Carbon Burial in two Greenland Fjords: Exploring the Influence of Glacier Type on Organic Carbon Dynamics

Buydens, Marius; De Borger, Emil; Meire, Lorenz; Bodé, Samuel; Schirone, Antonio; Soetaert, Karline; Vanreusel, Ann; Braeckman, Ulrike

doi:10.5194/egusphere-2025-102

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https://doi.org/10.5194/egusphere-2025-102

Preprints

21 May 2025

| 21 May 2025

Carbon Burial in two Greenland Fjords: Exploring the Influence of Glacier Type on Organic Carbon Dynamics

Marius Buydens, Emil De Borger, Lorenz Meire, Samuel Bodé, Antonio Schirone, Karline Soetaert, Ann Vanreusel, and Ulrike Braeckman

Abstract. Fjord systems are crucial for the burial and long-term storage of organic carbon (OC), contributing significantly to global blue carbon sequestration. Despite their importance, Greenland's fjords remain underrepresented in global carbon budgets, even though accelerated melt of the Ice Sheet alters these ecosystems through increased freshwater discharge and iceberg calving, ultimately leading to glaciers retreating inland. This study compares organic carbon burial rates (OCBRs) in two neighbouring Greenland fjords – Nuup Kangerlua, influenced by marine-terminating glaciers (MTGs), and Ameralik, dominated by land-terminating glaciers (LTGs) – to explore the effects of both types of glaciers on sediment carbon dynamics. Since subglacial discharge-driven upwelling in Nuup Kangerlua (MTG) has been shown to support higher summer phytoplankton blooms, we expected higher sediment organic carbon content and burial in this MTG fjord. However, our observations show higher OC content in sediments of Ameralik's (LTG) outer and mid fjord section and a similar OCBR in both fjords. This unexpected finding may be linked to differences in pelagic grazing pressure, organic carbon transport, and sediment preservation mechanisms. The findings call for further research to unravel the complex interactions between primary production, organic carbon transport, and preservation processes in different glacial fjord systems.

Received: 13 Jan 2025 – Discussion started: 21 May 2025

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Journal article(s) based on this preprint

09 Feb 2026

Carbon burial in two Greenland fjords shows no direct link to glacier type

Marius Buydens, Emil De Borger, Lorenz Meire, Samuel Bodé, Antonio Schirone, Karline Soetaert, Ann Vanreusel, and Ulrike Braeckman

Biogeosciences, 23, 1159–1179, https://doi.org/10.5194/bg-23-1159-2026,https://doi.org/10.5194/bg-23-1159-2026, 2026

Short summary

Marius Buydens, Emil De Borger, Lorenz Meire, Samuel Bodé, Antonio Schirone, Karline Soetaert, Ann Vanreusel, and Ulrike Braeckman

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-102', Anonymous Referee #2, 22 May 2025

General comments:
The manuscript by Buydens et al. investigates how organic carbon burial is affected by the presence of marine versus land terminating glaciers (MTG vs LTG). The authors compare two fjord systems close to Nuuk in Southwest Greenland. They present data for sediment cores collected during two separate cruises in late summer 2021 and late spring 2022 at 9 locations, 6 of them in Nuup Kangerlua, which receives meltwater from both MTG and LTG, and 3 in Ameralik, which only receives meltwater from one LTG. For all sites, they determined sedimentation rates using 210Pb and 137Cs, grain size distributions, porosity, carbon and nitrogen concentrations and stable isotopes, as well as pigments. From these data, they derive proportions of marine versus terrestrial organic carbon (OC), its freshness and burial rates. They find that despite more productive waters in Nuup Kangerlua (according to earlier publications), OC concentrations are significantly higher at the mid and outer stations of Ameralik and OC burial rates are similar for both systems. These findings are then compared to other glacial fjords and explained by a more complex foodweb in the MTG fjord, which supposedly recycles much of the enhanced primary production and therby prevents burial, in addition to possibly contrasting effects of OC transport and preservation mechanisms at play at these locations.
This study provides important insights into carbon dynamics in glacial fjords from a region that’s undergoing rapid change. The authors present an impressive wealth of data, combining sedimentological analyses with organic matter quantification and characterization. The manuscript is written very well and the figures nicely illustrate the results. My only major issue concerns the sedimentation rates, which are central to the investigation. In Fig. A1, it looks as if 210PB_xs was very low (and almost constant) for e.g. GF13 and GF6, how were sedimentation rates derived there? “Log transformed 210Pbxs activities were plotted against the cumulative dry mass depth (g cm-2) of the sediment per station.” It would be good to show these plots. Also, why was the CRS (not CSR) model applied to some of the cores? For consistency (and if only an average sedimentation rate is required), for all cores the CF:CS model should be used. (Moreover, in Table 2, AM8 and AM5 have uncertainties of 0.0.) Furthermore, how was the lack of bioturbation determined? From the Cs profiles in Fig. A1, it does look as if mixing may have affected at least some of the cores.
Another, smaller point: the authors state that their findings regarding higher OC content in the LTG fjord came as a surprise – yet comparisons with literature data (Figure 5) reveal that this is not so different from earlier studies in other regions.
Specific comments:
Abstract: The last two sentences could be combined as they are a bit redundant at the moment. The available space could then be allocated to mention e.g. results for OC composition or provide more detail (orders of magnitude) for sedimentation rates and/or OCBR, or some info on which parameters were measured for this study.
Figure 1: Sample IDs could be shown on the map.
Figure 4: A second panel with C:N vs d13C could be added, as C:N ratios have also been commonly employed to distinguish terrestrial from marine OC.
L161-170: For the proportions of terrestrial vs marine OC, the selection of endmember values need more explanation: “These end-members were derived from Northern and Mid-Norway fjord sediments” – how? Also, those values should have some uncertainties, which would then propagate to the fraction estimates. In Figure 4, the endmembers appear to have uncertainties.
Technical corrections:
Short summary: “necessarily” instead of “necessary”
Abstract: L21, 23 & 25 “organic carbon” should be “OC”
Figure A2: In the caption, it reads “Orange and black colors represent end of summer 2021 and spring 2022”, yet there is no orange in the figure.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC1
- AC2:
  'Reply on RC1', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC2
  - AC4: 'Reply on AC2', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC4-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC4
RC2:
'Comment on egusphere-2025-102', Anonymous Referee #3, 01 Jun 2025

