ALBERT

Description: A major trough (Belgica Trough) eroded by a palaeo-ice stream crosses the continental shelf of the southern Bellingshausen Sea (West Antarctica) and is associated with a trough mouth fan (Belgica TMF) on the adjacent continental slope. Previous marine geophysical and geological studies investigated the bathymetry and geomorphology of Belgica Trough and Belgica TMF, erosional and depositional processes associated with bedform formation, and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments.Here, we present multi-proxy data from sediment cores recovered from the shelf and uppermost slope in the southern Bellingshausen Sea and reconstruct the ice-sheet history since the last glacial maximum (LGM) in this poorly studied area of West Antarctica. We combined new data (physical properties, sedimentary structures, geochemical and grain-size data) with published data (shear strength, clay mineral assemblages) to refine a previous facies classification for the sediments. The multi-proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings: 1) subglacial, 2) proximal grounding-line, 3) distal sub-ice shelf/sub-sea ice, and 4) seasonal open-marine. In the seasonal open-marine facies we found evidence for episodic current-induced winnowing of near-seabed sediments on the middle to outer shelf and at the uppermost slope during the late Holocene.In addition, we obtained data on excess 210Pb activity at three core sites and 44 AMS 14C dates from the acid-insoluble fraction of organic matter (AIO) and calcareous (micro-) fossils, respectively, at 12 sites. These chronological data enabled us to reconstruct, for the first time, the timing of the last advance and retreat of the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet (APIS) in the southern Bellingshausen Sea. We used the down-core variability in sediment provenance inferred from clay mineral changes to identify the most reliable AIO 14C ages for ice-sheet retreat. The palaeo-ice stream advanced through Belgica Trough after not, vert, similar36.0 corrected 14C ka before present (B.P.). It retreated from the outer shelf at not, vert, similar25.5 ka B.P., the middle shelf at not, vert, similar19.8 ka B.P., the inner shelf in Eltanin Bay at not, vert, similar12.3 ka B.P., and the inner shelf in Ronne Entrance at not, vert, similar6.3 ka B.P. The retreat of the WAIS and APIS occurred slowly and stepwise, and may still be in progress. This dynamical ice-sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass-balance changes in the study area. The glacial history of the southern Bellingshausen Sea is unique when compared to other regions in West Antarctica, but some open questions regarding its chronology need to be addressed by future work.

Description: Studies using geophysical techniques have contributed to knowledge of past Antarctic ice masses in a variety of ways. Here we review how geophysical results have helped constrain past ice extents, glacial sediment depocentre locations, the dynamic behaviour of formerly more extensive ice sheets, their basal hydrology, and different styles of retreat. Geophysical methods employed include multibeam echo sounding, sidescan sonar imaging, seismic reflection profiling, sub-bottom acoustic profiling, measurement of physical properties on sediment cores, interpretation of gravity anomalies from shipboard surveys and inversion of satellite altimetry data, and outcrop-scale investigations using a remotely operated vehicle. We consider, however, that the best way to reconstruct past ice masses is to use geophysical methods as part of a multidisciplinary approach. To produce robust reconstructions geophysicists must work closely with marine geologists, geomorphologists, terrestrial Quaternary geologists, ice core researchers, ice sheet modellers, glacial isostatic adjustment modellers and biologists. An important challenge for the future is to integrate insights from past ice sheet changes into ice sheet models, and thus improve constraints on the contribution Antarctic ice sheets will make to sea-level rise in a warming climate.

Description: We present a new, high resolution (300 m) bathymetric grid of the South Orkney Islands and surrounding continental shelf, northeast of the Antarctic Peninsula. The new grid, derived from a compilation of marine echo-sounding data offers significant and demonstrable improvements over previous regional bathymetric representations and helps to visualise the morphology of the shelf in unrivalled detail. With multiple end users (oceanographers, glacial modellers, biologists and geologists) the new compilation forms important baseline information for a range of scientific applications. In particular, due to our limited understanding of glacial history in this region, the new bathymetry grid provides the first detailed insights into past glacial regimes. The continental shelf is dominated by seven glacially eroded troughs, marking the pathways of glacial outlets that once drained a former ice cap centered on the South Orkney Islands. During previous glacial periods, grounded ice extended to the shelf break to the north of the islands. A large, 250 km long sediment depocenter, interpreted as a maximum former ice limit of one or more Cenozoic glaciations, suggests that ice was only grounded to the 300 m contour in the South. Using observations from the new bathymetric grid, we propose a preliminary ice cap reconstruction for maximum glaciation of the South Orkney plateau suggesting an areal ice coverage in the region of 19000 km^2. The Timing of maximum ice extent, number of past advances and pattern of subsequent deglaciation(s) remain uncertain and will require further targeted marine geological and geophysical investigations to resolve.

