Relationship Between Continental Rise Development and Palaeo-Ice Sheet Dynamics, Northern Antarctic Peninsula Pacific Margin

ARTICLE IN PRESS

Quaternary Science Reviews 25 (2006) 933–944

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

David Amblasa, Roger Urgelesa, Miquel Canalsa,Ã, Antoni M. Calafata, Michele Rebescob, Angelo Camerlenghia, Ferran Estradac, Marc De Batistd, John E. Hughes-Clarkee

aGRC Geocie`ncies Marines, Universitat de Barcelona, Martı´ i Franque`s s/n, E-08028 Barcelona, Spain bIstituto Nazionale di Oceanograﬁa e di Geoﬁsica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico, Trieste, Italy cCSIC Institut de Cie`ncies del Mar, Passeig Marı´tim Barceloneta 37-49, 08003 Barcelona, Spain dRenard Centre of Marine Geology, Ghent University, Krijgslaan 281 S8, B-9000 Gent, Belgium eOcean 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, 1956–1962.] 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 sensitive glacial systems (Canals et al., 2000, 2002, 2003; O´ Cofaigh et al., 2002). Local ice caps form the present The location of the boundary from sub-polar to polar day glacial cover of islands around the peninsula, climatic conditions in the Northern Antarctic Peninsula whereas an ice sheet of variable thickness covers most (NAP) and its relatively warm maritime setting (Grifﬁth of the Antarctic Peninsula itself. Currently, the ice and Anderson, 1989) have led to very dynamic and drains perpendicular to the Peninsula axis through valley glaciers and ice streams that erode and transport ÃCorresponding author. Tel.: +34 93 402 13 60; sediment to the coast and the inner shelf. In contrast, fax: +34 93 402 13 40. during glacial periods grounded ice sheets reached the E-mail address: [email protected] (M. Canals). shelf edge (Bentley and Anderson, 1998; Anderson et al.,

2002) eroding deep troughs beneath fast flowing ice first stage, recorded by a basal regional unconformity, streams on the NAP inner shelf (Alley et al., 1989; Pope has been interpreted as having been caused by an and Anderson, 1992; Pudsey et al., 1994; Rebesco et al., enhancement in near-bottom currents associated with 1998; Canals et al., 2000; Domack et al., in press) and the opening of the Drake Passage during the Late depositing prograding sequences on the outer shelf Oligocene (Tucholke, 1977). This critical geodynamic (Larter and Barker, 1989; Larter and Cunningham, event facilitated the establishment of Neogene glaciation 1993). During these cold periods, large volumes of in Antarctica (Kennet, 1977) and hence, led to increased unsorted glacigenic sediments were delivered to, and sediment supply to the Antarctic margins. Sediment then bypassed, the upper slope as small volume but gravity flows funneled by the gullies and canyon-channel frequent events feeding the depositional systems of the systems draining the west of the NAP outer margin NAP Pacific continental rise (McGinnis and Hayes, largely contributed to the supply of fines to the deep 1995; Rebesco et al., 1996). margin and basin environments. Southwesterly flowing Large sediment mounds are characteristic features bottom currents originated in the Weddell Sea redis- along the continental rise of the NAP Pacific margin. tributed such fines along the continental rise (Rebesco Twelve mounds have been studied over the last decade, et al., 1996). This allowed the main drift growth phase including Ocean Drilling Program Leg 178 (Barker (from 15 to about 5 Ma BP) that corresponds with the et al., 1999), mainly because of the presumed value of second stage of drift development (Rebesco et al., 1996, their sediment record for understanding the Neogene 1997). The third stage (from about 5 Ma to present) was Antarctic glaciation. These sediment mounds have been characterised by reduced bottom current activity. These interpreted as sediment drifts produced by bottom three stages have been identified from the seismic currents redistributing the fine-grained component of stratigraphy and can be correlated over large distances channelised turbidity currents (Rebesco et al., 1996). (Rebesco et al., 2002). Numerous erosional unconformities observed in seismic In this paper we present in unprecedented detail, the reflection profiles suggest complex interactions between swath bathymetry of the NAP Pacific margin, including down-slope and along-slope processes throughout the the six northernmost continental rise mounds located history of these mounds. Conceptual models suggest between 631S and 651400S off the Biscoe and Palmer that these deposits formed in three major stages. The 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. ARTICLE IN PRESS D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 935 the continental slope and part of the adjacent con- 66,000 km2 (Fig. 2). The Simrad EM 12-S echosounder tinental shelf (Fig. 2). These data have been combined transmits 81 beams across a total swath angle of 1201 with the predicted sea-floor topography of Smith and producing a maximum swath width that is 3.5 times the Sandwell (1997), which on the whole gives an integrated water depth. The system is hull mounted and works at a view of the study area (Fig. 3A). The analysis of the frequency of 12.5 kHz, resolving features of a few meters geomorphic relationships between continental shelf and in height. Multibeam data were logged using Simrad’s rise morphosedimentary features allows proposing a Mermaid system and processed with the Swathed glacio-sedimentary model that links palaeo-ice sheet software. The bathymetry data set was then merged dynamics with outer margin sedimentation. The under- with the predicted sea-floor topography of Smith and standing of such a genetic relationship is significant to Sandwell (1997). Final 100 m grid spacing maps of the interpret the climatic record contained in the large NAP study area were generated using the Generic Mapping Pacific margin sediment mounds. The proposed model Tools (GMT) software (Wessel and Smith, 1991). represents a step forward with respect to former models Topographic parametric sonar seismic reflection (e.g. Rebesco et al., 1998), which were based on fewer profiles (TOPAS) were simultaneously acquired. The and lower resolution data. 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 2. Material and methods secondary very narrow beam with a frequency of 3.5 kHz, which gives a resolution better than 1 m and a The data set was acquired during BIO Hesperides typical penetration depth between 50 and 200 ms in cruises GEBRAP’96 and COHIMAR’01 with a Simrad deep-sea unconsolidated muds. Pulse triggering of EM EM 12-S multibeam echosounder in the austral sum- 12-S and TOPAS systems was controlled by a Simrad mers of 1996–1997 and 2001–2002, respectively. The synchronicity unit during COHIMAR’01 cruise. The data acquired during both cruises covers an area of 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. ARTICLE IN PRESS 936 D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944

