Grounding-zone Wedges Research Articles

Greenland ice sheet retreat history in the northeast Baffin Bay based on high-resolution bathymetry

New swath-bathymetric data acquired in 2010 and 2015 indicate a variety of glacial landforms in cross-shelf troughs of the Melville Bay (northeast Baffin Bay). These landforms reveal that, at their maximum extent, ice streams in the troughs crossed the shelf all the way to the shelf edge. Moraines, grounding-zone wedges (GZWs) and subglacial till lobes on the continental shelf define a pattern of variable ice stream retreat in the individual troughs. On the outer shelf, in the northern cross-shelf trough, ice-stream retreat was slow compared to more episodic retreat in the central (at least one stabilization on the outer shelf) and southern cross-shelf trough (re-advances at the shelf edge and fast retreat thereafter). Large GZWs on the mid-to inner shelf of the troughs indicate periods of grounding-zone stabilization. According to glacial landforms, the final retreat across the inner shelf (before 8.41 ka BP) was episodic to slow. Furthermore, evidence has been found for localized ice domes with minor ice-streams on inter-trough banks. The glacial landforms in Melville Bay, thus, indicate the varying and discontinuous ice sheet retreat history across the Northwest Greenland continental shelf.

Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland

The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was drained by multiple large ice streams and is thus a useful analogue for marine-based areas of modern ice sheets. However, despite recent advances from investigating the offshore record of the BIIS, the dynamic history of its marine margins, which would have been sensitive to external forcing(s), remain inadequately understood. This study is the first reconstruction of the retreat dynamics and chronology of the western, marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf geomorphology and core sedimentology and chronology enable a reconstruction of the Late Midlandian history of the BIIS west of Ireland, from initial advance to final retreat onshore. Five AMS radiocarbon dates from marine cores constrain the timing of retreat and associated readvances during deglaciation. The BIIS advanced without streaming or surging, depositing a bed of highly consolidated subglacial traction till, and reached to within ∼20 km of the shelf break by ∼24,000 Cal BP. Ice margin retreat was likely preceded by thinning, grounding zone retreat and ice shelf formation on the outer shelf by ∼22,000 Cal BP. This ice shelf persisted for ≤2500 years, while retreating at a minimum rate of ∼24 m/yr and buttressing a >150-km long, 20-km wide, bathymetrically-controlled grounding zone. A large (∼150 km long), arcuate, flat-topped grounding-zone wedge, termed here the Galway Lobe Grounding-Zone Wedge (GLGZW), was deposited below this ice shelf and records a significant stillstand in BIIS retreat. Geomorphic relationships indicate that the BIIS experienced continued thinning during its retreat across the shelf, which led to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone retreat past the GLGZW. At this stage of retreat the western BIIS was comprised of several discrete, asynchronous lobes that underwent several readvances. Sedimentary evidence of dilatant till deposition suggests that the readvances may have been rapid and possibly associated with ice streaming or surging. The largest lobe extended offshore from Galway Bay and deposited the Galway Lobe Readvance Moraine by <18,500 Cal BP. Further to the north, an ice lobe readvanced at least 50 km offshore from Killary Harbour, possibly by ≤15,100 Cal BP. The existing chronology currently does not allow us to determine conclusively whether these readvances were a glaciodynamic (internally-driven) response of the ice sheet during deglaciation or were climatically-driven. Following the <18,500 Cal BP readvance, the Galway Lobe experienced accelerated eastward retreat at an estimated rate of ∼113 m/yr.

In situ foraminifera in grounding zone diamict: a working hypothesis

AbstractThe ice-proximal diamict sediment deposited on the foreset of a grounding zone wedge in Glomar Challenger Basin on the eastern Ross Sea continental shelf yielded a low abundance assemblage of foraminifera at two piston core sites. We found 302 small well-preserved specimens representing 18 species of benthic foraminifera from 825 ml of sediment. Only three poorly preserved specimens of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral) were found. Our combined analyses of preservation state, assemblage composition and stable isotopes suggest that the benthic foraminifera may be in situ. This possibility is of interest to palaeoclimatologists who use ice-proximal sediments on the Antarctic continental shelves to radiocarbon date the post-glacial retreat history.

