Karin Andreassen

Glacigenic sediments on a passive continental margin as exemplified by the Barents Sea

Abstract Reflection seismics data, borehole data and gravity cores have been used to map the sedimentary facies of the epicontinental southwestern Barents Sea, its continental margin and adjacent coastal areas. An upper unconformity with a glacially eroded morphology extends over most of the continental shelf. Up to 300 m of stratiform glacigenic sediments overlie this unconformity. These sediments can be grouped into seismic units which are separated by smooth or irregular erosional surfaces. Each sequence may be up to 150 m thick, normally with a lateral extent of 100–300 km. Three dominant seismic signatures may be discerned: (semi)-transparent, stratified and chaotic reflection patterns. These sediments are partly glacifluvial in origin, but on the whole they represent more-or-less tectonized glacimarine deposits. The deeper troughs on the continental shelf contain an undeformed postglacial glacimarine/marine trough-fill. The greatest thickness (up to 1000 m) of glacigenic sediments occurs at the shelf break and below the upper slope. The depocentres are situated at trough-mouth fans. The sediments comprise several prograding sequences with a complex sigmoid-oblique character, indicating alternating upbuilding and depositional bypass/erosion in the topset segment. Small- and large-scale slide scars are observed on the upper slope, and debris lobes on the lower slope. A marked progradation seems to have occurred during the Plio-Pleistocene glacial periods. This progradation also resulted in an unstable slope with various types of sediment gravity flows. Currents flowing along the slope were established during interglacials, and gullying by cold, dense, shelf water and downslope transport occurred. Maximum net sedimentation rates are: ≈0.2 m/ka (continental slope), ≈0.4 m/ka (shelf break), ≈0.1 m/ka (shelf stratiform diamictons), ≈1 m/ka (shelf trough fills) and ≈60 m/ka (fjord trough fills).

Three-dimensional seismic data from the Barents Sea margin reveal evidence of past ice streams and their dynamics

Three-dimensional seismic data from the southern flank of the outer Bear Island Trough at the western Barents Sea margin show former ice-stream activity; the interpretation of such activity is based on megascale lineations and long chains of megablocks and rafts buried in thick till units between glacially eroded horizons. In several units the sediment blocks and rafts are aligned in chains up to 2 km wide and 50 km long. Individual megablocks have an areal extent of up to 2.2 km 2 , and their internal structure is well preserved, providing information about the processes involved in their incorporation into the ice, subsequent detachment, and partial disintegration. Previous discussions of sub-ice-stream processes have focused on ductile deformation of weak sediments and molding of subglacial landforms. This work also demonstrates the importance of brittle sediment deformation in these settings, adding to the understanding of ice streams as agents of glacial erosion, transport, and sedimentation.

Amplitude versus offset modeling of the bottom simulating reflection associated with submarine gas hydrates

Abstract A bottom simulating seismic reflection (BSR) that parallels the sea floor occurs worldwide on seismic profiles from outer continental margins. The BSR coincides with the base of the gas hydrate stability field and is commonly used as indicator of natural submarine gas hydrates. Despite the widespread assumption that the BSR marks the base of gas hydrate-bearing sediments, the occurrence and importance of low-velocity free gas in the sediments beneath the BSR has long been a subject of debate. This paper investigates the relative abundance of hydrate and free gas associated with the BSR by modeling the reflection coefficient or amplitude variation with offset (AVO) of the BSR at two separate sites, offshore Oregon and the Beaufort Sea. The models are based on multichannel seismic profiles, seismic velocity data from both sites and downhole log data from Oregon ODP Site 892. AVO studies of the BSR can determine whether free gas exists beneath the BSR if the saturation of gas hydrate above the BSR is less than approximately 30% of the pore volume. Gas hydrate saturation above the BSR can be roughly estimated from AVO studies, but the saturation of free gas beneath the BSR cannot be constrained from the seismic data alone. The AVO analyses at the two study locations indicate that the high amplitude BSR results primarily from free gas beneath the BSR. Hydrate concentrations above the BSR are calculated to be less than 10% of the pore volume for both locations studied.

Late Cenozoic erosion of the high-latitude southwestern Barents Sea shelf revisited

The southwestern Barents Sea has experienced profound erosion during the last ∼2.7 m.y. that has resulted in the development of a characteristic glacial morphology of the continental shelf and deposition of a several-kilometer-thick sediment fan along the western margin prograding into the deep sea. In the period from ca. 2.7 to 1.5 Ma, proglacial processes, including fluvial and glaciofluvial erosion, dominated. For this period, the total average erosion of the shelf was 170–230 m, the average erosion rate was 0.15–0.2 mm/yr, and the average sedimentation rates on the fan were 16–22 cm/k.y. Subglacial erosion affected an area of ∼575,000 km 2 during the period from ca. 1.5 to 0.7 Ma. Total average erosion is estimated at 330–420 m for this interval, and the average erosion rate was 0.4–0.5 mm/yr. Average sedimentation rates were 50–64 cm/k.y. During the last ∼0.7 m.y., glacial erosion mainly has occurred beneath fast-flowing paleo-ice streams topographically confined to troughs (∼200,000 km 2 ). The total average erosion is estimated at 440–530 m, average erosion rate is 0.6–0.8 mm/yr, and average sedimentation rate on the continental slope is 18–22 cm/k.y. The amount of erosion was mainly determined by the duration of the glaciations and the location, velocity, and basal properties of the ice streams. In total, glacial erosion of the troughs has been relatively high throughout the last ∼2.7 m.y. at ∼1000–1100 m. For the banks, erosion is inferred to have increased from ca. 2.7 Ma to a peak between 1.5 and 0.7 Ma. Subsequently, little erosion occurred in these areas, which implies a total of 500–650 m of erosion. Compared with other high-latitude areas, our rates are among the highest so far reported. This comparison also demonstrates that there have been large variations in the rate of sediment delivery to the glaciated continental margins.

Late Pliocene–Pleistocene development of the Barents Sea Ice Sheet

The late Pliocene–Pleistocene paleoenvironment has been reconstructed based on three-dimensional seismic data from the southwestern Barents Sea continental margin. During the late Pliocene–early Pleistocene, continental slope sediments were predominantly deposited from meltwater overflows and underflows. The seismic stratigraphy of the early–middle Pleistocene shows both glaciomarine sediment input from channelized meltwater discharge and the first indications of large debris-flow deposits on the continental slope, originating from input of subglacial deformation till eroded and transported by ice streams. During the middle–late Pleistocene, large debris flows dominated the slope succession. From the above results we infer the following evolution in the Barents Sea: (1) a temperate Barents Sea Ice Sheet with channelized meltwater flow developed during the late Pliocene–early Pleistocene; (2) alternating glacial periods of ice with channelized meltwater flow and the first periods of ice, including ice streams, characterized the early and middle Pleistocene; and (3) more polar ice conditions and a Barents Sea Ice Sheet that mainly included large ice streams, with little or no channelized meltwater flow, occurred in the middle and late Pleistocene.

Gas hydrates at the Storegga Slide: Constraints from an analysis of multicomponent, wide-angle seismic data

Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 , outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.

Signature of ice streaming in Bjørnøyrenna, Polar North Atlantic, through the Pleistocene and implications for ice-stream dynamics

Abstract The geomorphology of palaeo-ice-stream beds and the internal structure of underlying tills can provide important information about the subglacial conditions during periods of fast flow and quiescence. This paper presents observations from three-dimensional seismic data, revealing the geomorphology of buried beds of the Bjørnøyrenna (Bear Island Trough) ice stream, the main drainage outlet of the former Barents Sea ice sheet. Repeated changes in ice dynamics are inferred from the observed successions of geomorphic features. Megablocks, aligned in long chains parallel to inferred ice-stream flowlines, and forming dipping plates that are thrust one on top of another, are taken as evidence for conditions of compressive ice flow. Mega-scale glacial lineations (MSGL) and pull-apart of underlying sediment blocks suggest extensional flow. The observed pattern of megablocks and rafts overprinted by MSGL indicates a change in ice dynamics from a compressional to an extensional flow regime. Till stiffening, due to subglacial freezing, is the favoured mechanism for creating switches in sub-ice-stream conditions. The observed pattern of geomorphic features indicates that periods of slowdown or quiescence were commonly followed by reactivation and fast flow during several glaciations, suggesting that this may be a common behaviour of marine ice streams.

