the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Sep 2026
Estimates of sediment organic carbon burial and benthic-pelagic nutrient cycling in subarctic fjords (northern Nunatsiavut, eastern Canada)
Abstract. Fjords provide an important ecological service by burying and storing organic carbon (OC) more rapidly than most other marine environments. Quantifying this OC sink regionally requires an understanding of both the burial and remineralization processes that determine net OC sequestered. Within two sites from two Nunatsiavut fjords (Nachvak Fjord and Saglek Fjord, Labrador, Canada), we measured geochemistry associated with carbon, oxygen, and nutrient cycling and used a 1-D reactive transport model with an inverse modelling approach to simulate the geochemistry in the sampled sediment. A lower OC burial rate in sediment from Nachvak Fjord likely reflected significantly lower OC content in surface sediments and bottom sediments. Nachvak Fjord sediment modelling yielded a higher nitrification rate and a higher nitrate-dependent iron oxidation rate despite low heterotrophic denitrification compared to Saglek Fjord model results. Findings from microbial 16S rRNA gene sequencing were consistent with modelled biogeochemistry results from both fjords considering relative contributions from nitrate and sulfate reduction. Significantly higher OC content in Saglek Fjord sediments aligned with ~2X OC burial annually compared to Nachvak Fjord sediments. Differences in OC content and biogeochemical cycling might result from relative location chosen for sampling within each fjord, noting that we sampled outer fjord sediments in Nachvak Fjord, and inner fjord sediments in Saglek Fjord. Nevertheless, despite cold temperatures and good ventilation, both fjord basins utilized denitrification pathways (heterotrophic or nitrate-dependent iron oxidation) as the dominant nitrogen sink. Using both OC burial rates as end members, we estimate that northern Nunatsiavut fjords collectively store 8,000–33,000 tonnes OC yr−1. Our findings underscore the importance of additional studies with process-based measurements paired with OC burial estimates to understand further how climate change may alter benthic cycling and OC storage.
- Preprint
(7998 KB) - Metadata XML
-
Supplement
(1062 KB) - BibTeX
- EndNote
Status: open (until 27 Nov 2026)
- RC1: 'Comment on egusphere-2026-5292', Anonymous Referee #1, 27 Oct 2026 reply
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 387 | 202 | 72 | 661 | 99 | 70 | 69 |
- HTML: 387
- PDF: 202
- XML: 72
- Total: 661
- Supplement: 99
- BibTeX: 70
- EndNote: 69
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Overall:
This research shows strong potential to be a novel contribution to the field by providing estimated of OC burial in a previously understudied region. With some significant re-scoping and framing of the results, I can see it being a very useful foundation for future studies on benthic-pelagic interactions in Arctic systems. I would like to note that I am not a process modeler and therefore focused more on the benthic/sedimentary aspects of this manuscript and did not comment heavily on the model methods or results.
Major comment 1: The extrapolation of the OC rates calculated in these two fjords to represent the upper and lower bounds of all fjords in Nunatsiavut is under constrained and should be scaled back. Based on the data points available, I would recommend that the paper should be reframed as a process study rather than a framework for broader regional estimations.
Major comment 2: The order of materials throughout, particularly in the introduction and conclusion, seem disjointed. There are also materials placed in sections where they do not fit (e.g., results in discussion, etc.). Reconsider how these sections are ordered to more clearly tell the story.
Major comment 3: How were the core sites within each fjord chosen (i.e., one in the inner basin and one in the outer basin). Can you also indicate in which basin of each fjord the sample was taken for context, having previously outlined the number of basins in each? Particularly as the sample site from Nachvak fjord is far away from the Bentley and Kahlmeyer sites. This could also have implications for the OC burial rates you calculated as the higher rate from Saglek was from a site in the inner fjord and the lower rate from Nachvak fjord is from an outer basin site, where OC is presumably less preserved, but this effect is not discussed at all.
Specific Comments:
Please note that I have not focuses on identifying typographic errors, but please do ensure that the manuscript is thoroughly spell-checked following review.
Methods
Line 72 – The title of this section is sampling sites, but the sites themselves are not introduced until section 2.2 Core sampling. Consider renaming this section to “study site” or “Setting”
Line 97 – Relevance of the introduction of the potential MCA?
