Grid Cell Size Research Articles

Mud volcanoes and ice-keel ploughmarks, Beaufort Sea shelf, Arctic Canada

The shallow shelf waters of the Beaufort Sea have experienced marine transgression during the Holocene (Hill et al. 1985; Taylor et al. 2013). This has led to a warming of what was terrestrial permafrost by water incursion, and to the dissociation of subsurface gas hydrates which now vent into marine waters. Accompanying this change is the development of conical submarine landforms produced through the extrusion of sediments, combined with the continuing reworking of the seafloor by the ploughing action of the keels of drifting ice (Fig. 1). Fig. 1. Multibeam swath bathymetry and bathymetric profiles of mud volcanoes and ice-keel ploughmarks on the Beaufort Sea shelf, Arctic Canada. ( a ) Sun-illuminated multibeam-bathymetric image showing mud volcanoes and ice-keel ploughmarks. Acquisition system Kongsberg EM302. Frequency 30 kHz. Grid-cell size 5 m. White arrows denote a curvilinear ploughmark cross-cutting its previous track. The image is a compilation of data from CCGS Amundsen -based surveys in 2009 and 2011. The NW–SE narrow, linear artefacts arise from instrumentation employed in the initial survey. Wide, curvilinear artefacts arise from different instrumentation employed in a subsequent survey. ( b ) Sun-illuminated multibeam-bathymetric image showing a swarm of mud volcanoes. Acquisition system Kongsberg EM3002. Frequency 300 kHz. …

Pockmarks in the SW Barents Sea and their links with iceberg ploughmarks

Pockmarks and ploughmarks are observed to coexist on the floor of some high-latitude continental shelves (Fig. 1). Pockmarks are rounded to oval-shaped depressions up to few 100 m in diameter and few metres in depth and are often related to subsurface fluid-flow (King & MacLean 1970). They may appear as single features, in groups or as long chains (Hovland 1981). Ploughmarks are linear to curvilinear depressions in shallow seas and shelves, formed by iceberg keels impinging on the sedimentary seafloor (e.g. Woodworth-Lynas et al. 1991). Fig. 1. ( a ) Multibeam image of pockmarks and ploughmarks from the SW Barents Sea collected during the 2009 HU Sverdrup II cruise (funded by Lundin Norge AS). Acquisition system Kongsberg EM710. Frequency 70–100 kHz. Grid-cell size 5 m. ( b ) Enlarged image of pockmarks, some aligned along a ploughmark. ( c ) Backscatter image of the area shown in (b). ( d ) Sub-bottom profiler data of a pockmark showing stratified marine (above yellow line) and glacimarine sediments (above green line). Blue line is modern seafloor. Acquisition system HUGIN HUS AUV. Frequency 4–24 kHz. ( e ) Location of study area (red box; map from IBCAO v. 3.0). Many crater-like features are observed on the seafloor in the SW Barents Sea along the NW boundary of the Ingoydjupet depression …

Deglacial landform assemblage records fast ice-flow and retreat, Inner Hebrides, Scotland

High-resolution bathymetric data have been central to recent advances in the understanding of past dynamics of the former British–Irish Ice Sheet (BIIS). As approximately two-thirds of the former BIIS was probably marine-based during the Last Glacial Maximum (LGM) ( c. 29–23 ka), geomorphic observations of the seabed are required increasingly to understand the extent, pattern and timing of past glaciation. Until recently, glacial reconstructions for the Inner Hebrides, offshore of western Scotland, have been based primarily on terrestrial observations. Previous workers have proposed generalized reconstructions in which the Inner Hebrides are located within a significant former ice-sheet flow pathway that drained the western Scottish sector of the BIIS, feeding the Barra Fan during the LGM and earlier glaciations (Fig. 1). Results from numerical ice-sheet modelling suggest that former ice-flow velocities within the region were on the order of hundreds to thousands of metres per year, but yield further insight by demonstrating how dynamic binge/purge cycles may have affected ice-sheet mass balance over time (Hubbard et al. 2009). Following the LGM, ice-sheet retreat through the area is estimated to have been in the order of 20 m per year (Clark et al. 2012). Here we present swath-bathymetric data from the Inner Hebrides that provide in situ constraints on ice-sheet flow and subsequent retreat dynamics from within this important sector of the BIIS. Fig. 1. Swath-bathymetric images. ( a ) Inner Hebrides, Scotland (acquired with both multibeam (e.g. Kongsberg EM3002D) and interferometric systems for the Maritime and Coastguard Agency's UK Civil Hydrography Programme © Crown copyright. Grid-cell size 5 m). ( b ) Superimposed flow sets of streamlined landforms. Landforms associated with black flow arrows post-date features associated with white arrows. ( c ) Convergence zone of two ice-stream tributaries overlain by a series of recessional moraines, indicating later ice margin retreat to the NE. Inset shows interpreted glacigenic-landform map highlighting …

