Comment on cp-2021-19 Anonymous Referee # 2 Referee comment on " Elucidating modern West Antarctic sea surface conditions : An intercomparison of lipid biomarker proxies , instrumental and numerical-model data

The work proposed by Lamping and co-authors is focusing on the application of several lipid biomarkers to improve reconstructions in sea-ice conditions in West Antarctica. Part of this work corresponds to an extension of the Vorrath et al. (2019) study, but now include a new model-data comparison. In addition, the authors are also presenting a couple of other lipid tracers, including the GDGTs and their use as an ocean temperature proxy. The comparison between the proxy and instrumental data with the modelling simulations is absolutely fundamental to better understand the meaning of these molecular proxies, their locus of production, their transport, etc. and that will definitely serve the scientific community for future investigations on sea-ice conditions and to scrutinize other parameters using marine sediments.

extent (Liu et al., 2004) and rapid atmospheric warming (Vaughan et al., 2003). The Larsen Ice Shelves A and B, located at 40 the East Antarctic Peninsula, collapsed in 1995, respectively, which Massom et al. (2018 linked to the loss of a seaice buffer, enabling an increased flexure of the ice shelf margins by ocean swells. The Bellingshausen and Amundsen Seas are also affected by a stark sea-ice decline (Hobbs et al., 2016;Parkinson, 2019). Glaciers draining into the Amundsen Sea are thinning at an alarming rate, which has been linked to basal melting caused by relatively warm Circumpolar Deep Water (CDW) incursions into sub-ice shelf cavities, thinning the adjacent ice shelves from below (e.g., Jacobs et al., 2011). The 45 disintegration of ice shelves reduces the buttressing force for the West Antarctic Ice Sheet, which may lead to a partial collapse of ice shelves in these catchments, eventually impacting global sea level rise significantly (Pritchard et al., 2012;Vaughan, 2008).
State-of-the-art climate models are not yet fully able to depict sea-ice seasonality and sea-ice cover, which the 5 th Assessment Report of the Intergovernmental Panel on Climate Change (Stocker et al., 2013) explains by a lack of validation efforts using 50 proxy-based sea-ice reconstructions. Knowledge about (paleo-) sea-ice conditions in the climate sensitive areas around the West Antarctic Ice Sheet is hence considered as crucial for understanding past and future climate evolution.
To date, the most common proxy-based sea-ice reconstructions in the Southern Ocean are conducted by the use of sympagic diatom assemblages, which are strongly dependent on their preservation within the sediments (Allen et al., 2011;Armand and Leventer, 2003;Crosta et al., 1998;Esper and Gersonde, 2014;Gersonde and Zielinski, 2000;Leventer, 1998). Dissolution 55 effects within the water column or after deposition determine the preservation state of the small, lightly silicified microfossils and may alter the diatom record, leading to inaccurate sea-ice reconstructions (Leventer, 1998;Zielinski et al., 1998). To avoid ambiguous interpretations, the molecular remains of certain diatoms, specific organic geochemical lipids, have recently emerged as a robust proxy for reconstructing past (and present) Antarctic sea ice (Barbara et al., 2013;Collins et al., 2013;Denis et al., 2010;Etourneau et al., 2013;Lamping et al., 2020;Massé et al., 2011;Vorrath et al., 2019;2020). Specifically, a 60 di-unsaturated highly branched isoprenoid (HBI) alkene (HBI diene, C 25:2 ) has been detected in both sea-ice diatoms and sediments in the Southern Ocean (Johns et al., 1999;Massé et al., 2011;Nichols et al., 1988) and the sympagic (i.e. living within sea ice) tube-dwelling diatom Berkeleya adeliensis was recently identified as producer, which preferably proliferates in platelet ice (Belt et al., 2016;Riaux-Gobin and Poulin, 2004). However, B. adeliensis seems rather flexible concerning its habitat, since it was also recorded in the bottom ice layer and seems to be well adapted to changes in texture during ice melt 65 (Riaux-Gobin et al., 2013). Belt et al. (2016) introduced the term IPSO 25 ("Ice Proxy of the Southern Ocean with 25 carbon atoms") because of the structurally close relationship of this lipid to the counterpart IP 25 in the Arctic. Hitherto, only a relatively small number of studies based on IPSO 25 for recent and Holocene sea-ice reconstructions is available in the Southern Ocean Belt et al., 2016;Collins et al., 2013;Denis et al., 2010;Etourneau et al., 2013;Lamping et al., 2020;Massé et al., 2011;Tesi et al., 2020;Vorrath et al., 2019;2020). Commonly, for a more detailed assessment of sea-70 ice conditions, IP 25 in the Arctic Ocean and IPSO 25 in the Southern Ocean have been measured alongside complementary phytoplankton derived lipids, such as sterols and/or HBI-trienes, which are indicative of open-water conditions (Belt and Müller, 2013;Lamping et al., 2020;Müller et al., 2011;Vorrath et al., 2019;2020). The combination of the sea-ice biomarker and a phytoplankton biomarker, the so called PIP 25 index for the Arctic (Müller et al., 2011) and the PIPSO 25 index for the Antarctic (Vorrath et al., 2019), allow for a more quantitative differentiation of contrasting sea-ice settings. A misinterpretation 75 of an absent sea-ice biomarker, which can be the result of either no sea-ice cover or a severe sea-ice cover, that prevents light penetration hence limiting ice algae growth, can be circumvented with this approach.
Mechanisms contributing to ice shelf instability are manifold. As previously mentioned, relatively warm CDW is considered one of the main drivers for ice shelf thinning in the Amundsen Sea Embayment (Jacobs et al., 2011;Jenkins and Jacobs, 2008).
Accordingly, changing ocean temperatures are another crucial factor for the fate of West Antarctic Ice Sheet stability (e.g., 80 Colleoni et al., 2018). As for sea-ice reconstructions, organic geochemical lipids for reconstructing past and recent ocean temperatures in high latitudes have come into focus in the past decades, since the preservation of calcium carbonate https://doi.org/10.5194/cp-2021-19 Preprint. Discussion started: 1 March 2021 c Author(s) 2021. CC BY 4.0 License. microfossils is not continuous in high latitude sediments (e.g., Zamelczyk et al., 2012). Archaeal isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), sensitive to temperature change and relatively resistant to degradation processes, are wellpreserved in marine sediments (Huguet et al., 2008;Schouten et al., 2013). Hence, isoGDGTs are considered to be valuable 85 tools for reconstructing ocean temperatures (Schouten et al., 2002).
Our aim with this study is to provide insight into the application of biomarkers for sea ice as well as ocean temperature reconstructions. Estimates on recent sea-ice conditions along the eastern and western Antarctic Peninsula, the Amundsen and Weddell Seas, are based on the analyses of IPSO 25 , HBI-trienes and phytosterols in surface sediment samples from these areas.
We further address the potential connection between IPSO 25 and platelet ice formation under near-coastal fast ice, which is 90 related to the presence of near-surface ice shelf basal melt water. An intercomparison of sea ice as well as temperature reconstructions (based on GDGT analyses) with (1) sea-ice distributions obtained from satellite observations and (2) estimated sea-ice distribution and SSTs deduced from modelled data provides for an evaluation of the proxy approaches. For a more semi-quantitative sea-ice estimate, the relatively new approach of PIPSO 25 has been used to further assess the advantages and limitations of the sea-ice index as a potential tool to validate and improve numerical climate models to better understand 95 current and past trends in sea-ice development in the Southern Ocean.

