Relationship between continental rise development and palaeo-ice sheet dynamics, Northern Antarctic Peninsula Pacific margin

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1 Quaternary Science Reviews 25 (2006) Relationship between continental rise development and palaeo-ice sheet dynamics, Northern Antarctic Peninsula Pacific margin David Amblas a, Roger Urgeles a, Miquel Canals a,, Antoni M. Calafat a, Michele Rebesco b, Angelo Camerlenghi a, Ferran Estrada c, Marc De Batist d, John E. Hughes-Clarke e a GRC Geociències Marines, Universitat de Barcelona, Martí i Franquès s/n, E Barcelona, Spain b Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, Sgonico, Trieste, Italy c CSIC Institut de Ciències del Mar, Passeig Marítim Barceloneta 37-49, Barcelona, Spain d Renard Centre of Marine Geology, Ghent University, Krijgslaan 281 S8, B-9000 Gent, Belgium e Ocean Mapping Group, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Received 17 December 2004; accepted 10 July 2005 Abstract Acquisition of swath bathymetry data west of the North Antarctic Peninsula (NAP), between 631S and 661S, and its integration with the predicted seafloor topography of Smith and Sandwell [Global seafloor topography from satellite altimetry and ship depth soundings. Science 277, ] reveal the links between the continental rise depositional systems and the NAP palaeo-ice sheet dynamics. The NAP Pacific margin consists of a wide continental shelf dissected by several troughs, tens of kilometres wide and long. The Biscoe Trough, which has been almost entirely surveyed with multibeam sonar, shows spectacular fan-shaped streamlining sea-floor morphologies revealing the presence of ice streams during the Last Glacial Maximum. In the study area the continental rise comprises the six northernmost sediment mounds of the NAP Pacific margin and the canyon-channel systems between them. These giant sediment mounds have developed since the early Neogene by southwest flowing bottom currents, which have redistributed along the margin the fine-grained component of the turbiditic currents flowing down canyon-channel systems. The widespread evidence of shallow slope instability within the sediment mounds has been identified from both swath bathymetry and topographic parametric sonar seismic reflection profiles. Bathymetric data show that the heads of all the rise canyon-channel systems coincide geographically with the mouths of the major glacial troughs on the continental shelf edge. This suggests a close genetic link between these morphological features and allows considering a glacio-sedimentary model for the western NAP outer margin seascape development. This model considers the availability of depositional space on the continental rise as the limiting factor for mound development. The depositional space, in turn, is controlled by the spacing between glacial maxima shelf-edge reaching ice streams. This model takes into account both glacial and interglacial scenarios and gives new insights on evaluating the palaeoenvironmental record of the continental rise sediment mounds. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction The location of the boundary from sub-polar to polar climatic conditions in the Northern Antarctic Peninsula (NAP) and its relatively warm maritime setting (Griffith and Anderson, 1989) have led to very dynamic and Corresponding author. Tel.: ; fax: address: miquelcanals@ub.edu (M. Canals). sensitive glacial systems (Canals et al., 2000, 2002, 2003; O Cofaigh et al., 2002). Local ice caps form the present day glacial cover of islands around the peninsula, whereas an ice sheet of variable thickness covers most of the Antarctic Peninsula itself. Currently, the ice drains perpendicular to the Peninsula axis through valley glaciers and ice streams that erode and transport sediment to the coast and the inner shelf. In contrast, during glacial periods grounded ice sheets reached the shelf edge (Bentley and Anderson, 1998; Anderson et al., /$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi: /j.quascirev

2 934 D. Amblas et al. / Quaternary Science Reviews 25 (2006) ) eroding deep troughs beneath fast flowing ice streams on the NAP inner shelf (Alley et al., 1989; Pope and Anderson, 1992; Pudsey et al., 1994; Rebesco et al., 1998; Canals et al., 2000; Domack et al., in press) and depositing prograding sequences on the outer shelf (Larter and Barker, 1989; Larter and Cunningham, 1993). During these cold periods, large volumes of unsorted glacigenic sediments were delivered to, and then bypassed, the upper slope as small volume but frequent events feeding the depositional systems of the NAP Pacific continental rise (McGinnis and Hayes, 1995; Rebesco et al., 1996). Large sediment mounds are characteristic features along the continental rise of the NAP Pacific margin. Twelve mounds have been studied over the last decade, including Ocean Drilling Program Leg 178 (Barker et al., 1999), mainly because of the presumed value of their sediment record for understanding the Neogene Antarctic