Ice-sheet related landforms at the continental shelf edge, Albatross Bank, Alaska

Albatross Bank is a flat glaciated bedrock bank (Fig. 1a) located on the distal edge of the Alaskan continental shelf south of Kodiak Island (Fig. 1b). The bank illustrates the glacial geomorphology from an active ice edge during Marine Isotopic Stage (MIS) 2 ( c. 22–19 ka) sea-level regression (Briner & Kaufman 2008). The multibeam echo-sounder (MBES) images presented here provide the first detailed morphological evidence that the furthest extent of glacial advance in the late Pleistocene reached the shelf edge of the Gulf of Alaska. Other disparate areas of MBES imagery from the distal edge of the SE Alaska shelf to the western part of the Gulf of Alaska show evidence of glacial advance to the shelf break and support earlier depictions of Wisconsinan glaciation (Kaufman & Manley 2004; Briner & Kaufman 2008). Fig. 1. Multibeam swath bathymetry and bathymetric profiles of glacial features on Albatross Bank, Alaskan continental shelf. ( a ) Sun-illuminated multibeam-bathymetric image showing a band of seafloor iceberg ploughmarks (red inset, box f), bedrock outcrops and subglacial meltwater channels (red inset, box c). Acquisition system Reason SeaBat 8111. Frequency 100 kHz. Grid-cell size c. 10 m. ( b ) Location of study …

Moraine ridges in fjord-type, perialpine Lake Lucerne, central Switzerland

Overdeepened valleys and troughs on the northern and southern margins of the central European Alps have been shaped during the Quaternary glaciations (Preusser et al. 2010). These valleys host a number of fjord-type lakes, remnants of formerly larger water bodies, which show various morphological features that reflect their glacial origin, similar to marine fjords. Among the most prominent of these features are large-scale transverse moraine ridges in Lake Lucerne, a perialpine lake situated in a branched valley system on the northern margin of the Alps (Fig. 1b, c). Fig. 1. Swath-bathymetric data and reflection-seismic profiles from Lake Lucerne, central Switzerland. ( a , d ) Shaded-relief images of the Nase Moraine and Chindli Moraine. Colour represents depth; greyscale for areas outside the lake. Acquisition system GeoAcoustics GeoSwath Plus. Frequency 125 kHz. Grid-cell size 1 m. Greyscale derived from swisstopo swissALTI3D elevation model. Labels: ch, chutes and channels; fa, fan-shaped deposit; cr, secondary crest. ( b ) Location of study area (red box; map from GEBCO_08). ( c ) Overview of Lake Lucerne; areas shown in (a) and (d) are marked by red rectangles. ( e , f ) Reflection-seismic profiles across the moraine ridges. VE×4. Acquisition system RCMG Centipede sparker, SIG single-channel streamer, IXSEA Delph acquisition unit. Vertical scale …