Buydens et al, quantifies the accumulation of organic carbon in two Greenland fjords, with one being fed by a marine and the other by a land terminating glacier. The manuscript is a good addition to the literature that generally lacks data from Greenland systems. However, prior to publication the manuscript needs to be refined to improve clarity and potentially rethink the discussion.
The initial framing of the research in the Blue Carbon space feels forced. I would try to avoid this, as it is still a grey area if sediments can be considered Blue Carbon.
I believe the methodology employed in the research is sound, but the lack of detail present makes this difficult to assess. More detail is required for all methods used, currently they are inadequate. Additionally, this section lacks citation. Please utilise the wider literature to support the methods. Specific comments outlined below/
I also believe that the discussion needs reworked. This section is largely focused on reasons why the results do not match what was predicted. The study goes down multi avenues of enquiry but these a largely subjective as there is no data as there is no data available. Much of this could be condensed into a single section opposed to being four sub-sections.
Below are minor comments, I have not identified typos.
Introduction
Line 27 states that fjords OC burial contributes to around 10% of blue carbon burial. The work of Smith et al. (2015) states that fjord OC accumulation is equivalent to 10% of carbon buried in other marine sedimentary systems which does not include blue carbon environments (saltmarsh, seagrass and mangroves).
Line 45 – refrain from using “references therein”. Cite the relevant studies.
Lines 40 – 51 – In this paragraph you discuss how the different types of glaciers enhance or reduce nutrients entering the fjords. Which nutrients?
Line 66 – Throughout the manuscript the term “carbon sequestration” is used. Sequestration is when carbon is removed from the atmospheric pool. Fjords are donor environments (Middleburg, 2019) which means the receive and store carbon sequestered in other environments (terrestrial or marine). Accumulates or buries are more accurate terms to use when describing these processes.
Lines 81-85 – This line does not add anything to the introduction, please remove.

Methodology
Line 104 – is Stuart Lee et al., 2021 correctly referenced.
124 – Solid phase – would sediment be a better description. I don’t understand the need for this as there is no liquid phase sampling.
Line 125 – the n=3 and n=1 is not required as you state in the text there were 3 deployments for geochemistry and sedimentology and 1 for dating.
Line 128 – remove “thick”
Line 129 – expand on why the sampling intervals was increased from 1 to 2 cm downcore. Would it not have been more useful to be consistent throughout.
Table 1 – reduce the number of decimals point the BWT is reported to. “ decimal points will be suitable.
Line 139 – 153 needs significantly expanded; not enough information is provided to replicate the study and must be supported by references.
Line 139 – How was the sediment prepared for analysis. Was there any preparation such as removal of organics or carbonate using Hydrogen peroxide and HCl. Was any detergent (i.e. Calgon) used to prevent flocculation of the clays?
Line 140 states Porosity was obtained by dividing the porewater volume by the wet sediment volume. How was the porewater volume calculated?
Line 141 – What temperature were the sample dried and for how long?
Line 143 states “After decalcification with 37 % HCl, total organic carbon (TOC) was also measured” – Was this acid fumigation or direct acidification. I assume it is acid fumigation as 37% conc, HCl would not be used for direct acidification.
Line 144 is not clear - From these data, the molar CN ratios were calculated, and inorganic carbon (IC) was determined by subtracting TOC from TC. State that the molar C/N ratio was calculated using the OC.
Line 145 – What does “see further” refer to?
Line 146 – for the stable isotope analysis provide a preparation method, I assume the same as the EA.
Line 151 – Briefly expand on the methodology of Wright and Jeffrey (1997), how were the pigments extracted and what tool was used to measure them?
Line 185 – Both the linear and CRS calculation methods were used to determine the OCAR for a mix of sites. Please state why two different methods were used. I understand the Cs peaks allowed the CRS to be used on some cores, but would it not been useful for comparison to use the linear method on all cores.
Line 187 state there are clear 137Cs peaks in cores GF9, GF7, AM8 and AM5. From figure A1 that these peaks could be heavily distorted by mixing how confident are you these are unimpacted records.
Line 187 – State which year you are assigning to the 137Cs peak, Chernobyl or weapons testing?
Line 193 states “No bioturbated or mixed upper layer was observed in the profiles”. The radionuclide profiles would suggest significant mixing, which the authors later discuss in the text. There suitability of these cores for dating and the quality of the resultant outputs must be discussed.