glaciation. These sediment mounds have been interpreted as sediment drifts produced by bottom currents redistributing the fine-grained component of channelised turbidity currents (Rebesco et al., 1996). Numerous erosional unconformities observed in seismic reflection profiles suggest complex interactions between down-slope and along-slope processes throughout the history of these mounds. Conceptual models suggest that these deposits formed in three major stages. The first stage, recorded by a basal regional unconformity, has been interpreted as having been caused by an enhancement in near-bottom currents associated with the opening of the Drake Passage during the Late Oligocene (Tucholke, 1977). This critical geodynamic event facilitated the establishment of Neogene glaciation in Antarctica (Kennet, 1977) and hence, led to increased sediment supply to the Antarctic margins. Sediment gravity flows funneled by the gullies and canyon-channel systems draining the west of the NAP outer margin largely contributed to the supply of fines to the deep margin and basin environments. Southwesterly flowing bottom currents originated in the Weddell Sea redistributed such fines along the continental rise (Rebesco et al., 1996). This allowed the main drift growth phase (from 15 to about 5 Ma BP) that corresponds with the second stage of drift development (Rebesco et al., 1996, 1997). The third stage (from about 5 Ma to present) was characterised by reduced bottom current activity. These three stages have been identified from the seismic stratigraphy and can be correlated over large distances (Rebesco et al., 2002). In this paper we present in unprecedented detail, the swath bathymetry of the NAP Pacific margin, including the six northernmost continental rise mounds located between 631S and S off the Biscoe and Palmer archipelagos (Fig. 1). The swath data also cover most of Fig. 1. 3D view of the Northern Antarctic Peninsula region constructed from Smith and Sandwell (1997) predicted topography. View is from southwest (2301) and the illumination is from north northeast (0201). Scale calculated at 651S. The bold yellow line shows the boundaries of the study area. Colour code is as follows: grey: emerged landmasses; light blue: continental shelf and slope; dark blue: continental rise and deep basin. See also colour bar for altitudes.

3 D. Amblas et al. / Quaternary Science Reviews 25 (2006) the continental slope and part of the adjacent continental shelf (Fig. 2). These data have been combined with the predicted sea-floor topography of Smith and Sandwell (1997), which on the whole gives an integrated view of the study area (Fig. 3A). The analysis of the geomorphic relationships between continental shelf and rise morphosedimentary features allows proposing a glacio-sedimentary model that links palaeo-ice sheet dynamics with outer margin sedimentation. The understanding of such a genetic relationship is significant to interpret the climatic record contained in the large NAP Pacific margin sediment mounds. The proposed model represents a step forward with respect to former models (e.g. Rebesco et al., 1998), which were based on fewer and lower resolution data. 2. Material and methods The data set was acquired during BIO Hesperides cruises GEBRAP 96 and COHIMAR 01 with a Simrad EM 12-S multibeam echosounder in the austral summers of and , respectively. The data acquired during both cruises covers an area of 66,000 km 2 (Fig. 2). The Simrad EM 12-S echosounder transmits 81 beams across a total swath angle of 1201 producing a maximum swath width that is 3.5 times the water depth. The system is hull mounted and works at a frequency of 12.5 khz, resolving features of a few meters in height. Multibeam data were logged using Simrad s Mermaid system and processed with the Swathed software. The bathymetry data set was then merged with the predicted sea-floor topography of Smith and Sandwell (1997). Final 100 m grid spacing maps of the study area were generated using the Generic Mapping Tools (GMT) software (Wessel and Smith, 1991). Topographic parametric sonar seismic reflection profiles (TOPAS) were simultaneously acquired. The TOPAS is a hull-mounted sub-bottom profiler based on the parametric interference principle. It uses two primary frequencies of 21.5 and 18 khz leading to a secondary very narrow beam with a frequency of 3.5 khz, which gives a resolution better than 1 m and a typical penetration depth between 50 and 200 ms in deep-sea unconsolidated muds. Pulse triggering of EM 12-S and TOPAS systems was controlled by a Simrad synchronicity unit during COHIMAR 01 cruise. The absence of such a system during the GEBRAP 96 cruise, Fig. 2. Ship tracks from BIO Hesperides cruises GEBRAP 96 (white dashed lines) and COHIMAR 01 (white lines) on the northern Bellingshausen Sea. Bathymetric contours are plotted from Smith and Sandwell (1997) predicted bathymetry. Contour interval is 100 m. A small box with dotted edges centred at 66.31W and 64.21S corresponds to erroneous Smith and Sandwell (1997) data.

