Geomorphological Criteria of Ice Stream Activity

Ice streams are discrete features within an ice sheet and so we may expect them to leave behind a unique suite of glacial landforms. By coupling the known characteristics of existing ice streams (outlined above) with traditional theories of glacial geomorphology, it is possible to predict several geomorphological products of palaeo-ice stream activity. While individual criteria are not

Ice Stream type Name

Length

Width

Thickness

Velocity

Drainage

References

(km)

(km)

(m)

(m.a-1)

basin area (km2)

Pure Ice Streams Ice Stream A

>200

-50

-1,000

217-254

No data

Rose (1979); Shabtaie et al. (1987); Echelmeyer et al. (1994)

Ice Stream B

500

35

1,000->2,000

450

163,000

Rose (1979); Shabtaie and Bentley (1987);

300

30-80

>500 -500

Alley et al. (1989); Scambos and Bindschadler (1993); Whillans and Van der Veen (1993); Engelhardt and Kamb (1998)

Ice Stream C

<400

50-60

50 5

122,000

Rose (1979); Fastook (|987); Shabtaie and Bentley (1987); Whillans et al. (1987); Whillans and Van der Veen (1993)

Ice Stream D

550

46-59

800

420-670

104,000

Rose (1979); Scambos and Bindschadler

30-82

927-1,432

100-370

(1993); Scambos et al. (1994); Hodge and Doppelhammer (1996)

Ice Stream E

-320

75-100

975-1,091

400-550 -400

131,000

Rose (1979); Bentley (1987); Scambos and Bindschadler (1993)

Topographic Ice Pine Island Glacier

200

26

1,564

1300-2600

220,000

Bentley (1987); Lucchitta et al. (1995);

Streams

-300

30

-2,000

Jenkins et al. (1997)

Thwaites Glacier

c. 300

c. 83

-2,000

2,200-3,400

121,000

Bentley (1987); Rosanova et al. (1998); Ferrigno et al. (1998)

Rutford Ice Stream

>150

18-26

1,640-2,250

360-400 302-377

-36,000

Doake et al. (1987); Frolich and Doake (1988)

Jutulstraumen Ice

c. 300

c. 40

2,000

443

124,000

H0ydal (1996)

Stream

Jakobshavns Isbr®

70-80

-6

2,500

800-7,000

10,000

Lingle et al. (1981); Echelmeyer and

85-90

1,900-2,600

8,360

Harrison (1990); Echelmeyer et al. (1991); Fastook et al. (1995); Clarke and Echelmeyer (1996)

Table 9.1

Table 9.1

2l2 GLACIAL LANDSYSTEMS

necessarily exclusive to ice-stream activity, collectively they can be viewed as an idealized palaeo-ice stream imprint. The criteria are used to assemble a conceptual landsystem model of the bed of former ice streams, and it is envisaged that such models can provide an observational template upon which hypotheses of palaeo-ice streams can be better based.

9.6.1 Characteristic Shape and Dimensions

Palaeo-glaciological reconstructions rely heavily on the use of palaeo-flow indicators such as striae, roches moutonnées, flutes, drumlins, ribbed (Rogen) moraine, and mega-scale glacial lineations, to constrain ice-sheet flow geometry. If such indicators are found to record a pattern resembling the geometry and size of typical ice streams then this may be the most obvious clue as to the existence of an ice stream. Subglacial bedforms are the most widespread flow indicators and may cover around 70 per cent of the beds of former ice sheets. In contemporary ice streams, the onset of streaming flow (or onset zone) is characterized by a large convergence zone (Fig. 9.3), whereby slower moving ice is gradually incorporated into the ice-stream trunk (probably by tributaries). Thus, subglacial bedforms produced by an ice stream should exhibit a large degree of convergence in the onset zone, leading to a much narrower and well-defined trunk.

Any such pattern should be at a scale appropriate to ice streams, typically greater than 20 km wide and 150 km long. We acknowledge that contemporary ice streams may not be a representative sample of the population of ice streams that have ever existed, and so there is plenty of scope for an ice stream to be bigger or even smaller, particularly if the shape is consistent with ice-stream activity.

9.6.2 Bedform Signature of Fast Ice Flow

Mega-scale glacial lineations (MSGL) are elongate streamlined ridges of sediment (Fig. 9.4) produced subglacially and are similar to flutes and drumlins but much larger in all dimensions

Figure 9.4 Mega-scale glacial lineations observed from a satellite image. Ice flow was towards the northwest. These ridge-groove systems, expressed in surficial sediment, extend for tens of kilometres, and are a record of fast ice flow. Note the highly parallel nature of landforms over great distances and with no discordances in flow pattern. Image is about 30 km across and is centred at 101° 51' W, 64° 02' N, in the Dubawnt Lake region of northern Canada.

