Submarine Landforms Research Articles

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 …

Rhombohedral crevasse-fill ridges at the marine margin of a surging Svalbard ice cap

The ice cap of Austfonna in eastern Svalbard is the largest in the Eurasian Arctic at 8000 km2 and has about 200 km of marine-terminating ice cliffs (Dowdeswell et al. 2008). Several of its drainage basins are of surge type (Meier & Post 1969) and, between 1936 and 1938, one of these basins (Brasvellbreen; 1100 km2) increased its velocity rapidly and underwent an advance of about 20 km along a 30 km wide front (Schytt 1969). Since that time the ice-cap terminus has stagnated and retreated, losing mass by a combination of surface melting, thinning and iceberg production. Retreat has revealed several distinctive and well-preserved submarine landforms (Fig. 1) linked to this recent surge activity (Solheim & Pfirman 1985; Solheim 1991). Fig. 1. ( a ) Rhombohedral ridges and a terminal moraine ridge on the seafloor offshore of Brasvellbreen, an outlet of the Austfonna ice cap in eastern Svalbard. Image courtesy of the Norwegian Hydrographic Service (Permission no. 14/G754). Acquisition system Kongsberg EM1002. Frequency 97 kHz. Grid-cell size 3 m. The ridges are interpreted to have been formed by the filling of basal crevasses with deforming diamictic sediment. ( b ) Oblique aerial photograph of the surge and rapid advance of Brasvellbreen in 1938 (Photo S38-1958; courtesy of Norsk Polarinstitutt). ( c ) Location of study area (red box; map from IBCAO v. 3.0). ( d …

The variety and distribution of submarine glacial landforms and implications for ice-sheet reconstruction

Glacimarine processes affect about 20% of the global ocean today, and this area expanded considerably under cyclical full-glacial conditions during the Quaternary (Fig. 1) (Dowdeswell et al. 2016 b ). Many of the submarine landforms produced at the base and margin of past ice sheets remain well preserved on the seafloor in fjords and on high-latitude continental shelves after the retreat of the ice that produced them. These glacial landforms, protected from subaerial erosion and beneath wave-base and tidal currents in water that is often hundreds of metres deep, are gradually buried by both hemipelagic and glacimarine sedimentation; they may be preserved over long periods in the geological record if palaeo-continental shelves are not reworked by subsequent glacier advances or bottom currents (Dowdeswell et al. 2007). This means that, first, submarine glacial landforms can be observed at or close to the modern seafloor after retreat of the last great ice sheets from their most recent Quaternary maximum about 18–20 000 years ago using swath-bathymetric mapping systems and, secondly, buried glacial landforms may also be identified and examined within glacial-sedimentary sequences from Quaternary and earlier ice ages using seismic-reflection methods. Fig. 1. The global distribution of glaciers and ice sheets and the glacier-influenced, or glacimarine, environment. The approximate modern (yellow dotted line) and Quaternary full-glacial (yellow dashed line) limits of ice-rafting and ice-keel ploughing of the seafloor are shown (modified from Anderson 1983). GEBCO World Map: Gall projection. Numbered yellow dots refer to the locations of subsequent figures. The development of multibeam echo sounding over the past two decades, coupled with high-accuracy GPS positioning, has allowed morphological mapping of the seafloor at an unprecedented level of detail. In this paper, the variety of submarine glacial landforms observed in modern, Quaternary and more ancient sediments is described. Landforms produced subglacially, those formed at and beyond …

Ice-proximal fans in Dexterity Fjord, Buchan Gulf, Baffin Island, Canadian Arctic

