Significance of Palaeo Ice Streams

9.2.1 Palaeo-Ice Streams and Ice-Sheet Reconstructions

The large flux within ice streams has a profound effect on ice sheet configurations, including drainage basin and ice divide locations, and local and regional ice sheet topography. This is demonstrated by the effect that ice streams have on the mass balance, flow configuration and ice

Figure 9.1 An Antarctic ice stream (Byrd Glacier) viewed from a satellite image. Flow is towards the top of the image. Note the upstream convergence of flow leading into the main trunk, and the sharp margins between fast- and slow-moving ice. At the top of the image, flow diverges as it spreads into the floating ice shelf. Image is about 100 km in width.

Figure 9.1 An Antarctic ice stream (Byrd Glacier) viewed from a satellite image. Flow is towards the top of the image. Note the upstream convergence of flow leading into the main trunk, and the sharp margins between fast- and slow-moving ice. At the top of the image, flow diverges as it spreads into the floating ice shelf. Image is about 100 km in width.

divide positions of the Greenland and Antarctic ice sheets. It is estimated, for example, that around 90 per cent of Antarctica's drainage is accounted for by ice streams (Morgan et al., 1982) even though they occupy only around 13 per cent of the ice-sheet perimeter (Paterson, 1994). Clearly, the control that ice streams have is disproportionate to their size. Given that ice streams appear to be an intrinsic part of the ice-sheet system (i.e. will always tend to occur as 'release valves' within large ice sheets), then any reconstruction of former ice-sheet configuration and dynamics that neglects ice streams is likely to be seriously flawed. This applies both to reconstructions built by geomorphological inversion or by numerical modelling. For the former we need to be able to recognize former ice stream tracks, and for the latter we need numerically defined flow laws governing fast-flowing ice and must know the processes that initiate streaming flow and restrict its spatial extent. We regard that locating the positions of former ice streams is of paramount importance when reconstructing ice sheets, and their elucidation is likely to radically alter our views of ice-sheet geometries and their tempo and pattern of change.

9.2.2 Palaeo-Ice Streams and Climate Change

The marine geological record has provided important evidence of the role of ice streams in forcing 'abrupt' (decadal to millennial) climate changes. Episodes of ice streaming, particularly from the eastern margin of the former North American (Laurentide) Ice Sheet, were responsible for large iceberg discharge events into the North Atlantic between 60,000 and 10,000 years ago (Bond et al., 1992). Evidence for these events comes from bands of ice-rafted debris found in ocean cores, known as Heinrich layers. Andrews and Tedesco (1992) have specifically linked the carbonate detritus associated with the two most recent Heinrich events to sedimentary rocks eroded by an ice stream in Hudson Strait. It has been postulated that the influx of freshwater resulting from massive iceberg discharge was sufficient to cause changes in sea surface temperature and salinity, which had a considerable impact on ocean circulation and northern hemispheric climate (Broecker and Hemming, 2001). Although the trigger for the ice streaming itself remains unclear, it is now recognized that ice streams are instrumental in driving abrupt changes in high latitude climate and oceanographic circulation (Bond and Lotti, 1990; Andrews, 1998).

In addition, climate modelling by Manabe and Broccoli (1984) has demonstrated the important coupling between ice-sheet elevation and atmospheric circulation, and Shin and Barron (1989) suggested that North Atlantic climate is highly sensitive to ice-sheet elevation. Because ice streams have the ability to rapidly drain large portions of ice sheets, they play a critical role in controlling overall ice-sheet thickness and therefore, may impact on atmospheric circulation.

9.2.3 Palaeo-Ice Stream Beds and Basal Processes

Current research in western Antarctica is striving to ascertain the basal characteristics and processes of the ice streams (e.g. Kamb, 2001). While providing invaluable insights into the nature of the ice stream bed, borehole investigations are limited by the scale of the investigation, and seismic studies suffer from a lack of detailed resolution. In an ideal world, we would like to view the whole ice stream bed at a variety of scales. Studying former ice stream tracks allows us to do just that, and if we can confidently find a former ice stream bed, we have an unprecedented opportunity to investigate its basal characteristics on a variety of scales, from large scale mapping of geomorphology to micromorphological till analysis.

Palaeo-ice stream beds can provide information with regard to the thermal conditions of the ice sheet, the basal topography, the bed roughness, the hard-bed-to-soft-bed fraction and the geology and lithology, all of which have been hypothesized to be critical with respect to ice-stream location and functioning. However, it should be noted that the bedform record of a palaeo-ice stream may only be related to the final stages of ice-stream operation (i.e. conditions immediately prior to shut down), and that this may have suffered from post-depositional modification, particularly during deglaciation.

9.2.4 Palaeo-Ice Streams and Sediment Transport

While some ice streams may only 'smear' or redistribute sediments, others are extremely powerful erosional agents. In particular, some marine-terminating ice streams may be the most important mechanism by which sediment is delivered to continental margins, and their fluxes have been compared with the efficacy of the largest fluvial systems (e.g. Mississippi delta), despite their far shorter duration (Elverhei et al., 1998). A consequence of this is that ice-stream location and vigour determines the distribution and volume of major accumulations of sediments, or fans. The location, deposition rate, sediment volume and facies architecture have strong practical implications for mineral exploration and geohazards because gravity-driven slumping may occur within these fans.

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