Regional setting
The areas of investigation in this study include the southern Drake Passage, the continental shelves of the West and East Antarctic Peninsula (~ 60° S) and the more southerly located Amundsen and Weddell Seas (~ 75° S; Fig. 1). The different study areas are all connected by the only current circumnavigating the globe, the Antarctic Circumpolar Current (ACC; 100 Meredith et al., 2011;Rintoul et al., 2001). The ACC is the largest current system in the world characterised by a strong eastward flow, which finds its narrowest constriction in the Drake Passage. It is mainly composed of CDW, which is generally divided into the Upper CDW with low oxygen and high nutrient concentrations, and Lower CDW with high salinities (Rintoul et al., 2001). Along the Bellingshausen Sea, the Amundsen Sea and West Antarctic Peninsula (i.e., the Bransfield Strait), where the ACC flows close to the continental shelf edge, CDW is upwelling onto the shelf via bathymetric troughs, contributing to 105 basal melt and retreat of the adjoining ice shelves (Jacobs et al., 2011;Jenkins and Jacobs, 2008;Klinck et al., 2004). In the Weddell Sea, where the ACC is located sufficiently far from the Antarctic continent, a subpolar cyclonic circulation is present south of the ACC, the Weddell Gyre. The Weddell Gyre is the main circulation in the Weddell Sea and the most important source of Antarctic Bottom Water (Deacon, 1979), with sea-ice formation as an important factor in generating these dense water masses (Harms et al., 2001). Wind and currents force a northward sea-ice drift in the western Weddell Sea along the 110 coast of the East Antarctic Peninsula (Harms et al., 2001) until leaving it to melt in warmer waters to the North and up to the Powell Basin (Vernet et al., 2019). At the northern tip of the Antarctic Peninsula, Transitional Weddell Sea Water (TWW) branches off into the Bransfield Strait and is characterised by colder temperatures and higher salinities as a result of extended sea-ice formation in the Weddell Gyre (Collares et al., 2018;Thompson et al., 2009). Here, it encounters the well-stratified, warm, and fresh Bellingshausen Sea Water (BSW; Fig. 1), which is entering the Bransfield Strait from the West (Sangrà et al., 115 2011). Since 1978, satellite observations show strong seasonal shifts of sea-ice cover at the Antarctic Peninsula, which is less pronounced in the more southerly Amundsen and Weddell Seas (Fig. 2a-c). Mean monthly sea-ice concentrations (SIC) for winter (JJA), spring (SON) and summer (DJF) reveal a permanently ice-free Drake Passage, while the West and East Antarctic Peninsula shelf areas are influenced by a changing sea-ice cover in the course of a year (Fig. 2a-c). For the Amundsen and Weddell Seas, satellite data reveal a closed seasonal sea-ice cover with up to ~ 90 % concentration during winter and spring 120 ( Fig. 2a+b), and a late break-up of sea-ice cover to a minimum concentration of ~ 30 % during summer (Fig. 2c).

Sediment material
In total, we analysed a set of 41 surface sediment samples from different areas of the Southern Ocean (Fig. 1), all have been retrieved by multicorers during RV Polarstern expeditions in the past years. 16 surface sediment samples from the Amundsen 125 Sea continental shelf were collected during RV Polarstern expeditions PS69 in 2007 (Gohl, 2007) and PS104 in 2017 (Gohl, 2017). 25 surface sediment samples from the southeastern and southwestern Weddell Sea continental shelf were collected during RV Polarstern expeditions PS111 in 2018 (Schröder, 2018) and PS118 in 2019 (Dorschel, 2019). This set of samples was complemented by 26 surface sediment samples from the Bransfield Strait/West Antarctic Peninsula for which the analytical results were already published by Vorrath et al. (2019). 130

