Colm Ó Cofaigh

Evolution of subglacial bedforms along a paleo-ice stream, Antarctic Peninsula continental shelf

Geophysical data from the Antarctic Peninsula continental shelf reveal streamlined subglacial bedforms in a cross-shelf trough. Bedforms exhibit progressive elongation with distance along the trough, and record flow of a paleo-ice stream from the Antarctic Peninsula Ice Sheet during the last glacial maximum. Downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf. This, in turn, is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf. Although streaming flow operated across both substrates, the highest flow velocities occurred over the soft bed. Spatial variation in the inferred nature of fast-flow, from sliding to subglacial sediment deformation and/or ploughing, was also lithologically controlled. These data highlight the control of subglacial geology on ice-stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo-ice streams.

Thickness and extent of the subglacial till layer beneath an Antarctic paleo–ice stream

Fast-flowing ice streams and outlet glaciers currently account for as much as 90% of the discharge from the Antarctic and Greenland Ice Sheets. Although the deformation of subglacial material has been proposed as the mechanism for this rapid motion, such sediment is usually hidden under several kilometers of ice. Marine-geophysical records have allowed reconstruction of the three-dimensional thickness of the sedimentary bed beneath a large Antarctic paleo-ice stream for the first time. Fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength, unsorted sediment, averaging 4.6 m thick. Rapid motion of the paleo-ice stream was a result of subglacial deformation within this layer.

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.

Laminated sediments in glacimarine environments: diagnostic criteria for their interpretation

Abstract Fine-grained laminated deposits are often reported from contemporary and ancient glacimarine sedimentary sequences. Laminated glacimarine sediments form by a variety of processes, including suspension settling from turbid overflow plumes, turbidity current deposition and contour current activity. Subglacial deformation of pre-existing sediments may also produce laminated deposits in the form of glacitectonite and deformation till. The development of diagnostic criteria for genetic discrimination of laminated glacimarine sediments is important, both for their correct interpretation in sedimentary sequences, and also for the accurate reconstruction of former glacier extent and geometry, as laminated sediments may form in subglacial, ice-proximal and ice-distal environments. This paper reviews the processes by which laminated glacimarine sediments form and their resulting sedimentology. The aim is to provide a synthesis of diagnostic sedimentological criteria for laminated glacimarine deposits that will assist in the interpretation of such sediments in the geological record. Laminated sediments are genetically differentiated on the basis of sedimentary structures, bedding contacts, stratigraphic relationships and macrofaunal content/bioturbation. These attributes also facilitate discrimination between glacimarine and glacilacustrine rhythmites. The paper concludes by discussing the glaciological significance of laminated glacimarine sediments in terms of glacier hydrology, reconstructions of former ice-dynamics, and basal thermal regime.

Palaeo‐ice streams, trough mouth fans and high‐latitude continental slope sedimentation

The classical model of trough mouth fan (TMF) formation was developed in the Polar North Atlantic to explain large submarine fans situated in front of bathymetric troughs that extend across continental shelves to the shelf break. This model emphasizes the delivery of large volumes of subglacial sediment to the termini of ice streams flowing along troughs, and subsequent re-deposition of this glacigenic sediment down the continental slope via debris-flow processes. However, there is considerable variation in terms of the morphology and large-scale sediment architecture of continental slopes in front of palaeo-ice streams. This variability reflects differences in slope gradient, the relative contributions of meltwater sedimentation compared with debris-flow deposition, and sediment supply/geology of the adjacent continental shelf. TMF development is favoured under conditions of a low (<1D) slope gradient; a passive-margin tectonic setting; abundant, readily erodible sediments on the continental shelf - and thus associated high rates of sediment delivery to the shelf edge; and a wide continental shelf. The absence of large sediment fans on continental slopes in front of cross-shelf troughs should not, however, be taken to indicate the former absence of palaeo-ice streams in the geological record.

