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• The internal architecture of seafloor undulations and the initial growth interface are investigated. • Seafloor undulations are interpreted as a result of Quaternary sediment transport processes. • Inherited faults provide preexisting relief, allowing the initial growth of undulations. Sediment undulations are widely distributed along continental shelf and slope worldwide. The driving mechanisms of undulations are crucial for assessing the long-term stability of continental slope, which, however, remain poorly understood. We used high-resolution 3D seismic data to investigate the internal architecture and origin of a field of seafloor and subsurface undulations along the slope of South China Sea (SCS). The undulations have narrow troughs and broad ridges. The reflection geometries in the troughs between undulations tend to be continuous and not offset by faults. These troughs terminate at different subsurface depths, commonly ranging from 50 to 400 ms in two-way travel time (TWTT) below the seafloor, often at locations related to subsurface geological structures, such as buried faults and steps within mass transport deposits (MTDs). These subsurface terminations therefore suggest that the undulations are probably depositional features not linked to creep-related basal detachment. The troughs between undulations display an absence of growth features, while the undulations increase in amplitude from base to top (characteristics that are also incompatible with gravitational deformation). Erosional and depositional features are commonly found in the undulations; thus, we interpret the narrow troughs between undulations to represent the areas where sediment-laden bottom currents undergo passage from supercritical to subcritical conditions. These troughs nucleated mostly from fault scarps and seafloor steps induced by past slope failures. Previous alternative interpretations report that these features originate from subsurface creep; alternatively, the present interpretation as sediment transport structures has significant implications for seafloor management and engineering projects. [ABSTRACT FROM AUTHOR]

One of the eddy "hotspots" in the Black Sea is the area of stationary Sevastopol anticyclone. Here this eddy can reside for >3–6 months significantly affecting the cross-shelf exchange and nutrient fluxes in the Black Sea. Detaching from its stationary location, the eddy moves westward, where it finally dissipates. In this article, we use altimetry measurements and a high-resolution (1 km) Nemo numerical model to investigate in detail the physical processes responsible for the observed evolution of the Sevastopol eddies. The main reason for the stationarity of such eddies is the sharp turn of the continental slope, which plays the role of the meridional wall blocking the vortex propagation in the cyclonic direction. Such a wall is situated at depths of 200–500 m and acts only on sufficiently intense eddies with large vertical extensions, while weak eddies with a small thickness do not feel it. The eddy trajectory near the obstacle represents a cyclonic spiral, which displaces to the southeast before the eddy separation. The process of vortex separation from the wall can prolong for several months and can be divided into 4 stages: 1) Movement to the north after the collision with the meridional wall due to the action of the reactive forces; 2) The formation of an attached submesoscale cyclonic lens-like eddy (SCE) at 200–400 m caused by the rise of horizontal shear due to the interaction of currents with the local uplift of the bathymetry near the continental slope; 3) The movement of the formed vortex dipole to the southwest due to self-propelling force; 4) Displacement of the dipole to the south beyond the meridional wall and further movement of the Sevastopol eddy to the west. During the separation process, Sevastopol AE loses a large amount of energy due to leakage of the mass during its interaction with the wall and transfer of a large part of its energy to the attached SCE. This process explains the observed decrease in the intensity of the Black Sea eddies to the west of the places of their stationarity. Modeled (top panels) and schematic (bottom panels) stages of the stationarity and separation of the Sevastopol eddies. [Display omitted] • Meridional "wall" at 200 m causes stationarity of the Sevastopol anticyclone (SA). • Only intense and vertically-elongated eddies became stationary. • Eddy-topographic interaction causes formation of the submesoscale cyclonic eddy. • The vortex dipole displaces southward and SA separates from topographic obstacle. • SA loses large part of the energy during the separation process. [ABSTRACT FROM AUTHOR]

