Description: The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

Description: The SCAR expert group on the International Bathymetric Chart of the Southern Ocean (IBCSO) was inaugurated in 2004. IBCSO is a regional mapping project of the General Bathymetric Chart of the Ocean (GEBCO) under the joint auspices of the Intergovernmental Oceanographic Commission (IOC) (of UNESCO) and the International Hydrographic Organization (IHO). The project aim was to create the first seamless bathymetric compilation for the entire Southern Ocean south of 60°S. In 2013, finally the first Version of IBCSO was published in the Journal Geophysical Research Letters (Arndt et al., 2013). IBCSO Version 1.0 is a prime example for an international collaboration in Antarctic Science. Over 30 institutions from 15 countries contributed data and shared their expertise to generate the, so far, largest database of bathymetric data of the Southern Ocean. From this database a digital bathymetric model (DBM) was produced. The DBM covers the entire Antarctic Treaty area in a resolution of 500 m. It is available in several formats and projections. In addition, a new map has been created of the Southern Ocean and Antarctica and now is also available to the SCAR community. Both, the DBM and the map, can be downloaded free of charge from the IBCSO web site (www.ibcso.org). In my poster presentation I will present the map product of the IBCSO project and give information about its included data sets, its distribution and its design.

Description: The SCAR expert group on the International Bathymetric Chart of the Southern Ocean (IBCSO) was inaugurated in 2004. IBCSO is a regional mapping project of the General Bathymetric Chart of the Ocean (GEBCO) under the joint auspices of the Intergovernmental Oceanographic Commission (IOC) (of UNESCO) and the International Hydrographic Organization (IHO). The project aim was to create the first seamless bathymetric compilation for the entire Southern Ocean south of 60°S. In 2013, finally the first Version of IBCSO was published in the Journal Geophysical Research Letters (Arndt et al., 2013). IBCSO Version 1.0 is a prime example for an international collaboration in Antarctic Science. Over 30 institutions from 15 countries contributed data and shared their expertise to generate the, so far, largest database of bathymetric data of the Southern Ocean. From this database a digital bathymetric model (DBM) was produced. The DBM covers the entire Antarctic Treaty area in a resolution of 500 m. It is available in several formats and projections. In addition, a new map has been created of the Southern Ocean and Antarctica and now is also available to the SCAR community. Both, the DBM and the map, can be downloaded free of charge from the IBCSO web site (www.ibcso.org). In my presentation I will give an overview of the IBCSO V1.0 compilation methods and highlight the improvements of the IBCSO DBM compared to global datasets. Furthermore, some tips and hints for the usage of IBCSO including the use of the Source Identifier grid (SID) will be given.

Description: The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 is a new digital bathymetric model (DBM) portraying the seafloor of the circum-Antarctic waters south of 60° S. IBCSO is a regional mapping project of the General Bathymetric Chart of the Oceans (GEBCO). IBCSO Version 1.0 DBM has been compiled from all available bathymetric data collectively gathered by more than 30 institutions from 15 countries. These data include multibeam and single beam echo soundings, digitized depths from nautical charts, regional bathymetric gridded compilations, and predicted bathymetry. Specific gridding techniques were applied to compile the DBM from the bathymetric data of different origin, spatial distribution, resolution, and quality. The IBCSO Version 1.0 DBM has a resolution of 500 x 500 m, based on a polar stereographic projection, and is publicly available together with a digital chart for printing from the project website (www.ibcso.org) and at http://dx.doi.org/10.1594/PANGAEA.805736

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: 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: 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.

Description: The coastal bathymetry of Thwaites Glacier (TG) is poorly known yet nearshore sea-floor highs have the potential to act as pinning points for floating ice shelves, or to block warm water incursions to the grounding line. In contrast, deeper areas control warm water routing. Here, we present more than 2000 km2 of new multibeam echo-sounder data (MBES) acquired offshore TG during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project on the RV/IB Nathaniel B. Palmer (NBP19-02) in February-March 2019. Beyond TG, the bathymetry is dominated by a 〉1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some sea-floor highs having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf. In addition, submarine landform evidence indicates at least some unconsolidated sediment cover on the highs, as well as in the troughs that separate them. Knowing that this offshore area of ridges and troughs is a former bed for TG, we also used a novel spectral approach and existing ice-flow theory to investigate bed roughness and basal drag over the newly-revealed offshore topography. We show that the sea-floor bathymetry is a good analogue for extant bed areas of TG and that ice-sheet retreat over the sea-floor troughs and ridges would have been affected by high basal drag similar to that acting in the grounding zone today. Comparisons of the new MBES data with existing regional compilations show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the oceanic heat flux may be significantly underestimated. This work supports the findings of recent numerical ice-sheet and ocean modelling studies that recognise the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.