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The deep water in the ridge segment between 10°18'S and 10°57'S of northern Central Indian Ridge was surveyed with CTD and AUV for seafloor hydrothermal activities. During the survey, 25 CTD hydrocasts were made, and seawater samples were collected for analyses of chemical tracers like dissolved manganese (DMn). In the rift valley between 10°40'S and 10°57'S, consistent backscatter anomalies (up to 0.013 ΔNTU) were found in deep water, indicating excess of suspended particulate at the depth between 3050 and 3500 m. The seawater samples from this turbid layer was characterized with high concentration of dissolved manganese (maximum DMn = 39 nmol/L) and excess helium-3 (maximum δ3He = 17.1%) and thus confirmed the possible hydrothermal activities in that area. The distribution of turbidity and DMn in deep water apparently suggests that there could be active vent(s) located close to the eastern wall (10°47.5'S; 66°38.6'E) of the valley, near southern end of the ridge segment. The physiochemical characters of the plume suggest the source fluid might have originated from reactions involving both mafic and ultramafic rocks, and its nature is comparable to other plumes in the Indian Ocean. [ABSTRACT FROM AUTHOR]

An analogy between submarine channels and fluvial rivers has existed for long, especially on the basis of planform and morphometry. Underlying this broad resemblance are the minute disparities that shape and control these systems. In order to observe and quantify the variations between submarine channels and subaerial rivers, we present a first-ever geomorphometric investigation of one single system, where the fluvial river is compared with its offshore counterpart from source-to-sink. With exhaustive data from the submarine fan, parameters like longitudinal profile, width, sinuosity, slope and planform of the Indus Fan channel-levee complex (CLC) are estimated and compared on the basis of the same parameters estimated for the fluvial Indus River. Our new data analyses offers key insights into the variable geomorphometric patterns prevalent from the source of the Indus River until the margins of the submarine Indus Fan. Channel width and sinuosity vary from high-to-low downstream in the submarine system and from low to high in the fluvial basin. Characteristic depositional features of either system are mutually exclusive. Longitudinal profiles of the submarine fan and the river basin do not conform—principally due to the difference in intensity of erosional and depositional processes active in both regions. These differences are primarily attributed to a single-point (canyon-fed) distributary flow and a multi-point (tributary-fed) cumulative flow source system, and density contrasts between river flows and turbidity currents. By quantifying this variation, our attempt is to dissuade the long-standing morphometric analogy between fluvial rivers and submarine channels. [ABSTRACT FROM AUTHOR]

The Saronikos Gulf hosts the northwestern end of the South Aegean Active Volcanic Arc. It is affected by extensional back-arc tectonism in a prominent N-S direction, while older E-W rifting is also manifested. Recently acquired swath bathymetry and high-resolution seismic profiles led to the precise morphological mapping of five Quaternary continental shelf basins, along with their neotectonic delineations and their cross-correlation to the onshore active fault zones. Aegina and Methana basins, in the southeast Saronikos Gulf, display subsidence along ENE-WSW and NW-SE axes, and continuous marine sedimentation throughout the Upper Quaternary. Salamina basin in the northeast Saronikos Gulf is a shallow, relatively stable sedimentary basin, intermittently connected to the SE and NW Saronikos Gulf through the Aegina-Fleves and the Aegina-Salamina plateau respectively. In the western and most active region of Saronikos Gulf, the onshore E-W–trending faults of Kechrees and Agios Vassileios extend eastwards until the Salamina-Aegina plateau. The Megara basin is WNW-ESE oriented, possibly affected by the N-S Korfos fault valley, which connects the Megara and Epidavros basins. E-W–trending faults tilt the Quaternary sedimentary blocks, in compatibility to the N-S extensional tectonic regime of the adjacent Gulf of Corinth. Prograding clinoforms in the Megara and Epidavros basins reveal an Upper Quaternary-age intermittent connection to the open sea. Mass transport deposits and volcanic flows in-between the Upper Quaternary sediments of the Epidavros basin manifest the 500–400 ka tectonic phase that affected both the NE Peloponnese coast and the active N-S and NW-SE to WNW-ESE striking faults of the Epidavros basin margin. [ABSTRACT FROM AUTHOR]

Quaternary marine deposits of the Ischia Island have been analyzed based on marine geological survey and sedimentological data. The Ischia Island represents the emerged part of a large volcanic field, extending from the Procida island to the submerged volcanoes of the western Ischia offshore. The volcanic field develops over an area of about 42 km2, E-W trending, composed of calderas, such as the Ischia caldera, linked to the great explosive eruptions, plinian and ignimbritic ("Sintema del Rifugio di S. Nicola"). The grain-size analyses of sea bottom samples have shown that gravelly sands, sands, silty sands, muddy sands, sandy silts, and silts all occur. The comparison of the obtained results with previous sedimentological data collected in the Ischia offshore, coupled with the construction of marine geological maps at the 1:10,000 scale, has shown that sands seem to prevail in the eastern Ischia offshore, while gravelly sands, mainly bioclastic, are located at the top of relict volcanic edifices, such as the Forio Bank (western Ischia offshore) and the Ischia Bank (south-eastern Ischia offshore). Moreover, the obtained data show that rhodolith beds occur on the relict volcanic edifice of the Ischia Bank, on the related parasitic vent, and in the Ischia Channel, a morphological saddle located between the Ischia and Procida islands. [ABSTRACT FROM AUTHOR]

Two areas with cold methane seeps on the outer shelf of the Laptev Sea were studied by two interdisciplinary expeditions onboard the RV Akademik Mstislav Keldysh in August–September of 2017 and 2018. These fields lie in water between depths of 63 and 73 m, and in a region of growing interest to the international community. Characteristic features of the methane seeps were obtained, which include their distribution and appearance on the seabed based on acoustic anomalies and seafloor observations. The cold seeps are part of a domain striking in a SW–NE direction along the Laptev Sea Rift System, Khatanga–Lomonosov Fracture Zone, and the Gakkel Ridge junction, and its structure was determined by shallow faults on the outer shelf. These faults are related to subsidence of the outer shelf cutting the caprock formed by permafrost and gas hydrates. Faults serve as conduits for an intense bubble methane discharge at the seabed. Shallow-water methane seep fauna were described for the first time in the Siberian Arctic. The frenulate siboglinid tubeworm Oligobrachia haakonmosbiensis was among the dominant species of the methane seep communities. A newly discovered gastropod species Frigidalvania sp. was also found in abundance at the seeps as well as an ophiuroid Ophiocten sericeum. Significant differences were observed between benthic communities of the two seep fields and background fauna including integral community parameters and the presence/absence of certain species. Development of shallow methane seep communities in the Laptev Sea apparently is related to extremely oligotrophic conditions in this area. [ABSTRACT FROM AUTHOR]

