the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2025
Sedimentary organic carbon dynamics in a glaciated Arctic fjord: tracing contributions of terrestrial and marine sources in the context of Atlantification over recent centuries
Abstract. In this study, we investigated sedimentary organic carbon (OC) dynamics in Kongsfjorden, Svalbard, using three multicores collected during the HH22 and HH23 cruises aboard the RV Helmer Hanssen in 2022 and 2023. We assessed the relative contributions of petrogenic, soil-derived, plant-derived, and marine OC by applying a four-source apportionment approach based on Δ14Corg, δ13Corg, and lignin parameters, including the (Ad/Al)ᵥ ratio and lignin phenol concentrations, with Monte Carlo (MC) analysis. Age-depth models based on 210Pb and 226Ra data were used to evaluate temporal variations in the accumulation rates (ARs) of sedimentary OC. Our findings revealed a significant increase in marine OC ARs in recent decades, which appears to be closely linked to enhanced Atlantic Water (AW) inflow. This trend suggests that changes in the fjord’s biogeochemical cycles, driven by AW, are influencing sedimentary OC dynamics. Consequently, the increasing influence of AW underscores the potential for ongoing Arctic warming to further amplify AW inflow into Arctic fjords, with significant implications for carbon cycling and fjord ecosystems. By providing a historical perspective on AW trends and their effects on sedimentary OC dynamics, this study offers valuable insights into the potential consequences of future climate change.
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RC1: 'Comment on egusphere-2025-957', Anonymous Referee #1, 08 Apr 2025
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This paper is well-written and structured. It presents a novel dataset from three multicores along a transect in Kongsfjorden (Svalbard) and provides relative contributions of organic matter sources. To qualify the organic matter, the paper uses bulk parameters (carbon and nitrogen) and lignin phenols. The relative age of the cores is constructed using 210Pb geochronology. I highly enjoyed reading the manuscript and the ideas were easy to follow.
The introduction clearly laid out the importance of this study and the methods were succinct and well described. I would have enjoyed a slightly more detailed discussion in section 4.5 about increase surface runoff and impacts on the fjord. The paper suggests important influence from river discharge at the middle site, but does not support/compare with the existing literature on terrestrial OM inputs. Here are some key points that I would like clarification on, followed by minor changes.
Main recommendations
- Clarifying how this manuscript builds on from the published work in Kim et al., (2023)
This paper builds on finding from previously published work by the authors (Kim et al., 2023). Due to the similarities in analysis and research location – the present study would benefit from presenting how it builds on the previous findings. I would recommend adding 1-2 sentences in the introduction to present the findings of Kim et al., 2023 and specify how this new manuscript differs and expands from the previous work.
- Extrapolating Norg from Ntot
I recommend that the authors further justify extrapolating Norg from Ntot. How precise is this correlation? The authors describe different sources of OM in the fjord system. Wouldn’t these different sources also differ in partition of N? I think it is misleading to present both Norg and Ntot since Norg is just offset by a correction factor (as I understood it). I think this part in the paper deserved a bit more explanation. Also, couldn’t the age of the sediment affect the partitioning of N (i.e. is using a correction factor from surface sediment accurate)?
- Presenting Ntot/TOC as opposed to C/N
I have not encountered this ratio before. Typically, C/N ratios are presented in this format. Why have the authors decided to present the ratio as Ntot/TOC? Unless I am missing something, I would recommend that the authors present the bulk parameters in a more standard way for ease of comparison with other studies.
- Describing the study sites in greater detail
I believe section 2.1 could be made into its own section 2 and methods could be section 3. Would it be possible to showcase the biogeochemistry of Kongsfjorden in more details here? Otherwise, Krossfjorden is discussed quite heavily in the discussion. The study site section would benefit from having 2-3 more sentences describing how Kongsfjorden and Krossfjorden are similar/different.
- Discussing the historical/spatial dynamics
Section 4.5 begins to explore the potential OM dynamics at the middle site in a historical context. I think this section would benefit from being expanded a bit. This is the main objective of the manuscript. Why is the middle core seeing the largest changes in OM source. If it because it is more influenced by surface runoff, and therefore glacier dynamics, then perhaps citing sources of increase river discharge is more relevant than just temperature changes. The argument could be made more convincingly while citing the existing literature. Also, a comment on which change is more significant in Kongfjorden (more surface runoff or Atlantification? Or both and why?) would make a more impactful conclusion. The manuscript only mentions greater terrestrial input and then focuses on Atlantification.
Minor recommendations
Abstract: Perhaps omit the use of acronyms in the abstract for ease of read.
