Description: In recent years several previously undiscovered grounding-zone wedges (GZWs) have been described within the Abbot-Cosgrove palaeo-ice stream trough on the easternmost Amundsen Sea Embayment shelf. These GZWs document both the Last Glacial Maximum (LGM; 26.5-19 cal. ka BP) grounding-line extent and the subsequent episodic retreat within this trough that neighbors the larger Pine Island-Thwaites trough to the west. Here we combine bathymetric, seismic, and geologic data showing that 1) the grounding line in Abbot Trough did not reach the continental shelf break at any time during the last glacial period, and 2) a prominent stacked GZW constructed from six individual wedges lying upon another was deposited 100 km upstream from the LGM grounding-line position. The available data allow for calculating volumes for most of these individual GZWs and for the entire stack. Sediment cores were recovered seawards from the outermost GZW in the trough, and from the individual wedges of the stacked GZW in order to define the LGM grounding-line extent, and provide minimum grounding-line retreat ages for the respective positions on the stacked GZW. We present implications of a grounded-ice free outer shelf throughout the last glacial period. Furthermore, we assess the significance of the grounding-line stillstand period recorded by the stacked GZW in Abbot Trough for the timing of post-LGM retreat of the West Antarctic Ice Sheet from the Amundsen Sea Embayment shelf.

Description: Subglacial meltwater largely facilitates rapid but nonlinear ice flow beneath concurrent ice streams, and there is widespread evidence for a dynamic subglacial water system beneath the Antarctic Ice Sheet. It steers and affects the pattern of ice flow and is a direct result of boundary processes acting at the ice sheet bed, i.e. pressure-induced basal melting. Consequently, the occurrence of subglacial meltwater plays an important role in bedrock erosion, subsequent re-deposition, and in shaping the topography of ice-sheet beds. Here we present new geological, geophysical, and geochemical data from sediment cores recovered from the continental shelf in Pine Island Bay. We interpret the data as reliably indicating palaeo-subglacial lake deposition beneath the formerly expanded West Antarctic Ice Sheet, presumably during and/or subsequent to the Last Glacial Maximum (LGM) (Kuhn et al. 2017). Characteristic changes of sedimentary facies and geochemical profiles within these cores that were recovered on RV Polarstern expeditions ANT-XXIII/4 (2006) and ANT-XXVI/3 (2010), support the presence of an active and expanded subglacial lake system in at least five basins. The basins, which also have been targeted by sediment coring during the recent RV Polarstern cruise PS104 (2017), had been carved into bedrock over previous glacial cycles and were then filled with several meters of sediments. These findings have important implications for palaeo ice-sheet dynamics, suggesting the presence of considerable amounts of water lubricating the ice-bed interface, eventually leading to the subglacial deposition of lake sediments and water-saturated soft tills. Based on our recent findings, we conclude that the transition from the subglacial lake to an ocean-influenced environment took place during deglaciation at the transition from the LGM to the Holocene. We suggest that the ice sheet thinned and the subglacial lake basins successively transformed to sub-ice cavities, flushed by tidal currents at this time. We will present estimates of ice thickness for buoyancy at the grounding line for the time when the grounding line retreated landward across the rim of the subglacial lake. These estimates are based on the bathymetric setting, a glacial isostatic adjustment model, a global sea level curve, and the available chronological information. Our findings have implications for ice sheet models, which need to consider the predominantly non-linear effects related to subglacial hydrology. Keywords: West Antarctic Ice Sheet (WAIS), Sub-Ice processes, Deglaciation processes References Kuhn, G., Hillenbrand, C.-D., Kasten, S., Smith, J. A., Nitsche, F. O., Frederichs, T., Wiers, S., Ehrmann, W., Klages, J. P., Mogollón, J. M., 2017. Evidence for a palaeo-subglacial lake on the Antarctic continental shelf. Nature Communications, 8, 15591.

Description: One of the major questions in palaeoclimate research is whether or not the West Antarctic Ice Sheet (WAIS) collapsed during the warmest phase of the last interglacial period (Marine Isotope Stage 5e; 130-116 ka). Several numerical models and sea-level reconstructions suggest such a collapse, but critically it has not been confirmed by WAIS proximal empirical data, yet. Answering this question by analyzing sedimentary sequences from the West Antarctic shelf requires areas that remained unaffected by 1) erosion by grounded ice, and 2) scouring by iceberg keels since MIS 5e. During RV Polarstern Expedition ANT-XXVI/3 in early 2010 we discovered such an area on the Amundsen Sea shelf for the first time. We demonstrate that the outermost section of the Cosgrove-Abbot palaeo-ice stream trough in the easternmost Amundsen Sea Embayment was not covered by grounded ice during the last glacial period, and that a part of this area was largely protected from iceberg scouring by its large water depth and a grounding zone wedge located further landward. We present combined geophysical and geological data revealing this unique location at which the state of the WIAS during the last peak interglacial may be reconstructed, and give an outlook on how this could be achieved.

