Bounding cross-shelf transport time and degradation in Siberian-Arctic land-ocean carbon transfer

The burial of terrestrial organic carbon (terrOC) in marine sediments contributes to the regulation of atmospheric CO2 on geological timescales and may mitigate positive feedback to present-day climate warming. However, the fate of terrOC in marine settings is debated, with uncertainties regarding its degradation during transport. Here, we employ compound-specific radiocarbon analyses of terrestrial biomarkers to determine cross-shelf transport times. For the World’s largest marginal sea, the East Siberian Arctic shelf, transport requires 3600 ± 300 years for the 600 km from the Lena River to the Laptev Sea shelf edge. TerrOC was reduced by ~85% during transit resulting in a degradation rate constant of 2.4 ± 0.6 kyr−1. Hence, terrOC degradation during cross-shelf transport constitutes a carbon source to the atmosphere over millennial time. For the contemporary carbon cycle on the other hand, slow terrOC degradation brings considerable attenuation of the decadal-centennial permafrost carbon-climate feedback caused by global warming.

I would also like to see the authors to acknowledge that the rate constants presented in their manuscript are averaged over a kyr timeframe. In reality, the highest rates of decomposition occur soon after deposition and averaging over long timescales likely underestimates the reactivity of terrOC. Another important point is that degradation of terrOC such as lignin only occurs when oxygen is present so oxygen exposure time (OET), a parameter that may be embedded in transport time, is an important determinant of the role of terrOC in the carbon cycle.
Overall, this is a unique and interesting data set that provides new insights about the transport time and fate of terrOC across the Siberian-Arctic shelf. I recommend publication after considering the issue mentioned above as well as some rewriting of the manuscript. In some places, the manuscript is worded awkwardly and the word choices could be more direct. I've provided detailed comments below that the authors should consider when revising their manuscript.
Editorial Comments: Abstract should be rewritten. Choice of wording is awkward and the findings should be presented more directly and succinctly.
Line 21-22 and Line 41. Change to "that regulates atmospheric CO2" or "contributes to the regulation of atmospheric CO2" Line 24-25. Revise to, "compound specific radiocarbon analyses of terrestrial biomarkers to date cross-shelf transport times" Line 27-28. Revise to, "TerrOC was reduced by 85% during transit resulting in a degradation rate constant of 2.4 + 0.6 kyr-1" Line 28. What do the authors mean by "protracted transport"? Line 42. Omit "only".
Line 62 and throughout text including figure captions . The correct language is "long-chain n-fatty acids, LCFA", not "long-chained". The authors may want to note that long-chain fatty acids derive from plant waxes, but I suggest using an alternative to "wax lipids", which could be confused with "wax esters".
Line 66. Use alternative wording for "clock", which is not customarily used as a verb (e.g., "date", or "determine the net transport time") Line 73. Authors should define what they mean by "potentially significant in situ aging". This language is vague and should be omitted or revised.
Line 74. Revise to, "and are not unidirectional".
Lines 102-103. This paragraph opens by discussing hydrogen isotopes as source tracers but the authors discuss their findings as "concentrated weighted average n-alkanes". Instead, I recommend that the authors describe their findings as "concentrated weighted average values of 2H for C27, C29 and C31 n-alkanes". Revise description of findings to be more direct (e.g., "remained constant with water depth" rather than "does not display a significant trend with water depth").
Line 114. Revise to "are deposited in shallow waters" rather than "trapped".
Line 119. Define "fine" vs. "ultra-fine". Line 121. Avoid using "significant" here and throughout text unless statistical support is provided.
Line 133. The authors present "first-order rate constants" (units of per time) rather than "rates" (units of changes in concentration per unit time). The text and equation terms should be revised accordingly.
Line 137. I think this should read terrOC and biomarker analyses. The present wording suggests the ratio of terrOC to biomarkers (terrOC/biomarkers).
Line 149-150. Specify that cross-shelf transport FOR THIS SYSTEM is a millenial scale process. Without additional information, it is unclear whether rates for the Laptev Sea shelf can be extended to other systems. Figure 1. Revise y-axis label to read, "δ2H of long-chain n-alkanes [‰]. "HMW" is not defined and is unnecessary because the n-alkanes are already defined as "long-chain".

GENERAL COMMENTS:
"This paper reports the timescale of cross shelf transport of terrestrial organic matter on the continental shelf off the Lena River mouth. They successfully conducted difficult tasks and I have positively read it. I think this paper is acceptable for the publication in Nature Communications." We appreciate and are encouraged by the positive assessment and have paid attention to the constructive suggestions. Please find below our detailed responses (and actions in response) to each of the raised issues.

