Rebuttal Letter to Referee #1

Dear Editor, dear Referee #1,

we appreciate your constructive and helpful comments on our original submission, which have significantly contributed to the improvement of the manuscript. Below, please find our responses to your comments. Regarding the new TEX^L₈₆ calibration and additional modelled and WOA13 data (see supplementary material), we also restructured the subsections in Sect. 4 Results and discussion.

RC1.1: I was wandering why authors decided to present (and discuss!) data only for specific (HBI triene Z and Brassicasterol) phytoplankton derived HBI indices in the main text while moving others into the supplementary. Do data in supplementary add anything to the study? Are there any key outcomes? If so which ones etc. I think it would be nice to comment on those additional data.It also implies that those outcomes presented in the main text (based on HBI Triene Z and Brassicasterol) have shown most promise (reflect environmental settings best) in previous calibrations and have been applied most extensively, while I`m not convinced that`s the case in Southern Ocean. While they have been utilised fairly extensively in the Arctic and subjected to several calibration studies, It`s not been the case in the Southern Ocean (as author also point out) and applicability of approach utilising any of these pelagic lipids is vastly unexplored.

Author´s response: The dinosterol and HBI E-triene concentrations and thereof derived PIPSO₂₅ indices show very similar patterns when compared to the brassicasterol and Z-triene data presented in the main text. In order to avoid repetition while describing these results, we prefer to publish the data as supplement. This also allows other researchers to consider the applicability of dinosterol and/or HBI E-triene as phytoplankton markers for own studies. We now add a sentence in Sect. 4.1, commenting the similarity of the datasets.

RC1.2: There seems to be weighting towards HBIs, which somewhat detracts from GDGT outcomes. It made me wonder if perhaps "less" could be more. Should authors concentrate either on GDGTs or on HBIs? While they carry out the evaluation between the individual indicies and satelite/modelling data I was missing an intercomarison between lipid derived proxies and where outcomes from one support/contradict those derived from other.

Author´s response: We recognize your point here, which has similarly been mentioned by Referee #2. We have now, based on the suggestion by Referee #2, used a different calibration for subsurface oceans by Kim et al. (2012) and compared those new temperature reconstructions to instrumental and model data. This enabled us to emphasize the GDGT data in the manuscript. In order to prevent repetitions when splitting the data into two papers, we prefer to keep both proxies (HBIs and GDGTs) in the manuscript and publish the data set as one. We note that we do not directly compare IPSO₂₅ and TEX^L₈₆ as these proxies relate to different environmental variables (i.e. sea ice and subsurface ocean temperature). However, we now comment on the relation between WOA-derived sea surface temperatures and PIPSO₂₅ values.

RC1.3: Have authors considered including any taxonomy work? It seems like biomarkers are depicting some regional differences (e.g. EAP vs WS or EAP vs WAP or even WS vs AS) and I was wandering to what extent these could be observed via differences in diatom distributions. Could taxonomy/diatom work also provide some indications about productivity or phytoplankton composition differences that authors refer to in text (e.g. lines 247-249, 282 etc.)?

Author´s response: We agree that taxonomy work would add to a more detailed assessment of the environmental conditions in the different regions and it would be interesting to see, whether diatom distributions follow a similar pattern than the biomarker reconstructions. Taxonomy work, however, is not within the scope of this manuscript but we now address this point in Sect. 7 regarding future work: “Further taxonomy work, the composition of the proxy´s source habitat (basal sea ice, platelet ice, brine channels) and its connection to platelet ice formation via in situ or laboratory measurements are required to better constrain the proxy´s potential for sea ice reconstructions.“ An important aspect that should be mentioned here as well concerns the preservation of diatoms. Particularly in coastal (often heavily sea ice covered) areas the application of diatoms as environmental proxies can be affected by opal dissolution. This would certainly impact comparisons and/or correlations with other environmental proxies and we suspect that such a study would benefit from a larger data set that also contains sample material from more distal ocean areas (off the continental shelf).

RC1.4: Introduction seems to be rather generous towards HBIs but relatively scarce on GDGTs. I think it would be worth expanding this part and provide overview of current knowledge with respect to use of GDGTs in Southern Ocean.

