Deglacial and Holocene sea ice and climate dynamics at the Western Antarctic Peninsula

Vorrath, Maria-Elena; Müller, Juliane; Cárdenas, Paola; Mieruch, Sebastian; Esper, Oliver; Opel, Thomas; Lembke-Jene, Lester; Etourneau, Johan; Vieth-Hillebrand, Andrea; Lahajnar, Niko; Lange, Carina B.; Leventer, Amy; Evangelinos, Dimitris; Escutia, Carlota; Mollenhauer, Gesine

doi:https://doi.org/10.5194/egusphere-2022-804

Preprints

https://doi.org/10.5194/egusphere-2022-804

Preprints

24 Aug 2022

| 24 Aug 2022

Deglacial and Holocene sea ice and climate dynamics at the Western Antarctic Peninsula

Maria-Elena Vorrath, Juliane Müller, Paola Cárdenas, Sebastian Mieruch, Oliver Esper, Thomas Opel, Lester Lembke-Jene, Johan Etourneau, Andrea Vieth-Hillebrand, Niko Lahajnar, Carina B. Lange, Amy Leventer, Dimitris Evangelinos, Carlota Escutia, and Gesine Mollenhauer

Abstract. The reconstruction of past sea ice distribution in the Southern Ocean is crucial for an improved understanding of ice-ocean-atmosphere feedbacks and the evaluation of Earth system and Antarctic ice sheet models. The Western Antarctic Peninsula (WAP) is experiencing rapid warming and the associated decrease in sea ice cover contrasts the trend of growing sea ice extent in eastern Antarctica. To reveal the long-term sea ice history at the WAP under changing climate conditions we examined a marine sediment core from the eastern basin of the Bransfield Strait covering the last Deglacial and the Holocene. For sea ice reconstructions, we focused on the specific sea ice biomarker lipid IPSO₂₅, a highly branched isoprenoid (HBI), and sea ice diatoms, whereas a phytoplankton-derived HBI triene (C_25:3) and open ocean diatom assemblages reflect predominantly ice-free conditions. We further reconstruct ocean temperatures using glycerol dialkyl glycerol tetraether (GDGTs) and diatom assemblages, and compare our sea ice and temperature records with published marine sediment and ice core data. Our results document a retreat of the WAP ice shelf at 13.9 ka BP (before present). Maximum sea ice cover is observed during the Antarctic Cold Reversal, while a still extended but variable sea ice coverage characterized the core site during the early Holocene. An overall decreasing sea ice trend throughout the Middle Holocene is accompanied by a successive ocean warming and increasing phytoplankton productivity. The Late Holocene is characterized by unstable (winter) sea ice conditions and a further sea ice decline until 0.5 ka BP.

Received: 17 Aug 2022 – Discussion started: 24 Aug 2022

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Journal article(s) based on this preprint

30 May 2023

Deglacial and Holocene sea-ice and climate dynamics in the Bransfield Strait, northern Antarctic Peninsula

Maria-Elena Vorrath, Juliane Müller, Paola Cárdenas, Thomas Opel, Sebastian Mieruch, Oliver Esper, Lester Lembke-Jene, Johan Etourneau, Andrea Vieth-Hillebrand, Niko Lahajnar, Carina B. Lange, Amy Leventer, Dimitris Evangelinos, Carlota Escutia, and Gesine Mollenhauer

Clim. Past, 19, 1061–1079, https://doi.org/10.5194/cp-19-1061-2023,https://doi.org/10.5194/cp-19-1061-2023, 2023

Short summary

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-804', Anonymous Referee #1, 25 Oct 2022

Vorrath and co-authors present a new palaeoceanographic record based on micropalaeontological and biomarker records from a marine sediment core (PS97/072-1) at the north-eastern end of the Bransfield Strait, Northern Antarctic Peninsula (NAP). The Bransfield Strait is an important region of the NAP, with oceanographic links to the Weddell and Bellingshausen Seas. The area is also characterised by declining sea ice cover and warming sea surface temperatures over recent decades. The core record covers the late glacial (Antarctic Cold Reversal) to the beginning of the 20th century. Vorrath et al. use biomarkers and diatom ecological groups to reconstruct changes in local sea ice cover and sea surface temperatures to inform their oceanographic and climatic reconstruction for the past 13.9 ka.

I commend the authors for the considerable time and effort that has been invested to produce these micropalaeontological and biomarker data. The record has the potential to be a valuable addition to the literature on past ocean and climate conditions in the NAP. Unfortunately, this potential is not realised in the current manuscript. While the general structure and style of the text is appropriate, greater attention to detail and a more nuanced consideration of the palaeoceanographic conditions are required throughout the manuscript, especially with regards to the sea ice environment.

MAJOR REVISIONS

> Study Area:

The paragraph on the oceanographic setting mostly describes the sources and distribution of sub-surface water masses which is difficult to follow and too detailed considering the water masses are not mentioned in any other part of the paper except figure 1. There is no mention of the modern sea ice conditions or seasonal variability.

Consider whether the Northern Antarctic Peninsula would be a better description of the regional context of this study and be aware that WAP (West Antarctic Peninsula) is currently used to describe: 1) the ocean/seas over the continental shelf west of the AP landmass; 2) the western coast of the AP landmass; and 3) the whole area west of the AP spine (land and ocean). Please provide an accurate descripton of WAP and/or NAP and use the term consistently throughout the manuscript.

> Sea ice:

Descriptions of sea ice variability are too vague throughout. Whilst the relative changes in sea ice could appear to make sense in the time slice sections of the discussion, the sequence of Holocene sea ice changes summarised in the conclusions highlights the poor choice of terms used to describe the reconstruction:

• Post ACR: ‘retreat in spring sea ice’ - spring sea ice retreats each year, what do you mean here?

• Early Holocene: ‘slightly decreasing spring sea ice and highly variable WSI’

• Mid-Holocene: followed by ‘lower spring sea ice… sea ice seasons were short and sea ice cover was significantly reduced to a minimum around 5.5 ka BP, even though high seasonal amplitudes and short-term, centennial changes in sea ice conditions occurred’.

