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Over the last decade major advances in the understanding of palaeo-ice streams with respect to their dynamics, controls and basal processes have been made through marine geophysical and geological investigations of the Antarctic continental margin. Multibeam swath bathymetry has been used to map the geomorphological record of streaming flow over wide geographical areas of the continental shelf, notably around the Antarctic Peninsula and in the Bellingshausen and Ross seas. Palaeo-ice streams are recorded by subglacial bedforms within cross-shelf bathymetric troughs. These bedforms characteristically show a downflow evolution across the shelf, reflected by an increase in elongation ratios, from bedrock drumlins and ice-moulded bedrock on the inner shelf, to more elongate drumlins and lineations on the mid-shelf, to mega-scale glacial lineations up to 20 km long on the outer shelf. Initiation of streaming flow appears to have been controlled by subglacial geology with the onset of streaming being located at the transition from crystalline bedrock to a sedimentary substrate. The most elongate subglacial bedforms occur over the soft, sedimentary bed.
Cores from the mega-scale glacial lineations show that they are composed of till. Characteristically this till is weak (0-20 kPa) and porous. In core section the till appears massive but observations from x-radiography and micromorphology, combined with measurements of shear strength and reworked diatom abundance, shows that it contains well developed shear zones 0.1-0.9 m thick. These ?ice stream tills' were formed by a combination of subglacial deformation and lodgement. The base of the till layer ranges from flat and continuous to deeply grooved, implying the operation of different mechanisms of till mobilization and entrainment. Although deformation partitioning into relatively thin deforming layers occurred within the ice-stream till, evidence for large-scale till advection to the ice stream margin implies that these localized shear zones integrated to transport significant volumes of sediment beneath palaeo-ice streams.
The style of ice stream retreat varied between different bathymetric troughs, even where water depths along these troughs increased inshore. Three styles of retreat are inferred from geomorphological evidence: (1) rapid, (2) episodic between successive grounding-zone positions, and (3) slow retreat of grounded ice.
Antarctic palaeo-ice streams therefore did not respond uniformly to external forcing at the end of the last glaciation, but rather their diachronous retreat reflects the dominance of local controls such as bathymetry, drainage basin size and probably sediment supply to the grounding line. Such variations in retreat dynamics demonstrate that the deglaciation of marine-based ice sheets in areas of reverse bed slope is not necessarily catastrophic and they provide important constraints for numerical models that attempt to predict the dynamics of large polar ice sheets.
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