Ice-proximal fans are found where subglacial streams reach the margins of grounded tidewater glaciers. As the streams enter the marine environment, they lose energy and deposit their sedimentary load (Powell 1990; Mugford & Dowdeswell 2011). The primary factors influencing the development of ice-proximal fans in a submarine setting are sediment supply and the stability of the parent glacier terminus, which provides the time needed for fan formation (Dowdeswell et al. 2015). Several arcuate fan-like submarine landforms are observed in swath-bathymetric data beyond tidewater glaciers entering Dexterity Fjord, NE Baffin Island (Fig. 1). Fig. 1. ( a ) Swath-bathymetric data from Dexterity Fjord, Baffin Island combined with a Landsat 7 image from 2002. Four ice-proximal fans and their parent glaciers are shown (glacier numbers in brackets from Glacier Atlas of Canada). Acquisition system Kongsberg Simrad EM302. Frequency 30 kHz. Grid-cell size 10 m. ( b ) Location of study area (red box; map from GEBCO_08). ( c , d ) Enlarged images of ice-proximal fans and lateral moraines marking former ice positions. Orientation and colour bar as in (a). TMP, turbid meltwater plume; m, Little Ice Age lateral moraine. ( e ) Acoustic sub-bottom profile from ice-proximal fans (located in (a)). VE×33. Acquisition system …

Channels and gullies on the continental slope seaward of a cross-shelf trough, Labrador margin, eastern Canada

The Labrador Shelf is characterized by several cross-shelf troughs separated by intervening shallower banks. The troughs were probably occupied by fast-flowing ice streams in the Late Pleistocene. Hopedale Saddle trough has a long Quaternary history of till progradation at the shelf edge, and the modern continental slope developed over a major 0.3 Ma shelf-edge failure complex. The upper slope exhibits a series of relatively narrow and deep gullies, whereas the mid-slope contains wider and shallower channels that are locally anastomosing (Fig. 1a). The erosional submarine landforms on the slope are likely to be linked to the delivery of dense sediment-rich meltwater to the shelf edge from a full-glacial ice stream (Piper et al. 2012). Fig. 1. Multibeam bathymetry and seismic profile of gullies and channels on the continental slope in the central Hopedale Saddle region, offshore of Labrador. ( a ) Swath-bathymetric image. Examples of areas of convergent (C) and divergent (D) channel sections are labelled. IL, inner levee; T, incised-thalweg channel. White arrows indicate examples of artefacts. Acquisition system Kongsberg EM300. Frequency 30 kHz. Grid-cell size 10 m. ( b ) Location of study area (red box; map from GEBCO_08). ( c ) Seismic-reflection profile x–x′ across several channels along the mid-slope (located in (a)). VE×17. Acquisition system Sleeve-gun. Frequency 120–850 Hz. MIS, Marine Isotope Stage; GFD, glacigenic debris-flow; H, Heinrich Layer. ( d ) Enlarged oblique …

A new bathymetry of the Northeast Greenland continental shelf: Constraints on glacial and other processes

AbstractA new digital bathymetric model (DBM) for the Northeast Greenland (NEG) continental shelf (74°N–81°N) is presented. The DBM has a grid cell size of 250 m × 250 m and incorporates bathymetric data from 30 multibeam cruises, more than 20 single‐beam cruises and first reflector depths from industrial seismic lines. The new DBM substantially improves the bathymetry compared to older models. The DBM not only allows a better delineation of previously known seafloor morphology but, in addition, reveals the presence of previously unmapped morphological features including glacially derived troughs, fjords, grounding‐zone wedges, and lateral moraines. These submarine landforms are used to infer the past extent and ice‐flow dynamics of the Greenland Ice Sheet during the last full‐glacial period of the Quaternary and subsequent ice retreat across the continental shelf. The DBM reveals cross‐shelf bathymetric troughs that may enable the inflow of warm Atlantic water masses across the shelf, driving enhanced basal melting of the marine‐terminating outlet glaciers draining the ice sheet to the coast in Northeast Greenland. Knolls, sinks, and hummocky seafloor on the middle shelf are also suggested to be related to salt diapirism. North‐south‐orientated elongate depressions are identified that probably relate to ice‐marginal processes in combination with erosion caused by the East Greenland Current. A single guyot‐like peak has been discovered and is interpreted to have been produced during a volcanic event approximately 55 Ma ago.