• Late Holocene: ‘variable WSI’

Recurring phases of ‘decreasing/lower spring sea ice’, ‘short sea ice seasons’ and ‘variable WSI’ are difficult to put into context without some qualification. I suggest you add detail to convey the likely duration of sea ice cover (eg. 3-6 months), timing of break up/melt (eg. early/mid/late spring) or use the values of your IPSO/WSI results.

Also need to make it clear whether ‘more(less) sea ice’ relates to duration/extent etc. or use more precise terms.

> Diatom groups:

Composition of the various ecological groups is not given in either the main manuscript or supplementary files; frequently refer to ‘open ocean’ group without explanation as to whether it is the ‘warm’ or ‘cold’ ocean group or a total or both groups.

> Literature:

Very sparse reference made to other Bransfield Strait/NAP studies. Since your study is based on single site it would be useful to establish whether the climate signals are 'locally' coherent before comparing them with WAIS/EPICA, Palmer Deep etc.

> Comparison of proxy records:

Would be useful for the results or discussion to have a dedicated section comparing the different PS97/72-1 proxy records and providing potential explanations to reconcile results. Especially with regards to:

- different signals/trends between proxies

- reliability and accuracy of proxies at the site

- how amplitude of signals relates to changes in sea-ice/temperature (broadly linear relationships or not? unknown?)

COMMENTS & SUGGESTIONS

See annotated PDFs for specific comments and suggestions on the text, figures and supplementary information.

MINOR COMMENTS

Disordered use and introduction of abbreviations

Inconsistent use of sea-ice/sea ice

Inconsistent use of spacing between signs/units and numbers eg. < 3% and <3 %

Check whether citations should be listed in date order

Citation: https://doi.org/10.5194/egusphere-2022-804-RC1
- AC1:
  'Reply on RC1', Maria-Elena Vorrath, 06 Mar 2023
  Dear Referee #1,
  we appreciate your constructive comments and remarks on our original submission, which have helped to clarify and improve the manuscript substantially. We now provide an annotated text of the manuscript which includes the comments and corrections received as well as a “clean” and “reader-friendly” version of the revised manuscript. As requested, we now also provide more detailed information in the supplementary material. Below, please find our responses to your comments.
  RC1.1: I commend the authors for the considerable time and effort that has been invested to produce these micropalaeontological and biomarker data. The record has the potential to be a valuable addition to the literature on past ocean and climate conditions in the NAP. Unfortunately, this potential is not realised in the current manuscript. While the general structure and style of the text is appropriate, greater attention to detail and a more nuanced consideration of the palaeoceanographic conditions are required throughout the manuscript, especially with regards to the sea ice environment.
  Author´s response: We agree with the reviewer that the palaeoceanographic discussion in the previous version of our manuscript was not very detailed and we note that we, in the revised version, pay more attention to the meaning of our own proxy records and other records published in the study area.
  RC.1.2 - Study Area: The paragraph on the oceanographic setting mostly describes the sources and distribution of sub-surface water masses which is difficult to follow and too detailed considering the water masses are not mentioned in any other part of the paper except figure 1. There is no mention of the modern sea ice conditions or seasonal variability. Consider whether the Northern Antarctic Peninsula would be a better description of the regional context of this study and be aware that WAP (West Antarctic Peninsula) is currently used to describe: 1) the ocean/seas over the continental shelf west of the AP landmass; 2) the western coast of the AP landmass; and 3) the whole area west of the AP spine (land and ocean). Please provide an accurate description of WAP and/or NAP and use the term consistently throughout the manuscript.
  Author´s response: We thank the reviewer for the comment on the geographical definition of the West Antarctic Peninsula and accordingly changed the title and the text of the manuscript. We also revised the description of the study area and now provide information on the modern sea ice conditions.
  RC.1.3 - Sea ice: Descriptions of sea ice variability are too vague throughout. Whilst the relative changes in sea ice could appear to make sense in the time slice sections of the discussion, the sequence of Holocene sea ice changes summarised in the conclusions highlights the poor choice of terms used to describe the reconstruction:
  Post ACR: ‘retreat in spring sea ice’ - spring sea ice retreats each year, what do you mean here?
  
  Early Holocene: ‘slightly decreasing spring sea ice and highly variable WSI’
  
  Mid-Holocene: followed by ‘lower spring sea ice… sea ice seasons were short and sea ice cover was significantly reduced to a minimum around 5.5 ka BP, even though high seasonal amplitudes and short-term, centennial changes in sea ice conditions occurred".
  
  Late Holocene: ‘variable WSI’
  
