Description: Measuring temperature and salinity profiles in the world's oceans is crucial to understanding ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker Polarstern has been the platform of numerous CTD (conductivity, temperature, depth instrument) deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD data. In total 131 data sets (1 data set per cruise leg) containing data from 10 063 CTD casts are now freely available at doi:10.1594/PANGAEA.860066. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation, etc.) are described in detail. This compilation is special not only with regard to the quantity but also the quality of the data – the latter indicated for each data set using defined quality codes. The complete data collection includes a number of repeated sections for which the quality code can be used to investigate and evaluate long-term changes. Beginning with 2010, the salinity measurements presented here are of the highest quality possible in this field owing to the introduction of the OPTIMARE Precision Salinometer.

Description: The past ice sheet conditions in the southern Weddell Sea Embayment (WSE) are only poorly known. Studies from this area have led to two contradicting scenarios of maximum ice extent during the Last Glacial Maximum (LGM). The first scenario is mainly based on terrestrial data indicating only very limited ice sheet thickening in the hinterland and suggests a grounding-line position on the inner shelf. The alternative scenario is based on marine geological and geophysical data and concludes that the LGM grounding line was located on the outer shelf, about 650 km further offshore than in the other scenario. Three hypotheses have been brought forward to explain these two apparently contradictory scenarios. A) An ice plain was present on the shelf that enabled a large ice extent while maintaining little ice thickness in the hinterland. B) The maximum grounded ice advance lasted for a short period only and was probably caused by a short-termed touch down of an ice shelf on the outer shelf, which did not cause sufficient ice sheet thickening in the hinterland to be traced today. C) Due to an ice flow switch, Filchner Trough was fed by an area further to the west where ice had thickened at the LGM. Besides the poorly constrained LGM ice extent, studies suggest a complex development of its retreat speed and drainage pattern in succession of the LGM that needs to be further constraint. For example, radar data from ice rises in the southwestern hinterland of the WSE suggest that ice flow switches occurred as late as the Mid-Holocene and cosmogenic exposure ages indicate an early Holocene ice sheet thickness in the Ellsworth Mountains comparable to that of the LGM. We investigated multibeam bathymetry data (ATLAS Hydrosweep DS3), acoustic sub-bottom profiles (ATLAS Parasound P-70) and marine sediment cores collected from Filchner Trough during RV “Polarstern” expedition PS96 in Dec 2015-Feb 2016. Our key finding is a previously unknown stacked grounding zone wedge (GZW) located on the outer shelf. This GZW shows that the Filchner palaeo-ice stream stabilized at this position at least two times. Two sediment cores were recovered seaward of the GZW and on top of the lower part of the GZW, respectively. Radiocarbon dates from these cores indicate that (i) the GZW was formed in the Early Holocene and (ii) grounded ice did not extend seaward of the GZW at the LGM. Hence, our data provide evidence that the grounding line in Filchner Trough experienced dynamic changes in the Holocene and that no linear ice sheet retreat occurred within this trough after the LGM.

Description: Grounding-zone wedges (GZW) have been mapped on the sea floor in various sectors of the formerly glaciated continental shelf around Antarctica. In most cases, these wedges record periods of grounding-line stillstands during ice-sheet retreat following the Last Glacial Maximum (~26-19 ka BP). The presence of GZWs along the axis of a palaeo-ice stream trough therefore indicates episodic retreat of the grounding line from its LGM to modern position. However, information about their internal structure is sparse, and precise chronological constraints for both the onset and the duration of the stillstands they represent are still lacking. Consequently, the role of GZW formation in modulating post-LGM ice-sheet retreat cannot be reliably quantified. This information is vital, however, for calculating reliable retreat rates during the past, which are essential for evaluating and understanding the significance of modern retreat rates, particularly for the rapidly changing Amundsen Sea sector. Here we present a novel combination of swath bathymetric, reflection seismic, and sub-bottom sediment profiler data from a newly discovered stacked GZW in the Cosgrove-Abbot palaeo-ice stream trough in the eastern Amundsen Sea Embayment. In total, six generations of overlapping GZWs were mapped over a distance of ~40 km. We will present first estimates of GZW volumes through integration of the different geophysical datasets. Additionally, we recovered eight sediment cores, sampling most of the individual GZWs within the stack, which may allow us to establish age constraints for each grounding-line retreat episode. Together with the estimated GZW volumes, the ages from sediment cores may also enable the calculation of sediment flux rates at grounding lines, which remain elusive for Antarctic grounding lines. This knowledge will help refine available post-LGM retreat chronologies for the Amundsen Sea Embayment, which, in turn, serve as a basis for validating and improving ice-sheet models in an area where precise simulations of future retreat are urgently needed.

