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: 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: 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

Description: We will present new multibeam bathymetry data that make the Anvers-Hugo Trough west of the Antarctic Peninsula one of the most completely surveyed palaeo-ice stream pathways in Antarctica. We interpret landforms revealed by these data as indicating that subglacial water availability played an important role in facilitating ice stream flow in the trough during late Quaternary glacial periods. Specifically, we observe a set of northward-shoaling valleys that are eroded into the upstream edge of a sedimentary basin, extend northwards from a zone containing landforms typical of erosion by subglacial water flow, and coincide spatially with the onset of mega-scale glacial lineations. Water was likely supplied to the ice stream bed episodically as a result of outbursts from a subglacial lake previously hypothesized to have been located in the Palmer Deep basin on the inner continental shelf. In a palaeo-ice stream confluence area, close juxtaposition of mega-scale glacial lineations with landforms that are characteristic of slow, dry-based ice flow, suggests that water availability was also an important control on the lateral extent of these palaeo-ice streams. These interpretations are consistent with the hypothesis that subglacial lakes or areas of elevated geothermal heat flux play a critical role in the onset of many large ice streams. The interpretations also have implications for the dynamic behaviour of the Anvers-Hugo Trough palaeo-ice stream and, potentially, of several other Antarctic palaeo-ice streams. Keywords: multibeam bathymetry, ice stream, subglacial water, landform

Description: Subglaziale Seen sind unter dem Antarktischen Eisschild weit verbreitet. Sie bilden eine Quelle von subglazialem Schmelzwasser und modulieren weitgehend die Fließgeschwindigkeiten des überlagernden Eisstromes. Der Ausbruch von subglazialem Schmelzwasser am Rande des Eisschildes kann die globale ozeanographische Zirkulation, den Meeresspiegelanstieg und geochemische Zyklen beeinflussen. Trotz ihrer Bedeutung sind subglaziale Seen eine der am wenigsten erforschten Umgebungen auf unserem Planeten. Ihre Auswirkungen auf die Dynamik der Eisbedeckung und ihr Potential primitives aber autarkes Leben zu beherbergen, sind weitgehend schlecht charakterisiert. Wir präsentieren den ersten direkten Beweis für einen paläo-subglazialen See auf dem antarktischen Kontinentalschelf. Sedimentablagerungen beweisen, dass subglaziales Schmelzwasser während oder kurz nach der letzten Eiszeit unter dem Antarktischen Eisschild vorhanden war und erst bei der Enteisung entleert wurde. Im Pine Island Bay (Amundsen Sea Embayment, ASE) haben wir zum ersten Mal in der Antarktis mehr als drei Meter dieser typischen Sedimentfazies, die auf wenig dynamische Ablagerungsbedingungen in einem subglazialen See schließen lassen, beprobt. Niedrige Chloridkonzentrationen im Porenwasser des Sedimentkernes und diffus-advektive Modellierung der zeitlichen Entwicklung dieser Chloridkonzentrationen im Porenwasser zeigen eindeutig die Genese der Sedimente in einem subglazialen, schmelzwassergefüllten See. Basierend auf bathymetrischen Daten können wir zeigen, dass die Lage des subglazialen Sees mit einer prognostizierten Verteilung von subglazialen Seen in diesem Gebiet übereinstimmt. Dieser Befund ermöglicht Modellierungsstudien der Eisdynamik bei bekannter Geometrie des Eisuntergrundes, des Volumens der subglazialen Seen, der Eigenschaften des unterlagernden Substrates und der Form des Eisstroms. Dies ist besonders wichtig für den Pine Island Gletscher, der als "Weak Underbelly" des Westantarktischen Eisschildes (WAIS) gilt. Bisher wurde nur die Eis/Wasser Grenzfläche im subglazialen Wostoksee und wenige Zentimeter diamiktischen Sedimentes vom Whillans-See beprobt. Ansonsten wurde das antarktische subglaziale hydrologische System durch Fernerkundung und numerische Modelle rekonstruiert und die Anzahl der potenziellen subglazialen Seen auf mehr als 12000 geschätzt. Unsere Studie liefert nicht nur den ersten Nachweis für eiszeitliche subglaziale Seen und Beprobung dieser Sedimente in der Antarktis, sondern auch einen Rahmen für weitere Untersuchungen dieser einzigartigen subglazialen See-Umgebung in einer kostengünstigen Variante durch Schiffsexpeditionen auf dem saisonal eisfreien inneren Kontinentalschelf.

Description: The MARUM-MeBo (abbreviation for Meeresboden-Bohrgerät, the German expression for seafloor drill rig) is a robotic drilling system that is developed since 2004 at the MARUM Center for Marine Environmental Sciences at the University of Bremen in close cooperation with Bauer Maschinen GmbH and other industry partners. The MARUM-MeBo drill rigs can be deployed from multipurpose research vessel like, RV MARIA S. MERIAN, RV METEOR, RV SONNE and RV POLARSTERN and are used for getting long cores both in soft sediments as well as hard rocks in the deep sea. The first generation drill rig, the MARUM-MeBo70 is dedicated for drilling depths of more than 70 m (Freudenthal and Wefer, 2013). Between 2005 and 2017 it was deployed on 18 research expeditions and drilled more than. 3 km into different types of lithologies including carbonate and crystalline rocks, gas hydrates, sands and gravel, glacial till and hemipelagic mud with an average recovery rate of 67 %. In February and March 2017 the MeBo70 was used on the West Antarctic continental shelf in the Amundsen Sea Embayment for the first time. The goal of the deployment on RV Polarstern expedition PS104 was to recover a series of sediment cores from different ages that will provide material for investigating the glaciation history of this area known as the most dynamic drainage area of the West Antarctic Ice Sheet. In this presentation we will focus on the operational experiences of this first deployment of a multi-barrel sea floor drill rig on the Antarctic continental shelf. References: Freudenthal, T and Wefer, G (2013) Drilling cores on the sea floor with the remote-controlled sea floor drilling rig MeBo. Geoscientific Instrumentation, Methods and Data Systems, 2(2). 329-337. doi:10.5194/gi-2-329-2013

Description: It is becoming increasingly apparent that bathymetry plays a crucial role in determining the behavior of marine-terminating glaciers. This is because variations in the shape of the bed can produce both pinning points where glaciers (or their floating tongues) can ground and stabilize, as well as pathways for warm waters to move across the shelf and access the grounding line. Ahead of the first ITGC field season we present the existing state of knowledge about the bed in front of Thwaites Glacier (TG). We have compiled existing multibeam-bathymetric datasets from the UK, the USA and international partners (Korea, Germany) to produce a high-resolution grid (50-m cells) for the area. From this grid we identify possible pathways for warm Circumpolar Deep Water to the TG grounding line, a topographic high – as shallow as 130 m in places - that likely acted as a pinning point and is less than 18 km from the current eastern ice-shelf margin, and landforms indicative of the past behavior of the glacier (e.g. meltwater channels and basins, streamlined landforms). This exercise also highlights important data gaps to target for surveying in 2019, including for example, the area left vacant by the calving of the B-22 iceberg. Secondly, we explore existing sub-bottom and seismic-reflection profiles from the Amundsen Sea Embayment to investigate the nature of the substrate in front of TG. Unlithified sediment cover is generally thin (〈5 m) over scoured crystalline bedrock but thickens to up to 40 m in basins. We discuss potential coring targets close to pathways for warm water incursions, and former stability points including the possibility of unknown basins in front of TG.