High-resolution Multibeam Bathymetry Research Articles

Characterization of overbanking features on the lower reach of the Gioia-Mesima canyon-channel system (southern Tyrrhenian Sea) through integration of morpho-stratigraphic data and physical modelling

The collection of high-resolution multibeam bathymetry, single-channel seismic profiles, TOBI side scan sonar data, and gravity cores allowed the characterization of the main morpho-sedimentary processes acting along the lower reach of the shelf-indenting Gioia-Mesima canyon-channel system (GMS) and the surrounding continental slope (southern Tyrrhenian Sea). This last area, developing across a depth range of 1000–1700 m, shows a complex morphology due to the interaction between downslope gravitative processes (mainly turbidite sheet flows) and abrupt changes in slope gradients related to tectonically-controlled scarps. Particularly, several erosive-depositional features (levee deposits, sediment undulations, channels) have been related to overflow processes from the northern flank of the GMS, although the lower reach of the GMS is characterized by strong entrenchment (canyon height ranging from 120 to 270 m) and low sinuosity. Morphological and seismic stratigraphy data indicate that the distribution and dimension of these features vary in response both to the proximity to the external levee of the GMS and to the topographic gradient of the lower continental slope. Particularly, we were able to discriminate between a gently sloping sector (on average 1.5°) dominated by sedimentary bypass of the turbidity currents and a steeper sector (about 3°), where the erosional capability of these currents seems to increase. Indeed, three channels, 4,3–6,5 km long and up to 20 m deep, incise this steeper sector, running parallel to each other at a distance of 1250–1500 m. To support the capability of overbanking flow in producing these channels, we used a physical model for the ignition of turbidity currents that provides realistic values for the ignitive state of the overbanking turbidity flows. More generally, the methodological approach used in this study may be useful to provide constraints on the genesis and evolution of erosive-depositional features in other tectonically-controlled margins, where sedimentary gravity flows interact with an uneven morphology.

Submarine Depositional Terraces at Salina Island (Southern Tyrrhenian Sea) and Implications on the Late-Quaternary Evolution of the Insular Shelf

The integrated analysis of high-resolution multibeam bathymetry and single-channel seismic profiles around Salina Island allowed us to characterize the stratigraphic architecture of the insular shelf. The shelf is formed by a gently-sloping erosive surface carved on the volcanic bedrock, mostly covered by sediments organized in a suite of terraced bodies, i.e. submarine depositional terraces. Based on their position on the shelf, depth range of their edge and inner geometry, different orders of terraces can be distinguished. The shallowest terrace (near-shore terrace) is a sedimentary prograding wedge, whose formation can be associated to the downward transport of sediments from the surf zone and shoreface during stormy conditions. According to the range depth of the terrace edge (i.e., 10–25 m, compatible with the estimated present-day, local storm-wave base level in the central and western Mediterranean), the formation of this wedge can be attributed to the present-day highstand. By assuming a similar genesis for the deeper terraces, mid-shelf terraces having the edge at depths of 40–50 m and 70–80 m can be attributed to the late and early stages of the Post-LGM transgression, respectively. Finally, the deepest terrace (shelf-edge terrace) has the edge at depths of 130–160 m, being thus referable to the lowstand occurred at ca. 20 ka. Based on the variability of edge depth in the different sectors, we also show how lowstand terraces can be used to provide insights on the recent vertical movements that affected Salina edifice in the last 20 ka, highlighting more generally their possible use for neo-tectonic studies elsewhere. Moreover, being these terraces associated to different paleo-sea levels, they can be used to constrain the relative age of the different erosive stages affecting shallow-water sectors.

