Present Seafloor Research Articles

Research on the classification of seabed sediments sonar images based on MoCo self-supervised learning

The discrimination of seafloor substrate type is an extremely important part of seafloor science, and the substrate information is of great significance for the development of marine science and the protection of the marine environment. Current sonar equipment can efficiently generate seafloor images and present seafloor information visually, so the seafloor substrate classification technology based on sonar images has become a hot research topic. Convolutional neural network, as one of the most important classification algorithms in seabed substrate sonar image classification, has excellent performance in most cases. However, the size of the convolutional kernel of convolutional neural network limits the global feature extraction ability, and the ability to discriminate global features in sonar images is weak. In addition, seabed substrate sonar images have labelled data acquisition difficulty and high cost, and acoustic seabed substrate classification in practice generally belongs to small sample classification scenarios. Aiming at the above problems, this thesis selects Swin Transformer, which has strong global feature extraction ability, as the classifier, and uses MoCo self-supervised learning to pre-train the unlabeled data in order to achieve better results.

Fossilized silica diagenetic fronts: Implications for palaeoceanographic evolution across the Falkland/Malvinas plateau

A set of newly collected 2D seismic reflection data allows the mapping of two distinct cross-cutting reflectors across the Falkland/Malvinas Plateau. Reflector XR-F/MB in the Falkland/Malvinas Basin appears as a bottom simulating reflector that mimics the geometry of the present seafloor, whereas reflector XR-F/MT in the Falkland/Malvinas Trough is a non-bottom simulating reflector that mimics a shallower reflector representing the Early-Middle Miocene unconformity. The discordant geometry of these two reflectors with respect to the host-stratigraphy is argued to be associated with Opal-A to Opal-CT diagenesis, which is primarily a function of temperature. However, the estimated temperature at the present depth for reflector XR-F/MB lies below the minimum temperature for the onset of silica diagenesis. Based on their geometry and seismic characteristics, the two reflectors are interpreted to be fossilized silica diagenetic fronts, formed under palaeo-thermal conditions different from today. We hypothesize that the erosional action of intensified deep and bottom water masses subsequent to Antarctic glaciations during the Early-Middle Miocene may have driven the fossilization of the diagenetic front in the study area. It is estimated that erosion of a minimum of 270 m of overburden would account for the temperature drop driving the fossilization of the silica diagenetic fron.

An updated Weichselian chronostratigraphic framework of the Kongsfjorden Trough Mouth Fan and its implications for the glacial history of Svalbard

The Arctic is a climate‐sensitive area, responding rapidly to present changes, but for the past changes, the record is still incomplete. For instance, the Weichselian glacial history of the Svalbard–Barents Sea Ice Sheet (SBIS) has largely been reconstructed based on studies of the fragmentary Spitsbergen terrestrial and shelf records. However, the sediments removed from the land and shelf areas during peak glacials were deposited on trough mouth fans located along the continental slope. By studying the stratigraphy and processes of the trough mouth fans, comprising a more complete sediment archive, our new data have allowed gaps in the Weichselian glacial history of the SBIS to be refined and filled. Here we present new lithological and geochronological data from the Kongsfjorden Trough Mouth Fan, closely linked to the advance and decay of the SBIS. High‐resolution TOPAS seismic profiles reveal three distinct packages of glacigenic debris flows (GDFs) within its upper stratigraphy, each interpreted to represent an advance of the SBIS to the shelf edge. A radiocarbon dated, 12.6‐m‐long core from the southern flank of the Kongsfjorden Trough Mouth Fan penetrates trough sediments directly linked to the youngest GDF package and terminates in the second GDF, allowing us to study the last two Kongsfjorden ice‐stream advances in greater detail than was previously possible. The age model of core GS10‐164‐09PC, based on combining 14C‐, 18O‐stable isotope and magnetic susceptibility data, spans the last ~54 ka. An Early Weichselian glacial advance is tentatively dated to have ended at ~90 ka. A second peak glaciation is estimated at ~70 ka, followed by a deglaciation from ~54 ka. An ice rafted debris‐rich unit (U7) dated between 38 and 34 ka, followed by a plumite (U6), indicates an advance of unknown extent. The Last Glacial Maximum advance is dated to before 24 ka BP, followed by a rapid deglaciation at ~15 ka. The presence of coarser‐grained sorted sediments at the present seafloor is attributed to the influence of the West‐Spitsbergen Current, acting on water depths of at least 846 m, and is thought to have worked in the vicinity of the coring site since ~14 ka BP.