Bulk sediment and organic geochemical analyses
The sediment material was freeze-dried and homogenized with an agate mortar and stored in glass vials at -20 °C before and after these initial preparation steps to avoid degradation of targeted molecular components. The analysis of total organic carbon (TOC) contents was conducted on 0.1 g of sediment after removing inorganic carbon (total inorganic carbon, carbonates) with 500 µl 12 N hydrochloric acid. Measurements were conducted by means of a carbon-sulphur determinator (CS 2000; Eltra) 135 with standards being measured for calibration before sample analyses and after every tenth sample to ensure accuracy (error ± 0.02 %).
Lipid biomarker extraction of the sediment (4 g for PS69 and PS104; 6 g for PS111 and PS118) was done by ultrasonication (3 x 15 min), using dichloromethane:methanol (3 x 6 ml for PS69 and PS104; 3 x 8 ml for PS111 and PS118; 2:1 v/v) as solvent. Prior to this step, the internal standards 7-hexylnonadecane (7-HND; 20 µl/sample for PS69 and PS104 and 30 140 µl/sample for PS111 and PS118), 5⍺-androstan-3-ol (40 µl/sample) and C 46 (100 µl/sample) were added to the sample for quantification of HBIs, sterols and GDGTs, respectively. Via open-column chromatography, with SiO 2 as stationary phase, fractionation of the extract was achieved by eluting the apolar fraction (HBIs) and the polar fraction (sterols and GDGTs) with 5 ml n-hexane and 5 ml DCM/MeOH 1:1, respectively. The polar fraction was subsequently split into two fractions (sterols and GDGTs) for further processing. The sterol fraction was silylated with 300 µl bis-trimethylsilyl-trifluoroacetamide 145 (BSTFA; 2h at 60 °C). Compound analyses of HBIs and sterols were carried out on an Agilent Technologies 7890B gas chromatograph (GC; fitted with a 30 m DB 1MS column; 0.25 mm diameter and 0.25 µm film thickness) coupled to an Agilent Technologies 5977B mass selective detector (MSD; with 70 eV constant ionization potential, ion source temperature of 230 °C). The GC oven was set to: 60 °C (3 min), 150 °C (rate: 15 °C/min), 320 °C (rate: 10 °C/min), 320 °C (15 min isothermal) for the analysis of hydrocarbons and to: 60 °C (2 min), 150 °C (rate: 15 °C/min), 320 °C (rate: 3 °C/min), 320 °C (20 min 150 isothermal) for the analysis of sterols. Helium was used as carrier gas. The identification of HBI and sterol compounds is based upon their GC retention times and mass spectra (Belt, 2018;Belt et al., 2000;Boon et al., 1979). Lipid quantification was obtained by setting the individual, manually integrated, GC-MS peak area in relation to the peak area of the respective internal standard and normalization to the amount of extracted sediment. Quantification of IPSO 25 and HBI Z-triene was achieved using their molecular ion (IPSO 25 : m/z 348 and HBI Z-triene: m/z 346) in relation to the fragment ion m/z 266 of the internal 155 standard 7-HND (Belt, 2018). Quantification of sterols was achieved by comparison of the fragment ion of the individual sterol with the fragment ion m/z 348 of the internal standard 5⍺-androstan-3-ol. Instrumental response factors for the target lipids were considered as recommended by Belt et al. (2014) and Fahl and Stein (2012). All biomarker concentrations were subsequently normalized to the TOC content of each sample to account for different depositional settings within the different study areas. (1) where c (c = mean IPSO 25 /mean phytoplankton marker) is applied as a concentration balance factor to account for high concentration offsets between IPSO 25 and the phytoplankton biomarker (see Table S1 for c-factors of individual PIPSO 25 calculations). 165 Following the approach by Müller and Stein (2014) and Lamping et al. (2020), samples with exceptionally low (at detection limit) concentrations of both biomarkers have been assigned a PIPSO 25 value of 1 (see chapter 4.1.2). This accounts for five sample stations in the Weddell Sea PS111/13-2, /15-1, /16-3, /29-3; /40-2 (marked as orange x in Fig. 1).
The GDGT fraction was dried under N 2 , redissolved with 120 µl hexane:isopropanol (v/v 99:1) and then filtered using a polytetrafluoroethylene (PTFE) filter with a 0.45 µm pore sized membrane. GDGT measurements were carried out using high 170 performance liquid chromatography (HPLC; Agilent 1200 series HPLC system) coupled to an Agilent 6120 mass spectrometer (MS), operating with atmospheric pressure chemical ionization (APCI). The injection volume was 20 µl. For separating the GDGTs, a Prevail Cyano 3 µm column (Grace, 150 mm * 2.1 mm) was kept at 30 °C. Each sample was eluted isocratically for 5 min with solvent A = hexane/2-propanol/chloroform; 98:1:1 at a flow rate of 0.2 ml/min, then the volume of solvent B = hexane/2-propanol/chloroform; 89:10:1 was increased linearly to 10 % within 20 min and then to 100 % within 10 min. The 175 column was back-flushed (5 min, flow 0.6 ml/min) after 7 min after each sample and re-equilibrated with solvent A (10 min, flow 0.2 ml/min). The APCI was set to the following: N 2 drying gas flow at 5 l/min and temperature to 350 °C, nebulizer pressure to 50 psi, vaporizer gas temperature to 350 °C, capillary voltage to 4 kV and corona current to +5 µA.
Temperature calculations based on OH-GDGTs were carried out according to Lü et al. (2015): To determine the relative influence of terrestrial organic matter input, the BIT-index was calculated following Hopmans et al. with traditional approximation. We used T63 Gaussian grid which has a spatial resolution of about 1.9 x 1.9 degree (1.9 ° or 210 km). There are 47 vertical layers in the atmosphere.
Momentum transport arising from boundary effects is configured using the subgrid orography scheme as described by Lott (1999). Radiative transfer in ECHAM6 is represented by the method described in Iacono et al. (2008). ECHAM6 also contains a Land-Surface Model (JSBACH) which includes 12 functional plant types of dynamic vegetation and 2 bare-surface types 205 (Loveland et al., 2000;Raddatz et al., 2007). The ice-ocean module in AWI-ESM2 is based on the finite volume discretization formulated on unstructured meshes. The multi-resolution for the ocean is up to 15 km over polar and coastal regions, and 135 km for far-field oceans, with 46 uneven vertical depths. The impact of local dynamics on the global ocean is related to a number of FESOM-based studies (Danilov et al., 2017). The multi-resolution approach advocated by FESOM allows one to explore the impact of local processes on the global ocean with moderate computational effort (Danilov et al., 2017). AWI-210 ESM2 employs the OASIS3-MCT coupler (Valcke, 2013) (Sidorenko et al., 2019) and has been applied for marine radiocarbon concentrations (Lohmann et al., 2020), the latest Holocene (Vorrath et al., 2020), and the Last Interglacial (Otto-Bliesner et al., 2021). 215