Author´s response: We agree that GDGTs have not been sufficiently introduced and now add a paragraph on the state of the art concerning GDGTs in the Southern Ocean and we also consider ocean temperatures in the description of the study area to streamline the further presentation and discussion of GDGT data.

RC1.5: Title: perhaps consider rephrasing. “..sea surface..” Authors state that it`s not clear if GDGT based temperatures they`ve derived represent SST, near-surface or sub-surface (e.g. lines 448-449).

Author´s response: In regard of the newly calibrated TEX^L₈₆ data and the consideration of instrumental (Word Ocean Atlas) as well as model-derived subsurface ocean temperature data and the respective modifications to the manuscript, we also changed the title to: “Evaluation of lipid biomarkers as proxies for sea ice and ocean temperatures along the West Antarctic continental shelves”.

Line specific comments and amendments:

Previously line 56: `…emerged as a robust proxy…` seems to contradict with authors conclusions (c.f. line 532).

Author´s response: We now express it more carefully by changing the term robust to potential, which also better agrees with our conclusions. 

Previously line 65: I think authors might have wanted to say “by analogy” rather than “because of the structurally close relationship of this lipid…” e.g. HBI trienes are also structurally similar to IP₂₅ and IPSO₂₅

Author´s response: We followed your suggestion and changed the sentence to: “Belt et al. (2016) introduced the term IPSO₂₅ (“Ice Proxy of the Southern Ocean with 25 carbon atoms”) by analogy to the counterpart IP₂₅ in the Arctic.”

Previously line 68: change “…reconstructions is…” to “..reconstructions are ..”

Author´s response: We changed the sentence to: “Hitherto, only a relatively small number of studies based on IPSO₂₅ for recent and Holocene sea-ice reconstructions are available in the Southern Ocean…”

Previously lines 140-141: internal standards-provide mass equivalent added

Author´s response: We now provide the mass equivalents added to the sediments.

Previously lines 247-249: “… ,pointing to elevated productivity…” somewhat misleading given follow up sentence. Rephrase to … pointing either to elevated productivity or reworked terrigenous organic matter

Author´s response: We merged the two sentences and changed it as suggested to: “…pointing either to elevated productivity or reworked terrigenous organic matter in these areas…”

Previously lines 267-269: sentence beginning “Here productivity of the source diatoms..” any studies or diatoms data that could provide some support if that is the case?

Author´s response: While Moore and Abbott (2002) mainly focus on satellite estimates of surface chlorophyll concentrations to assess phytoplankton blooms at the polar front, Kemp et al. (2006), for example, report on higher concentrations of certain diatom species (incl. Rhizosolenia spp - a producer of HBI trienes; Belt et al., 2017) at oceanic frontal zones. A recent study by Cardenas et al. (2019) using surface sediments from the Drake Passage, however, documents an only minor abundance of Rhizosolenia spp. in those samples. We now mention this study and conclude that biosynthesis of HBI trienes by other pelagic diatoms should be considered.

Previously lines 283: 2 magnitudes – add orders of  after 2

Author´s response: We changed it accordingly to: “… (more than 2 orders of magnitudes) …”

Previously lines 337-341: “This difference between..” seems like really important point to consider! Where does this leave sterols as a phyto counterpart in PIP index?

Author´s response: We note that the use of sterols and HBI trienes to determine PIPSO₂₅ indices partly owes to previous observations that the latter may be absent in some environmental settings and using e.g. dinosterol as phytoplankton marker still enables a proper assessment of surface conditions (Lamping et al., 2020). Accordingly, consideration of both HBI trienes and sterols - though belonging to structurally different compound classes - seems reasonable and this has also been documented within Arctic Ocean studies where brassicasterol- and HBI Z-triene-based PIP₂₅ records exhibit very similar patterns over the last deglacial suggesting an overall reliable response to large-scale climate-driven environmental changes. The herein documented differences in sterol- and HBI triene-based PIPSO₂₅ indices accordingly contribute to the evaluation of these still relatively new approaches for Southern Ocean sea-ice reconstructions.