Late Quaternary ice flow in a West Greenland fjord and cross-shelf trough system: submarine landforms from Rink Isbrae to Uummannaq shelf and slope

Sea-floor landforms and acoustic-stratigraphic records allow interpretation of the past form and flow of a westward-draining ice stream of the Greenland Ice Sheet, Rink Isbrae. The Late Pliocene–Pleistocene glacial package is several hundred metres thick and down-laps onto an upper Miocene horizon. Several acoustic facies are mapped from sub-bottom profiler records of the 400 km-long Uummannaq fjord-shelf-slope system. An acoustically stratified facies covers much of the fjord and trough floor, interpreted as glacimarine sediment from rain-out of fine-grained debris in turbid meltwater. Beneath this facies is a semi-transparent deformation-till unit, which includes buried streamlined landforms. Landform distribution in the Uummannaq system is used to reconstruct past ice extent and flow directions. The presence of streamlined landforms (mega-scale glacial lineations, drumlins, crag-and-tails) shows that an ice stream advanced through the fjord system to fill Uummannaq Trough, reaching the shelf edge at the Last Glacial Maximum. Beyond the trough there is a major fan built mainly of glacigenic debris flows. Turbidity-current channels were not observed on Uummannaq Fan, contrasting with well-developed channels on Disko Fan, 300 km to the south. Ice retreat had begun by 14.8 cal. ka ago. Grounding-zone wedges (GZW) in Uummannaq Trough imply that retreat was episodic, punctuated by several still-stands. Ice retreat between GZWs may have been relatively rapid. There is little sedimentary evidence for still-stands in the inner fjords, except for a major moraine ridge marking a Little Ice Age maximum position. On the shallow banks either side of Uummannaq Trough, iceberg ploughing has reworked any morphological evidence of earlier ice-sheet activity.

The marine-based NW Fennoscandian ice sheet: glacial and deglacial dynamics as reconstructed from submarine landforms

The configuration of the marine-based NW Fennoscandian ice sheet during the Last Glacial Maximum (LGM) and deglaciation is reconstructed using detailed swath bathymetry and high-resolution seismic data. The investigated area covers about 10,000 km2 of the continental shelf off Troms, northern Norway. Large scale morphology is characterized by cross-shelf troughs, coast-parallel troughs and banks. Based on mega-scale glacial lineations (MSGL), lateral and shear zone moraines and grounding zone systems, the extent and dynamics of the ice sheet during the LGM are deduced. MSGL indicate fast-flowing ice streams in the cross-shelf troughs, while the glacial morphology on the banks indicates more sluggish ice here. The marine-based part of the Fennoscandian ice sheet was sourced from ice domes in the east via fjord and valley systems inshore. Using a balance flux approach, we estimate palaeo-ice stream velocities during the LGM to be approximately 350 m/year. Three deglaciation events have been reconstructed: i) During the Torsken-1 event the ice sheet halted or readvanced to form groundings zone wedges (GZW) and the Torsken moraine, ii) Several still-stands or readvances characterized the ice behaviour on the shallower banks during the Torsken-2 event, iii) During the Flesen event, prominent end moraines in the inner parts of the troughs and banks were deposited. The locations of the end moraines and GZW in the troughs indicate that the retreat of marine-based ice streams in areas of reverse bed slope was episodic, probably mainly due to the variation in widths of the cross-shelf troughs.

Detection of the submerged topography along the Egyptian Red Sea Coast using bathymetry and GIS-based analysis