SPECIFIC POINTS:
1) "Authors seem to assume that the 14C age of a minor component (FAs) represents a major fraction of sediment. Please clearly explain the assumptions reside background of this study." We chose these biomarkers because previous work had shown that FAs (as well as other lipids) are mainly associated with the mineral-bound fine fraction that is preferentially transported (e.g., Tesi et al., 2016). Also, long-chain FAs are common biomarkers for terrOC and the method for compound-specific radiocarbon analysis on these compounds is well established (e.g., Eglinton et al., 1996;Mollenhauer and Rethemeyer, 2009).
Importantly, we are not assuming that the 14 C ages of these terrestrial biomarkers are representing the 14 C ages of the bulk OC. Instead, we actually show in Fig. 2 that the opposite is the case: while the 14 C ages of the FAMEs increase with increasing water depth, the bulk OC 14 C ages decrease due to a growing proportion of marine OC. This difference illustrates the power in the source-specificity provided by molecular-14 C analysis. Furthermore, we acknowledge that biomarkers in general only make up a small portion of the total OC (Line 65-67).
2) "FA age presented in Fig. 2  This is a good clarifying point; any CSRA measurement is representing the average for a population of the target molecule, which naturally has a distribution of 14C ages. Hence, one cannot entirely rule out changing proportions of young vs old FAs along the transect. However, the associated lack of changes in their δ 13 C signature ( Supplementary Fig. 2), the relative contributions of the different long chain-lengths ( Supplementary Fig. 1B) and in the δD of the long-chain alkanes ( Supplementary Fig. 3) all provide pieces of evidence/information, supporting a fairly constant proportion along the transect. Taken all these data together, it is therefore reasonable to hypothesize that the trend towards higher FA ages with increasing water depth is indeed caused by ageing of the entire population during transport rather than changing proportions (as two identical molecules of different 14 C ages still have the same physical and chemical properties yielding similar processing). We agree that Δ 14 C can provide useful extra information, however, to measure the cross-shelf transport time, years (or in this case kyr) seemed the more appropriate unit. Since we are using calibrated 14 C ages there is no linear conversion to Δ 14 C; therefore, we could not simply add a Δ 14 C scale to Fig. 2 One expects that, with ongoing warming and continued permafrost thaw, increasing amounts of pre-aged terrOC will be released and eventually end up in shelf sediments. With transport times on the order of millennia, recent climate change events (on the order of decades) have likely not yet progressed far across the shelf. It is conceivable that the station close to land could have received a higher proportion of old permafrost material in recent times. However, the stable hydrogen isotope values for long-chain n-alkanes and the mineralogical composition suggest that any such changing input is at most of minimal influence to the make-up of the current system (Supplementary Fig. 3 and 4, Supplementary Table 1).

5) "The authors should mention about contamination from GC column bleed. I think open column chromatography is advisable after GC/PFC to remove column bleed."
This is a good point. Open column chromatography as a mean to remove potential contamination from column bleed has been tested on some of these samples. However, small sample sizes, low recoveries and comparably little effect on the purity of the isolated compounds ultimately prevented us from using it. We carefully monitored the purity of the isolated compounds by running the samples on the GC-FID prior to AMS analyses, the purities were fortunately high, and these results are reported in Supplementary Overall, I feel that this paper will be of interest to the community and provides a novel contribution to the literature, which helps to quantitatively address the contribution of a possible 'loss mechanism' for terrestrial organic matter during along-shelf transport. I have outlined some clarification questions for the authors below. Particularly, I have outlined some details that should be included in the main body of the manuscript (some of these are addressed in the methods section, but I feel should be moved to the main discussion) to strengthen the claims the authors make in this paper." We are pleased about the overall positive evaluation and would like to thank the reviewer for the clear, concise and constructive comments. We agree with the majority of the comments and suggestions for further clarification and have revised the manuscript accordingly, as outlined in the responses to the more detailed comments below.
Regarding the first specific comment here on the source apportionment, we have now included an expanded explanation in the main text (Line 138-142). In brief, we are working with two different sets of radiocarbon data: The radiocarbon ages of the terrestrial biomarkers (FAs) are used to determine the cross-shelf transport time, while the Δ 14 C values, along with the δ 13 C, of the total organic carbon (TOC) are used in the source apportionment calculations, following the well-established method applied also in earlier studies (e.g., Bröder et al., 2016;Tesi et al., 2016a;Vonk et al., 2012).
The second general comment concerned the statistical source apportionment method. The Markov chain Monte Carlo approach used in several earlier and ongoing studies is detailed also in the earlier papers, yet perhaps best in the paper by Andersson et al. (2015). It builds on the observation that to obtain statistically sound source estimates from mass balance relations the endmember distributions and data uncertainties need to be taken into account, not only for uncertainty estimation but also for correct estimates of the central values (Andersson, 2011). For the present case, the absolutely largest uncertainty comes from the numerical spread of the endmember distributions. With the current setup, using 1 000 000 iterations, a burn-in of 10 000 iterations and a data thinning of 10, the variability of the runs is much smaller than the standard deviations of the estimated parameter distributions. Running the same simulation 10 times provides a relative standard deviation of less than 1% for the estimated parameter value. Thus, the convergence of the simulations does not affect our interpretations. These factors have been well examined in our previous publications (e.g., Andersson et al., 2015;Fang et al., 2017). More details on the Monte Carlo strategy have now also been included in the Materials and Methods section (Line 335-342).

