Subglacial decoupling at the sediment/bedrock interface: a new mechanism for rapid flowing ice
Introduction
Interest in fast ice flow behaviour is stimulated by recent events along the periphery of contemporary ice sheets, where interior collapse subsequent to ice shelf disintegration is suddenly a realistic possibility (De Angelis and Skvarca, 2003). In the present state of rapid global environmental changes there is a growing wish to understand the causal mechanisms behind ice sheet instability and its contribution to global sea level rise (Alley and Bindschadler, 2001; Clark et al., 2002; Domack et al., 2005). Fast flowing ice streams and surging glaciers exert a strong control on the discharge of the Antarctic and Greenland ice sheets acting as regulators on their configuration and stability (Dowdeswell et al., 2004; Rignot and Kanagaratnam, 2006). Consensus in the literature dictates that mechanisms behind ice flow variability and distribution of meltwater are linked to subglacial processes influencing basal motion and not to ice-mechanical processes (Boulton and Hindmarsh, 1987; Fischer and Clarke, 2001). An important issue is, however, to identify and quantify the different hydro-mechanical processes beneath the ice and in particular the significance of deep-seated sediment deformation as it is directly linked to predictions of ice sheet stability (Clarke, 2005). Surge-type glaciers provide an opportunity to probe this problem as they experience major fluctuations in velocity between phases of active surging and quiescence. Between surge events as the glacier ice retreats—a landform association and sediment succession re-emerges imprinted with vital information on subglacial driving processes.
Section snippets
Setting
Brúarjökull, a northern outlet of the Vatnajökull ice cap in eastern Iceland, has experienced major velocity fluctuations switching between active winter surging of some 3 months duration and quiescent phases lasting from 70 to 90 years (Todtmann, 1960; Thorarinsson, 1969; Raymond, 1987). During the most recent surges initiated in 1890 and 1963 the glacier advanced respectively, 10 and 8 km affecting an area of roughly 1400 km2 (Fig. 1). Recent surges in 1890 and 1963 were documented in the field
Earlier models for fast ice flow
Rapid ice flow velocities reached either by ice streams or surging glaciers, have hitherto been explained by two modes of basal motion largely dependent on ice/bed coupling (Fischer and Clarke, 2001). Decoupling of a glacier from its bed enables fast ice flow through enhanced basal sliding across the ice/bed interface or very shallow subglacial deformation, i.e. the basal sliding model in Fig. 3A (Engelhardt and Kamb, 1998). Alternatively, fast ice flow is sustained by deformation of
Glaciodynamic interfaces
The continued recession of Brúarjökull by frontal retreat reveals a streamlined till plain superimposed on larger bedrock features and subglacial landforms developed across the ice/till interfaces (Schomacker et al., 2006) during the 1890 and 1964 surges (Fig. 1). Narrow, regularly spaced flutes are traced across the entire area from the present-day ice margin to the end moraines that mark former surge terminations (Fig. 1). Some flutes continue for more than 1.5 km with an elongation ratio
The dual-coupled model and its wider implications
The sediment succession and its properties at Brúarjökull provide evidence as to where decoupling and displacement leading to fast ice flow must occur: at the ice/till interface, within the LPT-sequence, or at the interface between the bedrock and the LPT-sequence. Rapid displacement along the interface between the till and LPT-sequence is excluded as it precludes the flute formation that demonstrably took place during the entire surge phase. Weak clast orientation in flutes has previously been
Acknowledgement
Financial support for this study was received from the Swedish National Research Council (Kurt H. Kjær, contract no. 621-2002-4753), The Royal Physiographic Society in Lund, Crafoord Foundation, Landsvirkjun, the University of Iceland Research Fund, Icelandic Research Council (Rannís) and the Danish Natural Research Council.
The National Museum of Iceland is thanked for providing permission and access to the photo archive of Sigurður Thorarinsson, and his son, Sven Sigurðsson, is acknowledged
References (33)
- et al.
Groundwater flow beneath ice sheets: part II—its impact on glacier tectonic structures and moraine formation
Quaternary Science Reviews
(1995) - et al.
Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system
Quaternary International
(2001) - et al.
Review of subglacial hydro-mechanical coupling: Trapridge glacier, Yukon Territory, Canada
Quaternary International
(2001) - et al.
Reflections on soft subglacial beds as a mosaic of deforming and stable spots
Quaternary Science Review
(2004) Glacier stress patterns and sediment transfer associated with the formation of superimposed flutes
Sedimentary Geology
(1989)- et al.
Subglacial till: the deforming glacier bed
Quaternary Science Reviews
(2003) - et al.
Water-pressure coupling of sliding and bed deformation: III. Apllication to ice stream B, Antarctica
Journal of Glaciology
(1989) Hydrological characteristics of the drainage system beneath a surging glacier
Nature
(1998)- et al.
Sediment deformation beneath glaciers: rheology and geological consequences
Journal of Geophysical Research
(1987)
Sea-level fingerprinting as a direct test for the source of global meltwater pulse IA
Science
Subglacial processes
Annual Review of Earth and Planetary Science
Glacier surge after ice shelf collapse
Science
Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch
Nature
Thickness and extent of the subglacial till layer beneath an Antarctic paleo-ice stream
Geology
Jakobshavn Isbræ, west Greenland: seasonal variations in velocity—or the lack thereof
Journal of Glaciology
Cited by (108)
Seasonal subglacial ponding deposits in a thick till sequence, Dösebacka drumlin, southwest Sweden
2022, Sedimentary GeologyDepositional Processes
2022, Treatise on GeomorphologyA litho-tectonic event stratigraphy from dynamic Late Devensian ice flow of the North Sea Lobe, Tunstall, east Yorkshire, UK
2020, Proceedings of the Geologists' Association