Sediment Transport Pathways

Transport routeways through valley glaciers are varied (Fig. 15.2). Boulton (1978) distinguished two main sediment transport pathways:

1. active subglacial transport, and

2. passive supraglacial or englacial transport.

Subglacial debris transport was termed active because sediment in the basal shear zone of glaciers is subjected to high inter-particle contact forces and consequently undergoes significant abrasion, fracture and comminution. Boulton (1978) argued that, in contrast, sediment in higher-level transport undergoes little modification and thus retains the characteristics of the parent debris. While this distinction is a useful one (e.g. Vere and Benn, 1989; Benn and Ballantyne, 1994), it is an oversimplification because supraglacial transport is not always 'passive'. Boulders may undergo edge-rounding as debris is redistributed by ablation of the underlying ice (Benn and Evans, 1998; Owen et al, 2002).

Glacifluvial processes were overlooked in Boulton's classification, though they may transport large amounts of sediment over, beneath and through many valley glaciers. Glacifluvial transport is particularly important on low-gradient glaciers with extensive debris cover, where reservoirs of sediment can be accessed by meltwater (Kirkbride and Spedding, 1996; Spedding, 2000). Subglacial conduits can entrain sediment from the glacier bed, and englacial conduits collect debris by wall melting. Glacifluvial sediment can then be delivered to the supraglacial transport zone where conduits emerge at the surface, or following conduit closure or freezing and debris melt-out. Fluvially rounded cobbles and sorted sand occur in the debris covers of New Zealand valley glaciers, Ngozumpa Glacier (Nepal) and other glaciers. Such facies probably represent a tiny

Figure 15.1 Three examples of mountain glaciers, showing the diversity of glaciated valley landsystems. A) Chola Glacier, a debris-covered glacier in the Khumbu Himal. Note avalanche cones below the headwall, and large lateral moraines. B) Slettmarkbreen, a cirque glacier with little supraglacial debris, Jotunheimen, Norway. C) Un-named hanging glacier and reconstituted lower tongue, Lahul Himalaya. Note avalanche track leading to the lower glacier, and dissected moraines at lower right.

Figure 15.1 Three examples of mountain glaciers, showing the diversity of glaciated valley landsystems. A) Chola Glacier, a debris-covered glacier in the Khumbu Himal. Note avalanche cones below the headwall, and large lateral moraines. B) Slettmarkbreen, a cirque glacier with little supraglacial debris, Jotunheimen, Norway. C) Un-named hanging glacier and reconstituted lower tongue, Lahul Himalaya. Note avalanche track leading to the lower glacier, and dissected moraines at lower right.

fraction of the debris flushed out of such glaciers during the ablation season, which is supplied directly to the proglacial outwash system (Kirkbride, 2002). In terms of sediment discharge, the apparent dominance of supraglacial debris is misleading, because the supraglacial load represents an inefficient pathway. Englacial and supraglacial fluvial pathways, although rarely observed, may dominate landsystem development at the termini of large debris-covered glaciers, as shown by the volume of Holocene outwash valley fills, many orders of magnitude greater than the volume of Holocene ice-marginal moraines.

376 GLACIAL LANDSYSTEMS

376 GLACIAL LANDSYSTEMS

Picture Sediment Transport Paths

Figure 15.2 Debris transport paths in a valley glacier. 1 = burial of rockfall debris in accumulation area, 2 = englacial transport and melt-out in ablation area, 3 = basal traction zone, 4 = suspension zone, 5 = basal till (may undergo deformation), 6 = elevated debris septum below glacier confluence, 7 = diffuse cluster of rockfall debris, 8 = debris elevated from the bed by compressive flow and shear near the margin, 9 = ice-stream interaction medial moraine, 10 = ablation-dominant medial moraine, 11 = avalanche-type medial moraine, 12 = supraglacial lateral moraine. (From Benn and Evans (1998).)

Figure 15.2 Debris transport paths in a valley glacier. 1 = burial of rockfall debris in accumulation area, 2 = englacial transport and melt-out in ablation area, 3 = basal traction zone, 4 = suspension zone, 5 = basal till (may undergo deformation), 6 = elevated debris septum below glacier confluence, 7 = diffuse cluster of rockfall debris, 8 = debris elevated from the bed by compressive flow and shear near the margin, 9 = ice-stream interaction medial moraine, 10 = ablation-dominant medial moraine, 11 = avalanche-type medial moraine, 12 = supraglacial lateral moraine. (From Benn and Evans (1998).)

Debris passes between transport pathways by several processes, including melt-out, burial by snow, and ingestion by crevasses. In high-relief terrain, steep icefalls above low-gradient ablation zones commonly elevate debris from basal transport to high-level transport by avalanching and glacier reconstitution, supplying large volumes of debris to supraglacial covers

Medial moraines are distinctive features of many valley and cirque glaciers. Eyles and Rogerson (1978) proposed a comprehensive classification based upon the relationship between debris supply and the morphological development of the moraine. Three main types were recognized:

1. ablation-dominant (AD) moraines, which emerge at the surface as the result of the melt-out of englacial debris

Figure 15.3 Avalanche-reconstitution of a glacier tongue transferring englacial and basal debris septa into a supraglacial debris cover. Kaufmann Glacier, Mt Haidinger, New Zealand.