Description: Subglacial meltwater facilitates rapid ice flow beneath concurrent ice sheets, and there is widespread evidence for a dynamic subglacial water system beneath the Antarctic ice sheet. It steers and affects the pattern of ice flow and is a direct result from boundary processes acting at the base of the ice sheet, i.e. pressure-induced basal melting. Consequently, the occurrence of subglacial meltwater plays an important role in bedrock erosion, subsequent resedimentation, and in shaping the topography of ice-sheet beds. Here we present new geological and geochemical data from sediments recovered on the West Antarctic continental shelf in Pine Island Bay that we interpret as reliable indicators for deposition in a palaeo-subglacial lake beneath the formerly expanded West Antarctic ice sheet, presumably during or following the Last Glacial Maximum. Characteristic changes of sedimentary facies and geochemical profiles within these cores taken on RV Polarstern expeditions ANTXXIII/ 4 (2006) and ANT-XXVI/3 (2010) support the presence of an active subglacial lake system during the late stages of the last glacial period. These findings have important implications for palaeo ice-sheet dynamics, suggesting there was considerable water available to lubricate the bedrock–ice interface and deposit water-saturated subglacial sediments (soft tills). Based on our investigations performed so far, we suggest that the transition from subglacial lake to contact with the ocean took place in the early Holocene. During this time we speculate that the ice sheet thinned and successively transformed into an ice shelf with sub-ice cavities flushed by tidal currents. Based on bathymetric maps and relative sealevel curves we will aim to estimate ice thickness as the grounding line retreated across the subglacial lake threshold further inland. Our findings may also have implications for ice-sheet models, which have to consider the predominantly non-linear effects related to subglacial hydrology.

Description: Subglacial meltwater facilitates rapid ice flow beneath concurrent ice sheets, and there is widespread evidence for a dynamic subglacial water system beneath the Antarctic Ice Sheet. It steers and affects the pattern of ice flow and is a direct result from boundary processes acting at the base of the ice sheet, i.e. pressure induced basal melting. Consequently, the occurrence of subglacial meltwater plays an important role in bedrock erosion, subsequent resedimentation, and in shaping the topography of icesheet beds. Here we present new geological and geochemical data from sediments recovered on the West Antarctic continental shelf in Pine Island Bay that we interpret as reliable indicators for deposition in a palaeosubglacial lake beneath the formerly expanded West Antarctic Ice Sheet, presumably during or following the Last Glacial Maximum. Characteristic changes of sedimentary facies and geochemical profiles within these cores taken on RV Polarstern expeditions ANTXXIII/ 4 (2006) and ANTXXVI/ 3 (2010) support the presence of an active subglacial lake system during the late stages of the last glacial period. These findings have important implications for palaeo icesheet dynamics, suggesting there was considerable water available to lubricate the bedrockice interface and deposit water saturated subglacial sediments (soft tills). Based on our investigations performed so far, we suggest that the transition from subglacial lake to contact with the ocean took place in the early Holocene. During this time, we speculate, that the ice sheet thinned and successively transformed into an ice shelf with subice cavities flushed by tidal currents. Based on bathymetric maps and relative sea level curves we will aim to estimate ice thickness as the grounding line retreated across the subglacial lake threshold further inland. Our findings may also have implications for ice sheet models, which have to consider the predominantly nonlinear effects related to subglacial hydrology.

Description: High-resolution swath bathymetry data collected during several research cruises over the past two decades reveal a palaeo-ice stream trough (Abbot Glacial Trough) crossing the middle and outer shelf of the easternmost Amundsen Sea Embayment, east of the main Pine Island Trough. Regions of both fast palaeo-ice flow (within the central trough) and slow palaeo-ice flow (on adjacent seafloor highs referred to as inter-ice stream ridges) bear glacial landforms indicative of phases of grounding-line stabilization of the ice sheet. We associate a grounding-zone wedge situated within the outer Abbot Glacial Trough with a grounding-zone wedge in outer Pine Island Trough and suggest a synchronous grounding-line halt in both troughs. New sediment echosounder and sediment core data collected from outer Abbot Glacial Trough, between the seaward limit of the grounding-zone wedge and the shelf edge, reveal an up to 6 m-thick well stratified drape that is composed of unconsolidated glaciomarine sediments occasionally bearing calcareous microfossils. In order to decipher whether this unusually thick sediment drape might indicate sub-ice shelf and/or seasonal-open marine deposition throughout or since the Last Glacial Maximum, we used a multi-proxy approach to characterize its lithofacies and applied radiocarbon dating of calcareous microfossils. Here we present our initial results and discuss since when the outer shelf in the eastern Amundsen Sea has been free of grounded-ice. Such information will 1) improve ice sheet models that aim to reconstruct the flow and extent of the West Antarctic Ice Sheet during the Last Glacial Maximum, 2) help to quantify the ice volume of the West Antarctic Ice Sheet during this time, and 3) prove or reject the possibility that Antarctic benthic biota endured glacial periods in outer shelf refugia.