  Recurring phases of ‘decreasing/lower spring sea ice’, ‘short sea ice seasons’ and ‘variable WSI’ are difficult to put into context without some qualification. I suggest you add detail to convey the likely duration of sea ice cover (eg. 3-6 months), timing of break up/melt (eg. early/mid/late spring) or use the values of your IPSO/WSI results. Also need to make it clear whether ‘more(less) sea ice’ relates to duration/extent etc. or use more precise terms.
  Author´s response: In the revised version of our manuscript, we tried to be more specific with the description of the sea ice conditions and to eliminate vague wording. In the Discussion chapter, we now provide information on how ‘high’ and ‘low’ PIPSO25 values are interpreted in terms of the duration and concentration of sea ice.
  RC.1.4 - Diatom groups: Composition of the various ecological groups is not given in either the main manuscript or supplementary files; frequently refer to ‘open ocean’ group without explanation as to whether it is the ‘warm’ or ‘cold’ ocean group or a total or both groups.
  Author´s response: Details on the ecological groups can now be found in the supplement. We also clarified that the “open ocean” group refers to either cold open ocean species or warmer open ocean species. We also added the surface temperature range for each ecological group. The main text and figure 3 refer to the warm open ocean species assemblage.
  RC1.5 - Literature: Very sparse reference made to other Bransfield Strait/NAP studies. Since your study is based on single site it would be useful to establish whether the climate signals are 'locally' coherent before comparing them with WAIS/EPICA, Palmer Deep etc.
  Author´s response: In the revised version of the manuscript, we paid more attention to other records from the NAP and the WAP revealing similarities as well as differences in the local environmental development during the Holocene. Specifically, we now refer to records published by Barcena et al. (1998), Heroy et al. (2008), Milliken et al. (2009), Kyrmanidou et al. (2018) and Roseby et al. (2022).
  RC1.6 - Comparison of proxy records: Would be useful for the results or discussion to have a dedicated section comparing the different PS97/72-1 proxy records and providing potential explanations to reconcile results. Especially with regards to:
  - different signals/trends between proxies
  - reliability and accuracy of proxies at the site
  - how amplitude of signals relates to changes in sea-ice/temperature (broadly linear relationships or not? unknown?)
  Author´s response: We revised the text and now discuss discrepancies or contrasting trends of the different proxies. Hereby we preferred to discuss the paleoclimate events in chronological order and to include the discussions of contrary proxies in these chapters. Regarding the divergent trends observed in RI-OH’ and TEX86L ocean temperatures we argue that RI-OH’ seems to be mainly driven by summer insolation, while TEX86 records (Bransfield Strait and Palmer Deep) rather depict the spring insolation trend. On account of the different trends in the sea ice diatom assemblage and WSI, we note that the discrepancy may be due to silica dissolution. Many of the species that we use as sea ice indicators, such as F. curta, can dominate the marginal ice zone adjacent to a retreating ice edge. They seem to do equally well in the sea ice and then are successful in seeding the water column, as contrasted with sea ice obligate taxa.
  RC1.7 - COMMENTS & SUGGESTIONS:
  See annotated PDFs for specific comments and suggestions on the text, figures and supplementary information.
  Author´s response: We wish to thank the reviewer for the very detailed and thorough comments provided in the annotated PDFs. They have been considered in the revised version of the manuscript.
  RC1.8 - MINOR COMMENTS: Disordered use and introduction of abbreviations; Inconsistent use of sea-ice/sea ice; Inconsistent use of spacing between signs/units and numbers eg. < 3% and <3 %; Check whether citations should be listed in date order
  Author´s response: We corrected all editing inconsistencies/errors.
  
  Citation: https://doi.org/10.5194/egusphere-2022-804-AC1
RC2:
'Comment on egusphere-2022-804', Anonymous Referee #2, 21 Nov 2022

Study by Vorrath and colleagues presents a new sedimentary record (PS97/072-1), utilising multiproxy approach to provide sea ice and temperature reconstructions in Bransfield Strait (Antarctic Peninsula) over the past ca14ka. Antarctic Peninsula has experienced some profound climatic changes over the last decades and thus paleoclimatic studies in this important region of the Southern Ocean are of real significance.

While manuscript is presented well and supported with good quality figures, I feel that it requires more clarity, particulalrly in relation to descriptions of climate variability, throughout the text (e.g. late-Holocene is a characterised by stable environmental conditions… ), and would benefit from more elaborate discussions which includes cases when e.g. proxies do not agree (see below).

Major comments

Age model – I was wandering if authors could perhaps elaborate on the strength of the age model presented? There are quite large changes in the sedimentation rates, but with no further age points, I was wandering how sure authors can be of a linear nature of the age model between ca 5-12ka and beyond. There is a 10-fold difference between the highest and lowest sedimentation, which surely if present between 5-12ka, would have an impact on the reconstructed ages and overall climatic reconstructions

Climatic reconstructions - There were times where proxies disagreed or more, data did not support authors climatic reconstructions which I think provides a room for further elaboration. For example,

4.1 – L338-338 – seemingly a difference between sea ice diatoms vs WSI.

4.2 – L361-362 – diverging trends in SOTs vs SSST

4.2 – L357-358 – I don`t think this is really supported by the data, particularly with WSI. Sea ice associated diatoms show an increase, at least during the early parts of an Early Holocene. Room for further discussion?

4.4. – L410 – Again, I think there is a room for further discussion. Looking at PIPSO, WSI and temperature records, I don`t think data quite support the statement of the stable environmental conditions.

L413 – 414 – “Minimum PIPSO25 values…” – Could authors elaborate on this please. There are low HBI concentrations during ACR yet PIPSO was considered as an indication of the continuous sea ice cover.

Minor edits

L95-96 – “..as reliable proxies..” – this is not such a clear cut as authors state!

L96 – diunsaturated; change to di-unsaturated please

L104 – “..robustly reconstruct” – again, I think this statement needs rephrasing.

L186 - ..”slightly overestimated..” – please elaborate by how much

L189 – “..identifications of HBIs..” – and GDGTs?

L190 – internal standards – please state quantifies added

L197 – Please, could you state the retention indices for HBIs. Also I might be wrong, but I think only mass spectra of HBI triene are presented in Belt et al 2000.

L212 – please provide details of the n-alkanes standards.

L237 – m/z 1296

L295 – “..and shows high..” – to use elevated instead of high would be more appropriate I think

L304 – “Sea ice concentration estimates..” ?

L324 – significant – is it supported by statistical treatment of data. Perhaps consider using an alternative wording.

L333 – “..a very thick or permanent..” – what does very thick mean? Continuous sea ice cover might be more appropriate?

L433 – we note..

Figures – I think a schematic would be a fitting addition as well.

Figure 4 – I didn`t quite get a point of diene/triene plots (d and e). Why not convert the data to either PIPSO (authors have both IPSO25 and triene data)?

Citation: https://doi.org/10.5194/egusphere-2022-804-RC2
- AC2: 'Reply on RC2', Maria-Elena Vorrath, 06 Mar 2023
  
  Dear Referee #2,
  we appreciate your constructive comments and remarks on our original submission, which have helped to clarify and improve the manuscript. We now provide an annotated text of the manuscript which includes the comments and corrections received as well as a “clean” and “reader-friendly” version of the revised manuscript. As requested, we now also provide more detailed information in the supplementary material. Below, please find our responses to your comments.
  RC2.1: Study by Vorrath and colleagues presents a new sedimentary record (PS97/072-1), utilising multiproxy approach to provide sea ice and temperature reconstructions in Bransfield Strait (Antarctic Peninsula) over the past ca14ka. Antarctic Peninsula has experienced some profound climatic changes over the last decades and thus paleoclimatic studies in this important region of the Southern Ocean are of real significance. While manuscript is presented well and supported with good quality figures, I feel that it requires more clarity, particularly in relation to descriptions of climate variability, throughout the text (e.g. late-Holocene is a characterised by stable environmental conditions…), and would benefit from more elaborate discussions which includes cases when e.g. proxies do not agree (see below).
  Author´s response: We revised the text and the wording to describe the environmental conditions in more detail. We now also discuss discrepancies between the applied proxies. Regarding the divergent trends observed in RI-OH’ and TEX86L ocean temperatures we argue that RI-OH’ seems to be mainly driven by summer insolation, while TEX86 records (Bransfield Strait and Palmer Deep) rather depict the spring insolation trend.
  Major comments
  RC2.2 - Age model: I was wandering if authors could perhaps elaborate on the strength of the age model presented? There are quite large changes in the sedimentation rates, but with no further age points, I was wandering how sure authors can be of a linear nature of the age model between ca 5-12ka and beyond. There is a 10-fold difference between the highest and lowest sedimentation, which surely if present between 5-12ka, would have an impact on the reconstructed ages and overall climatic reconstructions.
  Author´s response: We agree with the reviewer and point out in the text “that (as) the lack of age constraints between 12 ka and 6 ka BP may introduce chronological uncertainties, we only focus on overall trends reflected in our data and refrain from detailed allocations of known climatic events in this time period.“ The overall climatic trends depicted in core PS97/072, however, are in general agreement with other records from the study area.
  RC2.3 - Climatic reconstructions: There were times where proxies disagreed or more, data did not support authors climatic reconstructions which I think provides a room for further elaboration. For example,
  4.1 – L338-338 – seemingly a difference between sea ice diatoms vs WSI.
  4.2 – L361-362 – diverging trends in SOTs vs SSST
  4.2 – L357-358 – I don`t think this is really supported by the data, particularly with WSI.
  Sea ice associated diatoms show an increase, at least during the early parts of an Early Holocene. Room for further discussion?
  Author´s response: We revised the text and now discuss discrepancies and contrasting trends of the different proxies.
  RC2.4 - 4.4. – L410: Again, I think there is a room for further discussion. Looking at PIPSO, WSI and temperature records, I don`t think data quite support the statement of the stable environmental conditions.
  Author´s response: We revised also this part of the manuscript and provide a more detailed assessment of the sea ice and ocean temperature conditions.
  RC2.5 - L413 – 414 – “Minimum PIPSO25 values…”: Could authors elaborate on this please. There are low HBI concentrations during ACR yet PIPSO was considered as an indication of the continuous sea ice cover.
  Author´s response: We thank the reviewer for spotting this. We now conclude that the low HBI concentrations seem to refer to a reduction in spring sea ice – also since elevated TOC, biogenic opal and warm open ocean diatoms suggest higher primary productivity. In general, in the revised version of our manuscript, we tried to be more specific with the description of the sea ice conditions. In the Methods chapter, we now also provide information on how ‘high’ and ‘low’ PIPSO25 values are interpreted in terms of the duration and concentration of sea ice.
  RC2.6: Minor edits
  L95-96 – “..as reliable proxies..” – this is not such a clear cut as authors state!
  L96 – diunsaturated; change to di-unsaturated please
  L104 – “..robustly reconstruct” – again, I think this statement needs rephrasing.
  L186 - ..”slightly overestimated..” – please elaborate by how much
  L189 – “..identifications of HBIs..” – and GDGTs?
  L190 – internal standards – please state quantifies added
  L197 – Please, could you state the retention indices for HBIs. Also I might be wrong, but I think only mass spectra of HBI triene are presented in Belt et al 2000.
  L212 – please provide details of the n-alkanes standards.
  L237 – m/z 1296
  L295 – “..and shows high..” – to use elevated instead of high would be more appropriate I think
  L304 – “Sea ice concentration estimates..” ?
  L324 – significant – is it supported by statistical treatment of data. Perhaps consider using an alternative wording.
  L333 – “..a very thick or permanent..” – what does very thick mean? Continuous sea ice cover might be more appropriate?
  L433 – we note..
  Author´s response: We considered and addressed your comments in the revised manuscript.
  RC2.8 - Figure 4: I didn`t quite get a point of diene/triene plots (d and e). Why not convert the data to either PIPSO (authors have both IPSO25 and triene data)?
  Author´s response: We agree with the referee and accordingly changed the plot now showing PIPSO25 values for JPC10.
  
  Citation: https://doi.org/10.5194/egusphere-2022-804-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-804', Anonymous Referee #1, 25 Oct 2022

Vorrath and co-authors present a new palaeoceanographic record based on micropalaeontological and biomarker records from a marine sediment core (PS97/072-1) at the north-eastern end of the Bransfield Strait, Northern Antarctic Peninsula (NAP). The Bransfield Strait is an important region of the NAP, with oceanographic links to the Weddell and Bellingshausen Seas. The area is also characterised by declining sea ice cover and warming sea surface temperatures over recent decades. The core record covers the late glacial (Antarctic Cold Reversal) to the beginning of the 20th century. Vorrath et al. use biomarkers and diatom ecological groups to reconstruct changes in local sea ice cover and sea surface temperatures to inform their oceanographic and climatic reconstruction for the past 13.9 ka.

