the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2024
Organic Carbon, Mercury, and Sediment Characteristics along a land – shore transect in Arctic Alaska
Abstract. Climate warming in the Arctic results in thawing permafrost and associated processes like thermokarst, especially in ice-rich permafrost regions. Since permafrost soils are one of the largest organic carbon reservoirs of the world, their thawing could lead to the release of greenhouse gases, further exacerbating climate warming. To enhance predictions of potential future impacts of permafrost thaw, we studied how soil characteristics change in response to permafrost landscapes affected by thermokarst processes in an Arctic coastal lowland. We analysed six sediment cores from the Arctic Coastal Plain of northern Alaska, each representing a different landscape feature along a gradient from upland to thermokarst lake and drained basin to thermokarst lagoons in various development stages. For the analysis, a multiproxy approach was used including sedimentological (grain size, bulk density, ice content), biogeochemical (total organic carbon (TOC), TOC density (TOCvol), total nitrogen (TN), stable carbon isotopes (δ13C), TOC/TN ratio, mercury (Hg)), and lipid biomarker (n-alkanes, n-alkanols, average chain length (ACL), 𝑃𝑎𝑞, 𝑃𝑤𝑎𝑥, carbon preference index (CPI), higher plant alcohol index (HPA)) parameters. The results showed highest TOC contents in samples of the thermokarst lake and the drained thermokarst lake basin. Lowest TOC contents were measured in the samples of the semi-drained thermokarst lagoon. The comparison of unfrozen and frozen deposits showed significantly higher TOCvol and TN in the unfrozen deposits. Indicated by the ACL, δ13C and the 𝑃𝑎𝑞, 𝑃𝑤𝑎𝑥 we found a stronger influence of aquatic organic matter (OM) in the OM composition in the soils covered by water, compared to those not covered by water. Moreover, it was indicated by the results of the δ13C, TOC/TN ratio, and the CPI that the saline deposits contain stronger degraded OM than the deposits not influenced by saltwater. Additionally, we found positive correlations between the TOC and TOCvol and the Hg content in the deposits. The results indicate that thermokarst-influenced deposits tend to accumulate Hg during thawed periods and thus contain more Hg than the upland permafrost deposits that have not been impacted by lake formation. Our findings offer valuable insights into the dynamics of carbon storage and vulnerability to decomposition in coastal permafrost landscapes, reflecting the interplay of environmental factors, landform characteristics, and climate change impacts on Arctic permafrost environments.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3683', Anonymous Referee #1, 07 Feb 2025
The manuscript titled ‘Organic Carbon, Mercury, and Sediment Characteristics along a land – shore transect in Arctic Alaska’ by Giest et al. presents results from several sediment cores covering the variable coastal permafrost landscape in northern Alaska. The study comprises of downcore geochemical and biomarker dataset showing differences in quality and sources of organic carbon along the transect and provides insights on carbon decomposition on the variable coastal landscape covering different salinities and thermokarst influence. In its current form, the manuscript is rather heavy to read and could use shortening/sharpening of the text, especially the discussion. Below specific comments and a few suggestions also on how to improve the layout and figures.
Specific comments:
Line 23-26: This reads as a list of results, I suggest adding interpretation or removing from the abstract.
Line 45: Use only the abbreviation of IPCC as the abbreviation already explained previously.
Line 48: Add a reference for the permafrost temperature increase.
Line 115: Give a temperature for the cooled samples.
Line 117: Were all the analyses executed at AWI Potsdam? Provide this information e.g., in the first paragraph of this chapter.
Line 118: This first sentence is not necessary.
Line 122: Add here that these ‘other laboratory analyses’ include hydrochemical analyses as they are not mentioned anywhere in the actual manuscript.
Line 132: Refer to the supplementary text S1.1 in this paragraph.
Line 160: This heading is missing a number (same on line 176). I suggest also changing this heading to ‘Extraction and Analysis’ or something similar.
Line 165: Define the abbreviation NSO.
Line 183: I would advise to remove the ‘higher plants’ here or rephrase as bryophytes should not be classified as higher plants, and referencing ‘leaf waxes’ as higher plants does not sound correct.
Line 221. Results. The results are reported in a very detailed manner and could benefit of shortening in places. I advise to report the main/significant results and trends seen in the data and then refer to the supplement/database for more details.
Line 222: Bulk density and water content are mentioned in the methods, but the results are not reported in this paragraph. Add a phrase to this section reporting the main results, or just refer to the database where sedimentological data is made available. The authors could also move the bulk density and water content method section to the supplement as these results do not seem to be the most essential and just refer to them in the main document.
Lines 223-235: It seems that mostly the cores consist of silt and clays, except the marine one. I would shorten this section to report the main trends, exceptions, and then refer to a table/supplement for detailed results.
Line 322. Discussion. I encourage the authors to sharpen the discussion and reduce the result reporting in this section. The text in the conclusions flows well so I would suggest writing the discussion in a more similar style to the conclusion.
Line 323: I advise to remove this heading and instead split the 5.1.1 in two by separating the discussion on the biomarkers under a separate heading.
Line 327: The authors write that their data is comparable to other studies, so I advise to add more than one reference study.
Line 343: Is Strauss et al 2015 the most suited reference for this?
Lines 374-375: This is not necessary.
