Reviewer Comments

Jongejans et al. present a study that uses bulk and organic geochemical measurements to investigate the organic matter stored in the ~55 m headwall of the Batagay slump. This is quite intriguing as this slump is a unique insight into a long history of permafrost accumulation (and degradation) that may be useful for both understanding the evolution of permafrost landscapes as well as predicting future impacts of carbon stored in permafrost sediments as the environment is transformed by anthropogenic climate change. Overall, I find their manuscript to be well-written and quite detailed, but not overly long. I deeply appreciate the technical detail presented in the biomarker work (Table 2 is a delight) that is sometimes overlooked or omitted by organic geochemists.

I have relatively few questions and comments as this paper seems to fit nicely into the recent series of papers involving the ongoing investigation of the Batagay slump. Here are a few worth considering:

• The authors note that the interglacial units appear to have “decreased OM quality” whereas the glacial periods have “variable but overall higher OM quality”. This makes sense when thinking about relative temperature and rates of cycling and, perhaps, the different residence time of OM within the active layer where OM degradation occurs. However, the OM stock sizes between the interglacial and glacial periods must be quite different. Therefore, we might consider the differing consequences of releasing a relatively small amount of “fresh” glacial-era OM compared to relatively large amounts of “degraded” interglacial-era OM. Additionally, regardless of the characteristics of sediment-bound particulate OM, the Woody Layer contains, of course, wood and other plant detritus that will be readily remineralized upon thaw.
• Related to the above, I think a worthwhile and interesting calculation would be to estimate (even roughly) the relative sizes of C stocks within each of the types of units. If we could estimate the average C stock (i.e., organic C density per unit area) of these units, could we then also estimate (again, roughly) the amount of C mobilized by the Batagay slump since its formation?
• Combined with stock estimates, the authors could incorporate some of the biomarker-based degradation insights to categorize the pools of carbon mobilized as either “pre-processed” or “fresh” to perhaps get some insight into if we expect the mobilized material to be quickly remineralized or simply redeposited downstream. Combining this with knowledge of other thaw slumps could be useful for developing some insights into the consequences of this type of extreme thaw into local (nutrient loading), regional (source of deltaic organic matter), and even global (atmospheric) carbon cycles.
• This may be more appropriate for a different article (perhaps one with a stronger focus on cryostratigraphy and geomorphology), but, is the size/scale of Batagay a unique feature? Retrogressive thaw slumps are well-studied and widely documented, but the scale of Batagay is quite impressive. While we can expect that as we warm the Arctic, we will have more thaw-related features, will we expect more Batagay-scale slumps? And, do we think there is anything unique in terms of biogeochemical cycling and/or consequences for local/downstream ecosystems of a single Batagay-scale slump versus multiple, smaller slumps whose total volume of mobilized permafrost might be similar to Batagay?
• The authors note that an unconformity exists between the Lower Sand Unit and the Woody Layer and that the Woody Layer occupies erosional gullies that formed during the last interglacial. While I realize precise dating of accumulation rates is difficult, I would be curious to see an estimate of the amount of carbon mobilized from the Lower Sand Unit due to the warming-induced erosional processes during the last interglacial.

Permafrost soils store more than 30% of the global surface organic carbon. The thaw-induced carbon release in the form of greenhouse gases would create a positive feedback to amplify climate warming. In this paper, Jongejans et al., present the valuable records of TOC, TN and multiple lipid biomarkers to assess organic matter quality in a 650-ka ancient permafrost of east Siberia. Such old records cover glacial-interglacial climate variations, which provides a valuable opportunity to explore potential influence of climate changes on permafrost carbon cycling. Their biomarker evidences show higher organic matter decomposition during interglacial periods. This will be very useful for understanding of carbon cycles in permafrost regions as global climate warms. I have no major comments on this valuable paper and hence recommend to accept it after the following minor/moderate comments have been covered.

L22: Add what lipid biomarkers did you analyze in this study, such as alkanes, fatty acids.

L29: Delete “or bacterial”. Microbial origin contains bacterial origin.

L36: the world’s surface soil carbon?

L57-58: Biomarker tools as tracing permafrost thaw and carbon cycling are very important in this study. I suggest more previous publications are needed to introduce here. Multiple lipid biomarkers have been applied to sediment records for reconstructing carbon perturbations of permafrost (e.g., Evert et al. 2016, https://doi.org/10.1177/0959683616645942; Yao et al., 2021, https://doi.org/10.1130/G48891.1).

L115: What solvents (including solvent volume) do you use for separation the aliphatic, aromatic and polar NSO?

L127: The first mention of abbreviation “FA” is fatty acids?

L127: ACL can be also affected by climate changes and resulting alkane degradation, such as temperature and relative humidity. Terrestrial plants tend to produce longer n-alkanes to protect their water loss under higher temperature and drier conditions. Moreover, higher temperature and wetter conditions may facilitate higher microbial activities, may resulting in the faster degradation of organic matter.

L148: Add “may” before “contain”.

