SW Barents Sea Research Articles

Buried subglacial landforms in the SW Barents Sea imaged using high-resolution P-Cable seismic data

A large grounded ice sheet covered the epicontinental Barents Sea multiple times during the Late Pleistocene and left its erosional imprint (Solheim et al. 1990; Knies et al. 2009). Erosion involved the removal of large amounts of sediments from the Tertiary and Cretaceous sedimentary successions of the Barents Sea. The Upper Regional Unconformity (URU) represents the erosional base for continental-shelf glaciations (Vorren et al. 1988). Here, we present palaeo-landforms from this erosional base imaged using high-resolution P-Cable seismic data acquired along the border between the Ringvassoya Fault Complex and the Loppa High in the SW Barents Sea (Fig. 1). P-Cable seismic data have a frequency bandwidth of up to 300 Hz and spatial resolution of 6×6 m, which is much better than conventional 3D seismic data. Hence, interpretation of P-Cable data allows visualization of buried geomorphic landscapes in exceptional detail. In addition, amplitude information is readily available and complements grid-based interpretations. Fig. 1. Buried subglacial landforms in …

Glacially related gullies on the upper continental slope, SW Barents Sea margin

During the Late Weichselian glaciation an ice stream flowed westwards along the Bear Island Trough in the Norwegian Barents Sea, carrying large amounts of glacial debris to the shelf edge to be deposited on the Bear Island Trough–Mouth Fan (e.g. Andreassen et al. 2008). On heavily iceberg-ploughed shallower banks such as Tromsoflaket, south of the Bear Island Trough (Fig. 1), the ice sheet was less active although not necessarily frozen to its bed. Well-developed systems of gullies occur on the upper continental slope west of Tromsoflaket and on the southern flank of the Bear Island Fan, with their initiation points at or close to the shelf edge. Fig. 1. Gullies on the upper slope of the SW Barents Sea. ( a ) Shaded-relief image from MAREANO (www.mareano.no) of relatively shallow gullies on the southern part of the Bear Island Fan (ch is a channel at the distal end of a gully system). ( b ) Shaded-relief image of deeper gullies on the upper slope west of Tromsoflaket (ss, slide scars; dg, deepest gully of about 120 m). Acquisition system for (a) and (b) Kongsberg EM710. Frequency 70 kHz. Grid-cell size 10 m. Sun illumination from the NW. ( c ) Bathymetric cross-profiles of gullies in …

Iceberg ploughmarks in the SW Barents Sea imaged using high-resolution P-Cable 3D seismic data

Iceberg ploughmarks are common features of glaciated continental margins worldwide. They form when iceberg keels plough through sediments on the ocean floor. They occur in various sizes and shapes, but generally exhibit a linear or curvilinear geometry. The SW Barents Sea is a heavily glaciated margin and its seafloor shows numerous ploughmarks among other glacial landforms (Andreassen et al. 2008). We present seafloor features formed during the late Weichselian glaciation (Solheim et al. 1990) imaged using high-resolution P-Cable 3D seismic data. P-Cable 3D seismic data have a spatial resolution of 6×6 m which is better than many hull-mounted multibeam systems, particularly in deep water. However, the cumulative seismic response of the seafloor is slightly different due to the lower-frequency bandwidth (30–350 Hz) of P-Cable compared to multibeam systems (up to 500 kHz). In addition, amplitude information is readily available and complements grid-based interpretations. Numerous linear and curvilinear depressions occur on the seafloor near …