Organic-rich aquatic sediments are a significant source of methane to the atmosphere. In situ remote quantifications of gas content in shallow sediment is a complex task due to its large spatiotemporal heterogeneity. The spatial and multiannual changes of free gas (methane) content (Θ) in shallow sediments were studied in deep subtropical Lake Kinneret. We implemented recently developed acoustic methodology that allows estimating Θ in sediment based on assessment of sound speed. This method is based on measurement of reflection coefficient of acoustic signal at low-to-mid frequencies from the water-sediment interface using geoacoustic inversion technique. The used approach provides an assessment of the mean gas content over certain bottom area near the sampling location. Analysis of acoustic measurements carried out in 2015–2018 shows distinct changes in Θ with bottom depth along the offshore transects. We found the inverse relationship between Θ and lake level. The observed patterns in sedimentary gas content are supported by previous observations showing (a) changes of acoustic sediment properties with depth and (b) inverse relationship between rate of gas ebullition from bottom and lake level. The obtained absolute values of Θ at different locations were the same order of magnitude as those estimated directly from frozen cores. The validity and reliability of the method and its further development are discussed. [ABSTRACT FROM AUTHOR]

Neodymium isotopic composition (εNd) has enjoyed widespread use as a palaeotracer, principally because it behaves quasi-conservatively in the modern ocean. However, recent bottom water εNd reconstructions from the eastern North Atlantic are difficult to interpret under assumptions of conservative behaviour. The observation that this apparent departure from conservative behaviour increases with enhanced ice-rafted debris (IRD) fluxes has resulted in the suggestion that IRD leads to the overprinting of bottom water εNd through reversible scavenging. In this study, a simple water column model successfully reproduces εNd reconstructions from the eastern North Atlantic at the Last Glacial Maximum and Heinrich Stadial 1, and demonstrates that the changes in scavenging intensity required for good model-data fit is in good agreement with changes in the observed IRD flux. Although uncertainties in model parameters preclude a more definitive conclusion, the results indicate that the suggestion of IRD as a source of non-conservative behaviour in the εNd tracer is reasonable and that further research into the fundamental chemistry underlying the marine neodymium cycle is necessary to increase confidence in assumptions of conservative εNd behaviour in the past. [ABSTRACT FROM AUTHOR]

Anoxic marine sediments contribute a significant amount of dissolved iron (Fe2+) to the ocean which is crucial for the global carbon cycle. Here, we investigate iron cycling in four Arctic cold seeps where sediments are anoxic and sulfidic due to the high rates of methane-fueled sulfate reduction. We estimated Fe2+ diffusive fluxes towards the oxic sediment layer to be in the range of 0.8 to 138.7 μmole/m2/day and Fe2+ fluxes across the sediment-water interface to be in the range of 0.3 to 102.2 μmole/m2/day. Such variable fluxes cannot be explained by Fe2+ production from organic matter–coupled dissimilatory reduction alone. We propose that the reduction of dissolved and complexed Fe3+ as well as the rapid formation of iron sulfide minerals are the most important reactions regulating the fluxes of Fe2+ in these cold seeps. By comparing seafloor visual observations with subsurface pore fluid composition, we demonstrate how the joint cycling of iron and sulfur determines the distribution of chemosynthesis-based biota. [ABSTRACT FROM AUTHOR]

This research explores the Messinian record in a western Mediterranean basin at the eastern end of the Betic Cordillera (the Garruchal section, southern sector of the Bajo Segura basin). Sedimentary facies development and foraminiferal assemblages were used as palaeoenvironmental proxies to characterize the palaeoceanographic evolution of the basin's progressive restriction. Therefore, a succession of palaeoenvironmental phases can be established as follows. The "start phase" (until ~ 6.72 Ma) records the first ecological evidence of the restriction, evidenced by the sudden appearance of common benthic stress markers. The "amplification phase" of the restriction (up to ~ 6.35 Ma) marks a progressive increase in low bottom oxygen levels related to water-mass stratification in the evolution from upper epibathyal to outer shelf environments. The "consolidation phase" (until the intra-Messinian discontinuity ~ shortly after 6.03 Ma) records basin shallowing (from outer to inner shelf environments) in a context of a restricted basin. And lastly, the "climax phase" (until the end-Messinian discontinuity) records coastal sedimentation (marine coastal lagoon and sandy beaches). The findings of this research reveal that the Garruchal section is a good illustration of the sedimentary and palaeoenvironmental response of the peripheral basins to the progressive restriction of the Mediterranean before the Messinian Salinity Crisis. Finally, since it is widely accepted that the Mediterranean marine restriction has been developed in a stepwise mode, we here propose to establish the following terms for implementation at Mediterranean regional level: "start," "amplification," and "consolidation" phases, which finally led to the "climax" phase, that is, the Messinian Salinity Crisis stricto sensu. [ABSTRACT FROM AUTHOR]

Tidal ecosystems like the Wadden Sea are particularly valuable for their ecological and economic importance. Here, the natural dynamics of the abiotic and biotic processes is threatened by the human pressure, and great efforts are made on mapping and monitoring programs. Remote sensing techniques (e.g., satellite and airborne sources) are commonly used on land and intertidal areas, whereas hydroacoustic devices are deployed in the subtidal zones. The overlap of hydroacoustics (sidescan sonar) and airborne Lidar data in such sensitive transitional zone (inter- to subtidal) is rather uncommon. In order to test the limitations of both techniques in extremely shallow waters (0.7 m min, water depth) and to find the most efficient methods for the spatial classification of intertidal areas, a portion of the backbarrier tidal flat of Norderney was investigated. Lidar bathymetric data were used for extracting high resolution morphological information. Sidescan sonar mosaics were collected in two following years under contrasting weather conditions. An expert classification based on sidescan sonar backscatter intensity, seafloor texture, morphology, and surface sediment data subdivided the research area into 10 classes. The outcomes were compared with an existing RapidEye-based classification. The tested methods showed both advantages and limitations, which were discussed based on statistical analyses. Satellite and Lidar approaches were most suitable for mapping biogenic features (e.g., shellfish beds) over large areas, whereas sidescan sonar was superior for detail detection and discrimination of morpho-sedimentary regions. As an outlook, it is postulated to perform ground-truthed hydroacoustic mapping on small testing areas, and to use the obtained classification for training satellite-based classification algorithms. [ABSTRACT FROM AUTHOR]

Crabs are ecosystem engineers that dig burrows, which can trap sediment. We used a field experiment to explore how burrow aspect ratio (depth/diameter) affects the trapping efficiency of sediments and organic material. Arrays of burrows mimics were constructed using tubes of similar depths but two different opening diameters: fat tubes (aspect ratio of 3.8) and thin tubes (7.1). Different arrays were tested to examine whether the combination of burrows with two different aspect ratios affects the material capture, while retaining the same total area of openings per array. The results showed that, in general, the fat tubes trapped more organic material, especially large pieces, and more sediment than thin tubes. Furthermore, the silt-clay content of the sediment trapped in the tubes was up to 50% greater than the surrounding surficial sediment. Hence, we conducted 2D numerical simulations of flow around, and into, a single burrow to elucidate the mechanisms behind particle capture. Results showed that the flushing rate and the turbulent kinetic energy were greater in the fat tubes. However, turbulence persisted for a longer distance downstream of the thin tubes than the fat tubes. The enhanced turbulence may increase the flux of sediment into the tubes where particles can settle and, consequently, promote the sediment capture and sedimentation rate. Our results demonstrate that the aspect ratio of burrows significantly affects capture processes of organic material and sediment particles. Moreover, this capture mechanism depends on the flow regime, as faster flows are associated with larger amounts of material in the water column. [ABSTRACT FROM AUTHOR]

