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ABSTRACT The northern continental margin of the South China Sea (SCS) is an important component of deep‐water circulation, providing excellent conditions for studying bottom currents in a marginal sea. Seismic data were employed to discern the sedimentary patterns prevalent in the deep‐water continental slope sediments on the northern continental margin of the SCS, encompassing gravity flow, contourite and mixed depositional systems. The contourite depositional system includes various types of deposits (such as separated mounded drifts, patch or channel‐related drifts, deformed sheeted drifts, composite drifts, bottom current sediment waves, plastered contourite drifts) and various morphologic erosional features eroded by the bottom current (such as moats, non‐depositional surfaces, troughs and scarps). These contourite features are related to the continental slope's morphology and its sources. The Dongsha slope exhibits distinctive characteristics marked by intense bottom current erosion and deposition, featuring separated mounded drifts and deformed sheeted drifts along its lower slope. The lower slope of the Pearl River showcases a spectrum of bottom current‐induced features, including sediment wave fields, erosion fields and contourite drifts. The southern flank of the Shenhu slope is characterised by a bottom current erosion field, a non‐depositional surface, a sediment wave field and isolated mounded drifts. On the Yingqiong slope, the contourite drifts are limited to its southern flank where gravity flow action is absent, and the complex geomorphology interacts with the bottom current, forming a complex contourite depositional system. The results of this study serve as a foundational framework for further global research on bottom current circulation and hydrodynamics. [ABSTRACT FROM AUTHOR]

Bottom currents play a major role in deep‐sea sedimentation, but their significance in the burial of organic carbon is poorly quantified at a global scale. Here we show that Holocene fluxes of organic carbon into the contourite drifts are high, with a global average of 0.09 g cm−2 Kyr−1. At individual drift sites, fluxes are commonly 1–2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zones. These high fluxes of organic carbon into the contourite drifts are due to high rates of sedimentation. Over the past 50 million years, sedimentation rates at the studied contourite drift sites have overall increased, coincident with decreasing atmospheric CO2 and a cooling global climate. Our work suggests that a ramp‐up of the bottom‐current carbon pump has accelerated removal of CO2 from the atmosphere and oceanic water, thus contributing to the overall global cooling after the Eocene Thermal Maximum. Plain Language Summary: Bottom currents play a major role in deep‐sea sedimentation, but their significance in the burial of organic carbon is poorly quantified at a global scale. Here we examine data from modern contourite drifts (large‐scale, alongslope‐trending bottom‐current deposits) across the globe and show modern fluxes of organic carbon into the drifts are high, with a global average of 0.09 g cm−2 Kyr−1. At individual drift sites, fluxes are commonly 1 to 2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zones. These high fluxes of organic carbon into the drifts are due to high rates of sedimentation in these deepwater environments, which are driven primarily by vigorous bottom currents—in other words, by a bottom‐current pump that is highly efficient at burying organic carbon. Our work suggests that a ramp‐up of the bottom‐current carbon pump, attributable to progressive intensification of global ocean circulation over the past 50 million years, has accelerated removal of CO2 from the atmosphere and oceanic water, thus contributing to the global cooling after the Eocene Thermal Maximum. Sedimentary records of past organic carbon fluxes in contourite drifts over geologic time could well prove useful in informing predictions of future climate. Key Points: Modern fluxes of organic carbon into the contourite drifts are high, with a global average of 0.09 g cm−2 Kyr−1The fluxes into the drifts are commonly 1–2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zonesOver the past 50 million years, the bottom‐current pump has accelerated removal of carbon from the oceanic water [ABSTRACT FROM AUTHOR]

Abstract Bottom currents play a major role in deep‐sea sedimentation, but their significance in the burial of organic carbon is poorly quantified at a global scale. Here we show that Holocene fluxes of organic carbon into the contourite drifts are high, with a global average of 0.09 g cm−2 Kyr−1. At individual drift sites, fluxes are commonly 1–2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zones. These high fluxes of organic carbon into the contourite drifts are due to high rates of sedimentation. Over the past 50 million years, sedimentation rates at the studied contourite drift sites have overall increased, coincident with decreasing atmospheric CO2 and a cooling global climate. Our work suggests that a ramp‐up of the bottom‐current carbon pump has accelerated removal of CO2 from the atmosphere and oceanic water, thus contributing to the overall global cooling after the Eocene Thermal Maximum.

Mapping sediment deposition and erosion by thermohaline ocean bottom currents is important for the development of ocean infrastructure, future geo resources and understanding the sedimentology of contourites and abyssal sediment wavefields. However, only a limited percentage (estimated 20%) of the ocean floor has been mapped directly through seismic or sonar imaging. To better delineate where zones of bottom current deposition and erosion exist, we develop a prediction from numerical model solutions and sedimentological measurements of the ocean floor. This is achieved by integrating three types of data, which include the following: (1) bottom current shear stress from a model run of the HYCOM numerical ocean model; (2) sedimentation rates from ocean lithospheric age and sediment thickness from the GlobSed Model; (3) the measured extents of bottom current deposits from sonar observations. Shear stresses and sedimentation rates inside and outside the mapped extents of bottom current deposits allow us to quantify the conditions that are conducive for bottom current deposition. These conditions are then extrapolated and displayed on a 1/12° arcsecond resolution map of the world's oceans and validated through comparison with known, mapped systems. Based on our prediction, around 12% of the ocean has significant deposition by bottom currents while only 1% has erosion. Most bottom current activity occurs where thermohaline currents impinge upon the ocean floor like on continental slopes or some areas of the abyssal plain. Deposition and erosion also occur where constriction of ocean bottom currents takes place as in straits and seaways. Inland basins (i.e. seas) and continental shelves are mostly disconnected from global‐ocean thermohaline bottom current conveyors and, therefore, have limited bottom current deposition and erosion. Mid‐ocean ridges also have little bottom current deposition due to low sediment supply. [ABSTRACT FROM AUTHOR]

The land-detached Gollum Channel System (GCS) is one of very few large-scale canyon-channel systems on the Northwest European margin and thought to be of high importance in both along-slope and downslope sediment transport processes in the Porcupine Seabight basin. It is, however, unknown when this system was formed and how active it has been throughout its evolution, making it difficult to assess its regional impact. Here, using well data integrated with airgun seismic reflection data, a seismic stratigraphy (Cretaceous to present) is built for the GCS for the first time. We find that, contrary to what was thought before, the GCS was formed before Quaternary glaciations occupied the continental shelf and its origin is tentatively associated to a phase of Northeast Atlantic margin tilting in the early Pliocene. Each of the channels that make up the GCS was initiated by incision from erosive downslope gravity flows originating on the Celtic Sea Shelf. Gravity flows from Quaternary glacial processes reused the channels and mostly bypassed the upper slope or contributed to the channel fill, though some flows were capable of erosion of existing channel flanks and incision of several smaller channels. Additionally, we show that this margin was incised by erosive gravity flows on several occasions through time and that these incisions seem to follow preferential pathways. Interaction with along-slope bottom currents from the start of the Quaternary onwards was crucial to distribute sediments and nutrients to sediment drifts and cold-water coral mounds further north (downstream) along the Irish margin.