Results
Line 220 – What is the reason for such shifts in porosity and dry bulk density.
Line 37 – remove “while”
I have not commented on the discussion on conclusion as I believe the need significantly reworked, see initial comment.
Figures
Figure 1 – Add site numbers to the map. Change colour of marker for Nuuk it is currently to close to the sampling site symbol. Define the inner, middle and outer sections of the fjord on the maps.
Figure 2 has dry bulk density data. But this is not mentioned in the method section. Lines joining the points would be useful as it is hard to follow some of the downcore data (i.e., GF10).
Could Figure 2 and 3 be combined.
Figure 4 – add the end-member values used in the study to the plot. Also, it would be useful to plot d13C vs CN
Table 2 – in the caption state which model was used to determine these rates.
Figure 2A – Colours missing, there should be orange and black according to the caption.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC2
- AC1:
  'Reply on RC2', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC1
  - AC5: 'Reply on AC1', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC5-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC5
RC3:
'Comment on egusphere-2025-102', Anonymous Referee #1, 15 Jun 2025

General Comments:
This study from Buydens et al. contributes to an area of sedimentary carbon storage that is currently under-represented. It is important to understand these systems which are undergoing profound changes due to climate change. The study looks to understand the differences between marine-terminating glaciers (MTGs) and land-terminating glaciers (LTGs) on sedimentary OC burial in Greenland fjords. MTGs have an upwelling nutrient mechanism which drive extended primary production rates and could therefore contribute higher amounts of OC to the sediment store. The justification for the study is sound – a better understanding of the carbon cycling system will help to improve predictions about how climate change may impact the long-term storage capacity of Greenland fjords. Two fjord systems are compared; Nuup Kangerlua has 3 MTGs and 3LTGs while Ameralik has 3 LTGs – this provides an interesting comparison, however there are many similarities in the geomorphology and characteristics. It is not clear what the terrestrial vegetation is and if it is different between these systems.
Generally, this is a well-written and well-structured manuscript. The dataset is comprehensive for one study with an appropriate range of analytical measurements, however much more detail is required in the methods. It would be hard for anyone else to recreate the analyses. The discussion and conclusion rely heavily on hypothetical scenarios due to a lack of data/observations for what is a very complex system. I appreciate the thought that has gone into trying to explain the results found within this study, however more data would be needed to do this. I think more could be done to look at the spatial distribution of sediments within the fjord systems as this can help to better understand hydrodynamics of the system as well as the benthic communities and carbon processing. There are more questions than answers resulting from this study, which is fine, however the authors may wish to refine their discussion to focus on what could be understood/ concluded from the results and provide recommendations for Further Work/ Research to build a stronger picture.
Specific Comments:
Abstract –
L1 – Fjords play a crucial role in the burial of OC – not they are crucial – same as L26 of introduction.
L23-25 – duplicated sentences – could be combined.
Introduction -
L28 ‘Blue carbon’ – this isn’t defined but should be. Sediments are currently understood to be ‘emerging’ blue carbon systems and are not accepted as ‘actionable’ blue carbon systems (See Howard et al., 2023)? The term isn’t mentioned again so why use it at all?
Regarding the reference to Smith et al., 2015: Does the annual blue carbon burial include mangroves, seagrass and saltmarsh? Smith’s paper discussed fjords in the context of global sedimentary carbon only.
L32 – could also mention southern-hemisphere countries that have looked at OC burial in fjords -e.g. Chile.
L57 – depends on terrestrial vegetation found within the catchment – rather than growth specifically?
L60 – 62 – sediment type also important. The type and spatial distribution of sediment will be somewhat controlled by fjord geomorphology and current dynamics already mentioned – but %clay and presence of Fe minerals within the sediment has a control on the availability of OC and its degradation potential (see: Lalonde et al., 2012; Hunt et al., 2020; Moore et al., 2023)
Line 66 – sequestration refers to the removal od carbon dioxide from the atmosphere. Within sediments, as passive receivers of organic matter, the term OC accretion of burial is more appropriate.
Materials and Methods -
Figure 1 – could the red dots instead be changed to one colour to represent the LTGs and another colour to represent the MTGs? Station labels would be helpful
Section 2.2.1 – more appropriate to call this ‘Core Sampling’?
L127 - Why was the 2021 campaign more exploratory?
2.3 - More detail needed here. Think about how someone would replicate your analysis exactly. E.g., not sure what has been done here: ‘Porosity was obtained by dividing the porewater volume by the wet sediment volume.’
L155 – some more description as to why these parameters help to differentiate between terrestrial and marine-derived material would be helpful.
L166 – Why were the top 2 cm of sediment only used?
L168 – is the same terrestrial end member as Faust’s appropriate here? What are the dominant vegetation type within these study fjords?
L197 – Expand on how the 10 cm of sediment column was averaged.
L199 – How did you know the seasonal difference was insignificant?
Results -
L210-215 – I don’t agree that there is much difference between the sediment grain between GF13, GF12, GF10 and GF6 (inner and outer).
L220 – Would be helpful to expand upon why porosity porosity and dry bulk density (not mentioned in the methods) have been measured and what they tell us about the sediment dynamics?
Discussion -
L277 – introduces the term lability for the first time. Can this be expanded upon somewhere – what does it mean, what parameters have measures it and why does it matter?
L320 – the temperature difference between the two fjord systems is a tangible difference and could be better highlighted. The discussion about Fe and Mn is interesting, but currently limited data to tease the two fjords apart.
L389 – No mention about the biomass of benthic invertebrates which may adapt to respond to the variable MAR rates at the two fjords – i.e. the increase in MAR at Nuup Kangerlua may fuel a higher biomass benthos which results in similar OCBRs within both fjords. The benthic biology may be important here?
A section on Recommendations for further research would be helpful.
Conclusion
L399 – There isn’t enough evidence to suggest that MTGs DO function as carbon pumps – Perhaps change wording to COULD function as carbon pumps.
Technical Corrections:
L125 – TOC and TN not defined yet.
L127 – grammatical point – fewer stations rather than less stations. Although Table 1 shows that there were the same number of stations sampled in both years?
L140 – text missing – change to ‘conforming to the Wentworth scale’
Figure 4 – Can the values for the terrestrial and marine end members be shown – the range for 15N for marine is very broad – is that right?
Figure 5 – Data in blue and yellow represent the MTGs and LTGs respectively (red used in error). The mixed type is Red. Where have all the Greenland data points come from in 5b between 70-75N?
Figure 6 – write out the word transport
A2 – dots are grey, not orange.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC3
- AC3:
  'Reply on RC3', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC3
  - AC6: 'Reply on AC3', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC6-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC6

