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Abstract Mesoscale eddies are ubiquitous in the ocean and play a key role in exchanges across continental slopes. In this study the properties of wind-driven baroclinic turbulence are investigated using eddy-resolving process simulations, focusing on the case of retrograde winds that arises around the margins of the subtropical gyres. In contrast to a flat-bottomed ocean, over steep slopes eddies develop from baroclinic instabilities are confined to the top few hundred meters. Deeper in the water column baroclinic instability and vertical momentum transfer are suppressed, so wind-input momentum is exported toward the open ocean by eddies before traversing down to the ocean bed. Close to the sloping topography, eddy energy sourced from the upper ocean is converted to potential energy, steepening isopycnals and driving bottom-trapped prograde flows. This process is associated with upgradient lateral buoyancy fluxes and downgradient isopycnal potential vorticity fluxes, and cannot be reproduced via linear stability calculations. These properties of wind-driven shelf/slope turbulence are contrasted against simulations with flat bathymetry. The key differences described above hinge on the flow close to the steep topographic slope, which may be sensitive to the model’s vertical coordinate system. The simulations are therefore replicated using models that employ geopotential coordinates, terrain-following coordinates, and isopycnal coordinates. Quantitative inter-model discrepancies in the momentum and energy budgets are much more pronounced in the presence of a steep bottom slope. However, the key findings of this study are consistent across the models, suggesting that they are robust and warrant incorporation into parameterizations of eddy transfer across continental slopes.

Abstract Synoptic bathymetric surveys and current meter data collected over a sandwave field in Adolphus Channel (20 m water depth), Australia, yield average estimated celebrities of 0.75 and 0.25 m day−1, respectively. The sandwaves average 3.9 m in height, 102 m in wavelength and are comprised of up to 96% carbonate, consisting primarily of intact and fragmented calcareous alga Halimeda, benthic foraminifers, bryozoans and molluscs. The sand has a modal grain size of 0.8 mm. Current speeds measured 1 m above the bed averaged 0.42 m−1 and reached a peak of 1.36 m−1. Surveys carried out in September and February show that the sandwaves reversed their asymmetric orientation over this time interval, which is attributed to a change in the direction of the wind-driven currents during the monsoon season. The reversal of asymmetry was accompanied by a statistically significant change in the degree of sandwave asymmetry (ratio of stoss and lee slope lengths) whereas no change in mean wavelength was detected. The reversal is estimated to have required 47 days to occur based upon estimates of average sandwave cross-sectional area and bedload transport rates predicted from the current meter data.

Abstract Several knickpoints have been identified along the present-day thalweg of a sinuous submarine channel–levee system (CLS) on the slope of the western Niger Delta using 3D seismic data. The knickpoints form as a result of gradient changes caused by the uplift of a thrust and fold belt orthogonal to the CLS. The channel gradient is lower locally upstream of folds causing turbidity currents within the channel to decelerate and deposit the coarsest sediment load. The basinward dipping fold limb causes local steepening of the gradient, which leads to increased flow velocity and turbulence within the turbidity currents. This enhances erosion at the base of the channel and leads to the formation of a knickpoint. If preserved, e.g., as a result of channel avulsion or abandonment, the deposits upstream of the knickpoints could constitute an important hydrocarbon reservoir element. They can, however, also be partially eroded by headward-migrating knickpoints, as the channel strives to regain its equilibrium profile, leaving remnant sand pockets preserved on channel margins. Although knickpoints are difficult to recognise from subsurface seismic or outcrop data, it is anticipated that they can form at any stage of the evolution of a channel–levee system and may be particularly important in controlling 3D channel architecture where channels intersect dynamically changing seabed bathymetry.

Bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins, but only a small fraction of the seafloor has been sampled by ships. At the present exploration rate it will take several centuries to map significant portions of the seafloor topography. Satellite altimetry, which maps the topography of the equipotential sea surface, is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years. Using a model of a Gaussian‐shaped seamount loading a thin elastic lithosphere, we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile. The model predicts that the seamount diameter is equal to the peak‐to‐trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed. Moreover, the model suggests that these two measurements are relatively insensitive to the cross‐track location of the seamount. We confirm these model predictions using Seasat altimeter profiles crossing 14 well surveyed seamounts in the Pacific. We then apply the measurement technique to 26 × 106 million kilometers of Seasat profiles resulting in a new global set of seamount locations. Approximately one quarter of the seamounts identified in Seasat profiles were previously uncharted. Modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid; therefore the set of locations is not a straightforward sampling of the total seamount population, but is weighted toward seamounts which are poorly compensated. A preliminary analysis indicates considerable variations in population density and type across the oceans; most notable among them are the absence of seamounts in the Atlantic, variations in population density across large age‐offset fracture zones in the Pacific, the prevalence of small signatures in the Indian Ocean, and the existence of linear trends in the large seamounts of the west Pacific.

