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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,, Antoni M. Calafata, Michele Rebescob,
Angelo Camerlenghia, Ferran Estradac, Marc De Batistd, John E. Hughes-Clarkee
a
GRC Geociències Marines, Universitat de Barcelona, Martı´ i Franquès s/n, E-08028 Barcelona, Spain
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico, Trieste, Italy
c
CSIC Institut de Ciències del Mar, Passeig Marı´tim Barceloneta 37-49, 08003 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
b
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
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.: +34 93 402 13 60;
fax: +34 93 402 13 40.
E-mail address: [email protected] (M. Canals).
0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2005.07.012
sensitive glacial systems (Canals et al., 2000, 2002, 2003;
Ó 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.,
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2002) 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 651400 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.
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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 1996–1997 and 2001–2002, respectively. The
data acquired during both cruises covers an area of
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66,000 km2 (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.
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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.
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together with persistent bad weather conditions, resulted
in poorer data quality being acquired on this cruise than
COHIMAR’01.
3. Geophysical data
3.1. Seafloor and sub-seafloor deep margin
characteristics
Swath bathymetry data show that between 631S and
651400 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 500–1000 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
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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 60–120 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.
3.2. 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 450–500 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 23–70 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.
3.3. 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
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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).
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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
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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
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; Ó 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
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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.
Ó 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
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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
Acknowledgements
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
This study was supported by the Spanish (COHIMAR project, ref. REN2000-0896/ANT), Italian and
Belgian Antarctic programs, Fullbright Commission
GEMARANT project and Generalitat de Catalunya
Grant 2001SGR-00076 to excellence research groups.
Support from the Spanish ‘‘Ministerio de Educación,
Cultura y Deporte’’ (D.A.) and ‘‘Ministerio de Ciencia y
Tecnologı́a’’ (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 Ó Cofaigh and
Phil O’Brien, and the editor, Jim Rose.
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