Tropical marginal seas host important sedimentary archives that may be exploited to reveal past changes in continental erosion, chemical weathering, and ocean dynamics. However, these records can be challenging to interpret due to the complex interactions between climate and particulate transport across ocean margins. For the southern South China Sea over the last 90 Kyr, we observe a contrasting temporal relationship between the deposition of clay minerals and magnetic minerals, which were associated with two different hydrodynamic modes. Fine‐grained clay minerals can be carried in suspension by ocean currents, leading to a rapid response to regional climate‐driven inputs. In contrast, changes in magnetic mineralogy were linked to glacial‐interglacial sea level variability, from which we infer a control by bedload transport and resuspension. Overall, this study indicates that the transfer pathways and mechanisms imparted by varying hydrodynamic conditions exert a substantial influence on the distribution of terrigenous material in continental margin sediments. Plain Language Summary: Sediments that accumulate in the ocean along continental margins contain the minerals that were eroded and transported by rivers on the nearby land masses. Past changes in the composition of such sediments can be used to reconstruct the history of physical erosion and chemical weathering on the continents, but can also be affected by ocean dynamics over orbital and millennial timescales. Here, clay mineralogy, sediment grain size, and rock magnetic signatures are measured in a core from the continental slope of the southern South China Sea, spanning the last 90 thousand years. Differences between the clay mineral and magnetic records, which are linked to finer and coarser sediment fractions respectively, indicate that specific mechanisms and timescales influenced sediment transport to the core site. This depositional framework must be considered when interpreting the timing of changes in weathering and erosion proxies hosted in sediment records from marginal seas. Specifically, fine‐grained clay minerals may reveal a rapid response to continental weathering changes, whereas the magnetic records are influenced by the transport of denser coarser‐grained minerals under the influence of sea level change. In addition, this framework could potentially serve as an indicator of past sea level change on wide continental shelves. Key Points: In the southern South China Sea, clay mineralogy responds rapidly to climate‐driven changes in inputs on orbital timescalesMagnetic content within the coarser and denser fraction is influenced by sea level change, through bedload and resuspension transportThis depositional framework may serve as a guideline for evaluating sea level change on wide continental shelves over orbital timescales [ABSTRACT FROM AUTHOR]

The North Atlantic deep water (NADW), according to the classic ocean circulation theory, moves southward as a deep western boundary current (DWBC) even though it may veer into interior and then rejoin DWBC when encountering regional circulation features, such as eddy‐driven recirculation. In potential vorticity dynamics, the eastern side of the Mid‐Atlantic Ridge (MAR) may provide a similar topographic support as the continental slope off the western boundary for a southward transport of NADW. In this article, we quantify the mean meridional NADW transports on both sides of the MAR using a data‐assimilated product and find that the flow in the eastern basin contributes about 38±14% $38\pm 14\%$ of the net southward transport of NADW from 50° to 35°N. Our study points to the importance of observing NADW transport variations on the eastern side of the MAR in order to monitor the transport strength of Atlantic Meridional Overturning Circulation. Plain Language Summary: Classic theory suggested that the Deep Western Boundary Current (DWBC) carries the cold and fresh North Atlantic deep water (NADW) from the high latitude to the south. However, float and tracer observations, albeit sparse, have revealed a possible southward pathway in the eastern North Atlantic, that is, east of the Mid‐Atlantic Ridge (MAR). Using an ocean reanalysis product, the Estimating the Circulation and Climate of the Ocean (ECCOV4r4), we show that the steady‐state NADW meridional transport in the basin east of the MAR accounts for about 38±14% $38\pm 14\%$ of the meridional transport in the whole basin between 35° and 50°N. It is found that the NADW circulation pathways are strongly influenced by ocean bottom topography. For example, the eastern flank of the MAR acts as a "western boundary" in the eastern basin against which NADW flows southward. The topographic constraint from the MAR weakens south of 35°N due to changes in the MAR height and smaller planetary vorticity f $f$. The current study has improved our understanding of NADW transport structure at mid‐latitude by quantifying the equatorward transport east of the MAR. Key Points: The eastern Atlantic basin supports nearly 38±14% $38\pm 14\%$ of the southward North Atlantic deep water (NADW) transport in mid‐latitudesThe meridional transport of NADW in the eastern basin is due to the potential vorticity constraint imposed by the Mid‐Atlantic Ridge [ABSTRACT FROM AUTHOR]

During the evolution of shrimp, essential aspects of their organization have remained unchanged. Approximately the same adaptations that have generated a large number of parallelisms within this relatively small group of arthropods have been used in the process of their adaptive evolution. This evolutionary process is typical telogenesis. The ancient shrimp fauna of both Dendrobranchiata and Pleocyemata may have originated in shallow waters not later than in the Jurassic. This is confirmed by the remains of Dendrobranchiata found in the shallow-water sediments of the Cenomanian (early Upper Cretaceous) and the remains of Pleocyemata as inhabitants of shallow-water refugia and representatives of the most primitive caridean shrimp family Procarididae. The last group is morphologically very similar to fossil shrimp of the Jurassic (for example, Udorella agassizi). Later, due to adaptive radiation at the generic level, shrimp occupied both the continental slope and the pelagic zone. These shallow-water species may have then been ousted by a new shrimp fauna during colonization of the shelf. The previous shrimp fauna may have been displaced to the continental slope, perhaps bordering on it. Some part of the species from various genera could have moved deeper along the slope down to the abyssal zone, whereas some others might have shifted to occupy the pelagic zone. This movement may have been associated with food resources, which were limited on the slope in the places with low levels of terrigenous sedimentogenesis. Numerous species seem to have switched from the detrital food chain to a grazing one. This process was repeated several times; as a result, the surviving representatives of the most ancient and subsequent fauna occurred at even greater depths. It follows from the above information that a pulsing type of telogenesis is characteristic of shrimp. Therefore, the modern shrimp fauna consists of young shelf groups with intensive cladogenesis, groups that have persisted since the Cretaceous (or even Jurassic), and remains of earlier fauna that have survived at great depths or in shallow-water refugia. [ABSTRACT FROM AUTHOR]

