New insights into landslide processes around volcanic islands from Remotely Operated Vehicle (ROV) observations offshore Montserrat

Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition, and emplacement are hindered by the challenges of investigating deposits directly. Here we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment core and geophysical data. These complementary approaches enable a more comprehensive view of large-scale mass-wasting processes around island-arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5–14 ka (Deposit 1; 1.7 km3) and formed a radially spreading hummocky deposit that is morphologically similar to many subaerial debris-avalanche deposits. Hummocks comprise angular lava and hydrothermally altered fragments, implying a deep-seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km3) occurred at ∼130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris-avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris

Geomorphology and sedimentary features in the Central Portuguese submarine canyons, western Iberian margin

The Central Portuguese submarine canyons (Nazaré, Cascais and Setúbal–Lisbon canyons) dissect the Western Iberian margin in an east–west direction from the continental shelf, at water depths shallower than 50 m, down to the Tagus and Iberian abyssal plains, at water depths exceeding 5000 m. We present an analysis of the geomorphology of the canyons and of the sedimentary processes that can be inferred from the observed morphology of the three canyons, based on a compilation of swath bathymetry data and TOBI deep-towed side-scan sonar imagery. This first complete detailed mapping of the Central Portuguese canyons reveals substantial differences in their morphologies and downslope evolution. The canyons are divided into three sections: 1) canyon head and upper reach, 2) middle canyon, and 3) canyon mouth and distal part. The canyon heads and upper reaches are severely indented into the continental shelf, and they are characterised, in the Nazaré and Setúbal–Lisbon canyons, by sinuous V-shaped valleys entrenched within high canyon walls occupied by rock outcrops dissected by gullies. The Cascais upper canyon is complex, with multiple branches with high axial gradients and signs of mass wasting. Middle canyon sections, indented in the slope, display axial incisions with perched, stacked terraces, and are affected by debris avalanches originating from the canyon walls. At the base of slope, the distal Cascais and Setúbal–Lisbon canyons show many characteristics of channel-lobe transition zones: erosional features such as isolated to amalgamated chevron scours, and depositional bedforms such as mud to gravel waves. Pervasive scouring occurs up to 95 km beyond the canyon mouths. By contrast, the Nazaré canyon opens into a 27 km wide and 94 km long channel, whose flat-bottomed thalweg is occupied by sediment waves, irregular, comet-shaped and crescentic scours, and a second-order channel. Transverse, kilometre-scale sediment waves occupy the overbank area of the southern channel margin. The present morphology of the Central Portuguese canyons is the result of erosive processes, subsequent sediment transport and deposition, and sediment instability, whereas inherited tectonic fabric controls their location. Morphological differences between the canyons are explained by the main mechanisms driving their activity. Overall, these morphological features suggest that these canyons have acted as an efficient conduit of sediment to the deep basin, transporting large quantities of material to the deep sea during high-energy events

Emplacement of pyroclastic deposits offshore Montserrat: Insights from 3D seismic data

During the current (1995–present) eruptive phase of the Soufrière Hills volcano on Montserrat, voluminous pyroclastic flows entered the sea off the eastern flank of the island, resulting in the deposition of well-defined submarine pyroclastic lobes. Previously reported bathymetric surveys documented the sequential construction of these deposits, but could not image their internal structure, the morphology or extent of their base, or interaction with the underlying sediments. We show, by combining these bathymetric data with new high-resolution three dimensional (3D) seismic data, that the sequence of previously detected pyroclastic deposits from different phases of the ongoing eruptive activity is still well preserved. A detailed interpretation of the 3D seismic data reveals the absence of significant (> 3 m) basal erosion in the distal extent of submarine pyroclastic deposits. We also identify a previously unrecognized seismic unit directly beneath the stack of recent lobes. We propose three hypotheses for the origin of this seismic unit, but prefer an interpretation that the deposit is the result of the subaerial flank collapse that formed the English's Crater scarp on the Soufrière Hills volcano. The 1995–recent volcanic activity on Montserrat accounts for a significant portion of the sediments on the southeast slope of Montserrat, in places forming deposits that are more than 60 m thick, which implies that the potential for pyroclastic flows to build volcanic island edifices is significant

Mingulay Reef Complex : an interdisciplinary study of cold-water coral habitat, hydrography and biodiversity

The Mingulay reef complex in the Sea of the Hebrides west of Scotland was first mapped in 2003 with a further survey in 2006 revealing previously unknown live coral reef areas at 120 to 190 m depth. Habitat mapping confirmed that distinctive mounded bathymetry was formed by reefs of Lophelia pertusa with surficial coral debris dating to almost 4000 yr. Benthic lander and mooring deployments revealed 2 dominant food supply mechanisms to the reefs: a regular rapid downwelling of surface water delivering pulses of warm fluorescent water, and periodic advection of high turbidity bottom waters. Closed chamber respirometry studies suggest that L. pertusa responds to seawater warming, such as that seen during the rapid downwelling events, with increases in metabolic rate. Lipid biomarker analysis implies that corals at Mingulay feed predominantly on herbivorous calanoid copepods. Integrating geophysical and hydrographical survey data allowed us to quantify the roles of these environmental factors in controlling biodiversity of attached epifaunal species across the reefs. Longitudinal structuring of these communities is striking: species richness (α) and turnover (β) change significantly west to east, with variation in community composition largely explained by bathymetric variables that are spatially structured on the reef complex. Vibro-cores through the reef mounds show abundant coral debris with significant hiatuses. High resolution side-scan sonar revealed trawl marks in areas south of the coral reefs where vessel monitoring system data showed the highest density of local fishing activity. The interdisciplinary approach in this study allowed us to record the food supply and hydrographic environment experienced by L. pertusa and determine how it may be ecophysiologically adapted to these conditions. Improved basic understanding of cold-water coral biology and biodiversity alongside efforts to map and date these long-lived habitats are vital to development of future conservation policies

Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles

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

Insights into the emplacement dynamics of volcanic landslides from high-resolution 3D seismic data acquired offshore Montserrat, Lesser Antilles

We present results from the first three-dimensional (3D) marine seismic dataset ever collected over volcanic landslide deposits, acquired offshore of the Soufrie?re Hills volcano on the island of Montserrat in the Lesser Antilles. The 3D data enable detailed analysis of various features in and around these mass wasting deposits, such as surface deformation fabrics, the distribution and size of transported blocks, change of emplacement direction and erosion into seafloor strata. Deformational features preserved on the surface of the most recent debris avalanche deposit (Deposit 1) reveal evidence for spatially-variant deceleration as the mass failure came to rest on the seafloor. Block distributions suggest that the failure spread out very rapidly, with no tendency to develop longitudinal ridges. An older volcanic flank collapse deposit (Deposit 2) appears to be intrinsically related to large-scale secondary failure of seafloor sediments. We observe pronounced erosion directly down-slope of a prominent headwall, where translational sliding of well-stratified sediments was initiated. Deep-reaching faults controlled the form and location of the headwall, and stratigraphic relationships suggest that sliding was concurrent with volcanic flank collapse emplacement. We also identified a very different mass wasting unit between Deposit 1 and Deposit 2 that was likely emplaced as a series of particle-laden mass flows derived from pyroclastic flows, much like the recent (since 1995) phase of deposition offshore Montserrat but at a much larger scale. This study highlights the power of 3D seismic data in understanding landslide emplacement processes offshore of volcanic islands

Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard

Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites

Combinations of volcanic-flank and seafloor-sediment failure offshore Montserrat, and their implications for tsunami generation

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 smoothsurfaced 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