Ice-stream flow switching during deglaciation of the southwestern Barents Sea

Ice streams dominate the discharge of continental ice sheets. Recent observations and reconstructions have revealed that large-scale reorganizations in their flow trajectory (flow switching) can occur over relatively short time scales. However, the underlying causes of such behavior, and the extent to which they are predictable, are poorly known. This paper documents a major episode of ice-stream flow switching during the late Weichselian deglaciation of the southwestern Barents Sea and explores various hypotheses for its causation. Regional bathymetric data show that two ice streams that had similar, adjoining, topographically constrained source areas had very different trajectories and dynamics on the outer shelf. At the late Weichselian maximum, the Hakjerringdjupet ice stream flowed westward along the cross-shelf trough of Hakjerringdjupet, while the Soroya Trough ice stream flowed northward into Ingoydjupet, forming a tributary of the Bjornoyrenna ice stream. Initial retreat of the Hakjerringdjupet ice stream was rapid but with episodic periods of grounding. As it retreated onto the higher, rougher topography of the inner shelf, we infer a reduction in ice velocity and a dramatic decrease in the pace of retreat, as recorded by nested sequences of recessional moraines. Following (and probably in response to) this, we suggest that there was a short-lived surge/readvance of an adjacent lobe onto Fugloybanken. In contrast, the adjacent Soroya Trough ice stream remained active throughout deglaciation, before retreating rapidly, with no stillstands or readvances. We argue that the different retreat histories of the ice streams were determined by variations in bed topography/bathymetry, which modulated the grounding line response to sea-level variation. Such a mechanism is likely to be an important control on the long-term behavior of marine-based ice streams and outlet glaciers in Antarctica and Greenland and suggests that gathering data on their subglacial topography should be a priority.

Imprints of former ice streams, imaged and interpreted using industry three-dimensional seismic data from the south-western Barents Sea

Abstract Former ice-stream activity is shown from industry three-dimensional (3D) seismic data from the south-western Barents Sea. Although designed for deeper targets, the data allow, due to high spatial sample rate and three-dimensional migration techniques, construction of detailed plan view images. The integration of sea-floor geomorphology with stratigraphy documents the importance of glacial processes in the seascape evolution of this area. Fast-flowing ice streams occupying the cross-shelf troughs during the Late Weichselian glaciation caused large-scale erosion, and also left their imprints in the form of mega-scale glacial lineations on the sea floor as indicators of ice-flow direction. Various types of 3D seismic attributes, combined with detailed geomorphology and seismic stratigraphy, are used to investigate the 2–3 km of stratigraphic record that corresponds to over a million years of ice-stream activity. The appearance of mega-scale glacial lineations on various 3D seismic attribute maps indicates, together with other characteristics of ice streams, that they are formed by erosion beneath fast-flowing grounded ice. Bedform records of former ice streams may, however, be related only to the final stages of ice-streaming, immediately prior to shut down. Because we here have preserved up to several hundred metres of sediments between the buried, glacially eroded surfaces, we have the opportunity to study ice-stream imprints and associated processes covering longer time spans than just the last stages. Seismic volumetric attribute maps reveal that megablocks and rafts, often aligned in chains, commonly occur within the till units, implying that glaciotectonic erosion by fast-flowing ice streams was an important process in the transfer of sediments from the continental shelf to the Bjørnøya Trough Mouth Fan and the deep sea during the Plio-Pleistocene glaciations.

Geophysical constraints on the dynamics and retreat of the Barents Sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation

Our understanding of processes relating to the retreat of marine-based ice sheets, such as the West Antarctic Ice Sheet and tidewater-terminating glaciers in Greenland today, is still limited. In particular, the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales. Over numerous glaciations during the Quaternary, a marine-based ice sheet has waxed and waned over the Barents Sea continental shelf, characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming. Increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this Barents Sea ice sheet (BSIS), particularly so from its Late Weichselian maximum extent. We present a review of existing geophysical constraints that detail the dynamic evolution of the BSIS through the last glacial cycle, providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior. Although constraining data are generally spatially sporadic across the Barents and Kara Seas, behaviors such as ice sheet thinning, major ice divide migration, asynchronous and rapid flow switching, and ice stream collapses are all evident. Further investigation into the drivers and mechanisms of such dynamics within this unique paleo-analogue is seen as a key priority for advancing our understanding of marine-based ice sheet deglaciations, both in the deep past and in the short-term future.