4 936 D. Amblas et al. / Quaternary Science Reviews 25 (2006) Fig. 3. (A) Shaded relief image constructed from Simrad EM12-S swath bathymetry data supplemented with Smith and Sandwell (1997) data outside multibeam coverage. Illumination is from east north east (0601). White capital letters mark the location of the northeast to southwest topographic profiles illustrated in Fig. 6. Northeast-trending striping is an acquisition artifact. The black rectangles mark the location of Figs. 4 and 5. The small box with dotted edges centred at 66.31W and 64.21S corresponds to erroneous Smith and Sandwell (1997) data. (B) Interpretative drawing based on Fig. 3A. The sediment mounds (1 4A, in violet) and canyon-channel systems (letter labelled, with red colour) of the continental rise and the main glacial troughs (T1 T8 in black, and blue for T6), inter-trough areas and prograding lobes (1 3, in pink) on the shelf are shown. Black arrows mark the main glacial troughs as well as the inferred paleo-ice streams. Blue arrows mark the Biscoe Trough system as imaged by multibeam data. Note the correspondence between (1) main glacial troughs on the outer continental shelf and (2) deep-sea canyon-channel systems. C channel; I, Is island; C.F.Z. C Fracture Zone.

5 D. Amblas et al. / Quaternary Science Reviews 25 (2006) together with persistent bad weather conditions, resulted in poorer data quality being acquired on this cruise than COHIMAR Geophysical data 3.1. Seafloor and sub-seafloor deep margin characteristics Swath bathymetry data show that between 631S and S the continental rise west of the NAP consists of six large elongate mounds oriented perpendicular to the shelf edge. The mounds are separated by north west trending canyon-channel systems (Fig. 3A). From north to south the mounds here described are 1, 2, 3, 3A, 4 and 4A, and the canyon-channel systems are North Anvers, South Anvers, Palmer, Renaud, Biscoe and Lavoisier channels, as named by Rebesco et al. (1996, 2002) (Fig. 3B). The canyon-channel systems start down cutting between 2800 and 2900 m water depth. The northernmost canyon-channel systems (North Anvers and South Anvers channels) display a complex dendritic pattern at their catchment areas with many tributaries extending back to the very base of the continental slope. They evolve downslope, both converging into a large channel of over 8 km in width past the northwestern tip of mound 2. This channel finally vanishes on the abyssal plain in a water depth of about 3800 m. The Biscoe and Lavoisier channels, separated by mound 4A, also present a well developed dendritic morphology on their upper courses. On the contrary, the Renaud and Palmer channels, between mounds 4, 3A and 3, show fewer tributaries at their upper courses and a more linear shape downslope (Fig. 3A and B). Mound tops lay at about m above the canyon-channel system axes. The mounds can be divided into different morphological types. Mound 3 is strongly asymmetric with a steep and irregular side facing southwest and a gently-dipping smooth side facing northeast (Fig. 3A). Mounds 1 and 4A are also asymmetric though to a lesser extent. Mound 4 has quite a symmetric morphology and consists of two asymmetric sub-mounds, with a steeper side facing northeast in the northern one and southwest in the southern one (Fig. 3A). Mounds 2 and 3A are roughly symmetric. Mound 3, which is the largest mound imaged, attains an elevation above the surrounding seafloor of up to 1 km, a length of at least 130 km and a width of over 100 km (Fig. 4A). The top of all these sediment mounds is characterised