Experimental design
One transient experiment was conducted using AWI-ESM2, which applied the boundary conditions, including orbital parameters and greenhouse gases. Orbital parameters are calculated according to Berger (1978), and the concentrations of greenhouse gases are taken from ice-core records as well as from recent measurements of firn air and atmospheric samples (Köhler et al., 2017). The model was initialized from a 1,000-year spin-up run under mid-Holocene boundary conditions as 220 described (Otto-Bliesner et al., 2017). In our modeling strategy, we follow Lorenz and Lohmann (2004) and use the climate condition from the preindustrial state as spin-up and initial state for the subsequent transient simulation covering the period 1950-2014 CE. Topography including prescribed ice sheet was kept constant in our transient simulation. All model data are provided in Table S2.

Satellite SIC and SSTs 225
Satellite data are derived from Nimbus-7 SMMR and DMSP SSM/I-SSMIS passive microwave data and downloaded from the National Snow and Ice Data Center (NSIDC; Cavalieri et al., 1996). The sea-ice data represent mean monthly SIC, which are expressed to range from 0 % to 100 % and are averaged over a period of the beginning of satellite observations in 1978 to the individual year of sample retrieval. The monthly mean SIC were then split into different seasons: winter (JJF), spring (SON) and summer (DJF) (Fig. 2a-c) and the data are considered to represent the recent mean state of sea-ice coverage. All 230 satellite data are provided in Table S3.

Environmental settings of the Southern Ocean depicted by proxy data 235
In the following, we describe the biomarker and model data assembled during this study from North (Antarctic Peninsula) to South (Amundsen and Weddell Seas) and draw conclusions about the environmental settings deduced from the data set. All biomarker data collected during this study are provided in Table S1 and are available via the PANGAEA data repository (in prep.). https://doi.org/10.5194/cp-2021-19 Preprint. Discussion started: 1 March 2021 c Author(s) 2021. CC BY 4.0 License.

TOC content, HBIs and sterols in Antarctic surface sediments 240
TOC contents in marine sediments in a first approximation are often viewed as an indicator for primary productivity in surface waters (Meyers, 1997), however we are aware that additional factors, such as different water depths or depositional regimes, may exert control on sedimentary TOC as well. The TOC contents of the herein investigated surface samples are lowest in the Drake Passage with values around 0.12-0.54 %, increasing in a northwest-southeast gradient into the Bransfield Strait, ranging between 0.59-1.06 % ( Fig. 3a; WAP). 245 At the East Antarctic Peninsula, higher TOC contents (0.57-0.86 %) prevail around the Larsen Ice Shelf with a decreasing trend towards the Powell Basin (0.22-0.37 %) and an increase to 0.50 % around the area of the South Orkney Islands, pointing to elevated productivity in these areas ( Fig. 3a; EAP). The elevated TOC contents in this area may, however, also be attributable to higher inputs of reworked terrigenous organic matter.  (Smik et al., 2016) and hence suggested to be a proxy for MIZ conditions (Belt et al., 2015;Collins 265 et al., 2013;Schmidt et al., 2018). Vorrath et al. (2019), however, relate the high concentrations of HBI Z-triene at the northernmost stations in the permanently ice-free eastern Drake Passage to their proximity to the Antarctic Polar Front. Here, productivity of the source diatoms of HBI-trienes may be enhanced by meander-induced upwelling leading to increased nutrient flux to surface waters (Moore and Abbott, 2002). Moderate concentrations along the continental slope of the West Antarctic Peninsula and in the Bransfield Strait have been associated with elevated inflow of warm BSW which lead to a 270 retreating sea-ice margin during spring and summer (for more details, see Vorrath et al. (2019) and Vorrath et al. (2020).
Samples from the East Antarctic Peninsula continental shelf and the Powell Basin are characterised by relatively low concentrations of HBI Z-triene ( Fig. 3c; where IPSO 25 concentrations are highest; 0.1-2.37 µg*g OC -1 ; Fig. 3b), showing a southwest-northeast gradient, while the northernmost sample closest to the South Orkney Islands is characterized by higher HBI Z-triene concentration of ~ 8.49 µg*g OC -1 (Fig. 3c; EAP). This relatively high concentration may be related to an "Island 275 Mass Effect", coined by Doty and Oguri (1956), which refers to an increased primary production around oceanic islands in comparison to surrounding waters. Nolting et al. (1991) found extraordinarily high dissolved iron levels (as high as 50-60 nM) on the shelf of the South Orkney Islands and Nielsdóttir et al. (2012) also observed enhanced iron and Chl a concentrations in the vicinity of the South Orkney Islands. They connect, among others, the increased iron levels with input from seasonally retreating sea ice, which is recorded by satellites (Fig. 2a-c) and leads to a substantial annual phytoplankton bloom, which may 280 also cause the elevated TOC contents in that sample (Fig. 3a). We assume that these conditions are favourable for the growth of the source diatoms of HBI Z-triene, leading to elevated concentrations. In the Drake Passage and the East Antarctic https://doi.org/10.5194/cp-2021-19 Preprint. Discussion started: 1 March 2021 c Author(s) 2021. CC BY 4.0 License. Peninsula, brassicasterol displays a similar pattern as the HBI Z-triene, with relatively higher concentrations (more than 2 magnitudes) ranging between 1.86 and 5017.44 µg*g OC -1 (Fig. 3d). In the sample closest to the South Orkney Islands, however, brassicasterol concentrations are not elevated, unlike the HBI Z-triene ( Fig. 3d; EAP), which could refer to different 285 environmental preferences of the source organisms producing the individual biomarkers.
In the Weddell Sea, TOC contents are generally lower (< 0.4 %), with slightly elevated values in the West (up to 0.50 %) and right in front of the Filchner Ice Shelf (up to 0.52 %) (Fig. 3a). The Amundsen Sea is characterized by slightly higher TOC contents, with concentrations of up to 0.91 % in the West and lower values in the East (0.33 %; Fig. 3a; AS).
In the samples from the Amundsen and Weddell Seas, dominated by a strong winter sea-ice cover lasting until spring (Fig. 2a-290 c), all three biomarkers are low concentrated. An exception can be observed in samples from right in front of the Filchner Ice Shelf with significantly higher concentrations of IPSO 25 (7.09-73.87 µg*g OC -1 ; Fig. 3b; WS). Concentrations of IPSO 25 on the Amundsen Sea shelf are relatively low (0.04-3.3 µg*g OC -1 ) with slightly higher values towards the north-east ( Fig. 3b; AS). HBI Z-triene is also very low concentrated, showing slightly higher concentrations within the Filchner Trough (0.04-1 µg*g OC -1 ) and towards the more distal locations in the northeast of the Amundsen Sea (0.01-1.88 µg*g OC -1 ; Fig. 3c). 295 Brassicasterol generally shows similar patterns as the HBI Z-triene, with concentrations ranging between 1.86 and 220.54 µg*g OC -1 ( Fig. 3d; for HBI E-triene and dinosterol distribution, see Fig. S1).