Coastal hazard due to submarine canyons in active insular volcanoes: examples from Lipari Island (southern Tyrrhenian Sea)

The recent high-resolution multibeam bathymetry surveys around Lipari Island allowed to evidence several submarine canyons, whose head often cut back up to very shallow water and at a few tens of meters far from the coast. These canyons are mainly located in the eastern and southern side of the island and are characterized by an ongoing retrogressive (landward) erosion, that also controlled the shape and the evolution of the coastline. The canyon heads are formed by minor slide scars. By coupling slide scar morphometry and simple numerical model we have been able to roughly estimate the potential tsunami wave amplitudes generated by related slope failures. Moreover, the retrogressive erosion of canyon heads can be claimed as a cause of the enhanced subsidence reported in the last few thousand years in the eastern part of Lipari, where the main villages are located. Based on these evidence, we propose a first assessment of the coastal hazard due to marine retrogressive activity in the largest and most densely populated island of the Aeolian Archipelago.

HIGH-RESOLUTION GEOMORPHOLOGICAL MAPPING OF THE SHALLOW CONTINENTAL SHELF WEST OF THE KAVALA BAY, NORTH AEGEAN

Prominent geomorphological features of the shallow continental shelf west of the Kavala Bay (Loutra Eleftheron-Nea Peramos) were mapped using the data from a hydrographic survey (June 2014) of 320 nautical miles during which high resolution multibeam bathymetry and seismic-reflection subbottom profiling were carried out simultaneously. A fault zone comprised by a set of two primary sigmoidal gravity faults (recorded lengths and measured offsets: 12 km, 5 km and > 40 m, 25 m, respectively), with distinct expression on the seabed, and three other secondary gravity faults situated southern of the major faults, revealing synsedimentary tectonics, was identified. The striking difference between the texture of the footwall block sediments of the northern major fault and the texture of the sediments occupying the deep hanging wall block of the southern major fault emphasizes the impact of local tectonics on the sedimentary evolution of the study area. Concerning the observed bedforms, the most interesting were the sand dunes occurring at depths from 25 m to 65 m at least and occupying the northeast part of the study area. Their large dimensions and orientation in relation to the coastline position imply as a mechanism for their formation intense bottom-current activity.

The transtensional offshore portion of the northern San Andreas fault: Fault zone geometry, late Pleistocene to Holocene sediment deposition, shallow deformation patterns, and asymmetric basin growth

We mapped an ~120 km offshore portion of the northern San Andreas fault (SAF) between Point Arena and Point Delgada using closely spaced seismic reflection profiles (1605 km), high-resolution multibeam bathymetry (~1600 km 2 ), and marine magnetic data. This new data set documents SAF location and continuity, associated tectonic geomorphology, shallow stratigraphy, and deformation. Variable deformation patterns in the generally narrow (∼1 km wide) fault zone are largely associated with fault trend and with transtensional and transpressional fault bends. We divide this unique transtensional portion of the offshore SAF into six sections along and adjacent to the SAF based on fault trend, deformation styles, seismic stratigraphy, and seafloor bathymetry. In the southern region of the study area, the SAF includes a 10-km-long zone characterized by two active parallel fault strands. Slip transfer and long-term straightening of the fault trace in this zone are likely leading to transfer of a slice of the Pacific plate to the North American plate. The SAF in the northern region of the survey area passes through two sharp fault bends (∼9°, right stepping, and ∼8°, left stepping), resulting in both an asymmetric lazy Z–shape sedimentary basin (Noyo basin) and an uplifted rocky shoal (Tolo Bank). Seismic stratigraphic sequences and unconformities within the Noyo basin correlate with the previous 4 major Quaternary sea-level lowstands and record basin tilting of ∼0.6°/100 k.y. Migration of the basin depocenter indicates a lateral slip rate on the SAF of 10–19 mm/yr for the past 350 k.y. Data collected west of the SAF on the south flank of Cape Mendocino are inconsistent with the presence of an offshore fault strand that connects the SAF with the Mendocino Triple Junction. Instead, we suggest that the SAF previously mapped onshore at Point Delgada continues onshore northward and transitions to the King Range thrust.