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-102', Anonymous Referee #2, 22 May 2025

General comments:
The manuscript by Buydens et al. investigates how organic carbon burial is affected by the presence of marine versus land terminating glaciers (MTG vs LTG). The authors compare two fjord systems close to Nuuk in Southwest Greenland. They present data for sediment cores collected during two separate cruises in late summer 2021 and late spring 2022 at 9 locations, 6 of them in Nuup Kangerlua, which receives meltwater from both MTG and LTG, and 3 in Ameralik, which only receives meltwater from one LTG. For all sites, they determined sedimentation rates using 210Pb and 137Cs, grain size distributions, porosity, carbon and nitrogen concentrations and stable isotopes, as well as pigments. From these data, they derive proportions of marine versus terrestrial organic carbon (OC), its freshness and burial rates. They find that despite more productive waters in Nuup Kangerlua (according to earlier publications), OC concentrations are significantly higher at the mid and outer stations of Ameralik and OC burial rates are similar for both systems. These findings are then compared to other glacial fjords and explained by a more complex foodweb in the MTG fjord, which supposedly recycles much of the enhanced primary production and therby prevents burial, in addition to possibly contrasting effects of OC transport and preservation mechanisms at play at these locations.
This study provides important insights into carbon dynamics in glacial fjords from a region that’s undergoing rapid change. The authors present an impressive wealth of data, combining sedimentological analyses with organic matter quantification and characterization. The manuscript is written very well and the figures nicely illustrate the results. My only major issue concerns the sedimentation rates, which are central to the investigation. In Fig. A1, it looks as if 210PB_xs was very low (and almost constant) for e.g. GF13 and GF6, how were sedimentation rates derived there? “Log transformed 210Pbxs activities were plotted against the cumulative dry mass depth (g cm-2) of the sediment per station.” It would be good to show these plots. Also, why was the CRS (not CSR) model applied to some of the cores? For consistency (and if only an average sedimentation rate is required), for all cores the CF:CS model should be used. (Moreover, in Table 2, AM8 and AM5 have uncertainties of 0.0.) Furthermore, how was the lack of bioturbation determined? From the Cs profiles in Fig. A1, it does look as if mixing may have affected at least some of the cores.
Another, smaller point: the authors state that their findings regarding higher OC content in the LTG fjord came as a surprise – yet comparisons with literature data (Figure 5) reveal that this is not so different from earlier studies in other regions.
Specific comments:
Abstract: The last two sentences could be combined as they are a bit redundant at the moment. The available space could then be allocated to mention e.g. results for OC composition or provide more detail (orders of magnitude) for sedimentation rates and/or OCBR, or some info on which parameters were measured for this study.
Figure 1: Sample IDs could be shown on the map.
Figure 4: A second panel with C:N vs d13C could be added, as C:N ratios have also been commonly employed to distinguish terrestrial from marine OC.
L161-170: For the proportions of terrestrial vs marine OC, the selection of endmember values need more explanation: “These end-members were derived from Northern and Mid-Norway fjord sediments” – how? Also, those values should have some uncertainties, which would then propagate to the fraction estimates. In Figure 4, the endmembers appear to have uncertainties.
Technical corrections:
Short summary: “necessarily” instead of “necessary”
Abstract: L21, 23 & 25 “organic carbon” should be “OC”
Figure A2: In the caption, it reads “Orange and black colors represent end of summer 2021 and spring 2022”, yet there is no orange in the figure.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC1
- AC2:
  'Reply on RC1', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC2
  - AC4: 'Reply on AC2', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC4-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC4
RC2:
'Comment on egusphere-2025-102', Anonymous Referee #3, 01 Jun 2025