Description: Sea ice is a key component of the climate system as it exerts a strong control on the heat and gas exchange between the ocean and the atmosphere and the formation of dense deepwater driving thermohaline circulation processes. Changes in sea ice cover are also considered to affect the calving rate of icebergs at marine-terminating glaciers. So far, sea ice reconstructions in the Southern Ocean are mainly based on diatom assemblage studies and specific transfer functions permitting even quantitative estimates of sea ice concentrations. The sea ice biomarker IPSO25, a highly branched isoprenoid akin to the well-established Arctic sea ice proxy IP25, offers an additional method for the reconstruction of past sea ice conditions in the Southern Ocean. This may be particularly valuable in areas (or time intervals) where the preservation of diatoms is limited due to silica dissolution. Here we present first results of the new Helmholtz Young Investigator Group PALICE, settled at the Alfred Wegener Institute in Bremerhaven and the University of Bremen. Main objectives of the group are the assessment of past sea ice-ocean-atmosphere (and ice-sheet) interactions and the further evaluation and application of highly branched isoprenoids (including IPSO25) for robust sea ice reconstructions across different time scales. Specifically, direct comparisons between biomarker and diatom data may provide for an evaluation of both approaches. Current focus is on recently collected surface sediments and long cores from the Bransfield Strait, the southern Drake Passage (PS97) and the Amundsen Sea (PS104) as well as on upcoming expeditions in the Weddell Sea (PS111) and the Ross Sea (IODP 374).

Description: The West Antarctic Ice Sheet (WAIS) plays a key role in the global climate system and its collapse could contribute up to 4.3 m of sea-level rise. Mass loss of this marine-based ice sheet is largely caused by ocean-driven melting of ice shelves. This is confimed by modern observational data which show significant glacier thinning and retreat of grounding lines, particularly in the Amundsen Sea area. We here apply an integrated approach to determine provenance of marine sediments, which enables us to trace erosion of different bedrock lithologies, ultimately tied to the location of the eroding ice through time. We present provenance analysis on detrital Holocene seafloor sediments from the Amundsen Sea Embayment as well as from two marine cores PS58/254 (69°19´S, 108°27´W) and PC493 (71°09´S, 119°57´W), located on the continental rise of the Amundsen Sea and covering glacial-interglacial cycles of the past 800 kyrs. We use strontium (Sr) and neodymium (Nd) isotopic compositions of fine terrigenous grains (〈63μm), and 40Ar/39Ar ages on ice-rafted (〉150μm) hornblende and biotite grains. Our Holocene mapping results reveal drainage pathways with distinct signatures in the eastern and western Amundsen Sea Embayment. The western embayment records a homogenous provenance signature, pointing to a local source area in the hinterland, while the eastern embayment shows a range of compositions indicating erosion of the eastern coastal margin and a distinct, but unexposed source lithology under Pine Island Glacier and/or its drainage basin. Systematic isotope variations are detected between glacial and interglacial stages in both downcore records. Core PS58/254 exhibits a radiogenic fingerprint throughout the Late Pleistocene and systematic glacial-interglacial fluctuations in the order of three εNd units. They correlate with physical properties of the sediments (i.e. magnetic susceptibility) and trend towards lower values during interglacials, notably during Marine Isotope Stage (MIS) 5 and MIS 7. Core PC493 exhibits similar radiogenic Nd isotope composition, but a slightly reduced magnitude of glacial-interglacial changes. Detailed analysis of our results will offer a framework for interpreting sediment records from the area, including those from a recent MeBo expedition (PS104) and upcoming IODP expedition 379.