Morphology, structure, composition and build-up processes of the active channel-mouth lobe complex of the Congo deep-sea fan with inputs from remotely operated underwater vehicle (ROV) multibeam and video surveys

The detailed structure and composition of turbiditic channel-mouth lobes is still largely unknown because they commonly lie at abyssal water depths, are very thin and are therefore beyond the resolution of hull-mound acoustic tools. The morphology, structure and composition of the Congo turbiditic channel-mouth lobe complex (90×40km; 2525km2) were investigated with hull-mounted swath bathymetry, air gun seismics, 3.5kHz sub-bottom profiler, sediment piston cores and also with high-resolution multibeam bathymetry and video acquired with a Remote Operating Vehicle (ROV). The lobe complex lies 760km off the Congo River mouth in the Angola abyssal plain between 4740 and 5030m deep. It is active and is fed by turbidity currents that deposit several centimetres of sediment per century. The lobe complex is subdivided into five lobes that have prograded. The lobes are dominantly muddy. Sand represents ca. 13% of the deposits and is restricted to the feeding channel and distributaries. The overall lobe body is composed of thin muddy to silty turbidites. The whole lobe complex is characterized by in situ mass wasting (slumps, debrites). The 1-m-resolution bathymetry shows pervasive slidings and block avalanches on the edges of the feeding channel and the channel mouth indicating that sliding occurs early and continuously in the lobe build-up. Mass wasting is interpreted as a consequence of very-high accumulation rates, over-steepening and erosion along the channels and is therefore an intrinsic process of lobe building. The bifurcation of feeding channels is probably triggered when the gradient in the distributaries at the top of a lobe becomes flat and when turbidity currents find their way on the higher gradient on the lobe side. It may also be triggered by mass wasting on the lobe side. When a new lobe develops, the abandoned lobes continue to collect significant turbiditic deposits from the feeding channel spillover, so that the whole lobe complex remains active. A conceptual lithostratigraphic model is proposed for five morpho-sedimentary environments: lobe rims, lobe body, distributaries, levees, feeding channel. This study shows that high-resolution bathymetry ROV observations are necessary to fully understand the build-up processes of modern channel-mouth lobes.

Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from adjacent continents and islands. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the inner continental shelf of central Mozambique. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high-resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea-level change. Results show that sediment transport and delivery to the abyssal plains is partitioned into three distinct domains; southern, central and northern. Sediment partitioning is primarily controlled by changes in continental shelf and shelf-break morphology under the influence of a clockwise rotating shelf circulation system. However, changes in sea-level have an overarching control on sediment delivery to particular domains. During highstand conditions, such as today, limited sediment delivery to the submarine Zambezi Valley and Channel is proposed, with increased sediment delivery to the deepwater basin being envisaged during regression and lowstand conditions. However, there is a pronounced along-strike variation in sediment transport during the sea-level cycle due to changes in the width, depth and orientation of the shelf. This combination of features outlines a sequence stratigraphic concept not generally considered in the strike-aligned shelf-slope-abyssal continuum.

Submarine canyon-head morphologies and inferred sediment transport processes in the Alías-Almanzora canyon system (SW Mediterranean): On the role of the sediment supply