Relict proglacial deltas in Bradshaw and George sounds, Fiordland, New Zealand

New Zealand fjords (Fig. 1e) contain submerged, relict, proglacial lacustrine and marine deltas that formed after glaciers retreated from the west coast by 17 ka (Pickrill et al. 1992). Many of these deltas, for example those in Bradshaw and George sounds, are exceptionally well preserved because postglacial sedimentation rates have been low. Fig. 1. Relict proglacial deltas in Bradshaw and George sounds, Fiordland, New Zealand. ( a ) Swath-bathymetric image of the head of George Sound superimposed on a hill-shaded DEM. Acquisition system Kongsberg EM300. Frequency 30 kHz. Grid-cell size 5 m. EB, Elder Basin; WB, Whitewater Basin; SWB, South-West Basin; AD, Anchorage Delta; WD, Whitewater Delta; SWAD, South-West Arm Delta; ALD, Alice Delta; AF, Alice Falls. ( b ) Head of Bradshaw Sound. BB, Bradshaw Basin; GAD, Gaer Arm Delta; PD, Precipice Delta. ( c ) Long-axis profile from EB to Lake Alice (LA), across the relict proglacial ALD and WB. VE×11. ( d ) Long-axis profile across the modern and relict proglacial components of GAD. VE ×10. ( e ) Location of study area (red box; map from GEBCO_08). ( f ) Enlargement showing bathymetric details of the AD foreset slope off Anchorage Cove. ( g ) Head …

Glacial landform assemblages in Spitsbergen fjords from the last full-glacial, deglaciation and the late Holocene

Fjords are characteristic features of the western and northern coasts of Spitsbergen. They formed during repeated glacial cycles when grounded glaciers drained large ice sheets occupying the Svalbard–Barents Sea region. Since grounded glaciers often erode and remove pre-existing deposits from fjord basins, depositional sequences in fjords are typically limited to the last glacial–interglacial cycle (e.g. Forwick & Vorren 2010). Holocene sediment drapes in Spitsbergen fjords are not usually thick enough to obscure glacial landforms and deposits from the last full-glacial and deglacial period. Glacier advances related to climatic cooling during the late Holocene, together with glacier surges, have led to the formation of complex landform assemblages close to many fjord heads (e.g. Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Streuff et al. 2015). The floors in the central and outer parts of several Spitsbergen fjords, including Kongsfjorden, Isfjorden, Billefjorden, Sassenfjorden and Tempelfjorden, reveal multiple streamlined sedimentary lineations orientated sub-parallel to fjord axes (Figs 1a, b, c, 2a; e.g. Ottesen et al. 2005; Baeten et al. 2010; Forwick et al. 2010). The ridges are≤5 km long, 15 m high and<500 m wide, with a spacing of up to 800 m; length-to-width ratios exceed 10:1. Elongate landforms with length-to-width ratios smaller than 10:1 are also found in Kongsfjorden and Raudfjorden (e.g. Ottesen & Dowdeswell 2009; MacLachlan et al. 2010). Fig. 1. Examples of glacial landforms and sediments in Spitsbergen fjords. ( a ) Location of study areas (red boxes; map from IBCAO v. 3.0). Red boxes show locations of (b)–(d) and Figure 2. ( b ) Sun-illuminated multibeam-bathymetric image from Billefjorden (after Baeten et al. 2010). Acquisition system Kongsberg EM300. Frequency 30–34 kHz. Grid-cell size 5 m. ( c ) Sun-illuminated multibeam-bathymetric image from Kongsfjorden and southern Krossfjorden (courtesy of Boele Kuipers, Norwegian Hydrographic Service; for details see MacLachlan …

Bouldery debris mantle deposited by cold-based ice offshore of the Vestfold Hills, East Antarctica

Streamlined subglacial sediments are common in seafloor bathymetric images from many glaciated margins. Less common are seafloor features left by cold-based ice. The shallow waters around the now largely ice-free Vestfold Hills in East Antarctica (Fig. 1a, e) show few streamlined sedimentary features, even though the area has clearly been glaciated as implied by striated bedrock, moraine ridges and abundant erratic boulders on adjacent exposed land. A likely explanation for the absence of streamlined subglacial landforms is that the area was glaciated by ice frozen to its bed. Fig. 1. Multibeam swath bathymetry, outcrop photo and topographic profiles of boulder-rich debris mantle and sand ribbons from the Vestfold Hills coastal zone, East Antarctica. ( a ) Sun-illuminated bathymetric image of Airport Beach Embayment showing the debris mantle. Ice-keel ploughmarks are shown outside the embayment. Acquisition system Kongsberg EM3002. Frequency 300 kHz. Grid-cell size 2 m. ( b ) Close-up of debris mantle showing individual boulders. ( c ) Photograph of bouldery debris mantle above sea level. Large boulder is 4 m in diameter. ( d ) Multibeam image of sand ribbons and boulders. ( e ) Location of study area (red …

