ALBERT

Description: [1] The present geological setting west of Svalbard closely parallels the situation off mid-Norway after the last glaciation, when crustal unloading by melting of ice induced very large earthquakes. Today, on the modern Svalbard margin, increasing bottom water temperatures are destabilizing marine gas hydrates, which are held in continental margin sediments consisting of interlayered contourite deposits and glacigenic debris flows. Both unloading earthquakes and hydrate failure have been identified as key factors causing several megalandslides off Norway during early Holocene deglaciation. The most prominent event was the Storegga Slide 8200 years B.P. which caused a tsunami up to 23 m high on the Faroe and Shetland islands. Here we show by numerical tsunami modeling that a smaller submarine landslide west of Svalbard, 100 m high and 130 km wide, would cause a tsunami capable of reaching northwest Europe and threatening coastal areas. A tsunami warning system based on tiltmeters would give a warning time of 1–4 h.

Description: Spreading is a common type of ground failure in subaerial environments. However, this type of mass movement has hardly been documented in submarine settings. In this paper we show that spreading covers at least 25% of the Storegga Slide scar area, a giant submarine slide located offshore mid-Norway. The morphological signature of spreading is a repetitive pattern of ridges and troughs oriented perpendicular to the direction of movement. Two modes of failure can be identified: retrogressive failure of the headwall and slab failure and extension, both involving the breakup of a sediment unit into coherent blocks. These blocks are displaced downslope along planar slip surfaces. Limit equilibrium modeling indicates that loss of support and seismic loading are the main potential triggering mechanisms. The extent of displacement of the spreading sediment is controlled by gravitationally induced stress, angle of internal friction of the sediment, pore pressure escape, and friction. The resulting block movement pattern entails an exponential increase of displacement and thinning of the failing sediment with distance downslope. Sediment properties explain the remaining spatial variation of ridge and trough morphologies associated with spreading.

Description: In comparison to subaerial and planetary landscapes, submarine environments are rarely investigated using quantitative geomorphological techniques. Application of traditional geomorphometric techniques is hindered by the spatial variability in bathymetric data resolution and the extensive scale over which changes in topography occur. We propose a novel methodology for the improved quantitative analysis of submarine elevation data by adapting numerical techniques, developed for subaerial analyses, to submarine environments. The method integrates three main morphometric techniques: (1) morphometric attributes and their statistical analyses, (2) feature-based quantitative representation, and (3) automated topographic classification. These techniques allow useful morphological information to be extracted from a digital elevation model. Morphometric attributes and their statistical analyses provide summary information about an area, which can be used to calibrate computer-generated geomorphometric maps. In these maps the boundaries of geomorphological features are delineated, and they can thus be used as the basis for geomorphological interpretation. Ridge patterns and their morphological characteristics provide an accurate representation of specific aspects of terrain variability. Moment statistics are used as proxies of surface roughness to differentiate between surface types. Unsupervised classification, carried out using ridge characteristics and moment statistics, reliably segments the surface into units of homogeneous topography. A case study of debris flow lobes within the Storegga Slide shows that the techniques work robustly and that the new methodology integrating all the techniques can significantly enhance submarine geomorphological investigations.

Description: More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150– 400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1° C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic contine ntal margins, tens of Teragrams of methane per year could be released into the ocean.