Julian A. Dowdeswell

The International Bathymetric Chart of the Arctic Ocean (IBCAO) Version 3.0

The International Bathymetric Chart of the Arctic Ocean (IBCAO) released its first gridded bathymetric compilation in 1999. The IBCAO bathymetric portrayals have since supported a wide range of Arc ...

Submarine landforms and the reconstruction of fast-flowing ice streams within a large Quaternary ice sheet: The 2500-km-long Norwegian-Svalbard margin (57°–80°N)

Morphological interpretation of regional and detailed bathymetric data sets on the 2500-km-long Norwegian shelf from the North Sea (57°N) to Svalbard (80°N) has revealed a dynamic ice-flow pattern along the western marginof the Scandinavian and Barents/Svalbard ice sheets. About 20 cross-shelf troughs with megascale glacial lineations (MSGL; elongate ridges and grooves oriented parallel to trough long axes) are interpreted as former pathways for fast-flowing ice streams. Studies of large-scale margin morphology and seismic profiles have identified large submarine fans at the mouths of several major cross-shelf troughs. Less dynamic ice probably existed on shallower banks. The two largest paleo-ice streams were the Norwegian Channel Ice Stream and Bear Island Trough Ice Stream, each 150-200 km wide at the mouth. The lengths of individual MSGL vary from hundreds of meters to several tens of kilometers, and the distance between ridges varies from 0.1 to 3 km. MSGL amplitudes reach 15 m, but are commonly <10 m. The onset of MSGL and, hence, fast ice flow is generally close to the outer coast, at the border zone between crystalline rocks and softer sedimentary rocks. Transverse submarine ridges on various scales, commonly parallel to the shelf edge, reflect either the maximum ice-sheet position or the recessional pattern of the ice sheet. Lateral ice-stream moraines several tens of kilometers long have also been mapped along the sides of several cross-shelf troughs, identifying the border zone between fast ice flow and stagnant or slow-flowing ice on intervening banks.

THE NORWEGIAN–GREENLAND SEA CONTINENTAL MARGINS: MORPHOLOGY AND LATE QUATERNARY SEDIMENTARY PROCESSES AND ENVIRONMENT

The continental margins surrounding the Norwegian–Greenland Sea are to a large degree shaped by processes during the late Quaternary. The paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features: slides, trough mouth fans and channels. Several large late Quaternary slides have been identified on the eastern Norwegian–Greenland Sea continental margin. The origin of the slides may be due to high sedimentation rates leading to a build-up of excess pore water pressure, perhaps with additional pressure caused by gas bubbles. Triggering might have been prompted by earthquakes or by decomposition of gas hydrates. Trough mouth fans (TMF) are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves. In the Norwegian–Greenland Sea, seven TMFs have been identified varying in area from 2700 km2 to 215 000 km2. The Trough Mouth Fans are depocentres of sediments which have accumulated in front of ice streams draining the large Northwest European ice sheets. The sediments deposited at the shelf break/upper slope by the ice stream were remobilized and transported downslope, mostly as debris flows. The Trough Mouth Fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge. Two large deep-sea channel systems have been observed along the Norwegian continental margin, the Lofoten Basin Channel and the Inbis Channel. Along the East Greenland margin, several channel systems have been identified. The deep-sea channels may have been formed by dense water originating from cooling, sea-ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents.

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.

Iceberg production, debris rafting, and the extent and thickness of Heinrich layers (H-1, H-2) in North Atlantic sediments

The pattern of Heinrich-layer distribution for the last two events (H-1, ∼14.5 and H-2, ∼21.1 ka), mapped from magnetic susceptibility analysis of more than 50 North Atlantic Ocean cores, provides the most detailed information to date on their extent and thickness. An integrated spatial average thickness for the layers is 10–15 cm, and there is a strong distance decay eastward. The pattern of deposition over the North Atlantic is similar for events H-1 and H-2, indicating that icebergs followed similar drift tracks. Rates of iceberg production and sediment flux from the Hudson Strait drainage basin of the North American Laurentide ice sheet, the major iceberg source for the events, were calculated by using a mass-balance approach. This provides an envelope of sedimentation rates and the prediction that it would take between 50 and ∼1250 yr of iceberg sediment delivery to accumulate a Heinrich layer averaging 10 cm thick over the North Atlantic, depending on the model assumptions used. The most likely duration of Heinrich events is 250–1250 yr.

