4.1. Ice Sheet Dynamics and Relationship to the Subglacial Substrate
 Ice flow through Belgica Trough is inferred to have been in the form of a fast flowing ice stream, based on several lines of evidence. (1) The bed forms are located in a cross-shelf bathymetric trough and such troughs are commonly the loci of modern and Quaternary ice streams [Stokes and Clark, 1999; Vaughan et al., 2001, 2003; Ó Cofaigh et al., 2005; Evans et al., 2005]. (2) There is a convergent pattern of ice flow feeding into the head of the trough [Stokes and Clark, 1999]. (3) A downflow transition in subglacial bed forms within the trough, from drumlins to MSGL has been observed [cf. Wellner et al., 2001; Ó Cofaigh et al., 2002; Stokes and Clark, 2002, 2003; Dowdeswell et al., 2004]. Crescentic overdeepenings around the upstream ends of several drumlins in Eltanin Bay are regarded as the product of localized meltwater erosion. Such elongate subglacial bed forms have now been observed beneath the region of accelerating ice flow in the upstream part of the modern Rutford Ice Stream in Antarctica [King et al., 2003]. King et al.  have also reported evidence of channelized subglacial meltwater flow beneath a neighboring part of this ice stream, which is consistent with our interpretation of localized meltwater erosion in Eltanin Bay. (4) MSGL are the most elongate subglacial bed form that we document and they are formed in the upper part of an acoustically transparent sediment unit in the trough. A similar acoustic facies has been described from other paleoice stream troughs around Antarctica, and sediment cores show that it is typically a weak, massive diamict that is the product of subglacial deformation [Ó Cofaigh et al., 2002, 2005; Dowdeswell et al., 2004; Evans et al., 2005]. On the basis of the acoustic properties of the sediment unit within Belgica Trough, we similarly interpret it as a subglacial till that may, at least partially, be the product of sediment deformation. A subglacial till interpretation is also supported by core GC374 which was recovered from the acoustically transparent unit in outer Belgica Trough. At least the lower section of the structureless diamict, which exhibits high shear strength, is interpreted as a subglacial till [cf. Wellner et al., 2001; Dowdeswell et al., 2004; Ó Cofaigh et al., 2005; Evans et al., 2005]. MSGL formation is therefore associated with the presence of a soft till layer within the trough.
 In addition, the outward bulging bathymetric contours in front of Belgica Trough (Figure 8) and the acoustically transparent sediment lenses imaged by TOPAS on the slope are interpreted as representing a trough mouth fan composed of debris flow deposits [cf. Laberg and Vorren, 1995, 2000; Taylor et al., 2002; Ó Cofaigh et al., 2003]. In conjunction with seismic profiles from the outermost shelf and upper slope [Cunningham et al., 1994; Nitsche et al., 1997], these data imply that progradation of the margin occurred in front of Belgica Trough [cf. Vorren et al., 1989, 1998; Dowdeswell et al., 1996]. Trough mouth fans are formed where ice sheets reach the continental shelf break, typically as fast flowing ice streams, and deliver large volumes of glaciogenic sediment directly to the upper slope by debris flow processes [Vorren and Laberg, 1997; Vorren et al., 1998; Ó Cofaigh et al., 2003].
 GZWs on the midshelf and inner shelf comprise localized sediment accumulations, characterized by dipping subbottom reflectors truncated by a gently dipping reflector and overlain by acoustically transparent sediment (Figure 7). Such GZWs are formed by the deposition of unconsolidated, saturated till that is transported subglacially by an ice stream and deposited at the grounding line [cf. Alley et al., 1989]. Top set beds are a product of direct deposition as basal till at the ice stream base, while foreset beds are formed by release of till at the grounding line and subsequent remobilization of this sediment by debris flow processes. GZWs can be formed during either ice sheet advance, ice sheet retreat or during a readvance of the ice margin. Swath bathymetric records of GZWs in inner Belgica Trough show that MSGL are incised into the surface of the wedges but do not continue across them; that is the fronts of the GZWs (transverse sediment scarps) interrupt the MSGL (Figure 4a). This indicates that the episodes of grounding zone stabilization did not occur during ice sheet advance because, in that case, the MSGL would be continuous across the scarps that mark the former grounding line, whereas the scarps interrupt the MSGL. This implies that the GZWs formed during either ice sheet retreat or during readvances of the ice margin. Such readvances are most likely to have occurred during regional deglaciation, hence the timing of GZW formation would have been broadly similar in both cases. Iceberg scours occur in the outer part of Belgica Trough in water depths of greater than 600 m (Figure 5a). The depth of these scours within the trough suggests that they are associated with ploughing by large icebergs calved from the retreating ice sheet margin during regional deglaciation.