The interaction of sedimentary systems with oceanographic processes in deep‐water environments is not well understood yet, despite its importance for palaeoenvironmental reconstructions, and for a full understanding of source‐to‐sink sediment transport. The aim of this study is to improve the understanding of how contourite moats, elongated depressions formed by bottom currents associated with contourite drifts, develop and of the link between moat‐drift system morphology and bottom current dynamics. This study provides a systematic comparison of 185 cross‐sections of moat‐drift systems distributed at 39 different locations worldwide, and a detailed analysis of the morphology of six moats that cover a wide range of typical geological and hydrodynamic settings. Additionally, in situ measured current data were analysed to better link hydrodynamics to moat morphology. The median of all profiles across all moat‐drift systems reveals a 50 m relief, a width of 2.3 km, a relief to width ratio of 0.022, a slope angle of 6°, a drift angle of 3° and a concave‐up shaped morphology. Moats can be over 100 km long. Some moats are driven by sediment erosion while others are depositional and primarily exist due to differential sedimentation inside the moat compared to the drift alongside the moat. A new sub‐classification of moat‐drift systems based on their stratigraphy is proposed. This classification distinguishes moats depending on the degree of erosion versus deposition. No relation is found between latitude and moat‐drift morphology or stratigraphy in the analysed examples. The combined data indicate that a steeper slope focuses the current more than a gentle slope, resulting in an increase of the relief–width ratio and drift angle. Thus, this study provides new insights into the interaction of ocean currents with sedimentary morphology, which thereby affects the evolution of a poorly understood deep‐water sedimentary system. [ABSTRACT FROM AUTHOR]

Bottom currents are key processes that contribute to the shaping of submarine slopes, with the redistribution of sediments in contourite systems. Despite numerous recent studies on contourite systems, the complexity and diversity of these sedimentary systems are still not fully understood and often underestimated. Their understanding requires comprehensive works integrating all scales from seismic architecture to microfacies. This paper focuses on a contouritic ridge located between 2000 m and 2500 m water depth on the Mozambique margin. Bathymetry, seismic data and piston cores collected during the PAMELA‐MOZ3 cruise allow a multi‐scale study from large depositional geometries to sedimentary facies. At the seismic scale, the contouritic ridge shows three stages of evolution with: (i) initiation and development of the drift/moat system; (ii) an intermediate stage with successive incisions and aggradations; and (iii) moat infill and drift erosion during the Plio‐Quaternary. Plio‐Quaternary deposits are composed of hemipelagic, turbiditic and contouritic facies filling the moat. Coarse‐grained contouritic facies, dominated by planktonic foraminifera, are identified on the western flank of the ridge between the moat infill and the erosional area at the top of the ridge. They consist of condensed deposits, with sedimentation rate about 0.3 cm/ka, indicating a strong and stable bottom current that winnows away the fine‐grained component. This facies could be present more generally in an intermediate position between erosion and depositional areas in contourite systems. At present, the contourite system is located at the transition depth between North Atlantic Deep Water and Antarctic Intermediate Water. Trajectories of bottom currents are complex and interact with sporadic turbidity currents and anti‐clockwise eddies that participate in reshaping the sea floor morphology. Although Plio‐Quaternary depositional geometries indicate the end of drift/moat development, the moat filling and drift erosion are also related to bottom currents and constitute atypical contouritic sedimentation. [ABSTRACT FROM AUTHOR]

The analysis of sedimentary deposits influenced by bottom currents in glaciated continental margins provides crucial insights into paleo-depositional and oceanographic conditions. These reconstructions enable the assessment of interactions between advance and retreat of grounded ice sheets and past ocean circulation patterns. However, questions regarding these interactions and their specific mechanisms remain largely unanswered due to a lack of data in this remote area. In this study, we conducted a comprehensive analysis by integrating marine geophysical data, surficial sediment cores, oceanographic measurements, and ocean circulation models. Our aim was to understand spatial and temporal variations in sedimentary and oceanographic conditions during the past glacial and interglacial periods in combination with the long-term stratigraphic evolution. By integrating and cross-referencing diverse datasets, we were able to infer how bottom-current-controlled deposits (i.e., contourites) developed along the western bathymetric high of the Central Basin in the northwestern Ross Sea margin, Antarctica. Contouritic deposits lying over and along the flanks of bathymetric highs were identified through their mound-shaped external geometry and acoustically stratified facies, characterized by reflectors pinching toward the moat. Acoustic facies and multi-beam backscatter results, in conjunction with sedimentary core data, revealed contrasting patterns. Bathymetric highs exhibited thin (10 m thick), finer-grained stratified sediments with lower backscatter. These findings indicate that seabed winnowing occurred by strong bottom current during past glacial periods as supported by sedimentological analysis. The pathways of the westward-deflected dense shelf water outflow and the westward-flowing along-slope current, as simulated by oceanographic models, explain the distinctive development of contourites influenced by bottom-current processes. Moreover, the large accumulations of sediment in the contourites, resulting from bathymetric barriers in the north of the Central Basin, may contribute to submarine slope failures.

The bottom current is an important component of the three-dimensional ocean circulation, which is of significance for the safeties of ocean bottom engineering and facilities, the research on sediment and pollutant transports, and the ecological environment. Due to the lack of observation data, research on the bottom current in the South China Sea (SCS) has been limited. This study systematically analyzes bottom currents from 0.5 to 5 m above the seafloor based on 20-month-long mooring observations on the shelf slope west of the Dongsha Atoll. The spectral analysis results indicate that currents induced by the internal tides and the internal solitary waves (ISWs) comprise dominant constituents of bottom currents with velocity amplitudes up to O(50–90) cm/s. Dominant internal tide-induced bottom currents of the velocity amplitude of O(50) cm/s occupy 53% of the total horizontal kinetic energy. The pulsating ISW-induced bottom currents reach a maximum amplitude of 93 cm/s, which has intrinsic relation to the amplitude of the ISWs, according to the soliton propagation speed solutions of the Korteweg–De Vries (KdV) equation. The background bottom current is strongest in winter, followed by spring, and weak in summer and autumn, which is closely correlated with the behavior of mesoscale eddies. These results are of significance for understanding the dynamics of the bottom boundary layer.

In the late Miocene, giant ancient pockmarks, which are fairly rare globally, developed in the Qiongdongnan Basin. In this paper, to determine the sedimentary characteristics and genetic mechanism of these giant ancient pockmarks in the Yinggehai Formation of the Qiongdongnan Basin, based on high-resolution 3D seismic data and multiattribute fusion technologies, we analyzed the planar distribution and seismic facies of the ancient pockmarks and compared the characteristics of the ancient pockmarks with those of channels, craters, and hydrate pits. Moreover, we also discussed the implications of the fluid escape system and paleo-bottom current activity in the ancient pockmark development area and analyzed the influence of the ancient pockmarks on the paleoclimate in this region. Finally, an evolutionary model was proposed for the giant ancient pockmarks. This model shows that the giant ancient pockmarks in the southern Qiongdongnan Basin were affected by both deep fluid escape and lateral transformation of paleobottom currents. In addition, the giant ancient pockmarks contributed to the atmospheric CO2 concentration in the late Miocene and played a great role in the contemporary evaluation of deepwater petroleum exploration. [ABSTRACT FROM AUTHOR]