Trawl marks, iceberg ploughmarks and possible whale-feeding marks, Barents Sea

Humans, other animals and geological processes leave their imprint on the seafloor. The interpretation of such features may be challenging. Very-high-resolution synthetic aperture sonar (SAS) data and seabed photographs (TFish) have recently been acquired from the Barents Sea using the HiSAS 1030 and TFish systems on the autonomous underwater vehicle (AUV) HUGIN HUS. These data demonstrate the clear morphological distinction between iceberg ploughmarks and trawl marks produced during fishing, and also enable the interpretation of small, irregular depressions as whale-feeding marks. A 13 km2 area was imaged with a very-high-resolution SAS deployed from an AUV (Fig. 1a–c), nested within 175 km2 of conventional hull-mounted multibeam data (Fig. 1d) on the Loppa High, SW Barents Sea (Fig. 1h). Water depths here range from 420 to 440 m. SAS data were processed to give imagery with a resolution of 7×7 cm and bathymetric grids at 20×20 cm. TFish photographs (resolution 5×5 mm/pixel) were also acquired from the AUV from selected sites. Fig. 1. Multibeam bathymetry, SAS imagery and micro-bathymetry, and seafloor photographs of trawl marks, possible whale-feeding marks and iceberg ploughmarks. ( a ) SAS imagery of trawl marks and whale marks. Acquisition system Kongsberg HiSAS 1030 (wideband SAS sonar, frequency 70–100 kHz). ( b ) SAS micro-bathymetry …

Crescentic submarine hills and holes produced by iceberg calving and rotation

Late Weichselian maximum ice covered the entire Barents Sea continental shelf and a major ice stream delivered large amounts of glacial debris to the western slope (Fig. 1a). During deglaciation, one of several dynamic components of the ice sheet occupied Djuprenna, a glacially eroded trough offshore of the northernmost coast of mainland Norway (Fig. 1a). Mega-scale glacial lineations (MSGLs) indicate major ice flow when the main ice source had shifted eastwards and most of the SW Barents Sea was ice free (e.g. Ottesen et al. 2008). Spectacular seabed pits with adjacent crescentic hills are imaged on multibeam bathymetry of Djuprenna. Although morphologically similar to hill–hole pairs formed by glacitectonic processes (e.g. Saettem 1990) some critical differences suggest another genesis. Seafloor features of this type are probably uncommon and, to our knowledge, have not been described previously. Fig. 1. Images, metrics and interpretation of a field of over 60 hill–hole pairs located in the glacial trough, Djuprenna. ( a ) Location of study area (red box; map from GEBCO_08). Sun-illuminated multibeam-bathymetric images depict their morphological variation: ( b ) a non-crescentic hill; ( c ) a doublet with MSGLs; ( d ) adjoining but mirrored hills; ( e ) very concentric hill, up-ice side; ( f ) typical form; ( g ) hole both sides, hill convex down-ice; ( h ) no hole (infilled); ( i ) hole both sides, current scour-related, convex up-ice; ( j ) largest form, convex down-ice. Acquisition system Kongsberg EM1002. …

Middle to late Holocene paleoproductivity reconstructions for the western Barents Sea: a model-data comparison

In this study we focus on late Holocene primary productivity (PP) variability in the western Barents Sea and its response to variable sea ice coverage by combining PP reconstructed from several sediment cores with regional PP trends simulated with a well-constrained organic facies model, OF-Mod 3D. We find that modern production rates reconstructed from buried marine organic matter (“bottom-up”) resemble simulated export production at 50 m water depth inferred from numerical simulations of surface water PP in a 3D ocean model, SINMOD (“top-down”). Paleoproductivity rates in the northern Barents Sea are more variable and generally higher (30–150 gC m−2 year−1) than in the SW Barents Sea region (<75 gC m−2 year−1) throughout the last 6000 years BP. In the SW Barents Sea, PP rates and terrestrial organic matter (TOM) supply remain constantly low indicating present-day-like oceanographic conditions with only marginal influence of sea ice related processes during the last 6000 years BP. PP rates in the northern Barents Sea indicate a shift from stable modern-like conditions prior to 2800 BP to denser, more permanent sea ice coverage along the marginal ice zone (MIZ) between 2800 and 1000 years BP and low PP rates. PP rates increase around 1000 years BP indicating a northward shift of the MIZ and accelerated export towards the seabed. During the last 500 years a pronounced decline in PP rates towards the present day indicates reduced annual duration of the MIZ in the area due to global warming. Our results suggest that a combination of first-year ice and higher PP in a warming pan-Arctic may point to a potential Arctic carbon sink while sea ice is still present.

