the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Feb 2025
Accelerated export of terrestrial organic carbon to the Chukchi Sea since pre-industrial times
Abstract. The on-going global warming is causing rapid changes in the carbon cycle of the Arctic. Yet the response of the Arctic to these environmental changes are not fully understood. In this study, we investigate bulk organic parameters (TOC, TN, δ13Corg, δ15N, C/N) and terrestrial n-alkanes (TERR-alkanes) from 19 surface sediments and 5 210Pb-dated sediment cores covering up to the last three centuries from the Chukchi Sea. Downcore profiles indicate increasing OC since the beginning of the Industrial Era in all cores. They also show higher TOC and TN values in southern Chukchi Sea cores coincident with decreasing δ13Corg indicating an increasing contribution of terrestrial OC that is confirmed by TERR-alkanes. Comparison with regional paleo-records and instrumental data emphasize the key role of air temperature and sea ice cover on the OC cycle and vegetation of the surrounding landmasses.
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RC1: 'Comment on egusphere-2025-325', Anonymous Referee #1, 28 Feb 2025
The authors have obtained a solid data set with great potential to enhance our understanding of arctic carbon dynamics. Thus, I recommend reconsidering the article after major changes have been made.
My biggest issue is the presentation of the data. Figures 2 and 3 (especially 3a!) may be the most careless figures I have ever seen, and I am not even color blind. Further, there are 21 figures and 3 maps in this manuscript, this is a crazy overload! I like that the authors keep their color coding throughout the manuscript, but it would be very helpful if: a) the same colors would be used in the overview of figure 1; b) the N-S legend would actually match the locations on figure 1) – there it looks like the order is N-S: 14R09 – R1 – 14S03 – C07 – LV77-4; c) they come up with names like Alsaka core, or Chukchi Sea core, or Chukotka core – something that one can remember and not just a code that even does not match the overview figure. d) Indicate uncertainties – not just in their figures but in the entire manuscript.
My second big issue is the way that conclusions are drawn. With the amount of computing power and the easy access to scripts/data pipelines, I am struggling to call "here it goes up, here down", still scientific. Especially if the data are not presented in an easy-to-digest way (like mentioned above). For example, they have 2 isotopes and define 3 endmembers, this is perfect for an endmember model like a Markov chain Monte Carlo. Or to use a principle component analysis with all their proxies (FYI: Paq abbreviation is not defined at all). Another suggestion would be to plot your TERR index over precipitation vs the time of your age model to determine what environmental parameters are related to the proxy change. They start to do this in the second part of the manuscript (content and discussion of figures 4 and 5). However, to bin the 5 core sites which lay in completely different locations – several hundreds of km apart (~800km NS and EW!!!) – and are influenced by different river systems and oceanic currents, seam hardly appropriate when comparing it with sea ice cover.
I was excited to read the manuscript, based on your abstract, and I believe it can be a good paper, as they have set the foundation for it. But the current state looks more like a draft to me. Their conclusions are important, but I cannot follow how they got there in the way that the manuscript is structured and presented. However, I think they have all needed to make it a howlsome manuscript, and getting it into a different format should allow them to publish it. I do not attach a supplement file, as I think the entire manuscript needs to be reworked, and as they usually would have to reply to every comment, this would make things just more tedious for them and for me.
Citation: https://doi.org/10.5194/egusphere-2025-325-RC1 -
RC2: 'Comment on egusphere-2025-325', Anonymous Referee #2, 23 Apr 2025
The authors present a multi-proxy study using δ¹³C, δ¹⁵N, C/N ratios, and terrestrial n-alkane biomarkers from surface sediments and sediment cores to reconstruct terrestrial organic carbon (OC) inputs to the Chukchi Sea over the last three centuries compared with published datasets on temperature, sea ice and precipitation. This work contributes to a growing body of literature documenting the land-ocean carbon fluxes in the Arctic and the role of sea ice and climate forcing in shaping sedimentary OC signatures. However, several significant methodological and interpretive issues must be addressed to strengthen the manuscript's validity and clarity. As written, the manuscript needs substantial work to be publication-quality and contribute to advancing our understanding of these topics.
A few major issues that need to be addressed:
Title and Abstract
The abstract summarizes results but overstates interpretive certainty. Please consider softening the statement "provide evidence of a significant link..." to reflect that this is an inferred relationship based on proxy correlation, not direct causality.
Line 18 – “the response of the Arctic”, please be more specific here. Response of marine primary production or biological pump in Arctic marine ecosystems
Line 24 – from increased fluvial sources
Line 25 – Not entirely convinced that air temperature and sea ice are the key role here but rather increased precipitation, runoff from boreal rivers, and seasonal pulses from the coastal currents.
Line 35 – The most recent Arctic Report Card (2024) indicates a shift of the Arctic permafrost as a source rather than a sink. Relevant to include/discuss.
Line 40 - There is a more recent and relevant article indicating a third to half of NPP in the AO is from rivers and coastal erosion. This work should be considered and incorporated here:
Terhaar, J., Lauerwald, R., Regnier, P. et al. Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion. Nature Comm, 12, 169 (2021). https://doi.org/10.1038/s41467-020-20470-z
Line 41 – last few decades? Please be more specific.