Previously line 364: “.., with up to 11C..” – add temperatures after with

Author´s response: We added temperatures: “…with temperatures up to ~11 °C.”

Previously line 395: ˚C.5 – should .5 precede ˚C ?

Author´s response: This must have been an error in the formatting, but we corrected it from “-0.5 to -1 °C” to “-1 to -0.5 °C.”

Previously line 402: “The sea ice biomarker IPSO25 is hence..” Are there any detailed time series sea ice studies to support this statement? Analogy is based on what is known about IP25 in the Arctic, but I don`t think this is yet true for IPSO25 in Southern Ocean.

Author´s response: Time-series studies (such as Brown et al. (2011) on IP₂₅) focusing on IPSO₂₅ are not yet available, but the general consensus, based on the main algae bloom in the Southern Ocean and, more importantly, the main bloom of the source diatom of IPSO₂₅ B. adeliensis, is that IPSO₂₅ can be interpreted as spring sea-ice indicator (see Belt, 2016; Riaux-Gobin et al., 2013).

Previously line 458: I think authors shouldn`t use reference which have not been at least accepted for publication.

Author´s response: The work by Spencer-Jones et al. (2020) is now published in the journal Biogeosciences (https://doi.org/10.5194/bg-2020-333).

Previously line 477: remove `HBI diene`

Author´s response: We removed the term HBI diene as suggested.

References

Belt, S.T., Smik, L., Brown, T.A., Kim, J.H., Rowland, S.J., Allen, C.S., Gal, J.K., Shin, K.H., Lee, J.I., Taylor, K.W.R., 2016. Source identification and distribution reveals the potential of the geochemical Antarctic sea ice proxy IPSO25. Nature Communications 7, 12655.

Belt, S.T., Brown, T.A., Smik, L., Tatarek, A., Wiktor, J., Stowasser, G., Assmy, P., Allen, C.S., Husum, K., 2017. Identification of C25 highly branched isoprenoid (HBI) alkenes in diatoms of the genus Rhizosolenia in polar and sub-polar marine phytoplankton. Organic Geochemistry 110, 65-72.

Brown, T., Belt, S., Philippe, B., Mundy, C., Massé, G., Poulin, M., Gosselin, M., 2011. Temporal and vertical variations of lipid biomarkers during a bottom ice diatom bloom in the Canadian Beaufort Sea: further evidence for the use of the IP25 biomarker as a proxy for spring Arctic sea ice. Polar Biology 34, 1857-1868.

Cárdenas, P., Lange, C.B., Vernet, M., Esper, O., Srain, B., Vorrath, M.-E., Ehrhardt, S., Müller, J., Kuhn, G., Arz, H.W., Lembke-Jene, L., Lamy, F., 2019. Biogeochemical proxies and diatoms in surface sediments across the Drake Passage reflect oceanic domains and frontal systems in the region. Progress in Oceanography 174, 72-88.

Kemp, A.E.S., Pearce, R.B., Grigorov, I., Rance, J., Lange, C.B., Quilty, P., Salter, I., 2006. Production of giant marine diatoms and their export at oceanic frontal zones: Implications for Si and C flux from stratified oceans. Global Biogeochemical Cycles 20.

Kim, J.-H., Crosta, X., Willmott, V., Renssen, H., Bonnin, J., Helmke, P., Schouten, S., and Sinninghe Damsté, J. S.: Holocene subsurface temperature variability in the eastern Antarctic continental margin, Geophysical Research Letters, 39, https://doi.org/10.1029/2012GL051157, 2012.

Lamping, N., Müller, J., Esper, O., Hillenbrand, C.-D., Smith, J.A., Kuhn, G., 2020. Highly branched isoprenoids reveal onset of deglaciation followed by dynamic sea-ice conditions in the western Amundsen Sea, Antarctica. Quaternary Science Reviews 228, 106103.

Moore, JK, & Abbott, MR. (2002). Surface chlorophyll concentrations in relation to the Antarctic Polar Front: Seasonal and spatial patterns from satellite observations. Journal of Marine Systems, 37(1-3), 69-86.

