B. J. Todd

Mapping submarine glacial landforms using acoustic methods

The mapping of submarine glacial landforms is largely dependent on marine geophysical survey methods capable of imaging the seafloor and sub-bottom through the water column. Full global coverage of seafloor mapping, equivalent to that which exists for the Earths land surface, has, to date, only been achieved by deriving bathymetry from radar altimeters on satellites such as GeoSat and ERS-1 (Smith & Sandwell 1997). The horizontal resolution is limited by the footprint of the satellite sensors and the need to average out local wave and wind effects, resulting in a cell size of about 15 km (Sandwell et al. 2001). A further problem in high latitudes is that the altimeter data are extensively contaminated by the presence of sea ice, which degrades the derived bathymetry (McAdoo & Laxon 1997). Consequently, the satellite altimeter method alone is not suitable for mapping submarine glacial landforms, given that their morphological characterization usually requires a much finer level of detail. Acoustic mapping methods based on marine echo-sounding principles are currently the most widely used techniques for mapping submarine glacial landforms because they are capable of mapping at a much higher resolution. Although the accuracy and resolution of echo-sounding methods are continually being improved, the portion of the worlds ocean floor that has been acoustically surveyed is increasing only slowly. This lack of coverage is particularly true for those areas of the oceans covered by sea ice and infested with icebergs, where glacial landforms are an abundant component of continental shelf and fjord morphology. This is illustrated by the fact that only about 11% of the Arctic Ocean had been mapped using modern multibeam sonar technology by 2012 when the latest International Bathymetric Chart of the Arctic Ocean (IBCAO) was compiled (Jakobsson et al. 2012). A similar estimate of the mapped portion of the seafloor …

Atlas of Submarine Glacial Landforms: Modern, Quaternary and Ancient

New geophysical techniques (multibeam echo sounding and 3D seismics) have revolutionized high-resolution imaging of the modern seafloor and palaeo-shelf surfaces in Arctic and Antarctic waters, generating vast quantities of data and novel insights into sedimentary architecture and past environmental conditions. The Atlas of Submarine Glacial Landforms is a comprehensive and timely summary of the current state of knowledge of these high-latitude glacier-influenced systems. The Atlas presents over 180 contributions describing, illustrating and discussing the full variability of landforms found on the high-latitude glacier-influenced seafloor, from fjords and continental shelves to the continental slope, rise and deep-sea basins beyond. The distribution and geometry of these submarine landforms provide key information on past ice-sheet extent and the direction and nature of ice flow and dynamics. The papers discuss individual seafloor landforms, landform assemblages and entire landsystems from relatively mild to extreme glacimarine climatic settings and on timescales from the modern margins of tidewater glaciers, through Quaternary examples to ancient glaciations in the Late Ordovician.

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 …

Drumlins in Lake Ontario

Lake Ontario (74 m above sea level) is located 150–250 km north of the Late Wisconsinan southern limit of the Laurentide Ice Sheet in North America (Figure 1). Multibeam bathymetric mapping in deep eastern Lake Ontario (Figures 2, 3) revealed a set of parallel, straight, narrow ridges trending 235±5° which commonly rise 10–20 m above the surrounding lakefloor. The mapped ridges range in length to 6 km and from 60 to 600 m in width.

Seabed Habitat of a Glaciated Shelf, German Bank, Atlantic Canada

Publisher Summary German Bank is located off southern Nova Scotia on the Scotian Shelf in the eastern Gulf of Maine and is the offshore extension of the southern Nova Scotia landmass. Much of German Bank is exposed bedrock comprising Cambro–Ordovician metasedimentary rocks intruded by Late Devonian–Carboniferous granitoid plutons. Bedrock has been modified by glacial erosion and is separated by a rugged erosional surface from the discontinuous overlying Quaternary sediments. German Bank can be characterized as bathymetrically smooth glaciated continental shelf, with a regional gradient of less than 1° to the southwest. At distances of tens to hundreds of meters (large scale), geomorphic features are evident in outcropping bedrock and within the overlying glacial and postglacial sediments. High-resolution seafloor imagery was obtained on German Bank using Campod, an instrumented tripod that includes forward- and downward-looking video cameras and a downward-looking 35 mm still camera. The Campod provides a very large-scale (tens of centimeters) view of the seafloor. All visible species of megabenthos were identified to the highest possible taxonomic resolution. Statistical analysis of the relationship between biological factors of megafaunal community data as assessed from large-scale seabed photographs and environmental variables (BioEnv) revealed that the single variable that best explains the distribution of bottom fauna is summer oxygen saturation at the seabed. The best combination of variables related to benthic community composition is water depth, oxygen saturation, seabed cover by cobbles and boulders, and seabed cover by sand.

