Storegga Research Articles

Habitat heterogeneity influences cold‐seep macrofaunal communities within and among seeps along the Norwegian margin. Part 1: macrofaunal community structure

AbstractHåkon Mosby mud volcano (HMMV) is one of the most active and most studied seep sites in European waters. Many authors have described its thermal activity, dynamic of mud flows, and geochemical and microbial processes. It is characterised by a concentric zonation of successive biogenic habitats related to an activity and geochemical gradient from its centre to its periphery. Around the central area covered by mud flows, white and grey microbial mats occur among areas of bare sediment, whereas siboglinid tubeworm fields of Sclerolinum contortum and/or Oligobrachia haakonmosbiensis colonise the peripheral areas. The meiofaunal community is known to be structured among habitats, but the macrofauna has rarely been investigated and has never been sampled in situ. As part of the European project HERMES, using the ROVs Victor 6000 and Quest 4000, we sampled quantitatively the different habitats of the volcano for macrofauna sensus lato, retained on a 250‐ or 500‐μm sieve. We also sampled a newly discovered pockmark on Storegga slide (cne 5.6) and two pockmarks (G11, G12) in the Nyegga area. Macrofauna was identified and counted from phylum to family level. Our results on HMMV showed a gradient of increasing density and diversity from the volcano centre (1–3 taxa; 260 ind·m−2) to the peripheral siboglinid fields (8–14 taxa, 93,000 ind·m−2), with an intermediate situation for microbial mats. For macrofauna ≥500 μm, non‐siboglinid polychaetes dominated the communities of the central mud volcano area, white mats and S. contortum fields (83, 89 and 37% of the total, respectively), whereas gastropods dominated grey mats and O. haakonmosbiensis fields (89 and 44% of the total, respectively). Polychaete families followed the same pattern of diversity according to habitats within HMMV. Of 23 polychaete families identified, only one occurred in the centre, and three in the microbial mats. Capitellidae and Dorvilleidae (typical of organically and sulphide‐enriched areas) occurred at remarkably high densities in white microbial mats and in O. haakonmosbiensis fields. The S. contortum fields were the most diverse habitat with 12 polychaete families. The 250‐μm fraction showed similar taxa dominating the habitats, but taking meiofauna into account, nematodes became the major taxon in white mats and in S. contortum fields, where they were particularly large in size, whereas copepods dominated in other habitats. Meiofauna and macrofauna did not show the same patterns of density according to habitats. Using principal components analysis the habitats at HMMV were clearly distinct, and clustered according to dominant species of siboglinids and type of microbial mats. Pockmarks at Nyegga showed a similar concentric pattern of habitats around fluid sources as on the volcano, which seemed similarly to influence macrofauna composition, but at a much smaller scale. Total taxa and polychaete diversity are high in the S. contortum fields in these pockmarks as well. Regional‐scale comparisons including HMMV and Storegga suggested a higher influence of habitat‐type than seep‐site on the community structure.

Tanaidomorph Tanaidacea (Crustacea: Peracarida) from Mud-Volcano and Seep Sites on the Norwegian Margin

Recent sampling of the benthos has been undertaken around the Håkon Mosby Mud Volcano (HMMV) at around 1280 m depth on the Norwegian-Barents-Spitsbergen continental margin and at two seep-sites further south on the Norwegian Margin, the Nyegga seep-site and the Storegga Slide, at about 730 m depth. The collected material included eleven species of tanaidomorph tanaidacean, ten of which (from eight genera) are analyzed herein. Four of these species, one each in the genera Pseudotanais, Cryptocopoides, Obesutanais, and Akanthophoreus, are new to science. Supplementary descriptions are given for two species originally discovered during the 19th century Ingolf expedition, Typhlotanais mixtus and Meromonakantha irregularis. The genus Magotanais is synonymized with Cryptocopoides; the subfamily Akanthophoreinae, Sieg is restricted and raised to family-rank. The evolution of potentially endemic taxa at ventand seep-sites is discussed.

