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Radio-echo sounding data are used to investigate bed roughness beneath the three enhanced-flow tributaries of Slessor Glacier, East Antarctica. Slow-moving inter-tributary areas are found to have rough beds, while the bed of the northernmost tributary is relatively smooth. A reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary, and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times, so marine sediments may have accumulated here. Together, these factors are further support for the dominance of basal motion in this tributary, reported elsewhere. Conversely, although the other two Slessor tributaries may have water routed beneath them, they would not have been below sea level before the growth of the ice sheet, so cannot be underlain by marine sediments. They are also found to be rough, and, within the range of uncertainties, it is likely that basal motion does not play a major role in the flow of these tributaries. Perhaps the most interesting area, however, is a deep trough where flow rates are currently low but the bed is as smooth as the northern Slessor trough. It is proposed that, although ice deformation currently dominates in this trough, basal motion may have occurred in the past, when the ice was thicker.

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The Antarctic Climate Evolution (ACE) project is a new international research initiative to study the climate and glacial history of Antarctica by linking climate and ice sheet modeling studies with geophysical surveys and geological studies on and around the Antarctic continent (Fig. 1). The rationale for the ACE programme, outlined herein, was developed and refined, before, during and after the Antarctic Earth Science Symposium in Erice, Italy, in September 2001 (Cooper et al., 2002, Florindo et al., 2003).

An airborne radio-echo sounding campaign carried out in the upper reaches of Bailey Ice Stream and Slessor Glacier, in Coats Land, East Antarctica, has revealed that tributaries of enhanced flow lie within well-defined basal troughs and are separated from each other by bed highs. These new data indicate significant differences in ice thickness compared with those estimated in the Bedmap database. A numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas. Most flow in the enhanced flow tributaries of Slessor Glacier may also be explained by ice deformation alone. However, although ice deformation is also significant in the Bailey Ice Stream tributary, a large amount of basal motion is also required to fully explain flow velocities here. It is proposed that the trough in which Bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times. Along with water produced by geothermal heating, frictional heating, and through a reduction in the pressure melting point, basal motion may therefore be facilitated by the presence of a deformable, saturated till layer at the bed.