Subglacial decoupling at the sediment/bedrock interface: a new mechanism for rapid flowing ice

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Abstract

On millennial or even centennial time scales, the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system. At the surge-type glacier Brúarjökull, an outlet of the Vatnajökull ice cap, eastern Iceland, extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier. This newly discovered mechanism has far reaching consequences for our understanding of fast-flowing ice and its integration with sediment discharge and meltwater release.

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)

  • P.U. Clark et al.

    Sea-level fingerprinting as a direct test for the source of global meltwater pulse IA

    Science

    (2002)
  • G.K.C. Clarke

    Subglacial processes

    Annual Review of Earth and Planetary Science

    (2005)
  • H. De Angelis et al.

    Glacier surge after ice shelf collapse

    Science

    (2003)
  • E. Domack et al.

    Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch

    Nature

    (2005)
  • J.A. Dowdeswell et al.

    Thickness and extent of the subglacial till layer beneath an Antarctic paleo-ice stream

    Geology

    (2004)
  • K. Echelmeyer et al.

    Jakobshavn Isbræ, west Greenland: seasonal variations in velocity—or the lack thereof

    Journal of Glaciology

    (1990)
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