Riaux-Gobin, C., Dieckmann, G.S., Poulin, M., Neveux, J., Labrune, C., Vétion, G., 2013. Environmental conditions, particle flux and sympagic microalgal succession in spring before the sea-ice break-up in Adélie Land, East Antarctica. Polar Research 32.

Spencer-Jones, C.L., McClymont, E.L., Bale, N.J., Hopmans, E.C., Schouten, S., Müller, J., Abrahamsen, E.P., Allen, C., Bickert, T., Hillenbrand, C.D., Mawbey, E., Peck, V., Svalova, A., Smith, J.A., 2021. Archaeal intact polar lipids in polar waters: a comparison between the Amundsen and Scotia seas. Biogeosciences 18, 3485-3504.

Rebuttal Letter to Referee #2

Dear Editor, dear Referee #2,

we appreciate your constructive comments and remarks on our original submission, which have helped to clarify certain issues and improve several sections in the manuscript. Below, please find our responses to your comments. Regarding the new TEX^L₈₆ calibration and additional modelled and WOA13 data (see supplementary material), we also restructured the subsections in Sect. 4 Results and discussion.

RC2.1: The manuscript is overall well written and worth to be published. However, I am honestly wondering if all the data presented here should not be the focus of two distinct papers. The authors have sometimes real difficulties to connect the HBI and GDGT proxies, which is not surprising given the fact that they both record different physical variables (sea ice vs ocean temperatures). The manuscript title is a good proof of that as the authors are clearly proposing to “elucidate modern West Antarctic sea-surface conditions”, opening a perspective for paleoclimate studies, which is not really the link we would immediately make when reading the presentation and interpretation of the two proxies. In addition, I have several comments and concerns regarding the GDGT section which needs further considerations and improvement and should include more recent work on this proxy around Antarctica.

Author´s response: We thank Referee #2 for the mostly positive assessment of our manuscript. We considered all comments carefully and generally agree with the referees point, concerning the splitting of the manuscript into two distinct papers and understand the reasoning behind it. However, we would rather like to keep the manuscript as one, which enables us to prevent repetitions and to publish the dataset as one. In the revised manuscript, we put a stronger focus on GDGTs and also provide additional data based on a new calibration and instrumental data. We also changed the title of the manuscript in order to clarify the scope of the paper, both aspects have also been suggested by Referee #1. In the following, we address all the specific changes in the revised manuscript and provide additional responses to the comments and questions of Referee #2 in detail.

RC2.2: HBIs and other productivity biomarkers - I have no major comments on the interpretation of their data. I have nevertheless one main point that should be clarified further than the authors maybe do. The surface sediment samples selected for this study are probably spanning a “large” period of time than what one would ideally expect when strictly looking at the modern conditions (<40 years). We all know how hard it is to get very recent surface sediments and therefore we can only support this work, despite significant uncertainties surroundings the proxy interpretation as underlined by the authors. As they mentioned, “Vorrath et al. (2019) conducted radiocarbon dating on selected surface sediment samples from the Bransfield Strait, concluding that their biomarker data reflect the past two centuries “, explaining why the authors justify that “the different time periods covered by the different methods need to be considered and kept in mind when interpreting the results ». I presume that Lamping and colleagues have no or very few radiocarbon or 210Pb dating on their selected samples in the different investigated areas. I would strongly suggest to mention that regional sea ice has been probably quite variable over the last two decades and centuries which means that comparing concentrations of the HBIs and other sterol concentrations and ratios with satellite and model data can significantly differ owing to the difference of ages. In addition, I would also suggest the authors to clearly state that many studies have shown that significant degradation of organic compounds occurs both within the water column and surface sediments as a result of microbial activity and this might change from one area to another. It means that variations in concentrations between two sectors might not strictly reflect a real change in production of these compounds in the surface waters but might also report two different degradation states. Two surface sediment samples with two very different ages, about few decades, may exhibit two different concentrations which do not necessary mean that sea-ice concentration was higher or lower between them during a specific period of time but instead that the organic compounds could have been more degraded in one area, especially where the oldest sediment are found. In conclusion, I would insist more on this point in addition to the others convincingly raised by the authors.