Widespread hydrothermal vents and associated volcanism record prolonged Cenozoic magmatism in the South China Sea

Abstract The continental margin of the northern South China Sea is considered to be a magma-poor rifted margin. This work uses new seismic, bathymetric, gravity, and magnetic data to reveal how extensively magmatic processes have reshaped the latter continental margin. Widespread hydrothermal vent complexes and magmatic edifices such as volcanoes, igneous sills, lava flows, and associated domes are confirmed in the broader area of the northern South China Sea. Newly identified hydrothermal vents have crater- and mound-shaped surface expressions, and occur chiefly above igneous sills and volcanic edifices. Detailed stratigraphic analyses of volcanoes and hydrothermal vents suggest that magmatic activity took place in discrete phases between the early Miocene and the Quaternary. Importantly, the occurrence of hydrothermal vents close to the present seafloor, when accompanied by shallow igneous sills, suggest that fluid seepage is still active, well after main phases of volcanism previously documented in the literature. After combining geophysical and geochemical data, this study postulates that the extensive post-rift magmatism in the northern South China Sea is linked to the effect of a mantle plume over a long time interval. We propose that prolonged magmatism resulted in contact metamorphism in carbon-rich sediments, producing large amounts of hydrothermal fluid along the northern South China Sea. Similar processes are expected in parts of magma-poor margins in association with CO2/CH4 and heat flow release into sea water and underlying strata.

Evolution of deepwater turbidite bedforms in the Huaguang channel–lobe transition zone revealed by 3D seismic data in the Qiongdongnan Basin, South China Sea

The theoretical model of turbidite bedform evolution has been verified based on laboratory tests and outcrop surveys. However, the evolution of the bedforms associated with flow regime variation within the channel–lobe transition zone (CLTZ) is poorly documented and generally extremely poorly understood. In this study, a combination of seismic sedimentological and topographic analyses, based on high-resolution 3D seismic data within the Qiongdongnan Basin of the South China Sea, was used. Continuous bedforms varied with the depositional elements within the CLTZ of the Huaguang leveed channel system along the flow direction. The depositional elements within the CLTZ change from the main channel to the distributary channels as well as to the sub-distributary channels. Numerous bifurcation and confluence points of gravity flows illustrated the complexity of the drainage network within the CLTZ. The average asymmetry index of the bedforms increased from 0.8 to 1.6, with the average wavelength increasing from 803 m to 1545 m along the thalweg of the channels. Crevasse splays at the starting reach of the CLTZ are likely to form the initial state of the new CLTZ in the future, because the slope gradient starting from the avulsion point was higher than that inside the channel. The deep scours at the bifurcation points and high steps at the confluence points demonstrate the critical roles of supercritical flows and hydraulic jumps in reworking the bedforms of the CLTZ. The high steps at confluence points were simultaneously formed along the margin and thalweg of the main channel. The acceleration mechanisms of flow velocity forming the high steps were the increase of the elevation difference along the internal slope of the main channel margin and promotion of discharge along the main channel thalweg. This paper presents newly completed deepwater observations of the present seafloor bedforms based on the turbidity currents within the CLTZ. These observations update the evolution model of local supercritical flows and hydraulic jumps under the global decreasing trend of the Froude numbers within CLTZs.