I commend the authors for the considerable time and effort that has been invested to produce these micropalaeontological and biomarker data. The record has the potential to be a valuable addition to the literature on past ocean and climate conditions in the NAP. Unfortunately, this potential is not realised in the current manuscript. While the general structure and style of the text is appropriate, greater attention to detail and a more nuanced consideration of the palaeoceanographic conditions are required throughout the manuscript, especially with regards to the sea ice environment.

MAJOR REVISIONS

> Study Area:

The paragraph on the oceanographic setting mostly describes the sources and distribution of sub-surface water masses which is difficult to follow and too detailed considering the water masses are not mentioned in any other part of the paper except figure 1. There is no mention of the modern sea ice conditions or seasonal variability.

Consider whether the Northern Antarctic Peninsula would be a better description of the regional context of this study and be aware that WAP (West Antarctic Peninsula) is currently used to describe: 1) the ocean/seas over the continental shelf west of the AP landmass; 2) the western coast of the AP landmass; and 3) the whole area west of the AP spine (land and ocean). Please provide an accurate descripton of WAP and/or NAP and use the term consistently throughout the manuscript.

> Sea ice:

Descriptions of sea ice variability are too vague throughout. Whilst the relative changes in sea ice could appear to make sense in the time slice sections of the discussion, the sequence of Holocene sea ice changes summarised in the conclusions highlights the poor choice of terms used to describe the reconstruction:

• Post ACR: ‘retreat in spring sea ice’ - spring sea ice retreats each year, what do you mean here?

• Early Holocene: ‘slightly decreasing spring sea ice and highly variable WSI’

• Mid-Holocene: followed by ‘lower spring sea ice… sea ice seasons were short and sea ice cover was significantly reduced to a minimum around 5.5 ka BP, even though high seasonal amplitudes and short-term, centennial changes in sea ice conditions occurred’.

• Late Holocene: ‘variable WSI’

Recurring phases of ‘decreasing/lower spring sea ice’, ‘short sea ice seasons’ and ‘variable WSI’ are difficult to put into context without some qualification. I suggest you add detail to convey the likely duration of sea ice cover (eg. 3-6 months), timing of break up/melt (eg. early/mid/late spring) or use the values of your IPSO/WSI results.

Also need to make it clear whether ‘more(less) sea ice’ relates to duration/extent etc. or use more precise terms.

> Diatom groups:

Composition of the various ecological groups is not given in either the main manuscript or supplementary files; frequently refer to ‘open ocean’ group without explanation as to whether it is the ‘warm’ or ‘cold’ ocean group or a total or both groups.

> Literature:

Very sparse reference made to other Bransfield Strait/NAP studies. Since your study is based on single site it would be useful to establish whether the climate signals are 'locally' coherent before comparing them with WAIS/EPICA, Palmer Deep etc.

> Comparison of proxy records:

Would be useful for the results or discussion to have a dedicated section comparing the different PS97/72-1 proxy records and providing potential explanations to reconcile results. Especially with regards to:

- different signals/trends between proxies

- reliability and accuracy of proxies at the site

- how amplitude of signals relates to changes in sea-ice/temperature (broadly linear relationships or not? unknown?)

COMMENTS & SUGGESTIONS

See annotated PDFs for specific comments and suggestions on the text, figures and supplementary information.

MINOR COMMENTS

Disordered use and introduction of abbreviations

Inconsistent use of sea-ice/sea ice

Inconsistent use of spacing between signs/units and numbers eg. < 3% and <3 %

Check whether citations should be listed in date order

Citation: https://doi.org/10.5194/egusphere-2022-804-RC1
- AC1:
  'Reply on RC1', Maria-Elena Vorrath, 06 Mar 2023
  Dear Referee #1,
  we appreciate your constructive comments and remarks on our original submission, which have helped to clarify and improve the manuscript substantially. We now provide an annotated text of the manuscript which includes the comments and corrections received as well as a “clean” and “reader-friendly” version of the revised manuscript. As requested, we now also provide more detailed information in the supplementary material. Below, please find our responses to your comments.
  RC1.1: I commend the authors for the considerable time and effort that has been invested to produce these micropalaeontological and biomarker data. The record has the potential to be a valuable addition to the literature on past ocean and climate conditions in the NAP. Unfortunately, this potential is not realised in the current manuscript. While the general structure and style of the text is appropriate, greater attention to detail and a more nuanced consideration of the palaeoceanographic conditions are required throughout the manuscript, especially with regards to the sea ice environment.
  Author´s response: We agree with the reviewer that the palaeoceanographic discussion in the previous version of our manuscript was not very detailed and we note that we, in the revised version, pay more attention to the meaning of our own proxy records and other records published in the study area.
  RC.1.2 - Study Area: The paragraph on the oceanographic setting mostly describes the sources and distribution of sub-surface water masses which is difficult to follow and too detailed considering the water masses are not mentioned in any other part of the paper except figure 1. There is no mention of the modern sea ice conditions or seasonal variability. Consider whether the Northern Antarctic Peninsula would be a better description of the regional context of this study and be aware that WAP (West Antarctic Peninsula) is currently used to describe: 1) the ocean/seas over the continental shelf west of the AP landmass; 2) the western coast of the AP landmass; and 3) the whole area west of the AP spine (land and ocean). Please provide an accurate description of WAP and/or NAP and use the term consistently throughout the manuscript.
  Author´s response: We thank the reviewer for the comment on the geographical definition of the West Antarctic Peninsula and accordingly changed the title and the text of the manuscript. We also revised the description of the study area and now provide information on the modern sea ice conditions.
  RC.1.3 - Sea ice: Descriptions of sea ice variability are too vague throughout. Whilst the relative changes in sea ice could appear to make sense in the time slice sections of the discussion, the sequence of Holocene sea ice changes summarised in the conclusions highlights the poor choice of terms used to describe the reconstruction:
  Post ACR: ‘retreat in spring sea ice’ - spring sea ice retreats each year, what do you mean here?
  