2. ice-stream interaction (ISI) moraines, which find immediate surface expression downstream from glacier confluences, often by the merging of two supraglacial lateral moraines, and

3. avalanche-type (AT) moraines, which are transient features formed by exceptional rockfall events onto a glacier surface (Fig. 15.2).

Although there are shortcomings with this classification (Small et al., 1979; Vere and Benn, 1989), no satisfactory alternative has been proposed, and it remains in common use (Benn and Evans,

Where supraglacial sediment is high relative to snow inputs, continuous debris covers typically form in glacier ablation zones. Such debris-covered glaciers are distinctively different from clean glaciers (Higuchi et al., 1980), both in terms of their dynamics and their associated depositional landsystems.

15.4 DYNAMICS OF DEBRIS-COVERED GLACIERS

Thin debris cover (<~5 cm) enhances ablation due to reduced albedo and increased absorption of short and longwave radiation, whereas thicker debris insulates the underlying ice and reduces ablation, because of its low thermal conductivity (Nakawo and Young, 1981). On debris-covered glaciers, debris thickness generally increases towards the glacier terminus, reversing the ablation gradient and causing ablation rates to be very small on the lower part of the glacier. The reduced ablation causes ablation zones to enlarge to offset mass gains in the accumulation zone. As a consequence, debris-covered glaciers in equilibrium have accumulation-area ratios (AARs) of

0.2.0.4, compared with values of 0.6-0.7 for clean glaciers (Benn and Evans, 1998).

Glacier response to climate fluctuations is strongly influenced by the degree of supraglacial debris cover. For clean glaciers, ice volume changes are reflected in oscillations of the glacier terminus. The response of debris-covered glaciers to climatic warming is dampened by the insulating effect of debris. However, if warming is sustained, such glaciers can enter a phase of very rapid ablation if ice-contact lakes expand by calving (Kirkbride, 1993; Reynolds, 2000; Benn et al., 2001). Retreat of debris-covered glaciers and the cessation of sediment delivery to terminal moraines may thus significantly lag climate changes (Benn and Owen, 2002).

During periods of glacier stability or thickening, the termini of heavily debris-loaded glaciers are foci of dramatic sediment aggradation, forming some of the most impressive glacial depositional landforms in the world (Owen and Derbyshire, 1993; Kirkbride, 2000). Considerable variation in landsystem development occurs between glaciers, ranging from steep fronted lateral-frontal moraines (sometimes referred to as latero-frontal or lateral-terminal) to lower gradient ice-contact debris fans and outwash heads. This variation largely depends on the relative supply of ice and debris to the terminal area, and the efficiency of its removal by meltwater. Shroder et al. (2000) contrast the terminus environments of three glaciers in the Nanga Parbat massif, Pakistan, and identify three primary controls on landform development:

1. overall sediment supply to the glacier by rockfall and avalanching, which determines the amount of debris available for ice-marginal deposition

2. the velocity of ice in the ablation zone, which controls whether debris accumulates supraglacially or is transferred to the ice-margin for deposition, and

3. the ability of fluvial processes to remove sediment from the ice margin, which determines whether sediment accumulation is focused in the ice-marginal or proglacial zones.

The common view that debris-covered glaciers are unresponsive to climate is not strictly true. Large lateral-terminal moraines can act as significant barriers to glacier advance, particularly if depositional rates are high, so mass balance variation on debris-covered glaciers is commonly manifest as thickening and thinning instead of advance and retreat. Research on such glaciers emphasizes negative mass balance conditions where ice is increasingly insulated under thickening supraglacial debris (Kirkbride and Warren, 1999; Nakawo et al., 1999; Naito et al., 2000). Under positive balance, when gradients and velocities are increased, debris covers may accentuate the effects of kinematic waves (Thomson et al., 2000), enhancing an expansionary tendency over multiple mass balance cycles.

Kirkbride (2000) suggested that supraglacial load increases over several mass balance cycles, which complicates the response of covered glaciers to climate variation. Indeed, it is debatable whether a true equilibrium between glacier volume and climate can ever be achieved. If mean specific ablation rates decline due to supraglacial loading, continued expansion of the ablation zone is a necessary consequence. Thus, under constant climate, a debris-covered glacier will have to advance to maintain equilibrium mass balance. The slow, sustained advances of glacier ice-cored rock glaciers similarly reflect glaci-dynamic influences and an expansionary tendency.

In summary, the terminus positions of debris-covered glaciers tend to be stable for long periods. Debris delivered to the ice margin from glacial transport is concentrated into large landforms with high preservation potential. Because high supraglacial loads are largely offset by low terminus velocities, it is the long-term stability of the ice-contact zone that is a key determinant on landsystem evolution.

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