Author´s response: We fully agree with the arguments concerning the general problem with core top calibrations presented here by Referee #2 and would like to thank her/him for the well summarized issues about this topic and suggestions on which points to add to the manuscript. We now address the topic on degradation in more detail and also comment on regionally different core top ages (Sect. 5). Regarding the latter aspect, we now refer to e.g., Hillenbrand et al. (2010), Smith et al. (2011) and Vorrath et al. (2020) reporting modern Amundsen Sea shelf and Bransfield Strait core top ages, respectively. In addition to the concerns outlined by Referee #2 we now also mention subglacial erosion as well as the input of ancient carbon affecting surface sediment composition and recommend that intensive Pb-dating efforts are an essential prerequisite for core top studies. We now also draw the connection between sedimentation rates and degradation of organic matter (being higher in low sedimentation regimes) and consider studies dealing with HBI degradation (e.g., Rontani et al., 2014; 2019a; 2019b). We further emphasize that this study is not intended to provide a calibration of PIPSO₂₅ values against satellite-derived sea ice concentrations - also due to the uncertainties mentioned above.

RC2.3: I have more concern regarding the GDGT interpretation. There is now an emerging consensus that GDGT are more reflecting along the Antarctic margin the subsurface ocean temperatures (SOT) (0-200m, 100-200m, 50-400m water depth depending on the studied area) rather than SSTs (Kim et al., 2012; Etourneau et al., 2019; Liu et al., 2020). This is mostly linked to the fact that the GDGTs might be more synthesized by Traumarchaeota living at the intersection between the cold and low saline surface waters with the subsurface warm waters (CDW and WDW) around Antarctica. I would first suggest to consider the calibration of Kim et al. 2012 for converting the GDGT ratios into SOT (SOT = 50.8 x TEX86L + 36.1), which may reduce the temperature range and the unrealistic warm values found in the north, and then compare with model and instrumental data at different subsurface water depths. Furthermore, I would also suggest to consider that the GDGT seems to be produced mostly during the late winter and early spring (Murray et al., 1998; Kalanetra et al., 2009), even though we clearly need more data from the water column to confirm such hypotheses. Therefore, mapping the ocean temperatures at different seasons and the most appropriate depths would be worth to try. This might provide further constrains on the use of the GDGTs as paleotemperature proxy.

Author´s response: We now compare our data (incl. the newly calibrated TEX^L₈₆-derived temperatures) with World Ocean Atlas (WOA) derived temperatures and modelled data for the sea surface and subsurface (410 m; see supplementary material to this comment). Correlations of TEX^L₈₆ SOTs with instrumental and modelled temperatures for different depth intervals (0 - 200 m, 100 - 200 m, 50 - 400 m) suggest that the GDGT signal reflects deep subsurface temperatures (410 m water depth) best. Interestingly, also the RI-OH'-based temperatures show a stronger relation to this depth interval than to the sea surface. We present these new results and provide a thorough discussion where we also address the (dis)similarities between instrumental, proxy-based and modelled temperatures.

RC2.4: In conclusion, I believe the authors should split their data in two different papers which would make their interpretation clearer and more focused.

Author´s response: As stated above, we would like to keep the manuscript as one, although we understand the reasoning behind this suggestion. Admittedly, based on the newly calibrated TEX^L₈₆ temperatures and consideration of WOA- and model-derived subsurface ocean temperatures we now extend the discussion of the GDGT part, which may lengthen the manuscript. However, this ensures a balance between the HBI and the GDGT part.

Line specific and minor comments and amendments:

Previously line 30: more recent references, especially in the context of the modern global warming and West Antarctica sea ice decline (eg. Wang et al., J. of climate, 2020)?

Author´s response: We now refer to more recent references such as Wang et al., 2019; Turner et al., 2020; Jenkins et al., 2018.

Previously lines 36-37: I would adequately include references in the right places (ie. In the 40-year satellite record, sea-ice extent in East Antarctica is increasing (Comiso et al., 2017; Parkinson and Cavalieri, 2012), experiencing an abrupt reversal from 2014 to 2018 (even exceeding the drastic decay rates reported in the Arctic; Parkinson, 2019).