Combining individual biomarker records: the PIPSO 25 index
Targeting at a more quantitative assessment of sea-ice conditions, Vorrath et al. (2019) have followed the PIP 25 index applied 300 in the Arctic (Belt and Müller, 2013;Müller et al., 2011;Xiao et al., 2015) and introduced an equivalent sea-ice index for the conditions, the thickness of sea ice hinders light penetration, thereby limiting the productivity of bottom sea-ice algae (Hancke et al., 2018). This scenario may result in the absence of both phytoplankton and sea-ice biomarkers in the sediment. The other scenario is dominated by a permanently open ocean, where the sea-ice biomarker is absent as well, but here, the phytoplankton biomarkers are present in variable concentrations (Müller et al., 2011). The presence of both biomarkers in the sediment is indicative of seasonal sea-ice coverage and/or the occurrence of stable ice margin conditions, promoting biosynthesis of both 310 biomarkers (Müller et al., 2011). We here distinguish between P Z IPSO 25 and P B IPSO 25 using HBI Z-triene and brassicasterol as phytoplankton biomarker, respectively ( Fig. 4; for PIPSO 25 values based on HBI E-triene and dinosterol see Table S1 and underestimating the sea-ice cover in that area. Regarding the satellite-derived sea-ice data, this area of the continental shelf is 325 influenced by a severe seasonal sea-ice cover. As previously mentioned, we followed the approach by Müller and Stein (2014) and Lamping et al. (2020) and assigned a maximum PIPSO 25 value of 1 to these samples to circumvent misleading interpretations and aid visualisation.
Interestingly, we obtained an intermediate PIPSO 25 value (~ 0.51) derived for one sample in front of the Brunt Ice Shelf, which may be indicative of a less severe sea-ice cover in that area. A possible explanation for the relatively lower PIPSO 25 value may 330 be the presence of a coastal polynya that has been reported by Anderson (1993) and which is further supported by Paul et al. (2015), who note that the sea-ice areas around the Brunt Ice Shelf is the most active in the southern Weddell Sea, with an annual average polynya area of 3516 ± 1420 km². The reduced SIC here are also captured by our model, which is further described in Sect. 4.1.4.
PIPSO 25 values in the Amundsen Sea point to different scenarios. While the P Z IPSO 25 index ranges around 0.9 with a slight 335 decrease to a value of 0.3 in the easterly, more distal location (Fig. 4a), the P B IPSO 25 index is generally lower, ranging around 0.6 in the coastal area and with a much steeper decline towards distal locations to 0.2 (Fig. 4b). This difference between P Z IPSO 25 and P B IPSO 25 may be explained by the different source organisms biosynthesizing the individual phytoplankton biomarkers. While the main origin of HBI-trienes seems to be restricted to diatoms (Belt et al., 2017), brassicasterol is known to be produced by several algal groups adapted to a wider range of sea surface conditions, such as dinoflagellates, diatoms, 340 haptophytes, among others (Volkman, 2006).