Canyon head regions are key areas for understanding the shelf-to-canyon sedimentary dynamics and assessing the predominant hydrodynamic and sedimentary processes shaping their morphology. High-resolution multibeam bathymetry surveys conducted on the shelf, upper slope and along the main thalwegs of the Alías-Almanzora submarine canyon system were used to recognize the specific morphological features of the various canyon heads and infer the main sediment transport mechanism operating through them. Off the Almanzora River, different canyon head morphologies were observed at 65–90m water depth, incising the outer prodeltaic deposits. A direct connection from the river supply is evidenced by the presence of a large radial field of sediment waves on the prodelta, presumably related to hyperpycnal flows during flash flood events, and their continuation towards trains of cyclic steps that merge into the Almanzora canyon head. Processes linked to seepage and groundwater sapping also seems apparent in the prodeltaic deposits, which results in pockmark alignments and formation of elongated narrow canyon heads. At the southern limit of the Almanzora prodelta, another type of canyon head results from the formation and merging of linear gullies. These have been interpreted as the morphological expression of the distal off-shelf transport of flood-related hyperycnal flows. However, most of the canyon heads of the main tributaries of this canyon system (i.e., Garrucha, Cabrera and Alías) are found at much shallower water depths (7–20m). They generally show a continuation in land with rivers or intermittent creeks (known as “ramblas”) that also experience flash flood events, but some of them are disconnected from any river source. These canyon heads cut into the infralittoral prograding wedge, displaying feeder channels that tend to oriented against the direction of the prevalent long-shore current. Some canyon head tributaries show sediment waves along their axis interpreted as the result of hyperpycnal flood events or storm-induced turbidity flows. This study focusses on the diversity of sediment transport processes that can co-exist in the various canyon tributaries of a given submarine canyon system, stressing on the importance of the canyon heads location with respect to the principal sedimentary source, which ultimately determines their main geomorphological traits.

Morphology, age and sediment dynamics of the upper headwall of the Sahara Slide Complex, Northwest Africa: Evidence for a large Late Holocene failure

The Sahara Slide Complex in Northwest Africa is a giant submarine landslide with an estimated run-out length of ~900km. We present newly acquired high-resolution multibeam bathymetry, sidescan sonar, and sub-bottom profiler data to investigate seafloor morphology, sediment dynamics and the timing of formation of the upper headwall area of the Sahara Slide Complex. The data reveal a ~35-km wide upper headwall,opening towards the northwest, with multiple slide scarps, glide planes, plateaus, lobes, slide blocks and slide debris. The slide scarps were generated by retrogressive failure events associated with two types of mass movements: translational sliding and gravitational spreading. As a result of this evolution, three different glide planes (GP I, II, and III) can be distinguished approximately 100m, 50m and 20m below the seafloor. These glide planes are widespread and suggest failure along pronounced, continuous weak layers. Our data suggest an age of only about 2ka for the failure of the upper headwall area, a date much younger than that derived for landslide deposits on the lower reaches of the Sahara Slide Complex, which are dated at 50–60ka. The young age of the failure contradicts the postulate of a stable slope offshore Northwest Africa during sea-level highstands. Such an observation suggests that submarine-landslide risk along the continental margin of Northwest Africa should be reassessed based on a robust dating of proximal and distal slope failures.

The Ventotene Volcanic Ridge: a newly explored complex in the central Tyrrhenian Sea (Italy)

New high-resolution geophysical data collected along the eastern margin of the Tyrrhenian back-arc basin, in the Pontine Islands area, reveal a ∼NW-SE elongated morphological high, the Ventotene Volcanic Ridge (VR), located on the northern edge of the Ventotene Basin. High-resolution multibeam bathymetry, combined with magnetic data, multi- and single-channel seismic profiles, and ROV dives, suggest that VR results from aggregation of a series of volcanic edifices. The summit of these volcanoes is flat and occurs at about 170 m water depth. Given their depths, we propose that flat morphologies were probably caused by surf erosion during Quaternary glacial sea level lowstands. Seismic stratigraphy together with magnetic data suggest that the volcanic activity in this area is older than 190–130 ka age and may be coeval with that of Ventotene Island (Middle Pleistocene). The submarine volcanoes, located 25 km north of Ventotene, are part of a ∼E-W regional volcanic alignment and extend the Pontine volcanism landward toward the Gaeta bay. Integration of structural data from multichannel seismic profiles in this sector of the eastern Tyrrhenian margin indicates that several normal and/or transtensional faults, striking WNW-ESE, NNW-SSE, and NE-SW, offset the basement and form alternating structural highs and depressions filled by thick, mostly undeformed, sedimentary units. Arc-related magmatism is widespread in the study area, where the VR is placed at the hangingwall of the west-directed Apennines subduction zone, which is undergoing tensional and transtensional tectonics. Bathymetric and topographic evidence shows that VR lies in between a major NE-SW trending escarpment east of Ponza and a NE-SW trending graben southwest of the Roccamonfina volcano, a NE-SW transfer zone that accommodate the extension along this segmented portion of the margin. This suggests that the interaction between NE-SW and NW-SE trending fault systems acts as a structural control on location of eruptive centers, given that main volcanic edifices develop along the NW-SE direction, compatible with the extensional setting of the Tyrrhenian basin.

Records of continental slope sediment flow morphodynamic responses to gradient and active faulting from integrated AUV and ROV data, offshore Palos Verdes, southern California Borderland

Variations in seabed gradient are widely acknowledged to influence deep-water deposition, but are often difficult to measure in sufficient detail from both modern and ancient examples. On the continental slope offshore Los Angeles, California, autonomous underwater vehicle, remotely operated vehicle, and shipboard methods were used to collect a dense grid of high-resolution multibeam bathymetry, chirp sub-bottom profiles, and targeted sediment core samples that demonstrate the influence of seafloor gradient on sediment accumulation, depositional environment, grain size of deposits, and seafloor morphology. In this setting, restraining and releasing bends along the active right-lateral Palos Verdes Fault create and maintain variations in seafloor gradient. Holocene down-slope flows appear to have been generated by slope failure, primarily on the uppermost slope (~100–200m water depth). Turbidity currents created a low relief (<10m) channel, up-slope migrating sediment waves (λ=~100m, h≤2m), and a series of depocenters that have accumulated up to 4m of Holocene sediment. Sediment waves increase in wavelength and decrease in wave height with decreasing gradient. Integrated analysis of high-resolution datasets provides quantification of morphodynamic sensitivity to seafloor gradients acting throughout deep-water depositional systems. These results help to bridge gaps in scale between existing deep-sea and experimental datasets and may provide constraints for future numerical modeling studies.

Geomorphology of the central Red Sea Rift: Determining spreading processes

Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

Subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of Central Ecuador

The uplift of the coastal cordillera of central Ecuador is a likely consequence of the subduction of the Carnegie Ridge, a 400-km-long and 2-km-high topographic asperity. This study aims at analyzing the impact of the subduction of the Carnegie Ridge on the continental shelf sedimentation during the Quaternary. We interpret high-resolution (50–450Hz frequency) multi-channel (72 channels) and single-channel (Sparker) seismic data, piston cores, sediment profiles (3.5kHz), and high-resolution multibeam bathymetry acquired during the ATACAMES cruise with the RV L’Atalante in 2012. In the La Plata Island–Cabo San Lorenzo Peninsula region, the results show a full and detailed record of the last ten Quaternary sedimentary sequences deposited in intra-shelf basins, at and -just seaward of the shelf break. These 100-ka-scale sequences are tied to piston core data, to well-dated Quaternary onshore exposures and to a flight of coastal marine terraces on Cabo San Lorenzo Peninsula and La Plata Island. These sequences correlate with the global ice volume and deep marine temperature changes (d18O) for the last 0.7Ma, which points to a strict climatic control on depositional sequence development and preservation. However, the subsidence of the continental shelf acoustic basement, estimated by the stepwise backstripping of the sedimentary record, exhibits a complex deformation pattern with uplifting and subsiding regions. Deep marine seismic data, currently under processing, show evidence for a subducted seamount beneath La Plata Island and GPS data indicate an important interplate coupling in the same area with the potential to generate a Mw 7–7.5 earthquake. The pattern of the continental shelf deformation is consistent with the shape of this seamount. We tested the hypothesis of a link between the margin deformation and the subduction of the seamount, by comparing a stepwise subduction of the seamount to the palinspastic restoration of the deformation of the continental shelf for the last 1Ma. This comparison shows that the collision probably started c.500ka ago, together with the syntectonic sedimentation, and drastically slowed down by c.50ka, with the sealing of most of the deformation on the shelf.

Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

Abstract. Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

Geomorphology and Neogene tectonic evolution of the Palomares continental margin (Western Mediterranean)

The Palomares continental margin is located in the southeastern part of Spain. The margin main structure was formed during Miocene times, and it is currently part of the wide deformation zone characterizing the region between the Iberian and African plates, where no well-defined plate boundary occurs. The convergence between these two plates is here accommodated by several structures, including the left lateral strike–slip Palomares Fault. The region is characterized by sparse, low to moderate magnitude (Mw <5.2) shallow instrumental earthquakes, although large historical events have also occurred. To understand the recent tectonic history of the margin we analyze new high-resolution multibeam bathymetry data and re-processed three multichannel seismic reflection profiles crossing the main structures. The analysis of seafloor morphology and associated subsurface structure provides new insights of the active tectonic features of the area. In contrast to other segments of the southeastern Iberian margin, the Palomares margin contains numerous large and comparatively closely spaced canyons with heads that reach near the coast. The margin relief is also characterized by the presence of three prominent igneous submarine ridges that include the Aguilas, Abubacer and Maimonides highs. Erosive processes evidenced by a number of scars, slope failures, gullies and canyon incisions shape the present-day relief of the Palomares margin. Seismic images reveal the deep structure distinguishing between Miocene structures related to the formation of the margin and currently active features, some of which may reactivate inherited structures. The structure of the margin started with an extensional phase accompanied by volcanic accretion during the Serravallian, followed by a compressional pulse that started during the Latemost Tortonian. Nowadays, tectonic activity offshore is subdued and limited to few, minor faults, in comparison with the activity recorded onshore. The deep Algero-Balearic Basin is affected by surficial processes, associated to halokinesis of Messinian evaporites.

A submarine landslide source for the devastating 1964 Chenega tsunami, southern Alaska

During the 1964 Great Alaska earthquake (Mw 9.2), several fjords, straits, and bays throughout southern Alaska experienced significant tsunami runup of localized, but unexplained origin. Dangerous Passage is a glacimarine fjord in western Prince William Sound, which experienced a tsunami that devastated the village of Chenega where 23 of 75 inhabitants were lost – the highest relative loss of any community during the earthquake. Previous studies suggested the source of the devastating tsunami was either from a local submarine landslide of unknown origin or from coseismic tectonic displacement. Here we present new observations from high-resolution multibeam bathymetry and seismic reflection surveys conducted in the waters adjacent to the village of Chenega. The seabed morphology and substrate architecture reveal a large submarine landslide complex in water depths of 120–360 m. Analysis of bathymetric change between 1957 and 2014 indicates the upper 20–50 m (∼0.7 km3) of glacimarine sediment was destabilized and evacuated from the steep face of a submerged moraine and an adjacent ∼21 km2 perched sedimentary basin. Once mobilized, landslide debris poured over the steep, 130 m-high face of a deeper moraine and then blanketed the terminal basin (∼465 m water depth) in 11±5 m of sediment. These results, combined with inverse tsunami travel-time modeling, suggest that earthquake-triggered submarine landslides generated the tsunami that struck the village of Chenega roughly 4 min after shaking began. Unlike other tsunamigenic landslides observed in and around Prince William Sound in 1964, the failures in Dangerous Passage are not linked to an active submarine delta. The requisite environmental conditions needed to generate large submarine landslides in glacimarine fjords around the world may be more common than previously thought.

Glacial landform assemblage reveals complex retreat of grounded ice in the Ross Sea, Antarctica

The Ross Sea Embayment is the largest drainage basin of the Antarctic Ice Sheet. Multiple ice streams sourced from both the East Antarctic and West Antarctic ice sheets merged in the Ross Sea during the Last Glacial Maximum (LGM). The continental shelf record is important for understanding processes that influence retreat, constraining deglacial patterns and assessing Antarctica's contribution to eustatic sea-level rise since the LGM. Newly acquired, high-resolution multibeam bathymetry provides a detailed record of the complex deglacial history of a sector in the western Ross Sea (Fig. 1a). Retreat of grounded ice is marked by small, ice-marginal retreat landforms that were poorly resolved prior to the use of an upgraded multibeam system. Dynamic grounding zone behaviour is influenced by physiography and the presence of subglacial meltwater. Fig. 1. ( a ) Multibeam survey south of Crary Bank between Franklin Island and the Central Basin, western Ross Sea shelf (basemap modified by L. Prothro from GeoMapApp). Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 20 m. ( b ) Ross Sea showing the location of the survey area ((a) in red box; map from IBCSO v. 1.0). LAB, Little American Basin; GCB, Glomar-Challenger Basin; RB, Ross Bank; PT, Pennell Trough; PB, Pennell Bank; JT, JOIDES Trough; MB, Mawson Bank; CB, Central Bank; CrB Crary Bank; FrIS, Franklin Island; DT, Drygalski Ice Tongue; VLB, Victoria Land Basin; McM, McMurdo Sound. ( c ) Two sets of linear features interpreted as glacial lineations separated by an abrupt boundary (grey dashed line). Inset shows seismic profile across glacial lineations. Acquisition system Knudsen CHIRP. Frequency 3.5 kHz. ( d ) Asymmetrical ridges, interpreted as GZWs, deposited along the banks of a channel and on top of a volcanic landform with lineated topsets. ( e ) Profile of 4 m GZW deposited on volcanic landform showing underlying surface reflection. ( f ) Example of channel cross-sectional profile. …

Morphologies, classification and genesis of pockmarks, mud volcanoes and associated fluid escape features in the northern Zhongjiannan Basin, South China Sea

Based on new high-resolution multi-beam bathymetry and multichannel seismic reflection data, two new groups of numerous pockmarks and mud volcanoes were discovered in the northern Zhongjiannan Basin at water depths between 600 and 1400m. Individual pockmarks are circular, elliptical, crescent-shaped or elongated, with diameters ranging from several hundreds to thousands of meters and tens or hundreds of meters in depth, and they often form groups or strings. Crescent pockmarks, approximately 500–1500m wide in cross-section and 50–150m deep, occur widely in the southern study area, both as individual features and in groups or curvilinear chains, and they are more widespread and unique in this area than anywhere else in the world. Conical mud volcanoes, mostly with kilometer-wide diameters and ca. 100m high, mainly develop in the northern study area as individual features or in groups. Seismic data show that the observed pockmarks are associated with different kinds of fluid escape structures and conduits, such as gas chimneys, diapirs, zones of acoustic blanking, acoustic turbidity and enhanced reflections, inclined faults, small fractures and polygonal faults. The mapped mud volcanoes appear to be fed from deep diapirs along two main conduit types: the conventional conduits with downward tapering cones and another other conduit type with a narrow conduit in the lower half and emanative leakage passages in the upper half. Various types of pockmarks are found and a comprehensive pockmark classification scheme is proposed, according to: (a) their shape in plan view, which includes circular, elliptical, crescent, comet-shape, elongated and irregular; (b) their magnitude, which includes small, normal, giant and mega-pockmarks; and (c) their composite pattern, which includes composite pockmarks, pockmark strings and pockmark groups. For the genesis of the crescent pockmark (strings), a 5-stage speculative formation model is proposed, implying possible controlling factors of gravity sliding/slumping, fluid escape activity and sandbodies intrusion. Seismic data suggest that the mud volcanoes have likely undergone two episodes of evolution, which include a widespread drastic formation of mud volcanoes with dome-like structures, a wide range of seepage occurrences and the formation of complex sediments in the first stage, and the appearance of pockmarks, conical mud volcanoes (groups), and the formation of deformed, filled or broken buried complex sediments by continuous fluid flow processes during stage 2, in the Pliocene and the Quaternary. Pockmark gullies are extensively found along the slope, due to the interaction of slope failure and fluid escape processes. The Cenozoic sedimentary cover is thin in the Northern Zhongjiannan basin, which evolved from a Late Cretaceous or Palaeogene–Oligocene rift to the Neogene–Quaternary post-rift thermal subsidence, and therefore intense deep thermogenic fluid flow can easily affect the soft Cenozoic sediments, produce complex sediment deformation, and form numerous fluid flow structures at the seafloor, in particular pockmarks and mud volcanoes that dominantly emerged during the period of post-rift thermal subsidence and neotectonic movement since ca. 5.5Ma.

The Influence of Data Resolution on Predicted Distribution and Estimates of Extent of Current Protection of Three 'Listed' Deep-Sea Habitats.

Modelling approaches have the potential to significantly contribute to the spatial management of the deep-sea ecosystem in a cost effective manner. However, we currently have little understanding of the accuracy of such models, developed using limited data, of varying resolution. The aim of this study was to investigate the performance of predictive models constructed using non-simulated (real world) data of different resolution. Predicted distribution maps for three deep-sea habitats were constructed using MaxEnt modelling methods using high resolution multibeam bathymetric data and associated terrain derived variables as predictors. Model performance was evaluated using repeated 75/25 training/test data partitions using AUC and threshold-dependent assessment methods. The overall extent and distribution of each habitat, and the percentage contained within an existing MPA network were quantified and compared to results from low resolution GEBCO models. Predicted spatial extent for scleractinian coral reef and Syringammina fragilissima aggregations decreased with an increase in model resolution, whereas Pheronema carpenteri total suitable area increased. Distinct differences in predicted habitat distribution were observed for all three habitats. Estimates of habitat extent contained within the MPA network all increased when modelled at fine scale. High resolution models performed better than low resolution models according to threshold-dependent evaluation. We recommend the use of high resolution multibeam bathymetry data over low resolution bathymetry data for use in modelling approaches. We do not recommend the use of predictive models to produce absolute values of habitat extent, but likely areas of suitable habitat. Assessments of MPA network effectiveness based on calculations of percentage area protection (policy driven conservation targets) from low resolution models are likely to be fit for purpose.

Integrated use of LiDAR and multibeam bathymetry reveals onset of ice streaming in the northern Bothnian Sea

Geomorphological mapping from the new LiDAR (Light Detection and Ranging)-derived digital elevation model for Sweden and a high-resolution multibeam bathymetry data-set for the Gulf of Bothnia reveals a continuous system of glacial landforms crossing the transition between the modern terrestrial and marine environments. A palaeo-ice stream in the northern Bothnian Sea is reconstructed, with an onset tributary over the present-day Ångermanland–Västerbotten coastline. Systematic contrasts in landform morphology and lineation length indicate that this ice stream comprised a relatively narrow (∼40 km) corridor of fast flow, flowing first SW then S, and likely fed by converging flow around the upper Bothnian Sea. The geometry and landform associations of this system imply that ice, at the time period represented here, did not flow across the Gulf of Bothnia: SSE-ward ice flow indicators on the northern Swedish coast do not correspond directly with landform assemblages of the large SE-oriented Finnish deglacial lobes. Instead, we suggest they may contribute to a late-stage fast-flow event to the S and SW. Multibeam bathymetry data offer entirely new access into the rich, landform-scale geomorphological record on the seafloor of the Gulf of Bothnia. The combination of offshore multibeam with the new terrestrial LiDAR data provides unprecedented insight into and renewed understanding of the glacial dynamics of the Bothnian Sea sector of the Fennoscandian Ice Sheet, hitherto interpreted over large areas of unmapped ice sheet bed.