SPECIFIC POINTS:
1) "Line 55-What is the proportion of carbon delivered to the Arctic Ocean from the Lena River (i.e. the Lena exports the most DOC ~5700 Gg yr-1 (Holmes et al., 2012), and POC ~800 Gg yr-1 (McClelland et al., 2016) of a single arctic river)? See eg. Holmes et al., 2012 (DOC) and McClelland et al., 2016 (POC) for Pan-Arctic and river specific estimates. This gives a bit of context to how much carbon is delivered to this system, and its importance on a larger basin-scale setting (i.e. Arctic Ocean)." We agree that these facts provide useful background information to the reader and have now inserted the following sentence (Line 53-56): "The Lena River in Northern Siberia is the 2nd largest freshwater source to the Arctic Ocean delivering the largest amounts of dissolved and particulate terrOC of a single Arctic river (~5.7 and ~0.8 Tg C per year, respectively), which corresponds to 18 % and 14 %, respectively, of the total delivery to the Arctic Ocean (Holmes et al., 2012;McClelland et al., 2016 We understand that the answers to the reviewer's questions were mainly found in the Materials and Methods section and not in the main manuscript. We have therefore lifted some information from the Materials and Methods section to the main text (Line 62-65 and Line 101-105) and hope that this clarifies matters more efficiently.
Regarding the first point, the exclusivity of the long-chained FA biomarkers: The FA chainlengths isolated in this study for radiocarbon measurements were 24, 26, 28 and 30 carbon atoms, commonly employed terrestrial biomarkers; FAs produced by marine plankton have lower chain-lengths. As can be seen in Supplementary Fig. 1A, the ratio between short-and long-chain FAs clearly changes with increasing water depth, due to a growing proportion of marine OC. The proportions of the long-chain FAs, on the other hand, do not display such a trend ( Supplementary Fig. 1B). The same pattern is observed for δ 13 C values ( Supplementary  Fig. 2). Marine OC is generally more enriched, explaining the trend towards higher δ 13 C values for bulk OC, while δ 13 C values for the FAs are constantly low throughout the transect.
Regarding the second question about other terrestrial biomarkers: For our study area a recent publication, Tesi et al., 2016, showed that long-chain FAs are mainly associated with (i.e., bound to) the fine sediment fraction that is transported across the shelf, while a large fraction of the lignin phenols is found in matrix-free plant fragments that are retained close to the coast due to the large diameters of these fragments. For this reason, even though both lignin phenols and FAs are terrestrial biomarkers, we focused on the mobile fraction which is better represented by FAs to measure the cross-shelf transport time. Also, compound-specific radiocarbon analysis on FAs is a well-established method that had been tested on several samples from this region in earlier studies (Gustafsson et al., 2011;Vonk et al., 2014), yet with a different scope/application than in this present study.
3) "Line 79-What is the total particle transport along this transect? How does this compare to other shelf regions in the Arctic and around the world? How might this affect your conclusions and their applications in other systems? (i.e. importance of deposition vs. lateral transport for net carbon sequestration)" All of these are valid and good questions. The only numbers on total particle transport we could find for the study area stem from box models by Stein and Fahl (in Stein and Macdonald, 2004). They estimate transport across the Laptev Shelf via sea ice, bottom currents, brines, etc. to add up to about 52 Pg yr -1 , which corresponds to about 60 % of the annual input. Sedimentation rates obtained from 210 Pb measurements similarly suggest a fairly high deposition also on the outer shelf (Salvadó et al., 2017;Vonk et al., 2012). As we 6 state in the main text, this study was conducted on one of the Earth's widest continental margins and the transport times for narrower shelves are likely to be shorter. For instance, for the Beaufort Sea, another Arctic shelf, rapid burial has been reported (Hilton et al., 2015) due to the different geomorphology of the margin. There, in contrast to the Laptev Sea, shelf sediments are viewed as a carbon sink. The separate question on the relative importance of deposition compared to degradation during lateral transport for the Laptev Sea was beyond the scope of the present study but is in focus for ongoing and future studies. We do not have such data and could not find any published values either. It must be stressed, though, that end-member CSRA data is not necessary for the current application of the CSRA data to "time" cross-shelf transport.