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

AbstractEvidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Optimizing Sensor Configurations for the Detection of Slow‐Slip Earthquakes in Seafloor Pressure Records, Using the Cascadia Subduction Zone as a Case Study

AbstractWe present seafloor pressure records from the Cascadia Subduction Zone, alongside oceanographic and geophysical models, to evaluate the spatial uniformity of bottom pressure and optimize the geometry of sensor networks for resolving offshore slow‐slip transients. Seafloor pressure records from 2011 to 2015 show that signal amplitudes are depth‐dependent, with tidally filtered and detrended root‐mean‐squares of <2 cm on the abyssal plain and >6 cm on the continental shelf. This is consistent with bottom pressure predictions from circulation models and comparable to deformation amplitudes from offshore slow slip observed in other subduction zones. We show that the oceanographic component of seafloor pressure can be reduced to ≤1‐cm root‐mean‐square by differencing against a reference record from a similar depth, under restrictions that vary with depth. Instruments at 100–250 m require depths matched within 10 m at separations of <100 km, while locations deeper than 1,400 m are broadly comparable over separations of at least 300 km. Despite the significant noise reduction from this method, no slow slip was identified in the dataset, possibly due to poor spatiotemporal instrument coverage, nonideal deployment geometry, and limited depth‐matched instruments. We use forward predictions of deformation from elastic half‐space models and hindcast pressure from circulation models to generate synthetic slow‐slip observational records and show that a range of slip scenarios produce resolvable signals under depth‐matched differencing. For future detection of offshore slow slip in Cascadia, we recommend a geometry in which instruments are deployed along isobaths to optimize corrections for oceanographic signals.

Chronology of post-Messinian faulting along the Levant continental margin and its implications for salt tectonics

Abstract Numerous normal faults, expressing thin-skinned salt tectonics, are detected along the Levant continental slope, 15–30 km west of the Israeli coast. Previous studies estimated that this faulting initiated during Late Pliocene to Early Pleistocene, which leaves an uncertainty of about 50% in the total duration of active faulting. The purpose of this study is to improve the dating of the onset of faulting, and to reconstruct displacement rates with time. This will allow better understanding of salt tectonics as well as better assessment of geohazards. To increase dating resolution, we use 2D and 3D seismic material integrated with well logs and cuttings to combine bio- and seismo-stratigraphic data and to divide the Plio-Quaternary section offshore Israel to 4 depositional units: Pliocene (5.33–2.6 Ma); Gelasian (2.6–1.8 Ma); Calabrian-Ionian (1.8–0.35 Ma); and Ionian-Holocene ( Our analysis shows that during the Pliocene (duration of 2.7 my) faulting activity was minor. In the Gelasian (duration of 0.8 my) faulting activity peaked alongside huge slumping. In the past 1.8 my, faulting and slumping had both decreased, but they are still active today. The observation that the onset of faulting significantly predates the progradation of the shelf edge to its present location indicates that differential loading was probably not the first trigger that initiated salt tectonics offshore Israel. The association of intense faulting and slumping in the Gelasian is interpreted as a result of basinwards tilting due to regional inland uplift and basin subsidence. Yet positive feedback between faulting and slumping cannot be ruled out. The presence of fault scars on the present seafloor, where current sedimentation rate is similar and even faster than the average displacement rate, apparently indicates that faulting is dominated by sudden seismic ruptures rather than by continued creeping. Hence, considering sedimentation rate of about 50 m/my, a 1 m high fault scarp should be roughly younger than about 20 ky.

Morphology, architecture, and evolutionary processes of the Zhongjian Canyon between two carbonate platforms, South China Sea

Two isolated Neogene carbonate platforms (Xisha and Guangle carbonate platforms) have developed in the rifted uplifts since the Early Miocene. A large-scale submarine canyon system, the Zhongjian Canyon (ZJC), has developed in the tectonic depression between the two platforms since the Middle Miocene. High-resolution bathymetry data and 2D and 3D seismic data reveal the existence of the ZJC on the present seafloor, as well as in Neogene intervals. It exhibits typical characteristics of deepwater canyons that cut the surrounding rocks and indicate strong erosional features. The ZJC resulted from northwest–southeast strike-slip fault activities during synrift and postrift stages, and it periodically grew during the development of carbonate platforms since the Middle Miocene. We identified four cycles of parallel to subparallel high amplitude and dim reflectors in seismic data, which we interpreted as alternating canyon fill, based on the interpretation of seismic facies. Thus, the sedimentary evolution of the ZJC can be divided into four typical stages, which were in the Middle Miocene, Late Miocene, Early Pliocene, and Pleistocene. Considering the tectonic background of the carbonate platforms, as well as the on-going igneous activities, the sediment filling the canyon could be derived from a mixture of carbonate clasts, igneous clasts, mud, and silt. The laminar high-amplitude reflectors and dim-reflector package represented a fining-upward sedimentary cycle. The coarse-grained sediment in canyon fillings could be turbidites, carbonate debrites, and even igneous clasts. In contrast, the fine-grained sediment is likely to be dominated by pelagic to hemipelagic mud, and silt. This case study describes a deepwater canyon under a carbonate-dominated sedimentary environment and has significant implications for improving our knowledge of periplatform slope depositional processes. Furthermore, the insight gained into periplatform slope depositional processes can be applied globally.