Subglacial processes on an Antarctic ice stream bed. 1: Sediment transport and bedform genesis inferred from marine geophysical data

ABSTRACTThe spatial pattern and morphometry of bedforms and their relationship to sediment thickness have been analysed in the Marguerite Bay Palaeo-ice stream Trough, western Antarctic Peninsula. Over 17 000 glacial landforms were measured from geophysical datasets, and sediment thickness maps were generated from acoustic sub-bottom profiler data. These analyses reveal a complex bedform pattern characterised by considerable spatial diversity, influenced heavily by the underlying substrate. The variability in length and density of mega-scale lineations indicates an evolving bedform signature, whereby landforms are preserved at different stages of maturity. Lineation generation and attenuation is associated with regions of thick, soft till where deformation was likely to be the greatest. The distribution of soft till and the localised extent of grounding-zone wedges (GZWs) indicate a dynamic sedimentary system characterised by considerable spatio-temporal variability in sediment erosion, transport and deposition. Formation of GZWs on the outer shelf of Marguerite Trough, within the error range of the radiocarbon dates, requires large sediment fluxes (upwards of 1000 m3 a−1 (m grounding line width)−1), and a &gt;1 m thick mobile till layer, or rapid basal sliding velocities (upwards of 6 km a−1).

Ice stream retreat following the LGM and onset of the west Greenland current in Uummannaq Trough, west Greenland

The deglacial history and oceanography of Uummannaq Trough, central West Greenland continental shelf, was investigated using foraminiferal, sedimentological, and bathymetric records together with a radiocarbon chronology, providing a timeline for the retreat of glacial ice after the Last Glacial Maximum (LGM). To map ice stream retreat, data were collected from cores from the outer (JR175-VC45 and JR175-VC43) and inner (JR175-VC42) Uummannaq Trough. A large ice stream, fed by confluent glaciers draining the interior of the Greenland Ice Sheet, extended across the outer shelf during the LGM and was in retreat by 15.0 cal kyr BP. Foraminiferal data indicate that the ‘warm’ West Greenland Current (WGC) was established prior to 14.0 cal kyr BP, which is the hitherto earliest record of Atlantic Water found on the West Greenland shelf. For each of the cores, foraminifera indicate that ice sheet retreat was followed quickly by incursion of the WGC, suggesting that the warm water may have enhanced ice retreat. Prior to the Younger Dryas cold event, the radiocarbon chronology indicates that the ice sheet retreated to the mid-shelf, where it subsequently stabilised and formed a large grounding-zone wedge (GZW). After the Younger Dryas, around 11.5 cal kyr BP, the ice retreated rapidly from the GZW and into the fjords.