Buydens et al, quantifies the accumulation of organic carbon in two Greenland fjords, with one being fed by a marine and the other by a land terminating glacier. The manuscript is a good addition to the literature that generally lacks data from Greenland systems. However, prior to publication the manuscript needs to be refined to improve clarity and potentially rethink the discussion.
The initial framing of the research in the Blue Carbon space feels forced. I would try to avoid this, as it is still a grey area if sediments can be considered Blue Carbon.
I believe the methodology employed in the research is sound, but the lack of detail present makes this difficult to assess. More detail is required for all methods used, currently they are inadequate. Additionally, this section lacks citation. Please utilise the wider literature to support the methods. Specific comments outlined below/
I also believe that the discussion needs reworked. This section is largely focused on reasons why the results do not match what was predicted. The study goes down multi avenues of enquiry but these a largely subjective as there is no data as there is no data available. Much of this could be condensed into a single section opposed to being four sub-sections.
Below are minor comments, I have not identified typos.
Introduction
Line 27 states that fjords OC burial contributes to around 10% of blue carbon burial. The work of Smith et al. (2015) states that fjord OC accumulation is equivalent to 10% of carbon buried in other marine sedimentary systems which does not include blue carbon environments (saltmarsh, seagrass and mangroves).
Line 45 – refrain from using “references therein”. Cite the relevant studies.
Lines 40 – 51 – In this paragraph you discuss how the different types of glaciers enhance or reduce nutrients entering the fjords. Which nutrients?
Line 66 – Throughout the manuscript the term “carbon sequestration” is used. Sequestration is when carbon is removed from the atmospheric pool. Fjords are donor environments (Middleburg, 2019) which means the receive and store carbon sequestered in other environments (terrestrial or marine). Accumulates or buries are more accurate terms to use when describing these processes.
Lines 81-85 – This line does not add anything to the introduction, please remove.

Methodology
Line 104 – is Stuart Lee et al., 2021 correctly referenced.
124 – Solid phase – would sediment be a better description. I don’t understand the need for this as there is no liquid phase sampling.
Line 125 – the n=3 and n=1 is not required as you state in the text there were 3 deployments for geochemistry and sedimentology and 1 for dating.
Line 128 – remove “thick”
Line 129 – expand on why the sampling intervals was increased from 1 to 2 cm downcore. Would it not have been more useful to be consistent throughout.
Table 1 – reduce the number of decimals point the BWT is reported to. “ decimal points will be suitable.
Line 139 – 153 needs significantly expanded; not enough information is provided to replicate the study and must be supported by references.
Line 139 – How was the sediment prepared for analysis. Was there any preparation such as removal of organics or carbonate using Hydrogen peroxide and HCl. Was any detergent (i.e. Calgon) used to prevent flocculation of the clays?
Line 140 states Porosity was obtained by dividing the porewater volume by the wet sediment volume. How was the porewater volume calculated?
Line 141 – What temperature were the sample dried and for how long?
Line 143 states “After decalcification with 37 % HCl, total organic carbon (TOC) was also measured” – Was this acid fumigation or direct acidification. I assume it is acid fumigation as 37% conc, HCl would not be used for direct acidification.
Line 144 is not clear - From these data, the molar CN ratios were calculated, and inorganic carbon (IC) was determined by subtracting TOC from TC. State that the molar C/N ratio was calculated using the OC.
Line 145 – What does “see further” refer to?
Line 146 – for the stable isotope analysis provide a preparation method, I assume the same as the EA.
Line 151 – Briefly expand on the methodology of Wright and Jeffrey (1997), how were the pigments extracted and what tool was used to measure them?
Line 185 – Both the linear and CRS calculation methods were used to determine the OCAR for a mix of sites. Please state why two different methods were used. I understand the Cs peaks allowed the CRS to be used on some cores, but would it not been useful for comparison to use the linear method on all cores.
Line 187 state there are clear 137Cs peaks in cores GF9, GF7, AM8 and AM5. From figure A1 that these peaks could be heavily distorted by mixing how confident are you these are unimpacted records.
Line 187 – State which year you are assigning to the 137Cs peak, Chernobyl or weapons testing?
Line 193 states “No bioturbated or mixed upper layer was observed in the profiles”. The radionuclide profiles would suggest significant mixing, which the authors later discuss in the text. There suitability of these cores for dating and the quality of the resultant outputs must be discussed.

