Return to the Table of Contents for this chapter.
Pennsylvania State University, University Park, Pennsylvania 16802
Ice flowing on a continuous, soft-sediment layer achieves high velocities despite low basal shear stress (e.g., Blankenship et al. 1987) through deformation of that sediment layer. Water-lubricated sliding over the substrate occurs whether or not that substrate is deforming; hence, bed deformation produces higher flow velocities than does sliding by itself. The Siple Coast ice streams of West Antarctica and the ice streams or lobes of the southern margin of the Laurentide ice sheet achieved high velocities with low basal shear stresses through the effects of soft subglacial sediments (e.g., Jenson et al. 1995). Ice streaming is possible although slowed if the soft sediment is locally discontinuous provided the bedrock "sticky spots" are water-lubricated (Rooney et al. 1987; Anandakrishnan and Alley 1994), but loss of that water may stop the rapid motion (Alley et al. 1994).
Continuous or nearly continuous soft-sediment glacier beds are most likely if ice advances over unconsolidated or poorly consolidated sediments. First, sediment generation clearly is easier from softer materials. The difference in abrasion rates between hard and soft materials may be two orders of magnitude (Cuffey and Alley 1996).
In addition, a thickening till layer on bedrock causes slowed abrasion but enhanced till export through deformation. If ice must act through intervening till on the clasts that abrade the bedrock, the abrading clasts can roll part of the time and abrasion is reduced by roughly an order of magnitude (Cuffey and Alley 1996). If a thickened till layer reduces or eliminates sliding of till over its substrate, a further order(s)-of-magnitude reduction in abrasion will result (Cuffey and Alley 1996). In general, abrasion rates beneath till are unlikely to produce till faster than 0.1 millimeter per year except on very soft or unconsolidated rocks (Cuffey and Alley 1996).
Several processes compete to remove subglacial sediment. Beneath warm glaciers with surface melt fed to the bed through moulins, subglacial stream transport can be exceptionally rapid because glaciers concentrate runoff in time and space and provide steep head gradients compared to subaerial streams, and sediment transport increases with head gradient and with runoff (see review by Alley et al. in press). Moulin-fed streams rising out of overdeepenings also can cause rapid sediment entrainment to the ice by freeze-on in response to the decreasing pressure-melting point along flow (Lawson et al. 1996; Strasser et al. 1996). These processes are unlikely to be significant, however, for most of the Antarctic unless storage and release of water in subglacial floods are important because in the absence of surface melt, not enough water is present to transport more than very fine sediment (e.g., Alley 1989).
Several other processes may remove sediment from beneath the antarctic ice sheet and other glaciers (reviewed by Alley et al. in press). One especially, regelation of ice into subglacial sediments, is important based on laboratory and model studies (Iverson and Semmens 1995). Entrainment rates are modeled to depend on basal water pressures, melt rates, and sediment grain size (Iverson and Semmens 1995). Export of sediment produced at a rate of 0.1-1.0 millimeters per year or even higher is likely, provided the sediments are not finer than fine silt (Alley et al. in press).
Thus, it is likely that material eroded from well-lithified bedrock will be entrained by the ice and transported away, rather than accumulating under the ice to form a thick, continuous deformable layer. In contrast, soft sediments can be eroded sufficiently rapidly to produce continuous or nearly continuous deforming layers. We thus expect to see a reasonably close correlation between ice streaming and the presence of sedimentary basins, as is observed (e.g. Rooney et al. 1991, pp. 261-265).
The general increase in continuity of subglacial soft sediment toward the ice margin may be involved in thermal surging, such that cooling may produce surges. Consider the common case of an ice sheet with a moderate surface slope over a thawed internal region, a steep surface slope in a marginal region frozen to bedrock (Weertman 1961), and a thick proglacial sediment accumulation. A cooling may trigger ice advance through sea-level fall or decreased marginal ablation or calving. As the ice thickens over the former margin, the bed there will warm and may thaw from the bedrock. At the same time, the advancing margin may tend to freeze to material over which it flows, maintaining the thawed-interior/frozen-margin pattern. In such a situation, the frozen margin will maintain a steep surface slope and high basal shear stress only if it freezes to bedrock. Otherwise, the sediment pile can deform below the freezing front, yielding the low surface slope of an ice stream.
A cooling-induced ice advance thus may cause a shift from a steep margin and moderate-slope interior to a low-slope margin and moderate-slope interior. This transition can cause mass loss from the ice sheet as a whole and might occur quite rapidly and appear surgelike. It is at least possible that this has relevance to former ice-sheet behavior, including the great discharges of debris in icebergs from the Laurentide ice sheet during Heinrich events, which appear to have occurred following oceanic coolings (Bond et al. 1992).