by a narrow and long crest orthogonal to the shelf edge and attached to the continental slope at their south-easternmost end. Multibeam data show that the flanks of the mounds are disrupted by a series of steps both on their steep and smooth sides (Fig. 3A). These steps are interpreted as scars left by slope failures (Fig. 3B). The internal configuration of the uppermost 100 ms of the mounds, revealed by TOPAS seismic reflection profiles, shows a variable acoustic character ranging from chaotic to transparent to semi-transparent and truncated stratified facies (Fig. 4B). The thickness of the individual deposits related to sediment instability is highly variable, and ranges from a few ms to a maximum of 40 ms. The largest scars observed in swath data are located on mound 4A (Fig. 3A) and extend at least over 60 km with a scar height of m. Southeastwards of the mounds, the continental slope is steep (181 on average) and is not divided by deep canyons. Instead, multibeam covered areas show the presence of series of gullies across the continental slope (Fig. 5A and B). These gullies are closely spaced and appear to initiate at the shelf break. At the base of the slope the gullies vanish and there is no apparent connection with the continental rise canyon-channel systems. Dowdeswell et al. (2004) describe similar features in the continental slope off Marguerite Bay, southwards of the present study area Inner margin seafloor characteristics The continental shelf is up to 150 km wide and 450 m deep on average, and deeps landwards. The shelf is incised by a series of glacial troughs that are predominantly oriented southeast northwest. These troughs reach a maximum depth of about 1400 m in the Palmer Deep area (Fig. 3A). The multibeam data covering almost the whole Biscoe Trough, immediately upslope of the Renaud Channel, show spectacular sets of linear subparallel ridges and grooves forming fan-shaped bundle structures (Canals et al., 2000, 2003) (Fig. 5A). These bedforms show a progressive increase of its elongation with distance along the trough, from streamlined bedrock and drumlins at inner-shelf areas, to mega-scale lineations at mid and outer-shelf areas. The seafloor morphology becomes progressively smoother towards the shelf edge, at m water depth. As a whole, the ESE-WNW Biscoe Trough system runs roughly orthogonal to the shelf break and conforms a system that is 130 km long and km wide. The main branch of the Biscoe Trough system is disrupted by a southwest northeast narrow and elongated structural high (Fig. 5A and C) corresponding to the feature previously known as the Mid-Shelf High (Larter and Barker, 1991). This structurally controlled feature causes a vertical shift of the seafloor of up to 300 m in this sector Inner to deep margin integrated geomorphic analysis The integration of multibeam and predicted seafloor topography data shows that the heads of all the canyonchannel systems initiate in front of the mouths of the

6 938 D. Amblas et al. / Quaternary Science Reviews 25 (2006) Fig. 4. (A) Shaded relief image of mound 3 area (contours every 100 m). For location see Fig. 3A. The bold yellow line shows the location of the very high resolution seismic reflection profile illustrated in Fig. 4B. (B) Topographic parametric sonar (TOPAS) seismic reflection profile across mound 3 and South Anvers Channel. Note the presence of transparent and chaotic seismic facies interpreted as mass-flow deposits overlying continuously stratified facies on the northeastern flank of the mound. Time units are in milliseconds (two way travel time).