A long time ago the Red Sea was only known by small-scale bathymetric, magnetic anomaly maps and a few seismic reflection or refraction profiles. Therefore, detection of the major submerged coastal features was unattainable. This study is based on the integration of different data sets of topography and bathymetry (e.g. the global bathymetry data set, the SRTM DTED® 2, the soviet military topographic maps of scale 1:200.000 and the US army topographic maps of scale 1:250.000) to reveal the main submarine landforms that marked the continental shelf and its related slopes along the Egyptian Red Sea Coast from latitude 27°43′N to the Egyptian-Sudanese border at latitude 22°00′N. The study deduced that the continental shelf is noticeably influenced by the surface fault system extending eastward into the main Red Sea depression, showing the continental edge mostly like a fault-scarp of ∼60° anticlockwise fault plane. Sea ridges and subbasins were distinguished at the lower toe of the continental slope, which seem to be a result of a regional fold system. Two sea peaks of extinct volcanoes were recognized. Two types of submarine canyons were recognized as deep incised Messinian canyons and shallow canyons. The deep incised canyons (∼500mbsl) carve the continental edge with remarkable steep walls. They might be formed as a result of the Messinian event (∼5.59Ma). The shallow canyons are mostly developed during the Pleistocene lower sea level (∼90–130mbsl) where the major wadis cut their water courses through the continental shelf. Some individual submerged deltas were identified, showing a close relationship with the present-day drainage system, although they were supposed to be produced by an ancestor drainage system. Notable submarine terraces were recognized at depths 20–25, 50–75, and 100–120mbsl that are in agreement with the generalized global curve of sea-level rise since the LGM (∼23–18ka bp). It is expected that results of this study will be helpful for shedding light on the geomorphologic evolution of the continental shelf and its surrounding landmass.

Submarine landforms and the behavior of a surging ice cap since the last glacial maximum: The open-marine setting of eastern Austfonna, Svalbard

Multi-beam swath bathymetry data and Topographic Parametric Sonar (TOPAS) 3.5 kHz sub-bottom acoustic profiles were acquired from the inlet of Hartogbutka, an open-marine setting offshore of Austfonna, northeastern Nordaustlandet, Svalbard, the largest ice cap in the Eurasian Arctic. Using these marine geophysical data, the seafloor morphological and sedimentary evidence of former ice extent and dynamics of the three drainage basins (Basins 3, 4, and 5) adjacent to Hartogbukta were examined. Submarine landforms and landform assemblages that provide evidence of past ice dynamics are described and interpreted in the context of previous studies of surge-affected submarine environments elsewhere in Svalbard. A sequence of four distinct ice-flow episodes in Hartogbukta, from the late Weichselian deglaciation to the present, is identified to explain the morphological evolution of the seafloor. A full-glacial ice sheet extended over Hartogbukta during the late Weichselian, flowing ESE. At intervals throughout the Holocene, the drainage basins adjacent to Hartogbukta probably underwent repeated surges. Most recently, Basins 3 and 4 probably surged as one cohesive ice mass in the late 1800s and Basin 5 most likely underwent a less extensive surge in the 20th century. These ice-flow events are interpreted in the context of the glaciology and known glacial history of Austfonna and the NW Barents Sea. The 200 km-long ice cliffs of eastern Austfonna provide an unusual and important analog for open-marine deposition that would have been much more prevalent under full-glacial conditions where ice sheets in many parts of the Arctic advanced from fjord systems onto the adjacent continental shelves.

Late Quaternary ice flow and sediment delivery through Hinlopen Trough, Northern Svalbard margin: Submarine landforms and depositional fan

The morphology and distribution of submarine landforms in Hinlopen Trough, Northern Svalbard margin, and the upper continental slope beyond, are investigated using swath-bathymetric data, together with side-scan sonar, acoustic profiler records and sediment cores. Sun-illuminated images reveal a number of landform assemblages on the sea floor of Hinlopen Trough and confirm that this depression was occupied by an ice stream during the Late Weichselian glaciation around 20 ka before present. The geomorphology of the inner-shelf is characterised by bedrock drumlins and ice-sculpted bedrock. There is a middle-shelf transition from a rock bed to an unconsolidated sedimentary bed that contains highly-attenuated drumlins and megaflutes formed within a dark grey, matrix-supported diamict, interpreted as subglacial till. Outer-shelf landforms are mainly iceberg ploughmarks. The geomorphological imprint identified from Hinlopen Trough is similar to that of many former ice streams; the trough is dominated by elongate landforms orientated parallel to the inferred flow direction and is lacking in transverse features indicative of inter-ice stream locations. A characteristic down-flow progression in landform elongation is also observed, suggesting an increase in ice velocity across the continental shelf. The lack of grounding-zone features imaged from swath-bathymetric data implies that deglaciation was probably rapid within the trough. A large depositional fan exists on the upper continental slope beyond the trough, characterised by debris flow deposits relating to the down-slope transport of glacigenic sediment. Order of magnitude calculations of the volume of glacigenic sediment on the continental slope indicate that the ice stream provided high rates of debris delivery of around 5–7.5 m/ka. This suggests that Hinlopen Trough ice stream represented a major route for the transfer of ice and debris to the Hinlopen Fan on the northern continental margin of Svalbard during the Late Weichselian glaciation.