Figure 9.4 Mega-scale glacial lineations observed from a satellite image. Ice flow was towards the northwest. These ridge-groove systems, expressed in surficial sediment, extend for tens of kilometres, and are a record of fast ice flow. Note the highly parallel nature of landforms over great distances and with no discordances in flow pattern. Image is about 30 km across and is centred at 101° 51' W, 64° 02' N, in the Dubawnt Lake region of northern Canada.

(Clark, 1993). Typical lengths are 6-100 km, widths of 200-1,300 m and spacing of 200 m to 5 km. Given the association of MSGLs with inferred high strain rates and fast ice velocity it has been argued that they can be be used as an indicator of palaeo-ice stream location (Clark, 1993; 1999; Stokes and Clark, 1999). Until recently, these assumptions could not be tested because the foregrounds of contemporary ice streams are largely inaccessible. However, using swath bathymetry and high-resolution seismic investigations, MSGLs have, for the first time, been discovered on the Antarctic continental margin (Shipp et al., 1999; Canals et al., 2000). We take these remarkable finds as validation of the association between MSGLs and fast flow as they are found to lie distal to positions of existing or inferred ice streams. MSGL can thus be used as reliable indicators of palaeo-ice stream tracks.

9.6.3 Proxies for Ice-Stream Velocity Field

Ice-stream velocities have two main characteristics. First, the velocity of a marine-based ice stream steadily increases all the way to the grounding line. Second, ice-stream velocities remain high all the way across the ice stream until there is an abrupt decrease in the marginal areas. This unique characteristic is described as plug flow and is exhibited by all ice streams. Figure 9.5 illustrates this pattern of surface velocity across an ice stream, from the centre line to the slower-moving adjacent ice.

A manifestation of the rapid velocity of an ice stream may be highly attenuated streamlined bedforms. Elongation ratio (length divided by width) is a useful way of quantifying the degree of attenuation of subglacial bedforms (such as flutes, drumlins and MSGLs). Although there is no method for recovering former flow velocities from elongation ratios, there are many studies that report correlations between inferred fast ice flow and high elongation ratios (Boyce and Eyles, 1991; Clark, 1994a; Hart, 1999; Stokes and Clark, 2002b). Thus, swarms of highly attenuated drumlins and MSGLs could record the velocity field of an ice stream. We would expect to find (i) highest elongation ratios in the trunk rather than the convergent onset zone, and (ii) higher elongation ratios along the central axis of the trunk. For marine-terminating ice

Figure 9.5 Typical variation in velocity across an ice stream, from the centre line to adjacent slower-moving 'sheet ice'. Note that the ice stream moves by plug flow, with slightly higher velocities in the centre, decreasing outwards and with a dramatic change across the shear margin. (Simplified from Echelmeyer et al., 1994).

Figure 9.5 Typical variation in velocity across an ice stream, from the centre line to adjacent slower-moving 'sheet ice'. Note that the ice stream moves by plug flow, with slightly higher velocities in the centre, decreasing outwards and with a dramatic change across the shear margin. (Simplified from Echelmeyer et al., 1994).

streams, elongation ratios should steadily increase downstream towards the grounding line. For terrestrially terminating ice streams elongation ratios should decrease towards the outer margins of the lobate terminus.

Further proxy evidence for velocity fields may be gleaned from specific erratic dispersal patterns. Dyke and Morris (1988) recognized two types of dispersal train, schematically represented in Fig. 9.6. The Boothia-type dispersal train forms when an abrupt lateral variation in ice velocity transports distinctive sediment from a large source area. Such a lateral variation in ice velocity is a unique characteristic of ice streams, and Boothia-type dispersal plumes may be a product of their activity. In contrast, the Dubawnt-type dispersal plume implies no lateral variation in velocity. Although it may appear similar to a Boothia-type dispersal plume, the source area of the sediment is the key control on the pattern and it can be formed by slow sheet-flow. Hence, it is important to identify the spatial extent of the source area from which the distinctive till is transported. There is not necessarily a blatant connection between ice streams and dispersal trains, but when found in conjunction with other criteria, it may be highly suggestive of ice-stream activity.