A glacial landform assemblage in Placentia Bay, Newfoundland, eastern Canada

Placentia Bay, located on the south coast of the island of Newfoundland, is bounded to the west by Burin Peninsula and to the east by Avalon Peninsula (Fig. 1a). At the Last Glacial Maximum (LGM) the Laurentide Ice Sheet extended south of the bay, reaching the edge of the continental shelf (Shaw et al. 2006). Placentia Bay was an onset region for fast-flowing ice in Halibut Channel, a cross-shelf trough beyond the bay. The bay became ice free at c. 14 ka, by which time retreat of Laurentide ice had isolated a Newfoundland ice cap. Placentia Bay has been mapped with multibeam sonar and surveyed with sub-bottom profilers. Results from these activities are contained in 1:50 000 scale shaded-relief maps, backscatter strength maps, a ‘seascape’ map (Shaw et al. 2011) and in various papers (e.g. Brushett et al. 2007; Shaw et al. 2009). The landform assemblage includes: (1) glacially overdeepened troughs; (2) streamlined sedimentary landforms; (3) rock drumlins; (4) flutes overprinting sedimentary drumlins; (5) De Geer moraines superimposed on streamlined landforms; (6) a complex landform suite comprising coast-parallel moraines, drumlins and megaflutes, eskers and hummocky moraine; (7) transverse moraines; (8) submarine slides; and (9) paraglacially modified terrain. Fig. 1. Multibeam sonar bathymetry from Placentia Bay, Newfoundland, eastern Canada (Fig. 2g). ( a ) Sun-illuminated multibeam sonar bathymetric image, with a cross-section across the bay. Acquisition systems Kongsberg EM1002 and EM300. Frequency 12 kHz. Grid-cell size 5 m. ( b ) Backscatter strength. White line indicates area of seafloor erosion (megaflutes). ( c ) Streamlined glacial-sedimentary landforms. ( d ) Rock drumlins. ( e ) Drumlins (top) overprinted by flutes (bottom), illustrating a readvance from the SW. The inner bay includes three narrow channels (Fig. 1a): Western Channel (maximum depth 450 m), Central Channel (265 m), and Eastern Channel (333 m). The outer bay has lower relief and encompasses …

Debris-flow deposits on the West Antarctic continental slope

Most of the West Antarctic continental margin has prograded during Neogene and Quaternary times, due largely to sediment delivery to the shelf break by ice sheets (Larter & Cunningham 1993; Nitsche et al. 2000; Cooper et al. 2008). Continental slope progradation is widely attributed to debris-flow deposition, but geophysical data that show the morphology of individual debris-flow deposits are rare. Morphologically, the continental slope can be divided into low seafloor gradient (<3°) trough-mouth fans (TMFs), developed at the mouths of some large palaeo-ice streams, and inter-fan areas with steeper slopes. We describe acoustic sub-bottom profiles and a sediment core from debris-flow deposits on Belgica Fan (Dowdeswell et al. 2008) and a sub-bottom profile of a debris-flow deposit on an inter-fan area on the Amundsen Sea continental slope (Fig. 1a). Fig. 1. Acoustic profiles from the West Antarctic continental slope and photographs and x-radiographs of a sediment core from the Belgica Fan (Dowdeswell et al. 2008). ( a ) Location of study areas (red box and red-filled rhombus; map from IBCSO v. 1.0). ( b ) Oblique view from NNE of a new regional bathymetry compilation over outer Belgica Trough (BT) and Belgica Fan (BF), incorporating all available multibeam and single-beam bathymetry, with shaded-relief illumination from ENE. Grid-cell size c. 900 m. White lines labelled x–x′ and y–y′ show locations of (c) and (d), respectively, and white oval shows location of (e). …

Tidal power generation in the glaciated, macrotidal Minas Scour Trough, Bay of Fundy, Atlantic Canada