Coarse-grained hard substrate areas with grain sizes up to very coarse boulder (> 2 m) are very rare in the German North Sea. The "Helgoländer Steingrund" is one of such highly biodiverse areas: it is characterized by a half-moon-shaped hard substrate ridge, which subdivides the site into a more exposed (westerly) and a less exposed (easterly) flank, characterized by a mixture of sand and gravel deposits. Sonar systems, underwater videos, and bottom samples were used for mapping and classifying the abiotic and biotic components in such very patchy and coarse-grained habitat. Three main seabed types (sand, gravel, and hard substrate) were identified, based on acoustic backscatter data. The additional information coming from underwater videos and sediment bottom sample analysis allowed the description of six different seabed types, which included both the abiotic (sediments, morphology, etc.) and biotic components. The flanks of the ridge and their transition to the surrounding soft-ground areas were characterized by a distinct dominance of the bryozoa F. foliacea and A. diaphanum on the western and on the eastern side, respectively. Morphology and hydrodynamics are likely responsible for such zonation. This is proved by the outcomes of the Acoustic Doppler Current Profiler data, which showed the general flow pattern across the ridge and even resolved the local variability of current pattern, dependent on the tidal stage and bottom relief. [ABSTRACT FROM AUTHOR]

Near-coastal marine sediments often provide high-resolution records of various anthropogenic influences such as the release of heavy metals, which pose a potentially negative influence on aquatic ecosystems because of their toxicity and persistence. In places, the gradual onset of man-made heavy metal emission dates back to ~ 4500 years BP and is difficult to distinguish from potential natural sources. New Zealand offers a perfect setting for studies on anthropogenic impact due to its well-defined three-step development: pre-human era (until ~ 1300 CE), Polynesian era (~ 1300–1800 CE) and European era (since ~ 1840 CE). However, hardly any information exists about the degree of heavy metal input to New Zealand's coastal areas and the 'pristine' natural background values. This study determines the natural background contents of lead (Pb) and zinc (Zn) in marine sediments of the Firth of Thames, a shallow marine embayment on New Zealand's North Island, and investigates anthropogenic inputs in historic times. Eight sediment cores were analysed by X-ray fluorescence (XRF) for their element composition and temporally resolved by a pollen and radiocarbon-based stratigraphic framework. Sharp increases in Pb and Zn contents occurred simultaneously with the onset of goldmining activities (1867 CE) in the nearby catchment area. The contents of Zn (Pb) increase from very stable values around 60 (13) ppm in the older sediments, interpreted to reflect the natural background values, to an average maximum of 160 (60) ppm near the core top, interpreted to reflect a significant anthropogenic input. These findings unravel the history of contamination in the Firth of Thames and provide an urgently needed database for the assessment of its current ecological state. [ABSTRACT FROM AUTHOR]

The Helgoland Mud Area (HMA) in the German Bight, covering an area of approximately 500 km2, is one of a few depocentres for finer sediments in the North Sea. Radiocarbon and 210Pb analyses revealed continuous sedimentation over the last several centuries. Zinc (Zn) and lead (Pb) contents in the sediments show a distinct increase towards the youngest most sediments with the thickness of the heavy metal enriched sediments ranging from 15 to 103 cm. Stratigraphic data indicate that the onset of heavy metal enrichment is diachronous progressing north-westward over the depocentre, paralleled by a decrease in the thickness of the enriched layer. Beginning already during medieval times, the enhanced input of Zn and Pb seemingly is related to silver and zinc mining in the Harz Mountains and the Erzgebirge, well-known mining areas since the Bronze Age. Both regions are directly connected to the HMA by the Elbe and Weser rivers. Zn and Pb enrichment began in the south-eastern HMA and progressed subsequently with an average of 10 m per year north-westward, most likely triggered by variations in river discharge and by the hydrodynamic setting. Quantitative assessments of the Zn and Pb content in the sediments suggest that since the onset of enhanced Zn and Pb deposition, the anthropic Zn and Pb input in the HMA amounts to ~ 12,000 t and ~ 4000 t, respectively. [ABSTRACT FROM AUTHOR]

The swash zone is the most dynamic part of the upper beach. Here breaking waves interact with sediments, leading to beach morphodynamics, and eventually to coastal erosion and flooding. An understanding of swash characteristics such as the vertical excursion and mean swash period is of vital importance to identify and predict the possible impact of storms in coastal areas. The prediction of these swash parameters is commonly based on bulk offshore spectral wave parameters and the beach slope, the latter being expensive to monitor. In this study, a dataset from an intermediate beach on the south-west coast of Sylt, Germany was derived to analyze swash characteristics. Field observations show high variability in swash excursion even for similar incident wave conditions if expressed by bulk parameters. Here, the role of frequency spread was further investigated. An inverse relationship between swash frequency spread and swash characteristics (excursion and mean period) was identified. A parameterization for the swash frequency spread based on incident wave parameters is proposed, and a new approach to obtain total swash excursion and mean swash period is developed. The parameterization is validated using two independent swash datasets (Duck 1982 and Duck 1994) and compared to the predictive ability of other parameterizations. It is highlighted that common parameterizations based on peak period do not perform well in characterizing incident conditions when bi-modal spectra are present. The choice of the right descriptor for wave periods may have a more pronounced influence on the predictive skill than the inclusion of the beach slope. [ABSTRACT FROM AUTHOR]

Based on the analysis of 14 short sediment cores, we present new insights into the distribution of surficial sediments in the central Hauraki Gulf, a semi-enclosed coastal embayment on the northeast coast of New Zealand's North Island. We identify and discuss the effects of interaction of modern wind-generated waves and currents with regard to deposition and reworking of sediments in the Gulf. The modern hydrodynamic regime is controlled by tidal currents, oceanic inflows, and wave-induced currents and it is responsible for a N-S gradient in sediment texture and elemental concentrations in the central Hauraki Gulf sediments. The present-day sediment input into the system is generally low and consists of fine-grained fluvial sediments mostly deposited in the southern study area and comparatively high inputs of relict carbonate material to the northern study sites. The central Hauraki Gulf sediments, which show numerous age reversals in the sedimentary record, can be characterised as palimpsest sediments, as a consequence of continuous reworking and storm-induced sediment transport. In view of the new data, a previously assumed significant post-transgression accumulation of sediments of > 10 m in the central Hauraki Gulf appears to be very unlikely. [ABSTRACT FROM AUTHOR]