Results
Line 220 – What is the reason for such shifts in porosity and dry bulk density.
Line 37 – remove “while”
I have not commented on the discussion on conclusion as I believe the need significantly reworked, see initial comment.
Figures
Figure 1 – Add site numbers to the map. Change colour of marker for Nuuk it is currently to close to the sampling site symbol. Define the inner, middle and outer sections of the fjord on the maps.
Figure 2 has dry bulk density data. But this is not mentioned in the method section. Lines joining the points would be useful as it is hard to follow some of the downcore data (i.e., GF10).
Could Figure 2 and 3 be combined.
Figure 4 – add the end-member values used in the study to the plot. Also, it would be useful to plot d13C vs CN
Table 2 – in the caption state which model was used to determine these rates.
Figure 2A – Colours missing, there should be orange and black according to the caption.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC2
- AC1:
  'Reply on RC2', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC1
  - AC5: 'Reply on AC1', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC5-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC5
RC3:
'Comment on egusphere-2025-102', Anonymous Referee #1, 15 Jun 2025

General Comments:
This study from Buydens et al. contributes to an area of sedimentary carbon storage that is currently under-represented. It is important to understand these systems which are undergoing profound changes due to climate change. The study looks to understand the differences between marine-terminating glaciers (MTGs) and land-terminating glaciers (LTGs) on sedimentary OC burial in Greenland fjords. MTGs have an upwelling nutrient mechanism which drive extended primary production rates and could therefore contribute higher amounts of OC to the sediment store. The justification for the study is sound – a better understanding of the carbon cycling system will help to improve predictions about how climate change may impact the long-term storage capacity of Greenland fjords. Two fjord systems are compared; Nuup Kangerlua has 3 MTGs and 3LTGs while Ameralik has 3 LTGs – this provides an interesting comparison, however there are many similarities in the geomorphology and characteristics. It is not clear what the terrestrial vegetation is and if it is different between these systems.
Generally, this is a well-written and well-structured manuscript. The dataset is comprehensive for one study with an appropriate range of analytical measurements, however much more detail is required in the methods. It would be hard for anyone else to recreate the analyses. The discussion and conclusion rely heavily on hypothetical scenarios due to a lack of data/observations for what is a very complex system. I appreciate the thought that has gone into trying to explain the results found within this study, however more data would be needed to do this. I think more could be done to look at the spatial distribution of sediments within the fjord systems as this can help to better understand hydrodynamics of the system as well as the benthic communities and carbon processing. There are more questions than answers resulting from this study, which is fine, however the authors may wish to refine their discussion to focus on what could be understood/ concluded from the results and provide recommendations for Further Work/ Research to build a stronger picture.
Specific Comments:
Abstract –
L1 – Fjords play a crucial role in the burial of OC – not they are crucial – same as L26 of introduction.
L23-25 – duplicated sentences – could be combined.
Introduction -
L28 ‘Blue carbon’ – this isn’t defined but should be. Sediments are currently understood to be ‘emerging’ blue carbon systems and are not accepted as ‘actionable’ blue carbon systems (See Howard et al., 2023)? The term isn’t mentioned again so why use it at all?
Regarding the reference to Smith et al., 2015: Does the annual blue carbon burial include mangroves, seagrass and saltmarsh? Smith’s paper discussed fjords in the context of global sedimentary carbon only.
L32 – could also mention southern-hemisphere countries that have looked at OC burial in fjords -e.g. Chile.
L57 – depends on terrestrial vegetation found within the catchment – rather than growth specifically?
L60 – 62 – sediment type also important. The type and spatial distribution of sediment will be somewhat controlled by fjord geomorphology and current dynamics already mentioned – but %clay and presence of Fe minerals within the sediment has a control on the availability of OC and its degradation potential (see: Lalonde et al., 2012; Hunt et al., 2020; Moore et al., 2023)
Line 66 – sequestration refers to the removal od carbon dioxide from the atmosphere. Within sediments, as passive receivers of organic matter, the term OC accretion of burial is more appropriate.
Materials and Methods -
Figure 1 – could the red dots instead be changed to one colour to represent the LTGs and another colour to represent the MTGs? Station labels would be helpful
Section 2.2.1 – more appropriate to call this ‘Core Sampling’?
L127 - Why was the 2021 campaign more exploratory?
2.3 - More detail needed here. Think about how someone would replicate your analysis exactly. E.g., not sure what has been done here: ‘Porosity was obtained by dividing the porewater volume by the wet sediment volume.’
L155 – some more description as to why these parameters help to differentiate between terrestrial and marine-derived material would be helpful.
L166 – Why were the top 2 cm of sediment only used?
L168 – is the same terrestrial end member as Faust’s appropriate here? What are the dominant vegetation type within these study fjords?
L197 – Expand on how the 10 cm of sediment column was averaged.
L199 – How did you know the seasonal difference was insignificant?
Results -
L210-215 – I don’t agree that there is much difference between the sediment grain between GF13, GF12, GF10 and GF6 (inner and outer).
L220 – Would be helpful to expand upon why porosity porosity and dry bulk density (not mentioned in the methods) have been measured and what they tell us about the sediment dynamics?
Discussion -
L277 – introduces the term lability for the first time. Can this be expanded upon somewhere – what does it mean, what parameters have measures it and why does it matter?
L320 – the temperature difference between the two fjord systems is a tangible difference and could be better highlighted. The discussion about Fe and Mn is interesting, but currently limited data to tease the two fjords apart.
L389 – No mention about the biomass of benthic invertebrates which may adapt to respond to the variable MAR rates at the two fjords – i.e. the increase in MAR at Nuup Kangerlua may fuel a higher biomass benthos which results in similar OCBRs within both fjords. The benthic biology may be important here?
A section on Recommendations for further research would be helpful.
Conclusion
L399 – There isn’t enough evidence to suggest that MTGs DO function as carbon pumps – Perhaps change wording to COULD function as carbon pumps.
Technical Corrections:
L125 – TOC and TN not defined yet.
L127 – grammatical point – fewer stations rather than less stations. Although Table 1 shows that there were the same number of stations sampled in both years?
L140 – text missing – change to ‘conforming to the Wentworth scale’
Figure 4 – Can the values for the terrestrial and marine end members be shown – the range for 15N for marine is very broad – is that right?
Figure 5 – Data in blue and yellow represent the MTGs and LTGs respectively (red used in error). The mixed type is Red. Where have all the Greenland data points come from in 5b between 70-75N?
Figure 6 – write out the word transport
A2 – dots are grey, not orange.

