The manuscript “Changing sources and burial of organic carbon in the Chukchi Sea sediments with retreating sea ice over recent centuries” by Su et al. describes the response of the organic matter in Chukchi Sea sediments to arctic environmental changes based on the biomarkers in their surface sediments and sediment core. Overall, their manuscript is generally well written and organized. Methods seem to be appropriate. Results are illustrated with relevant Graphs and Tables. Discussion is focused on the main findings and comprehensive. However, I would encourage the authors to slightly improve their manuscript according to my suggestions as follows:

Line 251-252: “The presence of IP25 throughout R1 indicates that sea ice cover has been seasonal at least since the 1820s at this location”- As IP25 can also be detected at sea ice edge (Müller et al., 2011), and under sea ice blooms have been reported (Arrigo et al., 2012), is it possible that the IP25 in R1 comes from the smpagic algae from sea ice edge, rather than from seasonal sea ice? Besides, this is contrary to the conclusion of permeant sea ice in line 288-289. This part needs further discussion.

Line 282-283: “This result further confirm that HBI-III producers proliferate at the sea ice edge rather than in ice free waters.”- Is it possible the low HBI-III is a result of limited nutrient which was deleted by the blooms, rather than different producers?

Line 379-380: “higher light penetration and nutrients supply (both form river and via wind driven mixing)” – do you mean river and mixing will result in higher light penetration and much more nutrient?

Line 437-440: Why would wind driven vertical mixing lead to nutrient limitation? Why would these changes in phytoplankton lead to CO₂ drawdown?

Line 296: spring/summer sea ice – should be summer/fall sea ice as the sea ice in this area starts to melt in summer (line 124 “receding as the summer season (July) begins”

- In Fig. 7d ii and iii, why was the winter sea ice edge (in March) in the Chukchi Sea (ARC04-C07), rather that the Bering Sea? And why was there abundance HBI-III (+) under the permanent sea ice cover in Fig. 7d i?

Line 26：“δ” should be italic throughout the text

Line 28：“were also” should be “are also”

Line 32：summer sea ice

Line 120：“is summer” should be “in summer”

Line 120: “seasonal” should be deleted.

Line 103-104: “The dynamics of the Beaufort Gyre (BG) also impacts on the characteristics of the CS water mass.” - provide references here

Line 107-109: “This basin is connected to the Pacific Ocean through…”- sentence needs to be restructured

Line 123-125: “Remote sensing data evidence strong seasonal variations, with sea…”- awkward sentence

Line 139: “excess ²¹⁰Pb (²¹⁰Pb_ex) ” should be put in line 137

Line 178: 3β-ol)).

Line 181: What is “cholesterol-d6”? Do you mean “cholest-5-en-3β-ol-D6”?

Line 192: “The H-print values were also calculated to infer the…”- should come before formula (3)

Line 202: Please using R1 for ARC11-R1 in the text after line 130.

Line 221: varies

Line 232: present

Line 236: why “they”?

Line 238: spans

Line 239-240: - sentence needs to be restructured

Line 244：“our core” should be instead by “R01” throughout the text

Line 250: lower export of sympagic OC

Line 268: found in North of Iceland

Line 292: Wu et al. (2019)

Line 333: freshwater discharge as lowering salinity water can suppress HBI-III production

Line 337: within our study area – should be deleted

Line 339-341: “Apart from permafrost thawing, sea ice retreat likely accelerated coastal erosion…” - provide references here

Line 353: Schubert and Calvert, 2001

Line 362: The abbreviation of SuSIC has already appeared in the previous paragraph

Line 365: Fig. A5

Line 404: “the concentrations of” should be deleted.

Line 424: northern CS

Line 418: Ardyna and Arrigo, 2020

- “Figure” or “Fig.” please be consistent throughout the text (but the figure caption).

- CO₂ with “2” in lower case, please check it throughout the text including references.

- “Pacific water inflow” or “Pacific Water Inflow”, “Siberian coastal current” or “Siberian Coastal Current”, “11^th” or “11th”, “sea ice edge” or “sea-ice edge”, “Figure” or “Fig.” - please keep these be consistent throughout the text

Liang Su and Co-Authors present a new proxy downcore record from the Chukchi Sea covering the past 200 years combined with results from surface sediments.  The aim is to investigated different sources (pelagic/sympagic) of organic carbon in the study area. With their record they show the relationship between sea ice and organic matter input to better the understanding of the mechanisms driving the marine carbon cycle in the Arctic Ocean. This study is relevant due to the ongoing and expected climate changes in the Arctic Ocean especially regarding the fate of organic carbon produced, delivered in the Chukchi Sea, an area of dramatic sea-ice loss in the recent years.

The manuscript is well written and the presented work is of importance to the scientific community. However, some points need clarification and/or correction before publication.

Apologies, in case I have misinterpreted anything.