A marine sediment core located off the southernmost part of the Baja California Peninsula, NE Pacific, is analyzed to investigate a submarine mass failure. We examine the lithofacies and sedimentary structures, the total organic (TOC) and inorganic (TIC) carbon, the grain size composition, and the degree of fragmentation in foraminifera tests. The core BB03 consists of five lithostratigraphic units, from bottom to top, U1 to U5. Radiocarbon dating (AMS 14C) indicates that this sequence is inverted, where the deepest strata U1–U2 date at ~ 4,162 cal yr BP, U3–U4 at ~ 14,406 cal yr BP, and the shallower strata U5 at ~ 18,759–19,445 cal yr BP. Analysis of core and multibeam bathymetric data evidenced a submarine mass failure of the slump type, characterized by abruptly inverted layers with soft-sediment deformation structures. The event's timing is constrained both with AMS 14C and the well-known stratigraphic identification of laminated or bioturbated layers in this highly productive upwelling zone. The event occurred during the Late Holocene as constrained by a maximum age of ~ 4,000 years. Although there is no explicit evidence on the cause of the submarine slump, the geomorphic characteristics and the close spatial connection with other submarine mass failures suggest a close relationship with the Tosco-Abreojos fault having acted either as a preconditioning factor or as a triggering mechanism. Finally, this study represents one of the few studies of submarine mass failures at the NE Tropical Pacific margin, aiming to identify how the transported sediments interact with the regional morphology and the formation of modern seafloor structures. [ABSTRACT FROM AUTHOR]

From the BIOPEARL 1 and 2 Programs to the Amundsen and Scotia Seas a large series of Tanaidacea was collected using an epibenthic sled. We carried out a thorough morphological analysis of 169 individuals and provide a description of 14 pseudotanaid species distributed in three genera. Twelve species of the genus Pseudotanais are represented by three morphogroups: "affinis+longisetosus", "denticulatus+abathagastor", and "forcipatus". We provide a redescription of Akanthinotanais gaussi, supplement the definitions of the genera Akanthinotanais and Beksitanais, and transfer Pseudotanais abyssi to the genus Beksitanais. Based on the literature and new data, we summarize knowledge of the Antarctic Pseudotanaidae and discuss this family's spatial and bathymetric distribution. [ABSTRACT FROM AUTHOR]

Approximately 11 400 h of acoustic recordings from two sites off the Oregon coast have been evaluated to characterize and model the frequency and wind dependence of wind noise in the northeast Pacific continental margin. Acoustic data are provided by two bottom-mounted broadband hydrophones (64 kHz sampling frequency) deployed at depths of 81 and 581 m at the continental shelf and slope, respectively. To describe the spectral level versus frequency relation, separate linear models for the 0.2–3 kHz and 3–25 kHz frequency range are fitted to the data. While spectral slopes for the 0.2–3 kHz range generally decrease with increasing wind speed, slopes remain constant (shallow location) or increase with increasing wind speed (deep location) above 3 kHz. The latter is in strong contrast to results from previous studies. The relation between spectral level and wind speed is described by a piecewise linear model where spectral levels are approximately constant below a critical wind speed v c and increase linearly with logarithmic wind speed above v c . It is shown that the critical wind speed and the slopes of the piecewise linear model strongly depend on the acoustic frequency. [ABSTRACT FROM AUTHOR]