The configuration of the Bay of Fundy results in large tides (up to 17 m) which give rise to strong currents (Fig. 1d). Harnessing these currents to generate electricity has been the focus of engineering schemes dating from 1910. Fundy tidal power generation has garnered attention periodically throughout the twentieth century (Gordon et al. 2014). The seabed and water column conditions in the Minas Scour Trough at the NE head of the Bay of Fundy (Fig. 1a, d) are now the subject of engineering studies to characterize the site for electricity generation. Test installations have been deployed on the seabed, the subsea cable to collect and transmit power to the electrical grid on land has been laid and future infrastructure instalments are planned. Fig. 1. Multibeam swath bathymetry and geophysical profile of the main Minas Scour Trough, Bay of Fundy. ( a ) Sun-illuminated multibeam-bathymetric image showing the tidal scour system developed in, from west to east, Minas Channel, Minas Passage and Minas Basin. Inset interpretation shows the extent of the scour trough. Acquisition system Kongsberg EM1000. Frequency 95 kHz. Grid-cell size 5 m. ( b ) Seismic-reflection profile in Minas Channel showing eroded glacimarine sediments and exposed bedrock cropping out in Minas Scour Trough. VE×67. Acquisition …

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 …

Iceberg–seabed interaction on northwestern Makkovik Bank, Labrador Shelf, Canada

The Labrador Shelf and upper continental slope are commonly referred to as ‘Iceberg Alley’, a reference to the 2500–3000 icebergs transported southeastwards past Makkovik Bank (Fig. 1a) every year in the Labrador Current (Fig. 1b) from outlet glaciers in Greenland and eastern Arctic Canada (Miller & Hotzel 1984). The larger icebergs are known to contact the seabed from ship-based and satellite observations of stationary grounded icebergs in the midst of currents, winds and moving sea-ice. The bathymetric details of seafloor relief arising from multiple generations of iceberg–seabed interaction events are evident in multibeam images of the NW crest and flank of Makkovik Bank from less than 90 m to more than 250 m water depth. Fig. 1. Iceberg ploughmarks on Makkovik Bank, Labrador Shelf. ( a ) Multibeam image showing iceberg ploughmarks. Acquisition system Kongsberg EM1000. Frequency 95 kHz. Grid-cell size 7 m. White line is 140 m isobath. ( b ) Location of study area (red box; map from GEBCO_08). MB, Makkovik Bank; LMT, Labrador Marginal Trough. Yellow arrows show path of Labrador Current. ( c ) Bathymetric profile x–x′, located in (a). VE×22. ( d ) Bathymetric profile y–y′, located in (a). The seabed has relief of up to 7 m. VE×11. ( e ) Detailed image …

The potential distribution of the woody weed Calotropis procera (Aiton) W.T. Aiton (Asclepiadaceae) in Australia

The potential spread of any invasive plant is a central concern in weed risk assessment. Calotropis procera is wind dispersed and forms extensive monospecific stands that reduce the productivity of pastoral land, but its potential distribution and drivers of its spread are not well known. Using maximum entropy methodology, we modelled current and future potential distributions of C. procera in Australia. Occurrence data (n = 5976 presence records) were collated from regional databases and a field survey. Of a set of ‘independent’ environmental correlates, those that best accounted for the observed distribution of C. procera in Australia were distance (km) to roads, average annual rainfall (mm), mean temperature (°C), average wind speed (km/h), beef density and vegetation type, in that order of importance. Current and potential distribution of C. procera was best explained by interactions between anthropogenic disturbance and climatic factors, all underpinned by species characteristics. Models were based on a grid cell size of 5 km × 5 km and model performance was good (mean AUC = 0.916; s.d. = 0.014; AUC = area under the curve; perfect fit = 1). The model showed that C. procera has not saturated its current potential distribution. Models of future spread derived from climate change projections, based on global circulation models in the ‘Representative Concentration Pathway 4.5 emissions scenario for 2035’, show the area suitable for C. procera will increase, increasing the risk the weed poses. Range expansion will occur into all three states surrounding the Northern Territory, but mostly into the north-eastern border regions of Western Australia and north-western Queensland. Joint management of rubber bush at a regional scale across jurisdictions, is urgently advised to avoid future spread of rubber bush and further reductions in pastoral productivity.