Boothia type Dubawnt type

Figure 9.6 Specific erratic dispersal patterns may be used for a proxy of relative ice velocity in order to identify an ice stream. Two rock types (A and B) experiencing stream flow may give rise to a plume of erratic dispersal beneath the faster flowing stream ice, called 'Boothia-type' dispersal. This must not be confused with the more usual 'Dubawnt-type' dispersal that produces an apparent plume simply because of a restricted source area. (From Dyke and Morris, 1988).

Ice-stream flow Ice-sheet flow

Figure 9.6 Specific erratic dispersal patterns may be used for a proxy of relative ice velocity in order to identify an ice stream. Two rock types (A and B) experiencing stream flow may give rise to a plume of erratic dispersal beneath the faster flowing stream ice, called 'Boothia-type' dispersal. This must not be confused with the more usual 'Dubawnt-type' dispersal that produces an apparent plume simply because of a restricted source area. (From Dyke and Morris, 1988).

9.6.4 Abrupt Lateral Margins

Ice streams are characterized by their abrupt lateral margins bordered by slower-moving ice (Figs. 9.1 and 9.5). The characteristic geomorphology inscribed by a former ice stream would be expected to exhibit an abrupt margin or an abrupt zonation of subglacial bedforms at the margin. We would expect a high density of bedforms within the ice stream to cease abruptly. This is in contrast to most drumlin patterns, which gradually diminish in drumlin density orthogonal to flow. For example, Hodgson (1994) noted a well-defined margin of a drumlin field when postulating the existence of an ice stream flowing northwards over the eastern portion of Victoria Island in the Canadian Arctic. Figure 9.7 illustrates the abrupt nature of this change, which can be taken to record the ice-stream margin. Dyke and Morris (1988) also noted an extremely abrupt margin to a bedform pattern formed by an inferred ice stream on eastern Prince of Wales Island. In addition, Kleman and Borgstrom (1994) and Kleman et al. (1999) suggested that an abrupt margin can be produced at the transition between cold- and warm-based ice. Given the fact that some ice-stream margins may well be characterized by such a transition, then abrupt marginal areas (<2 km) could be used to de-limit the width of a palaeo-ice stream (see Kleman et al., 1999).

Figure 9.7 Bed geomorphology across a palaeo-ice stream margin, viewed from a satellite image. Note the contrast between hummocky and slightly drumlinized terrain in the west and the strong ice-stream signature (flow towards the north) in the east. The image is around 40 km across and the extreme abruptness of the margin (<2 km) is apparent. The margin forms the western limit of the M'Clintock Channel Ice Stream on Victoria Island, Arctic Canada. (From Clark and Stokes, 2001).

Figure 9.7 Bed geomorphology across a palaeo-ice stream margin, viewed from a satellite image. Note the contrast between hummocky and slightly drumlinized terrain in the west and the strong ice-stream signature (flow towards the north) in the east. The image is around 40 km across and the extreme abruptness of the margin (<2 km) is apparent. The margin forms the western limit of the M'Clintock Channel Ice Stream on Victoria Island, Arctic Canada. (From Clark and Stokes, 2001).

9.6.5 Subglacial Ice Stream Shear Margin Moraines

An abrupt lateral margin associated with a change in ice velocity (as described above) may also be conducive to the generation of characteristic landforms. We might expect a concentration of sediment accumulation at this boundary, producing an ice stream shear margin moraine. High melting rates should exist at ice-stream margins arising from strain-heating and the shear experienced between slow- and fast-moving ice. In addition, crevassed ice may also allow surface meltwater to penetrate to the ice-sheet bed. Sub- or englacial drainage of these waters may serve to transport and concentrate sediment in the marginal zone, and upon deglaciation, leave prominent moraines, composed primarily of glacifluvial material (e.g. Punkari, 1997). Alternatively, if sediment is eroded and entrained in the onset zones of ice streams, then downstream convergence of flow will concentrate this englacial debris into higher densities. In order to produce sediment accumulations at the margin, we hypothesize that elevated levels of strain-heating could produce sufficient melting for debris to be preferentially deposited here.

On southeastern Prince of Wales Island, Dyke and Morris (1988) identified a single, narrow ridge of till that delineated the western side of a drumlin field. This ridge can be traced for up to 68 km but is less than 1 km in width (Dyke et al., 1992). In a discussion of its origin, the ridge was interpreted as being a 'lateral shear moraine', marking a shear zone at the side of an ice stream, separating fast-flowing ice from slower-flowing cold-based ice (Dyke and Morris, 1988).