Submarine landforms and ice-sheet flow in the Kvitøya Trough, northwestern Barents Sea

High-resolution geophysical and sediment core data are used to investigate the pattern and dynamics of former ice flow in Kvitøya Trough, northwestern Barents Sea. A new swath-bathymetric dataset identifies three types of submarine landform in the study area (streamlined landforms, meltwater channels and cavities, iceberg scours). Subglacially produced streamlined landforms provide a record of ice flow through Kvitøya Trough during the last glaciation. Flow directions are inferred from the orientations of streamlined landforms (drumlins, crag-and-tail features). Ice flowed northward for at least 135 km from an ice divide at the southern end of Kvitøya Trough. A large channel-cavity system incised into bedrock in the southern trough indicates that subglacial meltwater was present at the former ice-sheet base. Modest landform elongation ratios and a lack of mega-scale glacial lineations suggest that, although ice in Kvitøya Trough was melting at the bed and flowed faster than the likely thin and cold-based ice on adjacent banks, a major ice stream probably did not occupy the trough. Retreat was relatively rapid after 14–13.5 14C kyr B.P. and probably progressed via ice sheet-bed decoupling in response to rising sea level. There is little evidence for still stands during ice retreat or of ice-proximal deglacial sediments. Relict iceberg scours in present-day water depths of more than 350 m in the northern trough indicate that calving was an important mass loss mechanism during retreat.

Submarine landforms and shallow acoustic stratigraphy of a 400 km-long fjord-shelf-slope transect, Kangerlussuaq margin, East Greenland

Kangerlussuaq Fjord is a relatively uniform, steep-walled basin, whose floor has an almost smooth surface. Debris is supplied mainly from icebergs from the fast-flowing Kangerlussuaq Glacier. Sedimentation after iceberg release from multi-year sea ice is mainly by rain-out of fine-grained englacial debris. Streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into Kangerlussuaq Trough at the Last Glacial Maximum (LGM). Bedrock channels and crescentic overdeepenings indicate warm-based ice and free water beneath parts of the former ice sheet. Cross-cutting iceberg scour marks, which characterise outer Kangerlussuaq shelf, were produced not only during deglaciation, but also occasionally through the Holocene by deep-keeled icebergs from further north in East Greenland. The outward-convex contours of the shelf edge and slope beyond Kangerlussuaq Trough, and debris flows on the slope, suggest a glacier-influenced high-latitude fan. The distribution of streamlined subglacial landforms demonstrates that the Greenland Ice Sheet extended throughout Kangerlussuaq Fjord and reached at least 200 km across the shelf in Kangerlussuaq Trough at the LGM. Streamlined landform orientation indicates ice flow from the interior of Greenland down the axis of Kangerlussuaq Trough. There is little evidence for discrete sedimentary depocentres in the trough, implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation.