This study entails the characterization of the depositional environment of the Hupo Basin shelf. By means of sedimentary structure analysis, grain size, textures, sediment color, as well as optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) ages, a sediment core (19ESDP-101, 120 m) has been interpreted to be the product of shallow-water contourites. The shallow-water (165 m) contourite deposits observed in the core are divided into seven sedimentary facies grouped into four facies associations (FA): FA1) contourite drift, FA2) contourite drift/channel transition, FA3) contourite channel/drift transition, and FA4) contourite channel. FA1 resulted from the interaction between hemipelagic fallout, low-density gravity flow, and sedimentation under low velocity bottom currents. Compared to FA1, both FA 2 and FA3 are indicative of higher velocity bottom currents, owing to their relative increase in grain size and the presence of subtle indicators of bed-load transport. FA 4 portrayed massive to slightly bedded sand, representing a contourite channel environment with high-energy conditions. Fluctuations in the bottom current activity, related to the intensification and deceleration of the North Korean Cold Current (NKCC), have caused fluctuations between contouritic and hemipelagic-dominated periods. The vertical sedimentary facies stacking patterns observed at the Hupo Basin site suggested that, over time, the depositional processes changed at the site where the core was optained. The facies association stacking pattern suggests the lateral migration of the contourite depositional system and continuous flow of the NKCC. Our findings and interpretations can serve as a much needed reference for shallow-water contourite recognition in modern environments. Moreover, our proposed model can be used to more accurately interpret shallow-water contourite deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • Comparison between pelagic plain and seamounts porewater REEs. • Bottom current can influence the benthic source of REEs from sediments. • Regions with seamounts can account for an important source of REEs to the ocean. The rare earth elements (REEs) are a crucial resource frequently used as proxies for reconstructing paleomarine environmental conditions. However, there is limited understanding regarding the marine REEs cycling. In addition, the relationship between benthic diffusion of REEs and seamounts, remain unclear. Here we present the concentration profiles of REEs and trace elements in porewater and seawater from both the seamounts area and abyssal plain. Our results evidenced that the concentration of dissolved porewater REEs, Mn and Al in sediments in proximity of seamount are much higher than those at the abyssal plain. This is attributed to the substantial benthic diffusion of REEs to the ocean. The REEs enrichment processes in porewater at uneven seafloor are based mainly on re-suspension associated with the strong currents and/ or partial dissolution of detrital from seamounts. We suspect this process may occur in other world regions with seamounts and infer that these sediments may account for a significant contributor to the oceanic REEs budget. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) plays a key role in maintaining the circulation in the Pacific and Indian oceans. After entering the northeast South China Sea from the Luzon Strait, the Pacific Deep Water transforms into the Deep-water Current (DWC) and flows westward. The upwelling of the DWC in the SCS could outflow into both the Indian and Pacific oceans. However, when and how this modern circulation was established in the SCS remains unclear. By using seismic reflection data tied to the Ocean Drilling Program wells in the northeastern SCS margin, we have discovered fields of previously unreported sediment waves, of which the onset dates back to ∼2.6 Ma. The sediment wave heights increased from 2.0 to 7.5 m, in association with spatial extent from 630 km2 to 800 km2 between ∼2.6 Ma and ∼ 0.7 Ma. After that, the wave heights and spatial extent reduced to ∼5.5 m and 700 km2, respectively. Considering the location, morphological features and water depth, we propose that these sediment waves were formed by the DWC. The morphological changes of the sediment waves are linked to energy increase and decrease of the DWC within ∼2.6–0.7 Ma and ∼ 0.7–0 Ma, respectively. We interpret the intensification as caused by the narrowing and uplifting of the Luzon Strait that is the sole deep-water gateway of the SCS, and speculate that the post-0.7 Ma weakening was probably related to the reduced Kuroshio Current intrusion due to the middle Pleistocene climate transition. This study proposes a novel model for the evolution of the Quaternary DWC hydrodynamics, fostering our understanding of the paleo-oceanographic links between the SCS and the Pacific Ocean. • A sediment-wave field formed by the Deep-water Current is identified in the northeastern South China Sea. • The Quaternary Deep-water Current energy increased firstly and then gradually weakened. • The hydrodynamics variation of the Deep-water Current is linked to tectonic evolution and probably climate transition. [ABSTRACT FROM AUTHOR]

Small-scale contourite drift is an important component of continental margins that can record information about complex oceanographic processes. The Belgica Mound Drift is one example of a small-scale contourite drift. It is formed under the influence of cold-water coral (CWC) mounds and represents one of the most distal contouritic expressions influenced by the Mediterranean Outflow Water (MOW) in the NE Atlantic Ocean. Three distinct evolutionary stages have been identified from new high-resolution pseudo-3D reflection seismic data, each associated with a significant change in paleoceanography, affecting both bottom-current intensity and sediment input. The pre-drift stage (Pliocene–Early Pleistocene) corresponds to the regional RD1 erosive event, which was caused by the reintroduction of the MOW in the Porcupine Seabight, creating a distinct paleotopography that will influence all ensuing sedimentary processes. The second stage (Early Pleistocene–Middle Pleistocene) is the contourite drift inception in two distinct centres of growth, strongly steered by topographic obstacles such as the CWC mounds. During the third and final stage (Middle Pleistocene–present day), the contourite drift is developed under a more stable but less dynamic environment, characterised by more continuous and mounded aggradational stratification. The final stage of the contourite drift is related to the Middle Pleistocene Transition, with a spatially variable reduction in the MOW-related bottom currents and sediment input. The spatial and temporal evolution of this drift shows that its present-day morphology is controlled by the location of initial growth. Evolving moat morphology indicates that the intensity of the bottom currents generally increases during the drift evolution. This research presents a crucial paradigm for advancing our knowledge of elucidating the complexities of smaller-sized contourite systems in diverse oceanic environments. • High-resolution pseudo-3D dissection of a contourite drift. • Initial centres of growth still dominate present-day drift morphology. • Dynamic evolution of the drift influenced by bottom currents and CWC mounds. • Belgica Mound Drift documents temporal variability of a distal MOW. • Moderate MOW bottom current intensity from Middle Pleistocene onwards. [ABSTRACT FROM AUTHOR]

Based on multibeam bathymetric data and high-resolution shallow sub-bottom profiles acquired during China's 10th Arctic Scientific Expedition Cruise in 2019, a sediment wave field was found on the western slope of the Chukchi Rise, in the Arctic Ocean. This sediment wave field developed on the lower slope with water depths of between 1200 m and 1800 m and stretched 15 km in the downslope direction. It comprised several parallel sediment waves, with wavelengths ranging from 700 m to 3400 m and wave heights from 12 m to 70 m. In the vertical direction, well-stratified deposits, tens of meters thick, were affected by sediment waves, which exhibit asymmetric waveforms and upslope migration trends. The morphological and seismostratigraphic characteristics of the sediment waves suggested their genesis as a result of the interaction between the bottom current and seafloor morphology, which was also supported by hydrographical data adjacent to the sediment wave field. It was infered that this bottom current was related to the Arctic Circumpolar Boundary Current, which many researchers suggest flows through the study area. [ABSTRACT FROM AUTHOR]

Contourite drifts are significant sedimentary features and provide clues for the reconstruction of paleoceanography and paleoenvironment. Although they have been increasingly identified in the world’s ocean, shallow-water contourite drifts (< 300 m depth) remain poorly understood and the examples are rare. This study documents a Middle Miocene shallow-water contourite depositional system in the southwest South China Sea by interpreting seismic reflection data and calibrating results with the previous chronological framework. The depositional system consisted of six mounded drifts and six moats. The contourite features were generated in seismic unit III (16-10.5 Ma) and distributed adjacent to carbonate reefs. They were formed on the proto-continental shelf (50-200 m depth) and shaped by the wind-driven currents. Changes in the sedimentary stacking patterns suggest three evolutionary stages of the contourite features. Stage I represents the growth of the Middle Miocene contourite depositional system between 16 and 10.5 Ma. Stage II marks the termination of carbonate drifts and the burial of the Late Miocene sedimentation during 10.5-5.3 Ma. Stage III started with the development of modern deep-water sedimentary systems since 5.3 Ma. The contourite features are compared with the examples on other South China Sea margins. Significant changes in the paleoceanography occurred at 10.5 Ma and 6.5-5.3 Ma when the dominated bottom currents shifted from the monsoonal wind-driven currents to the North Pacific waters, and then the modern circulation system. The Middle Miocene mounded drifts were likely sourced by the coarse-grained carbonate sands. Fluid flow escaped from the coarse-grained contourite layers and natural gas leakage occurs on the seafloor. Shallow-water carbonate contourite drifts can be served as a good gas reservoir and have great economic potential.