Glacial and deglacial seafloor methane emissions from pockmarks on the northern flank of the Storegga Slide complex

The Storegga Slide complex is a multi-stage slope failure on the Norwegian continental margin where the most recent major event occurred 8.1 ka b.p. (calendar years before present). Its northern flank contains pockmark features that are commonly inferred to be related to the historical and modern venting of methane-bearing fluids. Three jumbo piston cores (JPC), one from a pockmark and two background cores at variable distances from this site (proximal, 5 km, and distal, 15 km) on the northern flank of the slide (806–1,524 m water depths), were sampled at 10 cm resolution to assess the geologic record of methane venting in the Nyegga pockmark field. Six down-core radiocarbon measurements on mixed planktonic foraminifer species reveal ages of 9.4–16.4 ka b.p. Bathymodiolus mussel shell horizons, indicators of methane-rich environments, have been dated at 15.8–17.6 and ~22 ka b.p. in the pockmark core. Stable isotope analyses on planktonic (Neogloboquadrina pachyderma sinistral) and benthic (Islandiella norcrossi, Melonis barleeanum) Foraminifera reveal δ18O values indicative of a clear glacial/deglacial transition (−1.5‰ shift in planktonic species). Both planktonic and benthic δ13C signatures record multiple excursions, interpreted to reflect the influence of methane in the environment; these δ13C excursions occur in the pockmark core and also in the distal background core. While authigenic calcite formation on the seafloor may play an important role in producing such excursions, these data together suggest the influence of methane seepage within the pockmark field over the past 25 ka, whereby seepage was particularly active between 13 and 15 ka. This is consistent with previously inferred regional increases in porewater pressure associated with glacial loading and higher sedimentation rates, which can cause gas hydrate and slope instability.

Postglacial uplift and relative sea level changes in Finnmark, northern Norway

The outer coast of Finnmark in northern Norway is where the former Fennoscandian and Barents Sea ice sheets coalesced. This key area for isostatic modelling and deglaciation history of the ice sheets has abundant raised shorelines, but only a few existing radiocarbon dates constrain their chronology. Here we present three Holocene sea level curves based on radiocarbon dated deposits from isolation basins at the outermost coast of Finnmark; located at the islands Sørøya and Rolvsøya and at the Nordkinn peninsula. We analysed animal and plant remains in the basin deposits to identify the transitions between marine and lacustrine sediments. Terrestrial plant fragments from these transitions were then radiocarbon dated. Radiocarbon dated mollusk shells and marine macroalgae from the lowermost deposits in several basins suggest that the first land at the outer coast became ice free around 14,600 cal yr BP. We find that the gradients of the shorelines are much lower than elsewhere along the Norwegian coast because of substantial uplift of the Barents Sea. Also, the anomalously high elevation of the marine limit in the region can be attributed to uplift of the adjacent seafloor. After the Younger Dryas the coast emerged 1.6–1.0 cm per year until about 9500–9000 cal yr BP. Between 9000 and 7000 cal yr BP relative sea level rose 2–4 m and several of the studied lakes became submerged. At the outermost locality Rolvsøya, relative sea level was stable at the transgression highstand for more than 3000 years, between ca 8000 and 5000 cal yr BP. Deposits in five of the studied lakes were disturbed by the Storegga tsunami ca 8200–8100 cal yr BP.

Propagation of the Storegga tsunami into ice‐free lakes along the southern shores of the Barents Sea

AbstractDeposits in coastal lakes in northernmost Norway reveal that the Storegga tsunami propagated well into the Barents Sea ca. 8100–8200 years ago. A tsunami deposit – found in cores from five coastal lakes located near the North Cape in Finnmark – rests on an erosional unconformity and consists of graded sand layers and re‐deposited organic remains. Rip‐up clasts of lake mud, peat and soil suggest strong erosion of the lake floor and neighbouring land. Inundation reached at least 500 m inland and minimum vertical run‐up has been reconstructed to 3–4 m. In this part of the Arctic coastal lakes are usually covered by >1 m of solid lake ice in winter. The significant erosion and deposition of rip‐up clasts indicate that the lakes were ice free and that the ground was probably not frozen. We suggest that the Storegga slide and ensuing tsunami happened sometime in the summer season, between April and October. Copyright © 2011 John Wiley & Sons, Ltd.

High-resolution seismic stratigraphy, sedimentary processes and the origin of seabed cracks and pockmarks at Nyegga, mid-Norwegian margin

Densely spaced high-resolution TOPAS seismic profiles and EM1002 bathymetric data reveal the presence of numerous pockmarks, mound-like structures and elongated seabed cracks at Nyegga, offshore mid-Norway. The seabed cracks are located adjacent to the northern escarpment of the Storegga Slide, appearing as graben-like structures in the TOPAS data. Unlike the cracks, pockmarks and mound-structures are largely associated with vertical zones of acoustic blanking at depth, interpreted as pathways for vertically migrating gaseous fluids. Based on the TOPAS data, a new seismostratigraphic framework has been established and correlated to previously published age models of IMAGES cores MD99-2291 and MD99-2289. Seismic facies interpretation suggests repeated and rapid deposition of up to 40 m thick glacigenic wedges in the eastern part of the study area around 18.2 14C ka BP (21.8 cal. ka), 17.5 14C ka BP (20.8 cal. ka) and 16.9 14C ka BP (20 cal. ka). Towards the west, glacimarine deposition has prevailed, characterized by progressively increasing sedimentation rates with peak values of 30 m/ka during the period from 15.0 14C ka BP (18.2 cal. ka) to 15.8 14C ka BP (19 cal. ka). As the distribution of the Nyegga pockmarks closely coincides with the main Late Weichselian sediment depocenters, we suggest a relation between rapid and repeated sedimentation and periodic overpressure generation at depth, ultimately leading to fluid expulsion at the seabed and the formation of the Nyegga pockmark field. In contrast, seabed cracks at Nyegga appear to have formed due to local extension which we relate to horizontal stress reduction as a consequence of the Storegga Slide event. Potentially, this event has been accompanied by renewed vertical fluid migration and the most recent stage of pockmark development.

The tail of the Storegga Slide: insights from the geochemistry and sedimentology of the Norwegian Basin deposits

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene-aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable-coloured, finely layered Holocene sediment caps a homogeneous, extremely well-sorted, poorly consolidated, very fine-grained, grey-coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re-deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub-bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re-deposited sediments.

Detection of seismic anisotropy using ocean bottom seismometers: a case study from the northern headwall of the Storegga Slide

SUMMARY Azimuthal seismic anisotropy has been identified from the analysis of S-waves generated by P to S mode conversion in the Pleistocene sediments that form the northern headwall of the Storegga Slide, which were investigated with a seismic experiment employing a seabed array of ocean-bottom seismometers and a grid of airgun shots. The principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components, after the application of moveout, to show the variations in amplitude, phase and cumulative traveltime effects of S-waves, ultimately providing information that identified the ‘fast’ and ‘slow’S-wave polarization orientations. Particle-motion analysis was used to corroborate the results and provide further information on the magnitudes of cumulative S-wave splitting. A 2-D ray-traced inversion of the traveltimes of pre-critical P and PS arrivals provided a velocity model from which the variation with depth of Vp, Vs and anisotropy could be compared with lithological and stratigraphic data from a borehole at the centre of the OBS array. Increased anisotropic response was observed to be coincident with high velocity units, which have high mica but low water content and are interpreted to be of glacial origin. The analysis of azimuthal seismic anisotropy shows clear evidence for horizontal transverse isotropy or an orthorhombic symmetry. The distribution in orientations of the fast plane of symmetry is broadly bimodal (E–W and NE–SW) across the OBS array. The E–W group showed correlation with the headwalls of old, buried slides and other faults visible within coherency attributes calculated from an accompanying 3-D seismic data set and with the strike of some of the headwalls of slides shown in multibeam bathymetry. However, the pattern of headwall fractures shown in the bathymetry is complicated and reticulate, and the NE–SW orientation is also well represented. We infer that the cause of the anisotropy is the presence of vertical to sub-vertical, fluid-filled fractures and micro-cracks, partially held open by high pore-fluid pressure. The fracture orientations are controlled primarily by the present-day gravitationally induced down-slope stress, which is mediated by the heterogeneous nature of sub-surface, causing local changes in the orientation of the principal stresses at the margins of incipient or failed slides. The fractures, if connected, are likely to increase vertical permeability within the sediment column significantly, and influence the distribution of gas hydrate within the strata.

Holocene relative sea levels and related prehistoric activity in the Forth lowland, Scotland, United Kingdom

Changes in relative sea level (RSL) and their effect on the distribution of human activity in the Forth lowland, Scotland, UK between ca 11,700 and ca 2000 calibrated years before present (BP) are examined. Block uplift of shorelines occurred over a longer period than previously thought, continuing to at least around 4700 BP. New graphs of RSL change, based upon an analysis of previous work and evidence from new sites using standard morphological and stratigraphical analyses, support a fall in RSL in the early Holocene during which buried estuarine surfaces were formed, followed by a marked reversal on a centennial, perhaps even decadal, scale sometime around 9900–9500 BP from a falling sequence to a rising one. There may have been a short-lived fluctuation in the rising RSL ca 8300 BP, and the Holocene Storegga Slide tsunami of ca 8100 BP was widespread in the lowland. The rising RSL culminated at the Main Postglacial Shoreline, reached ca 7800 BP, when Mean High Water Spring Tides (MHWS) lay within the range 14.6–16.5 m British Ordnance Datum (OD) in the western lowland. This is the highest published Holocene RSL in Britain and Ireland. The subsequent fall in RSL stabilised at a second shoreline, the Blairdrummond Shoreline, from which regression was occurring at ca 4700 BP. RSL later fell to a lower shoreline at ca 3900 BP before reaching present levels within the last three millennia. Comparison between the empirical evidence for RSL change in this area and glacial isostatic adjustment models indicates both similarities and differences. Records of archaeological sites and artefacts in the lowland and surrounding areas are discussed. Maps showing the evolution of the estuary are compared with the distribution of Mesolithic, Neolithic, Bronze Age and Iron Age activity. Evidence for Mesolithic people indicates the opportunistic use of available resources as RSL rose and the estuary expanded to its maximum extent. Neolithic people were also attracted to the estuary, building at least one platform from which to exploit resources and accumulating middens along the shoreline for over 2000 years. Bronze Age people were present across the emerging saltmarshes and mudflats before raised peat mosses, already extensive at the head of the lowland, developed more widely. In the Iron Age, the area could be accessed both between the peat mosses of the western lowland and across newly emergent land in the east as RSL continued to fall.

Holocene temperature and salinity variability of the Atlantic Water inflow to the Nordic seas

Piston core LINK14 from 346 m water depth on the eastern shelf of the Faroe Islands in the Faroe-Shetland Channel has been investigated in order to reconstruct temperature and salinity changes of the Atlantic surface Water during the Holocene. We have analyzed the distribution of benthic and planktic foraminifera, stable isotopes and calculated by transfer functions absolute surface, subsurface and bottom water temperatures and salinities. The summer sea surface temperature (SST) shows a stepwise decrease from an early-Holocene warm phase 10 300—8300 yr BP with temperatures around 12°C, over a mid-Holocene cooler phase 8300—4000 yr BP, to a late-Holocene relatively cool phase with SST around or slightly below 11°C. Bottom water temperatures at 346 m water depth show much less variation than the SST. The late-Holocene sea surface cooling was probably caused by increase in the influence of the polar East Icelandic Current. A distinct decrease in the subsurface salinity from about 8300 to 6800 yr BP is probably also due to a stronger East Icelandic Current. The benthic faunas indicate strong bottom current activity during the entire period. A single exceptional disturbance dated to approximately 8300 yr BP was probably caused by extraordinary high bottom current velocities. The event is marked by a significant coarsening of the sediments and a sorting by size of the foraminifera. The event is close in time to the 8.2-ka cooling event, but more likely correlates with the Storegga slide event.