7 D. Amblas et al. / Quaternary Science Reviews 25 (2006) Fig. 5. (A) Multibeam shaded relief image of the Biscoe margin merged with Smith and Sandwell (1997) data. Illumination is from east north east (0601). For location see Fig. 3A. Note the well-developed streamlined glacial landforms throughout the Biscoe Trough. Main ice flow was from ESE (right) to WNW (left). Note the spatial relationship between the main outer shelf and continental rise features. Red dashed lines mark the crests of mound 4 and the top of the adjacent outer shelf high. Insets show location of Figs. 5A and B. I, Is island. (B) Gradient map showing a steep continental slope and the sharp flanks of mound 4 (contours every 100 m). (C) Gradient map showing the changing undulating character of the seafloor along the Biscoe Trough central part. glacial troughs on the continental shelf (Figs. 3 and 6). The subdued morphological expression of glacial troughs at the shelf edge and the lack of upslope continuity of the canyon-channel systems on the continental rise have prevented the establishment of the links between both systems until now. The highs separating the glacial troughs on the shelf appear to correlate well with mound crests (Fig. 6). This is especially evident on the Biscoe margin where high quality multibeam data from the shelf, slope and rise exist (Fig. 5), but it is also evident along the whole study area (Fig. 3A) as inferred from the predicted seafloor topography of Smith and Sandwell (1997). The three broad lobes identified at the shelf edge of the study area (Fig. 3A) do not appear to directly relate to the features observed on the continental rise. However, there is a tendency for shelf edge lobes to

8 940 D. Amblas et al. / Quaternary Science Reviews 25 (2006) Fig. 6. Topographic profiles parallel to the continental margin (see location in Fig. 3A). Vertical scale is magnified 28 times. Black dashed lines mark the correspondence between main glacial troughs on the outer shelf (profile A A 0 ) with continental rise canyon-channel systems (profiles B B 0 and C C 0 ). White dashed lines mark the correspondence between outer shelf highs (profile A A 0 ) and sediment mound crests (profiles B B 0 and C C 0 ). develop along inter-trough segments and for shelf edge re-entrants to form where most of the trough mouths are. These large lobes are numbered from north to south according to Larter et al. (1997). 4. Discussion The high-resolution bathymetric data support the view that during the Last Glacial Maximum (LGM) the Biscoe Archipelago was drained by a westward-moving ice stream system fed by an ice cap on the NAP. Major evidence in support of this interpretation is provided by the observed mega-scale lineations within the cross-shelf Biscoe Trough (Fig. 5). These features are inferred to have originated by sub-glacial erosion and sediment deformation processes. The progressive elongation of the Biscoe Trough glacial bedforms, which is a recurrent phenomena in the Antarctic margins (Canals et al., 2000, 2002, 2003; O Cofaigh et al., 2005; Domack et al., in press), would indicate a progressive increase of the palaeo-ice stream velocity across the shelf. Together with the Gerlache bundle (Canals et al., 2000), the Biscoe bundle remains one of the largest streamlined glacial landform ever imaged offshore of the Antarctic Peninsula. Similar sea-floor morphologies have been described at the Northern Hemisphere, like those in the Norwegian continental shelf (Rise et al., 2004; Ottesen et al., in press). We propose that the main troughs on the continental shelf of the study area (Fig. 3A), as depicted by the Smith and Sandwell (1997) data, were also probably eroded by ice streams, as suggested by Hughes (1977). The main shelf troughs were thus the preferred pathways for ice stream drainage during glacial stages, while inter-trough highs would represent slower drainage areas (Fig. 5B). The distribution of these morphologic features (troughs and highs) on the outer continental shelf appears to have influenced the arrangement of mounds and canyon-channel systems on the continental rise as observed from the combination of multibeam and Smith and Sandwell (1997) data (Fig. 6). These geomorphic relationships allow us to propose a depositional model for mound growth and canyon-channel systems development from a glacio-sedimentary point of view (Fig. 7). Such a model does not consider a close genetic relationship between the large shelf edge lobes and the arrangement of the continental rise sedimentary systems, as proposed by Rebesco et al. (1998), since no geomorphic evidences support this view (Fig. 3A). Based on the size and shape of the mounds, a combination of large volumes of sediment in the source area and an efficient system of sediment distribution across and along the continental rise are necessary for these features to form. The amount and type of sediment supply are likely to have varied over glacial interglacial cycles. Pelagic and hemi-pelagic settling from the sea surface, including biogenic and glacially-derived

