Physical Conditions at the Base of a Fast Moving Antarctic Ice Stream

Abstract

Boreholes drilled to the bottom of ice stream B in the West Antarctic Ice Sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1.6 bars of the ice overburden pressure. These conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick. The mechanics of ice streaming plays a role in the response of the ice sheet to climatic change.

Subglacial Sediments as a Control on the Onset and Location of two Siple Coast Ice Streams, West Antarctica

“…The underlying questions are how strongly subglacial geology influences the transition from slow flow via internal deformation to streaming ice flow due to rapid basal motion, and how far the potential for ice streaming extends into the inland ice reservoir of West Antarctica. Previous work has shown that streaming ice in West Antarctica is underlain by sedimentary packages Rooney et al, 1991;Smith, 1997;Anandakrishnan et al, 1998;Bell et al, 1998;Doake et al, 2001;Studinger et al, 2001;Anandakrishnan, 2003;Vaughan et al, 2003], that a soft bed is present immediately beneath the ice to allow rapid basal motion [Engelhardt et al, 1990;Engelhardt and Kamb, 1998;Tulaczyk et al, 1998;Kamb, 2001], and that subglacial sediments appear to shape the inland extent of streaming ice flow [Anandakrishnan et al, 1998;Bell et al, 1998;Blankenship et al, 2001;Studinger et al, 2001]. However, much geological heterogeneity has been found across West Antarctica, indicating that the mechanisms for producing streaming ice flow may vary.…”

“…These fast flowing ice streams are conspicuous in satellite images [Stephenson and Bindschadler, 1990;Bindschadler, 1991, 1993;Hodge and Doppelhammer, 1996] and ice velocity maps [Joughin et al, 1999[Joughin et al, , 2002, with well-defined lateral margins, and extend several hundred kilometers into the ice sheet. Seismic imaging Rooney et al, 1991;Anandakrishnan et al, 1998] and borehole analysis [Engelhardt et al, 1990;Tulaczyk et al, 1998;Kamb, 2001] have shown that, in places, streaming ice is coincident with subglacial sediments. The upper meters of these sediments are soft till, with the potential for active deformation Engelhardt et al, 1990].…”

“…Seismic imaging Rooney et al, 1991;Anandakrishnan et al, 1998] and borehole analysis [Engelhardt et al, 1990;Tulaczyk et al, 1998;Kamb, 2001] have shown that, in places, streaming ice is coincident with subglacial sediments. The upper meters of these sediments are soft till, with the potential for active deformation Engelhardt et al, 1990]. Gravity and magnetic anomalies across the Siple Coast have been interpreted as thick sedimentary packages, which are believed to have a strong influence on streaming ice flow [Bell et al, 1998;Blankenship et al, 2001;Studinger et al, 2001].…”

“…These have especially targeted the trunks of ice streams B and C Rooney et al, 1987bRooney et al, , 1991Engelhardt et al, 1990;Munson and Bentley, 1992;Clarke et al, 1997;Anandakrishnan et al, 1998;Bell et al, 1998;Kamb, 2001;Studinger et al, 2001], although limited coverage has been extended to the trunk of ice stream D and to the onset regions [Kamb, 2001;Bindschadler et al, 2001;Studinger et al, 2001]. The sparseness of coverage with high-resolution subglacial imaging in the onset regions of the Siple Coast ice streams has led to poor geological constraints on physical controls of the onset of ice streaming.…”

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

“…Subglacial geology has been recognised as a strong influence on some ice streams, where their activity is affected by the underlying topography, deeper structure (Anandakrishnan et al, 1998), and geology at the ice-bed interface . Many ice streams are known to be underlain by a layer of soft, dilatant, deformable sediments which facilitates fast ice motion and high ice discharge (Boulton and Hindmarsh, 1987;Blankenship et al, 1987;Engelhardt et al, 1990;Kamb, 2001). Outside of these streams, regions of slower ice flow often correlate with rugged beds characterised by sedimentary rocks or crystalline bedrock.…”