  Early Holocene: ‘slightly decreasing spring sea ice and highly variable WSI’
  
  Mid-Holocene: followed by ‘lower spring sea ice… sea ice seasons were short and sea ice cover was significantly reduced to a minimum around 5.5 ka BP, even though high seasonal amplitudes and short-term, centennial changes in sea ice conditions occurred".
  
  Late Holocene: ‘variable WSI’
  
  Recurring phases of ‘decreasing/lower spring sea ice’, ‘short sea ice seasons’ and ‘variable WSI’ are difficult to put into context without some qualification. I suggest you add detail to convey the likely duration of sea ice cover (eg. 3-6 months), timing of break up/melt (eg. early/mid/late spring) or use the values of your IPSO/WSI results. Also need to make it clear whether ‘more(less) sea ice’ relates to duration/extent etc. or use more precise terms.
  Author´s response: In the revised version of our manuscript, we tried to be more specific with the description of the sea ice conditions and to eliminate vague wording. In the Discussion chapter, we now provide information on how ‘high’ and ‘low’ PIPSO25 values are interpreted in terms of the duration and concentration of sea ice.
  RC.1.4 - Diatom groups: Composition of the various ecological groups is not given in either the main manuscript or supplementary files; frequently refer to ‘open ocean’ group without explanation as to whether it is the ‘warm’ or ‘cold’ ocean group or a total or both groups.
  Author´s response: Details on the ecological groups can now be found in the supplement. We also clarified that the “open ocean” group refers to either cold open ocean species or warmer open ocean species. We also added the surface temperature range for each ecological group. The main text and figure 3 refer to the warm open ocean species assemblage.
  RC1.5 - Literature: Very sparse reference made to other Bransfield Strait/NAP studies. Since your study is based on single site it would be useful to establish whether the climate signals are 'locally' coherent before comparing them with WAIS/EPICA, Palmer Deep etc.
  Author´s response: In the revised version of the manuscript, we paid more attention to other records from the NAP and the WAP revealing similarities as well as differences in the local environmental development during the Holocene. Specifically, we now refer to records published by Barcena et al. (1998), Heroy et al. (2008), Milliken et al. (2009), Kyrmanidou et al. (2018) and Roseby et al. (2022).
  RC1.6 - Comparison of proxy records: Would be useful for the results or discussion to have a dedicated section comparing the different PS97/72-1 proxy records and providing potential explanations to reconcile results. Especially with regards to:
  - different signals/trends between proxies
  - reliability and accuracy of proxies at the site
  - how amplitude of signals relates to changes in sea-ice/temperature (broadly linear relationships or not? unknown?)
  Author´s response: We revised the text and now discuss discrepancies or contrasting trends of the different proxies. Hereby we preferred to discuss the paleoclimate events in chronological order and to include the discussions of contrary proxies in these chapters. Regarding the divergent trends observed in RI-OH’ and TEX86L ocean temperatures we argue that RI-OH’ seems to be mainly driven by summer insolation, while TEX86 records (Bransfield Strait and Palmer Deep) rather depict the spring insolation trend. On account of the different trends in the sea ice diatom assemblage and WSI, we note that the discrepancy may be due to silica dissolution. Many of the species that we use as sea ice indicators, such as F. curta, can dominate the marginal ice zone adjacent to a retreating ice edge. They seem to do equally well in the sea ice and then are successful in seeding the water column, as contrasted with sea ice obligate taxa.
  RC1.7 - COMMENTS & SUGGESTIONS:
  See annotated PDFs for specific comments and suggestions on the text, figures and supplementary information.
  Author´s response: We wish to thank the reviewer for the very detailed and thorough comments provided in the annotated PDFs. They have been considered in the revised version of the manuscript.
  RC1.8 - MINOR COMMENTS: Disordered use and introduction of abbreviations; Inconsistent use of sea-ice/sea ice; Inconsistent use of spacing between signs/units and numbers eg. < 3% and <3 %; Check whether citations should be listed in date order
  Author´s response: We corrected all editing inconsistencies/errors.
  
  Citation: https://doi.org/10.5194/egusphere-2022-804-AC1
RC2:
'Comment on egusphere-2022-804', Anonymous Referee #2, 21 Nov 2022

Study by Vorrath and colleagues presents a new sedimentary record (PS97/072-1), utilising multiproxy approach to provide sea ice and temperature reconstructions in Bransfield Strait (Antarctic Peninsula) over the past ca14ka. Antarctic Peninsula has experienced some profound climatic changes over the last decades and thus paleoclimatic studies in this important region of the Southern Ocean are of real significance.

While manuscript is presented well and supported with good quality figures, I feel that it requires more clarity, particulalrly in relation to descriptions of climate variability, throughout the text (e.g. late-Holocene is a characterised by stable environmental conditions… ), and would benefit from more elaborate discussions which includes cases when e.g. proxies do not agree (see below).

Major comments

Age model – I was wandering if authors could perhaps elaborate on the strength of the age model presented? There are quite large changes in the sedimentation rates, but with no further age points, I was wandering how sure authors can be of a linear nature of the age model between ca 5-12ka and beyond. There is a 10-fold difference between the highest and lowest sedimentation, which surely if present between 5-12ka, would have an impact on the reconstructed ages and overall climatic reconstructions

Climatic reconstructions - There were times where proxies disagreed or more, data did not support authors climatic reconstructions which I think provides a room for further elaboration. For example,

4.1 – L338-338 – seemingly a difference between sea ice diatoms vs WSI.

4.2 – L361-362 – diverging trends in SOTs vs SSST

4.2 – L357-358 – I don`t think this is really supported by the data, particularly with WSI. Sea ice associated diatoms show an increase, at least during the early parts of an Early Holocene. Room for further discussion?

4.4. – L410 – Again, I think there is a room for further discussion. Looking at PIPSO, WSI and temperature records, I don`t think data quite support the statement of the stable environmental conditions.

L413 – 414 – “Minimum PIPSO25 values…” – Could authors elaborate on this please. There are low HBI concentrations during ACR yet PIPSO was considered as an indication of the continuous sea ice cover.