5) "Lines 99-108-What is the likely source based on all of these results? (also, extra "and" in parenthetic statement on line 101-should be "…Siberia, ICD-PF and AL-PF")"
We have now changed that part to read: "We therefore infer that along the transect the terrOC sources, ICD-PF and AL-PF, stay fairly similar and that their relative contributions do not change considerably." (Line 11-112) Here, the extra "and" was meant to show that these are two different studies. For clarification, we have now changed that parenthetic statement to read: "Tumara Paleosol Sequence in Northeast Siberia 26 , as well as ICD-PF and AL-PF in Laptev Sea surface sediments 27 ".
6) "Line 117-Reference for the fine fraction carrying the majority of the OC load, especially in this region?" The missing reference (Tesi et al., 2016b) has been inserted here. The novel approach here of quantitatively constraining cross-shelf transport times of terrestrial tracers opens up the possibility to quantitatively deduce the relative importance of exposure to oxygen during lateral transport versus oxygen exposure times after burial. To this end, we focused on the mineral-bound phase that is largely transported across the shelf as opposed to the matrix-free coarse material that is predominantly deposited closer to the shore. One could expect this matrix-free fraction to be more reactive as it lacks mineral-phase associations that are thought to stabilize OC (e.g., Lalonde et al., 2012). In an earlier study, however, we did not see substantial degradation after burial on a centennial scale (Bröder et al., 2016). We therefore infer that the time spent during transport plays a crucial role for OC degradation. Hydrodynamic sorting may also affect OC degradation patterns, but it seems difficult to assess in what way without any further data. Hence, we would prefer to refrain from categorizing this study as an upper or lower boundary in terms of C release to the atmosphere.
8) "Line 128-131-How do you determine total terrOC? How do you account for taxa-related differences in production and contribution of different biomarker you measure to terrestrial OC? ie. contribution of n-alkanes to sediments is very different dependent on taxa (e.g. angiosperms vs gymnosperms (Lane, 2017 Organic Geochemistry))." Total terrOC was determined by means of source apportionment calculations using bulk carbon isotope values δ 13 C and Δ 14 C as in several earlier studies (e.g., Tesi et al., 2016a;Vonk et al., 2012). Biomarker data have not been used to calculate terrOC and thus taxarelated differences in biomarker production are therefore not/less relevant to our approach. We understand that more information on the source apportionment calculations should be included in the main text and not only in the Materials and Methods section. We have now added a short paragraph (Line 138-142).

9) "Line 168-Again, I believe how you determined the contribution of bulk terrestrial OC warrants discussion within the text of the paper!"
We acknowledge an apparent lack of clarity with regards to the source apportionment calculations and have now added and clarified information in the main text (Line 138-142). Please see also our previous and later comments on this topic.

10) "Lines 173-183 This discussion is a bit wordy and the argument isn't clear. Are you just setting precedence, or are you arguing that the transport time is greater on one of the largest margins in any ocean (this makes sense…)?"
This paragraph was meant to compare our study to previous attempts for constraining crossshelf transport times. We have now shortened it substantially for clarity.

11) "Line 187 "stimulated" should be "stimulates"?"
Correct, thank you for pointing this out. It has now been changed.

12) "Lines 191-193 How does this tie into the proportion of OM transported from the terrestrial to aquatic in this region? For example, if most of the organic matter (including permafrost OM) is transported as DOM, how does this impact this idea of carbon-climate feedbacks in the system?"
This is an interesting question, yet beyond the scope of the current paper, which has a focus on the transport and degradation in the sedimentary compartment during cross-shelf transport. We are currently working on estimates for a total "degradation flux" of terrOC in both the sedimentary and water column (DOC and suspended POC) systems on the East Siberian Arctic Shelf.

13) "Line 201 I would argue that this should be "an ultimate" CO2 source to the atmosphere. How do you know that this degradation is complete to CO2? And how (where, when) does the supersaturation of CO2 at depth outgas to the atmosphere?"
This is a valid point. We have changed that part since this study is not specifically investigating if the degraded sedimentary terrOC is completely turned into CO 2 and then outgassed. The sentence now reads as follows: "For the wide Eurasian Arctic shelves, on the other hand, long-lasting sediment transport allows for terrOC degradation, thereby constituting a carbon source to overlying water and atmosphere."

14) "Materials and Methods: The flow of this section (i.e ordering of subsections) seems a bit jumbled. For example, bulk organic carbon mineral surface area and biomarker analyses section could be after sampling, but before the compound-specific work; and the compound specific sections should be sequential."
The order of the different subsections has been changed according to these suggestions.

15) "Line 219-In this compound-specific radiocarbon analysis section, you cite the methods for the dating protocol before the extraction. This would flow better to move sentence from lines 220-221 down to the start of the paragraph at line 226."
We have now moved this sentence accordingly.

16) "Lines 297-301 While the description of usage of the Monte Carlo simulations for the transport timing, degradation and recalcitrant fraction fits was clear to me, I am not sure I understand how the Monte Carlo simulations were used for the source appropriation. Could the authors comment a bit more on their input variables, assumptions and the error associated with this methodology?"
The major uncertainties regarding the Monte Carlo based source apportionment calculations are associated with the underlying assumptions: that isotopic mass balance is fulfilled, that the endmembers values properly reflect the sources, that there is no isotopic fractionation etc. The Monte Carlo simulations in themselves are very robust with respect to the choice of input parameters: regardless of choice the runs converge within the burn-in phase. The stochastic perturbations are tuned to meet the well-established criterion of an acceptance rate of ~ 0.23, which theoretically has been found to be an optimal condition for a broad range of Markov chain Monte Carlo algorithms (Roberts et al., 1997). These factors were established in previous work (e.g., Andersson et al., 2015;Keskitalo et al., 2017). Please see also our response to the General Comments. We have included more details on the Monte Carlo strategy both in the main text (Line 138-142) and in the Materials and Methods section (Line 335-342). 9

Reviewer #3
GENERAL COMMENTS: "This paper describes the state-of-the-art application of compound specific radiocarbon analyses of n-alkanes to constrain the transport and degradation time for terrestrial OC (terrOC) on the Laptev Sea shelf. The authors analyze their data using a clever approach that provides important insights about the fate of terrOC in a rapidly changing system (Arctic Shelf). My main concern about the work is that the rate constants (not rates; see below) are fit to data for 0 to 4 kyr timeframes. However, in most cases, the loading data for TerrOC and biomarkers remain constant between ~1.5 to 4 kyr, which may artificially make the rate constants more conservative. Thus, I recommend that the authors recompute the rate constants for a 0 to 1.5 kyr timeframe and compare these rate constants with those computed over the 4 kyr timeframe.
Since organic matter decomposition changes over time (see Middelburg (1989) Geochimica et Cosmochimica Acta Vol. 53, pp. 1577-1581, averaging over 4 kyr likely underestimates the rate constants and transit times.