Mangrove habitats are complex systems, which are subjected to both natural and human external forces such as tidal variations, sediment supply, deforestation, and climate change, which in many locations are causing mangroves to decline and even disappear. Global measurements of surface elevations have been conducted at many locations to understand if sedimentation rates in mangrove areas will keep pace with sea level rise. However, extending results to other areas requires a detailed understanding of subsidence processes in mangrove areas. Here, we provide a detailed geotechnical investigation (sediment cone resistance and friction, coefficient of consolidation, grain size, normalised tip resistance and friction ratio, etc.), critical for understanding surficial sediment dynamics and vertical sediment accretion rates, of the mangrove forest edge and its surrounding mud flat in the Firth of Thames, New Zealand. Eight in situ samples were collected and tested in oedometer experiments to evaluate the coefficient of consolidation. Furthermore, a kinematic penetrometer, NIMROD, was used to estimate the resistance forces of the mud flat and mangrove forest, from which soil properties were evaluated. Our results show that mangroves are able to change soil properties to enhance sediment resistance towards erosion. An increase of sediment strength correlated with an increase of mangrove tree density as well as a decrease in the coefficient of consolidation. Hence, the increase of tree density to a decrease of the coefficient of consolidation correlates as well. In this study, our results suggest that the Firth of Thames mangroves are able to accrete enough sediment to keep up with local sea level rises if sediment supplies remain similar in the future. The correlation of tree density to the coefficient of consolidation can be used to assess future mangrove-soil interaction and mud flat progression. Finally, the applicability of literature soil classifications for the kinematic penetrometer was realized for the first time and applied to mangrove and mud flat areas. [ABSTRACT FROM AUTHOR]

The Brazilian continental margin includes several volcanic islands, submerged volcanic seamounts, and a unique non-volcanic archipelago located in a transform segment of the Equatorial South Atlantic. The mechanism of formation of these islands is related to post-breakup magmatic episodes dated as Late Cretaceous to Pleistocene. Diverse Late Cretaceous to Paleogene alkaline magmatic episodes are registered in southeast Brazil, resulting in igneous plugs onshore and volcanic structures offshore. The Abrolhos Volcanic Complex in eastern Brazil is characterized by several volcanic features on the continental shelf, including small islands that expose Paleogene sedimentary layers interbedded with volcanic sequences. The adjacent Vitória-Trindade Chain extends to oceanic crust forming basaltic to alkaline seamounts that outcrop at the Trindade Archipelago, the easternmost islands in Brazil with the youngest volcanic eruptions. The Fernando de Noronha lineament in northeast Brazil is characterized by Neogene alkaline igneous plugs. The small islets in the São Pedro—São Paulo archipelago, located near the mid-Atlantic ridge, are formed by exhumed mantle rocks related to compressional episodes a transform fault zone. The Rio Grande Rise in southern Brazil is characterized by shallow Paleogene seamounts and a large oceanic plateau probably related to subaerial spreading centers formed in the Late Cretaceous. Multiple mechanisms are responsible for the origin and evolution of the volcanic islands offshore Brazil in continental, transitional, and oceanic crust settings, including volcanic build-ups, leaking fracture zones, and hotspots. Some of the islands might be related to mantle plume activity, as indicated by comparisons with modern mantle plume analogues in the South Atlantic. [ABSTRACT FROM AUTHOR]

Research about the variability of sand size inside the hopper of a trailing suction dredger has yet to be done, despite its importance for beach nourishment projects. In this study, several samples were taken using different methodologies during dredging for a particular case of beach nourishment on board one dredger. Sampling was carried out with a tube core and a Van Veen grab at the surface when the hopper was full of sand. The results showed problems with the Van Veen grab, the standard methodology used for sampling aboard a dredger. This method should be reconsidered in the future because surface samples were not representative of the whole hopper due to the bias of higher percentage of shell pieces. According to the results, the core tubes introduced by percussion were the only useful method in determining average sand parameters closer to the values obtained for the borrow site. Moreover, cores also showed a linear correlation between sand size and depth: D50 increased 20% from the surface to 1 m depth; this is a sixth of the hopper depth of this study case. Regrettably, the manual percussion procedure could not reach depths greater than 1 m. [ABSTRACT FROM AUTHOR]

This study compares bathymetry extracted from 3D seismic data at two Australian study sites of differing morphological complexities to two sources of collocated multibeam data: 50-m and 5-m multibeam digital bathymetric models (DBMs). Seafloor horizons are extracted from the 3D seismic data and converted to depth using sound velocity profiles collected during seismic acquisition. The resulting seismic-derived DBMs are independent of the multibeam DBMs and are shown to be highly comparable. For the morphologically simple site, the seismic-derived DBM was within ± 2% of the multibeam DBMs and, at 2σ, 95% of differences are in the range − 1.22 to 0.10% (− 1.02 to 0.48%) for the 50-m (5-m) multibeam DBM. For the morphologically complex site, > 80% (> 99%) of seismic-derived depths were within ± 2% (± 5%) of multibeam DBMs. At 2σ, 94% of differences are in the range − 3.48 to 1.69% (− 2.73% to 2.44%) for the 50-m (5-m) multibeam DBM. Increasing morphological complexity and slope angle were the most important factors affecting DBM comparisons, with seismic-derived depths typically underestimated in canyon thalwegs. Despite these differences, the higher data density, multichannel stacking and migration of the 3D seismic data resulted in seismic-derived DBMs with high resolution and improved feature relief and clarity when compared to multibeam DBMs for the conditions in this study (depths of 120–1900 m), particularly for morphological features such as individual rills and gullies. This method has the potential to expand the spatial coverage of high-resolution DBMs, for example, in Australia, by over 150,000 km2. [ABSTRACT FROM AUTHOR]

Multibeam bathymetry, coupled with seismic reflection profiles and sediment cores, was collected to understand the sediment process mechanisms for active onshore-offshore sediment transfer system from coastal erosion to nearshore transfer of sediment into a submarine channel and fan system. Savary Island in the northern Salish Sea of British Columbia, Canada, emerged immediately after deglaciation and became exposed to winter storms that began a continual modification of the island. Subsequent to coastal erosion, sediments are moved from the south side of the island, by strong tidal currents and longshore drift, to the island's north side. The mobilized sands are then entrained into and swept down submarine channels during gravity flows, likely a response to enhanced hydrodynamic tidal flow. Downslope progradation within the channels is primarily a result of downslope migrating submarine dunes that transfer sand to small submarine fans. Further transport beyond the channels into the deeper basin occurs via turbidity flows that have been active throughout the late Holocene. The prospect for Savary Island is grim, as this sediment transfer system will likely continue until the island disappears altogether. [ABSTRACT FROM AUTHOR]

Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins. [ABSTRACT FROM AUTHOR]