Variability of present and past PAH (polyaromatic hydro carbons) concentrations in sediments of the SW Barents Sea

Variability of present and past PAH (polyaromatic hydro carbons) concentrations in sediments of the SW Barents Sea

Natural variability of benthic foraminiferal assemblages and metal concentrations during the last 150years in the Ingøydjupet trough, SW Barents Sea

Abstract While today the SW Barents Sea is a relatively un-impacted and uncontaminated area, industrial activities related to the petroleum industry are projected to increase in the coming decades. This makes the area a valuable natural laboratory to establish pre-impacted baselines as a precursor for future seabed monitoring programs. Here we present benthic foraminiferal assemblages and metal concentrations in four sediment cores from the Ingoydjupet trough, SW Barents Sea, covering approximately the last 150 years. This information supports the application of foraminiferal assemblages as a bio-monitoring tool applicable in high latitudes. At all stations, metal concentrations in the sediment correspond to no effect concentrations. The downcore metal concentrations are mainly attributed to natural variability of the clay fraction and total organic content of the sediments. Agglutinated foraminifera are poorly preserved down-core. Patterns in the calcareous foraminiferal assemblages suggest an enhanced food supply as a result of increased Atlantic Water inflow through the region during the last 150 years. At near-shore stations, the Norwegian Coastal Current additionally influences assemblages. Decadal scale climatic oscillations are indicated by increased calcareous fluxes and are attributed to variability in the food-rich Atlantic Water. This study serves as an important baseline data set prior to increasing industrial activities in the SW Barents Sea, and thereby contributes to a better understanding of natural environmental variability.

Evolution and character of supra-salt faults in the Easternmost Hammerfest Basin, SW Barents Sea

This study investigates the evolution of supra-salt faults in the Eastern Hammerfest Basin using high–quality seismic reflection data. Traditional techniques of displacement analysis, including the variation of fault displacement (throw) against distance (x), depth (z), expansion and growth indices were adopted. Fault reactivation was assessed using bivariate plots of a) cumulative throw vs. age and b) throw (t) vs. depth of nine (9) representative faults.The interpreted faults are supra-salt crestal and synclinal faults striking NE, E and SE. These faults have complicated t-x and t-z plots and are characterized by considerable stratigraphic thickening in their downthrown section. Faults in the study area have developed over the salt structure since latest Paleozoic times; some of them were reactivated by Early to Middle Triassic through dip linkage of initially isolated fault sets. Along strike, the fault exhibit complex segmentation through coalescence of several subunits linked by local throw/displacement minima. Expansion and growth indices show that the faults of the study area developed during the deposition of Paleozoic to Early Cretaceous sediments by polycyclic growth involving both blind and syn-sedimentary activity.An important piece of information from this study is that fault propagation is controlled by lithological heterogeneity and that both lateral and vertical segmentation of faults are important for hydrocarbon migration within the Triassic to Late Cretaceous interval.

Controls on gas hydrate system evolution in a region of active fluid flow in the SW Barents Sea

The location and stability of gas hydrates in the SW Barents Sea is poorly constrained due to complex geological, geochemical, and geophysical conditions, including poor controls on regional heat flow and gas chemistry. Understanding the stability of gas hydrates in this region is important, as recent studies suggest destabilizing hydrates may lead to methane discharge into the ocean and possibly in to the atmosphere. Here, we use high-resolution 3D P-Cable seismic data, combined with 3D heat flow and fluid flow models to place new constraints on gas hydrate stability in this region. The 3D P-Cable seismic data, acquired in 2009 west of Loppa High, show cross-cutting, reverse polarity, high-amplitude reflectors interpreted as the base of gas hydrate stability. To constrain heat flow, fluid flow, and gas hydrate stability within the 3D seismic volume, we use a 3D steady-state, finite difference diffusive thermal model that incorporates regional bottom water temperature from CTD casts, expected geothermal gradients, and gas composition derived from well data. In general, modelled bottom simulating reflectors are deeper than observed BSRs. Our analysis weighs multiple factors that might explain the discrepancy between observed and modelled bottom simulating reflector depths. From this analysis, we propose that the most significant discrepancies in BSR depth are likely related to changes in regional fluid/heat flow and fluid geochemistry. The anomalously shallow bottom simulating reflectors can be explained via vertical fluid flow that might include ensuing potential effects on gas composition, pore water salinity and temperature. Our estimate suggest that a maximum vertical fluid flux of approximately 12 mm/y is necessary to explain the most significant anomalies. Our study provides new insight into regional heat flow, geochemistry, and end-member vertical fluid flux rates in the Barents Sea. Moreover, it documents that the fluid flow system is active and most likely, very dynamic.

