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Thermal and mechanical processes at the beds of cold glaciers

Purpose

n the Wright ValleyThe purpose of this research project is to investigate and develop a greater understanding of the behaviour of cold-based glaciers by generating deductive depositional models that are based on glaciodynamic processes.  The development of such models requires an understanding of the relationships between the geological products and glaciological processes that occur at ice margins. The specific objectives therefore consist of two elements: i) study of the morphology, structure and sedimentology of geological products and ii) study of the dynamics and composition of basal ice, the glacier substrate and the processes that lead to debris entrainment. Despite a burgeoning volume of research in glacial geomorphology the relationships between these elements remain poorly identified because few studies have attempted to link geological products with observed processes of erosion and transportation and even fewer have attempted in situ studies of the mechanical processes at glacier beds.

 

Rationale

Before the 1990s our understanding of glaciers and their geological products was based on ice flowing over bedrock and few researchers had investigated erosion and deposition processes at glacier beds that consist of unlithified sediments. In 1979 Boulton and Jones suggested that the rheology of subglacial sediments can exert a major control on glacier dynamics (such as the velocity of ice flow). Subsequent studies of ice streams in the West Antarctic ice sheet and at other soft-bedded glaciers have provided empirical evidence for subglacial bed deformation (Alley et al. 1987), along with  increased rates of basal sliding, although it is often difficult to distinguish between the two processes (Truffer et al. 2001). In 1996 Boulton again proposed a theory of large-scale erosion and deposition for an ice sheet resting on unlithified sediment. The studies that have investigated the influence of thick unlithified sediments on glaciers and ice sheets have contributed to the generation of models that are used to predict the behaviour of the large ice sheets that covered Europe and North America during the Pleistocene period.

Most research on subglacial bed deformation has focused on conditions associated with basal ice at pressure melting point and unfrozen sediments. While these conditions are likely to be dominant beneath large ice masses, significant areas in the high latitudes and high altitude areas are thought to have experienced frozen (cold-based) bed conditions during the Pleistocene (Kleman 1992; Kleman and Borgstrom 1994). Wright GlacierThere are two different approaches to investigating the geological processes and products of glaciers. One has been primarily concerned with the description of Pleistocene glacial deposits and from these interpreted (inferred) the mode of origin using general principles. The second school of thought, proposed by the 'dirty-ice glaciologists' is that observation and monitoring of sedimentary processes at the base of glaciers will lead to a greater understanding of the relationships between the sedimentological processes and products (Boulton 1987). Those that advocate the former approach have concluded that cold-based glaciers are ineffective agents of erosion, and thus have generated inferences about the thermal processes beneath Pleistocene glaciers from landforms that appear to have survived glaciation e.g. Kleman (1992) and Kleman and Borgstrom (1994).

To date very little empirical research has been conducted at the ice-sediment interface of cold-based glaciers, although work by Shreeve (1984) has suggested that sliding is theoretically possible at sub-freezing temperatures. Four important field experiments have been undertaken at Meserve Glacier (Holdsworth, 1974), Urumqui No 1 Glacier (Echelmeyer and Wang, 1987), Suess Glacier (Fitzsimons, 1996; Fitzsimons et al., 1999; 2000; Sleewaegen et al., 2003) and again at Meserve Glacier (Cuffey et al., 2000). These studies indicate that sliding can occur at subfreezing temperatures, that unlithified frozen sediments can deform in some circumstances and that the behaviour of basal ice in cold glaciers is considerably more complicated than was previously thought. These findings have been reinforced in a recent review of the behaviour of cold-based glaciers (Waller, 2001) and have stimulated a reappraisal of the possibility of subglacial deformation at subfreezing temperatures in Icelandic glaciers (Bennett et al., 2003). It is now widely recognised that the possibility of active basal deformation at sub-freezing temperatures has profound implications for our understanding of the glacio-dynamics and geomorphological potential of cold-based glaciers (Bennett et al., 2003). 

As most erosion occurs at the glacier bed the characteristics of basal ice which forms at the ice-bed interface have become a focus for investigations of bed processes. Basal ice can be defined as a relatively thin layer of ice that forms through a combination of thermal and mechanical processes which link a glacier to its substrate. These thermal and mechanical processes create several characteristics including layers and/or lenses of debris entrained from the bed, deformation structures, relatively high solute concentrations and unusual gas composition, that collectively distinguish it from ice that has formed solely by the firnification of snow at the glacier surface (Hubbard and Sharp 1989; Knight 1997).