I would also like to see the authors to acknowledge that the rate constants presented in their manuscript are averaged over a kyr timeframe. In reality, the highest rates of decomposition occur soon after deposition and averaging over long timescales likely underestimates the reactivity of terrOC. Another important point is that degradation of terrOC such as lignin only occurs when oxygen is present so oxygen exposure time (OET), a parameter that may be embedded in transport time, is an important determinant of the role of terrOC in the carbon cycle.
Overall, this is a unique and interesting data set that provides new insights about the transport time and fate of terrOC across the Siberian-Arctic shelf. I recommend publication after considering the issue mentioned above as well as some rewriting of the manuscript. In some places, the manuscript is worded awkwardly and the word choices could be more direct. I've provided detailed comments below that the authors should consider when revising their manuscript." We are naturally delighted about the positive appraisal and appreciate the useful comments and suggestions. We fully agree with the notion of non-constant degradation rates and thus the need to be clearer on the fact that, in this study, the apparent first-order degradation rate constants refer to an average over a kyr scale. We have also followed the reviewer's suggestion of computing the degradation rate constants for data points where the transport time is <1.5 kyr, and compared those with our previous estimates using all data points: On a first sight, it seems the reviewer's hypothesis is correct: using the <1.5 kyr data alone provides higher rates, on average about a factor of three higher than when using all data points. However, the estimated standard deviations are also much larger, on average 15 times larger than the standard deviations estimated for the full data set. This is not surprising: In total, we have 10 data points, of which five are <1.5 kyr, which are fitted to an offset exponential function with three parameters. Thus, the <1.5 kyr case (with five data points) does not constrain the decay rate well enough for us to be able to draw the conclusion that we indeed observe a change in degradation rate constant over time. A much larger dataset would be needed to properly test this hypothesis. In order to avoid oversimplification on the other hand, we had introduced the "recalcitrant fraction" as a parameter quantifying the portion that remains, which is largely governed by the data points with ages >1.5 kyr. Overall, even if degradation rates were higher by a factor of three for the first millennia, this would not change the conclusions of our study that terrOC degradation on this wide Arctic shelf takes place on a timescale of centuries to millennia. Nevertheless, since we agree with the notion, we have added a paragraph on this topic to the main text (Line 180-189) and the table shown above to the Supplementary Information (Supplementary Table 2).
We agree that the oxygen exposure time (OET) plays a major role in terms of terrOC degradation. This is particularly relevant for carbon pools that are predominantly reactive in oxic conditions like e.g., lignin phenols. As explained in the text, the terrestrial material that reaches the outer Laptev Sea shelf experienced millennial-scale transport which translates into a prolonged oxygen exposure time. On the one hand, we cannot really asses the actual time spent in oxic conditions because we cannot entirely exclude temporary burial below the top oxic layer of the sediments before the next mobilization event. One could therefore argue that our transport time should be considered an upper OET limit. On the other hand, it is reasonable to assume that this fraction escaped burial to be transported all the way to the outer shelf, thus, the OET in first-order approximation likely scales with the transport time. More on the OET concept has now been included in the manuscript (Line 166-173).
We have followed all of the editorial comments and our respective responses can be found below.

SPECIFIC POINTS:
1) "Abstract should be rewritten. Choice of wording is awkward and the findings should be presented more directly and succinctly." We have revised the abstract according to the following points made by the reviewer. Please see our responses to the specific points 2-5.
2) "Line 21-22 and Line 41. Change to "that regulates atmospheric CO2" or "contributes to the regulation of atmospheric CO2"" These sentences have been changed accordingly.

3) "Line 24-25. Revise to, "compound specific radiocarbon analyses of terrestrial biomarkers to date cross-shelf transport times""
This has been changed to "compound specific radiocarbon analyses of terrestrial biomarkers to determine cross-shelf transport times". 4) "Line 27-28. Revise to, "TerrOC was reduced by 85% during transit resulting in a degradation rate constant of 2.4 + 0.6 kyr-1"" This sentence has been revised.

5) "Line 28. What do the authors mean by "protracted transport"?"
The word "protracted" has been changed to "long-lasting" for clarification.
8) "Line 62 and throughout text including figure captions. The correct language is "longchain n-fatty acids, LCFA", not "long-chained". The authors may want to note that longchain fatty acids derive from plant waxes, but I suggest using an alternative to "wax lipids", which could be confused with "wax esters"." Thank you for pointing this out. "Long-chained" has been changed to "long-chain" throughout the text, figure captions and Supplementary Information. Instead of "wax lipids" it now reads "These lipids are derived from plant waxes and preferably bound to the fine fraction of the sediment". This hopefully clarifies matters. 9) "Line 66. Use alternative wording for "clock", which is not customarily used as a verb (e.g., "date", or "determine the net transport time")" We have now changed "clock" into "determine".

10) "Line 73. Authors should define what they mean by "potentially significant in situ aging". This language is vague and should be omitted or revised."
This part has been revised and now reads as follows: "the material is thought to undergo repeated cycles of burial and resuspension with potentially in situ ageing of several centuries before the next leap". 11) "Line 74. Revise to, "and are not unidirectional"." The word "are" has been added here.
12) "Lines 102-103. This paragraph opens by discussing hydrogen isotopes as source tracers but the authors discuss their findings as "concentrated weighted average nalkanes". Instead, I recommend that the authors describe their findings as "concentrated weighted average values of δ2H for C27, C29 and C31 n-alkanes". Revise description of findings to be more direct (e.g., "remained constant with water depth" rather than "does not display a significant trend with water depth")." This part has been changed according to the suggested edits. It now reads: "The concentration-weighted average values of δ 2 H for n-alkanes with carbon-chain lengths 27, 29 and 31 for the surface sediments along this Laptev Sea transect remain constant with water depth (Supplementary Fig. 3)." 13) "Line 114. Revise to "are deposited in shallow waters" rather than "trapped"." The word "trapped" has been changed to "deposited".
15) "Line 121. Avoid using "significant" here and throughout text unless statistical support is provided." The word "significant" has been replaced here (Line 121) and also in lines 27, 75, 178, and 301.