Lines 383-387: Here one example where the text could be shortened. I advise to summarise all this information into one sentences or even combining with the last sentence of this paragraph.
Line 412: More descriptive title would help the reader to navigate the discussion (e.g., title of the chapter 5.4 is informative).
Line 413: ‘Other parameters’ here sounds vague, giving an example could be helpful.
Line 448-451: This paragraph is not necessary.
Lines 452-459: Here another example where the authors could consider shortening the text as in its current form it is lengthy when the main message is that high ACL and d13C, and low TOC/TN ratios in saline samples indicate the presence of a stronger aquatic OM proportion.
Line 494-502: Add references to this paragraph.
Figures:
Fig 2. Increase the font size of the axis titles. I would also consider moving this figure to the supplement.
Fig 3 and 4: These are showing in essence the same data, so one of these figures could be moved to the supplement.
Fig 5 and 6: I suggest combining these two figures to a four-panel figure as they show related data.
Supplement:
Fig. S1. I like how this figure clearly shows the differences between the sampling sites and thus, think that it would be useful to have a version of this as one of the main figures (excluding the detailed soil characteristics info). Perhaps the authors could add this as a panel on Fig. 1?
Citation: https://doi.org/10.5194/egusphere-2024-3683-RC1 - AC1: 'Reply on RC1', Jens Strauss, 23 Mar 2025
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RC2: 'Comment on egusphere-2024-3683', Anonymous Referee #2, 12 Feb 2025
Accept with moderate revisions
Comments on the paper “Organic Carbon, Mercury, and Sediment Characteristics along a land
– shore transect in Arctic Alaska” by Giest et al.:
Global comments:
- Overall the discussion is too centered on describing the results on not enough on interpreting them, although the conclusion is clear and concise.
- The abstract and discussion have a big part on mercury but there is no mention in the conclusion. Mercury feels like an added measurements without much justification. There is not much background on mercury, especially on the different form of mercury and their risks.
- In the supplementary methods, L29-45 the authors describe pore-water analysis that are not used or cited in the main manuscript. These results can be combined with the bulk TOC, TN ratios. If these analysis are not used for the interpretation of the results the methods can be deleted.
Detailed comments:
L16: “their thawing could lead to the release…” I think that multiple studies have shown that thawing permafrost does release GHG, you should reformulate the sentence, maybe removing “could”.
L16-17: I guess you did not do any modeling of future impacts of permafrost thaw in this paper, this sentence “warming. To enhance predictions of potential future impacts of permafrost thaw” is overselling the study. Rather you studied the impact of thermokarst processes, which in turn could be useful for predicting future effect of permafrost thaw.
L22-23: As it is the abstract I would shorten to “lipid biomarkers (n-alkanes, n-alkanols and their ratios).
L48: A reference is missing
L56: rather than “quality” you analyse the source of OM. If you refer to quality then you need to explain what you mean: lability?
L73-74: In this paragraph you talk about mercury in general without identifying if it is total mercury, methylmercury or elemental mercury that is measured. This is important when stating the danger to human health and ecosystems as the different mercury form will be more or less dangerous. It should be also mentioned that there is usually a correlation between TOC and total mercury (e.g. Chakraborty et al., 2015).
L76: OM rather than “OC”
L109: maybe marine coastal instead of just “marine”?
L153: I guess you measured d13C on bulk organic matter? If so it would be good to add “The measurement of the δ13C signature of organic matter”
L155: degradation is also a process affecting d13C, as you mention L391,
L153-158: Which reference standard were used to calculate the ratio? What was the instrumental error/accuracy?
L190-192: Paq has been used a lot and has many bias, especially in region where floating and submerged vegetation are not fully characterized, such as in the Arctic. In addition, it is rarely used in soils but rather in lake sediment cores. Since the authors are studying different types of environments: soil, coastal, lake, I feel like this ratio does not add much information as you would expect higher aquatic influence in the marine and lacustrine environment. Similarly for Pwax. In comparison, the degradation indices are much more useful for the study.
Fig. 3: d13C needs error bars
L259: small typo “Mercury [ug …]”
L382-383: This sentence is very hard to understand, could you rephrase it?
L412. The title of this section is quite hard to guess could you make it more detailed (e.g. Mercury content)? In this paragraph the biomarkers are not compared to mercury content, why not? Right now this paragraph does not seem linked to the rest of the study
L415: can you add the correlation factor between TOC and mercury in the text to make it directly visible to the reader. O.34 and 0.42 are quite weak correlation. I would rather point and explain the correlation with grain size (-0.77). Further down in the text, the correlation with d13C is also rather weak (-0.42) and should be mentioned directly in the text.
L427: not just “additionally” but rather “mainly”. See above
Figure 7: you can remove the 1:1 correlation point to make the figure more readable
L455-456: The ACL difference is mainly indicative of additional primary production on site no? Since the concentration of the alkane is known the authors can mention which alkane increase in the saline and unfrozen deposit compared to the others.
L458: “13” should be superscript
References
Chakraborty et al., 2015: https://doi.org/10.1016/j.marchem.2014.10.005
Citation: https://doi.org/10.5194/egusphere-2024-3683-RC2 - AC2: 'Reply on RC2', Jens Strauss, 23 Mar 2025
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