Coastal zones are among the most important features of the earth as they have social and economic importance for the environment. Thus, they should be frequently monitored for management issues, for example, shoreline erosion, and any essential precautions for safe management taken. Shoreline extraction is an important tool to determine coastal changes. Modern technologies such as remote sensing give an opportunity to gain up-to-date, accurate, and reliable information with fast data acquisition for coastal regions. In this study, two different coastal areas, one in the northern part of Turkey (Terkos/Istanbul) and the second from the southern part of the country (Kemer/Antalya), were studied. The main objective of this study is to extract shorelines from Sentinel-1A radio detection and ranging satellite data using a non-parametric fuzzy approach, by exploiting Turkish multispectral RASAT satellite images. For this purpose, a whale optimization algorithm was initially employed to create land and water classes from RASAT images. The created classes were used as inputs to determine fuzzy membership values for non-parametric shoreline extractions from Sentinel-1A images. The results are evaluated in comparison with manually digitized reference shorelines by calculating the perpendicular distances between the digitized and remotely sensed shorelines. The calculated average distances from the RASAT-integrated Sentinel-1A (RASAT + Sentinel-1A) for Terkos/Istanbul and Kemer/Antalya were 29.18 m and 16.78 m, respectively. Average differences were also calculated using only RASAT images as 13.37 m and 6.29 m for both areas, respectively. [ABSTRACT FROM AUTHOR]

A robust and accurate method is presented for the geostatistical mapping of marine grain-size data for the Aegean Sea, incorporating new grain-size data and making use of various marine-environmental proxy parameters for interpolation. This approach is based on an indicator kriging algorithm, which also considers marine environment parameters, such as bathymetry and sediment sourcing, through a co-kriging treatment. Data manipulation is carried out without the need of specialised software, in a geographic information system (GIS) environment, with the implementation of geostatistical analysis tools. The quality of the produced map was assessed based on MESH criteria, yielding a total score of 74%; comparison with other interpolation methods, such as IDW and local polynomials showed a better representation of the physical context of surficial sediments. The produced map, making use of all existing information from recent and legacy data, provides valuable information for the sedimentation of Northern Aegean continental shelf, an area of significant scientific interest for geomorphological, palaeoenvironmental and economic studies. [ABSTRACT FROM AUTHOR]

Active seafloor hydrocarbon seepage from three distinct, positive seafloor features, termed knolls, in the Sigsbee Knolls area, Gulf of Mexico (GOM), was investigated in March 2015. The study sites included the Challenger Knoll, which was drilled by the Deep Sea Drilling Program (DSDP) in 1969 and showed the influence of salt tectonics on its evolution and the presence of subsurface petroleum. This study used gas flare mapping in the water column, detailed seafloor bathymetry, backscatter mapping, sub-bottom profiling, and remote sensing of surface water to locate seafloor areas of hydrocarbon emissions. Flares of gas bubbles were detected in the water column above all three knolls, making them the deepest reported gas emission sites in the GOM. Although surface oil slicks above the Sigsbee Knolls have been detected by satellite imaging in the past; in this study, neither satellite imagery nor ship-based observations detected floating oil, this suggests that oil seepage in the region is intermittent. High-resolution video surveys of the seafloor at Challenger Knoll, using a camera system mounted on a lowered tow-vehicle, documented the presence of seep-related chemosynthetic fauna including clam fields and microbial mats at the western part of the knoll. These findings are consistent with previous investigations that link hydrocarbon seepage at Challenger Knoll to salt tectonics. During visual seafloor inspections, no asphalt deposits, such as those known from sites in the Campeche Knoll area in the southern GOM, were observed. These findings indicate that hydrocarbon seepage is less pronounced in the Sigsbee Knolls region, central GOM, than is the case in hydrocarbon systems in the northern or southern GOM. [ABSTRACT FROM AUTHOR]

The presence of methane gas in the shallow sub-surface sediments of the Sea of Galilee, northern Israel, has been well documented over the years. Numerous theories for the mechanism responsible for the release of this gas into the water column have been put forth. These include changes in lake levels, seasonality, and seismic activity. The presence of gas in the sediments has been shown to cause acoustic blanking of the seismic signal. In effect, this means that areas of good seismic penetration are areas where gas is not present in the underlying sediment. In this study, 30 years of high-resolution seismic reflection data from the northwestern corner of the lake was examined. This database has allowed for the examination of changes in the spatial extent of gas in the area over time. Results show that the presence or absence of gas in the sediments does not adhere to one clear governing mechanism as previously assumed, but is rather a combination of factors that must be taken into account. [ABSTRACT FROM AUTHOR]

Intertidal flats regularly emerge and submerge in accordance with changes to the water level occurring over tidal cycles. The alteration between wet and drained states may affect the sediment water content, and so the erosion properties of intertidal sediments. This study examined the influence of ambient temperature on the erodibility of exposed cohesive sediment from an intertidal mudflat in the Firth of Thames, New Zealand, in December 2017 and March 2018. The EROMES device was used to measure the erosion potential of sediment (erosion threshold, τcr N m−2, and erosion rate, ER g m−2 s−1). Samples were drained and exposed to temperatures of 0, 8, 25 and 40 °C, chosen to mimic natural exposed conditions, while submerged samples simulated natural flooded conditions. Results showed that the cohesive sediment became more resistant to erosion when exposed to air compared with submerged samples as a consequence of decreased water content. The water content of exposed sediments decreased by 1.01–1.78 times, a rate which was a function of increasing temperature. The τcr of exposed experiments was 1.2 to 2.2 times higher, whereas ER decreased 1.2 to 6.2 times. Both the December 2017 and March 2018 sampling dates showed a similar pattern of increasing resistance to erosion (higher τcr and lower ER), which corresponded to depleted water content of the exposed sediment at higher temperatures. [ABSTRACT FROM AUTHOR]

Sediment transport processes from the slopes to the deep basins were studied in the southern Cretan margin (Eastern Mediterranean) by means of swath bathymetry, seismic reflection (airgun) profiling, and gravity coring. Mud/silt turbidites, developed in structureless or laminated patterns, appear to dominate the continental slope and the 2700–3600-m deep depressions of Samaria, Messara, and Gortys. These deposits are the result of successive mass transport events and turbidity currents. Over-steepening caused by strike–slip shearing associated with extensional tectonic activity in the shallow crustal levels, shaking due to significant seismic activity, and the presence of weak sediment layers are responsible for frequent triggering of slope failures and the subsequent formation of turbidity currents on the Gavdos slope. On the Cretan slope, sediments channeled as turbidity currents through canyons proved to be the major delivery mechanism, together with debris flows. Subordinate contributions of hemipelagic sediments occur only in the Messara basin. Moreover, steep non-incised slope areas and slope-confined canyons of the Gavdos slope were found to provide comparable sediment contribution with the fluvially connected canyons of the Cretan slope. [ABSTRACT FROM AUTHOR]