Citation: https://doi.org/10.5194/egusphere-2025-102-RC3
- AC3:
  'Reply on RC3', Marius Buydens, 24 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-102-AC3
  - AC6: 'Reply on AC3', Marius Buydens, 24 Jul 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-102/egusphere-2025-102-AC6-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-102-AC6

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (30 Jul 2025) by Mark Lever

AR by Marius Buydens on behalf of the Authors (31 Jul 2025) Author's tracked changes Manuscript

EF by Vitaly Muravyev (05 Aug 2025) Author's response

ED: Referee Nomination & Report Request started (13 Aug 2025) by Mark Lever

RR by Anonymous Referee #3 (04 Oct 2025)

RR by Anonymous Referee #4 (07 Oct 2025)

Suggestions for revision or reasons for rejection

Introduction:
A nice review of the issue. Would be stronger if tied more tightly to scientific method – a hypothesis/mechanism is loosely stated (MTG upwelling vs. LTG absence of upwelling), the prediction is intimated but less concrete (percent OC in surface sediments??), the specific testing is mostly absent and it isn’t clear how the hypothesis would be falsified. The last paragraph promises “insights,” which is appropriate for human behavior/cognition but we want something more concrete for physical sciences. Be specific, tell us exactly what you are going to test and how it will be interpreted. “Insights” is too vague.

Discussion:
Ln 323-4 – Again, linking to scientific method, the hypothesis is falsified, and this could be stated as such. The problem with “not supporting” is that it implies that testing could have also “supported,” but that is not the goal of scientific method, to support or prove. Otherwise, it is subject to confirmation bias like religion and politics, and we’ve seen how well that is working right now. I apologize for the little editorial, but it is what makes me most proud about our discipline, that when done well we will root out the mistakes and errors instead of doubling down on them.

Section 4.1: I’m not sure why surface sediment OC content is used as the test of the hypothesis instead of carbon accumulation rate, which incorporates the sediment dilution that is discussed here as a complicating factor for percent OC.

Ln 372-3: Say more about how dilution leads to an underestimate. Shouldn’t this be accounted for by dating the cores?

Ln 378-9: I understand that limited data makes it hard to identify and rank controls, but it is also unsatisfying as a reader when the whole process is labeled as “complex” as the parting shot. The next section proceeds to pick apart some of this complexity, so why not make this a more obvious transition into that next section?

4.3.1 Stylistically, I don’t love the approach of setting up a possible explanation (differences in bottom water temperature) and then proceeding to shoot it down. I appreciated that the text got there eventually, but dangling those small differences in temperature as a possible explanation made me groan, and I don’t think you ever want to make your readers groan.

4.3.2 Doesn’t your core dating address lateral transport? The lead profile seems to minimize that possibility. It is absolutely true that resuspension and transport of sediments is a complication, but then you would also have to address why this laterally transported sediment has higher or lower OC content that is decoupled from the original primary productivity idea. You partially do this with cross “contamination” from one fjord to the next, but then again, your surface OC content was similar in both.

4.3.3 Do you have data either here or in the literature to compare an annual primary productivity to carbon burial rates in MTGs vs. LTGs? It seems like that would be an important parameter to compare – a carbon burial efficiency number. Another number to consider is the relative inputs of terrestrial OC vs. marine OC. I know that you use 13C to try to tease this apart, but conceptually, shouldn’t it be a higher fraction in the LTGs than MTGs since you actually have a little river flow in the former that can allow more terrestrial carbon loading onto sediment surfaces? It would be informative to know what the weight percent OC is on suspended sediments before they discharge.

Just in general, foodweb dynamics are really hard to tease apart, and I appreciate that this should be raised as a factor, but the section as a whole feels more speculative than helpful. You need vertical flux data, and you don’t have it.