General comments:

The interpretation of the IP25/PIP25 record needs some attention. I think the biggest weakness is, that you do not use your IP25 record from surface sediments to verify the interpretation of your down core record, e.g., to validate the PIP25 index and its environmental signal.

Further an age control of surface sediments should at least be discussed. Surface sediments may not always represent modern conditions or a mix of several hundred to thousand years, which makes the comparison with satellite data from a very specific time interval difficult.

Further details on this are given in the Specific Comments.

In the figures, the time on the x-axis is displayed from left/old to right/young. In my understanding, it is common to show old ages on the right and young ages on the left in the palaeoceanographic community. This may also be the reason for some inconsistencies in the order results are described, see comment below.

Specific Comments

Introduction

L79 Belt et al., 2007, deVernal et al., 2013 seem rather old and rather specific proxy studies. Further there have been updates on the given references and many other studies regarding this topic.

Fig 1: The blue dots are barely visible, please change the color. The black pentagram is too small. What is the source of the sea ice margins, please add a reference.

Oceanographic Setting

L123-125 What is the time interval of the sea-ice dataset you used?

Results

L231-235 One time you describe your records from old to young, the other time, from young to old. Please be consistent.

L238 IP25 concentrations span

Material & Methods

L128 In what year where the surface sediments taken? Have they all been measured to represent modern sediments? This is relatively important when comparing them to modern sea-ice concentrations.

L196-199 Why do you exclude other surface records, e.g., Wegner-Koch et al., 2020

Discussion

L246-252: Yes, the concentrations are lower in the southern cores from Bai et al. and Kim et al.. What is missing here is a discussion about potential differences in core storage, method as mentioned by Belt et al. (2014, Clim. Past). Are there other factors that may limit productivity, nutrients, depositional system?

L258 Cabedo-Sanz et al., 2013 worked in Barents Sea. I would recommend to mention that, as you adapt their interpretation to a new area.

L270 ff

I see a problem here with the interpretation of the PIP25 index.

• It does not make sense to use c-balance factors from other studies and other areas. As mentioned by previous surface studies Xiao et al., Kolling et al., a balance factor should be calculated based on data as there are differences in the concentrations of individual biomarkers varying between regions, (not to mention geological time intervals) depending on extraction method, storage etc.. All of these make it not valid to use balance factors from surface sediments from Barents Sea (Smik et al., 2016) and a dataset from the Nordic Seas & Arctic Ocean (Xiao et al., 2015a). Further, Xiao et al. (2015a) used a different extraction method. I advise to calculate a balance factor based on your surface and downcore data, which is also recommended by Xiao et al.
• You are using the calibration from Müller et al (2012) from Fram Strait to interpret your PIP25 results as percentages of sea ice cover. This is not correct. The calibration by Müller et al (2012) was done for Fram Strait which has not been validated for any other area, which is why Müller et al. and other surface studies (Xiao et al., 2015a, Kolling et al., 2020) recommend that this calibration is only roughly applicable for other regions. Hence, I would recommend that you do not use percentages in your interpretation but the general sea ice regime as introduced by Müller et al., 2012, e.g., ice-free, variable, permanent, which you also show in Fig 5

Fig 6: How do you define those margins for the different sea ice conditions? Are they based on your surface dataset, or any other published surface dataset? Or are they just estimates?

From my understanding below permanent sea ice, there should be no production of any biomarker, but you allow production of e.g., 20 mg/gTOC of Brassicasterol below permanent sea ice when maximum Brassicasterol concentrations are at 40mg/gTOC. This does not seem realistic.  In Fig 7 is becomes obvious that you barely have any PIP25 values that indicate permanent sea ice cover, however in Fig 6 is seems as if at least half of your datapoints lie within the range that indicates permanent sea ice cover.

L285: Even though I am not a native speaker I feel that the wording ‘icier’ is not the correct scientific term. You use it several times in your manuscript. I would suggest using ‘increase in sea ice’ or something similar.

Fig 7a: It is really hard to distinguish the different shades of green, especially between ARC-11-R01 and ARC4-C07. Please use different colors.

Fig7b: What is the effect of light availability in your record? In my understanding, there should be no production below winter sea ice due to the lack of sunlight in Chukchi Sea. Further, what is the main bloom season on Chukchi Sea? Please elaborate more on this topic.

L334 Is there a difference in light availability between ~70°N and ~80°N that could also influence the amount of biomarkers produced. How long are the general production periods in your working area, are there specific differences from North to South?

L351 & L355 ‘sea-ice edge’ and ‘sea ice carbon’, if you hyphenate be consistent throughout the manuscript

Fig 8: I would suggest that you write an endmember on the scales for H-print and 13C, e.g. sympagic/pelagic.

L392-395 If land-derived organic matter is transported to the core location by sea ice, why aren’t IP25 and terrigenous sterols not parallel? Could you include the biomarker records to this discussion?

Conclusions

L438 nutient limited

L440 CO₂ drawdown