16) "Line 133. The authors present "first-order rate constants" (units of per time) rather than "rates" (units of changes in concentration per unit time). The text and equation terms should be revised accordingly."
This is correct, we have made the according changes throughout the manuscript.

17) "Line 137. I think this should read terrOC and biomarker analyses. The present wording suggests the ratio of terrOC to biomarkers (terrOC/biomarkers)."
Thank you for pointing this out. It now says "terrOC and biomarker analyses".

18) "Line 149-150. Specify that cross-shelf transport FOR THIS SYSTEM is a millenial scale process. Without additional information, it is unclear whether rates for the Laptev Sea shelf can be extended to other systems."
We agree that this first sentence of the discussion was ambiguous. We have now revised it to read: "Our study provides observation-based evidence that cross-shelf transport can be a millennial scale process." 13 19) " Figure 1. Revise y-axis label to read, "δ2H of long-chain n-alkanes [‰]. "HMW" is not defined and is unnecessary because the n-alkanes are already defined as "long-chain"." Supplementary Fig. 3 (which was previously Fig. S1) and its caption have been revised accordingly. 20) " Figure 3. Cite Fig. S4 as support for statement, "due to a growing proportion of modern marine organic matter."" Thank you for this good suggestion. A reference to Supplementary Fig. 1 (which was previously Fig. S4) has now been added to the caption of Fig. 2 (which we assume was meant here instead of Fig. 3).
This paper describes the state-of-the-art application of compound specific radiocarbon analyses of nalkanes to constrain the transport and degradation time for terrestrial OC (terrOC) on the Laptev Sea shelf. The results presented in this manuscript provide important insights about the fate of terrOC in a rapidly changing system (Arctic Shelf). I have read the revised manuscript and author responses to the initial round of reviewer comments and find that the authors have addressed the reviewer comments satisfactorily.
As stated in my previous review, this manuscript provides a unique and interesting data set that provides new insights about the transport time and fate of terrOC across the Siberian-Arctic shelf. This is important work and will make an excellent contribution to Nature Communications.

Reviewer #2
SPECIFIC POINTS: 1) "Line 21-I feel like the sentence starting with "Here, we leverage…" is missing the object of what you are specifically leveraging, i.e. the utilization of a method to address a gap in knowledge. Perhaps would read better as "Here, we leverage the utility of compound-specific…"" We have changed the word "leverage" to "employ" for clarification. Due to the abstract word limit we could not add the full suggested edit.
2) "Line 24-Perhaps would be better as "TerrOC, as determined through isotopic mixing model, was…"" This is a good clarifying point, alas, as mentioned above, word limitations do not allow for this revision to be included.
3) "Line 25-26-"long-lasting" is a bit vague. Perhaps better as "cross-shelf" and/or "net"?" We have changed "long-lasting" to "cross-shelf". 4) "Line 34-do you need to redefine "terrOC" here, as it has only been defined in the Abstract?" We have followed this suggestion and added the explanation of the abbreviation. It is now: "In high-latitude regions, increasing soil permafrost thaw 3 , accelerating coastal and sea floor 4,5 erosion, and rising fluvial sediment discharge 6 are expected to amplify the delivery of terrestrial organic carbon (terrOC) to the Arctic Ocean." 5) "Line 41-42 I would say that this also depends on the type of shelf! Might be useful at some point in introduction or conclusions to tie in the idea that this type of work can be applied to other systems to better understand variability in these cross-shelf processes on terrOC "sequestration" Also, see comment on lines 210-216 below in regards to this suggestion." This is a good point and one of our main conclusions. We believe this message is conveyed clearly by (lines 224-226 and 228-231 in the revised manuscript): "…the current results document a potentially small, but persistent leakage of terrOC to short-term reservoirs on large continental margins, which stands in contrast to the rapid terrOC burial reported for e.g., tropical mountain rivers 47 or fjords around the globe 48 .
[…] For the Mackenzie River, the efficient transfer and burial of terrOC has been suggested as a geological CO 2 sink 9 . For the wide Eurasian Arctic shelves, on the other hand, long-lasting sediment transport allows for terrOC degradation, thereby constituting a carbon source to overlying water and atmosphere." 6) "Line 45 "attributed to" instead of "due"" We have revised that sentence accordingly. 7) "Line 48 comma after the citations "15,16" and "in" instead of "of"" We have now made the suggested changes.
10) "Line 68-add "here" before "we"" We have moved the word "here" to the position suggested by the reviewer.