In many modern deep‐water slope environments, deposits formed by bottom currents (contourites) are rather common. However, reports on ancient contourites in deep‐water environments are scarce and most of them remain contentious. Based on the study of high‐resolution sedimentological, sedimentary geological and magnetic fabric characteristics, this study presents the key evidence of bottom current activity in the Middle Ordovician Kelimoli Formation from the western margin of the North China craton, and reconstructs the deep‐sea circulation pattern and the direction of deep‐water bottom currents in the middle and low latitudes during the Middle Ordovician. Six typical carbonate microfacies are identified in the Kelimoli Formation, which can be grouped into three sedimentary facies types related to the main interpreted depositional processes (bottom current, undifferentiated bottom current and hemipelagic settling, and hemipelagic settling). The evidence of bottom current deposits are five‐fold. Firstly, it is possible to observe a variety of traction current structures, which mainly represent bed‐load‐dominated transport of sediments. Secondly, there are multi‐scale inverse grading and bi‐grading, as well as inversely‐graded and normally‐graded sedimentary sequences, which reflect the periodic change of bottom current activity. Thirdly, gradual or sharp contact boundaries and small internal erosional surfaces indicate omission surfaces caused by the oscillating variation of bottom current velocity. Fourthly, the continuous rhythmic change of sedimentary fabrics on the microscopic scale reflects sustained, waxing and waning current intensities. Fifthly, the anisotropy of magnetic susceptibility reveals the microscopic fabric characteristics of imbricated magnetic grains, which represent the continuous action of directional current, and the restored palaeocurrent direction indicates that the current is mainly parallel to the slope strike. After the palaeo‐north correction of the North China craton clockwise rotation of 120° to 150° since the Ordovician, anisotropy of magnetic susceptibility shows that the direction of bottom current in the study area is towards the north‐east/east. Therefore, it is speculated that there was a clockwise deep‐water circulation in the middle and low latitudes of the southern hemisphere during the Ordovician. This study not only provides sedimentological evidence for the Ordovician bottom current activity, but also contributes to a better understanding of the Ordovician palaeoceanographical environment, especially the Middle Ordovician deep‐sea circulation pattern. [ABSTRACT FROM AUTHOR]

Based on multibeam bathymetric data and high-resolution shallow sub-bottom profiles acquired during China’s 10th Arctic Scientific Expedition Cruise in 2019, a sediment wave field was found on the western slope of the Chukchi Rise, in the Arctic Ocean. This sediment wave field developed on the lower slope with water depths of between 1200 m and 1800 m and stretched 15 km in the downslope direction. It comprised several parallel sediment waves, with wavelengths ranging from 700 m to 3400 m and wave heights from 12 m to 70 m. In the vertical direction, well-stratified deposits, tens of meters thick, were affected by sediment waves, which exhibit asymmetric waveforms and upslope migration trends. The morphological and seismostratigraphic characteristics of the sediment waves suggested their genesis as a result of the interaction between the bottom current and seafloor morphology, which was also supported by hydrographical data adjacent to the sediment wave field. It was infered that this bottom current was related to the Arctic Circumpolar Boundary Current, which many researchers suggest flows through the study area.

A study utilizing a dense grid of seismic and piston-core data revealed that four different types of sediment wave fields have formed on the northern slope of the strongly enclosed and low-latitude marginal South China Sea (SCS). Their origins include turbidite flow, bottom current, gravity-induced deformation, and multi-origin. Three sediment wave fields formed by turbidite flow are neighboring to the channel or canyon, have the up-current migration and decreased size from a shallow depth to deeper water, have greater content of sand and decrease of grain size from upslope to downslope, and a lower content of CaCO3. Bottom current's sediment wave fields are parallel to the slope, with small-size erosive troughs, scours or furrows, and convex depositional system on the downslope of the erosive field, and with a lower content of sand, a finer grain size, and a greater content of CaCO3. Gravity deformation's sediment wave fields are adjacent to the canyon, and develop several sliding surfaces corresponding to wave troughs, wave variation of no regularity. One sediment wave field was more symmetrical, displayed a smooth and rounded morphology, and has not evident migration direction, which is the result of the interaction of turbidite flows and bottom currents. Every sediment wave field involves a complex interaction between geomorphology, tectonic setting of neighboring area, and oceanography of the SCS. This study assists us to understand deeply the sedimentation processes and correspondingly depositional system in the deep-water with strongly enclosed and low-latitude characteristics and analyze the context and propagation direction of the arc-continent between the Eurasian Continent and the Luzon Arc. [ABSTRACT FROM AUTHOR]

We have developed a broadband ocean bottom seismometer (BBOBS) and its new generation (BBOBS-NX) with the penetrator sensor system since 1999. With them, we performed many practical observations to create a new research category of ocean bottom broadband seismology. As the next step in seafloor geophysical observation, the BBOBS and the BBOBS-NX can be a breakthrough in realizing a geodetic observation network on the seafloor. Although vertical displacement observation by the absolute pressure gauge has been widely conducted in recent years, other geodetic observations are rarely performed. A few trials to measure the seafloor tilt were performed, but those looked inadequate for practical observations. Note that the broadband sensor in our BBOBSs has a mass position signal output, which can be used to measure the tilt change. As the horizontal component noise level of the BBOBS-NX is good at a long period range, we expected it to be adequate for the tilt measurement. At the first evaluation, we performed a comparison with a water-tube tiltmeter. The result was comparable with a resolution of better than 1 µ radian. A practical observation at the south of Boso Peninsula (KAP3 site) was conducted as the in-situ study from April, 2013. In January, 2014, a slow slip event (SSE) occurred near this site. The tilt data were processed by removing steps, mechanical relaxation, and tides. The results show a clear peak started from late December 2013. Two more 2 year-long tilt observations began in 2015: one was at the KAP3 site and another was off the Miyagi Prefecture at the slope to the Japan Trench. The latter was recovered in 2017 with about 1.5 years of data, which indicate a large continuous tilt up to several tens of µ radian. This amount of tilt can be explained by a similar already estimated SSE. Mobile tilt measurement at the seafloor can be a powerful tool to study SSEs, as they can be located above the source area and also possible to build an observation array for a practical study because of its low cost and ease of deployment compared with a seafloor borehole site.

The global boundary stratotype section and point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries is present at the base of the Byk‐E tephra bed in the Chiba section, located in the central Boso Peninsula, Japan. The Byk‐E tephra bed, which erupted from the older Ontake Volcano, is a conspicuous marker bed in the Chiba section; therefore, it provides a clear local and regional marker for the GSSP. However, the sedimentological characteristics of the Byk‐E tephra bed have not been well documented. Here, we provide a detailed description and an isopach map of the Byk‐E tephra bed based on the lithofacies, grain‐size compositions, and grain orientations in the Chiba section and other localities in the Boso Peninsula. The lithofacies and grain size compositions of the tephra bed show that the lower part of the bed show no evidence of flow‐induced deposition. These results suggest that the Byk‐E tephra bed is most likely deposited as fallout deposits under a weak bottom current that could not direct the heavy mineral grains. Therefore, the Byk‐E tephra bed is not to erode the underlying bed during the deposition, indicating that there is no stratigraphic discontinuity. The isopach map drawn from the observed thickness of the fallout unit, show that the distribution of the Byk‐E tephra extends more than 260 km but a limited azimuthal range. [ABSTRACT FROM AUTHOR]

Study of 98 crescent shape depressions over Alcock Rise, Andaman Sea were reported for the first time in between water depth -500 and -2000 m using multibeam swath bathymetry data. These gigantic depressions have crescent length (CL) varies from 600 to 3800 m and width (CW) varies from 200 to 2500 m with an average central depression of 500 m. Detailed parametric characterization reveals that slope and axial ratio of these crescentic structures have no direct relationship with general shape and steepness of their escarpment. Moreover, spatial distribution of these structures show a clustering of elongated crescent with higher crescent length to width ratio in NW margin of Alcock Rise compare to centre. This change in shape from open elliptical to semicircular depression probably suggests that earlier formed open crescents were modified at later stage to semicircular depressions. As observed in the seismic data, the formation of the crescentic depressions were initiated by the normal fault in-association with major dextral transform fault and subsequently its geometry was modified by local transpression along with seismicity induced slumping and bottom current scouring from the weaker zones. So, tectonics and bottom current activity provides simpler explanation for the formation of crescentic structure over Alcock Rise.

The response of the Antarctic ice sheet to climate warming is the main source of uncertainty regarding future global sea level rise, since little is known about its present and past dynamics. The last deglaciation is the most recent interval of large-scale climate warming, during which the Northern and Southern Hemisphere ice sheets retreated, and sea level rose globally, although at a non-uniform rate. Geologic records from the polar regions are fundamental in determining the factors that caused the major changes in ice sheets during the last deglacial under different boundary conditions. Here, we combine morpho-bathymetric and seismic data with sediment cores and oceanographic measurements to reconstruct the processes that influenced the deposition of the southernmost, most extensive, ultrahigh-resolution record of the Holocene in Edisto Inlet fjord (Ross Sea, Antarctica). We find that post-glacial sedimentation resulted in a layered diatom mud up to 110 m thick that was locally redistributed by bottom currents over confined drifts-moats in the central part of the fjord. After the Holocene climatic optimum, the fjord was not carved by ground ice, and there continued to be internal fjord water circulation associated with Ross Sea circulation. These results support a retreat of coastal glaciers by about 11 kiloyears ago (ka) from the continental shelf of North Victoria Land. • Sediment drift showing evidence of present and paleo bottom current in the fjord. • Early retreat of the western Ross Sea ice sheet after the Last Glacial Maximum (LGM). • High-resolution seismic stratigraphy and geomorphic data from Edisto Inlet fjord. • Sediment cores from Edisto Inlet fjord recovered a very expanded Holocene record. [ABSTRACT FROM AUTHOR]

Oceanographic conditions in the Southern Ocean are closely related to the oceanic frontal system. Glacial-interglacial changes in cryosphere influence variations in oceanic fronts in the Southern Ocean and vice versa, causing changes in surface water productivity, nutrient utilization, bottom water chemistry, and/or bottom current intensity. Here, we documented geochemical, nitrogen isotope, and grain size records over the last 600 kyrs from the Drake Passage with previously published data to compare between glacial and interglacial oceanic conditions from surface to bottom. Biogenic opal (diatom) and export productions were high during interglacial periods and low during glacial periods. Surface water production was dominated by open ocean species during both glacial and interglacial periods without a clear glacial-interglacial change in sea ice species. After Marine Isotope Stage (MIS) 9, interglacial biogenic opal and diatom abundance increased distinctively with decreased terrestrial influence. Nutrient utilization increased (decreased) during interglacial (glacial) periods with increased (decreased) surface water production, particularly after MIS 9. This indicates that surface water production was regulated by light availability in association with sea ice duration/extent. According to good correlation between sortable silt mean grain size (SS MGS) and %sortable silt, SS MGS can be used for bottom current intensity indicator in this study; increased (decreased) bottom current during glacial (interglacial) periods. Increased glacial bottom current is likely related to southwestward flowing bottom current. Interglacial bottom current intensity became also significantly weakened after MIS 9. However, outstanding interglacial warmth in the Drake Passage occurred from MIS 9, not following the global trend to occur after MIS 11. This implies that more studies from the Southern Ocean in response to the Mid-Brunhes Event are required in the future. • Surface and bottom oceanic conditions showed clear glacial-interglacial variations. • More open ocean condition prevailed during interglacial periods after MIS 9. • Nitrate utilization is related to light availability; high/low during interglacials/glacials. • Sortable silt mean grain size in GC05-DP02 can be used for bottom current intensity. • Influence of southwestward flowing bottom current increased during glacial periods. [ABSTRACT FROM AUTHOR]

Using echo sounders to detect gas plumes in seawater is common, especially in the context of hydrothermal circulation areas or gas hydrate-bearing cold seeps. To understand the distribution of gas plumes in the southernmost Okinawa Trough, we have conducted 13 cruises with a 38 kHz single-beam echo sounder (EK60). A total of 266 gas plumes of acoustic image, associated with the hydrothermal circulation, are detected. To estimate the near-seafloor bottom current speeds, 201 gas plumes are further used. As a result, the gas plumes around the axial depression of the Okinawa Trough generally tilt to the northeast at rising tides and high tides, suggesting a northeastward flow of the bottom current. However, the gas plumes in the Keelung continental slope tilt to the southwest at ebb tides and low tides, suggesting a southwestward flow of the bottom current. Our results significantly show a good estimation of the near-seafloor bottom currents from EK plume images in the case of lacking real observations. The directions of the bottom currents depend on semidiurnal tides. Assuming a quasi-constant speed of upward gas bubbles out of seabed, we have estimated the bottom current speeds in 6 hydrothermal circulation zones near the rifting center of the southernmost Okinawa Trough. The estimated bottom current speeds in submarine volcanic areas vary largely from 2 to 160 cm s-1, but bottom current speeds in relatively flat region are between 20 and 50 cm s-1. The large variation of the bottom current speeds in the submarine volcanic areas could be due to the variable emissions of the gases out of the submarine volcanic areas. [ABSTRACT FROM AUTHOR]

Deep‐marine deposits provide a valuable archive of process interactions between sediment gravity flows, pelagic sedimentation and thermohaline bottom‐currents. Stratigraphic successions can also record plate‐scale tectonic processes (e.g. continental breakup and shortening) that impact long‐term ocean circulation patterns, including changes in climate and biodiversity. One such setting is the Exmouth Plateau, offshore NW Australia, which has been a relatively stable, fine‐grained carbonate‐dominated continental margin from the Late Cretaceous to Present. We combine extensive 2D (~40,000 km) and 3D (3,627 km2) seismic reflection data with lithologic and biostratigraphic information from wells to reconstruct the tectonic and oceanographic evolution of this margin. We identified three large‐scale seismic units (SUs): (a) SU‐1 (Late Cretaceous)—500 m‐thick, and characterised by NE‐SW‐trending, slope‐normal elongate depocentres (c. 200 km long and 70 km wide), with erosional surfaces at their bases and tops, which are interpreted as the result of contour‐parallel bottom‐currents, coeval with the onset of opening of the Southern Ocean; (b) SU‐2 (Palaeocene—Late Miocene)—800 m‐thick and characterised by: (a) very large (amplitude, c. 40 m and wavelength, c. 3 km), SW‐migrating, NW‐SE‐trending sediment waves, (b) large (4 km‐wide, 100 m‐deep), NE‐trending scours that flank the sediment waves and (c) NW‐trending, 4 km‐wide and 80 m‐deep turbidite channel, infilled by NE‐dipping reflectors, which together may reflect an intensification of NE‐flowing bottom currents during a relative sea‐level fall following the establishment of circumpolar‐ocean current around Antarctica; and (c) SU‐3 (Late Miocene—Present)—1,000 m‐thick and is dominated by large (up to 100 km3) mass‐transport complexes (MTCs) derived from the continental margin (to the east) and the Exmouth Plateau Arch (to the west), and accumulated mainly in the adjacent Kangaroo Syncline. This change in depositional style may be linked to tectonically‐induced seabed tilting and folding caused by collision and subduction along the northern margin of the Australian plate. Hence, the stratigraphic record of the Exmouth Plateau provides a rich archive of plate‐scale regional geological events occurring along the distant southern (2,000 km away) and northern (1,500 km away) margins of the Australian plate. [ABSTRACT FROM AUTHOR]

The Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats. [ABSTRACT FROM AUTHOR]