Description: Subglacial lakes are widespread beneath the Antarctic Ice Sheet and as a source for subglacial meltwater they are assumed to modulate ice stream velocity. Further, the evacuation of subglacial meltwater at the ice sheet margin influences ocean circulation and geochemical cycles. However, despite their importance„ subglacial lakes are one of the least explored environments on our planet. As a consequence, their importance for ice sheet dynamics and their ability to harbour life remain poorly characterised. We present the first direct evidence for a palaeo-subglacial lake on the Antarctic continental shelf, document- ing that subglacial meltwater was stored during the last glacial period and evacuated during the subsequent deglaciation. A distinct sediment facies observed in a core recovered from a small bedrock basin in Pine Island Bay, Amundsen Sea, is indicative of deposition within a low-energy subglacial lake setting. Diffusive modelling demonstrates that low chloride concentrations in the pore water of this characteristic sediment facies can only be explained by original deposition in a freshwater setting. We also show that the location of the subglacial lake within a basin on the inner shelf is consistent with the predicted distribution of subglacial lakes based on bathymetric data. This finding will enable future modelling studies to investigate how the geometry and capacity of subglacial lake systems can influence ice dynamics when the substrate and profile of the ice sheet is known – especially in the highly sensitive area known as the "weak underbelly" of the WAIS. With the exception of a direct lake water access at Subglacial Lake Vostok, and some centimetres of sediment retrieval from Subglacial Lake Whillans, the subglacial hydrological system in Antarctica has hitherto mostly been explored using remote sensing and numerical models that suggest the number of potential lake sites to more than 12.000. Our study not only provides first empirical evidence for a palaeo-subglacial lake but also delivers a framework for investigating and refining exploration of these unique subglacial lake environments and their sediments beneath thick contemporary ice sheets. Our approach, however, is easier and cheaper to conduct by using ship borne coring equipment on the seasonal ice-free continental shelf.

Description: In order to understand the growth and retreat of glaciers in response to the glacial-interglacial changes, subglacial marine sedimentary sequences have been studied extensively in the continental shelf areas of the Ross Sea. The purpose is to comprehend the glaciomarine sedimentation change on the continental slope of eastern Pennell-Iselin Bank in the Ross Sea, using three gravity cores (C1, C2, C3) and three box cores (BC1, BC2, BC3) collected from sites (RS14-1, 2, 3), respectively, across the continental slope to the eastern side of the Pennell-Iselin Bank during XXIX° (2014) PNRA expedition (Rosslope Ⅱ project). Several sedimentological (grain size, magnetic susceptibility), elemental (XRF), geochemical (biogenic opal, total organic carbon, total nitrogen, C/N ratios, CaCO3), and isotopic (δ13C and δ15N of organic matter) parameters were measured along sediment cores with AMS 14C dating of bulk sediments. Core-sediments consist mostly of hemipelagic sandy clay or silty clay with scattered IRDs (Ice-Rafted Debris). A comparison of sediment properties between box cores and the top of gravity cores reveals that the loss of sediment during sampling is trivial. Sediment colors of gravity cores alternate between brown and gray downward. Based on the variation patterns of sediment properties, sediment lithology was divided into different units (A and B), and subunits (B1 and B2). AMS 14C dates and sediment properties assign Unit A, Unit B1, and Unit B2 to interglacial, deglacial, and glacial conditions, respectively. Unit A represents the Holocene and interglacial sediments deposited mainly by the suspension settling of biogenic particles with IRDs in the open marine condition. Unit B1 reflects the deglacial sediments with an increase in IRDs showing the transition of sediment properties from Unit B2 to Unit A by the retreat of subglacial ices. Unit B2 is characterized by different sediment