Description: Paleo-ice stream beds that are exposed today on the West Antarctic continental shelf provide unique archives of conditions at the base of the past ice sheet, that are difficult to assess beneath its modern, extant counterpart. During the last decade, several of these paleo-ice stream beds have been studied in detail to reconstruct the extent of the West Antarctic Ice Sheet (WAIS) at the Last Glacial Maximum (LGM), the patterns of ice drainage, and the timing of grounding-line retreat during the last deglaciation. However, despite significant advances, such information still remains poorly constrained in numerous drainage sectors of the WAIS. In particular, the maximum extent of ice at the LGM remains ambiguous for key drainage basins of the ice sheet. Whether the WAIS extended to the shelf break around the continent, or advanced only partially across its sea bed, is a crucial piece of information required for reconstructing and modeling patterns of ice-sheet change from past to present. Here we present marine geological and geophysical data that we collected on R/V “Polarstern” expedition ANT-XXVI/3 in early 2010 to investigate the extent, flow, and retreat of the WAIS, from an especially poorly studied part of the West Antarctic shelf, offshore from the Hobbs Coast in the western Amundsen Sea. Here, a landward deepening paleo-ice stream trough is incised into the shelf. The seafloor within the western-central part of the trough is characterized by a large trough-wide grounding zone wedge, ~70 m thick and ~17 km long, which overlies a high of seaward dipping sedimentary strata. The back-slope of the GZW is characterized by highly elongate streamlined bedforms suggesting fast paleo-ice flow towards NW. The crest of the wedge has been cross-cutted by iceberg keels. In contrast, the outer shelf seafloor offshore the GZW is predominantly smooth and featureless, although there is some evidence locally for iceberg scouring. A radiocarbon age from calcareous microfossils in a core from the inner shelf shows that ice had retreated landward from the GZW before 12.967 cal 14C yrs. There are two possible interpretations for the GZW: either (1) that it formed during a significant stillstand as the WAIS retreated from the shelf edge following the LGM, or (2) it marks the maximum extent of grounded ice at the LGM. Preliminary data appears to support the latter hypothesis. Specifically we point to i) the size and geometry of the GZW which is comparable to other grounding-line features marking LGM-positions around Antarctica (e.g. in the Ross Sea and in Prydz Bay), and less similar to GZWs deposited during episodic ice-stream retreat (e.g. in Pine Island Trough and Marguerite Trough); and ii) the lack of subglacial bedforms on the outer shelf, which may be explained by a thick hemipelagic sediment cover deposited over tens of thousands of years. In order to test these two hypotheses and constrain the timing and duration of GZW formation, additional ages will be obtained from seasonal open-marine sediments overlying subglacial till in cores seaward of the GZW as well as from the outer shelf. We will present preliminary interpretations of these data, which will aim to resolve the extent of the WAIS in this sector and, at the same time, provide new information on the dynamics of paleo-ice streams, which drained the former ice sheet.

Description: Inter-ice stream areas cover significant portions of Antarctica's formerly glaciated shelves, but have been largely neglected in past geological studies because of overprinting by iceberg scours. Here, we present results of the first detailed survey of an inter-ice stream ridge from the West Antarctic continental shelf. Well-preserved sub- and proglacial bedforms on the seafloor of the ridge in the eastern Amundsen Sea Embayment (ASE) provide new insights into the flow dynamics of this sector of the West Antarctic Ice Sheet (WAIS) during the Last Glacial cycle. Multibeam swath bathymetry and PARASOUND acoustic sub-bottom profiler data acquired across a mid-shelf bank, between the troughs of the Pine Island-Thwaites (PITPIS) and Cosgrove palaeo-ice streams (COPIS), reveal large-scale ribbed moraines, hill-hole pairs, terminal moraines, and crevasse-squeeze ridges. Together, these features form an assemblage of landforms that is entirely different from that in the adjacent ice-stream troughs, and appears to be unique in the context of previous studies of Antarctic seafloor geomorphology. From this assemblage, the history of ice flow and retreat from the inter-ice stream ridge is reconstructed. The bedforms indicate that ice flow was significantly slower on the inter-ice stream ridge than in the neighbouring troughs. While terminal moraines record at least two re-advances or stillstands of the ice sheet during deglaciation, an extensive field of crevasse-squeeze ridges indicates ice stagnation subsequent to re-advancing ice, which deposited the field of terminal moraines in the NE. The presented data suggest that the ice flow behaviour on the inter-ice stream ridge was substantially different from that in the adjacent troughs. However, newly obtained radiocarbon ages on two sediment cores recovered from the inter-ice stream ridge suggest a similar timing in the deglaciation of both areas. This information closes an important gap in the understanding of past WAIS behaviour in the eastern ASE. Our newly-documented bedforms will also serve as an important diagnostic tool in future studies for interpreting ice-sheet histories in similar inter-ice stream areas.