High-resolution 3D seismic exhibits new insights into the middle-late Pleistocene stratigraphic evolution and sedimentary processes of the Bear Island trough mouth fan

Arctic Ocean trough mouth fans (TMFs) represent a valuable archive of glacial-interglacial sedimentary processes that are especially important when reconstructing pre-Weichselian glaciations that may lack distinct imprints on the shelves. In 2011, we acquired the first high-resolution 3D seismic cube (~3 m vertical and 6 m horizontal resolution) on the continental slope of the SW Barents Sea by use of a P-Cable 3D system, to study in detail the seismic stratigraphy and glacial depositional history of the Bear Island Trough Mouth Fan. This technology provides data with a resolution that, for the first time on the western Barents Sea slope, enables detailed mapping of deposits of different glacial cycles. The dataset provides entire spatially coverage, allowing us to reconcile multiple generations of glacigenic deposits and channel systems. High-resolution 3D seismic data is crucial to describe buried channels, glacial units, as well as low relief landforms such as sediment waves accurately. The 30 km2 seismic cube is located at the southern flank of the Bear Island TMF at water depths from 592 to 660 m where sandwaves dominate the present seafloor. The data covers the glacially derived stratigraphy in the uppermost ~700 m below the seafloor. We establish a robust stratigraphic framework by interpreting seismic reflectors along 2D tie-in lines to previously well-constrained seismic and well data. We find that our data provide a record of progradation of glacigenic debris flows (GDFs) since MIS 12 (0.5 Ma) to present. Horizon slices reveal a range of gullies and channels at different depths overlying the GDFs. We describe the paleoenvironment and sedimentary processes throughout this time-span (that covers seven glacial cycles) and discuss the impact of the Barents Sea Ice Sheet waxing and waning on erosion, sedimentation, and deposition along the continental slope. Abundant buried gullies were hitherto unknown at the Bear Island TMF, with previous work describing this succession as a debris-flow dominated unit where meltwater-related features are lacking, and interpreting this to represent low average temperatures. By use of the relatively small high-resolution 3D seismic dataset, we provide new evidence for the presence of gullies and channels indicating that periods of ice sheet melting and meltwater runoff existed throughout the middle-late Pleistocene succession. The work offers new insight into the stratigraphic evolution of a continental margin dominated by GDFs and demonstrates the value of high-resolution seismic, such as the P-Cable system, in resolving important details of paleo-slope-environments.

Solid rocks on the nearshore seabed – the distribution and potential impact on coastal processes in the Kołobrzeg region, the Southern Baltic

Abstract The Southern Baltic seabed is mainly composed of marine sediments situated on glacial and fluvioglacial deposits. In the Kołobrzeg region, however, solid rocks are located at the shallow subbottom. In order to determine the exact distribution of the rocks, some analyses of sonar images, underwater photos, sediment samples, seismoacoustic profiles and bathymetric data have been performed. The outcrops of Jurassic sandstones and mudstones have been documented. The geological structure of the area is presented, while the fact that the present seafloor relief is determined by the shallow uneven top surface of the Jurassic rocks is emphasized. The seabed has been classified as rock platform-constrained. The seafloor is a sediment-poor abrasion platform, within which marine sandy sediments are transported, mainly toward offshore, on the firm, stiff or hard surface of glacial deposits and Jurassic solid rocks. This results in a lack of material for the reconstruction of the beach after storms. Many of the commonly used methods for studying the condition and changes of the coast cannot be used in this type of terrain due to the presence of a thin and discontinuous dynamic layer lying on the erosion-resistant rocks of the geomorphologically complex seafloor.