Minor edits

L95-96 – “..as reliable proxies..” – this is not such a clear cut as authors state!

L96 – diunsaturated; change to di-unsaturated please

L104 – “..robustly reconstruct” – again, I think this statement needs rephrasing.

L186 - ..”slightly overestimated..” – please elaborate by how much

L189 – “..identifications of HBIs..” – and GDGTs?

L190 – internal standards – please state quantifies added

L197 – Please, could you state the retention indices for HBIs. Also I might be wrong, but I think only mass spectra of HBI triene are presented in Belt et al 2000.

L212 – please provide details of the n-alkanes standards.

L237 – m/z 1296

L295 – “..and shows high..” – to use elevated instead of high would be more appropriate I think

L304 – “Sea ice concentration estimates..” ?

L324 – significant – is it supported by statistical treatment of data. Perhaps consider using an alternative wording.

L333 – “..a very thick or permanent..” – what does very thick mean? Continuous sea ice cover might be more appropriate?

L433 – we note..

Figures – I think a schematic would be a fitting addition as well.

Figure 4 – I didn`t quite get a point of diene/triene plots (d and e). Why not convert the data to either PIPSO (authors have both IPSO25 and triene data)?

Citation: https://doi.org/10.5194/egusphere-2022-804-RC2
- AC2: 'Reply on RC2', Maria-Elena Vorrath, 06 Mar 2023
  
  Dear Referee #2,
  we appreciate your constructive comments and remarks on our original submission, which have helped to clarify and improve the manuscript. We now provide an annotated text of the manuscript which includes the comments and corrections received as well as a “clean” and “reader-friendly” version of the revised manuscript. As requested, we now also provide more detailed information in the supplementary material. Below, please find our responses to your comments.
  RC2.1: Study by Vorrath and colleagues presents a new sedimentary record (PS97/072-1), utilising multiproxy approach to provide sea ice and temperature reconstructions in Bransfield Strait (Antarctic Peninsula) over the past ca14ka. Antarctic Peninsula has experienced some profound climatic changes over the last decades and thus paleoclimatic studies in this important region of the Southern Ocean are of real significance. While manuscript is presented well and supported with good quality figures, I feel that it requires more clarity, particularly in relation to descriptions of climate variability, throughout the text (e.g. late-Holocene is a characterised by stable environmental conditions…), and would benefit from more elaborate discussions which includes cases when e.g. proxies do not agree (see below).
  Author´s response: We revised the text and the wording to describe the environmental conditions in more detail. We now also discuss discrepancies between the applied proxies. Regarding the divergent trends observed in RI-OH’ and TEX86L ocean temperatures we argue that RI-OH’ seems to be mainly driven by summer insolation, while TEX86 records (Bransfield Strait and Palmer Deep) rather depict the spring insolation trend.
  Major comments
  RC2.2 - Age model: I was wandering if authors could perhaps elaborate on the strength of the age model presented? There are quite large changes in the sedimentation rates, but with no further age points, I was wandering how sure authors can be of a linear nature of the age model between ca 5-12ka and beyond. There is a 10-fold difference between the highest and lowest sedimentation, which surely if present between 5-12ka, would have an impact on the reconstructed ages and overall climatic reconstructions.
  Author´s response: We agree with the reviewer and point out in the text “that (as) the lack of age constraints between 12 ka and 6 ka BP may introduce chronological uncertainties, we only focus on overall trends reflected in our data and refrain from detailed allocations of known climatic events in this time period.“ The overall climatic trends depicted in core PS97/072, however, are in general agreement with other records from the study area.
  RC2.3 - Climatic reconstructions: There were times where proxies disagreed or more, data did not support authors climatic reconstructions which I think provides a room for further elaboration. For example,
  4.1 – L338-338 – seemingly a difference between sea ice diatoms vs WSI.
  4.2 – L361-362 – diverging trends in SOTs vs SSST
  4.2 – L357-358 – I don`t think this is really supported by the data, particularly with WSI.
  Sea ice associated diatoms show an increase, at least during the early parts of an Early Holocene. Room for further discussion?
  Author´s response: We revised the text and now discuss discrepancies and contrasting trends of the different proxies.
  RC2.4 - 4.4. – L410: Again, I think there is a room for further discussion. Looking at PIPSO, WSI and temperature records, I don`t think data quite support the statement of the stable environmental conditions.
  Author´s response: We revised also this part of the manuscript and provide a more detailed assessment of the sea ice and ocean temperature conditions.
  RC2.5 - L413 – 414 – “Minimum PIPSO25 values…”: Could authors elaborate on this please. There are low HBI concentrations during ACR yet PIPSO was considered as an indication of the continuous sea ice cover.
  Author´s response: We thank the reviewer for spotting this. We now conclude that the low HBI concentrations seem to refer to a reduction in spring sea ice – also since elevated TOC, biogenic opal and warm open ocean diatoms suggest higher primary productivity. In general, in the revised version of our manuscript, we tried to be more specific with the description of the sea ice conditions. In the Methods chapter, we now also provide information on how ‘high’ and ‘low’ PIPSO25 values are interpreted in terms of the duration and concentration of sea ice.
  RC2.6: Minor edits
  L95-96 – “..as reliable proxies..” – this is not such a clear cut as authors state!
  L96 – diunsaturated; change to di-unsaturated please
  L104 – “..robustly reconstruct” – again, I think this statement needs rephrasing.
  L186 - ..”slightly overestimated..” – please elaborate by how much
  L189 – “..identifications of HBIs..” – and GDGTs?
  L190 – internal standards – please state quantifies added
  L197 – Please, could you state the retention indices for HBIs. Also I might be wrong, but I think only mass spectra of HBI triene are presented in Belt et al 2000.
  L212 – please provide details of the n-alkanes standards.
  L237 – m/z 1296
  L295 – “..and shows high..” – to use elevated instead of high would be more appropriate I think
  L304 – “Sea ice concentration estimates..” ?
  L324 – significant – is it supported by statistical treatment of data. Perhaps consider using an alternative wording.
  L333 – “..a very thick or permanent..” – what does very thick mean? Continuous sea ice cover might be more appropriate?
  L433 – we note..
  Author´s response: We considered and addressed your comments in the revised manuscript.
  RC2.8 - Figure 4: I didn`t quite get a point of diene/triene plots (d and e). Why not convert the data to either PIPSO (authors have both IPSO25 and triene data)?
  Author´s response: We agree with the referee and accordingly changed the plot now showing PIPSO25 values for JPC10.
  