AbstractHyperpycnal flow deposits, one of the most important deep‐water gravity‐flow deposits in lacustrine basins, have become the research focus in recent years. However, the sedimentary characteristics and depositional model of hyperpycnal flow deposits in lacustrine basins remain unclear due to the differences of depositional settings between lacustrine and marine environments. Hyperpycnal flow deposits observed in the middle of the third member of the Shahejie Formation (Es3z) in the Bonan Sag, Bohai Bay Basin, Eastern China, provide a rare case study to reveal the characteristics and depositional model in lacustrine basins. For the first time, detailed core analysis, high‐resolution 3D seismic data, petrology and grain size analysis were used to unravel the characteristics and depositional model of hyperpycnal flow deposits in this study. Twelve lithofacies, six bed types and four bedsets (corresponding to feeder channel, distributary channel, levee and lobe) were recognized from detailed facies analysis. Plant fragment, an important identification mark for hyperpycnal flow deposits, can be classified into three types: completely broken plant fragments, partially broken plant fragments and complete leaves. The proximal part of the deposit develops a small amount of scattered and completely broken plant fragments in massive or spaced stratified pebbly sandstone and massive sandstone due to strong erosion of sustained high‐density turbidity current. The medial part of the deposit is dominated by laminated partially broken plant fragments in planar laminated or rippled sandstone due to suspended settling of sustained high‐density turbidity current and quasi‐steady low‐density turbidity current. Layered partially broken plant fragments with some complete leaves are common in the upper part mud rich division of hybrid event bed and laminated siltstone in the distal part of the deposit. The distribution pattern of hyperpycnites is controlled comprehensively by palaeogeomorphy and sediment supply. Palaeogullies determine the provenance direction of hyperpycnal flow. The formation of synsedimentary faults and troughs control the transport routing patterns. Local micro‐palaeogeomorphy and depression areas further restrict the distribution of sand bodies. During the early stage of deposition with insufficient sediment supply, sediments are transported to the deep basin along confined faulted troughs forming elongated sandy bodies. During the late stage of deposition with sufficient sediment supply, sediments are transported to the deep basin without confinement accumulating fan‐shaped sandy bodies. This study offers insight for enhancing the recognition criteria of hyperpycnites, as well as their depositional model in lacustrine basins.

Abstract Intensive economic and shipping activities in Singapore Strait have caused Singapore coastal waters to be under high risk of water pollution. A nested three-dimensional unstructured-grid SUNTANS model is applied to Singapore coastal waters to simulate flow and pollutant transport. The small domain (~ 50 m resolution) Singapore coastal model is nested within a large domain (~ 200 m resolution) regional model. The nested model is able to predict water surface elevations and velocities with high R2 values of 0.96 and 0.91, respectively. Model results delineate the characteristics of circulation pattern in Singapore coastal waters during the Northeast and Southwest monsoons. The pollutants are modeled as passive tracers, and are released at six key sailing locations Points 1–6 in Singapore coastal waters and are named as Passive Tracers 1–6, respectively. Our results show that the rate of dispersion is twice as large for the Northeast monsoon compared to the Southwest monsoon due to differences in large-scale monsoons and small-scale local winds. The volume averaged concentration (VAC) diminishes faster and the local flushing time is shorter during the Northeast monsoon than the Southwest monsoon. Dispersion coefficients K and the VAC decreasing rate are maximum for Tracers 2 and 3 with shortest local flushing time due to the strong surrounding currents and abrupt bathymetry changes near Senang and St. John Islands. Dispersion coefficients K and the VAC decreasing rate are minimum for Tracer 1 due to weak currents induced by the semi-enclosed coastline near Tuas. It is found that both the lateral dispersion coefficient Ky and the compound dispersion coefficient K obey a “4/3-law”, which defines a linear correlation between dispersion coefficients and 4/3-power of selected length scale.

ABSTRACTDeltaic sediments of the Billund and Bastrup sands were deposited in a ramp setting in the storm‐dominated North Sea during the early Miocene. A marked relief in the hinterland and the relatively high precipitation resulted in a high sediment supply to the sea and progradation of major delta‐coastal plains south of the present‐day Norway. The focus of this study is on the forced regressive wedge system tracts of the two delta complexes, which show remarkably well‐developed marine erosional surfaces associated with sand‐rich packages characterised by steeply dipping clinoforms (up to 10°). The well‐developed clinoformal packages indicate that deposition occurred in water depths of 60–100 m even under a sea‐level fall. The sand‐rich delta lobes also demonstrate that it was a high‐energy environment and that wave‐generated re‐suspension at the delta front effectively re‐sorted the sediments and sand‐rich systems became separated from mud‐dominated portions of the delta complexes. The evolution of the above occurred in a basin that has been exposed by inversion tectonism. The sediment supply was consequently high. During deposition, eustatic sea‐level changes strongly controlled the evolution of sequences. The results found in this study may be applicable for mapping reservoir sands in ramp settings and in rift basins especially when looking for reservoir rocks in the basinal setting or when carrying out detailed reservoir mapping in already existing hydrocarbon fields.

Abstract Time serial bathymetric data acquired between 2004 and 2009 are used to evaluate the morphological evolution of the coastal area offshore Pointe Odden, located on the Mandji Island (Gabon). Data analysis highlights the alternation between fast sedimentation periods at shallow water depth related to intense longshore drift and catastrophic erosional events. Because of sediment overloading and slope oversteepening, small-scale instabilities are generated (successive slide scars, channel formation and growth by retrogressive erosion). However, when critical stability conditions are reached, large failures occur (2005 submarine slide). Geotechnical measurements and sedimentological analyses on the study area suggest that flow liquefaction would be the triggering mechanism of the 2005 event. Moreover, our analysis shows that the associated slide scar is rapidly filled by compensation and that failure morphology could disappear from the seafloor in about 15–20 years.

Glide planes, the basal surface or failure surface upon which submarine landslides initiate, commonly develop along weak, distinctive stratigraphic horizons but their lithological/mechanical characteristics and genetic mechanisms remain largely unknown. We use 2-D multi-channel seismic reflection data, integrated with multibeam bathymetry and deep drilling data from the Ulleung Basin margins, East (Japan) Sea, to: (1) identify and characterize the nature of glide planes associated with submarine landslides; (2) understand the influence of climate-modulated factors in preconditioning slope failures; and (3) document the post-failure evolution of the landslides. 24 glide planes were identified among 38 submarine slides (SL1 – SL38), which correspond to regionally continuous, positive-polarity high-amplitude seismic reflections. Well-seismic integration support ca. 340 ka – 1,200 ka ages of formation of the major glide planes in the southwestern and western margins of the basin. These glide planes developed at the interface between clay-rich sediment deposited during glacial periods and biogenic diatom-rich sediments deposited during interglacial periods. Physical, mineralogical and geochemical properties determined by density, porosity, gamma-ray, shear strength, X-ray diffraction, and X-ray fluorescence data reveal significant lithological and mechanical changes at the interface between these two lithologies. We therefore infer that these interfaces dictate the position of failure surfaces, with the diatom-rich layers acting as a weak layer. Excess pore pressure in these layers is likely due to initial high-water contents (up to 75%) and high compressibility; this is considered an important pre-condition for failure. In contrast, the glide planes along the northwestern margin of the Ulleung Basin (SL34 – 37) are older (ca. 1,200 ka – 2,140 ka). Seismic data further reveal three distinct contrasting styles of landslide post-failure behavior throughout the margins: (1) evacuated slide scars with areas of smooth seafloor; (2) slide scars with residual debris consisting of blocky sediments; and (3) slide scars with buried intact sediment blocks in front of the headwalls. Lateral variability of fluid flow, sediment composition, and mechanical properties of basal ‘weak’ layer(s), or the magnitude of earthquakes may have contributed to forming different types of mass-transport deposits (MTDs). Overall, these results show that landslide formation in the East (Japan) Sea result from a complex climatic, volcanic and tectonic interplay that controlled the formation of weak layers. Some of these layers extend regionally and can be identified and mapped by remote geophysical methods and targeted drilling This study was supported by ‘Geological survey in the Korean Peninsula and publication of the geological maps’ Project (GP2020-009) funded by the Ministry of Knowledge Economy (MKE; currently Ministry of Trade, Industry and Energy: MOTIE), Korea, and the research fund of the Chungnam National University. D. Cukur was supported by the KIGAM project (research fund number: 22-3111-2). S.H. Lee is supported by the KIOST Basic Project (PE99941). R.U. is supported by project PID2020-114856RB-100 / AEI / 10.13039/501100011033 24 pages, 18 figures, 2 tables, supplementary material https://doi.org/10.1016/j.margeo.2022.106956.-- Data availability statement: Supporting of the data were provided by the Korea Institute of Geoscience and Mineral Resources (KIGAM) under confidential status and the restrictions do not allow open sharing of the proprietary data used in this research. The data can be available upon reasonable request made to the authors with permission from the KIGAM With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) Peer reviewed