On Palaeozoic–Mesozoic brittle normal faults along the SW Barents Sea margin: fault processes and implications for basement permeability and margin evolution

Palaeozoic–Mesozoic brittle normal faults onshore along the SW Barents Sea passive margin off northern Norway give valuable insight into fault and fluid flow processes from the lower brittle crust. Microstructural evidence suggests that Late Permian–Early Triassic faulting took place during multiple phases, with initial fault movement at minimum P–T conditions of c . 300 °C and c . 240 MPa ( c . 10 km depth), followed by later fault movement at minimum P–T conditions of c . 275 °C and c . 220 MPa ( c . 8.5 km depth). The study shows that pore pressures locally reached lithostatic levels (240 MPa) during faulting and that faulting came to a halt during early (deep) stages of rifting along the margin. Fault permeability has been controlled by healing and precipitation processes through time, which have sealed off the core zone and eventually the damage zones after faulting. A minimum average exhumation rate of c . 40 m Ma −1 since the Late Permian is estimated. It implies that the debated Late Cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation, which caused subsequent isostatic recalibration and uplift of the marginal crust. The studied faults may be used as analogues of basement-involved fault complexes offshore, revealing details about the offshore nature of faulting, including past and present basement and fault zone permeability.

Character of seismic motion at a location of a gas hydrate‐bearing mud volcano on the SW Barents Sea margin

AbstractThe Håkon Mosby mud volcano (HMMV) at 1270 m water depth on the SW Barents Sea slope has been intensively studied since its discovery in 1989. A variety of sensors monitored morphological, hydrological, geochemical, and biological parameters in the HMMV area. An ocean bottom seismometer deployment allowed us to register seismic motion for 2 years, from October 2008 to October 2010. The analysis of seismic records documents two types of seismic signals. The first type are harmonic tremors with frequency peaks around 4–5 and 8–10 Hz that occur in swarms. Their origin could be from fluid flow circulation or resonant vibrations of gas bubbles or from delayed movement of fluid‐rich sediments in the conduit or in a deeper pseudo‐mud chamber of the HMMV. Because swarms occur with a periodicity of ~ 6 h, tide‐related effects are suspected to influence the mechanism originating the tremors. The second type of signals are regional earthquakes that were in 15 cases recognized in seismic records. The activity of harmonic tremors was not significantly affected by earthquakes.

Constraints on the dynamics of pockmarks in the SW Barents Sea: Evidence from gravity coring and high-resolution, shallow seismic profiles

Abstract Newly acquired sediment cores and high-resolution seismic lines were studied in order to constrain the timing and mechanism of pockmark formation in the Loppa High area in the southwestern Barents Sea. The evolution of the pockmarks after their formation was also investigated, together with the possibility that they are currently active features. The cores retrieved within the pockmarks lack a layer of ice-rafted debris and an overlying sequence of laminated glaciomarine sediments that were deposited on the undisturbed seafloor at the inception of the Bolling interstadial (~ 15 cal kyr BP). The pockmarks are suggested to have formed in that period, probably due to climate change-induced destabilisation of methane hydrates and the subsequent release of free gas. The depressions deepened as a result of reduced sedimentation above active seeps. The pockmark cores contain a series of gravel-rich layers, which are attributed to the winnowing of sediment during episodic fluid venting activity. The seismic images reveal little disturbance of the most ancient sediments underneath the pockmarks, indicating that the pockmark chimneys are localised features and were not imaged. The Holocene marine sediment cover in the pockmarks is thinner than on the surrounding seafloor, indicating relatively low sedimentation rates inside the depressions. The gas seepage may thus have been active until the recent past, although no indications of present-day methane flux were found.