Deglaciation of a major palaeo-ice stream in Disko Trough, West Greenland

Recent work has confirmed that grounded ice reached the shelf break in central West Greenland during the Last Glacial Maximum (LGM). Here we use a combination of marine sediment-core data, including glacimarine lithofacies and IRD proxy records, and geomorphological and acoustic facies evidence to examine the nature of and controls on the retreat of a major outlet of the western sector of the Greenland Ice Sheet (GrIS) across the shelf. Retreat of this outlet, which contained the ancestral Jakobshavns Isbrae ice stream, from the outer shelf in Disko Trough was rapid and progressed predominantly through iceberg calving, however, minor pauses in retreat (tens of years) occurred on the middle shelf at a trough narrowing forming subtle grounding-zone wedges. By 12.1 cal kyr BP ice had retreated to a basalt escarpment and shallow banks on the inner continental shelf, where it was pinned and stabilised for at least 100 years. During this time the ice margin appears to have formed a calving bay over the trough and melting became an important mechanism of ice-mass loss. Fine-grained sediments (muds) were deposited alternately with IRD-rich sediments (diamictons) forming a characteristic deglacial lithofacies that may be related to seasonal climatic cycles. High influxes of subglacial meltwater, emanating from the nearby ice margins, deposited muddy sediments during the warmer summer months whereas winters were dominated by iceberg calving leading to the deposition of the diamictons. This is the first example of this glacimarine lithofacies from a continental-shelf setting and we suggest that the calving-bay configuration of the ice margin, plus the switching between calving and melting as ablation mechanisms, facilitated its deposition by channelling meltwater and icebergs through the inner trough. The occurrence of a major stillstand on the inner shelf in Disko Trough demonstrates that the ice-dynamical response to local topography was a crucial control on the behaviour of a major outlet in this sector of the GrIS during retreat.

Late Wisconsinan grounding-zone wedges, northwestern Gulf of St Lawrence, eastern Canada

Grounding-zone wedges (GZWs) are formed on high-latitude continental shelves by the accumulation of subglacial sediments from fast-flowing ice at the grounding zone of an ice sheet during temporary stillstands (e.g. Mosola & Anderson 2006; Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2015). GZWs are therefore recognized to record phases of episodic ice-sheet retreat (Dowdeswell et al. 2008). The morphology and distribution of a series of GZWs mapped using high-resolution multibeam sonar and airgun seismic data in the northwestern Gulf of St Lawrence, eastern Canada, are described here (Fig. 1a–d). These ice-contact submarine landforms were deposited when the marine-based southeastern margin of the Laurentide Ice Sheet stabilized for a few decades to centuries during its rapid retreat through the Gulf of St Lawrence. Fig. 1. Grounding-zone wedges (GZW) in the northwestern Gulf of St Lawrence, eastern Canada. ( a ) Location of study area (red box; map from GEBCO_08). ( b ) Oblique view of grounding-zone wedge at 180 m water depth (GZW 1) illustrating circular depressions along its steeper down-ice slope. ( c ) Seismic-reflection profile with two GZWs. Seismic profile shows bedrock surface which consists of gently dipping monoclinal Middle Ordovician sedimentary rocks of the St Lawrence Platform (Sanford 1993). Profile is located …

Geometry and volume of a middle shelf grounding-zone wedge in Ross Sea, Antarctica

A number of grounding-zone wedges (GZWs) have been identified in the cross-shelf troughs of Ross Sea from seismic data (Shipp et al. 1999). A combined analysis of a large-area multibeam survey and a grid of reflection-seismic lines reveals the planform, thickness and volume of a GZW on the middle shelf of the Glomar Challenger Basin. Combined with sediment flux estimates derived from the Whillans and Mercer ice streams that drain the West Antarctic Ice Sheet (WAIS), this geometric information allows the likely duration of the grounding event to be calculated. A well-defined set of dip-aligned mega-scale glacial lineations (MSGLs) is found on the middle shelf of the Glomar Challenger Basin (Fig. 1a). The seaward limit of the MSGLs corresponds to the outer extent of a GZW previously identified from seismic data (e.g. Mosola & Anderson 2006). The sinuous trend of the grounding zone shows that the GZW has two lobes (Fig. 1a, c). In contrast, the seaward termination of other GZWs often has a linear geometry (e.g. Jakobsson et al. 2011). Immediately seaward of the grounding zone, a narrow region without MSGLs corresponds to the GZW foreset, which dips seaward at 0.5°. The middle shelf GZW pinches out less than 5 km northwards of the grounding-zone boundary (Fig. …