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. ARTICLE IN PRESS D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 937 together with persistent bad weather conditions, resulted scars left by slope failures (Fig. 3B). The internal in poorer data quality being acquired on this cruise than configuration of the uppermost 100 ms of the mounds, COHIMAR’01. revealed by TOPAS seismic reflection profiles, shows a variable acoustic character ranging from chaotic to transparent to semi-transparent and truncated stratified 3. Geophysical data facies (Fig. 4B). The thickness of the individual deposits related to sediment instability is highly variable, and 3.1. Seafloor and sub-seafloor deep margin ranges from a few ms to a maximum of 40 ms. The characteristics largest scars observed in swath data are located on mound 4A (Fig. 3A) and extend at least over 60 km with Swath bathymetry data show that between 631Sand a scar height of 60–120 m. 651400S the continental rise west of the NAP consists of six Southeastwards of the mounds, the continental slope large elongate mounds oriented perpendicular to the shelf is steep (181 on average) and is not divided by deep edge. The mounds are separated by north–west trending canyons. Instead, multibeam covered areas show the canyon-channel systems (Fig. 3A). From north to south presence of series of gullies across the continental slope the mounds here described are 1, 2, 3, 3A, 4 and 4A, and (Fig. 5A and B). These gullies are closely spaced and the canyon-channel systems are North Anvers, South appear to initiate at the shelf break. At the base of the Anvers, Palmer, Renaud, Biscoe and Lavoisier channels, slope the gullies vanish and there is no apparent as named by Rebesco et al. (1996, 2002) (Fig. 3B). connection with the continental rise canyon-channel The canyon-channel systems start down cutting systems. Dowdeswell et al. (2004) describe similar between 2800 and 2900 m water depth. The northern- features in the continental slope off Marguerite Bay, most canyon-channel systems (North Anvers and South southwards of the present study area. Anvers channels) display a complex dendritic pattern at their catchment areas with many tributaries extending 3.2. Inner margin seafloor characteristics back to the very base of the continental slope. They evolve downslope, both converging into a large channel The continental shelf is up to 150 km wide and 450 m of over 8 km in width past the northwestern tip of deep on average, and deeps landwards. The shelf is mound 2. This channel finally vanishes on the abyssal incised by a series of glacial troughs that are predomi- plain in a water depth of about 3800 m. The Biscoe and nantly oriented southeast–northwest. These troughs Lavoisier channels, separated by mound 4A, also reach a maximum depth of about 1400 m in the Palmer present a well developed dendritic morphology on their Deep area (Fig. 3A). The multibeam data covering upper courses. On the contrary, the Renaud and Palmer almost the whole Biscoe Trough, immediately upslope of channels, between mounds 4, 3A and 3, show fewer the Renaud Channel, show spectacular sets of linear sub- tributaries at their upper courses and a more linear parallel ridges and grooves forming fan-shaped bundle shape downslope (Fig. 3A and B). structures (Canals et al., 2000, 2003)(Fig. 5A). These Mound tops lay at about 500–1000 m above the bedforms show a progressive increase of its elongation canyon-channel system axes. The mounds can be with distance along the trough, from streamlined bedrock divided into different morphological types. Mound 3 is and drumlins at inner-shelf areas, to mega-scale linea- strongly asymmetric with a steep and irregular side tions at mid and outer-shelf areas. The seafloor facing southwest and a gently-dipping smooth side morphology becomes progressively smoother towards facing northeast (Fig. 3A). Mounds 1 and 4A are also the shelf edge, at 450–500 m water depth. As a whole, the asymmetric though to a lesser extent. Mound 4 has quite ESE-WNW Biscoe Trough system runs roughly ortho- a symmetric morphology and consists of two asym- gonal to the shelf break and conforms a system that is metric sub-mounds, with a steeper side facing northeast 130 km long and 23–70 km wide. The main branch of the in the northern one and southwest in the southern one Biscoe Trough system is disrupted by a southwest– (Fig. 3A). Mounds 2 and 3A are roughly symmetric. northeast narrow and elongated structural high (Fig. 5A Mound 3, which is the largest mound imaged, attains an and C) corresponding to the feature previously known as elevation above the surrounding seafloor of up to 1 km, the ‘‘Mid-Shelf High’’ (Larter and Barker, 1991). This a length of at least 130 km and a width of over 100 km structurally controlled feature causes a vertical shift of (Fig. 4A). The top of all these sediment mounds is the seafloor of up to 300 m in this sector. characterised by a narrow and long crest orthogonal to the shelf edge and attached to the continental slope at 3.3. Inner to deep margin integrated geomorphic analysis their south-easternmost end. Multibeam data show that the flanks of the mounds The integration of multibeam and predicted seafloor are disrupted by a series of steps both on their steep and topography data shows that the heads of all the canyon- smooth sides (Fig. 3A). These steps are interpreted as channel systems initiate in front of the mouths of the ARTICLE IN PRESS 938 D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 otic seismic facies way travel time). . The bold yellow line shows the location of the very high resolution seismic reflection profile Fig. 3A . (B) Topographic parametric sonar (TOPAS) seismic reflection profile across mound 3 and South Anvers Channel. Note the presence of transparent and cha Fig. 4B Fig. 4. (A) Shaded relief image of mound 3 area (contours every 100 m). For location see illustrated in interpreted as mass-flow deposits overlying continuously stratified facies on the northeastern flank of the mound. Time units are in milliseconds (two ARTICLE IN PRESS D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 939