Abstract The bituminous marl formation (BMF) is an important source rock in Romania. At the study location, in the Eastern Carpathians, the BMF composition is dominated by calcite (coccolith debris, cement) and diagenetic silica, with the remainder consisting largely of detrital clays, quartz and minor feldspar. Abundant intervals with soft sediment deformation, likely slump deposits, alternate with thinly layered marls that may locally contain layers and lenses of rippled and cross-laminated sandstone. The latter suggest intermittent action of traction currents. Although the fine grained nature and abundant nanoplankton content of the marls suggest that they originated via pelagic settling through the water column, upon close inspection, they consist of flattened sand-size yet fine-grained aggregates (soft clasts). Experimental studies suggest that these were likely eroded from the seabed and transported in bedload by bottom currents. Inclined fabric elements within marls suggest that soft clasts formed ripples, and that marl layers are the depositional consequence of ripple migration. Whereas typical marl layers probably reflect migration of small ripples of a few cm's height, there are also cross-bedded marly beforms with ca. 20 cm pre-compaction relief that by size are muddy megaripples. Unlike in sand, where bedform hierarchy reflects increasing flow velocity, muddy megaripples more likely are a reflection of bottom current systems of substantial duration. With marl layers as well as interbedded sands recording bottom current activity, the BMF represents a combination of slope processes and bottom current activity. The likely environment of deposition is a lower slope to basin setting with contour currents reworking pelagic sedimentation and intermittent sediment supply by slumping. Highlights • The mudstones consist of abundant sand-size yet fine-grained lenticular aggregates. • The water-rich soft aggregates were derived from erosion of seafloor muds. • Bedload transport by bottom currents resulted in ripples and lenticular fabric. • Prolonged flow events produced muddy megaripples, a newly discovered bedform. • Instead of pelagic settling, mudstones indicate an energetic and mobile seabed. [ABSTRACT FROM AUTHOR]

Abstract Based on detailed sedimentological observations and geochemical analyses, four geological events have been identified, and a genetic depositional model has been established within a sequence stratigraphic framework. The organic-rich siliceous shale interbedded with dense bentonite layers in the transgressive systems tract 1 (TST 1) is attributed to the first stage of high-frequency volcanism (which started at ∼447.62 Ma) associated with the collision of the Cathaysia and Yangtze Blocks. The presence of the organic-fair lime mudstone/shelly limestone indicates that Hirnantian Glaciation in the highstand systems tract (HST). The occurrence of organic-rich siliceous shale interbedded with scarce bentonite layers in the transgressive systems tract 2 (TST 2) is attributable to the second stage of low-frequency volcanism (which started at ∼443.83 Ma) and to postglacial effects. The formation of organic-fair mixed shale with silty laminae is attributed to bottom-current activity in the early highstand systems tract (EHST). The occurrence of organic-poor clay-rich shale is due to terrigenous input in the late highstand systems tract (LHST). Our research also shows that the high marine productivity caused by volcanic ash and a high burial rate in an anoxic depositional setting led to organic matter and siliceous mineral enrichment in the TST 1 and TST 2. The favorable organic matter preservation conditions and the postglacial biotic recovery played significant roles in the formation of the organic-rich shale in the TST 2. Highlights • Five systems tracts are identified within two 3rd order sequences of Upper Ordovician-Lower Silurian shales. • Four geological events are identified within a sequence stratigraphic framework. • Volcanism and postglacial effects play significant roles in the formation of the organic-rich shales. • Bottom-current activity and terrigenous input result in relatively low biogenic silica and TOC contents of shales. [ABSTRACT FROM AUTHOR]

Highlights • Supplies from rivers cannot explain the supper high sedimentation rate of the RDSD. • Due to Dongsha Uplift since 3 Ma, roughly 1/2 formations since Miocene were denudated. • Annual denudation rate from Dongsha Uplift sine 3 Ma had been about 60 Mt/y. • Sediment denudated from the Dongsha Uplift can be the main source for the RDSD. Abstract The slope of the northern South China Sea (SCS) is one of the world's most active areas of modern marine sedimentary processes. A rapidly deposited sediment drift (RDSD) was identified on the slope near Dongsha Island. The sources of this body have been studied and discussed by several authors, but no consensus has been achieved. Based on multi-channel seismic profiles interpretation and data analysis of long-term observation of bottom currents, we identified that although the amounts of sediments input from the Pearl and Hanjing Rivers, and from Kao-ping and Tseng-Wen Rivers are large, considering the current situation of northern SCS, none of the Pearl River, the Hanjiang River, the Kao-ping River and the Tseng-Wen River can maintain the deposition pattern of the northern SCS margin as the single source, especially to maintain the supper high sedimentation rate at the RDSD area. The Miocene drape formation (MDF) calmly draped over an unconformity after rifting stage resulting in a relatively constant thickness of about 2000 m throughout the overall northern SCS continental margin, and followed by a standard configuration of a clinoform, the Pliocene formation and another clinoform of about another 700 m in total thickness. Due to Dongsha uplift since 3 Ma, roughly 1/2 of MDF and the N 2 formation in the Dongsha area measures 150 km by 200 km were denudated resulting in an average annual denudation rate of 60 × 106 t/y. If these sediments were deposited on a slope region within an area measuring 100 km by 100 km, the sedimentation rate could reach 250 cm/ky. This is clearly an impressive new sediment source that has never been discussed before. Moreover, the RDSD is near the Dongsha Uplift and on the adjacent slope. We believe that this new sediment source should be the main contributor for the RDSD development. [ABSTRACT FROM AUTHOR]

Abstract Based upon 3-D seismic reflection data, published drill core studies, oceanographic observations and numerical modelling, this study identifies a small scale contourite depositional system that is locally confined by canyons, slope failures, erosional terraces and erosional slopes on the Jianfeng slope, northern South China Sea (~1500 to ~2300 m water depth). Possible oceanic driving mechanisms for the spatial heterogeneity in sedimentation and erosion within the Quaternary deposits in this complex slope system are investigated. Numerical simulations show that the quasi-steady South China Sea deep water circulation, which affects the study area, is characterised by low velocities (maximum 4 cm/s), with a slight modulation by tides (±1 cm/s). This quasi-steady deep-water thermohaline circulation is able to keep sediment particles in suspension, rather than to erode seafloor sediment. The bottom hydrodynamic regime becomes energetic when mesoscale eddies (horizontal scale of 10 to 100 km) approach. Our modelling study indicates that both surface and bottom mesoscale eddies are essential to account for the spatial heterogeneity in sedimentation pattern along the northern South China Sea continental slope. According to simulation, the eddy front contains the highest flow velocity over a mesoscale eddy cycle (45 days), exceeding the threshold for resuspension of unconsolidated sediment (15 cm/s). As a consequence massive resuspension is produced at various sites, and redistributed by sub-mesoscale (horizontal scale of 1 to 10 km) circulations originated from an eddy-topography interaction. In contrast to the surrounding areas, which are subject to erosional forcing for a relatively long part (>7% of an eddy cycle), both the locally-confined drift and a further upstream large elongated-mounded drift experience little erosion (<2.5% of an eddy cycle), and serve as depositional centres for sediment from remote areas and erosion from adjacent areas. Our study demonstrates a promising new perspective for bridging the scales between short-term sediment dynamics and long-term sedimentation through a comparison of modelled scenarios between normal conditions and energetic events. Highlights • Depositional features of a complex deep-water sedimentary system are investigated. • Modelled contourite deposition and sedimentary structures are compared. • The SCS deep thermohaline circulations are insufficient to remobilise sediments. • Contourite deposition is controlled by the interaction of eddies and topography. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Fluid flow resulted in complex fluid escape structures, mainly after the Miocene. • The fluid compositions are likely multi-source. • Active fluid escape features are indicated by seafloor fluid pathways and plumes. • Gravity flows along gullies contributed to the observed asymmetric morphologies. • Bottom currents and eddies may play important roles in sediment remobilization. Abstract Mega-pockmarks, which can reach a few kilometers in width and a few hundred meters in depth, and associated fluid escape features in the Northern Zhongjiannan Basin, South China Sea, have received much attention in the recent years, due to their large variety of morphologies and sizes. In this work, an integrated interpretation of detailed multibeam bathymetry, conventional multi-channel seismic reflection and seismic oceanography sections covering the northern Zhongjiannan Basin, South China Sea, are used to image the seafloor morphology, the subsurface geology and the water column, in order to better understand the geological and oceanographic controls on the formation and evolution of the widespread fluid escape features in this region. Regional uplift and volcanism took place in this area near the Palaeogene–Neogene boundary and, during the Neogene–Quaternary post-rift thermal subsidence period, differential subsidence and diapirism deformed the overlying kilometer-scale successions. Mainly after the Miocene, focused flow of fluids originated at depth occurred along gas pipes, polygonal faults, tapered reflectors and faults into the shallow sediments, resulting in intense fluid blowouts and complex fluid escape structures on the seafloor. Our results show that elongated pockmarks, pockmark gullies and gullies in the northern slope are generally controlled by underlying buried channels and gullies. However, pockmark gullies and gullies in the southern slope are controlled by gravity sliding/slumping, along with crescent pockmarks developing parallel to water depth contours. To varying degrees, intense fluid escape activities associated with mud diapirism and gas escape in the study area resulted in mud extrusion and the formation of mud volcanoes, complex seabed morphologies and irregular pockmarks. Mega-pockmarks, collapse structures and blind valleys have formed at the apexes of tectonic uplifts related to volcanism and above basement highs, which suggests that these processes played a role in creating pathways for fluid escape. Bottom currents associated with the South China Sea Western Boundary Current, imaged here for the first time with the seismic oceanographic method, mainly flow to the south, and erode the seafloor, making pockmark walls and slopes of gullies and channels in the north steeper than those in the south. Some fluid escape structures, such as the mega-pockmarks and pockmark gullies, are buried by sediment transported by the bottom currents. Turbidity currents flowing downslope along the long pockmark gullies also erode the seafloor and make the NW slopes of the mud volcanoes less steep than those in the SE. Eddies imaged by the seismic sections may also play a role in transporting sediment to the deep sea in this area. Some of the reported fluid escape structures are still active, as indicated by the fluid pathways that reach the seafloor and plumes in the water column imaged by the seismic sections. [ABSTRACT FROM AUTHOR]

The post-rift succession of the Northern Carnarvon Basin (north-western Australia) has commonly been interpreted as a relatively simple and uniform sequence that records a transition from siliciclastic to carbonate sedimentation deposited on a passive margin. However, given the significant vertical and lateral variation in seismic facies visible on seismic data, this interpretation likely oversimplifies the depositional history of the margin. Regional composite seismic lines that cross most of the basin, integrated with lithological and biostratigraphic information from exploration wells, provide the context for a better understanding of the depositional processes and environments that characterize the post-rift continental margin succession. We show that the sedimentary sequences deposited above the Valanginian breakup unconformity contain a wide variety of seismic facies that can be linked to a number of different marine depositional environments, with the greatest lateral variation occurring in the Turonian – Rupelian and in the Tortonian – Present. The former interval consists of three dominant seismic facies, namely polygonally faulted, incised, and parallel bedded, which we interpret to indicate a lateral transition from an environment primarily dominated by fine-grained pelagic/hemipelagic deposition to one dominated by energetic bottom currents that created both depositional and erosional features, such as contourite drifts and associated moats. The latter interval is expressed by sigmoidal and continuous reflections which pass laterally into more chaotic reflection packages, which we interpret as clinoforms and mass-transport complexes (MTCs). The development of bottom currents may be connected to changes in circulation associated with the opening of oceans adjacent to the northwest margin of Australia, while the MTCs may indicate increased regional seismic activity and slope instability resulting from the development of collisional plate boundaries. Definition of these sequences highlights the significant changes that have occurred in the sedimentary processes that operated on the margin, and their potential link to its tectonic evolution. • We provide a detailed seismic-stratigraphic framework for sequences above the breakup unconformity on the North West Shelf of Australia. • We identify key depositional sequences and their constituent packages, and interpret their associated environments of deposition. • A stable shelf environment existed from the Valanginian up to the Turonian. • From the Turonian until the Rupelian, deposition was influenced by high energy bottom currents which created incisions and sediment drifts. • From the Rupelian bottom currentsactivity decreased and tectonic activity resulted in the development of mass-transport complexes. [ABSTRACT FROM AUTHOR]

The mudstone deposits, a major constituent of the Lesser Himalayan Krol succession, represents the terminal phase of Marinoan snowball melting, an important part of the most significant events of the Proterozoic time, the Cryogenian Snowball Earth. The Facies analysis suggests that the sediments were supplied to the shelf by meltwater flows, and were later dispersed to the deeper part of the shelf by current/wave-enhanced bottom currents. The upper part of the deposit bears the signature of gradual shallowing of the basin caused due to glacioisostatic rebound. Petrographic study of the samples, randomly collected from different parts of the deposit, reveals that the mud deposit is composed of silt-sized particles of quartz and dolomite, and argillaceous material. Besides the variation in proportion of these constituent components, no other material was found within this deposit. This compositional homogeneity of this huge deposit primarily suggest derivation from some preexisting sediments. Mineralogical similarity and perfect match in trace and Rare Earth elements composition suggest that this mud was derived from the periglacial loess deposits of the Nuccaleena Formation of South Australia. This is in agreement with the provenance of the underlying Krol Sandstone. The mudstones, along with the underlying Krol Sandstone, thus suggest that the Lesser Himalayan Krol succession represent an extended part of the Adelaide rift system of South Australia. The relative position of India and Australia during Neoproterozoic, as per the reconstructed configuration of the Rodinia supercontinent, corroborates this inference. • A mudstone deposit, constituting a major part of the Neoproterozoic Krol succession of Lesser Himalaya, is the subject of present study. • This mud deposit was formed during the terminal phase of Marinoan snowball melting. • The sediments, after being supplied to the shelf by meltwater flows, were dispersed to the deeper part of the shelf by current/wave-enhanced bottom currents. • The petrographic character and geochemical (trace and RE element) composition suggest that the mud was derived from the periglacial loess deposits of the Nuccaleena Formation of South Australia. • This succession suggests that the Lesser Himalayan Krol succession represent an extended part of the Adelaide rift system of South Australia. [ABSTRACT FROM AUTHOR]

A remarkable exposure of tubular authigenic carbonates was found on the seafloor in the Dongsha area of the South China Sea (SCS). The tubular carbonates, around 2–3 cm in diameter and usually less than 10 cm in length, represent broken fragments of once-larger pipes that now protrude from muddy sediments. The morphology, carbon and oxygen stable isotope compositions, and trace and rare earth element contents of the carbonates were analyzed to decipher the mode of carbonate formation. The tubular carbonates exhibit a dark brown coating of iron and manganese hydrous oxides, indicating prolonged exposure to oxic bottom waters. The carbonate content of the micritic pipes falls between 12.5 and 67.3 wt.% with an average of 42.0 wt.%, suggesting formation within the sediment. This inference is supported by trace and rare earth element patterns including a moderate enrichment of middle rare earth elements. Low δ13C values (as low as −50.3‰, Vienna Pee Dee Belemnite (VPDB)) suggest that carbonate precipitation was induced by the anaerobic oxidation of methane. The unusually positive δ18O values of the carbonates (as high as +5.3‰, VPDB) are believed to reflect the destabilization of locally abundant gas hydrate. Taken together, it is suggested that pipe formation was initiated by sediment-dwelling organisms, such as crustaceans or bivalves. The burrows subsequently acted as conduits for upward fluid migration. The lithification of the sediment directly surrounding the conduits and the partial filling of the conduits with carbonate cement resulted in the formation of tubular carbonates. Turbidity currents, sediment slumps, or the vigorous emission of fluids probably induced the fragmentation of tubular carbonates within the sediment. The carbonate fragments had been further subjected to winnowing by bottom currents. This study provides insight into the interaction of megafauna burrowing with fluid migration and carbonate formation at hydrocarbon seeps, highlighting the role of bottom currents and mass wasting on the formation of fragmented tubular carbonates.