A set of grounding-zone wedges in Vestfjorden, North Norway

At the Last Glacial Maximum (LGM), a 400 km long fast-flowing ice stream drained an interior basin of the Scandinavian Ice Sheet of about 150 000 km2 through Vestfjorden and the adjacent cross-shelf trough of Traenadjupet further downstream (Fig. 1) (Ottesen et al. 2005 a ). Evidence for past ice-stream activity comes from a variety of submarine sedimentary landforms that are streamlined in the direction of past ice flow (Ottesen et al. 2005 b ). Superimposed on these streamlined landforms are several wedge-like sedimentary features orientated transverse to flow. These submarine landforms suggest that deglacial retreat was punctuated by stillstands that produced substantial depocentres known as grounding-zone wedges (GZWs) (e.g. Anderson 1999; Dowdeswell et al. 2008; Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2015). Fig. 1. Grounding-zone wedges in Vestfjorden, North Norwegian margin. ( a ) Shaded-relief swath-bathymetric image of a 5500 km2 area of Vestfjorden showing two large GZWs which both override MSGLs and have further MSGLs on their surfaces. The shallowest parts of the GZWs have been affected by the ploughing action of iceberg keels. Acquisition system Kongsberg EM100. Frequency 97 kHz. Grid-cell size 50 m. ( b …

Grounding-zone wedges on the West Greenland shelf imaged from multibeam and seismic data

Grounding-zone wedges (GZWs) are asymmetrical depocentres built up beneath the grounding zone of marine-terminating ice streams through the delivery of soft, deforming subglacial till from upglacier. They are typically tens of metres thick, tens of kilometres in length and usually form subdued transverse-to-flow ridges across the long axes of fjords and cross-shelf troughs on high-latitude continental shelves (e.g. Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2015). The ridges, representing the relatively steeper ice-distal face of the wedge, usually appear as small scarps or steps in multibeam imagery. The wedges thin in an ice-proximal direction, often becoming difficult to identify except by using acoustic-stratigraphic methods. Relatively large sedimentary wedges have been identified in multibeam data from several Greenland cross-shelf troughs (e.g. Dowdeswell et al. 2014). The features generally have a scarp at their ice-distal end, and a subdued, low-gradient ice-proximal slope which makes their termination difficult to pinpoint from bathymetric data. Where a basal reflection can be identified, there is some evidence that the wedges thin progressively over a few tens of kilometres away from the former ice margin and have a characteristic asymmetry along trough axes. A multibeam image of a modern wedge from the outer shelf of …

Grounding-zone wedges on the western Svalbard shelf

When ice sheets decay and retreat across continental shelves during regional deglaciation, the glacial landforms preserved on the seafloor provide evidence of the nature and rate of change. The presence of streamlined sedimentary landforms implies that fast-flowing ice streams drained a full-glacial ice sheet that extended across the continental shelf (e.g. Ottesen et al. 2005). By contrast, observations of sedimentary depocentres such as grounding-zone wedges (GZWs; Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2015) in cross-shelf troughs are a clear indicator that halts in ice-sheet retreat, perhaps lasting centuries during which ice remained active delivering sediment to the grounding zone, took place between episodic phases of more rapid terminus retreat. The continental shelf west of Svalbard contains a number of sedimentary depocentres or wedges that are orientated across troughs, transverse to the direction of former ice flow (Fig. 1) (e.g. Ottesen et al. 2007; Forwick & Vorren 2010). Multibeam images of three sedimentary wedges are shown in Figure 1a–c. The wedges extend c. 5–20 km across the two fjords. Each …

Seismic character of possible buried grounding-zone wedges in the Late Ordovician glacial rocks of Algeria

