives”
by Joseph Novak (jobnovak@ucsc.edu)
Summary and overall significance
Brittingham et al. present a study of higher plant n-alkane transport in a small stream in the Armenian Caucasus. Their analysis of n-alkane distributions and isotopic composition in stream sediments and soils revealed that, contradictory to expectation, stream sediments did not have an n-alkane composition that approximated the area-weighted upstream vegetation. This is a valuable contribution to the field that provides needed insight into how the transport of n-alkanes may imprint upon sedimentary archives utilized for paleoclimate or palaeoecological study. Overall, the manuscript’s main findings are well-supported by the data, and I look forward to the article’s publication after revision to address my comments and technical corrections.
Recommendation: minor revision
General comments:
My main comment on the manuscript is a request that the authors include information about their mixing model, including necessary equations and values used for constants, as a subsection in the methods. The values for constants could be added as a table or in the text. The additional text and equations would substantially clarify the discussion in section 4.2. and, more importantly, are necessary for evaluating the comparison between measured and expected values shown in Figure 6. Much of the necessary text can be moved from lines 194–208 to the methods section.
I also strongly recommend appending the supporting data for this study as supplementary tables or as files uploaded to a publicly accessible database. The data may prove useful for future paleoclimate or palaeoecological work in this region, so it is important to make sure the data is accessible to those who may need it in the future.
Lastly, I recommend inserting more references to the figures within the text. As is, each figure is only referenced once or twice, even in sections where the data is discussed in detail. More figure references will make the text easier to follow.
Specific comments
Title
I suggest amending the title to better reflect the findings and conclusions of the study. As is, the title appears to suggest that the findings here are applicable to all high-altitude river (and lake?) catchments. However, this assertion is not made in the manuscript.
Two other things to point: (1) n-alkanes measured in this study should really be referred to as leaf wax or vegetation biomarkers since they are produced by plants, not produced by sediments; (2) I believe you mean downstream river transport, as in away from the river’s headwaters. As written, the word upstream implies transport of sediment in the opposite direction of the water flow, which I do not think is what you mean.
A potential alternative title could be:
“Locally Produced Leaf Wax Biomarkers in the High-Altitude Lesser Caucuses Outweigh Downstream Transport”
Abstract
Lines 18–22: these first two sentences are a bit repetitive. I suggest removing the first sentence that reads, “Sedimentary records…signals.” and starting the abstract with the second sentence that reads “The integration of…”. This still introduces the study effectively while making the language more succinct.
Line 25: I suggest removing “the existence of” and just stating “We utilize a treeline…”
Lines 35–36: Please clarify the language here, I am unsure what you mean by the “latter observation” since the language in lines 32–35 makes it seem like you do not have sufficient evidence to distinguish which mechanism is driving the pattern you observe in the stream sediments (this is also what I took away from reading the rest of the manuscript).
Line 38: I am unclear what “changes in upstream fractionation differences” means here. I suspect you are talking about fractionation of leaf wax hydrogen isotope composition caused by changes in landscape vegetation, please clarify the language as this confused me a bit.
Introduction
Line 45: Is “so called” necessary here?
Line 49: I suggest adding a sentence here briefly summarizing any paleoclimate records from this region that utilize leaf wax n-alkanes if there are any as that will clarify the importance of this study to your readers.
Line 62: I suggest replacing “though” with “although”
Line 64: I suggest rewriting as “The carbon isotope (δ13C) composition of plant tissue is primarily set by the photosynthetic pathway of the plant.
Line 72: Could you please comment on which of these factors is important for your study site? For example, are there C4 plants in the Caucuses?
Lines 83–86: I suggest rewriting this sentence to summarize the common findings of the studies you cite here. For example, I recall that the Feakins et al. 2018a study cited here found that riverine n-alkanes approximated area-weighted vegetation in the Amazon River catchment (the opposite of your finding, which highlights what makes your study interesting). Perhaps these other studies have found the same thing, or mixed results?
Lines 99–102: I suggest rewriting as “Comparison of the hillside and streambed sedimentary n-alkanes allows…”. The current wording is a bit repetitive and clunky.
Lines 104–108: I suggest splitting into two sentences, revising to: “…Tornero et al., 2016). Pleistocene sediments…”
Lines 118–119: I suggest removing the phrase “In order to extract n-alkanes” as it is repeating the end of the previous sentence.
Lines 120–121: please describe the solvent schedule, water content of the silica gel, and quantity of silica gel used for chromatographic separation.
Line 123: please describe any internal and external standards used for sample quantification. Please clarify whether and how the different response factors of the C25 – C33n-alkanes were accounted for during sample quantification or in calculation of the OEP and ACL values. This is important because, while the peak area of each compound is proportional to concentration, that relationship (response factor) is mass-dependent and so is slightly different for each of the n-alkanes.
Line 125: Did you forget to insert a citation here?