Line 43 – Arrigo has done substantial work on increasing NPP in the Arctic. The authors may want to consider including prior work (e.g. Arrigo and van Djiken 2015 https://doi.org/10.1016/j.pocean.2015.05.002)
Methods
- The authors state surface sediments were collected, but do not indicate how they were collected (e.g., Van Veen grab, box core, top of multi-corer). This is critical for understanding whether the samples represent intact, undisturbed surface material suitable for modern flux interpretation.
- It is unclear whether an internal laboratory standard was used in addition to the IAEA-certified reference materials. Internal standards are essential for monitoring instrument drift and analytical reproducibility. Please clarify.
- The methods mention the addition of 5α-cholestane but make no reference to derivatization (e.g., silylation using BSTFA). Derivatization is typically required for lipid compound analysis via GC-MS, particularly if alcohols or acids were present. Please clarify whether derivatization was performed and, if not, justify its omission.
Results and Discussion
- While the authors report measurements of total nitrogen (TN) and δ¹⁵N in sediments, these data are not meaningfully discussed in the manuscript. It remains unclear how these variables contribute to the study’s central questions or interpretations. δ¹⁵N is a valuable tracer of nitrogen sources and cycling processes (e.g., denitrification, organic matter origin), and TN content can inform on organic matter input and preservation. I strongly encourage the authors to either integrate these measurements into their broader discussion—particularly in relation to sediment diagenesis, productivity, or regional biogeochemical shifts—or clarify why they were measured if they are not central to the narrative. As it stands, the inclusion of these variables without interpretation detracts from the overall coherence of the manuscript.
- The proposed linkage between ACL₍₂₇₋₃₁₎ and evapotranspiration/drier conditions is speculative and not well-supported. ACL typically reflects changes in source vegetation or degradation, not moisture balance directly. Please revise this interpretation or support it with more appropriate references (e.g., compound-specific isotope data or vegetation studies).
- Although precipitation trends are mentioned and included in the supplementary materials, the authors dismiss the lack of correlation without much exploration. Given that precipitation and runoff are primary vectors for terrestrial OM export, more effort should be made to assess their potential role. Data from R-ArcticNET could be one option to explore. Additionally, the role of river discharge is sorely missing. The inclusion of Walsh’s sea ice reconstructions was interesting but there is something not quite connecting for a complete story here.
- While the discussion focuses on a potential link between the Siberian Coastal Current and the delivery of terrestrial biomarkers (e.g., long-chain n-alkanes and % sphagnum moss), it fails to adequately address the lack of observed relationships with the Alaska Coastal Current (ACC). This is a critical omission, especially given the ACC’s strong seasonal influence on shelf circulation and potential for transporting terrestrial material from Alaskan riverine sources. A more comprehensive analysis comparing the spatial trends of biomarker distributions with known current trajectories (e.g., ACC vs. SCC) would strengthen the interpretation and may reveal key regional differences in sediment provenance and deposition. Existing circulation models and observational studies (e.g., Danielson et al. 2017; Woodgate 2018) could provide useful context. Other studies focusing on riverine inputs (e.g., changes in Yukon River discharge) could also be considered more substantively (e.g., Bennett et al. 2015 https://doi.org/10.1016/j.jhydrol.2015.04.065)
- The discussion does not address bioturbation at all, which is known to affect Pb-210 and Cs-137 profiles in this shallow, highly productive, continental shelf region. Bioturbation can substantially obscure the vertical profile of unsupported 210Pb in Arctic shelf and slope environments. Given that the study region is known to experience intense bioturbation by benthic fauna — as documented in prior work (e.g., Cooper and Grebmeier, 2018, https://doi.org/10.1016/j.dsr2.2018.01.009). The assumption of a steady-state depositional model may be inappropriate or at least requires additional justification. I recommend the authors discuss whether signs of mixing (e.g., non-exponential decay profiles, homogenized surface layers) were observed and, if possible, support their dating interpretation with corroborating tracers such as 137Cs (which is briefly mentioned but not reported, line 72).
Conclusions
The statement that the record shows "strong terrestrial OC export events in pre-industrial times" needs to be tempered unless the dating uncertainties and bioturbation issues are directly addressed. Also, reinforce that ACL interpretations are tentative given the current uncertainty about their linkage to hydrologic regime. Consider expanding the conclusion to explicitly reflect the uncertainties in proxy interpretations (ACL, Paq, % Sphagnum) and their implications for reconstructing Arctic terrestrial OC dynamics.
In general, the paper also lacks some relevant references. I've included a few but suggest the authors do another thorough review of work that has been done in the region.
Other Minor/Specific Comments
- Line 93 – The authors used 2-4 g of sediment. This seems like a lot. Is there a published method the authors were following? Perhaps in the Bai or other Jalali paper?
- Line 127 – TEER should be TERR
- Line 143-44 – This statement is unclear and inaccurately described. All records are decreasing ? or are they more/less enriched or depleted. The latter language should be used to describe changing isotopic values. Additionally, it is unclear why there is a focus on 1850-1900. Why this short 50 year window? Do the data support this resolution of analysis? As stated above, there are some concerns about bioturbation that need to be addressed.
- Line 242 – Figure 4: The scale for sea ice concentration is not appropriate. Please limit the scale to 0–100%.
- Line 290 – statistically significant, or just “significant”?
Citation: https://doi.org/10.5194/egusphere-2025-325-RC2
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