A relatively brief ice age in the Late Ordovician, about 445 Ma ago, when North Africa was located near the South Pole led to the deposition of terrestrial and marine glacial sediments across northern Africa (e.g. Le Heron & Craig 2008). Several types of glacial landforms have been identified previously in these glacial rocks, including sedimentary features interpreted as ice-proximal fans and tunnel valleys produced by subglacial meltwater flow (e.g. Hirst 2012). These glacigenic rocks had been buried beneath about 2000 m of overlying sediment and have been deformed subsequently by multiple orogenic phases. A number of asymmetrical depocentres have been identified in three-dimensional (3D) reflection-seismic records from Illizi Basin, Algeria (Fig. 1). They are interpreted as grounding-zone wedges (GZWs), by analogy with similar features in Quaternary sediments on the Greenland margin (Dowdeswell & Fugelli 2012). Fig. 1. ( a , c , d ) Seismic profiles through the long axes of three Late Ordovician buried grounding-zone wedges (GZWs, blue lines mark upper surfaces) in Illizi Basin, Algeria. VE×25, ×10 and ×10, respectively. Time sections from ISS™ (Independent Simultaneous Sources). 3D survey shot in 2009. We acknowledge Sonatrach, BP's partner in Algeria, for its part in …

Grounding-zone wedges and mega-scale glacial lineations in Kveithola Trough, Barents Sea

Multibeam-bathymetric data from Kveithola Trough show a seafloor characterized by east–west-trending mega-scale glacial lineations (MSGLs) overprinted by transverse grounding-zone wedges (GZWs). GZWs are thought to form by deposition of subglacial till at temporarily stable ice-sheet marine termini between successive episodic retreats (Batchelor & Dowdeswell 2015). Sub-bottom profiles show that the present-day morphology of Kveithola Trough is largely inherited from the palaeo-seafloor topography of the GZWs, which is now draped by deglacial to early Holocene glacimarine sediment about 15 m thick. The ice stream that produced such subglacial morphology was flowing from east to west towards the shelf edge in Kveithola Trough during the Last Glacial Maximum (LGM). Its rapid retreat was probably associated with progressive lift-off and successive rapid melting of grounded ice induced by eustatic sea-level rise. The east–west-trending Kveithola Trough in the Barents Sea (Fig. 1) shows well-preserved subglacial landform morphologies (Rebesco et al. …

Grounding-zone wedges on Antarctic continental shelves

The concept of sediment accumulation at high rates in ice-stream grounding-zone settings originates with Alley et al. (1987) who envisioned ‘grounding zone deltas’ composed of sediment transported in a thin deformation-till layer beneath ice streams. This led to the discovery of sediment accumulations on Antarctic continental shelves that, in some instances, revealed foreset beds in seismic sections (e.g. Bart & Anderson 1995). Subsequently, with the advent of multibeam swath bathymetry, these wedges were identified in a number of shelf areas and the term grounding-zone wedge (GZW) was adopted (Shipp & Anderson 1997). Seismic images of GZWs show that they are wedge-shaped in a direction parallel to cross-shelf trough axes, tapering in a landward direction (Fig. 1) and lens-shaped in a transverse direction (Shipp et al. 1999; Howat & Domack 2003). GZWs typically range in thickness from a few metres to a few tens of metres and are occasionally stacked in a back-stepping …

Enigmatic needle-like seafloor features in the Bear Island Trough, central Barents Sea

Mapping of well-preserved submarine glacial landforms has been used to decipher the pattern and dynamics of the retreat of the Bear Island Trough ice stream that drained the Barents Sea Ice Sheet during the Late Weichselian deglaciation (e.g. Andreassen et al. 2008, 2014; Winsborrow et al. 2010; Ruther et al. 2011; Bjarnadottir et al. 2014). However, the formation processes of some of the landforms identified are not fully understood; one example is that of an enigmatic linear sedimentary seafloor feature termed ‘needle’ by Bjarnadottir et al. (2014). A variety of subglacial landforms co-exist in the Bear Island Trough (BIT), including landforms typically associated with both active ice streams, such as mega-scale glacial lineations (MSGLs) and grounding-zone wedges (GZWs), and ice stagnation (e.g. Ruther et al. 2011; Andreassen et al. 2014; Bjarnadottir et al. 2014). Included among these are remarkably straight and elongate features, termed ‘needles’ by Bjarnadottir et al. (2014), which are described here. Needle-like features occur in the upper BIT at water depths of 230–280 m (Fig. 1a). They are <40 km long, <700 m wide and protrude <40 m from …