HIE-FDTD Method With PML for 2-D Periodic Structures at Oblique Incidence

For analyzing the oblique incident plane wave on periodic structures with the fine structure only in one direction, a hybrid implicit-explicit finite-difference time-domain (HIE-FDTD) method with the perfectly matched layer (PML) is derived. First, the field transformation technique is adopted to eliminate the time delay, and the PML formulations are derived by using the auxiliary differential equation (ADE) method. Then, by combining the transformed Maxwell’s equations with the PML formulations and adopting the locally one-dimensional (LOD) scheme, the proposed method with two substep implementations is obtained, which only needs to solve one implicit equation. The Courant–Friedrichs–Lewy (CFL) condition of the proposed method is more relaxed and no longer limited by the fine grid cell size.

A large, glacially modified, shelf-edge canyon, Scotian Shelf, Atlantic Canada

Located on the Scotian Shelf, The Gully is the largest submarine canyon on the outer southeastern Canadian continental margin (Fig. 1a, b). It indents the continental shelf much deeper than other canyons on the Scotian margin, connecting the middle shelf to the continental slope. It is generally recognized that The Gully formed by fluvial, glacial and meltwater erosion that cut deeply into Cenozoic mudstone and sandstone units (Fader et al. 1998). It was excavated over the past million years as successive continental glaciations lowered sea level, exposing the shelf and allowing rivers to erode and deposit sediments near the top of the continental slope. The Gully was also partially eroded by turbidity currents that flowed down the canyon at times of glacial maxima. Fig. 1. Multibeam swath bathymetry and bathymetric profiles of The Gully, Scotian Shelf, Atlantic Canada. ( a ) Sun-illuminated multibeam-bathymetric image of The Gully and the adjacent Scotian shelf and slope showing feeder canyons, bathymetric profile locations and bathymetric isobaths that define the limits of The Gully. Acquisition system Kongsberg EM1000. Frequency 95 kHz. Grid-cell size 10–20 m. Isobaths are in metres. ( b ) Location of study area (red box; map from GEBCO_08). ( c ) Change in slope image featuring curvilinear moraines on the eastern flank of The Gully. ( d ) Bathymetric profile of …

The bundle: a mega-scale glacial landform left by an ice stream, Western Bransfield Basin

Ice masses were grounded at or near the shelf edge around the Antarctic Peninsula (AP) at the Last Glacial Maximum (LGM). Post-LGM warming led to ice ungrounding and retreat, which proceeded asynchronously from north to south (O Cofaigh et al. 2014). Bransfield Basin (BB) is one of the first places in Antarctica where multibeam bathymetry was systematically collected in the 1990s, resulting in the early imaging of glacial landforms on the modern seafloor (e.g. Canals et al. 2000). BB is a 450 km long marginal rift basin extending between the northern tip of the AP and the South Shetland Islands (SSI) (Fig. 1b). The Western Bransfield Basin (WBB) extends from the mouth of Croker Passage (CP) to Boyd Strait (Fig. 1a). Its deepest part is bounded by the volcanic high of Deception Island to the NE and several continental shelves. WBB bifurcates into SW–NE and SE–NW orthogonal branches (Fig. 1a). Fig. 1. ( a ) Swath-bathymetric image of the Western Bransfield Basin. HFZ: Hero Fracture Zone. Acquisition system Simrad EM12-S. Frequency 12.5 kHz. Grid-cell size 200 m (colour) and GEBCO_08 Grid (grey). Land area from Landsat Image Mosaic of Antarctica (US Geological Survey). ( b ) Location of study area (red box; map from IBCSO v. 1.0). …

Canyons and slope instability on the Lofoten–Vesterålen continental margin, North Norway