Introduction: an Atlas of Submarine Glacial Landforms

Glacial landforms and sediments exposed sub-aerially have been the subject of description, analysis and interpretation for more than a century (e.g. De Laski 1864; De Geer 1889). Indeed, such features provided important initial observations informing Louis Agassizs ideas that ice was a key instrument in sculpting the landscape and that glaciers and ice sheets had extended to mid-latitudes during the past, implying that Earths climate must have changed considerably through time (Agassiz 1840). It is only in the last few decades that attention has begun to focus on the marine evidence for the past growth and decay of ice sheets that is recorded in submarine landforms and sediments preserved on high-latitude continental margins. This interest has been driven, in part, by the recognition that sediments deposited below wave-base are often well preserved in the Quaternary geological record, and may be less subject to erosion and reworking than their terrestrial counterparts. In addition, new marine-geophysical technologies have enabled increasingly high-resolution imaging and penetration of the high-latitude seafloor, most notably using multibeam swath-bathymetric and three-dimensional (3D) seismic-reflection methods, and modern ice-strengthened and ice-breaking research vessels have allowed the effective deployment of these increasingly sophisticated instruments in the often ice-infested waters of the Arctic and Antarctic seas. The geological record from high-latitude continental margins is now recognized to provide key information on former ice-sheet extent, the direction and nature of past ice flow and dynamics, and a well-preserved window on the detailed form and composition of former ice-sheet beds (e.g. Ottesen et al. 2005; Anderson et al. 2014; Jakobsson et al. 2014). The geometry and distribution of submarine glacial landforms on the seafloor, and the underlying glacial-sedimentary stratigraphic record with which they are associated, is the topic of this volume. The aims and purpose of the Atlas are: (1) to …

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 …

De Geer moraines on German Bank, southern Scotian Shelf of Atlantic Canada

Numerous regularly spaced, parallel, linear to curvilinear ridges of sediment are recognized to form at or close to the grounding lines of water-terminating glaciers (Linden & Moller 2005). These ice-flow transverse ridges, sometimes known as De Geer moraines (De Geer 1889), have modest heights and widths and variable lengths. De Geers original description of Swedish moraines with these characteristics invoked an annual cycle. Present use of ‘De Geer moraine’ refers to closely spaced subaqueous moraine ridges which may or may not be annual. The distribution and pattern of ubiquitous De Geer moraines on German Bank provide insight into the direction and timing of regional deglaciation of the southern Scotian Shelf (Todd et al. 2007). Across a bathymetric range of over 100 m, southern German Bank is mantled by swarms of ridges; each ridge displays a quasi-linear or curved line in planform (Fig. 1). In topographic depressions, the ridges are subdued in relief or are absent (Fig. 1a). The strike of the ridge crests is generally WSW–ENE. Individual ridges can be traced horizontally from short segments of a few hundred metres up to almost 10 km. …

Electromagnetic studies across lakes and rivers in permafrost terrain, Mackenzie River Delta, Northwest Territories, Canada

Shallow and deep electromagnetic studies were conducted in early spring across the frozen surface of a 3.3 km-wide shallow lake on Richards Island in the Mackenzie River delta, N.W.T. A preliminary interpretation of the results suggests that the ground is unfrozen at shallow depths beneath the lake, but that there is a thin layer of frozen ground beneath the unfrozen material. At greater depths, the data are interpreted to indicate an unfrozen layer above the main body of permafrost which is several hundred me&es thick. Such studies are important in understanding the changes in permafrost conditions that will be encountered at lake or river crossings hould an oil or gas pipeline be constructed in this environment.

Canyons and slides on the continental slope seaward of a shallow bank, Labrador margin, eastern Canada

This is the author accepted manuscript. The final version is available from Geological Society of London via https://doi.org/10.1144/M46.133