Grounding-zone wedges and mega-scale glacial lineations in the Mertz Trough, East Antarctica

Glacial erosion and deposition have shaped the Mertz Trough, East Antarctica, where seafloor grounding-zone wedges (GZWs) are associated with mega-scale glacial lineations (MSGLs) (McMullen et al. 2006). GZWs form along grounded glacial margins constrained by ice shelves during stillstands and consist of wedge-shaped glacially transported sediment (Powell & Domack 2002). MSGLs are parallel elongate bedforms that typically form in soft sediments beneath rapidly flowing ice streams (Clark 1993; Canals et al. 2000; Clark et al. 2003). They are found in glacial troughs, usually parallel to trough margins. MSGLs are generally 6 to >100 km long, 200–1300 m wide and spaced 0.3–5 km apart, crest-to-crest (Clark et al. 2003; McMullen et al. 2006). Mertz Trough is located perpendicular to George V coast of Wilkes Land, East Antarctica (Fig. 1a, b). Multibeam-bathymetric imagery reveals distinct features on the seafloor including parallel elongate ridge and groove pairs, two sinuous sediment mounds with streamlined bedforms on their surfaces, and depressions (Fig. 1c). Fig. 1. Multibeam-bathymetric image of the central Mertz Trough, East Antarctica. ( a ) Location of study area (red box; map from …

Grounding-zone wedges on the northern Larsen shelf, Antarctic Peninsula

Prominent quasi-linear or lobate wedge-shaped sedimentary landforms, termed grounding-zone wedges (GZWs), are distributed widely on polar continental shelves. They are regarded as a product of the deposition of mainly subglacially transported sediment at the grounding zone of modern and palaeo ice sheets and ice streams (e.g. Shipp et al. 1999; Ottesen et al. 2005; Horgan et al. 2013). GZWs vary in shape, dimension and regional distribution across continental shelves, forming single or multiple and widely or closely spaced bedforms. The presence of these landforms is used to delimit the maximum and retreat positions of former ice-sheet grounding-zones (e.g. Shipp et al. 1999; Ottesen et al. 2005). During the Late Glacial transition, retreat of the Antarctic Peninsula Ice Sheet produced several distinctive grounding-zone landforms (Evans et al. 2005). Well-defined ramp-scarp, wedge-shaped bathymetric features are present in several places across the northern Larsen shelf south of Larsen Inlet and east of the deepest part of Greenpeace Trough, NE Antarctic Peninsula (Fig. 1a–h). The landforms are found at water depths of 510–580 m, are spaced c. 6 km or more apart and are up to 15–20 km long, 10–18 m high and …

Mega-scale glacial lineations and grounding-zone wedges in Prydz Channel, East Antarctica

The Prydz Bay continental shelf was sculpted by the Lambert Glacier–Amery Ice Shelf system, a large outlet glacier that drains 16% of the East Antarctic Ice Sheet (Allison 1979; Fig. 1a). Prydz Channel (71–73° E; Fig. 1b) is a NW-trending cross-shelf trough (500–700 m deep) in western Prydz Bay that formed in the Pliocene, when the Lambert-Amery system first developed a fast-flowing ice stream (Fig. 1b; Cooper & O'Brien 2004). In Prydz Channel mega-scale glacial lineations (MSGLs) delineate the direction and orientation of past ice flow, and large grounding-zone wedges (GZWs; Batchelor & Dowdeswell 2015) in the inner channel mark the limit of the Last Glacial Maximum (LGM) ice advance about 250 km from the shelf edge (Fig. 1c, d, e). Fig. 1. Multibeam swath bathymetry and seismic-reflection profile of Prydz Channel, East Antarctica. ( a ) Location of study area (red box; map from IBCSO v. 1.0). ( b ) General bathymetry of Prydz Channel, delineated by the 500 m contour, and other relevant regional features. ( c ) Multibeam swath bathymetry of mega-scale glacial lineations (MSGLs) in Prydz Channel and grounding-zone …