 The location of ice streams is commonly associated with areas of soft substrate, either in the form of unconsolidated sediments, or soft and easily erodible bedrock [Anandakrishnan et al., 1998; Studinger et al., 2001]. Streaming over these soft beds is either by subglacial sediment deformation or basal sliding, or some combination of these processes [Alley et al., 1986; Engelhardt and Kamb, 1998; Kamb, 2001]. It has also been suggested, based primarily on investigations of paleoice streams, that streaming can occur over areas of hard bed composed of crystalline bedrock [Evans, 1996; Ó Cofaigh et al., 2002; Stokes and Clark, 2003]. In some localities on the Antarctic continental shelf, a transition from streamlined crystalline bedrock to drumlins, and then to MSGL in soft sediments has been observed [Shipp et al., 1999; Wellner et al., 2001]. This geomorphological transition has been inferred to represent a downflow change from slow ice sheet flow over the crystalline bedrock, to a zone of flow acceleration recorded by the drumlins (representing the onset zone of an ice stream), to the high velocities of the main ice stream trunk as recorded by the MSGL [Wellner et al., 2001].
 On the basis of their rough and irregular appearance on the swath records, and the apparent absence of sediment on TOPAS records, the crudely streamlined forms in inner Eltanin Bay appear to be formed largely in bedrock. North of about 72°35′S and west of 81°W these crudely streamlined forms evolve into drumlins and then MSGL. The MSGL in Belgica Trough are formed in an acoustically transparent sediment unit that is interpreted as a till. Assuming the most elongate bed forms record the highest flow velocities [cf. Clark, 1993; Stokes and Clark, 1999, 2002; Ó Cofaigh et al., 2002], MSGL record streaming flow through Belgica Trough toward the shelf edge. Thus the highest inferred flow velocities occurred over the area of soft bed. Shorter bed forms in Eltanin Bay that exhibit a convergent orientation are predominantly formed in bedrock.
 We propose that the zone of crudely streamlined bedrock and drumlins in Eltanin Bay represents the onset zone of a paleoice stream in Belgica Trough. Streaming would be enhanced by the presence of a topographic trough, which would facilitate strain heating and an increase in velocity [cf. Iken et al., 1993]. However, the relationship of the MSGL to the area of the trough underlain by a soft bed implies that subglacial geology also acted as a major control on the development of streaming flow.
4.2. Paleoglaciology of the West Antarctic Ice Sheet on the Bellingshausen Sea Margin at the LGM
 The streamlined subglacial bed forms imaged by swath bathymetry in Belgica Trough, Eltanin Bay and the Ronne Entrance record flow of a grounded ice stream toward the edge of the continental shelf. Flow directions mapped from the orientation of the bed forms show that ice flow into the head of Belgica Trough was the result of convergence of ice emanating from Eltanin Bay and the Ronne Entrance (Figure 9). This coalescent ice mass then flowed along the trough, and reached at least as far north as 70°37′S on the outermost shelf. Belgica Trough was thus the pathway for a major ice sheet outlet that was fed by ice draining from the southern part of the Antarctic Peninsula Ice Sheet through the Ronne Entrance, as well as ice from the WAIS draining through Eltanin Bay. These ice masses coalesced in Belgica Trough and extended to the outermost continental shelf and probably the shelf edge (Figure 9). The presence of an additional outlet that flowed northward out of the Ronne Entrance, is implied by the NNW orientated MSGL and drumlins immediately west of Beethoven Peninsula (Figures 1 and 2).
Figure 9. Paleoice flow directions in the study area mapped from the orientation of streamlined subglacial bed forms. Note bifurcation of flow emanating from the Ronne Entrance and convergence of flow into the head of Belgica Trough. Area enclosed by solid line represents the interpreted extent of the Belgica Trough drainage basin assuming that the main ice divide during times of glacial maximum was in the same position as today. The area to the west also enclosed by a solid line is a possible additional part of the drainage basin. Much of the ice sheet bed in Ellsworth Land is near or below sea level, and the ice divide in this area may have been in a different position at glacial maximum. Map projection is polar stereographic.