Abstract According to the field experiment implemented by the cruise with the observation by Submersible Jiaolong in No. 37 and 38 Cruise of China Ocean Mineral Resources R&D Association, detailed investigations about northern Yap Trench, including the water properties, bottom currents, turbulent mixing were presented. The deep watermass properties indicate that the deep water originates from East Mariana Basin (EMB) and East Caroline Basin (ECB), as part of the west propagating Lower Circumpolar Deep Water (LCPW). The volume transport results further prove it. The variability of salinity, potential temperature presents semidiurnal variations. The isothermal level variations indicate the existence of semidiurnal internal tide. The phase of salinity lags about 12 h than that of potential temperature. The salinity serves as the compensation on the isopycnal. The bottom currents mainly consist of the low frequency motions, near inertial motions, semidiurnal and diurnal motions. The strong semidiurnal motions derived by Lanczos band-pass filter and reconstructed current indicate the existence of internal tide in the western side of Yap Trench. Enhanced turbulent mixing is observed at the central part and slope of the northern Yap Trench, and weakens the stratification. The strong mixing seems to be caused by internal tides dissipation. Though mesoscale eddies are active around the Yap Trench, the bottom currents are not directly associated with mesoscale eddies. Highlights • The watermass properties suggest the deep water originates from LCPW. • The bottom current is influenced by low frequency motions, near inertial motions, semidiurnal motions and diurnal motions. • Enhanced turbulent mixing is observed at the central part and slope of northern Yap Trench. [ABSTRACT FROM AUTHOR]

The number of anthropogenic substrata in the ocean – structures like oil rigs and offshore renewable energy generators – is increasing. These structures provide hard-bottom habitat in areas previously dominated by sand or mud, so they have the potential to alter species distributions or serve as “stepping-stones” between other hard-bottom habitats. It is thus important to understand what factors influence the composition and abundance of benthic fauna recruiting at these sites. We examined recruitment to hard substrata (fouling panels) deployed on sand at various distances from a large rocky reef (∼60 m isobath) on the southern Oregon coast in 2014–2015. Recruitment was dominated by the acorn barnacle Hesperibalanus hesperius. For the majority of the study period in 2014, an anti-cyclonic eddy was present near the deployment sites. However, anomalously high recruitment of H. hesperius during August – early October 2014 coincided with dissipation of the eddy, slower bottom currents, and a positive convergence index, suggesting that H. hesperius larvae from the adjacent area may have been accumulated and retained near our study sites. Other sessile species, including hydroids and bryozoans, recruited to the fouling panels in low abundances, and most of these species have long-range dispersal and fast growth. Mobile invertebrates observed on the fouling panels included gastropods and nudibranchs, most of which also have long-range dispersal and fast growth, and are predators as adults. Thus, a community with two trophic levels assembled on the fouling panels in a relatively short time period (<12 weeks). None of the common hard-bottom species from the adjacent rocky reef recruited to the panels, suggesting that there is a specialized assemblage of species that can exploit hard-bottom habitats surrounded by sandy plains. Our results raise many questions about the influences of dispersal and oceanographic conditions on recruitment to hard substrata. [ABSTRACT FROM AUTHOR]

Based on high-resolution reflection seismic and core data from IODP Expedition 359 we present a new channel-related drift type attached to a carbonate platform slope, which we termed delta drift. Like a river delta, it is comprised of several stacked lobes and connected to a point source. The delta drifts were deposited at the exit of two gateways that connect the Inner Sea of the Maldives carbonate platform with the open ocean. The channels served as conduits focusing and accelerating the water flow; Entrained material was deposited at their mouth where the flows relaxed. The lobe-shaped calcareous sediment drifts must have formed under persistent water through flow. Sediment supply was relatively high and continuous, resulting in an average sedimentation rate of 17 cm ka −1 . The two delta drifts occupy 342 and 384 km 2 , respectively; with a depositional relief of approximately 500 m. They have a sigmoidal clinoform reflection pattern with a particular convex upward bending of the foresets. In the Maldives the drift onset marks the transition from a sea-level controlled to a progressively current dominated depositional regime. This major event occurred in the Serravallian about 13 Ma ago, leading to the partial drowning of the carbonate platform and the creation of shallow seaways. The initial bank-enclosed topography resembles an “empty bucket” geometry which is rapidly filled by the drift sediments that aggrade and prograde into the basin. Thereby the depositional environment of the delta drifts changes from deep water (>500) to shallow-water conditions at their topsets, indicated by the overall coarsening upward trend in grain size and the presence of shallow water large benthic foraminifers at their top. [ABSTRACT FROM AUTHOR]

Contourite drifts are anomalously high sediment accumulations that form due to reworking by bottom currents. Due to the lack of a comprehensive contourite database, the link between vigorous bottom water activity and drift occurrence has yet to be demonstrated on a global scale. Using an eddy-resolving ocean model and a new georeferenced database of 267 contourites, we show that the global distribution of modern contourite drifts strongly depends on the configuration of the world's most powerful bottom currents, many of which are associated with global meridional overturning circulation. Bathymetric obstacles frequently modify flow direction and intensity, imposing additional finer-scale control on drift occurrence. Mean bottom current speed over contourite-covered areas is only slightly higher (2.2 cm/s) than the rest of the global ocean (1.1 cm/s), falling below proposed thresholds deemed necessary to re-suspend and redistribute sediments (10–15 cm/s). However, currents fluctuate more frequently and intensely over areas with drifts, highlighting the role of intermittent, high-energy bottom current events in sediment erosion, transport, and subsequent drift accumulation. We identify eddies as a major driver of these bottom current fluctuations, and we find that simulated bottom eddy kinetic energy is over three times higher in contourite-covered areas in comparison to the rest of the ocean. Our work supports previous hypotheses which suggest that contourite deposition predominantly occurs due to repeated acute events as opposed to continuous reworking under average-intensity background flow conditions. This suggests that the contourite record should be interpreted in terms of a bottom current's susceptibility to experiencing periodic, high-speed current events. Our results also highlight the potential role of upper ocean dynamics in contourite sedimentation through its direct influence on deep eddy circulation. [ABSTRACT FROM AUTHOR]

Depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide. A field of anomalous depressions with dome structures within the centers are documented for the first time using sub-bottom seismic profiles, multibeam bathymetry data, and well data in the northern Yellow Sea Basin. They have complex seabed morphologies (circular, elliptical, polygonal, linear and crescent shapes) with kilometer-scale diameters and shallow depths (10 s decimeters to meters). Focused fluid flow pathways, such as gas chimneys, faults and pipes, and free gas (enhanced amplitude anomalies) are observed within the shallow strata. The depressions are interpreted as 'immature pockmarks', possibly caused by insufficient fluid supply compared to fully developed 'normal pockmarks'. They are suspected to still be in the middle stage of pockmark evolution. Their formation processes are suspected to be controlled by sub-seabed plumbing/fluid flux and bottom currents. This study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution. [ABSTRACT FROM AUTHOR]

This study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion/infill cross-section, based on high-resolution three-dimensional/two-dimensional seismic data and bathymetric data. The honeycomb structures cover an area of more than 5000 km2 and are developed within the Late Miocene to recent deep-water sediments of the north-western South China Sea. Linear erosional troughs up to 10 km long and 1 km wide are widely developed in this area, are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the Late Miocene. The unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies. Considerable sea-floor topography was generated as a direct result of tectonic movements in the area since the Late Miocene, and this topography then influenced the pathways of strong bottom currents. This study highlights that: (i) an unsteady bottom current regime can be laterally extensive and persist for millions of years; (ii) structurally controlled sea-floor relief plays an important role in controlling the depositional pattern; and (iii) the bottom currents were active since the Late Miocene, flowing from the south-east through the Xisha-Guangle Gateway and crossing the honeycomb structure zone. This study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the South China Sea deep-water circulation which is at present still poorly understood. [ABSTRACT FROM AUTHOR]