Approximately 600 km of multichannel seismic reflection data and multibeam bathymetric data with an area of 2750 km2 indicate that offshore Cide–Sinop is filled by the sediments of four main seismic units dating from the Upper Cretaceous–Paleocene to Plio-Pleistocene. The deposits from the Eocene to modern times are highly eroded from their upper surfaces and widely outcropped at the seafloor. These units constitute the large flat shelf plain with an average depth of 100 m. Moreover, there are no significant marine onlaps identified in the shelf area. These stratigraphical and morphological features indicate that, until very recently, the area was a terrestrial landscape. Structurally, the shelf-plain and shelf-slope are primarily deformed by E–W-trending active reverse faults and strike–slip faults with a compressional component. These faults are densely spaced toward the outer shelf as fault segments bordering E–W-oriented dunes on the seafloor. A strike–slip fault with a reverse component delimits the shelf edge at long distances. Toward offshore İnebolu, the fault character changes to a negative flower structure that bends to the WNW direction, which indicates that the strike–slip fault is right lateral. The strike–slip faulting is also effective along the shelf slope deforming the slumps. These tectonic structures demonstrate that the shelf area is widely deformed by E–W-trending active transpressional strike–slip faults and reverse faults. This result reveals that the southern Black Sea shelf is under a recent N–S compressional tectonic regime. [ABSTRACT FROM AUTHOR]

Subsidence in south Louisiana is a fundamental process and major source of uncertainty influencing restoration planning and design and wetland habitat resilience. The primary goal of this study was to conduct an analysis of recent subsidence rates within the Barataria Basin. High-resolution geodetic GPS elevation measurements at 19 benchmarks were used to determine recent subsidence rates for the Barataria Basin. Net elevation changes at all stations equaled or exceeded predicted measurement uncertainties based on session duration. Water elevation change from two gauges in the southern part of the basin supplemented survey data, resulting in a range of subsidence velocities at 21 locations from about 2 to 7 mm/year. These data record short-term subsidence trends (6- to 16-year time series) that are expected to be representative of conditions at proposed restoration sites over the next 20 to 50 years. Foundation/rod depths for benchmarks ranged from near surface to about 30-m deep. None of these foundations/rods were sleeved or anchored and thus were not isolated from surrounding consolidating sediment and associated downdrag forces. Therefore, subsidence rates at these sites were considered representative for the entire sediment column. Subsidence rate comparisons for nearby deep and shallow rod benchmarks support this conclusion. Spatial variability in subsidence indicates a compelling relationship between subsidence rates and the age, composition, and thickness of Holocene deltaic deposits. The Coastal Protection and Restoration Authority of Louisiana intends to use this same approach to continue refining subsidence rates throughout coastal Louisiana to support coastal restoration planning and design. [ABSTRACT FROM AUTHOR]

Seagrass meadows are important benthic habitats contributing to many aspects of ecological community health, beach stability and sediment supply. Relationships between morpho-sedimentary features and distribution of seagrass meadows were investigated through an integrated analysis of geomorphology, sediments and benthic habitat structure in a temperate nearshore setting (Esperance Bay, Western Australia). The results demonstrate that seagrass distribution is related to gradients in sediment texture and composition, hydrodynamics and human impact. Dense seagrass meadows occurred in more sheltered regions of the bay, whereas sparser vegetation was found in areas of higher wave energy and artificial activities (like ship anchoring and dredging activities). A preferential retention of heavier siliciclastic sediments was recorded in the seagrass meadows especially in areas with high sediment supply resulting in elevated beds and formation of intermates. Sediment characteristics suggest that carbonate sediment is transported onshore from the seagrass meadow supplying the beach system. The combined analyses of geomorphological features and sediment characteristics at Esperance have allowed identification of a prevalent eastward oriented sediment transport current. Seagrass beds are confirmed to be a sediment source in terms of sediment budget contributing to beach stability. Integrated geomorphological data, sediment analyses and benthic habitat mapping provide useful information for the management of coastal environments characterized by the presence of seagrass meadows by providing new insights on coastal processes. [ABSTRACT FROM AUTHOR]

A new geotechno-stratigraphic model for Quaternary deposits in the Western Irish Sea Mud Belt area is presented. This area, located in the North Irish Sea, has been heavily influenced by the advance and retreat of the British and Irish Ice Sheet during the last glaciation and subsequent Holocene marine transgression. This study uses a synthesis of lithostratigraphy, geotechnical data (from in situ cone penetration testing) and seismic profiles to generate a stratigraphic framework describing the geometry, distribution and characteristics of Quaternary deposits in an area of the Western Irish Sea Mud Belt at increased resolution. Within a regional context, this stratigraphy is compared with other established offshore stratigraphies in the Irish Sea and terrestrial successions. Four stratigraphic units are identified consisting of a basal subglacial (lodgement) till (Unit 4) emplaced by the Irish Sea Ice Stream. Unit 4 overlies irregular bedrock and is overlain in turn by glaciomarine to glaciolacustrine, ice-proximal outwash and glaciomarine ice-proximal sand and sandy muds (Units 3 and 2 respectively; previously undifferentiated). A thick succession (up to 27 m) of Holocene marine muds (Unit 1) caps the sequence up to the contemporary seabed. Results show that these deposits, and their geotechnical properties, have significant implications for the anthropogenic use of the area, such as the development of offshore renewable energy infrastructure. Revealed are relatively thick (up to 30 m) weak, under-consolidated sediments at the surface and highly heterogeneous, often over-consolidated, sediments that have limited groundtruthing at depth. Furthermore, localised shallow gas is imaged on seismic profiles. The influence on the geotechnical properties of the sediments by this gas revealed no significant effect on shear strength values from cone penetration testing data, although it may have implications for the long-term behaviour of the sediments. [ABSTRACT FROM AUTHOR]

Hundreds of newly discovered gas seeps, craters, and mounds are presented from the central Barents Sea, Norway. Where they occur together, the crater-mound pairs have a preferred orientation towards the southeast. According to complex cross-cutting relationships with iceberg ploughmarks, both the craters and mounds formed during deglaciation (ca. 11–13 ka BP). Free gas, trapped beneath shallow gas hydrates in discrete areas, deformed or displaced overlying bedrock and sediment, forming mounds. Over-steepened slopes, weak layers, fractured bedrock, and unstable sediment resulted in small landslides towards the southeast: the direction of regional slope. In some cases, the highly buoyant, hydrate-cored sediment mounds may have instead, detached and floated with bottom currents before being re-deposited, either locally or distally. In this scenario, southeasterly flowing bottom currents would also explain the preferred orientation of the observed crater-mound pairs. Crater collapse would have occurred simultaneously or subsequently to landslide or detachment and floating due to the rapid dissociation of any remaining gas hydrates; a result of attendant regional changes in temperature and pressure following deglaciation and local removal of overlying sediment. The model of formation developed in this study explains the southeasterly trend in crater-mound orientation, but differs from other studies that interpret modern mounds, located elsewhere in the Barents Sea, as gas hydrate-cored. [ABSTRACT FROM AUTHOR]