TEX L 86 -and RI-OH' -derived temperatures
Isoprenoidal GDGTs are archaeal membrane lipid-derived proxies and valuable tools for reconstructing ocean temperatures (Schouten et al., 2002). These specific lipids, preserved in marine sediments, are sensitive to temperature change and relatively resistant to degradation processes (Huguet et al., 2008;Schouten et al., 2013). Schouten et al. (2002) found that the number of 345 cyclopentane rings in sedimentary GDGTs is correlated with surface water temperatures and developed the first archaeal lipid paleothermometer TEX 86 , a ratio of certain GDGTs, as a sea surface temperature (SST) proxy.
For a critical appraisal of the applicability and reliability of GDGT indices as temperature proxies in our investigated regions, we here make use of two temperature proxy approaches developed for the high latitude polar oceans: The TEX L 86 proxy by Kim et al. (2010) and the RI-OH' proxy by Lü et al. (2015), calculated and calibrated using Eq. 3 and 5, respectively. 350 The reconstructions represent annual mean ocean temperatures. In all samples, the BIT-index (Eq. 6) is < 0.3, indicating no significant contribution of terrestrial input influencing the distribution and hence applicability of GDGTs to estimate ocean temperatures. TEX L 86 and RI-OH' both show a similar pattern, but different temperatures, ranging between -4.23 to +10.57 °C and -2.62 to +4.67 °C, respectively (Fig. 5a+b). At the West Antarctic Peninsula, temperatures follow a northwest-southeast gradient with relatively higher temperatures in the permanently ice-free Drake Passage and the continental slope, influenced 355 by the ACC and relatively warm CDW (Orsi et al., 1995;Rintoul et al., 2001). Temperatures decrease towards the Bransfield Strait and the East Antarctic Peninsula, which are influenced by a seasonal sea-ice cover and the relatively colder and highly saline TWW, branching off the Weddell Gyre (Collares et al., 2018;Thompson et al., 2009). At the East Antarctic Peninsula, a southwest-northeast gradient can be observed with relatively lower temperatures around the Larsen Ice Shelf and higher temperatures towards the Powell Basin and the South Orkney Islands, towards the North. These general temperature patterns 360 align well with the decreasing sea-ice cover in that area towards the North.
Absolute temperature estimates derived from the two paleothermometers show significantly different ranges. While the TEX L 86 signal is reflecting temperatures in the Amundsen and Weddell Seas quite well, it seems to be significantly warm-biased further to the North, in the Drake Passage, with up to ~ 11 °C. This warm-biased TEX L 86 signal is a known caveat in that area and is, among others, assumed to be connected to GDGTs produced by deep-dwelling Euryarchaeota (Park et al., 2019) Interestingly, our reconstructions suggest that the TEX L 86 -derived temperatures (Fig. 5a) are only warm-biased in the relatively warmer Drake Passage but depict temperatures in the colder regions (Amundsen and Weddell Seas) reasonably well or only slightly warm-biased, if compared to the WOA13 temperatures (Fig. 5c).
Further to the South, in the Amundsen and Weddell Seas, temperatures are generally lower than at the Antarctic Peninsula. 370 Samples from the Weddell Sea record a temperature decrease from east to west, which may reflect an eddy-driven route in the north-eastern corner of the Weddell Gyre carrying relatively warm, salty CDW, which then advects westward along the southern edge of the Weddell Gyre (Vernet et al., 2019). While the origin of GDGTs is not yet fully understood and still debated (Ho et al., 2014), the biosynthesis of intact polar lipid GDGTs in CDW, as just recently suggested by Spencer-Jones et al. (2020), might, however, support the hypothesis of advected CDW in that area. In the Amundsen Sea, relatively higher 375 temperatures (~ 0.5 °C) at the sample locations in the north-eastern part of the embayment are reflected in the RI-OH'-derived temperatures but are not reflected in the TEX L 86 -based reconstruction.

Modelled SIC and SSTs
The global climate model setup AWI-ESM2 was used to simulate SSTs and SIC in the study area for modern conditions ; Fig. 5d and 6, respectively). Modelled SIC indicate an absence of sea ice in the permanently ice-free Drake Passage 380 ( Fig. 6a-c) and a northwest-southeast gradient from the continental slope to the Bransfield Strait during winter and spring ( Fig.   6a+b) with the latter as being ice-free during summer (Fig. 6c). During all three seasons (from winter through spring and summer), a southwest-northeast gradient at the East Antarctic Peninsula can be observed, highlighting the decreasing sea-ice influence towards the Powell Basin in the North. Absolute modelled SIC are decreasing from winter to summer, but still underestimate SIC observed by satellites (Fig. 2). In the Amundsen and Weddell Seas, the model shows a heavy sea-ice cover 385 (~ 90 %) during winter and spring. Interestingly, modelled SIC in the area in front of the Brunt Ice Shelf are as low as ~ 45 % (Fig. 6a+b), corresponding well with the reduced PIPSO 25 value of ~ 0.51 % and may reflect the polynya conditions in that region documented by Anderson (1993) and Paul et al. (2015). During summer, the model suggests a reduction in SIC in the Amundsen and Weddell Seas to about 15-25 % (Fig. 6c), slightly underestimating the satellite observations. Modelled annual mean SSTs (Fig. 5d) are highest with up to 5 °C in the permanently ice-free Drake Passage, influenced by 390 the relatively warm ACC. Decreasing temperatures are simulated towards the continental slope and the Bransfield Strait (~ 0.5-1 °C), coinciding with the intensifying influence of sea-ice cover in that area. At the East Antarctic Peninsula/northwestern Weddell Sea, the modelled SSTs show a southwest-northeast gradient towards the Powell Basin with temperatures increasing from -0.5 °C in the South to 0.5 °C in the North, aligning well with the other modelled records. In the Amundsen and Weddell Seas, annual mean SSTs are negative, with temperatures from -0.5 to -1 °C.5 395

Comparing biomarker data with satellite and numerical model data
Here, we discuss the advantages and caveats of the sea-ice biomarker IPSO 25 and the semi-quantitative sea-ice index PIPSO 25 by comparing the proxy data to satellite and numerical model data. The main ice algae bloom in the Southern Ocean occurs during spring, when temperatures increase, sea ice starts melting, which results in the release of nutrients and stratification of the water column and the increasing solar insulation stimulates the productivity of photosynthesizing organisms (Arrigo, 2017;400 Belt, 2018). The sea-ice biomarker IPSO 25 is hence commonly interpreted as a spring sea-ice indicator, which is why, in the following, we compare the biomarker-based sea-ice reconstructions to satellite-derived spring SIC and modelled spring SIC.