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.

Assemblages of submarine landforms produced by tidewater glaciers in Svalbard

[1] High-resolution swath bathymetry from the marine margins of several Svalbard tidewater glaciers shows an assemblage of submarine landforms that is probably linked to glacier surging. These landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity. Swath images comprise an assemblage of superimposed landforms, allowing reconstruction of relative age of deposition: (1) large transverse ridges, interpreted as recessional moraines overridden by a subsequent ice advance; (2) a series of curvilinear streamlined bedforms orientated parallel to former ice flow, interpreted as lineations formed subglacially during rapid advance; (3) large terminal ridges, marking the farthest extent of ice at the last advance, with flow lobes immediately beyond interpreted as submarine debris flows; (4) a series of interconnected rhombohedral ridges, interpreted as a product of soft sediment squeezing into crevasses formed at the glacier bed, probably formed during immediate post-surge stagnation; and (5) a series of fairly evenly spaced small transverse ridges, interpreted as push moraines produced annually at tidewater glacier termini during retreat. A simple descriptive landsystem model for tidewater glaciers of probable surge type is derived from these observations. We also show that megascale glacial lineations can form not only beneath large ice streams, but are also produced beneath surging tidewater glaciers lying on deforming sedimentary beds.

GLACIMARINE SEDIMENTARY PROCESSES AND FACIES ON THE POLAR NORTH ATLANTIC MARGINS

Abstract Major contrasts in the glaciological, oceanic and atmospheric parameters affecting the Polar North Atlantic, both over space between its eastern and western margins, and through time from full glacial to interglacial conditions, have lead to the deposition of a wide variety of sedimentary facies in these ice-influenced seas. The dynamics of the glaciers and ice sheets on the hinterlands surrounding the Polar North Atlantic have exterted a major influence on the processes, rates and patterns of sedimentation on the continental margins of the Norwegian and Greenland seas over the Late Cenozoic. The western margin is influenced by the cold East Greenland Current and the Svalbard margin by the northernmost extent of the warm North Atlantic Drift and the passage of relatively warm cyclonic air masses. In the fjords of Spitsbergen and the northwestern Barents Sea, glacial meltwater is dominant in delivering sediments. In the fjords of East Greenland the large numbers of icebergs produced from fast-flowing outlets of the Greenland Ice Sheet play a more significant role in sedimentation. During full glacials, sediments are delivered to the shelf break from fast-flowing ice streams, which drain huge basins within the parent ice sheet. Large prograding fans located on the continental slope offshore of these ice streams are made up of stacked debris flows. Large-scale mass failures, turbidity currents, and gas-escape structures also rework debris in continental slope and shelf settings. Even during interglacials, both the margins and the deep ocean basins beyond them retain a glacimarine overprint derived from debris in far-travelled icebergs and sea ice. Under full glacial conditions, the glacier influence is correspondingly stronger, and this is reflected in the glacial and glacimarine facies deposited at these times.

Calibrating the Late Ordovician glaciation and mass extinction by the eccentricity cycles of Earth's orbit

A process-based sedimentological analysis of Upper Ordovician glacial-marine rocks in Africa suggests that full glaciation of the continental shelf started in the late extraordinarius Zone of the Hirnantian. Two cycles of ice-sheet growth are represented during full glaciation. Initial terrestrial ice-sheet growth in the early extraordinarius Zone influenced the first event of the Late Ordovician mass extinction. Retreat of the ice sheet from the shelf ended by the persculptus Zone, when the second event of the Late Ordovician mass extinction began. Eccentricity controlled ice-sheet growth is assumed (periodicity 0.1 m.y.). Hence, two cycles of full glaciation lasted 0.2 m.y. The duration of the extraordinarius Zone is estimated as 0.5 m.y. Therefore, the minimum duration of the first extinction event was 0.3 m.y.