The coupled behavior of a deforming but slightly discontinuous subglacial till and of a water layer that lubricates sliding over the till and any sticky spots may be quite complex. Iverson et al. (1995) documented that a transient increase in water pressure forced by moulin drainage through Storglaciaren, Sweden, a glacier with a discontinuous deforming bed, caused a transient decrease in the rate of bed deformation. Presumably, the water caused enhanced separation between ice and till and so faster sliding. The decreased deformation of the till then is explainable as a decreased basal shear stress on it, with force-balance for the glacier maintained by increased side drag or increased drag on sticky spots. Because of differences in roughness characteristics between till and bedrock, sliding over till may be more sensitive to water pressure than is sliding over bedrock (Alley 1989).
It has been hypothesized that ice stream C, West Antarctica, stopped recently because its basal meltwater was diverted to ice stream B (Anandakrishnan and Alley 1994; Alley et al. 1994). If so, and if the Iverson et al. (1995) observations are relevant to the longer times and larger size of the ice-stream situation, then ice stream B now has faster sliding between ice and subglacial till than before the meltwater diversion. The basal shear stress on the till beneath ice stream B then would be less today than before the water diversion because the higher ice velocity will have increased the drag from the ice-stream sides, may have increased the drag from subglacial sticky spots, and may have caused thinning or reduced surface slope and thus reduced driving stress for ice flow. If this decrease in basal shear stress on the till is more important than the softening of the till caused by the increased water supply, as on Storglaciaren, then the deforming layer beneath ice stream B is thinner or deforms more slowly and transports less sediment now than before the water diversion.
This research was supported by National Science Foundation grant number OPP 93-18677.
References
Alley, R.B. 1989. Water-pressure coupling of sliding and bed deformation: I. Water system. Journal of Glaciology, 35(119), 108-118.
Alley, R.B., S. Anandakrishnan, C.R. Bentley, and N. Lord. 1994. A water-piracy hypothesis for the stagnation of ice stream C. Annals of Glaciology, 20, 187-194.
Alley, R.B., K.M. Cuffey, E.B. Evenson, J.C. Strasser, D.E. Lawson, and G.J. Larson. In press. How glaciers entrain and transport basal sediment: Physical constraints. Quaternary Science Reviews.
Anandakrishnan, S., and R.B. Alley. 1994. Ice stream C sticky spots detected by microearthquake monitoring. Annals of Glaciology, 20, 183-186.
Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1987. Till beneath ice stream B. 1. Properties derived from seismic travel times. Journal of Geophysical Research, 92(B9), 8903-8911.
Bond, G., H. Heinrich, W. Broecker, L. Labeyrie, J. McManus, J. Andrews, S. Huon, R. Jantschik, S. Clasen, C. Simet, K. Tedesco, M. Klas, G. Bonani, and S. Ivy. 1992. Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period. Nature, 360, 245-249.
Cuffey, K.M., and R.B. Alley. 1996. Erosion by deforming subglacial sediments: Is it significant? (Toward till continuity). Annals of Glaciology, 22, 17-24.
Iverson, N.R., B. Hanson, R. LeB. Hooke, and P. Jansson. 1995. Flow mechanism of glaciers on soft beds. Science, 267, 80-81.
Iverson, N.R., and D.J. Semmens. 1995. Intrusion of ice into porous media by regelation: A mechanism of sediment entrainment by glaciers. Journal of Geophysical Research, 100(B7), 10219-10230.
Jenson, J.W., P.U. Clark, D.R. MacAyeal, C. Ho, and J.C. Vela. 1995. Numerical modeling of advective transport of saturated deforming sediment beneath the Lake Michigan Lobe, Laurentide Ice Sheet. Geomorphology, 14, 157-166.
Lawson, D.E., E.B. Evenson, J.C. Strasser, R.B. Alley, and G.J. Larson. 1996. Subglacial supercooling, ice accretion, and sediment entrainment at the Matanuska Glacier, Alaska. Abstracts with programs (Vol. 28). Boulder, Colorado: Geological Society of America. [Abstract]
Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1987. Till beneath ice stream B. 2. Structure and continuity. Journal of Geophysical Research, 92B(B9), 8913-8920.
Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1991. Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thompson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press.
Strasser, J.C., D.E. Lawson, G.J. Larson, E.B. Evenson, and R.B. Alley. 1996. Preliminary results of tritium analyses in basal ice, Matanuska Glacier, Alaska, U.S.A.: Evidence for subglacial ice accretion. Annals of Glaciology, 22, 126-133.
Weertman, J. 1961. Mechanism for the formation of inner moraines found near the edge of cold ice caps and ice sheets. Journal of Glaciology, 3(30), 965-978.