An inter–ice-stream glaciated margin: Submarine landforms and a geomorphic model based on marine-geophysical data from Svalbard

Well-preserved submarine landforms from the continental shelf and fjords of northwest- ernmost Svalbard provide an example of ice-sheet deposition in an inter-ice-stream setting. EM1002 swath-bathymetric imagery covering 1280 km 2 was examined. At the shelf edge, a distinctive and continuous belt of hum- mocky topography represents the grounding zone of a slow-moving Late Weichselian ice sheet. Active ice on the outer shelf is inferred from subtle lineations orientated parallel to fl ow. Low-amplitude transverse moraines crosscut the lineations, suggesting ice retreat across the outer shelf with brief stillstands. On the middle and inner shelf, large moraine ridges indicate multiple stillstands during de- glaciation. There are arcuate moraine ridges at fjord mouths. Streamlined crag-and-tail landforms are preserved from when ac- tive full-glacial ice fl owed out of the fjords. Clusters of smaller transverse ridges indi- cate slow retreat of grounded ice through the fjords. Holocene sedimentation is by rainout from sediment-rich meltwater, producing smooth basin fi ll. Small slides from the fjord walls are common. Little Ice Age glacier readvance produced another set of terminal moraines and smaller retreat moraines in the innermost fjords. A schematic model of this inter-ice-stream glacial landform assem- blage summarizes the geomorphic record. It is compared with a model derived from sev- eral Svalbard cross-shelf troughs occupied by fast-fl owing Late Weichselian ice streams. In general, the seafl oor morphology of continen- tal margins affected by ice streams is domi- nated by streamlined, subglacially produced landforms oriented in the former ice-fl ow direction, interrupted by major grounding- zone wedges formed during temporary halts in ice retreat. By contrast, between ice streams, shelf and fjord morphology records submarine landforms of various dimensions oriented mainly transverse to ice fl ow, pro- duced at slowly retreating, grounded ice- sheet margins. There is little evidence for channeled subglacial water fl ow in the form of eskers and ice-contact fans on the Sval- bard margin, implying that basal water is drained or advected mainly within soft sub- glacial sediments.

Morphology of Younger Dryas subglacial and ice‐proximal submarine landforms, inner Vestfjorden, northern Norway

The sea‐floor morphology of two pronounced across‐fjord bedrock thresholds located at the mouths of Ofotfjorden and Tysfjorden, northern Norway, has been analysed based on swath bathymetry and seismic data. The Younger Dryas ice front was located here during the recession of one of the large palaeo‐ice streams of the Fennoscandian Ice Sheet. The thresholds are several kilometres long and wide, rising to several hundred metres above the adjacent sea floor, and the slopes are steep, up to 25°. The Ofotfjorden threshold is draped by acoustically discontinuous to chaotic sediments partly infilling the bedrock relief. A pattern of well‐developed, subglacial bedforms (e.g. crag‐and‐tail formations, drumlins and glacial lineations) on top of both thresholds suggests fast‐flowing ice. A series of smaller transverse ridges is identified on both thresholds and probably records ice‐front oscillations during the final deglaciation. The distal parts of the sediments have been remobilized by slides that occurred after glacial retreat from the thresholds. Earthquake activity due to the isostatic rebound following ice retreat from this area was the most likely triggering mechanism for the slides. The location of the ice front on a prominent bedrock threshold indicates that the basin configuration was important in locating the maximum position of the climatically induced re‐advance, i.e. a topographic control on the maximum Younger Dryas position in the Ofotfjorden and Tysfjorden area is suggested.

Morphology of the upper continental slope in the Bellingshausen and Amundsen Seas – Implications for sedimentary processes at the shelf edge of West Antarctica

