Showing papers by "Jürgen Mienert published in 2005"

Rethinking Late Weichselian ice-sheet dynamics in coastal NW Svalbard

Abstract: New marine geological evidence provides a better understanding of ice-sheet dynamics along the western margin of the last Svalbard/Barents Sea Ice Sheet. A suite of glacial sediments in the Kongsfjordrenna cross-shelf trough can be traced southwards to the shelf west of Prins Karls Forland. A prominent moraine system on the shelf shows minimum Late Weichselian ice extent, indicating that glacial ice also covered the coastal lowlands of northwest Svalbard. Our results suggest that the cross-shelf trough was filled by a fast-flowing ice stream, with sharp boundaries to dynamically less active ice on the adjacent shelves and strandflats. The latter glacial mode favoured the preservation of older geological records adjacent to the main pathway of the Kongsfjorden glacial system. We suggest that the same model may apply to the Late Weichselian glacier drainage along other fjords of northwest Svalbard, as well as the western margin of the Barents Ice Sheet. Such differences in glacier regime may explain the apparent contradictions between the marine and land geological record, and may also serve as a model for glaciation dynamics in other fjord regions.

Fluid flow impact on slope failure from 3D seismic data: a case study in the Storegga Slide

Abstract: Analysis of three-dimensional (3D) seismic data from the headwall area of the Storegga Slide on the mid-Norwegian margin provides new insights into buried mass movements and their failure mechanisms. These mass movements are located above the Ormen Lange dome, a Tertiary dome structure, which hosts a large gas reservoir. Slope instabilities occurred as early as the start of the Plio-Pleistocene glacial–interglacial cycles. The 3D seismic data provide geophysical evidence for gas that leaks from the reservoir and migrates upward into the shallow geosphere. Sediments with increased gas content might have liquefied during mobilization of the sliding and show different flow mechanisms than sediments containing less gas. In areas where there is no evidence for gas, the sediments remained intact. This stability is inherited by overlying strata. The distribution of gas in the shallow subsurface (<600 m) may explain the shape of the lower Storegga headwall in the Ormen Lange area.

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

Abstract: 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.

Bottom-simulating reflections and geothermal gradients across the western Svalbard margin

Abstract: Seismic reflection data reveal prominent bottom-simulating reflections (BSRs) within the relatively young (<0.78 Ma) sediments along the West Svalbard continental margin. The potential hydrate occurrence zone covers an area of c. 1600 km2. The hydrate accumulation zone is bound by structural/tectonic features (Knipovich Ridge, Molloy Transform Fault, Vestnesa Ridge) and the presence of glacigenic debris lobes inhibiting hydrate formation upslope. The thickness of the gas-zone underneath the BSR varies laterally, and reaches a maximum of c. 150 ms. Using the BSR as an in-situ temperature proxy, geothermal gradients increase gradually from 70 to 115 °C km−1 towards the Molloy Transform Fault. Anomalies only occur in the immediate vicinity of normal faults, where the BSR shoals, indicating near-vertical heat/fluid flow within the fault zones. Amplitude analyses suggest that sub-horizontal fluid migration also takes place along the stratigraphy. As the faults are related to the northwards propagation of the Knipovich Ridge, long-term disturbance of hydrate stability appears related to incipient rifting processes.

Postglacial carbonate production by cold-water corals on the Norwegian Shelf and their role in the global carbonate budget

Abstract: Cold-water coral reefs have been neglected in calculations of global carbonate production. We present the first calculations of the amount of CaCO 3 produced by cold-water corals on the Norwegian Shelf in postglacial time, showing that the reef-building Lophelia pertusa in Norwegian waters contributes ∼54–188 g/m 2 / yr locally, or 0.03–0.38 g/m 2 /yr averaged over the entire shelf. These estimates indicate that cold-water reef CaCO 3 flux is 4%– 12% of the tropical reefs. A first tentative global estimate indicates that cold-water coral CaCO 3 production could add >1% to the total marine CaCO 3 production. The spatial and temporal evolution of cold-water reefs, and thus their influence on atmospheric CO 2 , is difficult to reconstruct, but factors that dominantly influence the rate of coral growth include sea-level and oceanographic changes, submarine slides, and anthropogenic activity. The results show that other shelf and slope environments should be assessed in a similar manner to further constrain and evaluate the global CaCO 3 production by cold-water corals and their possible contribution to the CaCO 3 budget.