11) "Line 104-Maybe give an average and standard deviation of the 13C values of the LCFA here?"
We have now included the average d 13 C value and standard deviation: (-31.2 ± 0.5‰) (line 102 in the revised manuscript).
12) "Line 106-I would recommend removing "Isotopic ratios of…", and start the sentence with "Stable hydrogen isotopes..."" This sentence has now been revised as suggested.
14) "Line 131-Add "Therefore" to start the sentence "The observed cross-shelf…"" This word has been moved to the start of the sentence.
15) "Line 132-add something along the lines of "as opposed to variations in source or hydrodynamic sorting" to the end of this sentence" 3 This sentence now reads as follows: "Therefore, the observed cross-shelf increase in LCFA ages can be attributed to terrOC ageing during lateral transport as opposed to variations in source material or hydrodynamic sorting." (revised manuscript line 128-130).
16) "Line 136-Add "net transport time" after "quantitative estimate"" We have now added "for the net transport time" at the end of this sentence.
17) "Lines 157-158 and lines 182-184 seemingly contradict each other. Clarify what is meant in each statement to resolve this apparent contradiction." We have revised lines 182-184 to now read as follows:" While these rates are all similar, the relative difference between the different carbon pools are consistent with previous findings." in order to resolve that apparent contradiction.
18) "Lines 187-195-These lines address one of the main concerns of reviewer #3, however, I feel the explanation in the text is still a bit confusing to follow. Perhaps can alter the order this is presented from the analytical progression you took to its overall implications -i.e. I understood this section as follows: you included the recalcitrant component to deal with the idea of multiple reactivates across the shelf, perhaps due to OET, mineral protection, etc., as evident by the 3X rate of degradation that might be occurring within the first 1.5kyr that the material is transported, compared to the net transport time of 3.6kyr of terrOC transport across the shelf." We agree this could be explained even better and have revised the text. Following the reviewer's suggestion, we have now changed the order of the arguments around in order to clarify this paragraph. It now reads: "Furthermore, we decided to include a recalcitrant component (i.e., a fraction with a degradation rate constant of 0 kyr -1 ) to account for the limited changes observed for transport times >1.5 kyr. When computing degradation rate constants for the subset of inner-mid shelf data points where the transport time is shorter than 1.5 kyr, the rate constants are on average a factor of three higher than when using the full set of observations across the entire shelf (Supplementary We have changed this paragraph to improve its connectivity to read as follows: "An earlier attempt to quantify cross-shelf transport times on the Washington margin by tracing the volcanic ash of the 1980 Mount St. Helen eruption resulted in a transport time of <1 year 21 . This study may have underestimated the true terrOC transport time as this value stands in sharp contrast to another estimate using bulk organic carbon 14 C measurements and assumptions on the proportion of terrOC in the bulk to derive a cross-shelf transport time of approximately 1800 years 22 . Furthermore, the use of compound-specific radiocarbon dating, as in our study on LCFAs in the mobile fraction (<63 μm) along the 600-km long Laptev Sea transect, circumvents uncertainties caused by e.g., changing proportions of marine organic matter and hydrodynamic sorting, and thus enables a quantitative constraint for terrOC transport across one of the widest ocean margins on Earth." (revised manuscript lines 204-213).

20) "Line 228-Add "on large continental margins""
This sentence has been revised accordingly.

21) "Line 235-
The effect of what? Add "terrOC has…" between "effect" and "on"" We have also changed this sentence and added "of terrOC" between "effect" and "on".
22) "Line 237-start the sentence "Sediment…" with a "Therefore," Or "Thus," Or "In conclusion,"" We have revised the sentence to now start with "In conclusion, …". 23) " Figure 3. I would just reiterate in this figure caption (especially C-F) that the biomarker data was corrected for what was 'present' in the fine fraction, which is outlined in Figure  S5." This sentence has been changed accordingly.