Structural and stratigraphic controls on subsurface fluid flow at the Veslemøy High, SW Barents Sea

Subsurface and seafloor fluid flow anomalies are gaining large interest after the finding of five new hydrocarbon discoveries and observation of large gas flares in the SW Barents Sea. In the present study, we have analysed structural and stratigraphic controls on fluid flow towards the seafloor using gravity cores selected based on subsurface gas anomalies observed on seismic data from the Veslemøy High, SW Barents Sea. The subsurface fluid flow at the Veslemøy High is observed to be controlled by 1) the morphology and orientation of regional faults, structural highs and sedimentary basins, 2) the presence of Paleocene silica ooze deposits that changes microstructure with temperature thereby controlling fluid flow and 3) the location of regional and local open faults formed by glacial loading and unloading. Analysis of extractable organic matter in subsurface Holocene sediments corroborates the active migration pathways inferred from seismic data. Micropalaeontological studies on benthic foraminifera reveal methane seep associated assemblages that confirm the interpretation of subsurface gas anomalies in seismic data. We ultimately link these new results to the geological evolution history of the region to give a comprehensive model for the fluid flow system within the study area.

Sediment transport and deposition in the Ingøydjupet trough, SW Barents Sea

In this study we have performed a baseline characterisation of sediment properties in four depositional areas of the Ingøydjupet trough in order to investigate the sediment transport and deposition by ocean currents in the SW Barents Sea. Sedimentation rates, by the 210Pb method, were 1.0–2.4mm/yr and decreasing with distance offshore. Stronger and more variable bottom currents associated with the Norwegian coastal current operate nearshore based on the down-core distribution of sortable silt (SS¯) and sediment fractions (clay and silt). These nearshore stations exhibit a trend of decreasing fine sediment supplies and increasing sand content toward present day. The most stable bottom currents are associated with the farthest offshore station which is under the influence of the North Atlantic Current as indicated by the distribution of (SS¯) and also higher TOC contents (0.6–0.9%) compared to nearshore stations (0.4–0.8%). These baseline findings are of relevance for the petroleum industry in planning environmental assessment and monitoring programs connected to the future expansion of industry activities in this region.

Gas hydrate systems in petroleum provinces of the SW-Barents Sea

3D seismic data of the SW-Barents Sea provided information about the existence of Structure II gas hydrates and shallow gas in marine sediments. Gas hydrates and shallow gas exist in the SW-Barents Sea in conjunction with deep-hydrocarbon reservoir leakage. Available conventional 3D seismic data show acoustic chimneys that extend to depths ∼1700 ms two-way travel time below the seafloor. The location of the chimneys coincides with structural boundaries and fault complexes cutting through mostly Jurassic and Triassic strata. Vertical fluid flow through these chimneys transports thermogenic gas to the gas hydrate stability zone, causing the formation of a bottom-simulating reflector (BSR). In addition, variations of fluid flux across deep-seated fault complexes cause changes in heat flow, which explains the shoaling of the BSR and the formation of a “tilted” BSR (TBSR) at the border between the Ringvassøya Fault complex and the Loppa High. The stability model using thermogenic gas composition (type I and II) shows gas hydrates are stable at ∼225 m–∼345 m depth, corresponding to variation in the geothermal gradient for upper BSR (28.7 °C/km and 37 °C/km) and lower BSR levels (23.4 °C/km and 28.3 °C/km). These lateral changes in the temperature gradient can be reconciled with changes in upward fluid flow rates of 1.8–2.2 mm yr−1. Such gas-hydrate and fluid flow systems on Arctic continental margins especially in shallow water depths are potential gas emitters due to their sensitivity to warming ocean temperatures.

Hydrocarbon plumbing systems above the Snøhvit gas field: Structural control and implications for thermogenic methane leakage in the Hammerfest Basin, SW Barents Sea

Based on the analysis of the high-resolution 3D seismic data from the SW Barents Sea we study the hydrocarbon plumbing system above the Snøhvit and Albatross gas field to investigate the geo-morphological manifestation and the dynamics of leakage from the reservoir. Fluid and gas escape to the seafloor is manifested in this area as mega-pockmarks 1–2 km-wide, large pockmarks (<100 m wide) and giant pockmarks 100–300 m-wide. The size of the mega pockmarks to the south of the study area may indicate more vigorous venting, whilst the northern fluid flow regime is probably characterised by a widespread fluid and gas release. Buried mega depressions and large-to-giant pockmarks are also identified on the base Quaternary and linked to deep and shallow faults as well as to seismic pipes. A high density of buried and seafloor giant pockmarks occur above a network of faults overlying an interpreted Bottom Simulating Reflector (BSR), whose depth coincides with the estimated base of the hydrate stability zone for a thermogenically derived gas hydrate with around 90 mol% methane. Deep regional faults provide a direct route for the ascending thermogenic fluids from the reservoir, which then leaked through the shallow faults linked to seismic pipes. It is proposed that the last episodic hydrocarbon leakage from the reservoir was responsible for providing a methane source for the formation of gas hydrates. We inferred that at least two temporally and dynamically different fluid and gas venting events took place in the study area: (1) prior to late Weichselian and recorded on the Upper Regional Unconformity (URU) and (2) following the Last Glacial Maximum between ∼17 and 16 cal ka BP and recorded on the present-day seafloor.