Lines 98-101 – Simply the description of the fjord basins. As currently written, it is hard to visualize the fjord structures. Consider adding a more detailed bathymetry indicating where the sills are located to Figure 2 and referencing Figure 2 in this section.
Line 115 – This statistic seems out of place – in the context of the fjord settings, why is such a general range being referenced? Is there a more site-specific value that can be used instead?
Line 127 – Were O2 measurements made during this time to ensure concentrations remained consisted with bottom water O2?
Line 128 – What does “subdivided cores” mean? Rewrite in more explicit language.
Line 146 – What was the point of the seawater core tube – I assume this was subsampled as a “blank” but this is not clear.
Line 189 – Was the overlying water syphoned off the cores before extrusion?
Line 190 – Justification for why the cores from the different sites were sliced to different intervals?
Line 201-215 – Consider describing in more detail how gamma counting and 210Pb activity measurements inform the calculation of sediment accumulation rates.
Line 203 – These cores are from more than 15 years ago – considering the fast SARs seen in the Bentley and Kahlmeyer paper, are these suitable to interpret sedimentation rates in a relatively short core collected in 2022/2023?
Line 204 – More core details needed – e.g., core length?
Line 209 – Justification for using the CFCS model? Was there an assumption of a uniform sediment supply over time and if so, can this be stated and referenced?
Line 254 – Regional habitat mapping data – how was this collected and where was it used – it doesn’t appear to ever be mentioned again.
Line 278 – Non-fine-grained sediment still contains OC – was this considered and if not please be explicit in describing why. Can the previously mentioned habitat mapping data be used to contextualize or validate the “30% mud cover” assumption being made?
Line 285-288 – What defines the three pools of each organic matter and iron oxides in the model?
Line 305 – “Sediment sampled from both fjords was consistently fine and soft.” Again, you previously mention that you assumed only 30% of surface sediments were muddy? These statements seem to be at odds.
Results
Lines 307-311 – These are previously published results, perhaps they belong in the introduction rather than your results section to clearly distinguish what new evidence you are presenting and what data is preexisting. Further, you are presenting sedimentation rates which does not fit the subtitle of “sediment appearance”. Reorganize and/or retitle accordingly.
Lines 349-353 – Significance of these decreases/increases? In some sections you seem to have assessed significance but not in others.
Line 586 – Is it reasonable to assume 30% cover of fine-grained sediment based on Scottish observations, since Arctic depositional settings are much more variable and different parts of the fjord are being measured?
Line 594-598 (and Lines 265-269) – Further to major comment 1: This assumption needs a much stronger justification. Why would you assume that these represent the lower and upper bounds of all northern Nunatsiavut fjords – where is the evidence for that? Do you know that these are the largest and smallest fjords with the highest/lowest sedimentation rates? This conclusion seems to be a stretch based on the results presented, and ultimately only two observational data points. Particularly as later in the discussion (Lines 652-653) you indicate that the OC content measured in the surface sediments from these fjords “greatly exceed the estimated average OC content in surface sediments from other Eastern Canada fjords”. You also contradict the assumptions behind these generalizations multiple times early in your discussion (e.g., Line 655, Lines 659-663). Please either consider removing this section of the paper or alternatively find a valid way to sanity-check these estimated and by calculating an error on this estimate.
Discussion
Further to Major Comment 2: Consider organizing the discussion in the same order as the results are presented and subdividing into subsections. The current order of discussion jumps around a lot.
Line 680 - Figure 11 presents results, should be moved to results section. Lines 723-733 also belong in results.
Line 736-738 – If comparing these two values, it would be useful to present them both with the same units (i.e., convert your regional estimate to km2/yr).
Lines 791 – 801 – This is a valid point and a good ambition – however it still perhaps does not justify expanding the assumptions from these two fjords to the entire Nunatsiavut coast, where the individual fjord settings may vary significantly. This could also be important to state at the beginning/in the intro to frame the purpose of the paper.
Lines 803-812 – This entire section introduces a lot of uncertainty into your results but does not resolve them. A reframing of the paper to focus on these two sites rather than extrapolating to the whole region means you could rework this section to eliminate some of these uncertainties.