Late Quaternary history of the Kap Mackenzie area, northeast Greenland

Wagner, B., Bennike, O., Cremer, H. & Klug, M. 2010: Late Quaternary history of the Kap Mackenzie area, northeast Greenland.Boreas, Vol. 39, pp. 492–504. 10.1111/j.1502‐3885.2010.00148.x. ISSN 0300‐9483.The Kap Mackenzie area on the outer coast of northeast Greenland was glaciated during the last glacial stage, and pre‐Holocene shell material was brought to the area. Dating of marine shells indicates that deglaciation occurred in the earliest Holocene, before 10 800 cal. a BP. The marine limit is around 53 m a.s.l. In the wake of the deglaciation, a glaciomarine fauna characterized the area, but afterc. one millennium a more species‐rich marine fauna took over. This fauna includedMytilus edulisandMysella sovaliki, which do not live in the region at present; the latter is new to the Holocene fauna of northeast Greenland. The oldestM. edulissample is dated toc. 9500 cal. a BP, which is the earliest date for the species from the region and indicates that the Holocene thermal maximum began earlier in the region than previously documented. This is supported by driftwood dated toc. 9650 cal. a BP, which is the earliest driftwood date so far from northeastern Greenland and implies that the coastal area was at least partly free of sea ice in summer. As indicated by former studies, the Storegga tsunami hit the Kap Mackenzie area atc. 8100 cal. a BP. Loon Lake, at 18 m a.s.l., was isolated from the sea atc. 6200 cal. a BP, which is distinctly later than expected from existing relative sea‐level curves for the region.

Sources of methane inferred from pore-water δ13C of dissolved inorganic carbon in Pockmark G11, offshore Mid-Norway

Pockmark G11 is the most spectacular one among the pockmarks located at the southern border of Vøring Plateau and 1–2 km away from the northern flank of the Storegga Slide, mid-Norwegian continental margin. For the first time, detailed pore-water geochemical studies were conducted to address methane hydrate occurrence, methane seepage and associated geochemical processes, and methane characteristics in the pockmark. Pore-waters collected from five sediments cores inside and one sediment core outside the pockmark, were analyzed for dissolved Cl−, sulfate (SO42−), total hydrogen sulfide (ΣH2S), Ca2+, Mg2+, dissolved inorganic carbon (DIC), δ13CDIC and δ18O. Methane hydrates were recovered in all sediments below 0.75m in a core inside pockmark G11, which is in good accordance with heavy oxygen isotope (1.9 to 2.3‰SMOW) and low Cl−concentrations (84.9 to 16.1mM) in pore-waters. Pore-water profiles indicate that upward-migrating methane fluids are spatially variable in the pockmark, with methane fluxes ranging from below detection in the center and outside, to 0.30–0.54molm−2 a−1 inside. In the cores with active methane fluxes, maximum DIC concentrations (19.4 to 21.5mM) and corresponding minimum δ13CDIC values (−52.3 to −54.6 ‰ PDB) occur within sulfate-methane-transition (SMT) zones from ∼0.40 to 0.50m below seafloor (mbsf), close to the seafloor. Simple mass balance modeling and δ13CDIC measurements within the SMT zones suggest that methane in shallow sediments within pockmark G11 is microbial in origin. Pore-water geochemistry and seabed observations suggest that methane fluxes inside pockmark G11 are episodic, and derived mostly from the recycling of methane hydrate at depth during sediment burial.