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). where high quality multibeam data from the shelf, slope The subdued morphological expression of glacial and rise exist (Fig. 5), but it is also evident along the troughs at the shelf edge and the lack of upslope whole study area (Fig. 3A) as inferred from the continuity of the canyon-channel systems on the predicted seaﬂoor topography of Smith and Sandwell continental rise have prevented the establishment of (1997). the links between both systems until now. The three broad lobes identiﬁed at the shelf edge of The highs separating the glacial troughs on the the study area (Fig. 3A) do not appear to directly relate shelf appear to correlate well with mound crests to the features observed on the continental rise. (Fig. 6). This is especially evident on the Biscoe margin However, there is a tendency for shelf edge lobes to ARTICLE IN PRESS 940 D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944

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–A0) with continental rise canyon-channel systems (profiles B–B0 and C–C0). White dashed lines mark the correspondence between outer shelf highs (profile A–A0) and sediment mound crests (profiles B–B0 and C–C0).

develop along inter-trough segments and for shelf edge We propose that the main troughs on the continental re-entrants to form where most of the trough mouths shelf of the study area (Fig. 3A), as depicted by the are. These large lobes are numbered from north to south Smith and Sandwell (1997) data, were also probably according to Larter et al. (1997). 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 4. Discussion inter-trough highs would represent slower drainage areas (Fig. 5B). The distribution of these morphologic The high-resolution bathymetric data support the features (troughs and highs) on the outer continental view that during the Last Glacial Maximum (LGM) the shelf appears to have influenced the arrangement of Biscoe Archipelago was drained by a westward-moving mounds and canyon-channel systems on the continental ice stream system fed by an ice cap on the NAP. Major rise as observed from the combination of multibeam evidence in support of this interpretation is provided by and Smith and Sandwell (1997) data (Fig. 6). These the observed mega-scale lineations within the cross-shelf geomorphic relationships allow us to propose a deposi- Biscoe Trough (Fig. 5). These features are inferred to tional model for mound growth and canyon-channel have originated by sub-glacial erosion and sediment systems development from a glacio-sedimentary point of deformation processes. The progressive elongation of view (Fig. 7). Such a model does not consider a close the Biscoe Trough glacial bedforms, which is a recurrent genetic relationship between the large shelf edge lobes phenomena in the Antarctic margins (Canals et al., and the arrangement of the continental rise sedimentary 2000, 2002, 2003; O´ Cofaigh et al., 2005; Domack et al., systems, as proposed by Rebesco et al. (1998), since no in press), would indicate a progressive increase of the geomorphic evidences support this view (Fig. 3A). palaeo-ice stream velocity across the shelf. Together Based on the size and shape of the mounds, a with the Gerlache bundle (Canals et al., 2000), the combination of large volumes of sediment in the source Biscoe bundle remains one of the largest streamlined area and an efficient system of sediment distribution glacial landform ever imaged offshore of the Antarctic across and along the continental rise are necessary for Peninsula. Similar sea-floor morphologies have been these features to form. The amount and type of sediment described at the Northern Hemisphere, like those in the supply are likely to have varied over glacial–interglacial Norwegian continental shelf (Rise et al., 2004; Ottesen cycles. Pelagic and hemi-pelagic settling from the et al., in press). sea surface, including biogenic and glacially-derived ARTICLE IN PRESS D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 941

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 are inferred to be the main processes forming the well- to the mounds growth during interglacial times (Pudsey developed canyon-channel systems observed on the and Camerlenghi, 1998; Lucchi et al., 2002). However, continental rise of where the troughs terminate at the main periods of mound generation occurred during the shelf. Similar scenarios have been described in cold phases and glacial maxima through the delivery of the Northern Hemisphere high latitude margins (e.g. terrigenous sediment by downslope processes associated O´ Cofaigh et al., 2004). Because of the considerable with a grounded ice sheet covering the continental shelf. hydraulic jump at the base of the NAP Pacific margin This interpretation is supported by the presence of large continental slope, where the slope gradient shifts from obliquely prograding wedges of unsorted sediments on more than 181 to less than 41, suspension clouds from the outer continental shelf along the Pacific margin of turbidity currents are inferred to form easily (Pudsey the NAP (Larter and Barker, 1989; Bart and Anderson, and Camerlenghi, 1998). While the upper continental 1995; Prieto et al., 1999; Canals et al., 2003). Ice streams, rise network of dendritic canyons carried the coarse- in particular, were efficient agents in eroding and grained particles to the lower rise and abyssal plain, transporting sediment to the outer continental shelf southwest flowing bottom currents redistributed the during cold periods (Alley et al., 1987). As suggested by suspended fine-grained components, both from melt- several authors (McGinnis et al., 1997; Pudsey and water plumes