Structure, distribution, and evolution history of the Early Holocene erosional mud ridge system on the inner East China Sea shelf near the Yangtze River estuary

Utilizing the collected high-resolution seismic dataset and accompanying borehole and bathymetric data, we systematically evaluated the morphology, architecture, sedimentology, and evolution of erosional mud ridges within the inner East China Sea (ECS) shelf. We identified 20 mud ridges, i.e., seismic reflection profile crossings of exposed or buried mud ridges, which are 3.0–30.1km in width and 2.5–17.3m in height. The mud ridges are composed predominantly of gray clayey silt, and on seismic profiles contain parallel to subparallel reflectors. They formed around ~10–12kaBP within an estuarine environment. Scouring features of some mud ridges on the eastern part of the study area can be recognized. Consideration of the relative positions of mud ridges, together with the topographical features, enables us to map four linear mud ridges (LMRs). The SE-NW oriented LMRs are >50km in length, 3.0–9.5km in width and running parallel to each other. They also display asymmetric shapes, with steeper slopes to the SW. The eastern segments of some LMRs are exposed on the present seafloor whereas other segments are mainly overlain by the mid- and late Holocene strata. Since the LMRs share similarities with the modern tidal sand ridges in shape and orientation, we hypothesize that they are formed under a uniform tidal current. Seismic data highlight that the internal reflectors of sand ridges consist of dipping clinoforms and are significantly different from LMRs, a feature which is largely due to the difference in grain-size composition of sediments between the inner and mid-outer ECS shelf. The mid- to outer ECS shelf is capped by coarser-grained sediments (i.e., medium to fine-grained), which were reworked and deposited at locations near the erosional areas under a polycyclic tidal current, thus forming multiphase sand ridges. However, fine-grained sediments (i.e., silty clay and clayey silt) overlain on the inner ECS shelf with light mass were carried far away from the erosional areas by the tidal currents, and the relict mud ridges were built. Evidence shows that the LMRs were formed by the early Holocene strata being scoured by a persistent southeasterly flowing current during ~8–10kaBP. The evolution of the LMRs occurred in three stages: (a) formation of the transgressional strata at ~10–12kaBP, (b) strong trough erosion at ~8–10kaBP, and (c) local adjustment (i.e., local erosion slightly and preservation) after ~8kaBP.

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay, Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.

Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea

AbstractWe show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2–2.7°/Myr, relative to Australia) of crustal blocks north of the rift, producing 10–15 mm/yr of extension in the continental rift, increasing to 20–40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low‐angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra‐High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30–50% slower than those from seafloor spreading‐derived magnetic anomalies. The 0.5 Ma to present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a reevaluation of these data in the context of the GPS rates is warranted.

Three‐dimensional seismic analysis of sediment waves and related geomorphological features on a carbonate shelf exposed to large amplitude internal waves, Browse Basin region, Australia

AbstractThis study analyses the three‐dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore Australia that is characterized by a pronounced internal wave regime. The data interpreted comprise an extensive, >12 500 km2 industrial three‐dimensional seismic‐reflection survey that images the northern part of the Browse Basin, Australian North West Shelf. The most prominent seismic–morphological features on the modern sea floor are submarine terrace escarpments, fault‐scarps and incised channels, as well as restricted areas of seismic distortion interpreted as mass wasting deposits. Besides these kilometre‐scale sea floor irregularities, smaller bedforms were discovered also, including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m. These sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces, along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise. Additional bedforms that characterize the more planar regions of the outer shelf are elongate, north‐west/south‐east oriented furrows and ridges. The formation of both sediment waves and furrow‐ridge systems requires flow velocities between 0·3 m sec−1 and 1·5 m sec−1, which could be generated by oceanic currents, gravity currents or internal waves. In the studied setting, these velocities can be best explained as being generated by bottom currents induced by internal waves, an interpretation that is discussed against oceanographic background data and modelling results. In addition to the documentation of three‐dimensional seismic–geomorphological features of the modern sea floor, it was also possible to map kilometre‐scale buried sediment wave fields in the seismic volume down to ca 500 ms two‐way‐time below the present sea floor, indicating the general potential for the preservation of such bedforms in the sedimentary record.

