Description: Deepwater landslides are often underestimated as potential tsunami triggers. The North Gorringe avalanche (NGA) is a large (∼80 km3 and 35 km runout) newly discovered and deepwater (2900 m to 5100 m depth) mass failure located at the northern flank of Gorringe Bank on the southwest Iberian margin. Steep slopes and pervasive fracturing are suggested as the main preconditioning factors for the NGA, while an earthquake is the most likely trigger mechanism. Near-field tsunami simulations show that a mass failure similar to the NGA could generate a wave 〉15 m high that would hit the south Portuguese coasts in ∼30 min. This suggests that deepwater landslides require more attention in geo-hazard assessment models of southern Europe, as well as, at a global scale, in seismically active margins.

Description: Landslides associated with flank collapse are volumetrically the most significant sediment transport process around volcanic islands. Around Montserrat, in the Lesser Antilles, individual landslide deposits have volumes (1 to 20 km3) that are up to two orders of magnitude larger than recent volcanic dome collapses (up to 0.2 km3). The largest landslide deposits were emplaced in at least two stages, initiated by the emplacement of volcanic debris avalanches which then triggered larger-scale failure of seafloor sediment, with deformation propagating progressively downslope for up to 30 km on gradients of 〈 1°. An unusually detailed seismic, side-scan sonar and bathymetric dataset shows that the largest landslide off Montserrat (forming Deposit 8) incorporated ~ 70 m of in-situ sediment stratigraphy, and comprises ~ 80% seafloor sediment by volume. Well-preserved internal bedding and a lack of shortening at the frontally-confined toe of the landslide, shows that sediment failure involved only limited downslope transport. We discuss a range of models for progressively-driven failure of in-situ bedded seafloor sediment. For Deposit 8 and for comparable deposits elsewhere in the Lesser Antilles, we suggest that failure was driven by an over-running surface load that generated excess pore pressures in a weak and deforming undrained package of underlying stratigraphy. A propagating basal shear rupture may have also enhanced the downslope extent of sediment failure. Extensive seafloor-sediment failure may commonly follow debris avalanche emplacement around volcanic islands if the avalanche is emplaced onto a fine-grained parallel-bedded substrate. The timing of landslides off Montserrat is clustered, and associated with the deposition of thick submarine pyroclastic fans. These episodes of enhanced marine volcaniclastic input are separated by relatively quiescent periods of several 100 ka, and correspond to periods of volcanic edifice maturity when destructive processes dominate over constructive processes. Highlights: ► Marine volcanic debris avalanche emplacement can lead to much larger sediment failure. ► Failure is progressive, through in situ-strata, and frontally non-emergent. ► Sediment failure propagates on very low gradients, dominating final deposit volume. ► Process involves undrained loading and/or shear rupture, and may be repeated widely. ► Landslide timing reflects timescales of volcanic edifice growth and destruction

Description: Ice streams are the fast-flowing zones of ice sheets that can discharge a large flux of ice. The glaciated western Svalbard margin consists of several cross-shelf troughs which are the former ice stream drainage pathways during the Pliocene–Pleistocene glaciations. From an integrated analysis of high-resolution multibeam swath-bathymetric data and several high-resolution two-dimensional reflection seismic profiles across the western and northwestern Svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin. Our results show that the northwestern margin of Svalbard experienced a switching of a major ice stream. Based on correlation with the regional seismic stratigraphy as well as the results from ODP 911 on Yermak Plateau and ODP 986 farther south on the western margin of Spitsbergen, off Van Mijenfjord, we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling (starting ∼2.8–2.6 Ma). A switch in ice stream flow direction to the present-day Kongsfjorden cross-shelf trough took place during a glaciation at ∼1.5 Ma or probably later during an intensive major glaciation phase known as the ‘Mid-Pleistocene Revolution’ starting at ∼1.0 Ma. The seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the Svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier. Highlights ► Reflection seismic data reveal two glacial fans at northwest Svalbard margin. ► The fans are result of ice stream activities during Pliocene–Pleistocene glaciations. ► Based on seismic and bathymetric data we find the flow directions of the ice streams. ► We find a switch in ice stream flow direction. ► The switch resulted as the ice sheet became thick and overcame a topographic barrier.

Description: High-resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw〉6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW-verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along-strike structural variability as well as fault trend suggests that the HF is composed by three main sub-segments: North (N25), Central (N50) and South (N45). Swath-bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20º) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre-stack depth-migrated multichannel seismic reflection profiles across the HF and neighboring areas allowed us to constrain their seismo-stratigraphy, structural geometry, tectono-sedimentary evolution from Upper Jurassic to present-day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst-case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behavior and near-shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.