Active Archaeal Communities at Cold Seep Sediments Populated by Siboglinidae Tubeworms from the Storegga Slide

Siboglinid tubeworms in cold seep sediments can locally modify the geochemical gradients of electron acceptors and donors, hence creating potential microhabitats for prokaryotic populations. The archaeal communities associated with sediments populated by Oligobrachia haakonmosbiensis and Sclerolinum contortum Siboglinid tubeworms in the Storegga Slide were examined in this study. Vertical distribution of archaeal communities was investigated using denaturing gradient gel electrophoresis based on 16S rRNA genes. The active fraction of the archaeal community was assessed by using reverse-transcribed rRNA. Archaeal communities associated with sediments colonized by tubeworms were affiliated with uncultivated archaeal lineages of the Crenarchaeota and Euryarchaeota. The composition of the active archaeal populations changed with depth indicating a reorganization of microbial communities. 16S rRNA gene libraries were dominated by sequences affiliated to the Rice Cluster V which are unusual in marine sediment samples. Moreover, this study provides the first evidence of living Crenarchaeota of the Rice Cluster V in cold seep sediments. Furthermore, the Storegga Slide sediments harbored a high diversity of other minor groups of uncultivated lineages including Terrestrial Miscellaneous Euryarchaeotal Group, Marine Benthic Group (MBG)-D, MBG-E, Deep-Sea Hydrothermal Vent Euryarchaeotal Group, Lake Dagow Sediment, Val Kotinen Lake clade III, and Sippenauer Moor 1. Thus, we hypothesize that the vertical geochemical imprint created by the tubeworms could support broad active archaeal populations in the Siboglinidae-populated Storegga Slide sediments.

Norwegian margin outer shelf cracking: a consequence of climate-induced gas hydrate dissociation?

A series of en echelon cracks run nearly parallel to the outer shelf edge of the mid-Norwegian margin. The features can be followed in a ~60-km-long and ~5-km-wide zone in which up to 10-m-deep cracks developed in the seabed at 400–550 m water depth. The time of the seabed cracking has been dated to 7350 14C years BP (8180 cal years BP), which corresponds with the main Storegga Slide event (8100 ± 250 cal. years BP). Reflection seismic data suggest that the cracks do not appear to result from deep-seated faults, but it cannot be ruled out completely that tension crevices were created in relation to past movements on the headwall of the Storegga slide. The cracking zone corresponds well to the zone where the base of the hydrate stability zone (BHSZ) outcrops. Evidence of fluid release in the BHSZ outcrop zone comes from an extensive pockmark field. We suggest that post-glacial ocean warming triggered the dissociation of gas hydrates while the interplay between dissociation, overpressure, and sediment fracturing on the outer shelf remains to be understood.

Distributions and carbon isotopic compositions of lipid biomarkers in authigenic carbonate crusts from the Nordic margin (Norwegian Sea)

We investigated the distributions and δ 13C values of lipid biomarkers in authigenic carbonate crusts from the Nordic margin (Norwegian Sea). Abundant, highly 13C-depleted archaeal and bacterial biomarkers revealed that anaerobic oxidation of methane (AOM) is associated with carbonate authigenesis at the Håkon Mosby Mud Volcano (HMMV) and the Nyegga pockmark field. No conclusive evidence for AOM was found in the crust sample from the Storegga Slide. ANME-2 methanotrophic archaea and sulfate-reducing bacteria from the Desulfosarcina/Desulfococcus cluster were inferred from lipid fingerprints. Distinct patterns of 13C-depleted, non-isoprenoid dialkyl glycerol diethers (DAGEs) are possibly associated with unknown microbial partners in the AOM.