Internal waves (IWs) are an ubiquitous phenomenon in natural stratified fluids, including oceanic and coastal waters. Turbulence caused by their breaking can trigger vertical mixing of sediments, nutrients, and other dissolved substances. A major generation mechanism of IWs in coastal waters is the interaction of currents with topographic features. Additionally, current shear can lead to the development of Kelvin-Helmholtz instabilities, independent of the estuarine morphology. These instabilities normally form stationary rolled up vortices, which can also be imaged in echograms. In this study, we present new indications that propagating IWs in the Ems estuary arise from Kelvin-Helmholtz instabilities. The Richardson number drops below 0.25 at lutocline at the beginning of IW generation. However, the subsequently appearing lutocline undulations did not roll up, like the typical Kelvin-Helmholtz billows, but evolve into propagating and growing IWs of Holmboe type. Some of these waves were even subject to wave breaking. IW breaking occurred mainly during the second half of the ebb tides and is accompanied by vertical up-mixing of fluid mud. The turbulence and vertical mixing, caused by IW breaking, strongly decreases the local stratification by up to 60%. [ABSTRACT FROM AUTHOR]

Seabed liquefaction occurs frequently under waves in coastal areas due to accumulation of excess pore water pressure. During wave-induced liquefaction, submarine silty soil fluctuates like a fluid. In this study, a set of falling-ball test devices was made and used to measure the viscosity coefficient of liquefied silty soil during silty soil fluctuation experiments. Smooth density balls (copper, iron, and zirconia) of three different densities were used. A laser displacement sensor was used to record the displacement of the density ball with time. Each density ball sank at a constant speed when the forces on the ball reached equilibrium. According to the result of the force analysis, the movement of a density ball in this period could be regarded as vertical uniform motion, and the ratio of displacement to time was used as the sinking speed. Therefore, the viscosity coefficient of liquefied soil was calculated via Stokes' law. Because of the smaller density and the effect of the movement of liquefied soil, there was a large error with the measurement of the zirconia ball in the falling-ball method. Therefore, the data for other density balls (excluding zirconia balls) were selected. The results showed that the range of the viscosity coefficient of the liquefied silty soil was 0.81–1.71 kPa s. The fluctuation amplitude of the liquefied soil and the effects of soil action on viscosity measurements were also evaluated. Thus, Stokes' law can be used to calculate the viscosity coefficient of liquefied silty soil by the falling-ball method. [ABSTRACT FROM AUTHOR]

In this study, the Cenozoic sedimentation and infill history of the Great South Basin (GSB), New Zealand, is analysed from a seismic geomorphologic perspective. A suite of sediment types, including mass-transport deposits (MTDs), deltaic clinoforms, contourite-drifts and turbidites, are documented based on high-quality 3-D seismic reflection data and multiple regional 2-D seismic profiles. The MTDs include older, highly compacted and deeper Palaeocene deposits that are markers of late Neogene tectonic reactivation, while the younger MTDs were translated over slopes eroded by drifts. Possible trigger mechanisms for mass wasting may include oversteepened margins, prolonged fluid dissipation and weak geological layers. Sedimentation from the Eocene to Recent was contemporaneous with regional plate reorganisation and syn-orogenic activity. As a result, three distinct Eocene deltaic systems with variably oriented channels and depositional elements provide evidence for changing plate kinematics during the Eocene. The Eocene deltaic systems are river-dominated and were deposited during relative rise in sea level under variable flow regimes. The passage of the Antarctic Circumpolar Current in the GSB from the Late Eocene to Oligocene led to the deposition of three elongate, detached contourite drifts. In a final phase of basin infill, hemipelagic sedimentation and deposition by turbidity currents dominated late Neogene sedimentation in the GSB. The analyses presented here demonstrate the importance of geomorphology in understanding the sediment infill history, their interactions and temporal organisation, which have wider implications for numerous geoscience disciplines. [ABSTRACT FROM AUTHOR]

The scope of the present work is to apply tested techniques of pattern recognition on bathymetric data from the Eratosthenes Seamount to (a) detect topographic highs and linear-curvilinear lineaments present on the seabed, (b) evaluate the morphostucture of the seamount and its relation with the degree of deformation the region has suffered, and (c) to check if shallow salt movements are associated with specific patterns on bathymetric data. Our scope is implemented by applying a two-step procedure on bathymetric data. Initially, the topographic highs in the study area are detected and their geomorphologic attributes (orientation, average slope, eccentricity, and shape complexity) associated with the degree of seabed deformation are computed. Then, quantitative attributes (slope, aspect, and their derivatives) are combined to automatically detect seabed morphologic structures (lineaments, highs, or depressions), which are potentially related to the presence of geological faults or triggered by other subsurface processes. Based on the analysis in this work, two major segments (the northern and the southern) are identified on the Eratosthenes Seamount and related to E-W trending oblique faults. These two segments are highly deformed according to the quantitative estimation of the geomorphologic attributes. Furthermore, ENE-WSW to NE-SW and WNW-ESE to NW-SE lineaments are identified and limit this prominent topographic high. Smaller topographic highs accompanied by topographic depressions, north and southwest of the Eratosthenes Seamount are also detected, showing medium to strong deformation and reflecting a similar geodynamic activity in this area. As a corollary, a comparison of the bathymetric patterns shows that the present seabed morphology north of the Eratosthenes Seamount relates to shallow salt movements, which create a chaotic pattern. This pattern is expressed by short and curved normal faults, shear zones, possible small-scale reverse faults, massive extrusion of salt bodies (possibly allochthonous), and Pliocene-Quaternary minibasins or peripheral sinks. [ABSTRACT FROM AUTHOR]

Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones. [ABSTRACT FROM AUTHOR]

Deep-sea fans have been proposed to act as carbon sinks, rapid deposition driving shallow methanogenesis to favor net storage within the gas hydrate stability zone (GHSZ). Here, we present new evidence of widespread gas venting from the GHSZ on the upper Amazon deep-sea fan, together with analyses of the first samples of gas hydrates recovered offshore NE Brazil. Multibeam water column and seafloor imagery over an 18,000-km2 area of the upper Amazon fan reveal 53 water column gas plumes, rising from venting features in water depths of 650-2600 m. Most gas vents (60%) are located along seafloor faults that record the ongoing gravitational collapse of the fan above deep décollements, while others (40%) are located in water depths of 650-715 m within the upper edge of the GHSZ. Gas compositions from hydrates recovered in vents at three locations on and north of the fan indicate biogenic sources (dominantly methane with 2-15% of CO2; δ13C from − 81.1 to − 77.3‰), whereas samples from vents adjacent to the fan proper include possible thermogenic contributions (methane 95%, CO2 4%, and ethane 1%; δ13C - 59.2‰). These results concur with previous findings that the upper edge of the GHSZ may be sensitive to temporal changes in water temperatures, but further point to the importance of gas escape from within areas of gas hydrate stability. Our results suggest the role of fluid migration along pathways created by faulting within rapidly deposited passive margin depocenters, which are increasingly recognized to undergo gravitational collapse above décollements. Our findings add to evidence that gas can escape from sediments to the sea in areas where gas hydrates are stable on passive margins, and suggest the need of further studies of the dynamics of deep-sea depocenters in relation to carbon cycling. [ABSTRACT FROM AUTHOR]

