Description: Previous reconstructions of ice-sheet changes in Antarctica’s Weddell Sea sector since the Last Glacial Maximum (LGM) at 19–23 cal. (calibrated) kyr B.P. suffered from large uncertainties and were partly contradictory. As a consequence, the contribution of this sector to the LGM sea-level lowstand and post-LGM sea-level rise was unclear. Furthermore, whether and how precursor water masses for Antarctic Bottom Water (AABW) were formed in the Weddell Sea Embayment under glacial conditions is unknown, as this today requires the existence of the floating Filchner-Ronne Ice Shelf. Here we present new marine geophysical and marine geological data from the outer shelf section of the Filchner paleo–ice stream trough documenting that grounded ice had advanced onto and retreated from the outer shelf prior to 27.5 cal. kyr B.P., i.e., >4500 yr before the LGM. The data reveal the presence of a stacked grounding-zone wedge (GZW) just south of 75°30′S. This GZW was formed during two episodes of grounding-line re-advance onto the outer shelf after 11.8 cal. kyr B.P., with data further inshore implying paleo–ice stream retreat from the GZW location prior to 8.7 cal. kyr B.P. Our findings show that (1) ice-sheet buildup in the Weddell Sea sector made only limited contributions to the LGM sea-level lowstand, (2) ice-ocean interaction below an ice shelf in outer Filchner Trough could have contributed to AABW production at the LGM, and (3) numerical models need to take into account a highly dynamic ice-sheet behavior in regions of the West Antarctic Ice Sheet and East Antarctic Ice Sheet confluence.

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: Amongst the geological community there is now consensus that the West Antarctic ice sheet reached outer continental shelf positions in most sectors during its last maximum extent. This conclusion is mainly formed on the basis of extensive palaeo-glaciological work that has been conducted during the past decade in various drainage sectors of the ice sheet. In these regions of the continental shelf, reconstructions of ice-sheet extent, relative flow velocities and retreat have mainly focused on deeper cross-shelf depressions, generally interpreted as pathways of fast-flowing palaeo-ice streams. Within these troughs, the prevalent glacial landforms consist of continuums of streamlined bedforms (e.g. mega-scale glacial lineations and drumlins) whose progressive increase in elongation is indicative of formation beneath wet-based streaming ice. However, extensive shelf areas surrounding these spatially restricted deeper corridors largely lack well-preserved glacial landforms, thereby inhibiting reliable shelf-wide ice-flow reconstructions. Recently, by analysing new high-resolution bathymetric data from these inter-ice stream ridges, former basal ice conditions could be illuminated for the first time. Subglacial landforms such as hill-hole pairs, sediment rafts and crevasse-squeeze ridges clearly indicate cold/dry-based, slow-flowing or even stagnant ice on the shelf areas outside the troughs. Here we present a new compilation of bathymetric datasets from the eastern Amundsen Sea Embayment covering both palaeo-ice stream troughs and adjacent inter-ice stream ridges. It allows a clear distinction between different basal ice regimes (dry-based vs wet-based) across the former ice-sheet bed. However, we additionally identified regions of apparent changes in basal conditions, i.e. regions where hill-hole pairs overprint mega-scale glacial lineations, thereby suggesting cold/dry-based basal conditions prior to final retreat rather than streaming flow. Generally, the ability to reconstruct the width and basal form of the West Antarctic ice sheet in this manner, across other parts of the Antarctic shelf, will greatly aid numerical ice-sheet models that aim to simulate past configurations of the ice sheet and its evolution to the present day.