and turbidity currents, forming the large Camerlenghi, 1998; Pudsey, 2000; Lucchi and Rebesco, sediment mound deposits of the upper continental rise in press), turbid meltwater plumes and mass-wasting (Rebesco et al., 1996). processes were common at the base of ice stream Since no significant migration of the canyon-channel terminus and would have facilitated increases in pelagic systems has been observed on along-strike seismic and hemipelagic settling rates. Therefore, the rate of reflection profiles across the continental rise in the study sediment delivery to the shelf edge and slope was higher area (Rebesco et al., 1996, 2002), it can be inferred that at ice-stream terminus than in inter-ice stream sectors. It the ice sheet drainage pattern on the shelf experienced implied higher frequency of sediment failure events in little change during the period in which the mounds front of fast-flowing ice systems than in slower, as developed. Multichannel seismic reflection profiles suggested for this margin by Rebesco et al. (1998) and perpendicular to the Biscoe Trough on the continental by Dowdeswell et al. (1998) and Vorren et al. (1998) for shelf also support this idea (Canals et al., 2003; Rebesco polar North Atlantic margins. In addition, sediment et al., in press). Presumably this has been a key factor in destabilisation was enhanced by the intrinsic physical allowing the sediment mounds to attain their large sizes. properties of the sediment delivered beyond the glacial The spacing between glacial troughs on the shelf appears trough mouth, which would have comprised low shear to control the spacing between major canyon-channel strength material as a consequence of shear remoulding, systems on the continental rise (Fig. 6), which at the under-consolidation, and some sorting if compared with same time bounds and determines the shape and size of sub-glacial tills (Elverhøi et al., 1997; Rebesco et al., the sediment mounds (Fig. 7). In other words, when 1998). Sediments slid off and evolved into debris flows larger spacing exists between two consecutive shelf edge and turbidity currents at the base of the slope, as reaching ice stream systems, larger mounds could suggested by Larter and Cunningham (1993). The later develop in the adjacent continental rise (Fig. 7). This ARTICLE IN PRESS 942 D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 is in particularly the case for Mound 3, which would mounds and canyon-channel systems along the NAP have been fed mostly by the glacial system labelled T2 Pacific margin. on Fig. 3B, with contributions from T3. In this case, the Ice streams are efficient agents for the erosion and large spacing between T2 (main sediment source to transport of sub-glacial debris to the outer continental Mound 3) and T4 (main sediment source to Mound 3A) shelf and shelf edge. Thus, when grounded ice reached determines the large size of Mound 3 (Fig. 3B). The the shelf edge during glacial maxima, mass-wasting asymmetric morphology of Mound 3 reflects the action processes became more frequent on the steep and of the southwest flowing bottom currents. On the other narrow slope in front of ice stream termini and hand, where relatively closely spaced ice streams reached determined the location of turbidity canyon-channel the shelf edge, smaller mounds could develop in the systems on the continental rise. Therefore, these canyon- adjacent continental rise, even though the rate of channel systems represent the downslope continuation sediment delivery to the shelf edge was relatively high. of the cross-shelf glacial troughs and would have This is because in the continental rise the sediment primarily operated during glacial times. Southwesterly deposited by bottom currents was eroded by the flowing bottom currents redistributed the suspended adjacent canyon-channel system to the south–west or fine-grained components from turbidity currents and was bypassed to the next sediment mound to the formed the sediment mounds. south–west. Typical mounds of this later case do not The shape and size of the mounds is controlled by the necessarily show the expected asymmetric morphology rate of sediment delivery from ice streams on the outer with a gently dipping smooth side facing northeast. This shelf and the spacing of the ice streams themselves. The is clearly exemplified by mounds 2, 3A and 4, being different shapes and sizes observed on the surveyed mound 4 the best example for this situation (Fig. 3). mounds 1–4A reflect different combinations of these As inferred from multibeam data (Figs. 3A and 4A) controls. We hypothesize that successive glacial ad- and TOPAS profiles (Fig. 4B), another significant vances to the outer shelf and shelf edge, combined with sedimentary process redistributing the sediment over the lack of migration of the ice streams and sediment the mounds was (or perhaps still is) mass wasting, which pathways both on the shelf and rise over the late is relevant not only on the steeper slopes but also on Tertiary and Quaternary times have been key factors in gentle slopes where the largest slide scars have been allowing the mounds to attain their large sizes. identified. Therefore, mass-wasting processes also play a The widespread presence of shallow slope instabilities decisive role in mound shaping, at least in the case of the within the sediment mounds is clear from the swath studied mounds. In this regard, recurrent destabilisation bathymetric data and seismic reflection profiles. Such and destruction of the mounds sedimentary record may mass-wasting processes also play a decisive role, though diminish their potential as valuable palaeoenvironmen- hitherto largely ignored, in shaping the mounds. tal archives.