Another property you could look at is sediment loading (using percent OC and grain size as a proxy for surface area if you want to get into the weeds more). Do you have any way of distinguishing between discrete OC particles vs. sediment-associated OC?

4.4 I appreciate the intent here of providing future direction, but this section comes across as a more complicated “more research is needed” statement, which undermines what you *have* accomplished in this study. It basically says, “what we did isn’t good enough, here is how we need to make it better.” It would be stronger to pick one or two new hypotheses (i.e. mechanisms) that you think are the most likely alternative hypotheses to your original hypothesis, and then be specific about the prediction and tests that could falsify. As is, there is a vagueness to these around “understanding,” which is only earned through concrete and specific data/tests/mechanisms/knowledge.

Conclusions: This section is highly redundant, and I think is unnecessary as long as the abstract captures all the same information. A significant shortcoming of both the abstract and the conclusions is that there is nothing quantitative to make it concrete and memorable. As in the introduction, I don’t like the focus on “insights,” and I would extend this to the title as well – “exploring” may accurately describe the exercise of carrying out this work, but it gives the impression of insecurity in what is presented here. Consider a bolder title, something like, “Fjords fed by marine- vs. land-terminating glaciers show no significant difference in organic carbon accumulation rate.” That would be memorable.

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RR by Anonymous Referee #5 (13 Oct 2025)

ED: Reconsider after major revisions (13 Oct 2025) by Mark Lever

AR by Marius Buydens on behalf of the Authors (13 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 Nov 2025) by Mark Lever

RR by Anonymous Referee #3 (17 Nov 2025)

RR by Anonymous Referee #2 (10 Dec 2025)

Suggestions for revision or reasons for rejection

I have reviewed an earlier version of this manuscript and find it much improved. The wealth of data remains impressive, while structure and figure quality now meet expectations for publication. I also wish to acknowledge that fact that the authors put a good amount of effort into sincerely replying to all of the different requests from the reviewers. My remaining issues with the manuscript are minor, as detailed below. I still struggle with the source apportionment calculations using only d13C values without uncertainties. With that simplistic approach, the authors might as well just postulate that all OC is marine for all stations except NK13. If they wanted to improve their approach, they would need to include uncertainties for the endmember values and propagate these. They could also include d15N and or C:N ratios as is commonly done (e.g. using the MixSIAR models). Anyhow, for the sake of this paper, simply assuming negligible contributions of terrestrial OC is probably fair enough.
Small error in line 204: "ancient marine carbon" is probably supposed to just be "ancient carbon"?
And there is a repetition of the following sentences "The distinct geomorphology of Ameralik and Nuup Kangerlua, particularly their differing sill depths, shapes bottom water exchange and temperature regimes, which can influence organic matter preservation. Both fjords have no anoxic deep-water
masses, and bottom water renewal occurs every one to two years (Mortensen, 2011; Stuart-Lee et al., 2021). However, Ameralik’s shallower sill depth (∼110 m) compared to Nuup Kangerlua (∼200 m) restricts the inflow of warmer, saltier coastal waters (Stuart-Lee et al., 2021), resulting in slightly lower bottom water temperatures (∼0.5 °C in Ameralik vs. ∼1.3 °C in Nuup Kangerlua during spring sampling). While such temperature differences may contribute to the higher pigment and OC preservation observed in the slightly colder bottom waters of Ameralik, their overall impact is likely modest. Arctic microbial communities are well adapted to low temperatures, and mineralization rates below 10 °C differ only minimally (Thamdrup et al., 2007; Scholze et al., 2020). Nevertheless, the relationship between temperature, water renewal, and preservation remains worth considering, as studies from Svalbard fjords have suggested higher pigment content in sediments associated with colder bottom waters (Krajewska et al., 2020)." Once starting in line 406 and once more in line 418.

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ED: Publish subject to minor revisions (review by editor) (14 Dec 2025) by Mark Lever

AR by Marius Buydens on behalf of the Authors (06 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 Jan 2026) by Mark Lever

AR by Marius Buydens on behalf of the Authors (15 Jan 2026) Manuscript

Journal article(s) based on this preprint

09 Feb 2026

Carbon burial in two Greenland fjords shows no direct link to glacier type

Marius Buydens, Emil De Borger, Lorenz Meire, Samuel Bodé, Antonio Schirone, Karline Soetaert, Ann Vanreusel, and Ulrike Braeckman

Biogeosciences, 23, 1159–1179, https://doi.org/10.5194/bg-23-1159-2026,https://doi.org/10.5194/bg-23-1159-2026, 2026

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Marius Buydens, Emil De Borger, Lorenz Meire, Samuel Bodé, Antonio Schirone, Karline Soetaert, Ann Vanreusel, and Ulrike Braeckman

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As the Greenland Ice Sheet retreats, it is important to understand how this impacts the carbon burial in Greenland fjords. By comparing a fjord with marine-terminating glaciers versus one fed by a land-terminating glacier, we see that the productive waters associated to marine-terminating glaciers not necessary lead to enhanced carbon burial. Instead, we highlight the complex interplay of physical, biological, and sedimentary processes that mediate carbon dynamics in these fjords.


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