Hans Petter Sejrup

A giant three-stage submarine slide off Norway

One of the largest submarine slides known, The Storegga Slide, is located on the Norwegian continental margin. The slide is up to 450 m thick and has a total volume of about 5,600 km3. The headwall of the slide scar is 290 km long and the total run-out distance is about 800 km. The slide involved sediments of Quaternary to Early Tertiary age and occurred in three stages. Earthquakes combined with decomposition of gas hydrates are believed to be the main triggering agents for the slides. The first slide event is tentatively dated to be about 30,000 to 50,000 years B.P. and the two last major events are dated to be at 6,000 to 8,000 years B.P.

The distribution of living benthic foraminifera on the continental slope and rise off southwest Norway

Abstract Surface sediment samples taken by ☐ corer from 45 stations on the Norwegian continental margin and in the Norway Basin have been investigated for their benthic foraminiferal content. Unlike previous studies, the living benthic foraminiferal fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Factor analysis of both the living and dead faunal data reveals six living species assemblages and five corresponding dead assemblages. The additional living assemblage is characterized by the arenaceous speciesCribrostomoides subglobosum that dominates between 1400 and 2000 m water depth, but is rare in the dead faunal data. Trifarina angulosa and, to a lesser extent,Cibicides lobatulus characterize the shallowest foraminiferal assemblage from 200 to 600 m water depth, where it is associated with strong bottom currents and warm, saline Atlantic water of the North Atlantic Drift. On the slope between 600 and 1200 m water depth, theMelonis zaandami Species Assemblage dominates, particularly in areas characterized today by rapid sedimentation of terrigeneous material. Between 1000 and 1400 m depth, where the slope is covered by fine grained, organic-rich, terrigeneous mud, the living foraminiferal assemblage is characterized byCassidulina teretis andPullenia bulloides. Below 1400 m, three foraminiferal assemblages are found:C. subglobosum is found from 1400 to 2000 m,Cibicidoides wuellerstorfi andEpistominella exigua predominantly live from 2000 to 3000 m water depth, and below 3000 m,Oridorsalis umbonatus andTriloculina frigida dominate the fauna. All of theElphidium excavatum tests found in this study and theCassidulina reniforme tests found above 500 m water depth were found to be reworked. Analysis of the sediment grain-size distribution and the organic carbon content in surface samples from the deepest stations suggest that the abundance ofC. wuellerstorfi andE. exigua is positively correlated to relatively coarse (caused by planktic foraminifera) and organic-rich sediments, whereas high frequencies ofO. umbonatus andT. frigida coincide with low organic carbon content. We suggest thatC. wuellerstorfi is adapted to deep-sea environments with relatively high food supply, tolerating relatively low interstitial water oxygen content, whereasO. umbonatus may tolerate lower food supply prefering well-oxygenated interstitial waters.

Quaternary seismic stratigraphy of the North Sea Fan: glacially-fed gravity flow aprons, hemipelagic sediments, and large submarine slides

Abstract Approximately 1000 km of high resolution sleeve-gun array transects on the North Sea Fan, located at the mouth of the Norwegian Channel, reveal three dominant styles of sedimentation within a thick (> 900 m) Quaternary sediment wedge comprising numerous sequences. These are interpreted as: terrigenous hemipelagic sedimentation, large scale translational slides, and aprons of glaciogenic debris flow deposits contributing to considerable fan construction. Four large, buried translational slides involved sediment volumes upwards of 3000 km 3 each and preceded the similarly dimensioned “first” Storegga Slide on the NE fan flank. Several thick (> 100 m) terrigenous hemipelagic deposits apparently represent long-lived (150–200 kyr) periods of sedimentation whose distribution indicates fan input via the Norwegian Channel. The upper sequences are each made upper sequences are each made up of one or several thick (> 100 m) aprons comprising stacked lensoid and/or lobate forms which range from 2 to 40 km in width and 15 to 60 m in thickness. They characterize debris flows attributed to periodic input from several phases of a Norwegian Channel ice stream reaching the shelf edge. Subsidence in the outer Norwegian Channel allowed preservation of several glaciation cycles represented by sheet erosion-bounded tills and progradational units. Much of the shelf/slope transition has been preserved, allowing a preliminary chronology of the fan sequences through correlation with borehole sediments in the Norwegian Channel. Debris flows, which signal the initial shelf-edge glaciation, are not recognized from the initial glaciation in the Channel (> 1.1 Myr) but are associated with a Middle Pleistocene and all following glacial erosion surfaces (GES) in the outer Norwegian Channel. This was followed by six further sequences, probably totalling over 13,000 km 3 of sediment. At least four of these were shelf-edge ice-maximum events the last of which was Late Weichselian age ( 14 C AMS). Considering earlier glaciation-related hemipelagic sedimentation, material since removed by the large slides, and extensive unmapped areas, total Quaternary fan sedimentation was in the vicinity of 20,000 km 3 .