Due to the characteristics of basal ice and its interactions with the bed it is thought to be an important component in the rheological behaviour of glaciers. Wright Valley AntarcticaMost models of glacier behaviour are founded on the assumptions of homogenous deformation behaviour that is described by Glen's flow law and on the presence of a sharp ice-substrate interface. These assumptions are increasingly at odds with field and laboratory observations that demonstrate that basal ice is characterised by compositional and mechanical properties that distinguish it from glacier ice formed by precipitation alone (Fitzsimons 2005). These observations suggest that unconsolidated deposits beneath glaciers are common and that these deposits may deform in response to shear stresses imposed by the flowing ice. The separation between modelling efforts and field and laboratory observations is compounded by a tendency to conceptualise the behaviour of basal ice and the glacier substrate as independent entities when such a separation may not be supported by field observations.

Despite a call for enhancement of our understanding of the behaviour of ice at glacier beds by Theakstone in 1967, and restatement of the same imperative in a landmark review of basal ice formation and deformation by Hubbard and Sharp (1986) we have not yet achieved a complete understanding of the formation and deformation of basal ice. Until such comprehension is achieved it seems likely that basal ice will continue to be treated as an isotropic material.

Although the recent studies demonstrate that cold-based glaciers can erode and deform frozen sediments we do not yet understand how this deformation occurs, how the deformation results in debris entrainment, or how changes in the properties of the resulting basal ice alter glacier behaviour. From these three questions our research programme can be derived.

 

Specific objectives

  • To understand the geometrical relationships between ice, liquid water and the glacier substrate.
  • To understand the processes involved in the deformation and entrainment of the substrate beneath and at the margins of cold glaciers.
  • To understand the principal controls of the mechanical behaviour of basal ice and how basal ice controls glacier behaviour.

Camping in Antarctica

One of the key techniques that is regularly used to determine if liquid water has been frozen onto glacier ice is the determination of the isotopic fractionation of oxygen and deuterium. We have employed a statistically-based approach to coisotopic analysis with considerable success (Souchez et al. 2004). Our recent research on the isotopic composition of basal ice from Taylor Glacier shows that some basal ice is almost certainly the product of regelation, as the isotopic signature is almost identical to the local meteoric water line (Mager 2005). Two possible explanations for the anomalous isotopic signature are that the scale of the regelation process is considerably finer than the sampling scale that we have employed or that there has been net conductive freezing without isotopic fractionation. During the upcoming field seasons we intend to extend our previous research by focusing on the problem of the use and interpretation of isotopic data from basal ice. To do this we will undertake very fine sampling of basal ice from several glaciers in the McMurdo Dry Valleys, such as the Joyce, Wright Lower and the Garwood Glacier, as well as examining the isotopic composition of an ice margin that has experienced net conductive freezing (the edge of the Southern McMurdo Ice Shelf).

The principal outcomes of the research are aimed at making a fundamental contribution to our understanding of glaciers and glaciation and to overthrow the idea that persists in the literature that cold glaciers are geomorphologically inert. The outcomes of the research will improve our understanding of how glaciers are coupled to their beds and how ice-bed interactions change the rheology of basal ice.

 

Statement of Outcomes

Two direct outcomes of the project will be to provide new field data to rewrite elements of existing glaciological theory concerning cold-based glaciers and/or to provide data on the particular processes associated with some glaciers in the McMurdo Dry Valleys. Either outcome will result in an advancement of knowledge concerning the dynamics of cold-based glaciers.

Mt Erebus, AntarcticaThe investigation has important implications for studies of Quaternary climate change because the reconstruction of past climates depends on the understanding of proxy data including glacial geology. As our understanding of contemporary climate change scenarios depends in part on records of past climate change this project forms a small element of the research on global change as identified by the International Geosphere-Biosphere Programme in the PAGES (Past Global Changes) project.

The project also contributes to the development of research skills in New Zealand through the involvement of several post-graduate students who will be provided with the opportunity of working within a unique environment and with distinguished overseas scientists.

 

References

Alley, R.B., Blankenship, D.D., Bentley, C.R. and Rooney, S.T., 1987. Till beneath ice stream B. 3. Till deformation : evidence and implications. Journal of Geophysical Research 92 (B9), 8921-8929.

Cuffey, K.M., H. Conway, A.M. Gades, B. Hallet, R. Lorrain, J.P. Severinghaus, E.J. Steig, B. Vaughn and White, J.W.C. 2000. Entrainment at cold glacier beds. Geology 28(4), 351-354.

Bennett, M.R., Waller, R.I., Midgley, N.G., Huddart, D., Gonzalez, S., Cook, S.J. and Tomio, A., 2003. Subglacial deformation at sub-freezing temperatures: Evidence from Hagafellsjokull-Eystri, Iceland. Quaternary Science Reviews 22, 915-923.

Boulton, G.S., 1996. Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation. Journal of Glaciology 42, 43-62

Boulton, G.S., 1987. Progress in glacial geology in the last fifty years. Journal of Glaciology, special issue, 1987, 25- 32.

Boulton, G.S. and Jones, A.S., 1987. Stability of temperate ice sheets and ice caps resting on beds of deformable sediment. Journal of Glaciology 24(90) 29-43.

Echelmeyer, K. and Wang, Z., 1987. Direct observations of basal sliding and deformation of basal drift at sub-freezing temperatures. Journal of Glaciology 33, 83-98.

Fitzsimons, S.J., 1996. Formation of thrust block moraines at the margins of dry-based glaciers, south Victoria Land, Antarctica. Annals of Glaciology 22, 68-74.

Fitzsimons, S.J., 2005. Mechanical behaviour and structure of the debris-rich basal ice layer. In Knight, P.G. (ed.) Glacier Science and Environmental Change. Blackwell, Oxford.

Fitzsimons, S.J. McManus, K.J. and Lorrain, R., 1999. Structure and strength of basal ice and substrate of a dry based glacier: evidence for substrate deformation at subfreezing temperatures. Annals of Glaciology 28 236-240.

Fitzsimons, S.J., Lorrain, R.D. and Vandergoes, M.J., 2000. Behaviour of subglacial sediment and basal ice in a cold glacier. In The Deformation of Glacial Materials. Geological Society of London Special Publication No 176, 181-190.

Holdsworth, G., 1974. Meserver Glacier, Antarctica: Part I Basal Processes. Institute of Polar Studies Report No. 37, 104 pp.

Hubbard, B., 2002. Direct measurement of basal motion at a hard-bedded temperate glacier: Glacier de Tsansfleuron, Switzerland. Journal of Glaciology 48, 1- 8.

Hubbard, B. and Sharp, M., 1989. Basal ice formation and deformation: a review. Progress in Physical Geography 13, 529-558.

Kleman, J., 1992. The palimpsest glacial landscape in northwestern Sweden - Late Weischselian boulder blankets and interstadial periglacial phenomena. Geografiska Annaler 74A, 63-78.

Kleman, J. and Borgstrom, I., 1994. Glacial land forms indicative of a partly frozen bed. Journal of Glaciology 40, 255-264.

Knight, P.G., 1997. The basal ice layer of glaciers and ice sheets. Quaternary Science Reviews 16, 975-993

Mager S., 2005. A compositional approach to understanding the formation of basal ice in Antarctic glaciers. Unpublished PhD thesis, University of Otago. 220 pp. [abstract]

Shreve, R.L., 1984. Glacier sliding at subfreezing temperatures. Journal of Glaciology 30, 341-347.

Sleewaegen S., Samyn, D., Fitzsimons, S., and Lorrain R., 2003. Equifinality of basal ice facies from an Antarctic cold-based glacier. Annals of Glaciology 37, 257-262.

Souchez, R, Samyn, D., Lorrain, R., Pattyn, F. and Fitzsimons, S., 2004. An isotopic model for basal freeze-on associated with subglacial upward flow of pore water. Geophysical Research Letters 31, L02401

Truffer, M. Echelmeyer, K.A and Harrison, W.D., 2001. Implications of till deformation on glacier motion. Journal of Glaciology, 47 123-134

Waller, R.I., 2001. The influence of basal processes on the dynamic behaviour of cold-based glaciers. Quaternary International 86, 117-128.

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Relevant Student Dissertations and Theses

Filmer, V. 2000. The basal ice layer of the Taylor Glacier, Antarctica. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 84 pp.

Hooker, B. 1998. Chemical signatures of clear basal ice facies at the margins of dry-based glaciers, South Victoria Land, Antarctica. Unpublished MSc thesis, in Geography, at the University of Otago, Dunedin, New Zealand. 96 pp. [abstract]

Lambourne, A. 1998. Physical and chemical composition of basal ice from the Suess Glacier, Antarctica. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 84 pp.

Larking, R. 1997. The composition, structure and dynamics of a dry-based glacier apron. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 68pp.

MacDonell, S
. 2003. Cold-based glacier behaviour: Victoria Upper Glacier, Antarctica. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 102 pp. [abstract]

Mager, S. 1996. Sediment transport in dry-based glaciers : an assessment of sedimentary signatures imprinted by glacial transportation. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 78 pp. [Abstract]

Mager, S. 2005. A compositional approach to understanding the formation of basal ice in cold Antarctic glaciers. Unpublished PhD thesis in Geography and Chemistry, University of Otago, Dunedin, New Zealand. 220 pp. [abstract]

Morgan, T. 1999. Chemical and physical characteristics of ice from the basal zone, Taylor Glacier, Antarctica. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 93 pp.

Sirota, P. 1999. The structure and strength of basal ice in the Suess Glacier, Antarctica. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 92 pp. [abstract] [PDF 2442kb]

Webb, N. 2003. Cold-based glacier apron morphology, structure, and composition : a multifaceted approach to testing basal ice formation by apron incorporation. Unpublished BSc (hons) dissertation, in Geography, at the University of Otago, Dunedin, New Zealand. 86 pp.

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© 2009 Department of Geography, University of Otago, Dunedin, New Zealand