Stokes and Clark (2002a) discuss ice stream shear margin moraines and report four examples associated with the former M'Clintock Channel Palaeo-Ice Stream (Hodgson, 1994; Clark and Stokes, 2001), Victoria Island, Arctic Canada. The moraines range in length from 11-22 km, maintain fairly constant widths of about 500 m and range in height from 10 to 50 m above the surrounding terrain. They are composed of carbonate drift of a similar composition to the ice-stream bedforms and have been laid down irrespective of local and regional topography. Two of the moraines display a lateral offset and are thought to reflect minor migrations of the ice-stream margin. Their mode of formation is explored and it is suggested that they occur when erosion at the ice-stream margin provides a surplus of sediment, which is 'smeared out' in the downstream direction.

Ice stream shear margin moraines are subglacial accumulations of sediment that form at the margins of active ice streams. They mark the shear zone between fast- and slow-moving ice, and may or may not coincide with an abrupt change in the basal thermal regime.

9.6.6 Spatially Focused Sediment Delivery

Although not necessarily indicative of ice-stream activity, focused accumulations of sediment on a continental shelf or slope may complement, and indeed strengthen terrestrial evidence for ice-stream flow. Vorren and Laberg (1997) have identified huge submarine till deltas, thought to have been produced by ice streams draining the northwestern part of the Fennoscandian Ice Sheet and the Barents Sea Ice Sheet.

Because a stable ice-sheet margin (i.e. without an ice stream) would not deliver such concentrated accumulations of sediment to the continental shelf, it is clear that offshore sediment accumulations can provide a valuable clue to the existence of marine-based ice streams. Clearly, if terrestrial evidence that suggests ice stream activity is available, the identification of offshore sediment accumulations serves to support such hypotheses. Furthermore, such fans, if well dated, can also be used as proxies for former ice velocities and discharges.

9.6.7 Summary

The above criteria are summarized in Table 9.2, which indicates how they are derived from the characteristics of existing ice streams. Flow indicators (e.g. drumlins and MSGLs) of a palaeo-ice stream need to have a distinctive well-defined pattern, or flow-set, that conforms to the characteristic shape and dimensions of ice streams, and display a large degree of convergence in the onset area. It is crucial that such bedforms are significantly different in morphology and elongation from surrounding flow indicators. Further verification may come from proxies for the velocity field such as the pattern of variation in bedform elongation ratios or by the distinctive 'Boothia-type' dispersal trains. For example, if we could find a pattern of distinct bedforms that conforms to the shape of an ice stream and if the bedform elongation ratios within it vary as would be expected both across and down-ice, then this is very strong evidence for a palaeo-ice stream. A manifestation of the sharply delineated margin found on ice streams may be an abrupt lateral margin to the bedform pattern and ice stream shear margin moraines. Evidence of a deforming till layer and sediment accumulation fans are not necessarily indicative of ice-stream flow but may provide substantial supporting evidence when found in conjunction with the other criteria.

The focus of this chapter has been on identifying 'pure' palaeo-ice streams from their bed imprint. Many topographic ice streams, discharging through large bedrock troughs, may be erosional in nature and leave an entirely different record. From erosional evidence (whalebacks, roches moutonees) it is probably much harder to demonstrate that these were definite ice-stream tracks, rather than simple topographic capture of flow leading to greater lengths of time of occupancy and slightly higher flow velocities. Evans (1996) argues that topographic ice streams are recognizable

Contemporary ice stream characteristic

Proposed geomorphological signature

A. Characteristic shape and dimensions

1.

Characteristic shape and dimensions (>20 km wide and >150 km long) of distinct flow pattern

2.

Highly convergent flow patterns leading into a trunk

B. Rapid velocity

3.

Bedform signature of fast flow; MSGLs and highly attenuated drumlins (length:width >10:1 100:1)

4.

Boothia-type erratic dispersal trains

C. Distinct velocity field (plug flow, downstream variation in velocity)

5.

Expected spatial variation in MSGL and drumlin elongation ratios

6.

Boothia-type erratic dispersal trains

D. Sharply delineated shear margin

7.

Abrupt lateral margins (<2 km)

8.

Ice stream shear margin moraines

E. Spatially focused sediment delivery

9.

Submarine till delta or sediment fan

Table 9.2

Table 9.2

from erosional evidence, but Mathews (1991) cautions against the simple equation that a glacially eroded trough equals an ice stream. For topographic ice streams the best lines of evidence are likely to be significant volumes of sediment deposited as trough mouth fans. If dating of these indicates high sediment efflux over short bursts of time, then the inference of an ice stream seems reasonable (i.e. Dowdeswell et al., 1996; Vorren and Laberg, 1997).

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