Line 136: How exactly did you correct for size effects? Also, I think you want to use the standard error of the mean, not standard deviation. In general, Polissar and D’Andrea (2011) [https://doi.org/10.1016/j.gca.2013.12.021] is an excellent guide on how to calculate uncertainties associated with leaf wax stable isotope measurements, particularly hydrogen isotopes.
Results
Line 144: could you please provide some example chromatograms or histograms showing the n-alkane distributions in your samples? This could be as a supplementary figure if you prefer.
Lines 144–145: I suggest reporting the range of carbon preference index (CPI) values of your samples to support this statement.
Line 149: please replace “averages” with “is”
Lines 150–151: Please replace “average” with “mean”
Lines 151–153: Please specify what kind of statistical test was done here and the N samples or degrees of freedom. Same goes for the test mentioned in lines 155–156.
Line 161: please replace “average” with mean
Lines 170–171: does the word “significant” imply a statistical test was done here? If so, please specify what kind, the p-value, and N samples or degrees of freedom
Lines 173–174: I see, this is what is referred to in 170–171. Please consider combining and condensing into a single sentence to clarify the language / keep the text concise.
Line 177: Please specify the N samples or degrees of freedom
Lines 178–179: Please revise to: “δD values were also more negative in stream sediment samples collected above the treeline (-175‰) than those collected below the treeline (-158‰).”
Please also specify the statistical test, p-value, and N samples / degrees of freedom
Line 180: Please revise to “…stream sediment samples…”
Discussion
Lines 194–208: as said in my general comments, please move these lines to a new section of the methods that includes the equations and constants used in the mixing model. In addition, please specify where the satellite images used for your mapping came from with appropriate references.
Line 209: I recommend beginning your revised section 4.2. here. Also, I suggest replacing “this mixing model” with “our mixing model”
Line 211: range of expected values for which measurement(s)? Please clarify.
Line 212: Please reference a figure to back up this assertion
Lines 213–214: Please also reference a figure here. Additionally, this is not really how ACL values (I assume that is what you are talking about here, please clarify) are used in practice. They tend to be thought of as a more qualitative indicator of vegetation / ecosystem composition. I would suggest rewriting this sentence to discuss that the n-alkane distributions do not show an expected “mixed” signal but are rather indistinguishable from the endmember values.
Line 223: I suggest replacing “though” with “although”
Lines 236–241: Please clarify the language here. I had to reread this section several times to understand what you mean.
Lines 245–254: this is a good idea, but the implementation here could use some improvement. More information is needed about how the lines in Figure 7 were calculated, as is the “expected” δD timeseries and the relative timing of the vegetation shift. It may be useful to expand this exercise into its own section in the Discussion with an accompanying short section in the Methods, but this is really just a suggestion.
Line 251: I suggest replacing “heavily affected” with “influenced”
Lines 255–258: I think this is better suited to the Introduction. Please see my comment regarding lines 83–86.
Conclusions
No comments
References
Line 294: why is this centered and not aligned to the left? Also, should it not be bolded and called “references”?
In general, please check the formatting of your citations and ensure that all article titles have proper typesetting (subscripts, superscripts, Greek characters, etc.), that journal names are italicized, and check for typos.
Figures
In general, please ensure to upload 300 DPI or higher images or vector files of the images. The current images are a bit fuzzy.
Figure 1: the points showing your sample sites in the right panel are very difficult to see. Could they be larger or a different shape? Perhaps a shape with a black border would help.
Figure 2: in the figure caption the samples are referred to as sediments while on the figure they are referred to as soils. Please make these consistent with each other. I also recommend switching to a colorblind friendly color palette (red-green is particularly hard to distinguish for many colorblind people).
Figure 3: Please remove the lines. They really clutter up the figure and make it difficult to see the pattern that is very clearly evident in the data (really nice result!). Also, I recommend making this figure square to match the style of figure 2. Finally, please consider using three different symbol shapes and a colorblind-friendly color palette.
Figure 4: please also remove the lines here. Same comments as figure 3 regarding the color palette and symbol shapes. Also, please use δ13C for your y-axis label for consistency with Figure 2.
Figure 5: please also remove the lines here. Same comments as figure 3 regarding the color palette and symbol shapes. Also, please use δD for your y-axis label for consistency with Figure 2.
Figure 6: Could this figure be made a little larger? It is very hard to read in this small format. Also, the low DPI of the image is really evident here. Please be sure to replace with a higher quality image. I also recommend switching to a colorblind friendly color palette and using different symbol shapes for the different panels.
Figure 7: please see my comment regarding lines 245–254 of the text
Technical Corrections
Line 34: Typo, shrubs is misspelled as “shurbs”
Line 47: Typo here, “supports” should be “supporting”
Line 78: Missing space between period and first word of following sentence “…(Gamarra et al., 2016).The fractionation…”
Line 172: Typo, “averages” should be “average”
Line 174: lower case delta is needed, not an upper case delta. It does not matter that this is the beginning of a sentence, the upper case delta means something different than the lower case delta and is not applicable here.
Lines 177–178: Typo here, sentence is divided by a random period
Citation: https://doi.org/10.5194/egusphere-2024-724-RC1
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