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.

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.

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.