Comparison of proxy-based, modelled and observed sea-ice conditions
Our satellite-derived SIC represent monthly mean (spring) SIC averaged from 1978 to the individual year of sample retrieval. their biomarker data reflect the past two centuries. We hence note that biomarker data from the Antarctic Peninsula, which is 410 affected by a very recent ice loss, may hence overestimate the sea-ice cover and underestimate ocean temperatures. Nonetheless we here correlate the biomarker data with satellite and model data to further investigate the quantitative significance of the sea ice proxy (Fig. 7). Following Esper and Gersonde (2014), who, assuming a non-linear response of sea-ice diatom productivity to sea-ice dynamics, propose the usage of a polynomial regression instead of a linear correlation, we here use a polynomial regression (third degree). 415 IPSO 25 concentrations in the surface sediments around the Antarctic Peninsula exhibit similar trends as the satellite-derived and modelled SIC, while they differ significantly in the Amundsen and Weddell Seas, where high SIC are revealed by satellites and the model but IPSO 25 is very low concentrated. The relatively low IPSO 25 concentrations in these areas highlight the uncertainty when considering IPSO 25 as a sea-ice proxy alone, since such low concentrations are not only observed under open water conditions, but also under a severe sea-ice cover. In this case, the low concentrations of IPSO 25 are the result of the 420 latter, where limited light availability hinders ice algae growth, leading to an underestimation of sea-ice cover. As a result, IPSO 25 and satellite/model data show low correlations (R² = 0.19/R 2 = 0.16; Fig. 7a+c), requiring caution when interpreting IPSO 25 as a sea-ice proxy alone. As stated in earlier sections, the combination of IPSO 25 and a phytoplankton marker may prevent this ambiguity. The perennial sea-ice cover in the Amundsen and Weddell Seas is better represented by the P Z IPSO 25 values than by the sea-ice proxy alone. However, we note that at the southern sampling sites, the PIPSO 25 index may not be 425 able to further resolve/detail sea-ice concentrations higher than 50 % reasonably well (see Fig. S3). This may be an indicator for a threshold (here ~ 50 % SIC) where the growth of the HBI triene and IPSO 25 producing algae is limited.
In general, however, the P Z IPSO 25 values correlate much better with satellite/modelled SIC (R² = 0.78/R 2 = 0.76; Fig. 7b+d) than IPSO 25 concentrations. For correlations of satellite/model data with PIPSO 25 calculated using the HBI E-triene, brassicasterol and dinosterol, respectively, we refer the reader to Fig. S4. There are, however, also limitations in the semi-430 quantitative sea-ice index PIPSO 25 , that need to be considered when interpreting this approach. A drawback may appear when the concentrations of the sea-ice proxy IPSO 25 and the phytoplankton marker are both low (due to unfavourable conditions for both ice algae as well as phytoplankton) or high (due to a significant seasonal shift in sea-ice cover and/or stable ice edge conditions), which may lead to similar PIPSO 25 values, although the sea-ice conditions are completely different from each other. This scenario was detected in five samples from the Weddell Sea (PS111/13-2, /15-1, /16-3, /29-3; /40-2; Fig. 3b+c), 435 where IPSO 25 and the HBI Z-triene concentrations are close to the detection limit, while P Z IPSO 25 values are very low, suggesting a reduced sea-ice cover. Satellite and model data, however, show that these sample locations are influenced by heavy, perennial sea-ice conditions. We conclude that biomarker concentrations of both biomarkers at or close to the detection limit, indicative of a severe ice cover, need to be treated with caution. As mentioned above, we assigned a maximum P Z IPSO 25 value of 1 to these samples and we note that such practice always needs to be made clear when applying the PIPSO 25 approach. 440 The coupling of IPSO 25 with a phytoplankton marker, nonetheless, provides the more robust and reliable sea-ice reconstructions. Regarding the above-mentioned ambiguities, we recommend to not only calculate the PIPSO 25 index, but also consider individual biomarker concentrations and, if possible, take other sea-ice measures, such as satellite data and/or wellpreserved diatom assemblage data (Lamping et al., 2020;Vorrath et al., 2019;2020) into account.

Temperature reconstructions 445
Concerning the different time frames covered by the proxy data and instrumental observations, caution must be taken when comparing GDGT-derived ocean temperature reconstructions (spanning decades to centuries) with modelled SSTs (spanning https: //doi.org/10.5194/cp-2021-19 Preprint. Discussion started: 1 March 2021 c Author(s) 2021. CC BY 4.0 License. ~ 63 years) and modern SSTs based on the WOA13 (covering the time period from 1955-2012; ~ 57 years). Since it is still not fully understood whether GDGT-based temperature reconstructions represent SSTs, near-surface or sub-surface ocean temperatures (Kalanetra et al., 2009;Kim et al., 2012;Park et al., 2019) we here refer to ocean temperatures. 450 GDGT-derived temperatures, annual mean SSTs (depicted by the WOA13) and modelled annual mean SSTs (Fig. 5a-d) show similar patterns at the Antarctic Peninsula and the Amundsen and Weddell Seas. TEX L 86 values correlate reasonably well with modelled annual mean SSTs (R 2 = 0.71; Fig. 8a). Following Park et al. (2019), stating that TEX L 86 values in the Southern Ocean have a polynomial correlation with modern annual SSTs, we here also used a polynomial correlation. As mentioned in Sect. 4.1.3, the temperatures are, however, significantly warm-biased (up to ~ 7 °C higher), which may speculatively be 455 attributable to GDGT contributions of Euryarchaeota in CDW of the Antarctic Polar Front (Park et al., 2019). These findings are supported by a study by Spencer-Jones et al. (2020) in review, who found that GDGTs may be actively synthesized at CDW depths in the Amundsen Sea. They may hence be a significant source of isoprenoidal GDGTs within the sediments in that area.
The correlation with RI-OH'-derived temperatures is slightly lower (R 2 = 0.46; Fig. 8b), the temperature ranges of RI-OH' 460 are, however, much more realistic (from -3 to 5 °C), supporting the study by Park et al. (2019). The addition of OH-isoGDGTs in the temperature index is a promising step towards high latitude temperature reconstructions and may improve our understanding of the temperature responses of archaeal membranes in Southern Ocean waters (Fietz et al., 2020;Park et al., 2019). Clearly, more data -ideally obtained from sediment traps, surface samples as well as longer sediment cores -and calibration studies will help to further elucidate the applicability of this approach. 465