Malangsdjupet: a cross-shelf trough on the North Norwegian margin

Cross-shelf troughs, linked to the presence of fast-flowing ice streams, are a common feature of high-latitude shelves (Batchelor & Dowdeswell 2014). On the North Norwegian shelf (Fig. 1a–e), ice advanced westwards as the Scandinavian Ice Sheet expanded during full-glacial conditions on a number of occasions during the Quaternary (Rise et al. 2005). As ice flowed out from the crystalline bedrock of Norway, it eroded deep cross-shelf troughs in the shelf sediments beyond and left evidence of former rapid flow in the form of streamlined landforms such as mega-scale glacial lineations (Ottesen et al. 2005). Glacial sediment was delivered to a submarine fan on the continental slope that built out from the mouth of the trough. Malangsdjupet on the North Norwegian margin (Fig. 1e) provides an example of a well-developed cross-shelf trough where there is clear evidence of occupation by an ice stream that drained the interior of the Scandinavian Ice Sheet under full-glacial conditions. Fig. 1. ( a ) MAREANO swath-bathymetry shaded-relief map of the Malangsdjupet cross-shelf trough, North Norway. Acquisition systems Kongsberg EM710 (70–100 kHz), EM1002 (95 kHz) and EM3000 (300 kHz). Grid-cell size 50 m. TMF, trough-mouth fan; GZW, grounding-zone wedge. ( b ) Sub-bottom profile across the northern lateral …

Landform assemblage produced by the Biscoe Trough ice stream, northern Antarctic Peninsula

The continental shelf off the Biscoe Archipelago has one of the largest cross-shelf troughs on the Pacific margin of the North Antarctic Peninsula (NAP), the Biscoe Trough (Fig. 1a, b). Grooved and streamlined bedrock, drumlins, mega-scale glacial lineations (MSGLs) and grounding-zone wedges (GZWs) define a continuum of landforms over 130 km that provide evidence that ice streams sourced in three different interior drainage basins converged and flowed along the trough during the Last Glacial Maximum (LGM). The Biscoe Trough is important in understanding the configuration of the Antarctic Peninsula Ice Sheet during the LGM (Lavoie et al. 2015) and the onset of the modern Antarctic ice sheet in the Late Pliocene (Rebesco et al. 2006). Fig. 1. ( a ) Location of study area (red box; map from IBCSO v. 1.0). ( b ) Multibeam-bathymetric image of Biscoe Trough collected during RV Hesperides COHIMAR/SEDANO cruise in 2002. Background bathymetry (in grey) from GEBCO\_08. Acquisition system Simrad EM12-S. Frequency 12.5 kHz. Grid-cell size 50 m. Land area from Landsat Image Mosaic of Antarctica, US Geological Survey. PD, Palmer Deep; HIT, Hugo Island Trough. ( c ) Enlarged multibeam image of Biscoe Trough. 100 m contours from GEBCO\_08. ( d ) Schematic diagram of the main geomorphic features of Biscoe Trough. The main trunk of the Biscoe Trough follows an ESE–WNW orientation. Two NE–SW- and SSE–NNW-orientated tributary troughs enter Biscoe Trough from the north and south, respectively (Fig. 1c). The elongate bedforms revealed in multibeam data are initially aligned parallel to these orientations, and show overall convergence towards the shelf edge. The north and south tributary troughs are deepest, with water depths >800 m and 1000 m, respectively (Fig. 1c). The trough boundaries are steep and straight, with a relief of 600 m with respect to the <200 m deep steep-sided banks. A third area of overdeepening, with …