Depositional characteristics and processes of alongslope currents related to a seamount on the northwestern margin of the Northwest Sub-Basin, South China Sea

This study infers, from 2D seismic data, the presence of a seamount-related deep-water contourite depositional system, located on the northwestern margin of the Northwest Sub-Basin of the South China Sea. Alongslope aligned erosive features and contourite drifts developed in water depths between 1000 and 1500 m on the present seafloor. A moat formed north of the seamount as a result of bottom current intensification after being deflected by an obstacle morphology, indicating a major eastward flowing current. Due to the Coriolis deflection, an elongated-mounded drift developed to the north of the moat and a modest plastered drift to the south (onlapping the north side of the seamount). Bottom currents away from the seamount are less or minimally intensified, generating contourite channels and furrows over the drift to the north of the moat. Combined with the known oceanographic setting, the bottom currents could belong to the anticyclonic South China Sea Intermediate Water circulation. From the seismic data, the architecture of contourite deposits during progressive burial of the seamount is documented. The first appearance of contourite channels in seismic Unit 1 is inferred to be the onset of the contourite depositional system, which could be traced back to the early Late Miocene. The following aggradational pattern of this system, from the base of seismic Unit 2 onwards, indicates relatively enhanced and stable South China Sea Intermediate Water circulation occurring on the northwestern margin of the northern South China Sea. Subsequently, the behaviour and depositional processes of bottom currents upstream and downstream the seamount are discussed. The depositional model suggests that a complex contourite channel system will replace the former moat-drift system once the seamount is buried. Finally, our findings may provide new challenges for investigating the South China Sea Cenozoic palaeoceanography, which are helpful to understand the sedimentary dynamic processes related to the South China Sea deepwater circulation. (C) 2014 Elsevier B.V. All rights reserved.

How to evacuate 10 km3 of mud: saturate with gas and decrease the pressure!

The crest of the Absheron anticline in the South Caspian Basin at a few hundred meters below the present seafloor shows a subcircular depression about 8 km in diameter and 200 m deep, bounded by steep edges dipping 15° to 45° into it. The depression and the surrounding series are respectively filled and overlain by a regional mass-transport deposit (MTD) 150 m thick outside the depression and 300 m thick inside, composed mostly of extensional blocks. Geometric and stratigraphic analyses indicate that 150 m of initially deposited sediment were removed from a closed area after burial. Seismic evidence of shallow gas accumulations below the crater-like feature suggests that gas likely played a significant role in its development. The model proposed for the emplacement of the crater is that the gas-bearing cover of a shallow gas reservoir underwent exsolution when its overburden thinned during an episode of extensional slope failure. This resulted in loss of resistance to shear and evacuation of the gas-bearing sediment, likely at the shearing base of the failed mass. This evacuation feature is considered an example where the presence of gas locally governs the morphology of an MTD. The interpreted process shows a positive feedback between slope failure and loss of strength at the base of the resulting MTD. ᅟ

Submarine volcanoes of the Kolumbo volcanic zone NE of Santorini Caldera, Greece

The seafloor northeast of Santorini volcano in Greece consists of a small, elongated rifted basin that has been the site of recent submarine volcanism. This area lies within the Cyclades back-arc region of the present Hellenic subduction zone where the seafloor of the eastern Mediterranean Sea is descending beneath the Aegean microplate. The Cycladic region and the Aegean Sea as a whole are known to be regions of north–south back-arc extension and thinning of continental crust. Nineteen submarine volcanic cones occur within this small rift zone, the largest of these being Kolumbo which last erupted explosively in 1650AD, causing significant damage and fatalities on the nearby island of Santorini. Previous SEABEAM mapping and seismic studies from HCMR indicate that many of the smaller v'olcanic cones have been built above the present seafloor, while others are partly buried, indicating a range of ages for the activity along this volcanic line. None of the cones to the northeast of Kolumbo had been explored in detail prior to a cruise of the E/V Nautilus (NA007) in August 2010. The ROV Hercules was used to explore the slopes, summits and craters of 17 of the 19 centers identified on multibeam maps of the area. Water depths of the submarine volcano's summits ranged from 18 to 450m. In general, the domes/craters northeast of Kolumbo were sediment covered and showed little evidence of recent volcanic activity. Outcrops of volcanic rock were found in the crater walls and slopes of some of the cones but they typically consisted of volcanic fragments of pumice and lava that have been cemented together by biological activity, indicative of the lack of recent eruptions. Geochemical analysis of samples collected on the northeast cones showed evidence of low temperature hydrothermal circulation on the summit and upper flanks in the form of stream-like manganese precipitates emanating from pits and fractures.