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 The subglacial bed forms that we document record the most recent episode of ice stream advance through Belgica Trough to the continental shelf edge. We estimate the area of the drainage basin that fed this ice stream by assuming that the main ice divides in West Antarctica and Palmer Land at times of glacial maximum were in the same positions as today. We inferred the positions of the lateral boundaries of the drainage basin on the basis of topographic data and paleoice flow directions interpreted from the multibeam data. By this method we conclude that the basin probably encompassed southwestern Palmer Land and parts of southern Alexander Island and the Bryan Coast of Ellsworth Land (Figure 9). An additional part of the Bryan Coast farther west may also have contributed ice to the Belgica Trough, but we only observe bed forms that suggest flow from this region in one small area. During periods when the Belgica Trough paleoice stream reached the continental shelf edge, the total area of the drainage basin feeding this outlet would have been about 217,000 km2 if it did not include the western part of the Bryan Coast, and about 256,000 km2 if it did include that region.
 Bedrock topography in central Palmer Land rises 1500–2000 m above sea level and has an important effect in concentrating precipitation [Turner et al., 2002]. Therefore we consider it unlikely that the main ice divide was displaced from central Palmer Land at times of glacial maximum. However, between 73°W and the Ellsworth Mountains (Figure 9), bedrock topography is generally of low elevation and three-dimensional models of the glacial maximum WAIS show the main divide in Ellsworth Land offset to the south of its present position [e.g., Stuiver et al., 1981; Huybrechts et al., 2002]. Our estimate of 217,000–256,000 km2 for the area of the drainage basin feeding the Belgica Trough ice stream compares with areas of 48,000–570,000 km2 for modern drainage basins within the WAIS, and a total area of grounded ice in the modern WAIS of about 2,000,000 km2 [Vaughan et al., 1999]. The only two modern drainage basins within the WAIS that exceed 300,000 km2 in area (Siple Coast and Pine Island-Thwaites) are composite basins with more than one outlet.
 The present net surface mass balance in the parts of Palmer Land and Ellsworth Land that we interpret as having been within the Belgica Trough drainage basin is >500 kg m−2 yr −1, which is more than three times the Antarctic average of 149 kg m−2 yr−1 [Vaughan et al., 1999; Turner et al., 2002]. If net surface mass balance was also higher than average in this area during glacial periods, then the outflow from the basin would have been disproportionately large compared to its area. Therefore the Belgica Trough probably represents one of the main outlets of the WAIS during late Quaternary glacial periods.
 Glacial trim lines on the northern part of the eastern flank of the Ellsworth Mountains (Figure 9) indicate a former ice surface elevation up to 1900 m above present, ignoring isostatic compensation [Denton et al., 1992]. This observation is consistent with the hypothesis that a major ice divide was close to the northern end of the range during glacial periods. However, the age of the trim lines remains unknown, so it is not clear whether or not they represent the LGM ice surface, and it is even possible that they formed in pre-Quaternary times [Denton et al., 1992].
 Although there is some crosscutting of bed forms in the outer shelf trough (Figure 4b), this is localized in occurrence and relatively minor. The trajectory of former ice flow through the trough was consistently toward the shelf edge (Figure 9). MSGL are formed in the upper part of a subglacial till unit, and in core section this till is overlain directly by deglacial and postglacial sediments (Figure 6). These sediments are typically ≤0.5 m in thickness. The streamlined subglacial bed forms that we image on the floor of Belgica Trough formed during the most recent episode of ice advance to the shelf edge. In conjunction with the thin sequence of deglacial and postglacial sediments that overlie the till associated with this advance, the simplest interpretation is that the timing of ice sheet advance occurred during the last glaciation and that grounded ice therefore reached the shelf edge at the LGM.
 The new data presented in this paper fill a major gap in reconstructions of the Antarctic Ice Sheet during the last glacial cycle [cf. Bentley and Anderson, 1998; Bentley, 1999]. They indicate an extensive WAIS at the LGM on the Bellingshausen Sea continental margin, which advanced to the continental shelf edge. In conjunction with data from further to the west in Pine Island Bay [Lowe and Anderson, 2002; Dowdeswell et al., 2005; J. Evans et al., Extent and dynamics of the West Antarctic Ice Sheet on the outer continental shelf of Pine Island Bay, Amundsen Sea, during the last glaciation, submitted to Marine Geology, 2005] and northeast around the Antarctic Peninsula [Pudsey et al., 1994; Ó Cofaigh et al., 2002, 2005; Evans et al., 2004, 2005] this implies an extensive ice sheet configuration during the LGM along the Antarctic Peninsula, Bellingshausen Sea, and Amundsen Sea margins, characterized by fast flowing ice streams which drained extensive basins of the WAIS and Antarctic Peninsula Ice Sheet through cross-shelf bathymetric troughs, and reached the outermost shelf or shelf edge.