Distribution of subsurface fluid-flow systems in the SW Barents Sea

The SW Barents Sea is a large hydrocarbon-prone epi-continental Sea of the Norwegian Arctic region. A significant portion of the hydrocarbon gases generated in deep source rocks has leaked or migrated into the shallow subsurface and is now trapped in gas hydrate and shallow gas reservoirs. The evolution of sedimentary basins of this region has controlled the leakage of these fluids through marine sediments. Understanding the distribution of various fluid-flow systems may enhance our knowledge of the evolution of different basins in the SW Barents Sea and could help find potential targets for future hydrocarbon exploration. We analyze approximately 3000 2D multi-channel seismic profiles and data from 60 wells covering the entire SW Barents Sea, to identify and classify fluid-flow features, and study their relationship to tectonic elements and geological history. Gas chimneys are the most abundant feature among various other fluid-flow features such as fluid leakage along faults and fractures, seepage pipes, and high amplitude anomalies potentially indicating trapped fluids. Large fluid-flow features, covering areas as large as 600 km2, occur close to known hydrocarbon fields such as Snøhvit, Skrugard, and Havis. The fluid-flow features occur above major deep-seated faults in the area suggesting a close relation to it. The number of fluid-flow features in the western part of the study area is significantly higher than in the eastern part. The amount of net erosion in the study area shows no direct control over the distribution of fluid-flow features, suggesting that the faults and distribution of mature source rocks control the fluid flow. The strong correlation between the locations of fluid-flow features and structural elements indicates that extensional tectonics, uplift and glaciations could have played major roles in the timing and activity of the fluid leakage, although erosion might have had an added effect.

Seismic velocities guiding geological interpretation in frontier areas: the Stappen High area, SW Barents Sea

O.A. Blaich, J.I. Faleide, M. Rieder, G.A. Ersdal and B.I. Thyberg describe how integration of long-offset seismic reflection data and inversion of seismic refraction first arrival traveltime shot-gathers improved geological interpretation of the Stappen High in the SW Barents Sea.

Sub-arctic Holocene climatic and oceanographic variability in Stjernsund, northern Norway: evidence from benthic foraminifera and stable isotopes

A high-resolution record, covering 9.3–0.2 ka BP, from the sub-arctic Stjernsund (70°N) was studied for benthic foraminiferal faunas and stable isotopes, revealing three informally named main phases during the Holocene. The Early- to Mid-Holocene (9.3–5.0 ka BP) was characterized by the strong influence of the North Atlantic Current (NAC), which prevented the reflection of the Holocene Climatic Optimum (HCO) in the bottom-water temperature. During the Mid-Holocene Transition (5.0–2.5 ka BP), a turnover of benthic foraminiferal faunas occurred, Atlantic Water species decreased while Arctic-Polar species increased, and the oxygen isotope record showed larger fluctuations. Those variations correspond to a period of global climate change, to spatially more heterogeneous benthic foraminiferal faunas in the Nordic Seas region, and to regionally diverging terrestrial temperatures. The Cool Late Holocene (2.5–0.2 ka BP) was characterized by increased abundances of Arctic-Polar species and a steady cooling trend reflected in the oxygen isotopes. In this period, our record differs considerably from those on the SW Barents Sea shelf and locations farther south. Therefore, we argue that regional atmospheric cooling triggered the late Holocene cooling trend. Several cold episodes centred at ∼8.3, ∼7.8, ∼6.5, ∼4.9, ∼3.9 and ∼3.3 ka BP were identified from the benthic foraminiferal faunas and the δ18O record, which correlated with marine and atmospherically driven proxy records. This suggests that short-term cold events may result from reduced heat advection via the NAC or from colder air temperatures.