Southwest Mallorca Island: A cool-water carbonate margin dominated by drift deposition associated with giant mass wasting

By means of new high-resolution Parasound, multi-channel reflection seismic and multibeam swath bathymetry data, we provide new insight into the sedimentation processes at the margin of a cool-water carbonate ramp system off southwestern Mallorca Island, western Mediterranean Sea. Here, we mapped 7 depositional units which we tied to the Messinian unconformity. The oldest units (1–3) are deposited from late Messinian to late Pliocene during phases of extreme sea-level fluctuations. 2.4Ma ago with the global climate cooling, when the estuarine water mass circulation switched into an anti-estuarine pattern in the Mediterranean Sea and sea-level highstands reached the today level, bottom currents started to shape the distal deposits of the cool-water carbonate system off southwestern Mallorca, represented by depositional units 4–7. Although, sediment supply from the carbonate shelf to the slope is low due to the insignificant input of terrigenous clastics and slow reef growth, slope sediments reach thicknesses >140m. Our study displays that the accumulation of sediments at the southwestern insular margin was mainly controlled by long-lived bottom currents and giant submarine landslides. The bottom currents formed elongated mounded drifts that contemporaneously developed in alongslope and upslope direction. They are probably controlled by an isobath-parallel bottom current in a present water depth of 350–550m which we associate with the Levantine Intermediate Water (LIW); and an upslope current acting in 250–600m associated with the Algerian mesoscale gyres that enter the Balearic straits from SE. Several scarps at the present sea-floor indicate the predominance of slope failures in the study area. Additionally, mass transport deposits occur basinwards in almost all depositional units. Accordingly, giant mass wasting events took place from the Pliocene into recent times.Locally, above the alongslope drift a field of sediment waves occurs in a water depth of 170–310m. They are oriented slightly oblique to the contour lines with their steep flank facing upslope, showing wavelengths of 400–800m and heights of 10–15m. We suggest that the waves migrate in upslope direction perpendicular to the main isobath-parallel flow.Furthermore, the results of this study indicate that sea-level fluctuations have only a minor influence on drift deposition; instead submarine landslides caused the creation of new drift units. They significantly altered the relief of the sea-floor and thereby initiate the realignment of the prevailing current pattern. The fault scarps and detachment surfaces left behind were the nucleolus for the development of new drift units.

Repeated fluid expulsion through sub-seabed chimneys offshore Norway in response to glacial cycles

Focused fluid flow through sub-seabed sediments is a common phenomenon on continental margins worldwide. However, the governing controls and timing of this fluid release have been difficult to understand, in particular, for fluid flow features buried beneath sub-surface sediments. A link between fluid flow activity, ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated Norwegian margin. New high-resolution P-Cable 3D seismic data from the Nyegga area on the mid-Norwegian margin reveal at least two more periods of fluid expulsion from sub-seabed sediments. The 3D seismic data show depositional patterns within chimney features, expressed as truncations of seismic horizons against the flanks of the chimneys. The truncations are interpreted, by analogy with present day observations, as evidence for buried carbonate mounds and/or sediment wash-out during formation of pockmarks in the past. The truncations are, hence, an indicator and a chronological marker for fluid expulsion in the past. The classification of chimneys results in three major groups: (1) chimneys that have been formed and consecutively reactivated one or two times during the last 200kyr and that have a fluid flow expression at the present seafloor; (2) chimneys that are approx. 125–160kaBP old without any associated fluid-flow expression at the present day seafloor; (3) chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum (18–25kaBP). The observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region. The emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the Naust formation at Nyegga.