Holocene relative sea‐level changes in Harris, Outer Hebrides, Scotland, UK

AbstractEvidence for relative sea‐level changes during the middle and late Holocene is examined from two locations on the Atlantic coast of Harris, Outer Hebrides, Scotland, using morphological mapping and survey, stratigraphical, grain size and diatom analysis, and radiocarbon dating. The earliest event identified is a marine flood, which occurred after 7982–8348 cal. a (7370 ± 80 14C a) BP, when the sea crossed a threshold lying at −0.08 m Ordnance Datum Newlyn (OD) (−2.17 m mean high water springs (MHWS)) before withdrawing. This could have been due to a storm or to the Holocene Storegga Slide tsunami. By 6407–6122 cal. a (5500 ± 60 14C a) BP, relative sea levels had begun to fall from a sandflat surface with an indicated MHWS level of between 0.08 and −1.96 m (−2.01 to −4.05 m). This fall reached between −0.30 and −2.35 m (−2.39 to −4.44 m) after 5841–5050 cal. a (4760 ± 130 14C a) BP, but was succeeded by a relative sea‐level rise which reached between 0.54 and −1.57 m (−1.55 to −3.66 m) by 5450–4861 cal. a (4500 ± 100 14C a) BP. This rise continued, possibly with an interruption, until a second sandflat surface was reached between 2.34 and −0.26 m (0.25 to −2.35 m) between 2952–3375 cal. a (3000 ± 80 14C a) and 1948–2325 cal. a (2130 ± 70 14C a) BP, before present levels were reached. The regressive episode from the earliest sandflat is correlated with the abandonment of the Main Postglacial Shoreline. It is maintained that the fluctuations in relative sea level recorded can be correlated with similar events elsewhere on the periphery of the glacio‐isostatic centre and may therefore reflect secular changes in nearshore sea surface levels. Despite published evidence from trim lines of differential ice sheet loading across the area, no evidence of variations in uplift between the locations concerned could be found. Copyright © 2009 John Wiley & Sons, Ltd.

Fluid distributions inferred from P-wave velocity and reflection seismic amplitude anomalies beneath the Nyegga pockmark field of the mid-Norwegian margin

Travel-time inversion of wide-angle ocean-bottom seismic (OBS) data results in detailed P-wave velocity models of the shallow sub-seabed beneath the Nyegga pockmark field. The area lies on the northern flank of the Storegga Slide on the mid-Norwegian margin. Velocity anomalies indicate two low P-wave velocity zones (LVZs) providing evidence for the presence of gas-rich fluids in the subsurface at Nyegga. Integrating the velocity results with 2D and 3D reflection seismic data demonstrates that LVZs coincide with zones of high-amplitude reflections that allow mapping the extent of the fluids in the subsurface. The upper fluid accumulation zone corresponds to a velocity inversion of ∼250 m/s and occurs at a depth of about 250 mbsf. The lateral extent is documented in two distinct areas. The westward area is up to 40 m thick where gas-rich fluids beneath a bottom-simulating reflection indicate that fluids may be trapped by gas hydrates. The eastward zone is up to 60 m thick and comprises a contourite deposit infilling a paleo-slide scar. On top, glacigenic debris flow deposits provide a locally effective seal for fluids. The second velocity inversion of ∼260 m/s extends laterally at about 450 mbsf with decreasing thickness in westward direction. Based on effective-medium theory the gas saturation of pore space in both layers is estimated to be between 0.5 and <1% assuming a homogeneous distribution of gas. Fluids probably originate from deeper strata approximately at the location of the top of the Helland-Hansen Arch. Fluids migrate into the second LVZ and are distributed laterally. Fluids migrate into shallower strata or are expelled at the seabed through the formation of vertical fluid migration features (VFMFs), so-called chimneys. The distribution of the chimneys is clearly linked to the two fluid accumulation zones in the subsurface. A conceptual model draws on the major controlling factors for fluid migrations at specific locations within Nyegga. Fluid migrations vary according to their actual position with respect to the prograding Plio–Pleistocene sedimentary wedge.