9 D. Amblas et al. / Quaternary Science Reviews 25 (2006) Fig. 7. Synthetic sketch showing the glacial setting and the active processes leading to the development of continental rise sediment mounds during glacial periods in connection with ice dynamics on the continental shelf. Note the influence of spacing between ice streams on mound development. When larger spacing exists between two consecutive shelf edge reaching ice stream systems, larger mounds could develop in the adjacent continental rise. material, was the main sedimentary process contributing to the mounds growth during interglacial times (Pudsey and Camerlenghi, 1998; Lucchi et al., 2002). However, the main periods of mound generation occurred during cold phases and glacial maxima through the delivery of terrigenous sediment by downslope processes associated with a grounded ice sheet covering the continental shelf. This interpretation is supported by the presence of large obliquely prograding wedges of unsorted sediments on the outer continental shelf along the Pacific margin of the NAP (Larter and Barker, 1989; Bart and Anderson, 1995; Prieto et al., 1999; Canals et al., 2003). Ice streams, in particular, were efficient agents in eroding and transporting sediment to the outer continental shelf during cold periods (Alley et al., 1987). As suggested by several authors (McGinnis et al., 1997; Pudsey and Camerlenghi, 1998; Pudsey, 2000; Lucchi and Rebesco, in press), turbid meltwater plumes and mass-wasting processes were common at the base of ice stream terminus and would have facilitated increases in pelagic and hemipelagic settling rates. Therefore, the rate of sediment delivery to the shelf edge and slope was higher at ice-stream terminus than in inter-ice stream sectors. It implied higher frequency of sediment failure events in front of fast-flowing ice systems than in slower, as suggested for this margin by Rebesco et al. (1998) and by Dowdeswell et al. (1998) and Vorren et al. (1998) for polar North Atlantic margins. In addition, sediment destabilisation was enhanced by the intrinsic physical properties of the sediment delivered beyond the glacial trough mouth, which would have comprised low shear strength material as a consequence of shear remoulding, under-consolidation, and some sorting if compared with sub-glacial tills (Elverhøi et al., 1997; Rebesco et al., 1998). Sediments slid off and evolved into debris flows and turbidity currents at the base of the slope, as suggested by Larter and Cunningham (1993). The later are inferred to be the main processes forming the welldeveloped canyon-channel systems observed on the continental rise of where the troughs terminate at the shelf. Similar scenarios have been described in the Northern Hemisphere high latitude margins (e.g. O Cofaigh et al., 2004). Because of the considerable hydraulic jump at the base of the NAP Pacific margin continental slope, where the slope gradient shifts from more than 181 to less than 41, suspension clouds from turbidity currents are inferred to form easily (Pudsey and Camerlenghi, 1998). While the upper continental rise network of dendritic canyons carried the coarsegrained particles to the lower rise and abyssal plain, southwest flowing bottom currents redistributed the suspended fine-grained components, both from meltwater plumes and turbidity currents, forming the large sediment mound deposits of the upper continental rise (Rebesco et al., 1996). Since no significant migration of the canyon-channel systems has been observed on along-strike seismic reflection profiles across the continental rise in the study area (Rebesco et al., 1996, 2002), it can be inferred that the ice sheet drainage pattern on the shelf experienced little change during the period in which the mounds developed. Multichannel seismic reflection profiles perpendicular to the Biscoe Trough on the continental shelf also support this idea (Canals et al., 2003; Rebesco et al., in press). Presumably this has been a key factor in allowing the sediment mounds to attain their large sizes. The spacing between glacial troughs on the shelf appears to control the spacing between major canyon-channel systems on the continental rise (Fig. 6), which at the same time bounds and determines the shape and size of the sediment mounds (Fig. 7). In other words, when larger spacing exists between two consecutive shelf edge reaching ice stream systems, larger mounds could develop in the adjacent continental rise (Fig. 7). This