Swath bathymetric and sub-bottom profiler data reveal a variety of submarine landforms such as gullies, slide scars, subtle shelf edge-parallel ridges and elongated depressions, and small debris flows along the continental shelf break and upper slope of West Antarctica. Gullies cutting through debris flow deposits on the Belgica Trough Mouth Fan (TMF) suggest formation after full-glacial deposition on the continental slope. The gullies were most likely eroded by sediment-laden subglacial meltwater flows released from underneath the ice margin grounded at the shelf edge at the onset of deglaciation. Scarcity of subglacial meltwater flow features on the outer shelf suggests that the meltwater reached the shelf edge mainly either through the topmost layer of soft diamict or in the form of dispersed flow beneath the ice, although locally preserved erosional channels indicate that more focused and higher-energy flows also existed. Concentration of gullies on the upper continental slope in front of the marginal areas of the major cross-shelf troughs, as contrasted to their axial parts, is indicative of higher-energy gully-eroding processes there, possibly due to additional subglacial meltwater flow from beneath the slow moving ice lying over the higher banks of the troughs. The shallow and sinuous gully heads observed on the outermost shelf within the Pine Island West Trough may indicate postglacial modification by near-bed currents resulting either from the subglacial meltwater flow from underneath the ice margin positioned at some distance landward from the shelf edge, or from the currents formed by brine rejection during sea ice formation. On the continental slope outside major troughs, slide scars as well as shelf-edge parallel ridges and elongated depressions indicate an unstable and failure-prone uppermost slope, although failures were probably mainly associated with rapid sediment loading during glacial periods. Complex, cauliflower- and amphitheatre-shaped gully heads biting back into the shelf edge suggest upslope retrogressive, multi-stage small-scale sliding as a contributing factor to the formation of gullies in these areas. Small debris fans immediately downslope of the slide scars suggest that small-scale debris flows have been the main downslope sediment transfer processes in the areas of weak or absent subglacial meltwater flow.

Submarine landforms characteristic of glacier surges in two Spitsbergen fjords

Well-preserved submarine landforms, all less than 100 years old, are imaged on high-resolution swath bathymetry obtained from Van Keulenfjorden and Rindersbukta (inner Van Mijenfjorden), Spitsbergen, Svalbard. Several tidewater glaciers in these fjords have surged in the last few hundred years. Streamlined landforms, found within the limits of known surges, are interpreted as mega-scale glacial lineations (MSGL) formed subglacially beneath actively surging ice. Large transverse ridges are terminal moraines formed by thrusting at the maximum position of glacier surges. Sediment lobes at the distal margins of terminal moraines are interpreted as glacigenic debris flows, formed either by failure of the frontal slopes of thrust moraines or from deforming sediment extruded from beneath the glacier. Sinuous ridges are eskers, formed after surge termination by the sedimentary infilling of subglacial conduits. Concordant ridges, parallel to former ice margins, are interpreted as minor push moraines, probably formed annually during winter glacier readvance. Discordant ridges, oblique to former ice margins, are interpreted as crevasse-squeeze ridges, forming when soft subglacial sediments are injected into basal crevasses. These submarine landforms have been deposited in the following sequence based on cross-cutting relationships between them, linked to stages of the surge cycle: (1) MSGL; (2a) terminal moraines and (2b) lobe-shaped debris flows; (3) isolated areas of crevasse-fill ridges; (4) eskers and (5) annual retreat ridges. A descriptive landsystem model for tidewater surge-type glaciers has been developed, whose wider applicability is emphasised by comparison with two areas in Isfjorden, Spitsbergen. The model also has a number of features in common with landsystem models for terrestrial surge-type glaciers, but is likely to be more complete since submarine landforms are particularly well preserved. The landforms discussed here may be produced and preserved in different proportions in the varied environmental settings where surging glaciers are found.

Submarine glacial landforms and rates of ice-stream collapse

The rate of deglacial ice-sheet retreat across polar continental shelves, and possible ice-stream collapse and sea-level rise, has been much debated. High-resolution imagery of seafloor morphology is available for many polar shelves and fjords. The rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms, produced at ice-stream sedimentary beds. These landforms, exposed by ice retreat across high-latitude shelves, demonstrate that deglaciation occurs in three main ways: rapidly, by flotation and breakup; episodically, by still-stands and/or grounding events punctuating rapid retreat; or by slower retreat of grounded ice. Submarine landform assemblages imply, through the presence of grounding-zone wedges overprinting mega-scale glacial lineations on many polar shelves, that ice-stream retreat is more often episodic than catastrophic. These observations provide a robust test of the ability of numerical models to predict the varied response of ice-sheet basins to environmental changes.

