the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Changing sources and burial of organic carbon in the Chukchi Sea sediments with retreating sea ice over recent centuries
Liang Su
Marie-Alexandrine Sicre
Youcheng Bai
Ruoshi Zhao
Xibing Han
Zhongqiao Li
Haiyan Jin
Anatolii S. Astakhov
Xuefa Shi
Jianfang Chen
Abstract. Decreasing sea ice extent in summer caused by climate change is affecting the carbon cycle of the Arctic Ocean. In this study, surface sediments across the western Arctic Ocean are investigated to characterize sources of sedimentary organic carbon (OC). Bulk organic parameters (total organic carbon, total nitrogen, δ13Corg and δ15N) combined with molecular organic biomarkers (e.g., sterols and highly branched isoprenoids (HBIs)) are applied to distinguish between sympagic, pelagic, and terrestrial OC. Furthermore, downcore profiles of these parameters were also generated from the Chukchi Sea R1 core (74° N) to evaluate changes in the relative contribution of these three components of sedimentary OC over the last 200 years with decreasing sea ice. Our data evidence that from 1820s to 1930s, prevailing high and variable sea ice cover inhibited in situ primary production resulting in prominent land-derived material stored in sediments. From 1930s to 1980s, with the gradual decline of sea ice, primary production increased progressively. The ratio of sympagic and pelagic OC began to rise to account for a larger portion of sedimentary OC. Since 1980s, accelerated sea ice loss led to enhanced primary production, stabilizing over the last decades due to freshwater induced surface ocean stratification in summer.
Liang Su et al.
Status: closed
-
RC1: 'Comment on egusphere-2023-64', Anonymous Referee #1, 14 Feb 2023
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 (210Pbex) ” 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”, “11th” or “11th”, “sea ice edge” or “sea-ice edge”, “Figure” or “Fig.” - please keep these be consistent throughout the text
Citation: https://doi.org/10.5194/egusphere-2023-64-RC1 - AC1: 'Reply on RC1', Jian Ren, 22 Mar 2023
-
RC2: 'Comment on egusphere-2023-64', Anonymous Referee #2, 24 Feb 2023
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
Citation: https://doi.org/10.5194/egusphere-2023-64-RC2 - AC2: 'Reply on RC2', Jian Ren, 22 Mar 2023
-
EC1: 'Comment on egusphere-2023-64', Nathalie Combourieu Nebout, 04 Apr 2023
Dear authors,
I have read your responses to reviewers and now I need your new version of the paper with the corrections in track mode changes.
Could you please send it to us and I will be able to post my final decision very quickly
Best regards
Nathalie Combourieu-Nebout
Citation: https://doi.org/10.5194/egusphere-2023-64-EC1
Status: closed
-
RC1: 'Comment on egusphere-2023-64', Anonymous Referee #1, 14 Feb 2023
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 (210Pbex) ” 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”, “11th” or “11th”, “sea ice edge” or “sea-ice edge”, “Figure” or “Fig.” - please keep these be consistent throughout the text
Citation: https://doi.org/10.5194/egusphere-2023-64-RC1 - AC1: 'Reply on RC1', Jian Ren, 22 Mar 2023
-
RC2: 'Comment on egusphere-2023-64', Anonymous Referee #2, 24 Feb 2023
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
Citation: https://doi.org/10.5194/egusphere-2023-64-RC2 - AC2: 'Reply on RC2', Jian Ren, 22 Mar 2023
-
EC1: 'Comment on egusphere-2023-64', Nathalie Combourieu Nebout, 04 Apr 2023
Dear authors,
I have read your responses to reviewers and now I need your new version of the paper with the corrections in track mode changes.
Could you please send it to us and I will be able to post my final decision very quickly
Best regards
Nathalie Combourieu-Nebout
Citation: https://doi.org/10.5194/egusphere-2023-64-EC1
Liang Su et al.
Liang Su et al.
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