10 942 D. Amblas et al. / Quaternary Science Reviews 25 (2006) is in particularly the case for Mound 3, which would have been fed mostly by the glacial system labelled T2 on Fig. 3B, with contributions from T3. In this case, the large spacing between T2 (main sediment source to Mound 3) and T4 (main sediment source to Mound 3A) determines the large size of Mound 3 (Fig. 3B). The asymmetric morphology of Mound 3 reflects the action of the southwest flowing bottom currents. On the other hand, where relatively closely spaced ice streams reached the shelf edge, smaller mounds could develop in the adjacent continental rise, even though the rate of sediment delivery to the shelf edge was relatively high. This is because in the continental rise the sediment deposited by bottom currents was eroded by the adjacent canyon-channel system to the south west or was bypassed to the next sediment mound to the south west. Typical mounds of this later case do not necessarily show the expected asymmetric morphology with a gently dipping smooth side facing northeast. This is clearly exemplified by mounds 2, 3A and 4, being mound 4 the best example for this situation (Fig. 3). As inferred from multibeam data (Figs. 3A and 4A) and TOPAS profiles (Fig. 4B), another significant sedimentary process redistributing the sediment over the mounds was (or perhaps still is) mass wasting, which is relevant not only on the steeper slopes but also on gentle slopes where the largest slide scars have been identified. Therefore, mass-wasting processes also play a decisive role in mound shaping, at least in the case of the studied mounds. In this regard, recurrent destabilisation and destruction of the mounds sedimentary record may diminish their potential as valuable palaeoenvironmental archives. mounds and canyon-channel systems along the NAP Pacific margin. Ice streams are efficient agents for the erosion and transport of sub-glacial debris to the outer continental shelf and shelf edge. Thus, when grounded ice reached the shelf edge during glacial maxima, mass-wasting processes became more frequent on the steep and narrow slope in front of ice stream termini and determined the location of turbidity canyon-channel systems on the continental rise. Therefore, these canyonchannel systems represent the downslope continuation of the cross-shelf glacial troughs and would have primarily operated during glacial times. Southwesterly flowing bottom currents redistributed the suspended fine-grained components from turbidity currents and formed the sediment mounds. The shape and size of the mounds is controlled by the rate of sediment delivery from ice streams on the outer shelf and the spacing of the ice streams themselves. The different shapes and sizes observed on the surveyed mounds 1 4A reflect different combinations of these controls. We hypothesize that successive glacial advances to the outer shelf and shelf edge, combined with the lack of migration of the ice streams and sediment pathways both on the shelf and rise over the late Tertiary and Quaternary times have been key factors in allowing the mounds to attain their large sizes. The widespread presence of shallow slope instabilities within the sediment mounds is clear from the swath bathymetric data and seismic reflection profiles. Such mass-wasting processes also play a decisive role, though hitherto largely ignored, in shaping the mounds. 5. Conclusions The integration of detailed multibeam bathymetry data and predicted seafloor topography from the NAP Pacific margin suggests a close genetic link between the main morphological features observed on the continental shelf and rise. Based on these, a model for the development of the large sediment mounds and intervening canyon-channel systems of the NAP Pacific continental rise is proposed. On the Biscoe shelf, mega-scale glacial lineations within a multibeam-surveyed glacial trough are interpreted as recording the former presence of an ice stream. The large continental shelf troughs observed from the lower resolution predicted seafloor topography are inferred to have originated also by fast flowing ice drainage systems. The distribution of these erosive physiographic elements over the shelf, and hence the presence of fast flowing ice streams grounded near or at the shelf edge during glacial times, played a decisive influence on the arrangement of the continental rise Acknowledgements This study was supported by the Spanish (COHI- MAR project, ref. REN /ANT), Italian and Belgian Antarctic programs, Fullbright Commission GEMARANT project and Generalitat de Catalunya Grant 2001SGR to excellence research groups. Support from the Spanish Ministerio de Educacio n, Cultura y Deporte (D.A.) and Ministerio de Ciencia y Tecnologı a (R.U.) through FPU and Ramon y Cajal fellowships, respectively, is greatly acknowledged. The manuscript greatly benefited from thorough and insightful comments by reviewers Colm O Cofaigh and Phil O Brien, and the editor, Jim Rose. References Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., Till beneath ice stream B. 4. A coupled ice-till flow model. Journal of Geophysical Research 92,

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