Dynamics of the Late Weichselian ice sheet on Svalbard inferred from high‐resolution sea‐floor morphology

High‐resolution bathymetric mapping of the fjords and continental shelf around the Svalbard archipelago shows an extensive pattern of large‐ and medium‐scale submarine landforms formed by differences in ice‐flow regimes. Mega‐scale glacial lineations, lateral moraines, transverse ridges and glaciotectonic features are superimposed on the large‐scale fjord, shelf and cross‐shelf trough morphology of the margin. From these landforms we have inferred the flow and dynamics of the last ice sheet on Svalbard. Major fjords and their adjacent cross‐shelf troughs have been identified as the main routes for ice streams draining the ice sheet. On the west coast of Svalbard major pathways existed along Bellsund, Isfjorden and Kongsfjorden. Along the northern Svalbard margin most of the ice drained through the Woodfjorden cross‐shelf trough and Wijdefjorden‐Hinlopen strait. Extensive areas with trough‐parallel glacial lineations in the cross‐shelf troughs suggest fast ice flow by palaeo‐ice streams. Lateral ice‐stream moraines, several tens of kilometres in length, have been mapped along the margins of some of the cross‐shelf troughs, identifying the border zone between fast ice flow and stagnant or slow‐flowing ice on intervening banks. Several general implications can be drawn from the interpretation of the glacier‐derived submarine landforms around Svalbard. Firstly, the Late Weichselian ice sheet was partitioned into fast‐flowing ice streams separated by slower moving ice. Secondly, our submarine morphological evidence supports earlier sedimentological, stratigraphical and chronological studies in implying that a large ice sheet reached the shelf edge around almost all of western and northern Svalbard in the Late Weichselian. The idea of a relatively restricted ice sheet over Svalbard, with ice‐free conditions in some areas of the west coast at the Last Glacial Maximum, is therefore unlikely to be correct. Thirdly, the ice sheet appears to have retreated more rapidly from the cross‐shelf troughs and outer fjords, although sometimes this occurred in a punctuated pattern indicated by grounding‐zone wedges, and more slowly from the intervening shallower banks. In addition, a grounding zone for the ice sheet has been mapped at the shelf edge 10–20 km off the northwest coast of Svalbard, suggesting that ice did not reach the adjacent Yermak Plateau during the Late Weichselian.

Evidence for Late Pleistocene ice stream activity in the Witch Ground Basin, central North Sea, from 3D seismic reflection data

Buried submarine landforms mapped on 3D reflection seismic data sets provide the first glacial geomorphic evidence for glacial occupation of the central North Sea by two palaeo-ice-streams, between 58–59°N and 0–1°E. Streamlined subglacial bedforms (mega-scale glacial lineations) and iceberg plough marks, within the top 80 m of the Quaternary sequence, record the presence and subsequent break-up of fast-flowing grounded ice sheets in the region during the late Pleistocene. The lengths of individual mega-scale glacial lineations vary from ∼5 to ∼20 km and the distance between lineations typically ranges from 100 to 1000 m. The lineations incise to a depth of 10–12 m, with trough widths of ∼100 m. The most extensive and best-preserved set of lineations, is attributed to the action of a late Weichselian ice stream which either drained the NE sector of the British–Irish ice sheet or was sourced from the SW within the Fennoscandian ice sheet. The 30–50 km wide palaeo ice-stream is imaged along its flow direction for 90 km, trending NW–SE. An older set of less well-preserved lineations is interpreted as an earlier Weichselian or Saalian ice-stream, and records ice flow in an SW–NE orientation. Cored sedimentary records, tied to 3D seismic observations, support grounded ice sheet coverage in the central North Sea during the last glaciation and indicate that ice flowed over a muddy substrate that is interpreted as a deformation till. The identification of a late Weichselian ice stream in the Witch Ground area of the North Sea basin provides independent geomorphic evidence in support of ice-sheet reconstructions that favour complete ice coverage of the North Sea between Scotland and Norway during the Last Glacial Maximum.