Based on 3D and 2D high-resolution multichannel seismic reflection data in the Western High-Sea of Marmara, this study reviews shallow gas occurrence and related structures and classifies gas conduit systems within the upper, few hundred meter-thick sediment layers below the seafloor. Acoustic anomalies including high amplitude-reverse polarity reflections (bright spots), low amplitude transparent zones, chaotic or discontinuous reflections, pull-down effects, and plumes in the water column are interpreted in terms of natural gas occurrence and fluid flow structures (e.g., mud volcanoes, pockmarks). The gas occurrence is thought to be mostly of thermogenic origin. Mud volcanoes are one of the primary gas conduits forming craters on the seabed due to overpressure of fluidized gassy sediment flows. Following the reach of the Northern Branch of the North Anatolian Fault (NAF-N) to the Western High, the thermogenic fluids are believed to migrate vertically and horizontally to shallow depths mainly through the faults. Natural gas most probably originates from the Thrace Basin Eocene source rock or the Eocene-Oligocene reservoir rock, which extends below the Western High. Shallow gas is distributed by minor faults and gas pipes. Gas, to some extent, emanates from the seafloor via pockmarks and mud volcanoes or is trapped by the crests of the anticlines coinciding with erosional surfaces, impermeable sediments, and gas hydrate-bearing layers. Shallow traps below the tectonized “Western High” structure are likely located in thin layers of sands imbedded with impermeable silty clay layers. However, there is no shallow reservoir in the usual sense within the upper layers imaged by the 3D seismic data (< 300 ms two-way travel time). The existence of gas is an indicator of hydrocarbon-rich layers at depth and of active tectonics, and it also impacts the global climate and marine life conditions. [ABSTRACT FROM AUTHOR]

The North Sicily continental margin is a very active region located in the Central Mediterranean. Strong seismicity, active tectonics and volcanism, fluid escape, high sediment supply, and widespread mass movements historically have exposed this region to marine geohazards, with a potential for tsunami generation. Morpho-bathymetric analysis revealed that one of the most common mechanisms associated with marine geohazards is due to submarine mass failure processes, genetically linked to the other processes active in this margin. With the aim to assess the risks associated with landslide-generated anomalous waves, we selected two sectors of this margin, Gulf of Palermo to the west and Patti offshore to the east. The workflow included analysis of the morpho-bathymetric data, morphometric characterization, calculation of parameters of landslide-generated waves, and computation of run-ups by using different algorithms. Assuming that each of the identified landslides could be a potential tsunamigenic source, we calculated the associated theoretical run-ups, referring to the main computation methodologies proposed in the scientific literature. In order to identify the methodology that better suits run-up values for landslide-generated tsunami, we compared the known run-up values of actual, historical cases with those calculated through the different methodologies. The values obtained with the most suitable equation, both for theoretical and historical events, fit a curve that we used to formulate an empirical law describing the relationships between amplitude and depth, at the source point, and relative run-up. It can be used to calculate easily and promptly the run-up associated with a generic landslide-generated tsunami. [ABSTRACT FROM AUTHOR]

A multibeam echosounder survey was conducted (deeper than ca. 300 m water depth, total area: ca. 1.8 × 103 km2) in the southern basin of Lake Baikal, Russia, in June 2015, 2016, and 2017. Characteristic morphology of the lake floor was mapped on the ancient Tankhoy stratum, covered with present sediment, by high-resolution bathymetry. Sediment core sampling operations were conducted in August 2015 and August 2016 at a characteristic mound-like landform named Kedr (after the Kedrovaya River). Sub-surface gas hydrates (GH), containing not only microbial but also thermogenic gases (Hachikubo et al. 2016), were retrieved. Core lithology and sub-bottom profiler survey suggest that Kedr is a mud volcano (MV). Hydrogen and oxygen isotopic anomalies were observed in the sediment pore waters. This suggests the water results from clay mineral dehydration, which is the first observation of this process in Lake Baikal sediment pore water. The thermogenic gases, mud breccia, and water from clay mineral dehydration suggest potential ascending gas as well as water from greater depths, presumably from the ancient Tankhoy stratum under the Kedr MV. [ABSTRACT FROM AUTHOR]

A dataset comprising geomorphological information (side-scan sonar, bathymetry, samples, drop camera, scuba diving, and a limited number of subbottom profiles) revealed a large number of low-relief buildups occupying almost half of the Gulf of Gera, a shallow land-locked environment. They probably represent mud mounds constructed of biogenic fragments and benthic assemblages consisting mainly of mollusks in a fine-grained matrix, and they are confined to the central part of the gulf. According to their morphometric characteristics and surface distribution, the mounds were separated into three subtypes with the highest (up to 2.5 m), larger coalescing features occurring close to the gulf entrance, whereas the smaller and more circular features appear in an elongated zone at the center of the gulf. This configuration coincides with the gulf’s circulation pattern that disperses and transports nutrients and fine-grained material. The high-resolution subbottom profiles, acquired in a restricted zone of the southeastern gulf, have shown the presence of paleo-mound surfaces in the Holocene highstand surface layer and fluids in the underlying sediments. Similar formations have been rarely reported in the literature, and those other reports are from different environmental settings. Consistent with the literature, they are probably developed due to the interplay of physical and biological processes. However, only an interdisciplinary study could shed light on the specific processes that drive their formation and their unique distribution pattern in the specific environment of the Gulf of Gera. [ABSTRACT FROM AUTHOR]

Very large (>60×106 m3) sackungen or deep-seated gravitational slope deformations occur below sea level along a steep fjord wall in central Douglas Channel, British Columbia. The massive bedrock blocks were mobile between 13 and 11.5 thousand radiocarbon years BP (15,800 and 13,400 BP) immediately following deglaciation. Deformation of fjord sediments is apparent in sedimentary units overlying and adjacent to the blocks. Faults bound the edges of each block, cutting the glacial section but not the Holocene sediments. Retrogressive slides, small inset landslides as well as incipient and older slides are found on and around the large failure blocks. Lineations, fractures and faults parallel the coastline of Douglas Channel along the shoreline of the study area. Topographic data onshore indicate that faults and joints demarcate discrete rhomboid-shaped blocks which controlled the form, size and location of the sackungen. The described submarine sackungen share characteristic geomorphic features with many montane occurrences, such as uphill-facing scarps, foliated bedrock composition, largely vertical dislocation and a deglacial timing of development. [ABSTRACT FROM AUTHOR]