Arctic gas hydrate provinces along the western Svalbard continental margin

Abstract: In 2001, a high-resolution seismic survey was conducted for the detailed study of the distribution, both spatially and vertically, of gas hydrate and free gas accumulations west of Svalbard, as part of the HYDRATECH and INGGAS projects. High-resolution single-channel seismic reflection and the 4-component ocean-bottom seismometer (OBS) data illustrate the widespread nature of gas hydrates and free gas accumulations north of the Knipovich Ridge off Western Svalbard, by the presence of a nearly-continuous polarity-reversed bottom-simulating reflection (BSR) on down-slope seismic profiles. In the absence of a distinct and/or a continuous BSR, it is the sudden change in reflection amplitude and frequency content that marks the base of the hydrate zone. The BSR coincides with the top of the free gas zone. Compressional wave velocity analyses and modelling reveal increased velocities above the BSR attributed to a gradual increase of partial hydrate saturation (6–10% of pore volume). A sharp drop in compressional-wave velocity across the BSR is due to free gas accumulation. The sub-bottom depth of the BSR closely matches the calculated stability limit for methane hydrates. To the east of the Knipovich Ridge, mud diapirism is observed in a deeper basin (∼2250 m water depths). The domes rise from an extensive chaotic source zone buried under a 200–400 ms thick sediment drape, and are more pronounced in the south. At some places, there is evidence of stratigraphically-controlled shallow gas accumulations (bright spots) and short cross-cutting BSR-like features that might point towards the presence of hydrate and/or free gas. The diapiric movement is believed to be a recent and still ongoing process of mass mobilisation. In both the cases, the nearby and tectonically-active slow-spreading, Knipovich Ridge is assumed to play an important role in the generation of elevated heat and methane fluxes as well as faulting and subsequent fluid migration. As a result, shallow subsurface hydrates (

Gas hydrate dissociation and seafloor collapse in the wake of the Storegga Slide, Norway

Abstract: Two-dimensional seismic data from the Mid-Norwegian margin provide evidence for sediment liquefaction and fluid mobilisation within the sediments that were located at the base of the hydrate stability zone before the Storegga Slide occurred. The disturbed subsurface sediments are overlain by a prominent roll-over structure and sea-floor collapse. This indicates fluid escape from the formerly hydrated sediment and suggests that the landslide caused a pressure drop strong enough to dissociate, the gas hydrates. We calculate that this fluid escape must have taken place within less than 250 years after the slide, as the effect of pressure decrease on hydrate stability was later compensated by a temperature decrease, related to the slumping process. The volume of expelled fluids from the collapse structure exceeds the volume of the gas hydrate dissociation products, implying that gas hydrate dissociation significantly affected the surrounding sediments.

Sedimentological and geochemical environment of the Fugløy Reef off northern Norway

Abstract: A number of reefs are found along the coast of northern Norway, and a cluster of particularly high reefs off Troms County at 70°N are known collectively as the Fugloy Reef. The reefs, up to 40 m high and more than 200 m wide, consist mainly of the reef-building Lophelia pertusa. Most of the reefs identified in the study area are located on moraine ridges in water depths of 140–190 m, and in water masses dominated by the relatively warm and saline Norwegian Current. Several of the reefs are located on the flanks of channels incising the moraine highs, where currents are tidally dominated and periodically reach velocities of 30 cm/s. Gravity cores were acquired from the reefs and their surroundings, and thorough analyses of the sampled sediments provide valuable information about three (paleo-) sedimentary environments surrounding the reefs. The immediate vicinity of the reefs consists of coarse and unsorted deposits that are interpreted to be moraine material deposited by the retreating inland ice. Elevated current velocities have prevented fine sediments from settling since the ice retreated. The second province is a pockmarked basin at water depths down to ∼300 m. Gravity cores from the basin reveal silty sand deposits of more than 4 m thickness representing postglacial sedimentation in the area. Gas analyses reveal that the hydrocarbons found in the sediments clearly are of biogenic origin, although it is somewhat enigmatic whether biogenic gas is the sole driving force behind the pockmarks in the area. No direct link between the reefs and the pockmarks is found. The third sedimentary province is characterised by resedimentation of coral debris, clearly illustrated by sorted deposits and U/Th-datings from the allochthonous deposits. Remobilisation of coral debris is modest in areal extent, but an important mechanism linked to the occurrence of the coral reefs.

ESONET: toward an European network of excellence on subsea observatories

Abstract: ESONET proposes a network of sea floor observatories around the European Ocean Margin from the Arctic Ocean to the Black Sea for strategic long term monitoring as part of a GMES with capability in geophysics, geotechnics, chemistry, biochemistry, oceanography, biology and fisheries. Long-term data collection and alarm capability in the event of hazards (e.g. earthquakes) will be considered. ESONET will be developed from networks in key areas where there is industrial sea floor infrastructure, scientific/conservation significance (e.g. coral mounds) or sites suitable for technology trials (e.g. deep water close to land).

Combining Seismic Stratigraphy, Multi Attribute Analysis And Rock Physical Modeling to Identify Carbonate Facies And Karst Along an Upper Palaeozoic Carbonate Platform, Barents Sea.

Abstract: A change from ramp geometry to shelf-slope geometry with prograding wedges (Colpaert et al., submitted), major karstification and sink-holes (Hunt et al., 2003) and largescale polygonal buildups (Elvebak et al., 2002) are characteristic features with a reservoir potential. They are detected from 3D-seismics in the Upper Palaeozoic carbonate interval. Well information is very rare or nonexistent and only in 2005 sink-holes have been penetrated first by an exploration well. No reserves were found but hydrocarbon traces are shown. Polygonal buildups and prograding wedges are so far only defined from seismics or outcrops on land.