5. Conclusions Acknowledgements

The integration of detailed multibeam bathymetry This study was supported by the Spanish (COHI- data and predicted seafloor topography from the NAP MAR project, ref. REN2000-0896/ANT), Italian and Pacific margin suggests a close genetic link between the Belgian Antarctic programs, Fullbright Commission main morphological features observed on the continen- GEMARANT project and Generalitat de Catalunya tal shelf and rise. Based on these, a model for the Grant 2001SGR-00076 to excellence research groups. development of the large sediment mounds and inter- Support from the Spanish ‘‘Ministerio de Educacio´ n, vening canyon-channel systems of the NAP Pacific Cultura y Deporte’’ (D.A.) and ‘‘Ministerio de Ciencia y continental rise is proposed. Tecnologı´ a’’ (R.U.) through FPU and ‘‘Ramon y Cajal’’ On the Biscoe shelf, mega-scale glacial lineations fellowships, respectively, is greatly acknowledged. The within a multibeam-surveyed glacial trough are inter- manuscript greatly benefited from thorough and in- preted as recording the former presence of an ice stream. sightful comments by reviewers Colm O´ Cofaigh and The large continental shelf troughs observed from the Phil O’Brien, and the editor, Jim Rose. lower resolution predicted seafloor topography are inferred to have originated also by fast flowing ice drainage systems. The distribution of these erosive References physiographic elements over the shelf, and hence the presence of fast flowing ice streams grounded near or at Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., 1987. Till the shelf edge during glacial times, played a decisive beneath ice stream B. 4. A coupled ice-till flow model. Journal of influence on the arrangement of the continental rise Geophysical Research 92, 8931–8940. ARTICLE IN PRESS D. Amblas et al. / Quaternary Science Reviews 25 (2006) 933–944 943

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