Lines 32-34 : Please elaborate on this positive feedback loop. How would a greater contribution of AW further amplify AW inflow in fjordal system?
Introduction: Since the scope of study is similar to Kim et al 2023 – perhaps define in the introduction how the present study builds on the previous paper.
Lines 52-53: “While modern observations have clear limitations in providing long-term datasets” – perhaps reword “datasets” to make this statement more meaningful (i.e. of course modern data isn’t old data…)
Line 57: Change “high-Arctic Svalbard archipelago” to High Arctic Svalbard archipelago
Methods: I would recommend adding a sentence at the beginning of each subsection to establish why these analyses were undertaken. This way, readers without prior knowledge of certain methods can understand why certain analyses were done. (i.e. to understand the source of carbon in the fjordal system, we…). Please also describe the indices used in for phenol composition analysis (S/V, C/V and 3,5-Bd/V, (Ad/Al)v).
Line 100: AW acronym has already been defined.
Lines 107-109: Perhaps the types of terrestrial inputs to Kongfjorden could be expanded a bit.
Line 128: Define KOPRI for readers that are not aware of this facility.
Line 139: samples were heated to what temperature?
Line 150: Define HPGe and KBSI.
Line 173: Replace “carbon isotopes” with “δ13Corg” for uniformity
Results: Why was the radioisotope analysis not performed at the inner site? This needs to be addressed somewhere in the manuscript.
Lines 227-230: What about the inner site? Also, where is the sedimentation rate reported by Zaborska in relation to the present study?
Line 243: Please state what is the significant correlation between Ntot and Norg derived from previous studies. (see comment in main recommendations)
Discussion:
Lines 392-394: “The poor sorting observed across all morphological zones highlights fluctuating energy conditions during deposition, likely driven by episodic glacial advances and retreats. These variations suggest that the prevailing climatic conditions in Kongsfjorden were neither stable nor persistent over extended periods” – I am not sure that this conclusion can be drawn from the lack of correlation between TOC and grain-size and poor-sorting. The present study doesn’t link specific advance/retreat events to sedimentary behaviour – so I would avoid overinterpretation and just state that poor sorting and lack of correlation between grain-size and TOC are indicative of a glacier-fed system.
Lines 492-499: This section could be developed further. Why do the river systems deliver soil-derived OM to the middle site more than the inner site? If this is due to surface runoff – then a map indicating where the river systems are in Kongfjorden would be highly relevant. There is also a large body of literature on the biogeochemical signature of surface runoff in Kongsfjorden. Perhaps it would be interesting to compare to some of these studies?
Lines 502-517: These equations could be placed in the methods section. The discussion loses a bit of momentum when you have to read through how the AR were calculated.
Figures:
Figure 1: It would be nice to show the elements discussed in section 2.1 on this map. Adding a third panel with labels of the different glaciers and river systems could be helpful to readers that don’t have prior knowledge of Kongsfjorden.
Figure 2: Change “Water contents (%)” to “Water content (%)” both in caption and in axis title.
Figure 3: The figure caption states 3 sites (A-C) – but only two are presented (A-B). Please explain in text why inner site is not present here. Add a note in the figure caption to explain the different symbols used in the slopes in the Pbex panels.
Figure 6: Having a color key, instead of textual annotations, would make the EM identification easier at first glance.
Figure 7: The key states that surface sediments have been taken for this study. However, the methods only states multi cores. Please clarify either in the key or in text where the square samples come from. I think that it would be beneficial to differentiate symbols here for the Konsfjorden multicores samples (the present manuscript) over the rest of the dataset to highlight the novelty of this paper of Kim et al., 2023. (see comment in main recommendation)
Figure 8: See comment for figure 7.
Citation: https://doi.org/10.5194/egusphere-2025-957-RC1 -
RC2: 'Comment on egusphere-2025-957', Anonymous Referee #2, 10 Apr 2025
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The manuscript by Kim et al provides a detailed study of 3 sediment cores from Kongsfjorden, Svalbard, with the aim to improve our understanding of the relationship between organic carbon (OC) deposition in the fjord, its different sources and climatic/oceanographic changes. The authors combined sedimentological data, bulk parameters, biomarkers and a robust geochronology based on 210Pb to estimate the relative contributions of sedimentary OC sources to different coring sites in Kongsfjorden. The manuscript is very well written and very clear, and the described dataset represents an important addition to our knowledge of the organic carbon cycle in Svalbard. I have just a few general comments and suggestions, and some minor comments.
General comments:
- It is unclear in the introduction which knowledge gaps you are trying to fill: is it the lack of observational data on AW inflow before the recent warming or during it? Or both? A few more words could be spent to underline and make clear the main aim of the work
- Since the aim of the work is strongly related to the inflow of AW in the fjord, I think a section briefly explaining the mechanisms of AW intrusions in the fjord, mentioning also its effects, should be included. With it, a better representation of WSC-ESC dynamics could be included in Fig. 1 (for example, like in De Rovere et al 2022).
- The EM modeling for grain size seems an excellent tool to study grain size distribution with a more accurate approach, but I do not understand why EM3, as you presented it, can not be simply a combination of EM1 and EM2. Could you further explain how the procedure for establishing the different end members work?
- S/V and C/V ratios, both in core and surface sediments, seem to point towards a quite strong contribution of gymnosperms, despite the major presence of angiosperm species in Svalbard archipelago (see for example https://npolar.no/en/themes/vegetation-svalbard/ and references therein). How does this compare to other lignin datasets from similar Arctic areas?
- You present really interesting results on the change of OC sources in the latest decades, especially the increased marine OC accumulation rates in the middle fjord, but why do you think the same pattern is not present in the outermost core also? I think in general that this entire section of the discussion could be expanded, elaborating further the interpretation of the changes in sedimentary OC deposition, both in time and between coring sites.
Detailed comments:
Line 32-33: this sentence is a bit confusing; it reads as if the increased influence of AW underscores the potential future amplification of AW inflow itself. I think what you meant is that the increased influence of AW underscores the likely important effects of an even stronger AW inflow
Line 71-72: I am not sure what “long-term changes” are when referring to post 1990 effects, do you mean also changes that will happen in the future?
Line 88: I would add past changes to present and future changes
Line 93: refer to Fig. 1 in this instance
Line 102-104: change position of this sentence before the previous one, in line 99
Line 174: the procedure for sample preparation before 14C analysis of TOC is missing
Line 180: remove “the” before CuO oxidation
Line 190: change “precision” to “uncertainty”
Line 223: why were only 2 cores analyzed for geochronology? Besides, this would agree with fig. 3 where only the depth profiles of cores 1161MUC and 1159MUC are displayed, but the caption of the figure states that all 3 cores have depth profiles (line 891-892)
Line 274-276: judging by the ranges and average values, it is dubious if the middle core or the outer core have different lignin phenols concentrations
Line 325: change “located closest” to “the closest one”
Line 331: why does the settling of fine particles contribute to the formation of EM2 near the glacier front? EM2 was described as an EM mainly constituted by coarser particles
Line 334-339: could it be that the mixed (fine + coarse) distribution that you observe in the middle of the fjord is generated by the combination of different (marine, fluvial and glacial) processes? This is one of the things which brings me to think that EM3 could be a combination of EM1 and EM2 (see also the 3rd general comment).
Line 349: remove the comma after 0.4
Line 362-364: please include in the text the range of values, or average values, for surface sediments, IRDs and coal to make it easier for the reader
Line 369-370: as for the previous comment, please include the δ13C values for surface sediments, IRDs and coal in the discussion
Line 390-392: this statement, which I fully agree with, also partly contrasts with the previous interpretation of EM3 in the middle fjord, where you stated that fluvial processes were the possible source of the bimodal distribution, while here it is stated that glacial processes were the key factor for the general lack of sorting of the sediments
Line 392-396: I am not sure I agree with this interpretation. Of course, it is likely that the fjord experienced changes in the energy of the depositional environments, but the poor sorting in all cores could be simply due to the always present glacial activity, that even in stable conditions will provide poorly sorted sediments, just as you stated in the previous sentence citing Singh et al. 2019.
Line 428: remove “The”
Line 436-438: how do you explain a higher contribution of degraded OC in the middle and outer fjord, when supposedly the major source of old and reworked (and thus also degraded) OC in the fjord should be the Kongsbreen/Kronebreen (as it is also suggested by radiocarbon data)?
Line 481-485: while describing the results from Method 1, you used the range of % for each EM. Here instead you used the average values with SDs. Change one of the two descriptions for better uniformity
Line 581: explain further what you mean with “two-step process”, this point is not clear from the discussion
Fig. 11D: the axis label states “Distance from Blomstrandbreen front in 2006 (m)”, but it is not clear from where this distance was measured. A figure in the Supplementary Materials showing the position of this glacier would also be useful for the readers
Citation: https://doi.org/10.5194/egusphere-2025-957-RC2
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