the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Deglacial and Holocene sea ice and climate dynamics at the Western Antarctic Peninsula
- 1University Hamburg, Institute for Geology, Hamburg, Germany
- 2Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
- 3MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany
- 4Department of Geosciences, University of Bremen, Germany
- 5Centro de Investigación Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Universidad Austral de Chile, Valdivia, Chile
- 6EPHE/PSL Research University, France
- 7UMR 5805 EPOC, CNRS, Université de Bordeaux, France
- 8Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
- 9Centro Oceanográfico COPAS Sur-Austral/COPAS-Coastal, Universidad de Concepción, Chile
- 10Departamento de Oceanografía, Universidad de Concepción, Chile
- 11Scripps Institution of Oceanography, La Jolla, CA 92037, USA
- 12Department of Geology, Colgate University, New York, USA
- 13Departament de Dinàmica de la Terra i de l'Oceà, Universitat de Barcelona, Spain
- 14Instituto Andaluz de Ciencia de la tierra, CSIC-Univ. de Granada, Spain
- 1University Hamburg, Institute for Geology, Hamburg, Germany
- 2Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
- 3MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany
- 4Department of Geosciences, University of Bremen, Germany
- 5Centro de Investigación Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Universidad Austral de Chile, Valdivia, Chile
- 6EPHE/PSL Research University, France
- 7UMR 5805 EPOC, CNRS, Université de Bordeaux, France
- 8Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
- 9Centro Oceanográfico COPAS Sur-Austral/COPAS-Coastal, Universidad de Concepción, Chile
- 10Departamento de Oceanografía, Universidad de Concepción, Chile
- 11Scripps Institution of Oceanography, La Jolla, CA 92037, USA
- 12Department of Geology, Colgate University, New York, USA
- 13Departament de Dinàmica de la Terra i de l'Oceà, Universitat de Barcelona, Spain
- 14Instituto Andaluz de Ciencia de la tierra, CSIC-Univ. de Granada, Spain
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 (C25:3) 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.
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Maria-Elena Vorrath et al.
Status: final response (author comments only)
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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
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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)?
Maria-Elena Vorrath et al.
Maria-Elena Vorrath et al.
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