Tunnel valley genesis

Tunnel valleys are elongate depressions with overdeepened areas along their floors cut into bedrock or unconsolidated sediment. They are frequently sinuous and form anastomosing networks, although they also exist as independent, straight valleys. Their sedimentary infill is variable but is characteristically dominated by sediment gravity flow facies and thick units of glaciofluvial sands. Till tends to be rare and, where present, occurs along valley sides. Three main theories of tunnel valley formation exist at present. The first ascribes the formation of tunnel valleys cut into unconsolidated sediment to the creep of deformable subglacial sediment into a subglacial conduit from the sides and below, followed by removal of this material through the conduit by meltwater flow. Tunnel valleys are thus created by lowering of the sediment surface on either side of the conduit. The second theory argues that tunnel valleys form during deglaciation, at or close to the ice margin, by subglacial meltwater erosion and that the valleys are time transgressive. The third theory also argues for an origin by subglacial meltwater erosion but claims that the discharges involved took the form of catastrophic channelized floods and that the tunnel valleys within anastomosing networks formed synchronously. Following an outline of tunnel valley geomorphol ogy and sedimentology, each theory is critically reviewed and conclusions are drawn. Emphasis is placed upon the importance of sedimentological and geomorphological field observations as a basis for formulating models of tunnel valley genesis.

Chapter 50 - The Greenland Ice Sheet During the Past 300,000 Years: A Review

Abstract The Greenland ice sheet‘s response to climate change is a major issue in the climate debate. This report reviews existing evidence on how the ice sheet margins reacted to climate change during the past 300,000 years—how it responded to the warm climate of the last interglacial and expanded on to the shelf during the last ice age. Compared to the other large ice sheets in the northern hemisphere, the Greenland ice sheet showed remarkable resilience to temperature change—a good omen for the future.

An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle

Considerable uncertainty surrounds the extent and timing of the advance and retreat of the Greenland Ice Sheet (GIS) on the continental shelf bordering Baffin Bay during the last glacial cycle. Here we use marine geophysical and geological data to show that fast-flowing ice sheet outlets, including the ancestral Jakobshavn Isbrae, expanded several hundred kilometers to the shelf edge during the last glaciation ca. 20 ka. Retreat of these outlets was asynchronous. Initial retreat from the shelf edge was underway by 14,880 calibrated (cal) yr B.P. in Uummannaq trough. Radiocarbon dates from the adjacent Disko trough and adjoining trough-mouth fan imply later deglaciation of Jakobshavn Isbrae, and, significantly, an extensive readvance and rapid retreat of this outlet during the Younger Dryas stadial (YD). This is notable because it is the first evidence of a major advance of the GIS during the YD on the West Greenland shelf, although the short duration suggests that it may have been out of phase with YD temperatures.

Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream

Abstract Continental-slope and shelf-edge morphology off Marguerite Bay, western Antarctic Peninsula, is investigated using swath-bathymetric data and parametric sub-bottom profiler records, together with sediment cores. Marguerite Bay has a well-defined cross-shelf trough, and a relatively steep continental slope. The slope beyond the trough mouth is convex in longitudinal profile, whereas to the north and south it is concave and reaches a maximum of 12°. There are no deep canyons cutting into the prograding outer shelf and slope. Instead, a series of gullies runs down the upper slope, reaching depths of >200 m south of the trough mouth but

Large-scale reorganization and sedimentation of terrestrial ice streams during late Wisconsinan Laurentide Ice Sheet deglaciation.

Glacial geomorphological mapping of Shuttle Radar Topography Mission (SRTM) data from the western Canadian Prairies demonstrates that during the last (late Wisconsinan) deglaciation of the Laurentide Ice Sheet terrestrial ice streams underwent a major reorganization of their flow configuration. This reorganization involved a 90° shift in flow direction and was accompanied by a corresponding increase in the influence of topography on streaming flow. Ice streams included both topographically confined and “pure” ice streams that flowed independent of topography. Streaming flow is recorded by suites of highly elongate (>60 km long) subglacial bedforms, bounded sharply at their lateral margins by prominent moraines. Initial streaming flow was unconfined by topography but was replaced progressively, and crosscut, by younger topographically confined flows. Flow reorganization is inferred to have been caused by temporal and spatial variations in the interaction between frozen and thawed bed conditions, with thinning and shutdown of one ice stream ultimately triggering initiation of others. This highlights the role of internal glaciodynamically driven reorganization in triggering streaming flow within large ice sheets and shows that large-scale flow reorganization can occur over the time scale of a single deglaciation in terrestrial ice streams.