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Basal hydrological systems play an important role in controlling the dynamic behaviour of ice streams. Data showing their morphology and relationship to geological substrates beneath modern ice streams are, however, sparse and difficult to collect. We present new multibeam bathymetry data that make the Anvers-Hugo Trough west of the Antarctic Peninsula the most completely surveyed palaeo-ice stream pathway in Antarctica. The data reveal a diverse range of landforms, including streamlined features where there was fast flow in the palaeo-ice stream, channels eroded by flow of subglacial water, and compelling evidence of palaeo-ice stream shear margin locations. We interpret landforms as indicating that subglacial water availability played an important role in facilitating ice stream flow and controlling shear margin positions. Water was likely supplied to the ice stream bed episodically as a result of outbursts from a subglacial lake located in the Palmer Deep basin on the inner continental shelf. These interpretations have implications for controls on the onset of fast ice flow, the dynamic behaviour of palaeo-ice streams on the Antarctic continental shelf, and potentially also for behaviour of modern ice streams. [ABSTRACT FROM AUTHOR]

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 were 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. [ABSTRACT FROM AUTHOR]

The Amundsen Sea embayment is a probable site for the initiation of a future collapse of the West Antarctic Ice Sheet. This paper contributes to a better understanding of the transport pathways of subglacial sediments into this embayment at present and during the last glacial period. It discusses the clay mineral composition of sediment samples taken from the seafloor surface and marine cores in order to decipher spatial and temporal changes in the sediment provenance. The most striking feature in the present-day clay mineral distribution is the high concentration of kaolinite, which is mainly supplied by the Thwaites Glacier system and indicates the presence of hitherto unknown kaolinite-bearing sedimentary strata in the hinterland, probably in the Byrd Subglacial Basin. The main illite input is via the Pine Island Glacier. Smectite originates from the erosion of volcanic rocks in Ellsworth Land and western Marie Byrd Land. The clay mineral assemblages in diamictons deposited during the last glacial period are distinctly different from those in corresponding surface sediments. This relationship indicates that glacial sediment sources were different from modern ones, which could reflect changes in the catchment areas of the glaciers and ice streams. [ABSTRACT FROM PUBLISHER]

The article discusses a study on the streaming flow of a palaeo-ice stream in the Antarctic Peninsula. It notes that ice streams tend to contribute to the mass balance and dynamic behaviour of ice sheets, adding that fast ice flow or ice streaming is a result of the deformation of soft substrate or by basal sliding across the bed. Findings indicate that basal sliding has caused the fast ice flow in the inner shelf area while deformation has been limited to discontinuous patches of soft till less than 40 centimeters (cm) thick. Results also show that the geomorphic imprint from the inner to outer shelf reflects the downstream transition of basal sliding to subglacial deformation as well as the increase of the velocity of ice-flow downstream.

Reinardy, B. T. I., Hiemstra, J. F., Murray, T., Hillenbrand, C.-D. & Larter, R. D. 2011: Till genesis at the bed of an Antarctic Peninsula palaeo-ice stream as indicated by micromorphological analysis. Boreas, Vol. 40, pp. 498-517. 10.1111/j.1502-3885.2010.00199.x. ISSN 0300-9483. Sediment cores from cross-shelf troughs on the NE Antarctic Peninsula shelf recovered tills with variable shear strengths that represent different subglacial depositional regimes. In addition to detailed qualitative micromorphological descriptions, a quantitative method was applied, which revealed a higher abundance of boudins and intraclasts and a lower abundance of crushed and fractured grains in samples from the soft till compared with samples from the underlying stiff till. This is the first evidence of significant (micro-scale) differences between the two types of till and thus strengthens previous interpretations that were based primarily on shear strength. The differences between the soft and stiff till relate to a deforming continuum whereby the initial deposition of till as ice advanced across the shelf produced ductile structures before dewatering and compaction led to the formation of brittle structures such as crushed and fractured grains in the now stiff till. A change in ice-flow dynamics led to streaming flow and the deformation of the upper parts of the stiff till that was being reworked into a soft till. The soft till facilitated ice streaming, and progressive shearing led to the homogenization of the ice stream substrate, which was partially advected downstream. The resulting till thus contains poly-deformational structures, with deformation structures inherited from the stiff till being generally poorly preserved. Our micromorphological analysis of the soft till provides the first widespread sedimentological evidence of deformation across the palaeo-ice stream bed on the NE Antarctic Peninsula shelf. [ABSTRACT FROM AUTHOR]

Abstract: The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary. [Copyright &y& Elsevier]

Abstract: The history of grounded ice-sheet extent on the southern Weddell Sea shelf during the Last Glacial Maximum (LGM) and the timing of post-LGM ice-sheet retreat are poorly constrained. Several glaciological models reconstructed widespread grounding and major thickening of the Antarctic Ice Sheet in the Weddell Sea sector at the LGM. In contrast, recently published onshore data and modelling results concluded only very limited LGM-thickening of glaciers and ice streams feeding into the modern Filchner and Ronne ice shelves. These studies concluded that during the LGM ice shelves rather than grounded ice covered the Filchner and Ronne troughs, two deep palaeo-ice stream troughs eroded into the southern Weddell Sea shelf. Here we review previously published and unpublished marine geophysical and geological data from the southern Weddell Sea shelf. The stratigraphy and geometry of reflectors in acoustic sub-bottom profiles are similar to those from other West Antarctic palaeo-ice stream troughs, where grounded ice had advanced to the shelf break at the LGM. Numerous cores from the southern Weddell Sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments. These data sets give evidence that grounded ice had advanced across the shelf during the past, thereby grounding in even the deepest parts of the Filchner and Ronne troughs. Radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited, but indicate that the ice grounding occurred at the LGM and that ice retreat started before ∼15.1 corrected 14C kyrs before present (BP) on the outer shelf and before ∼7.7 corrected 14C kyrs BP on the inner shelf, which is broadly synchronous with ice retreat in other Antarctic sectors. The apparent mismatch between the ice-sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles (similar to those of “ice plains”) that had advanced through Filchner Trough and Ronne Trough at the LGM. Considering the global sea-level low-stand of ∼130 m below present, a low surface slope of the expanded LGM-ice sheet in the southern Weddell Sea can reconcile grounding-line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland. This scenario implies that ice-sheet growth in the Weddell Sea sector during the LGM and ice-sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea-level fluctuations during these times. [Copyright &y& Elsevier]