L145-146: Could you show a supplementary figure or table for these correlations?

L227: Please specify what biomolecules or organic indices can give insight into different OM sources.

L240: Change to “higher ACL” and “lower P_aq”.

L260: Add a supplementary figure or table for these correlations. And elsewhere.

L280: Higher ACL does not indicate higher terrestrial source. Higher temperature or drier climate can also lead to higher ACL values.

L285: ACL can be affected multiple factors. Please see my comment “L127”.

L308: Are there aquatic plants grow around the study area? Higher P_aq ratios could also be due to input of mosses - they also produce lots of mid-chain n-alkanes.

L318 and 330: Again, delete “or bacterial”. Microbial origin contains bacterial origin.

L346: Change to “rivers”.

L357: Add more references (e.g., Evert et al. 2016, https://doi.org/10.1177/0959683616645942; Yao et al., 2021, https://doi.org/10.1130/G48891.1).

L359: Please specify what past environments.

L370: The impacts of findings should be described as well. E.g. why are these findings important and for whom?

Table 2: Add m/z data of molecular weight, base peak, and characteristic peak of each individual compounds.

This manuscript provides detailed lipid biomarker analyses of exposed permafrost (including ancient deposits) from the recently exposed Batagay Megaslump, East Siberia.  I generally found the paper to be a good read and the analyses were appropriate for the sample type, of which the deposit is quite novel and covers an impressive time window (up to 650 ka yr).  A number of existing/in review studies have worked on this exposure, so it is good to see one which focuses on organic geochemistry/biomarkers.

To improve the manuscript further the palaeoclimate discussion section would benefit from some revisions by placing each time period more clearly in place with the known climate and environmental conditions at the time (palaeoclimate). Placing the results within a critical framework explaining where these results agree with (or disagree with) existing findings in each time period a little bit more clearly would be helpful and enable the interpretation of any further palaeoclimate interpretations of significance.  A table might help here and a little bit of wider reading.

I was also wondering if the group had any compound-specific stable isotope analyses, for example on long chain alkanes or fatty acids to identify any differences between interglacial vegetation and its source/productivity? If they do, then these data should be included.  The discussion would also benefit from a synthesis plot plotting regional data from other palaeobotanical proxies from the site (e.g. Ashastina et al., 2018; Opel et al., 2019) and more widely regionally. This will also assist with improving the palaeoclimate discussion section and show to readers more directly the similarities. 

Finally, I note that the current title is quite descriptive in nature.  An alternative approach is to revise the title to make it more impactful and focus on the main finding or outcome from the paper.  More interrogation of the palaeoclimate discussion section will help here, but an example could be (from the abstract) to devise a title that focus on terrestrial character of the glacial periods, or the high microbial activity in the interglacial. The potential lability of the Holocene deposit (and its vulnerability) could also be an option as here it is possible to make comparisons with older deposits so effectively.

Detailed comments

31. Do you think that the most recent Holocene deposited permafrost is most vulnerable to warming, leading to OM degradation?

36-37. Sentences with the same reference (Strauss et al., 2021) could benefit from merging or rephrasing to improve readability.

74. Perhaps change ‘herbs occur’ to ‘herbs are present’?

96. Change ‘the Spring Expedition’ to ‘a spring expedition’

115-116. What instrument/technique is used for medium pressure liquid chromatography? I think you should include this.

118. “biogeochemical and alkane parameters” Alkane parameters are ‘biogeochemical’ so rephrase?

Table 1. References to the sources of these indexes could also be included in the table in brackets to enable easy source identification.

157- 158. Should it read “Here, we found…”?

Figure 2. You display a number of indices. You could also consider calculating and displaying TARFA (Meyers et al., 1993) and CPI for fatty acids (Matsuda and Koyama, 1977).

188. Please double check you are happy with use of the term ‘lenses’.

201. Is the comma (,) necessary after ‘Above’?

202 & 214. ‘medium high’ – how about moderately high?

Discussion first paragraph (223-229) – Make clear the differences are seen over contrasting climate periods (glacial-interglacial periods), showing how land-cover varied over different climate regimes.

233. ‘to some extent’ a bit vague.  Can you explain to what extent or specify?

240. Write ‘more terrestrial and less aquatic’ to correspond with preceding order of high ACL and low Paq.

248. Do we know which microbes were present if abundance and what is their function?  Check with Courtin et al, (in review).

249. Pollen findings – could these unpublished findings/companion papers data be introduced here in a summary synthesis plot (most impactful findings) to help with the biomarker comparison?

Discussion of the time periods would be helped by a table showing time period, deposit, references and interpretation to make this section clearer and easier to follow.

281. Suggest replace ‘stronger OM’ with greater or ‘higher OM’. 

309. I think the role of diagenesis/preservation in the Holocene deposits could be introduced here.

321. I suggest to change ‘stated’ to suggested and ‘are’ to ‘could be’.

365 onwards. Given the relatively wet conditions interpreted are you suggesting that the Holocene was a unique interglacial then, geographically at this position, compared with previous interglacial periods?