  Citation: https://doi.org/10.5194/egusphere-2022-804-AC2

Peer review completion

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

ED: Reconsider after major revisions (09 Mar 2023) by Xavier Crosta

Dear Dr. Vorrath,

Thank you for submitting your manuscript ''Deglacial and Holocene sea ice and climate dynamics at the western Antarctic Peninsula" to Climate of the Past special issue Reconstructing Southern Ocean sea-ice dynamics on glacial-to-historical timescales.

Based on the reviewers' evaluations and my own reading I recommend reconsideration of your manuscript following Major Revision. I encourage you to submit a revised version along the lines detailed in the initial responses to reviewers. I also recommend that you take my editorial comments (some been similar to the reviewers’ ones) into consideration in your revised version.

Major comments
The manuscript presents a wealth of proxies dealing with phytoplankton productivity, ocean temperature and sea ice conditions. The new data are compared to similar records from both side of the Antarctic Peninsula. However, all these data are discussed in a very descriptive way (cold, warm, more or less sea ice) and very little information on the potential drivers are provided: insolation (summer vs spring), ice-sheet retreat, regional oceanography (fronts’ migrations, upwelling of CDW), winds, etc…. There is almost no explanation about the differences observed between the different proxies (MAT vs PiPSO25 ; TEX vs RI-OH) in the same core or between the same proxies in the different cores. So I recommend to better use the distribution of the three cores to present a much better assessment of the external forcings and internal drivers that may explain the regional differences.

It is not clear why there are two sets of corrections for each 14C dates. The ventilation age of 1200 years represents the total reservoir age calculated locally on living benthic carbonate organisms. It embeds the 400 years global reservoir age and the local deltaR. This correction is commonly used in many Antarctic coastal regions when dealing with carbonate remains. The additional reservoir age correction derived from Butzin’s model represents the same metric in a temporal framework. In conclusion, it seems to me that you corrected twice the raw 14C dates by the same metric, which is obviously a mistake. If so, the whole age model must be revised, and the interpretations too. If I am wrong, you must explain why you correct twice you 14C dates and what represents each corrections. Please justify why you used SH20 that is generally adapted to terrestrial sequences?

Minor comments
A list of the diatom species included in each group (lines 261-262) would be welcome in the SOM.

I would say that IPSO25 decreases continuously until 5.5 ka BP when looking at the smooth curve (line 293).

Which species are present in the sea-ice group before12.8 ka BP (line 299). As there are only few diatoms, this high % of sea-ice diatoms might be related to preservation issues and might be tricky to interpret. Please justify.

Summer insolation is referred to only once (line 370). So Iwonder why presenting this curve in figure 5 ? Do you think that TEX and RI-OH can be produced during different seasons ? If so, present spring insolation and discuss all this more exhaustively.

Forcings and drivers must be discussed in more details as done lines 378-381. For example, there is no explanation for the observed differences between RI-OH and TEX lines 398-402. Only an observation. Either detail forcings/drivers in each chapter (period) or in a separate chapter as the precessional processes you are dealing with might be rather similar throughout the Holocene, though with the addition of the ice-sheet and bipolar seesaw during the late deglacial.

How do you explain still high BSi, TOC and total diatom abundances during the last 1 ka if sea ice was very thick and pervasive (lines 414-418)? Do you think that the slight temperature drop recorded in the regional marine and ice cores could conduct to a very heavy sea-ice cover ?

This reference is largely outdated as some records were produced since 1990 (line 420).

Figures
Figure 1 : I wonder if the CTD plot is useful as it not used in the discussion.
Figure 3-5 : Detail what the black curves represent. Repeat in figure 5 what the grey band represent.

Supplementary material
Table S1 : Detail on which type of calcareous material were performed the 14C dates.
Section S2 : How robust is the tie-point ? It could be moved upward the MUC by 5 cm. Would it change the interpretation about the LIA ?
Section S3 : How do you explain the very low C/N values in the bottom part of the core where BIT values are highest. BIT values suggest high terrigenous input from land despite what is mentioned lines 21-23. Could this affect GDGTs during the 14-12 ka BP period ?
Table S5 : Please remove the first line with the repetitive core name. This is very confusing. Add the core name in the table caption.

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AR by Maria-Elena Vorrath on behalf of the Authors (25 Mar 2023) Author's response

EF by Vitaly Muravyev (03 Apr 2023) Manuscript Author's tracked changes Supplement

ED: Publish subject to minor revisions (review by editor) (06 Apr 2023) by Xavier Crosta

AR by Maria-Elena Vorrath on behalf of the Authors (25 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (27 Apr 2023) by Xavier Crosta

AR by Maria-Elena Vorrath on behalf of the Authors (03 May 2023)

Journal article(s) based on this preprint

30 May 2023

Deglacial and Holocene sea-ice and climate dynamics in the Bransfield Strait, northern Antarctic Peninsula

Clim. Past, 19, 1061–1079, https://doi.org/10.5194/cp-19-1061-2023,https://doi.org/10.5194/cp-19-1061-2023, 2023

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Sea ice is important to stabilize the ice sheet in Antarctica. To understand how the global climate and sea ice were related in the past we looked on ancient molecules (IPSO₂₅) from sea ice algae and other species whose dead cells accumulated on the ocean floor over time. With chemical analyses we could reconstruct the history of sea ice and ocean temperatures of the past 14 thousand years. We found out that sea ice became less and less as the ocean warmed and more phytoplankton was growing.


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