Author´s response: We shifted the references accordingly in the right places.

RC2: I think it is important that the authors more clearly mention the warm water circulation in the Weddell Sea related to the Warm Deep Waters (the equivalent to the CDW along the WAP) and their origins.

Author´s response: We now mention the circulation of Warm Deep Waters in the Weddell Sea in Sect. 2 (Regional Setting) and consider these water masses for the discussion of the GDGT data.

RC2: Western and Eastern AP. Why not to use acronyms WAP and EAP for Western and Eastern Antarctic Peninsula, respectively?

Author´s response: We now use the acronyms WAP and EAP for western and eastern Antarctic Peninsula, respectively, throughout the manuscript.

Previously line 248: confusing between “these areas” and “this area”

Author´s response: We changed the sentence according to the suggestion of Referee #1 by combining the two sentences.

Previously lines 249; 275-280: or something else? Could be related to local upwelling forcing too

Author´s response: We add this as an alternative process stimulating primary productivity into the manuscript.

RC2: Missing several references (eg. Vernet et al. 2019, Cao et al 2019…)

Author´s response: Vernet et al., 2019 is included. Unfortunately, we were not able to identify the reference from Cao et al., 2019 suggested by you. We would appreciate it if you could give us more information on that publication.

Please also note the supplementary material.

References

Hillenbrand, C.-D., Smith, J.A., Kuhn, G., Esper, O., Gersonde, R., Larter, R.D., Maher, B., Moreton, S.G., Shimmield, T.M., Korte, M., 2010. Age assignment of a diatomaceous ooze deposited in the western Amundsen Sea Embayment after the Last Glacial Maximum. Journal of Quaternary Science 25, 280-295.

Jenkins, A., Shoosmith, D., Dutrieux, P., Jacobs, S., Kim, T.W., Lee, S.H., Ha, H.K., Stammerjohn, S., 2018. West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability. Nature Geoscience 11, 733-738.

Rontani, J.-F., Smik, L., Belt, S.T., 2019. Autoxidation of the sea ice biomarker proxy IPSO25 in the near-surface oxic layers of Arctic and Antarctic sediments. Organic Geochemistry 129, 63-76.

Rontani, J.-F., Smik, L., Belt, S.T., Vaultier, F., Armbrecht, L., Leventer, A., Armand, L.K., 2019. Abiotic degradation of highly branched isoprenoid alkenes and other lipids in the water column off East Antarctica. Marine Chemistry 210, 34-47.

Rontani, J.F., Belt, S.T., Vaultier, F., Brown, T.A., Massé, G., 2014. Autoxidative and Photooxidative Reactivity of Highly Branched Isoprenoid (HBI) Alkenes. Lipids 49, 481-494.

Smith, J.A., Hillenbrand, C.-D., Kuhn, G., Larter, R.D., Graham, A.G.C., Ehrmann, W., Moreton, S.G., Forwick, M., 2011. Deglacial history of the West Antarctic Ice Sheet in the western Amundsen Sea Embayment. Quaternary Science Reviews 30, 488-505.

Turner, J., Guarino, M.V., Arnatt, J., Jena, B., Marshall, G.J., Phillips, T., Bajish, C.C., Clem, K., Wang, Z., Andersson, T., Murphy, E.J., Cavanagh, R., 2020. Recent Decrease of Summer Sea Ice in the Weddell Sea, Antarctica. Geophysical Research Letters 47, e2020GL087127.

Vorrath, M.E., Müller, J., Rebolledo, L., Cárdenas, P., Shi, X., Esper, O., Opel, T., Geibert, W., Muñoz, P., Haas, C., Lange, C.B., Lohmann, G., Mollenhauer, G., 2020. Sea Ice dynamics at the Western Antarctic Peninsula during the industrial era: a multi-proxy intercomparison study. Clim. Past Discuss. 2020, 1-43.

Wang, Z., Turner, J., Wu, Y., Liu, C., 2019. Rapid Decline of Total Antarctic Sea Ice Extent during 2014–16 Controlled by Wind-Driven Sea Ice Drift. Journal of Climate 32, 5381-5395.