properties, mainly supplied by the continuously lateral melt-water plume or distal part of debris flow originating from the front of grounding floes in the subglacial continental shelf under the ice shelf during the glacial period. Thus, Unit B contains mostly reworked and eroded sediments from the continental shelf with scattered IRDs. The influence of subglacial continental shelf sedimentation in terms of melt-water transport and/or distal stage of debris flow was limited as far as to the middle slope areas (Site 2) during the deglacial and glacial periods. The deeper Site 1 remains in seasonally open marine conditions during the glacial period, due to the peaks of biogenic opal and TOC contents. Keywords: sediment property, subglacial activity, continental slope, Ross Sea

Description: The majority of glaciers draining the Antarctic Peninsula Ice Sheet are thinning and retreating rapidly1. It is widely understood that these changes are driven by both a warming ocean and atmosphere. However, there are other mechanisms, including pinning points created by bathymetric highs and a reverse bed gradient, that are thought to have an important control on ice stream behaviour (Weertman, 1974; Jamieson et al., 2012). Our understanding of the interplay between these mechanisms and time-scales over which they are important is currently limited in time to the advent of satellite monitoring. By reconstructing the cause and style of ice stream retreat following the Last Glacial Maximum (LGM; 25-19 ka BP), it is possible to gain a greater insight into the mechanisms which drive glacier retreat (Ó Cofaigh et al., 2014). Sedimentary sequences deposited during the LGM and the subsequent deglaciation on polar continental shelves, provide an important archive of past changes (Ó Cofaigh et al., 2014). Previous studies have typically identified three sediment facies assemblages; sub-glacial, transitional and open marine (Ó Cofaigh et al., 2014; Domack et al., 1988; Smith et al., 2011). Transitional sediment facies are deposited at the grounding line and are often targeted for radiocarbon dating, as they represent the onset of glaciomarine sedimentation following the retreat of grounded ice (Domack et al., 1988; Smith et al., 2014; Heroy et al., 1996). Despite the development of depositional models to help explain the processes occurring at grounding lines (Powell et al., 1995 and 1996), there is still significant uncertainty about the temporal and spatial variations in grounding line sedimentation along and across a palaeo-ice stream trough. Here we use a multi-proxy approach (water content, shear strength, magnetic susceptibility, density, contents of biogenic opal, Total Organic Carbon and CaCO3, grain size distribution and X-radiographs) on marine sediment cores recovered from the Anvers-Hugo Palaeo-Ice Stream Trough (AHT), western Antarctic Peninsula shelf, to identify variability in transitional sediment facies deposited along and across the trough. We discuss possible controls on the variability in transitional sediment facies and how this is related to the rate and style of ice stream retreat. Our data reveal systematic variability in the types and volume of transitional sediments deposited during the last deglaciation of AHT. A detailed analysis of the transitional sediment facies shows that this variability reflects different phases of ice stream behaviour. Large volumes of ice proximal sediment facies recovered seawards of grounding zone wedges are indicative of episodes of grounding line still-stands. Re-advances of the grounding line, concurrent with a shallowing of the reverse bed gradient and a narrowing of the trough, appear to have occurred during the final stages of deglaciation. This is indicated by interlaminated ice-proximal and ice-distal sediment facies within inner shelf cores. Transitional sediment variability additionally captures the evolution of the ice stream during deglaciation, including the formation of a small ice shelf on the inner shelf. Keywords: Antarctic Peninsula, Last Glacial Maximum, ice stream, sediment cores References Cook, A. J., Holland, P. R., Meredith, M. P., Murray, T., Luckman, A. & Vaughan, D. G, 2016. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 353, 283-286. Weertman, J, 1974. Stability of the Junction of an Ice Sheet and an Ice Shelf. Journal of Glaciology, 13, 3-11. Jamieson, S. S. R., Vieli, A., Livingstone, S. J., Cofaigh, C. O., Stokes, C., Hillenbrand, C.-D. & Dowdeswell, J. A, 2012. Ice-stream stability on a reverse bed slope. Nature Geoscience, 5, 799-802. Ó Cofaigh, C., Davies, B. J., Livingstone, S. J., Smith, J. A., Johnson, J. S., Hocking, E. P., Hodgson, D. A., Anderson, J. B., Bentley, M. J., Canals, M., Domack, E., Dowdeswell, J. A., Evans, J., Glasser, N. F., Hillenbrand, C.-D., Larter, R. D., Roberts, S. J. & Simms, A. R, 2014. Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum. Quaternary Science Reviews, 100, 87-110. Domack, E. W. & Harris, P. T, 1998. A new depositional model for ice shelves, based upon sediment cores from the Ross Sea and the Mac. Robertson shelf, Antarctica. Annals of Glaciology, 27, 281-284. 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. Smith, J. A., Hillenbrand, C.-D., Kuhn, G., Klages, J. P., Graham, A. G. C., Larter, R. D., Ehrmann, W., Moreton, S. G., Wiers, S. & Frederichs, T, 2014. New constraints on the timing of West Antarctic Ice Sheet retreat in the eastern Amundsen Sea since the Last Glacial Maximum. Global and Planetary Change, 122, 224-237. Heroy, D. C. & Anderson, J. B, 1996. Radiocarbon constraints on Antarctic Peninsula Ice Sheet retreat following the Last Glacial Maximum (LGM). Quaternary Science Reviews, 26, 3286-3297. Powell, R. D., Dawber, M., McInnes, J. N. & Pyne, A. R, 1996. Observations of the Grounding-line Area at a Floating Glacier Terminus. Annals of Glaciology, 22, 217-223. 1Powell, R. D. & Domack, E, 1995. Modern Glacimarine Environments. In: Glacial Environments, Volume 1 (ed. J Menzies). Butterworth-Heinemann, 445-486.

Description: Modern global change affects not only the polar north but also, and to increasing extent, the southern high latitudes, especially the Antarctic regions covered by the West Antarctic Ice Sheet (WAIS). Consequently, knowledge of the mechanisms controlling past WAIS dynamics and WAIS behaviour at the last deglaciation is critical to predict its development in a future warming world. Geological and paleobiological information from major drainage areas of the WAIS, like the Amundsen Sea Embayment, shed light on the history of the WAIS glaciers. Sediment records obtained from a deep inner shelf basin north of the Getz Ice Shelf document a deglacial warming in three phases. Above a glacial diamicton and a sediment package barren of microfossils that document sediment deposition by grounded ice and below an ice shelf or perennial sea ice cover (possibly fast ice), respectively, a sediment section with diatom assemblages dominated by sea ice taxa indicates ice shelf retreat and seasonal ice-free conditions. This conclusion is supported by diatom-based summer temperature reconstructions. The early retreat was followed by a phase, when exceptional diatom ooze was deposited between 12,000 and 13,000 cal. years B.P. Microscopical inspection of this ooze revealed excellent preservation of diatom frustules of the species Corethron pennatum together with vegetative Chaetoceros, thus an assemblage usually not preserved in the sedimentary record. Sediments succeeding this section contain diatom assemblages indicating rather constant Holocene cold water conditions with seasonal sea ice. The deposition of the diatom ooze can be related to changes in hydrographic conditions including strong advection of nutrients. However, sediment focussing in the partly steep inner shelf basins cannot be excluded as a factor enhancing the thickness of the ooze deposits. It is not only the presence of the diatom ooze but also the exceptional preservation and the species composition of the diatom assemblage, which point to specific scenarios involving e.g. changes in the food web that can be related to warmer surface water temperatures. Such warming of shelf waters may be related with an overshooting Atlantic Meridional Overturning Circulation (AMOC) and strong injection of warmer North Atlantic Deep Water into the Southern Ocean water masses at Termination I. Such finding may highlight the effects of AMOC changes on Antarctic ice shelf extent and coastal ecosystems. Keywords: WAIS, Amundsen Sea Embayment, diatoms, deglacial warming