Atle Nygård

Configuration, history and impact of the Norwegian Channel Ice Stream

The Norwegian Channel between Skagerrak, in the southeast, and the continental margin of the northern North Sea, in the northwest, is the result of processes related to repeated ice stream activity through the last 1.1 m yr. In such periods the Skagerrak Trough (700 m deep) has acted as a confluence area for glacial ice from southeastern Norway, southern Sweden and parts of the Baltic. Possibly related to the threshold in the Norwegian Channel off Jaeren (250 m deep), the ice stream, on a number of occasions over the last 400 ka, inundated the coastal lowlands and left an imprint of NW-oriented ice directional features (drumlins, stone orientations in tills and striations). Marine interstadial sediments found up to 200 m a.s.l. on Jaeren have been suggested to reflect glacial isostasy related to the Norwegian Channel Ice Stream (NCIS). In the channel itself, the ice stream activity is evidenced by mega-scale glacial lineations on till surfaces. As a result of subsidence, the most complete sedimentary records of early phases of the NCIS are preserved close to the continental margin in the North Sea Fan region. The strongest evidence for ice stream erosion during the last glacial phase is found in the Skagerrak. On the continental slope the ice stream activity is evidenced by the large North Sea Fan, which is mainly a result of deposition of glacial-fed debris flows. Northwards of the North Sea Fan, rapid deposition of meltwater plume deposits, possibly related to the NCIS, is detected as far north as the Voring Plateau. The NCIS system offers a unique possibility to study ice stream related processes and the impact the ice stream development had on open ocean sedimentation and circulation.

Extreme sediment and ice discharge from marine-based ice streams: New evidence from the North Sea

A major problem for understanding the dynamics of ice streams has been a lack of precise data on ice streaming longevity and sediment transport efficacy. Here we present the first well-constrained data on sediment flux from a paleoice stream. This has been achieved by computing the volume of sediment deposited as debris flows on the fan located at the outlet of the Norwegian Channel ice stream, and converting to a flux measurement by accounting for the duration of streaming in this episode (between 20 and 19 ka during the last glacial stage). In this period the ice stream delivered an average 1.1 Gt of sediment per year, equivalent to 8000 m 3 yr −1 per meter width of ice stream front. The calculated flux is an order of magnitude higher than most previous estimates for other paleoice streams and is comparable to the present sediment flux from the world9s largest rivers. The short period of debris-flow deposition suggests that the Norwegian Channel ice stream underwent rapid on-off switching, with punctuated iceberg delivery to the North Atlantic as a consequence.

Geometry and genesis of glacigenic debris flows on the North Sea Fan: TOBI imagery and deep-tow boomer evidence

Abstract Acoustic data have been used to study the upper sequence of glacigenic debris flows (GDFs) deposited during the last glacial maximum on the North Sea Fan. The presumed GDF release area at the shelf break (450 m water depth) exhibits a sequence up to 90 m thick, thinning to less than 45 m basinward at approx. 750–800 m depth, on a low slope (0.6–0.8°). A seismically featureless sequence at the shelf break changes distinctly the first kilometers basinward, showing a significant occurrence of features interpreted as infilled transport chutes. The straight or sinuous chutes feed into well-defined relief-building GDF lobes 10–40 km from the shelf break. A lack of failure scarps at the shelf break suggests that the GDFs have been sourced by a relatively continuous release of material. Flow lines concentrated along the chutes and center of the GDFs suggest a laminar movement pattern in the GDF core, supported by passive flanks. Based on the flow morphology high pore pressure and low sediment strength within a central core of the flow is assumed to have contributed to the mobility of the GDFs. Dissipation of pore pressure along the margins of the flow may have caused the margins freeze while the sediment in the core remained mobile.

A Weak Layer Feature on the Northern Storegga Slide Escarpment

The northern Storegga Slide esearpment and the area affected by the slide has been an area of extensive geological and geotechnical studies during recent years, mainly focusing on sediment transport processes and slope failure mechanisms. The well-preserved stratigraphy established north of the Storegga Slide, also partly exposed along the northern escarpment, is considered to be the closest parallel to the geological setting of the Storegga Slide prior to the slope failure (Fig. 1, 2). The stratigraphic setting is characterised by units consisting dominantly of diamicton and/or ice-proximal upper margin deposits, and hemipelagic and/or glaciomarine seismically well-layered deposits on mid and lower margin settings (Fig. 2). The boundary of these units is regional and usually characterised by a strong seismic reflector. Occasionally, the internal layering of these units also displays a pronounced seismic character, but they have a less distinctive regional appearance. Open image in new window Fig.1. Outline of the Storegga Slide area on the mid-Norwegian margin, with the TOBI sidescan survey area at the northern Storegga Slide escarpment. The location of the airgun seismic profiles presented in Fig. 2 is shown

Morphology of a Non-glacigenic Debris Flow Lobe in the Helland Hansen Area Investigated with 3D Seismic Data

Thick prograding debris flow packages of Late Pliocene to Pleistocene age, mainly of glacigenic origin, are commonly found on the mid-Norwegian continental margin. Such packages are often separated by hemipelagic sediments, and they are the major components of the large submarine fans found along the North Atlantic Margins (Vorren et al. 1998; King et al. 1996). On the continental slope in the Helland Hansen area, a particularly well developed debris flow lobe is investigated using 3D seismic exploration data (Fig. 1). The debris flow lobe examined here lies stratigraphically above acoustically layered hemipelagic sediments and below an acoustically transparent package of glacigenic material, presumably debris flow material (Fig. 2). Characteristic features distinguish this debris flow from debris flows of glacigenic origin (e.g, Nygard et al., this volume), suggesting that this is a ‘conventional’ debris flow. Open image in new window Fig. 1. Map of the working area

Morphology of Glacigenic Debris Flows on the Upper North Sea Fan

During the mid and late Pleistocene, ice streams draining through the Norwegian Channel (Fig. 1) have, through subglacial deformation of till, transported thousands of cubic kilometers of sediments to the shelf edge, building out a fan-shaped body dipping less than 0.7° basinward (King et al. 1996; Sejrup et al. 1996, 2000). This area strongly contrasts the irregular/hummocky slope to the northeast (Storegga Slide complex) and the more smooth topography to the southwest (Shetland slope). The stratigraphic succession in the fan is characterized by an alternation between intense deposition of glacigenic debris flows (GDFs) when an ice stream developed in the Norwegian Channel, and periods without shelf edge glaciation when hemipelagic sedimentation prevailed. After losing contact with the ice stream at the grounding line, the material moved hundreds of km down the continental slope as gravitational GDFs, forming continuous elongate bodies. Open image in new window Fig. 1. Map over the working area