Description: Spreading is a type of mass movement where a sediment unit is extended and broken up into coherent blocks that are displaced and tilted along a planar slip. High-resolution seafloor data demonstrate that spreading is a common style of submarine mass movement. Submarine spreading is clearly exemplified in the Storegga Slide, Norwegian margin (Fig. 1a, b). The slide occurred 8100 + 250 cal a BP as a retrogressive slope failure (Haflidason et al. 2005). It is one of the largest known submarine slides and the site of repeated sliding activity. Failures on the Norwegian margin are linked strongly to the growth and retreat of the Fennoscandian ice sheets, in particular to the alternating deposition of glacigenic debrites and basal and deformation tills during glacial maxima (e.g. O1–O2 30–15 ka and O4–O7 200–130 ka sub-units of the Naust Formation), and of fine-grained glacimarine, hemipelagic and contouritic sediments during interglacials (e.g. O3 130–30 ka sub-unit of the Naust Formation). The Naust sub-units are described in full in Berg et al. (2005). Differences in the geotechnical properties of these sediments, coupled with seismicity, rapid sediment deposition, associated high pore pressures and the regional topography and structural setting, are responsible for .20 slope failures across the region during the Quaternary (Solheim et al. 2005).

Description: Recent seafloor mapping around volcanic islands shows that submarine landslide deposits are common and widespread. Such landslides may cause devastating tsunamis, but accurate assessment of tsunami hazard relies on understanding failure processes and sources. Here we use high-resolution geophysical data offshore from Montserrat, in the Lesser Antilles, to show that landslides around volcanic islands may involve two fundamentally different sources of sediment (island-flank and larger seafloor-sediment failures), and can occur in multiple stages. A combination of these processes produces elongate deposits, with a blocky centre (associated with island-flank collapse), surrounded by a smoother-surfaced deposit that is dominated by failed seafloor sediment. The failure of seafloor sediment is associated with little marginal accumulation, and involves only limited downslope motion. Submarine landslide deposits with similar blocky and smooth-surfaced associations are observed in several locations worldwide, but the complex emplacement processes implied by this morphological relationship can only be revealed by high-resolution geophysical data. Such complexity shows that the volume of landslide deposits offshore of volcanic islands cannot simply be used in tsunami models to reflect a single-stage collapse of primary volcanic material. By applying predictive equations for tsunami amplitude to investigate general scenarios of volcanic island landslide generation, we show that the tsunami hazard associated with volcanic island collapse remains highly significant. Volcanic flank failures, even if relatively small, may generate large local tsunamis, but associated seafloor sediment failures, even if they have a much greater volume, have a substantially lower potential for tsunami generation.

Description: Deepwater landslides are often underestimated as potential tsunami triggers. The North Gorringe avalanche (NGA) is a large (~80 km 3 and 35 km runout) newly discovered and deepwater (2900 m to 5100 m depth) mass failure located at the northern flank of Gorringe Bank on the southwest Iberian margin. Steep slopes and pervasive fracturing are suggested as the main preconditioning factors for the NGA, while an earthquake is the most likely trigger mechanism. Near-field tsunami simulations show that a mass failure similar to the NGA could generate a wave 〉15 m high that would hit the south Portuguese coasts in ~30 min. This suggests that deepwater landslides require more attention in geo-hazard assessment models of southern Europe, as well as, at a global scale, in seismically active margins.

Description: Objective— Liver X receptors (LXRs) are oxysterol-activated nuclear receptors that are highly expressed in macrophages and regulate lipid homeostasis and inflammation. Among putative LXR target genes, lysophosphatidylcholine acyltransferase 3 (LPCAT3) involved in the Lands cycle controls the fatty acid composition at the sn-2 position of glycerophospholipids and, therefore, the availability of fatty acids, such as arachidonic acid (AA), used for eicosanoid synthesis. The aim of our study was to determine whether LXRs could regulate the Lands cycle in human macrophages, to assess the consequences in terms of lipid composition and inflammatory response, and to work out the relative contribution of LPCAT3 to the observed changes. Approach and Results— Transcriptomic analysis revealed that LPCAT3 was upregulated by LXR agonists in human macrophages. Accordingly, LXR stimulation significantly increased lysophospholipid acyltransferase activity catalyzed by LPCAT3. Lipidomic analysis demonstrated that LXR activation increased the AA content in the polar lipid fraction, specifically in phosphatidylcholines. The LXR-mediated effects on AA distribution were abolished by LPCAT3 silencing, and a redistribution of AA toward the neutral lipid fraction was observed in this context. Finally, we observed that preconditioning of human macrophages by LXR agonist treatment increased the release of arachidonate-derived eicosanoids, such as prostaglandin E 2 and thromboxane after lipopolysaccharide stimulation, with a significant attenuation by LPCAT3 silencing. Conclusions— Altogether, our data demonstrate that the LXR-mediated induction of LPCAT3 primes human macrophages for subsequent eicosanoid secretion by increasing the pool of AA, which can be mobilized from phospholipids.