Coupled response of the late glacial climatic shifts of northwest Europe reflected in Greenland ice cores: Evidence from the northern North Sea

The climatic regimes of the land areas adjacent to the North Sea are controlled by the influx of Atlantic water north of the British Isles and into the North Sea. A high-resolution record from the Norwegian Channel off western Norway covering the past 15 ka ( 14 C) shows that prior to the Younger Dryas, there were three periods of sea-surface conditions there similar to those of the present, interspersed with arctic conditions. These changes, corresponding to an increase of (Approx.)5 °C in summer sea-surface temperatures, took place on the scale of

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.

Large submarine slides on the Norwegian continental margin: Sediments, transport and timing

Abstract A large submarine slide area at Storegga on the continental slope off western Norway is described. Three main slide events have been recognized involving a total volume of 5580 km3 of Cenozoic sediments. The First Slide comprised 3880 km3, while the Second and Last Slides together comprised 1700 km3. The Second Slide covered 3500 m of water depth and a distance of at least 800 km. The sediments deposited by the slides consist of different types of debris flow deposits and turbidites. Smaller mass movements in the form of turbidites and submarine creep deposits have occurred after the main slide events. A continuous transition from coarse debris flows into turbidites is common. The First Slide comprised young, unconsolidated deposits, while the Second Slide cut deeper into Quaternary, Neogene and Paleogene sediments east of the Faeroe-Shetland Escarpment. The First Slide took place before 30,000 B.P., the Second and Last Slides took place in the Holocene between 8000 and 5000 B.P. Earthquakes possibly together with gas released from decomposition of gas hydrates are considered to be the most likely triggering mechanisms.

Glacial geology of outer Bjørnøyrenna, southwestern Barents Sea

The Upper Cenozoic glacigenic succession in outer Bjornoyrenna (Bear Island Trough), southwestern Barents Sea is investigated utilizing data from six shallow boreholes (< 143 m below sea bed) and seismic profiling. Overdeepened troughs in the lowermost part of a large Plio-Pleistocene wedge at the shelf margin are interpreted as glacial erosion forms. This suggests that the overlying succession, which basinward attains a thickness of several km, was deposited during a period with glaciations. Large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north-south trending zones parallel to the present shelf break. They probably formed in a continental shelf/coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations. A core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till. Most of the other cores are dominated by Middle and Upper Pleistocene muddy, massive, partly fissile diamictons interpreted to be mainly tills. Undrained shear strength of these cores ranges from about 20 kN m−2 to more than 300 kN m−2, and both underconsolidated intervals compared to the present burial depth are found. An underconsolidated interval comprising Eemian interglacial, biourbated marine sediments overlain by laminated Weichselian glaciomarine sediments on the outher shelf, is covered by 50–100 m of muddy diamicton. This diamicton is suggested to be mainly till deposited by a late Weichselian ice sheet which was grounded to more than 500 m below present sea level. Earlier Weichselian ice expansions did not reach the outer shelf. Based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca. 440 ka. The present study suggests that glacial erosion was a major process in the Late Cenozoic denudation of the Barents Sea, and in large areas perhaps as much as 200–250 m of preexisting strata could have been eroded by glaciers since about 440 ka.

The last 18 kyr fluctuations in Norwegian sea surface conditions and implications for the magnitude of climatic change: Evidence from the North Sea

A combined record of three cores spanning the last 18 kyr from the northern North Sea is investigated for content of benthic and planktonic foraminifera and stable oxygen isotopes. The paleoenvironmental development through this time period shows an early deglaciation (18–14.4 ka) and the Younger Dryas (12.7–11.5 ka) characterized by arctic/polar conditions and increased ice rafting in the Norwegian Channel. During the Bolling-Allerod period, warm sea surface temperature (9°C) conditions similar to present conditions are inferred, while bottom waters stayed cold (0–1°C) with normal salinity. The Bolling-Allerod period is interrupted twice at 13.9–13.6 ka (Older Dryas) and at 13.0–12.8 ka (Inter-Allerod Cooling Period) by reductions in sea surface temperatures and increased sea ice cover. The beginning of the Holocene period is marked by increases in surface and bottom water temperature. Superimposed on the broad climatic changes through the Holocene, a series of short-lived oscillations in the ocean circulation are recorded. The amplitude of these Holocene events appears larger in the early Holocene (prior to 8 ka) than compared with the remaining part of the Holocene. This amplification can possibly be attributed to a general increased freshwater budget in the North Atlantic at this time during the final stages of the deglaciation of the Laurentide and Scandinavian ice sheets.

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.

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (