How to cite: Nanson, R., Arosio, R., Gafeira, J., McNeil, M., Dove, D., Lilja, B., Margaret, D., Janine, G., Post, A., Webb, J., and Nichol, S.: A two-part seabed geomorphology classification scheme: Part 2 – a geomorphic classification framework and glossary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10006, https://doi.org/10.5194/egusphere-egu23-10006, 2023.

Sediment mobility is one of the key issues considered  during the design and construction process off offshore infrastructure ( Wind farms, cables, pipelines etc.).  Early understanding of the seabed mobility can significantly affect the project timelines, cost and, if not mitigated, can reduce the lifespan of already existing assets.  The most common approach to evaluate sediment mobility relies on repeated bathymetric surveys which aim to unravel the rate of change of the seabed over time. However, repeated surveys to be effective require to be performed over timelines allowing for confident detection of change above the uncertainty threshold and need to consider seasonal conditions within the area of interest. This time separation typically needs to be greater (several years)  the bigger the mobile bedforms across the area.  This means that it is unlikely that a result of a repeated bathymetric survey will be available early in the project life. Here, a public domain repeated bathymetric survey data from a deglaciated continental shelf area offshore N-E Scotland with moderate-to high-resolution(2-8m) data will be used to (1) identify mobile and immobile paleo bedforms, (2)quantify the rate of change of the seabed and (3) investigate the effect on different data resolution on the seabed mobility quantification.

How to cite: Kurjanski, B., Caruso, S., McGhee, C., Rea, B., and Spagnolo, M.: Effect of bathymetric data resolution on the understanding of sediment mobility: implications for offshore infrastructure projects on deglaciated continental shelves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16716, https://doi.org/10.5194/egusphere-egu23-16716, 2023.

Transits during oceanographic expeditions constitute a potential huge amount of acquired bathymetric data that could be systematically integrated to increase the knowledge on submarine morphology, especially for planned surveys in the equatorial Atlantic, the Arctic, the Indian and the Southern Oceans. The recent PNRA ISOBatA project aims to efficiently exploit seafloor datasets collected during transfer times within the Antarctic region and the Ross Sea. Along the route from New Zealand to the Italian Mario Zucchelli Station in the Ross Sea, the Emerald Fracture Zone and the Macquarie Triple Junction, located in the SW Pacific Ocean, represent two selected areas to test the strength of transit acquisition in remote areas, normally affected by adverse weather conditions.

The ISOBatA project has the main purpose to contribute to the mapping of Antarctic waters developing best practices and dedicated workflows to implement QA in multibeam data acquisition procedures during transit times, as well as in the processing, analysis and archiving of data and metadata.

The ISOBatA experience in the Southern Ocean suggests there are several critical issues associated with collection of multibeam data in remote and ice-infested waters. Operating procedures need more standardization, to avoid the acquisition of redundant data along common routes and unreliable data.

Our work aims to open a discussion to address the need for standardization in data acquisition during transit times, which should include priority in accordance with the geomorphological/geographical nature of the working areas. The integration of bathymetric data acquired during research vessel transfers to remote regions could imply a common international effort for a systematic exploration of the seafloor, sharing coverage in real time to avoid redundancy.

How to cite: Accettella, D., Burca, M., Cuffaro, M., Diviacco, P., Gasperini, L., Lodolo, E., Muccini, F., Savini, A., and Varzi, A. G.: Toward the systematic exploration of the seabed morphology during transits after the ISOBatA project experience in the Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15935, https://doi.org/10.5194/egusphere-egu23-15935, 2023.

The Northwest (NW) African continental margin is well known for the occurrence of large-scale submarine landslides with prominent scarps exposed at the seafloor. Previous studies primarily focused on major landslides, but rarely covered small scarps. It is unclear if the distribution of landslides along the NW African continental margin is biased by the availability of processed data because data were collected mainly in designated surveys in areas of special interest. Numerous multibeam bathymetric data sets, however, are available for the area as data were also collected during transits and cruises where seafloor mapping was not a primary objective. We compiled such various datasets in the open-source MB-System software and implemented a cloud-based auto-processing and adaptive filtering workflow to handle the large bathymetric datasets (15,476 survey lines). The results show that our auto-processed bathymetric data provide a much-improved view of the seafloor (50 × 50 m), compared to EMODnet2020 and GEBCO 2022 GRID without having manually edited the data. Such a workflow allows to process large underway multibeam datasets of the given kind and therefore it resolves the unknown submarine landforms. Our results from NW Africa offer not only new insights into small-scale submarine landslides but also fulfill the missing piece from previous studies that focused on large-scale submarine landslides. Minor scarps are mainly found close to areas with major landslides, supporting the hypotheses that the NW African continental margin is characterized by large-scale but infrequent landsliding. Minor scarps are additionally identified in some other areas, such as the walls of the Agadir Canyon. Associated landsliding may contribute to the well-known Moroccan Turbidite System. The additional information on minor scarps allows us to gain a more comprehensive understanding of submarine landslides and the associated tsunami risk along the NW African continental margin.

How to cite: Tang, Q., Schneider von Deimling, J., Geersen, J., and Krastel, S.: Compilation and processing of bathymetric data recorded along the Northwest African continental margin over several decades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8985, https://doi.org/10.5194/egusphere-egu23-8985, 2023.

Coralligenous (C) includes calcareous build-ups of biogenic origin, characterised by the association of calcareous algae and several invertebrates. This habitat is one of the most important at the Mediterranean scale; it is a hot-spot of biodiversity thriving from shallow waters down to the limit of the mesophotic zone. The Italian project “CRESCIBLUREEF - Grown in the blue: new technologies for the knowledge and conservation of the Mediterranean reefs” aims at studying  peculiar C outcrops found along a depth gradient offshore Marzamemi village (SE Sicily). 

During the first project expedition (June 2021), we performed a Multibeam Echosounder (MBES) survey of the target area by using a R2Sonic 2022 system. A new 17 km2 high-resolution morpho-bathymetric map was realised, which interpretation led to the identification and classification of five major habitats, including different C morphotypes. C habitat in the form of banks was found mainly distributed between 30 and 35 m of water depth (w.d.). This investigation allowed us to observe and quantify the overall C distribution along a depth gradient spanning between 20 and 100 m of w.d., giving us a broad-scale perspective of its extension at the seafloor. 

A third marine survey (September 2021) was focused on collecting video and still images by using a Sony α Alpha 7ii reflex coupled with the Easydive Leo3 Wi housing and the Easydive illumination system Smart Sea - Gold Plus 7000 Lumen, through scuba diving. Data collection was performed over selected areas suitable for the application of underwater photogrammetry, taking into account the presence of C build-ups  (as confirmed by the interpretation of the MBES dataset) and the operational depth (i.e.: no more than 35 m of w.d.). Data collected by adopting this technique and processed using Structure-from-Motion (SfM) algorithms allowed us to get more information at the community level of such complex habitats, coupling the seafloor scale with the smaller scale obtained by direct observations. 

In this work, our intention is to improve the understanding of the geospatial context of Coralligenous distribution and extent from a multiscale perspective. Specifically, we want to show how eco-geomorphological indexes calculated using the high-resolution outputs of the C photogrammetry (3D meshes, DTMs, and orthomosaics) may be used to perform resolute investigations of such habitat on a broader scale, by considering their spatial distribution extrapolated from the MBES data.

How to cite: Varzi, A. G., Fallati, L., Savini, A., Bazzicalupo, P., Bracchi, V. A., and Basso, D.: Underwater Scuba Photogrammetry VS. MBES Acoustic Sounding: how to integrate multiscale data for a better understanding of Coralligenous outcrops, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13496, https://doi.org/10.5194/egusphere-egu23-13496, 2023.

How to cite: Cukur, D., Kong, G.-S., Buchs, D., Lee, G.-S., Kim, S.-P., Um, I.-K., Chun, J.-H., and Horozal, S.: Upslope migrating sand waves on sediment-starved shelves: An example from the southeastern continental margin of the Korean Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1723, https://doi.org/10.5194/egusphere-egu23-1723, 2023.

Submarine canyons are the main conduits that transport material such as sediment, organic carbon, and litter from the continent to the deep sea. This transport of material is more efficient when the canyon heads incise into the shelf, as opposed to canyons that are confined to the continental slope. The specific controls on the distribution of these two canyon types along the world’s continental margins remain unquantified and we still lack knowledge about these seascape shaping processes.

Spatial statistics on a global scale help to reveal these processes.  In this study, we successfully predict the global patterns of submarine canyon occurrence along major continental margins based on terrestrial and marine environmental variables using point patterns on linear networks. We show that submarine canyon density of both types increases as a function of gradient of the continental slope which is the most important predictor. Subsequently, the locations of slope-confined canyons are best predicted by age of the adjacent ocean lithosphere with old ages corresponding to high canyon densities. Shelf-incised canyons are best predicted by the shelf gradient which correlates positively with shelf-incised canyon densities and, to a lesser extent, by high water discharge from the adjacent catchments.

Our results show that marine variables – primarily the continental slope gradient - are most crucial for spatially predicting submarine canyons while terrestrial variables are of lesser importance. The influence of terrestrial conditions and shelf morphology on slope-confined canyons is minimal. However, incision of canyons into the shelf is facilitated when shelves are steep and river discharge is high, highlighting the secondary role of canyon head erosion by terrestrially derived sediment. Our results underscore that the formation of submarine canyons worldwide is mainly governed by backward erosion along steep continental slopes by mass failure and/ or erosive sediment density currents.  Erosion by sediment flows carrying sediment directly from terrestrial sources is likely less important for the formation of submarine canyons.

How to cite: Bernhardt, A. and Schwanghart, W.: Controls on global submarine canyon occurrence and formation processe s– Insights from Spatial Point Pattern Analysis –, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7648, https://doi.org/10.5194/egusphere-egu23-7648, 2023.

Submarine slope failures pose a hazard to seafloor infrastructure and coastal communities. Given the high population densities, slope failures can have a particularly significant impact around river deltas, generating damaging tsunamis and breaking critical telecommunications connections. Despite the risks they pose, a lack of detailed monitoring means that the factors that lead to slope collapse remain poorly constrained. Numerical modelling is typically used to assess future slope stability. Still, sparse existing data ensure that we cannot yet determine how submerged delta slopes evolve and progress to failure at the field scale. Here, we aim to close this gap by analysing repeat seafloor surveys of the submerged Squamish prodelta, British Columbia, to determine the physical controls on slope instability. Multibeam bathymetric surveys were performed on 93 consecutive weekdays in 2011, during which time at least five large (>50,000 m³) delta slope collapses occurred, as well as numerous smaller slope failures. These surveys allow us to determine how the delta slope and geometry changes on an unusually detailed timeframe (i.e. daily) in the build-up to slope collapse and how it relates to variable sediment supply from the feeding river and tidal fluctuations. Analysis of the five large collapses reveals that a single mechanism is not responsible for every failure. So, we investigated how different parts of the delta encounter major failure at different times and locations by measuring and mapping out the delta head and associating it with sediment input and tide high. From this, we found that slope failure is likely due to a combination of enhanced slope geometry due to delta lip progradation and pore pressure fluctuations relating to sediment loading and tidal effects.

How to cite: Zulkifli, Z., Clare, M., and Minshull, T.: What are the controls for delta front slope failure? Insights from detailed monitoring at Squamish Delta, British Columbia., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4582, https://doi.org/10.5194/egusphere-egu23-4582, 2023.

Seafloor depressions, sometimes known as pockmarks, are commonly observed features on the ocean floor. Their shape and size can range from small, circular indentations (10s m) up to large, often irregularly shaped depressions (several kms in diameter). The origin of pockmarks is often attributed to focused fluid or gas seepage at the seafloor, but their formation mechanisms (e.g., gas/fluid composition, timing, physical processes) remain ambiguous in many cases. On the Chatham Rise, offshore New Zealand’s South Island, seafloor depressions cover an area >50,000 km², and appear to be bathymetrically controlled. For this region, it has been hypothesized that episodic release of geological CO₂ resulted in the recurring formation of pockmarks at glacial terminations. Seismo-acoustic surveys allow the investigation of potential fluid-flow pathways and buried paleo-pockmarks. High-resolution imaging of shallow subsurface features can be conducted using hull-mounted, parametric subbottom profilers that are available on most larger research vessels. Higher frequencies (>1 kHz) and narrow acoustic beams provide very high vertical resolution (decimetre range) and small lateral footprints capable of resolving smaller structures than using conventional seismic. A recent voyage in 2020 acquired an extensive grid of densely spaced (~25 m) 2D subbottom profiles over a dense pockmark field on the Chatham Rise.

Here we present a novel approach to create a comprehensive pseudo-3D cube from high-resolution 2D echosounder profiles using a recently developed processing workflow. Based on this generated cube, we perform a preliminary analysis of seafloor pockmarks and paleo-pockmarks in the shallow subsurface up to 150 m below the seafloor. Our analysis includes insights into the recurrence of pockmark formation at different geological times and an assessment of morphological changes and varying spatial locations over time. Additionally, we investigate a potential polygonal fault network beneath the lowermost layer of paleo-pockmarks that might channel upward fluid migration in the area.

How to cite: Warnke, F., Pecher, I., Hillman, J., Davy, B., and Strachan, L.: Spatial-temporal development of paleo-pockmarks on the Chatham Rise from 3D imaging with subbottom profiler data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5505, https://doi.org/10.5194/egusphere-egu23-5505, 2023.

Orographic reliefs are continuously created and modified in active continental tectonic settings, influencing the drainage pattern and interacting with it. It is not uncommon that stream capture occurs in these settings, causing major rearrangement of river courses. This process often produces a geomorphological feature known as wind gap, i.e. a gap through which a stream once flowed but that is now abandoned and dry as a result of the capture. Analizing a high resolution 3D seismic data set, kindly made available by ENI S.p.A., we discovered a similar feature in the Ionian offshore of the Crotone peninsula, northern Calabria. This underwater region is characterized by intense tectonic activity that is partly controlled by the occurrence of a mobile substrate, possibly overpressured shales. The relevant uplift affecting the nearby Calabria onshore can also contribute to promote gravitational instability. In this setting the "wind gap" is represented by a stretch of a downslope weakly incised channel that has soon been abandoned as a result of the growth of a tectonic structure. The course of the new submarine channel runs sub-parallel to the coast for a long strecth, before taking a downslope trajectory. The present-day submarine channel is deeply incised, showing at least two main phases: a deep valley incision containing an axial valley with a much lower relief, which likely represents the current route of turbidite flows. The main channel valley results from the major erosional episode that affected the continental slope offshore northern Calabria. The limited incision in the abandoned channel strecth suggests that drainage rearrangement occurred in the very early stage of channel incision. Therefore, the estimated age of the tectonic deformation that is responbile for originating the "wind gap" can offer a useful hint on the timing of onset of erosion in this area.

How to cite: Argnani, A., Pellegrini, C., and Rivere, M.: An underwater "wind-gap" in the Ionian offshore of northern Calabria, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15255, https://doi.org/10.5194/egusphere-egu23-15255, 2023.

The Túnel de la Atlántida (Atlantida Tunnel), located in Lanzarote Island (Canary Islands, Spain), with a length of about 2000 m and a depth of 64 m, is the largest submerged lava tunnel in the world. It corresponds to the submerged part of the lava tube complex of the La Corona volcano, with a length of about 10 km. During the development of the Sublantida Project, using diving techniques, various forms associated with the formation of the volcanic tube have been catalogued and a study of its sediments, minerals and speleothems has been carried out, including XRD, ESEM and petrological microscopy. It has been possible to propose a paleoenvironmental reconstruction from the formation of the volcanic tube, ca.21 Ka ago, to the present. The geomorphological, petrological, and sedimentary characteristics associated with the formation of the lava tube justify its importance as a World Geological Site of Interest.

Acknowledgments: This project has received funding from the Spanish Ministry of Science and Innovation and the Spanish State Research Agency (grants CGL2017-91218-EXP and PID2019-110603RB-I00-SUBSYST). It is a contribution to the IGCP Project 725.

How to cite: Lario, J., Martín-Pozas, T., Sanchez-Moral, S., Cañaveras, J. C., Fernandez-Cortes, A., Cano, R., Lopez-Tercero, C., Roldan, A., Martin, E., Perez-Mejias, C., and Cheng, H.: Geomorphological and sedimentary features of an underwater lava tube: the Túnel de la Atlántida (Lanzarote, Spain), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6059, https://doi.org/10.5194/egusphere-egu23-6059, 2023.

Submarine fans are deposits of coarse sediments of continental origin in the deep sea, and are generally characterized by a complex depositional architecture, due to the multiple triggering mechanisms of deep-water sediment gravity flows. Consequently, this poses great challenges to deep water petroleum exploration and development. We analyzed the geomorphologic evolution and architecture of Campanian, deeply buried, submarine fans in the Kribi-Campo sub-basin, offshore Cameroon. Using a 3D seismic reflection data set and logs from two wells, we mapped seven horizons, including the fan base, fan top and five internal horizons. In cross-section, the fan is characterized by a high amplitude seismic facies exhibiting an aggradational pattern with parallel and continuous reflectors. The stacked fan-shaped morphology is up to 340 ms TWT thick, extends over an area of 600 km² and oriented NE-SW, near the Kribi High. The analysis of lobes and channels on each horizon provided a timelapse that captures the major geomorphologic transformations of the submarine fan from its initiation, growth, and abandonment.  The submarine fan is composed of depositional lobes whose beds consist of sand, silt and mud. The detailed structure of these lobes has a finger-like morphology and is generally oriented at high angles to the channel that delivered the sediment to the lobes. The finger-like features are interpreted as thick massive sands, formed as a result of sediment-gravity flows which branched off the main flow eroding into pelagic clay substrate. Two types of channel morphology were identified (straight and sinuous). Our results show that channel and sand-body architecture evolve in a predictive manner, primarily controlled by fan aggradation. The elongated shape and morphology of the submarine fan may arise from the interaction of the fault-related folds and Kribi High, with sandstone deposition within the intervening topographic lows, sourced from the east. The 3D seismic geomorphological analysis of the submarine fan, as presented in this study, is essential to better understand their geometries, facies distribution, stacking patterns and depositional architecture to improve reservoir predictions.

How to cite: Secke Bekonga Gouott, B., Eruteya, O. E., Niyazi, Y., Yem, M., Yene Atangana, J. Q., Maloh, A. L., Makoube Etame, S., and Samankassou, E.: Submarine fans in the Kribi-Campo sub-basin, offshore Cameroon: Geomorphology and stratigraphic evolution during the Late Cretaceous, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6826, https://doi.org/10.5194/egusphere-egu23-6826, 2023.

Understanding the mechanism of turbidity currents is important for siting submarine cables and pipelines. It is because the turbidity currents can transport a large amount of sediment in long distance that causes severe damages to these buried linear structures. It is not clear why turbidity currents can gain and sustain such a large amount of kinetic energy. One of possibilities to explain this process is a drag reduction which reduces the turbulent energy due to the inclusion of fine particles as previous studies reports. However, the influence of fine particles to their flow characteristics has not been fully elucidated. Thus, in this study, a series of model tests were conducted to compare the horizontal steady flows of silica suspension and NaCl solution in a flume. The test results show that the flow characteristics of silica suspension were different from that of NaCl solution. These differences are considered to be caused by silica particles, and it is suggested that drag reduction by fine particles would be taken place in turbidity currents.

How to cite: Kyoi, M., Nomura, S., Oshima, I., Nishiura, D., Furuichi, M., and Tani, K.: Experimental comparison between the turbidity and density currents, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14045, https://doi.org/10.5194/egusphere-egu23-14045, 2023.

The Euro-Mediterranean Submarine landSlide (EMSS) database is a catalogue of submarine landslides of the Mediterranean Sea and the European continental margins of the Atlantic and Arctic Oceans. The catalogue is compiled from data available in the literature as well as information collected from geophysical data and so far not published in the scientific literature. A first version has been recently made available online (https://ls3gp.icm.csic.es/?page_id=553) and OGC services are being developed to be available soon through the EPOS data portal (https://www.ics-c.epos-eu.org/) in the frame of the EU funded project Geo-INQUIRE. Within Geo-INQUIRE we are currently working on a second version of the catalogue improving both areal coverage in the Atlantic Ocean and information relative to the source areas (as opposed to the previous version where only deposits and scars was considered). The aim of the latter improvement is to better characterize the failure and post-failure stages of submarine landslides. The new catalogue stores polygon and polyline geospatial features related to landslide deposits, landslide source areas and landslide scars as well as information relative to age, volume, area, runout, thickness, typology, scar elevation, relevant slopes and depths as well as related metadata. The catalogue includes submarine landslides that span from Miocene to Present day, although a clear bias exists towards submarine landslides of younger age, particularly for the smaller events. The reason for this is that the older and smaller events are difficult to identify on lower resolution geophysical data sets in deep-water and large sub-surface depths. The catalogue aims to offer improved understanding of mass-wasting processes, the potentially resulting tsunamis and derived geohazard. Recent case studies using a data subset (Gulf of Cadiz, SW Iberian Margin) portray the application of such type of databases in (probabilistic) analysis of submarine slope instability and tsunami-genesis from submarine landslides. We believe the current EMSS is the seed for the world ocean submarine landslide database. In this regard, we encourage the offshore geohazards community to contribute to enlarge the database. Shapefile templates will be made available to ease the task. This work is supported by the European Union’s Horizon Europe Research and Innovation Program under grant agreement No 101058518 (Geo-INQUIRE).

How to cite: Urgeles, R., Gamboa, D., León, R., Lovholt, F., Vanneste, M., Cattaneo, A., and Vila, C.: The EuroMediterranean Submarine Landslide database: towards offshore geohazard quantitative assessement from submarine landslides and derived tsunamis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2662, https://doi.org/10.5194/egusphere-egu23-2662, 2023.

Volcanic island sector collapses produce some of the volumetrically largest mass movements on Earth. They may trigger devastating tsunamis that pose hazards to coastal communities and endanger seafloor installations. However, very little is currently known about the interplay between volcanic activity and subsequent mass wasting (volume, source location, and trans­port dis­tance) as well as their specific em­place­ment pro­cesses (tim­ing, kin­emat­ics, and dy­nam­ics). Moreover, these are key information to de­vel­op­ a re­li­able tsunami haz­ard as­sess­ment for sec­tor col­lapses.

The volcanic island of Montserrat in the Lesser Antilles is an ideal target to study the timing, frequency, and kinematics of sec­tor col­lapses as well as subsequent mass wasting. In 2019, Meteor expedition M154 investigated the major landslide complex – Deposit 2, located in the southeast offshore sector of Montserrat and provided an outstanding geophysical (M154-1) and sedimentological dataset. Here, the second leg, M154-2, focused on sediment sampling. Within and in the vicinity of Deposit 2, drill cores were taken with the MeBo70 drill rig from up to 63 mbsf. Ad­di­tion­ally, 21 sup­ple­ment­ing grav­ity cores were taken in the vi­cin­ity of Me­Bo70 drill sites and along systematic transects across the slid masses. Sedimentological, geophysical, geotechnical as well as geochemical analyses of these sediment cores enable a unique opportunity to gain new insights into timing of mass wasting events and complement information on the volcanic island evolution.

Based on these sediment cores, this pro­ject aims at con­trib­ut­ing to the gen­eral com­pre­hen­sion of vol­canic is­land sec­tor col­lapses, par­tic­u­larly the in­ter­re­la­tion­ship of vol­canic pro­cesses and as­so­ci­ated mass move­ments by establishing an event chronostratigraphy for the marine sediment records off Montserrat volcanic island.

Samples from four MeBo70 drill sites at the undisturbed slope, the central and distal part of Deposit 2, and south of Montserrat were analyzed for their componentry and composition. The sediments predominantly comprise mud-rich facies interbedded with fine to coarse-grained, better-sorted sands. The sandy intervals sometimes show multiple units defined by normally-graded beds or sharp color changes with variable proportions of volcanic and biogenic clasts. In a small number, coarse volcanic sands to volcaniclastic gravels were encountered. Tuffaceous deposits are less frequent. Petrographic analyses of selected samples by polarized light microscopy enable the investigation of clast inventories to differentiate between sediment units. Geochemical fingerprinting of major elements of volcanic glasses by electron microprobe elucidates this differentiation. The geochemical analyses further show a mainly basaltic to rhyolitic volcanism in the range of Arc Tholeiitic and Calc-alkaline series. The analyzed samples represent different stages of volcanic island evolution with periods of increased volcanic activity and eruptions, flank collapses, submarine mass wasting events, and periods of relative inactivity. Moreover, trace element analyses by laser ablation inductively coupled plasma-mass spectrometry of selected potential primary volcanic layers enable the possibility to better distinguish between single eruptions and also to narrow down their source area(s) as well as that of the sedimentary material.

How to cite: Sass, K., Kutterolf, S., Freudenthal, T., Watt, S., Berndt, C., Krastel, S., and Huhn, K.: Volcanic Island Sector Collapse: Reconstruction of volcanic activity and implications for subsequent mass movements from marine records drilled with MeBo70 offshore Montserrat (Lesser Antilles), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11441, https://doi.org/10.5194/egusphere-egu23-11441, 2023.

Recent work has demonstrated elevated shear strength in the uppermost 100 meters below seafloor (mbsf) on seismically active margins. This observation is consistent with the seismic strengthening hypothesis that repeated exposure to earthquake shaking progressively dewaters and densifies sediment, which leads to increased shear strength and slope stability.  However, the relative contribution of seismic strengthening versus intrinsic properties on shear strength remain largely unknown. Here, we compare sediments from seismically active and passive margins from scientific ocean drilling sites that exhibit significant shear strength differences. Active margin sites are Nankai (Site C0001), Cascadia (Site 1054), and Southern Alaska (Site U1418), and passive margin sites are Amazon Fan (Site 942), North Carolina Slope (Site 1054), and New Jersey (Site 1073). From each site, we sampled 500 g of sediment equally distributed throughout the top 100 mbsf. We combined samples to create a representative bulk sample per continental margin and reconstituted them with saltwater that matched field-measured salinity. We measured particle size (hydrometer), plasticity states (Atterberg limits), mineralogy (powder X-ray diffraction), compression behavior and permeability (1-D resedimentation experiments), and undrained shear strength (fall cone device). All samples are siliciclastic marine mud that classify as silty clay or clayey silt. Despite the apparent similarity in lithology, sand fraction varies from 0.8 wt. % (Amazon) to 10.3 wt. % (N. Carolina) and clay fraction (<2 mm) varies from 37.7 wt. % (N. Carolina) to 56.0 wt. % (Amazon). Void ratios, measured in resedimentation experiments range from 1.6 (porosity = 62%) (Nankai) to 1.0 (porosity = 50%) (S. Alaska) at a vertical effective stress of 100 kPa. Resedimentation experiments are followed by consolidation to 1 MPa (equivalent to 100 meters of burial depth) and undrained shear strength measurements, which are compared with field-measured shear strengths. We find the previously observed strengthening effect observed in the active margin field- strength is no longer present in the lab-strengths. This suggests that the exposure to seismicity in the field is potentially leading to enhanced shear strength during early burial.

How to cite: Fitzgerald, B., Sawyer, D., Reece, J., and Scott, W.: Shear Strength Development During Early Burial on Seismically Active Margins: A Geotechnical Investigation into Seismic Strengthening, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8508, https://doi.org/10.5194/egusphere-egu23-8508, 2023.

Submarine landslides are common on all sediment bearing submarine slopes worldwide. They have the potential to damage expensive subsea infrastructure such as pipelines or telecommunication cables, and generate hazardous tsunamis. Numerous studies have shown that weak layers embedded within the slope stratigraphy play a crucial role in controlling the formation of submarine landslides; however, very little is known about their internal structure and composition. Although weak layers seem to be an essential pre-conditioning factor for slope failure, many questions remain unanswered, such as where with respect to weak layers do failure planes form: within the weak layer, above or below it? Previous studies usually relied on sedimentological and geotechnical sediment core and in-situ analyses to investigate weak layers. These analyses, however, do not provide insights into the internal structure of the sediments on a micro-scale level and hence, lack information needed to qualitatively and quantitatively investigate weak layers.

Here, we present a new approach towards weak layer investigation that is based on high-resolution micro-Computed Tomography (µCT) imaging. µCT is used to visualise, and qualitatively and quantitatively investigate selected sediment samples taken from within weak layers and the background sediment of submarine landslides. Our results show clear compositional and structural differences between individual sub-units of the investigated weak layers, as well as the background sediment. These differences can be attributed partly to different sediment types, i.e. coarse- versus fine-grained sediments, but also reveal a dependency on the sedimentation regime. We find that pore space distribution is highly spatially variable and works on a sub-millimetre scale. Such high variability may be masked by standard bulk porosity measurements, which require larger (several centimetre) sediment samples and only provide information averaged over the entire sample. The identification of small-scale changes, however, appears to be crucial for the formation of weak layers. Our results therefore demonstrate the huge potential of µCT to investigate the internal structure of weak layers, obtaining information that is not resolved and lost in other analytical methods.

How to cite: Gatter, R., BN Murthy, M., and Huhn, K.: Micro-structural characterisation of weak layers of submarine landslides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14647, https://doi.org/10.5194/egusphere-egu23-14647, 2023.

Mass-transport deposits often show a low-amplitude, “acoustically transparent” seismic response compared to unfailed sediments. This amplitude signature is often interpreted as a lack of coherent internal reflectivity caused by a loss of internal structure during transport and emplacement, and is widely used to delineate mass-transport deposits in sub-bottom profiler data. An apparent contradiction is that cores penetrating such “acoustically transparent” deposits can sometimes retrieve well-stratified sediments that show little evidence of deformation.

In this study we examine the variation in the single-channel seismic amplitude response with changing heterogeneity using synthetic seismic modelling. We model the internal structure of mass-transport deposits as a two-component binarised random medium, where the lateral correlation length is used to artificially control the degree of internal deformation/scale of internal structure, while maintaining the magnitude of the internal reflectivity constant. We construct two synthetic models: i) a simplified single-source marine example and ii) a multi-source example based on a real world “acoustically transparent” mass-transport deposit imaged by a dense network of AUV sub-bottom profiles in the Black Sea. We use 2-D elastic finite-difference modelling to model the seismic response (at sub-bottom profiler bandwidths) of an ensemble of both synthetic models with varying geostatistical parameters and random seeds for the mass-transport deposit zones. For the single-source synthetic model a reduction in observed amplitude with reduced lateral scale length is consistently observed across a range of vertical correlation lengths. For the real world Black Sea example, with realistic elastic and geostatistical parameters based on cone-penetration tests and physical property measurements from sediment cores, we find that when the lateral scale length of the random medium is around 1 m, recorded seismic amplitudes are, on average, reduced by ∼15% relative to unfailed sediments.

We conclude that relatively small amounts of deformation at scales larger than the dominant seismic wavelength are, in general, able to a generate significant decrease in seismic amplitude, without requiring a reduction in the average reflectivity. Our synthetic modelling results should discourage interpretation of the internal structure of mass-transport deposits based on seismic amplitudes alone as “acoustically transparent” mass-transport deposits may still preserve coherent, metre-scale internal structure. In addition, the minimum scale of heterogeneity required to produce a reduction in seismic amplitudes is likely much larger than the diameter of sediment cores, meaning that such mass-transport deposits may still appear well-stratified and undeformed when cored.

How to cite: Ford, J., Camerlenghi, A., Zolezzi, F., and Calarco, M.: Modelling the seismic amplitude response to internal heterogeneity of mass-transport deposits, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7464, https://doi.org/10.5194/egusphere-egu23-7464, 2023.