Description: Multibeam swath bathymetry datasets collected over the past two decades have been compiled to identify palaeoice stream pathways in the easternmost Amundsen Sea Embayment. We mapped 3010 glacial landforms to reconstruct palaeo-ice flow in the �250 km-long Abbot Glacial Trough that was occupied by a large palaeo-ice stream, fed by two tributaries (Cosgrove and Abbot) that reached the continental shelf edge during the last maximum ice-sheet advance. The mapping has enabled a clear differentiation between glacial landforms interpreted as indicative of wet- (e.g. mega-scale glacial lineations) and cold-based ice (e.g. hill-hole pairs) during the last glaciation of the continental shelf. Both the regions of fast palaeo-ice flow within the palaeo-ice stream troughs, and the regions of slow palaeo-ice flow on adjacent seafloor highs (referred to as inter-ice stream ridges) additionally record glacial landforms such as grounding-zone wedges and recessional moraines that indicate grounding line stillstands of the ice sheet during the last deglaciation from the shelf. As the palaeo-ice stream flowed along a trough with variable geometry and variable subglacial substrate, it appears that trough sections characterized by constrictions and outcropping hard substrate that changes the bed gradient, led the pace of grounding-line retreat to slow and subsequently pause, resulting in the deposition of grounding-zone wedges. The stepped retreat recorded within the Abbot Glacial Trough corresponds well to post-glacial stepped retreat interpreted for the neighbouring Pine Island-Thwaites Palaeo-Ice Stream trough, thus suggesting a uniform pattern of episodic retreat across the eastern Amundsen Sea Embayment. The correlation of episodic retreat features with geological boundaries further emphasises the significance of subglacial geology in steering ice stream flow. Our new geomorphological map of the easternmost Amundsen Sea Embayment resolves the pathways of palaeo-ice streams that were probably all active during the last maximum extent of the ice sheet on this part of the shelf, and reveals the style of postglacial grounding-line retreat. Both are important input variables in ice sheet models and therefore can be used for validating the reliability of these models.

Description: Marine and terrestrial geological and marine geophysical data that constrain deglaciation since the Last Glacial Maximum (LGM) of the sector of the West Antarctic Ice Sheet (WAIS) draining into the Amundsen Sea and Bellingshausen Sea have been collated and used as the basis for a set of time-slice reconstructions. The drainage basins in these sectors constitute a little more than one-quarter of the area of the WAIS, but account for about one-third of its surface accumulation. Their mass balance is becoming increasingly negative, and therefore they account for an even larger fraction of current WAIS discharge. If all of the ice in these sectors of the WAIS was discharged to the ocean, global sea level would rise by ca. 2 m. There is compelling evidence that grounding lines of palaeo-ice streams were at, or close to, the continental shelf edge along the Amundsen Sea and Bellingshausen Sea margins during the last glacial period. However, the few cosmogenic surface exposure ages and ice core data available from the interior of West Antarctica indicate that ice surface elevations there have changed little since the LGM. In the few areas from which cosmogenic surface exposure ages have been determined near the margin of the ice sheet, they generally suggest that there has been a gradual decrease in ice surface elevation since pre-Holocene times. Radiocarbon dates from glacimarine and the earliest seasonally open marine sediments in continental shelf cores that have been interpreted as providing approximate ages for post-LGM grounding-line retreat indicate different trajectories of palaeo-ice stream recession in the Amundsen Sea and Bellingshausen Sea embayments. The areas were probably subject to similar oceanic, atmospheric and eustatic forcing, in which case the differences are probably largely a consequence of how topographic and geological factors have affected ice flow, and of topographic influences on snow accumulation and warm water inflow across the continental shelf. Pauses in ice retreat are recorded where there are “bottle necks” in cross-shelf troughs in both embayments. The highest retreat rates presently constrained by radiocarbon dates from sediment cores are found where the grounding line retreated across deep basins on the inner shelf in the Amundsen Sea, which is consistent with the marine ice-sheet instability hypothesis. Deglacial ages from the Amundsen Sea Embayment (ASE) and Eltanin Bay (southern Bellingshausen Sea) indicate that the ice sheet had already retreated close to its modern limits by early Holocene time, which suggests that the rapid ice thinning, flow acceleration, and grounding line retreat observed in this sector over recent decades are unusual in the context of the past 10,000 years.