The role of platelet ice for IPSO 25 production
Platelet ice formation plays an important role in sea-ice generation along some coastal regions of Antarctica (Hoppmann et al., 2015;2020;Lange et al., 1989;Langhorne et al., 2015). In these regions, High Saline Shelf Water (HSSW) flows into sub-ice shelf cavities of Antarctica´s continental shelves, initiating basal melt of the adjacent ice shelves (Fig. 9). The surrounding water is cooled and freshened and is then transported towards the surface, where the pressure relief can cause this water, called 470 Ice Shelf Water (ISW), to be supercooled (Foldvik and Kvinge, 1974). The temperature of the supercooled ISW is potentially below the in-situ freezing point, which may eventually cause the formation of ice platelets that accumulate under landfast ice attached to adjacent ice shelves ( Fig. 9; Holland et al., 2007;Hoppmann et al., 2015;2020).
The sympagic, tube-dwelling, diatom B. adeliensis is a common constituent of Antarctic sea ice, preferably flourishing in the relatively open channels of sub-ice platelet layers in near-shore locations covered by fast ice (Medlin, 1990;Riaux-Gobin and 475 Poulin, 2004). Based on investigations of sea-ice samples from the Southern Ocean, Belt et al. (2016) detected this diatom species to be a source of the HBI diene IPSO 25 , which, according to its habitat, led to the assumption of the sea-ice proxy being a potential indicator for the presence of platelet ice. As stated above, B. adeliensis is not confined to platelet ice, but is also observed in bottom ice and described as well adapted to changes in the texture of sea ice during ice melt (Riaux-Gobin et al., 2013). 480 In an attempt to elucidate the relationship of IPSO 25 and platelet ice more clearly, we here regard our data in connection to observed platelet ice occurrences.
Elevated IPSO 25 concentrations in front of the Larsen Ice Shelves at the East Antarctic Peninsula could be linked to several processes. So far, it is hard to differentiate between in-situ IPSO 25 production or allochthonous input of IPSO 25 from drift ice.
According to Langhorne et al. (2015), sea-ice cores retrieved from that area did not incorporate platelet ice. The high IPSO 25 485 concentrations could hence be explicable by either drift ice by the Weddell Gyre or by bottom ice production. We do, however, note that our samples reflect much longer time frames than the sea-ice samples investigated by Langhorne et al. (2015). The https: //doi.org/10.5194/cp-2021-19 Preprint. Discussion started: 1 March 2021 c Author(s) 2021. CC BY 4.0 License. lack of platelet ice observed in their investigated sea-ice cores does not rule out the former presence of platelet ice, which may be captured in our investigated sediment samples, covering a much longer time interval.
There are several previous studies on IPSO 25 which report a close connection of the proxy to proximal, coastal locations and 490 polynyas in the seasonal ice zone (i.e., Collins et al., 2013;Smik et al., 2016). They do not, however, discuss the relation to adjacent ice shelves as possible "platelet ice factories". We note that the core locations investigated by Smik et al. (2016) are in the vicinity of the Moscow University Ice Shelf, where Langhorne et al. (2015) did not observe platelet ice within sea-ice cores. Hoppmann et al. (2020), however, report on a sea-ice core from that area, incorporating platelet ice. The different observations by Langhorne et al. (2015) and Hoppmann et al. (2020) show how variable the occurrence of platelet ice can be. 495 The absence of IPSO 25 in the Amundsen Sea ( Fig. 3b; AS) might in turn be explicable by the absence of platelet ice in that region. The Amundsen Sea shelf is classified as a warm shelf (Thompson et al., 2018) and characterized by the upwelling of warm CDW (Schmidtko et al., 2014), hindering the formation of ISW and making the presence of platelet ice in recent conditions highly unlikely (Hoppmann et al., 2020). This theory is also supported by Langhorne et al. (2015), stating that platelet ice formation is not observed, where thinning from basal melting of ice shelves is believed to be greatest, which applies 500 to the warm shelf of the Amundsen Sea (Thompson et al., 2018). Accordingly, if the formation and accumulation of platelet ice -up to a certain degree -is indicative of basal ice shelf melting on fresh shelves (Hoppmann et al., 2015;Thompson et al., 2018), high IPSO 25 concentrations determined in marine sediments may hence serve as indicator of past basal melting processes and associated ice shelf dynamics. This may, however, only be true up to a certain threshold where platelet ice formation is diminished/hampered due to warm oceanic conditions leading to a too intense basal melting (Langhorne et al., 505 2015).
While using IPSO 25 as a sea-ice proxy in Antarctica, it is hence important to also consider regional platelet ice formation processes as these may affect the IPSO 25 budget. Determining thresholds associated with platelet ice formation is challenging, therefore, further investigations, such as in-situ measurements of IPSO 25 concentrations in platelet ice or culture experiments in home laboratories are needed to better depict the connection between IPSO 25 and platelet ice formation (and ice shelf basal 510 melting).

Conclusion
Biomarker analyses focusing on IPSO 25 , HBI-trienes, phytosterols and GDGTs, in surface sediment samples from the continental shelves off West Antarctica were investigated to depict recent sea surface and temperature conditions in this climate sensitive region. Proxy-based reconstructions of the sea surface conditions were compared to (1) satellite observations and (2)  515 estimated sea-ice patterns and SSTs deduced from model data. The semi-quantitative sea-ice index PIPSO 25 , combining the sea-ice proxy IPSO 25 with an open-water phytoplankton marker, yielded reasonably good correlations with satellite observations and numerical model results, while correlations with the sea-ice proxy IPSO 25 alone are rather low. Minimum concentrations of both biomarkers, used for the PIPSO 25 calculations, however, may lead to ambiguous interpretations and significant underestimations of sea-ice conditions. The combination of different sea-ice measures when interpreting biomarker 520 data should hence be strived for.
The presumed relationship between IPSO 25 and platelet ice formation in connection to basal melting of ice shelves is supported by our data, showing high IPSO 25 concentrations in areas where platelet ice formation has previously been reported and low IPSO 25 concentrations where no platelet ice formation is occurring. Oceanic conditions and the intensity of basal melting, however, need to be considered when using IPSO 25 as an indirect indicator for basal melting processes and associated ice shelf 525 dynamics. more realistic, when compared to temperature data based on the WOA13 and modelled annual mean SSTs. Further investigations of HBI synthesis, transport, sedimentation and preservation within the sediments as well as the composition of 530 its sources habitat (bottom 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 as a robust sea-ice biomarker.

Data availability
Datasets related to this article can be found online on PANGAEA Data Publisher for Earth & Environmental Science (doi: in prep).

Competing interests 540
The authors declare that they have no conflict of interest.