Canyons are relatively unusual features of the 2500 km long Norwegian–Svalbard continental slope, where ice sheets have delivered large volumes of sediment to the upper slope a number of times during the Quaternary (Ottesen et al. 2005). The 250 km long, steep and topographically complex slope off Lofoten and Vesteralen in North Norway (Taylor et al. 2000; Laberg et al. 2007) has recently been mapped in detail using high-resolution multibeam methods (Fig. 1a–g) (www.mareano.no). The slope is dissected by 15 well-defined canyon systems and numerous slide scars (Rise et al. 2013), with the largest and deepest of these canyons incised by nearly 1100 m (Laberg et al. 2007). Fig. 1. Canyons and mass movements on the Lofoten–Vesteralen margin, North Norway. Acquisition system Kongsberg EM 120. Frequency 12 kHz. Grid-cell size 25 m. ( a ) Shaded-relief image. Blue dashed lines mark axes of cross-shelf troughs. Fifteen canyons are numbered. A, Andoya. ( b ) Elongate slide scars on the upper slope of canyon 3. L, debris lobe. ( c ) Upper part of canyons 8 and 9. Note amphitheatre-shaped head- and side-walls indicating retrogressive slide activity. White dashed lines mark ridges (R) between adjacent slide branches. ( d ) Slope gradient image combined with …

The Laurentian Channel: a major cross-shelf trough in Atlantic Canada

The Laurentian Channel is a wide, deep, U-shaped, glacially-excavated, cross-shelf trough in Atlantic Canada. It extends NW–SE approximately 700 km from the mouth of the St Lawrence River, across the Gulf of St Lawrence and terminates at the edge of the continental shelf south of Newfoundland (Fig. 1a, e). The Laurentian Channel was a major conduit of ice-stream flow on the SE margin during several advances of the Laurentide Ice Sheet. Fig. 1. Bathymetry and geophysical profiles of the Laurentian Channel, Atlantic Canada. ( a ) Multibeam image showing the extent of the Laurentian Channel. Image developed from digital elevation model of Shaw & Courtney (2004). Grid-cell size 789 m. BB, Burgeo Bank; HC, Hermitage Channel. ( b ) Seismic-reflection profile along the axis of the SE portion of the Laurentian Channel. Tie with oblique seismic profile in (c) indicated. U, an unconformity; LM, Laurentian Moraine; OLM, Outer Laurentian Moraine. Inset shows detail of the Laurentian Moraine. VE×123. Acquisition system 0.6 litre sleeve gun, single channel streamer. Frequency 150–650 Hz. ( c ) Oblique seismic-reflection profile from St Pierre Bank in the NW to Banquereau in the SE. Tie with longitudinal seismic profile in (b) indicated. Inset shows detail of till over unconformity. VE×26. Acquisition system air gun, single channel streamer. Frequency 150–650 Hz. ( d ) Multibeam …

Large sediment drifts on the upper continental rise west of the Antarctic Peninsula

Large sediment mounds on the continental rise around Antarctica have been interpreted as sediment drifts formed through interaction of downslope transport of mainly glacially derived sediments with alongslope bottom currents (Rebesco et al. 1996). Large sinuous channels between drifts represent turbidity current pathways that are probably most active during glacial periods. Drift sediments are mainly silt and clay, interpreted as fine-grained components of turbidity currents, entrained and redistributed by ambient bottom currents (Barker et al. 1999). ‘Drift 5’, west of the Antarctic Peninsula, is described here as an example of a large sediment drift (Fig. 1a). Fig. 1. Multibeam bathymetry, sea floor photographs and acoustic profiles of Drift 5, on the upper continental rise west of the Antarctic Peninsula. ( a ) Location of study area (red box; map from IBCSO v. 1.0). ( b ) Multibeam image compilation overlaid on interpolated IBCSO v 1.0 regional bathymetry, with shaded-relief illumination from NE. Grid-cell size 80 m. ( c ) Multibeam image compilation over the proximal part of the drift with shaded relief from NW. Acquisition systems various, but mostly Kongsberg EM120, 12 kHz. Grid-cell size 80 m. ( d ) Acoustic sub-bottom profile across the drift summit plateau. Acquisition system TOPAS PS018 parametric profiler. Secondary beam frequency 1.5–5 kHz. VE×25. ( e ) Photograph taken from ROV Isis at 2937 m water depth of cliff face on the proximal lower part of the NE flank of the drift exhibiting stratified sediments containing coarse ice-rafted debris. The two red dots …