Hazard assessment of underwater landslide-generated tsunamis: a case study in the Padang region, Indonesia

Submarine landslides can generate local tsunamis with high run-ups, posing a hazard to human lives and coastal facilities. Both ancient (giant Storegga slide off Norwegian coast, 8200 B. P.) and recent (Papua New Guinea, 1998) events show high potential danger of tsunamigenic landslides and the importance of mitigation efforts. This contribution presents newly discovered landslides 70 km off Padang (Western Sumatra, Indonesia) based on recent bathymetry measurements. This highly populated city with over 750,000 inhabitants exhibits high tsunami vulnerability due to its very low elevation. We model tsunamis that might have been induced by the detected landslide events. Estimations of run-up heights extrapolated from offshore tsunami amplitudes for Padang and other locations in the northern Mentawai fore-arc basin yield maximum values of about 3 m. We also provide a systematic parametric study of landslide-induced tsunamis, which allows us to distinguish potentially dangerous scenarios for Padang. Inside the fore-arc basin, scenarios involving volumes of 0.5–25 km³ could endanger Padang. Apart from slide volume, the hazard distribution mainly depends on three landslide parameters: distance to Padang, water depth in the generation region, and slide direction.

A subsurface evacuation model for submarine slope failure

ABSTRACTAnalysis of three‐dimensional (3D) seismic reflection data from the Norwegian continental margin provides an insight into an unusual, buried submarine slope failure, which occurred adjacent to the later Holocene‐age Storegga Slide. The identified failure, informally named the ‘South Vøring Slide’ (SVS), occurs in fine‐grained hemipelagic and contourite sediments on a slope of 0.5°, and is characterised by a deformed seismic facies unit consisting of closely spaced pyramidal blocks and ridges bound by small normal faults striking perpendicular to the slope. The SVS contrasts with other previously described submarine slope failures in that it cannot be explained by a retrogressive model. The defining characteristic is the high relative volume loss. The area affected by sliding has thinned by some 40%, seen in combination with very modest extension in the translation direction, with line length balancing yielding an extension value of only 4.5%. The volume loss is explained by the mobilisation of an approximately 40 m thick interval at the lower part of the unit and its removal from beneath a thin overburden, which subsequently underwent extensional fragmentation. Evidence for the mobilisation of a thick fine‐grained interval in the development of a submarine slope failure from a continental margin setting may have implications for the origins of other large‐scale slope failures on the Norwegian margin and other glacially influenced margins worldwide.

Effects of rapid sedimentation on developing the Nyegga pockmark field: Constraints from hydrological modeling and 3‐D seismic data, offshore mid‐Norway

Three‐dimensional (3‐D) seismic data in the Nyegga region expose hundreds of seafloor pockmarks and fluid escape chimneys in close proximity to the northern sidewall of the Storegga Slide, on the formerly glaciated Norwegian margin. As the up to 350 m wide individual craters of the Nyegga pockmark field postdate the Late Glacial Maximum (∼25 calendar (cal) ka B.P.) and because modern fluid flux is characterized as microseepage at most, it appears that the pockmarks developed nearly simultaneously with some specific event. External types of forcing such as earthquake loading, the Storegga Slide (8.1 cal ka B.P.), or rapid sediment loading represent potential trigger mechanisms for developing hundreds of pockmarks within a millennial time window. We integrate the 3‐D seismic observations with a two‐dimensional (2‐D) sedimentation–fluid flow model to quantify the effects of sediment loading and erosion on overpressure generation and fluid expulsion. The models simulate the temporal evolution of compaction‐driven overpressure and fluid expulsion based on sedimentation rates estimated from seismic data and hydrologic and sediment properties obtained from laboratory experiments and petrophysical data. The 2‐D model predicts rapid overpressure generation in response to high sedimentation rates during the last (Weichselian) deglaciation period (25–18 cal ka B.P.). The high pressures significantly increase fluid expulsion, which peaks between 19 and 16 cal ka B.P. The modeled high‐flux period and the following declining flux period exhibit an excellent correlation with existing age dates of methane‐derived authigenic carbonate retrieved from Nyegga pockmarks. Accordingly, rapid sediment loading provides a mechanism to drive extensive cold seep formation. Our model suggests that the Nyegga pockmark field developed between 19 and 16 cal ka B.P. and therefore predates the giant Storegga Slide event.