Idealized facies of bottom current deposits (contourites) have been established for fine‐grained contourite drifts in modern deep‐marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine‐grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non‐existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo‐Mediterranean Outflow Water. Foraminifera assemblages were used for age‐constraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain‐size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo‐Mediterranean Outflow Water, which is mainly driven by precession and millennial‐scale climate variations. Tides substantially modulated the palaeo‐Mediterranean Outflow Water on a sub‐annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process‐based interpretation of deep‐water bottom current deposits. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a collection of Northwest Pacific nudibranch molluscs by means of integrative taxonomy, including morphological analyses, molecular data from the cytochrome c oxidase subunit I, 16S rRNA, histone H3, 28S rRNA, 18S rRNA markers, and ecological data. Two new species, Bathydoris antoni sp. nov. and Dendronotus kurilensis sp. nov., are described, and their phylogenetic relationships reconstructed. We also document two potentially new species of the genera Cadlina (Cadlinidae) and Cuthona (Fionidae s.l.). For the first time, we report molecular data for the Northwest Pacific specimens of Colga pacifica (Polyceridae), Dendronotus patricki (Dendronotidae), Ziminella vrijenhoeki (Paracoryphellidae), and Zeusia herculea (Aeolidiidae). Our molecular data supports the existence of biogeographic connections between the shallow water nudibranch fauna and their continental slope counterparts and communities on both sides of the North Pacific with possible ongoing gene exchange between fauna of both regions. We found two general types of deep shelf and bathyal communities inhabited by nudibranchs in the Northwest Pacific, each characterized by a certain type of fauna and a connectivity with different bathymetric and geographic areas. [ABSTRACT FROM AUTHOR]

This paper reports new data on the composition of hydrocarbon gases in bottom sediments of the Laptev–East Siberian marginal shelf transition zone, the continental slope of the Lomonosov Ridge, and Podvodnikov Basin of the Arctic Ocean. Anomalous concentrations of methane and hydrocarbon gases (up to C5 inclusive) have been established. Gas-geochemical indicators of eight genetic groups of hydrocarbon gases have been determined. The oil-and-gas content of the study area has been predicted based on the data obtained. [ABSTRACT FROM AUTHOR]

Establishing the foundations for a sustainable use of deep‐sea resources relies on increasing knowledge on this inaccessible ecosystem, which is challenging with traditional methods. The analysis of environmental DNA (eDNA) emerges as an alternative, but it has been scarcely applied to deep‐sea fish. Here, we have analyzed the fish eDNA contained in oceanic vertical profile samples (up to 2000 m depth) collected throughout the continental slope of the Bay of Biscay. We detected 52 different fish species, of which 25 were classified as deep‐sea fish. We found an increase of deep‐sea fish richness and abundance with depth, and that eDNA reflects day–night community patterns and species‐specific vertical distributions that are consistent with the known diel migratory behavior of many mesopelagic fishes. These findings highlight the potential of eDNA to improve knowledge on the fish species inhabiting the dark ocean before this still pristine ecosystem is further exploited. [ABSTRACT FROM AUTHOR]

A Bottom Simulating Reflection (BSR) has been identified using 3D seismic data from the Cameroon continental slope margin. The BSR covers an area of c. 350 km2 in water depths ranging between 940 m to 1,750 m across an area characterized by high- and low-gradient slopes, gullies, scours, and fans. The thickness of the Gas Hydrate Stability Zone (GHSZ) is ~100 - 250 m, assuming an average velocity of 1,800 m/s. Pockmarks are intensively developed across the slope and most of them are observed in the BSR area. Geothermal estimation is based on hydrate stability conditions for pure methane - seawater system, hydrostatic pressure model, and a range of P-wave velocity models for the GHSZ, ranging from 1,600 -1,800 to 2,000 - 2,200 m/s. Geothermal gradient is calculated showing the range and distribution of thermal gradients in the BSR area from 0.046 oC/m to 0.094 oC/m with assumed GHSZ velocity of 1,800 m/s. Thermal gradient anomalies have been observed in association with gullies, vertically stacked channels and in some individual pockmarks. The highest anomalies of 0.08 oC/m - 0.094 oC/m are found in the depression areas of pockmark trains, within seafloor gullies. These positive anomalies are most likely controlled by active or recently active fluid advection and expulsion through the Cameroon slope. [ABSTRACT FROM AUTHOR]

The nominal spatial distribution of diapycnal mixing in the South China Sea (SCS) is obtained with Thorpe-scale analysis from 2004 to 2020. The inferred dissipation rate ε and diapycnal diffusivity Kz between 100 and 1500 m indicated that the strongest mixing occurred in the Luzon Strait and Dongsha Plateau regions, with ε ~ 3.0 × 10−8 W kg−1 (εmax = 5.3 × 10−6 W kg−1) and Kz ~ 3.5 × 10−4 m2 s−1 (Kzmax = 4.2 × 10−2 m2 s−1). The weakest mixing occurred in the thermocline of the central basin, with ε ~ 6.2 × 10−10 W kg−1 and Kz ~ 3.7 × 10−6 m2 s−1. The ε and Kz in the continental slope indicated that the mixing in the northern part [O(10−8) W kg−1 and O(10−4) m2 s−1, respectively] was comparatively stronger than that in the Xisha and Nansha regions [O(10−9) W kg−1 and O(10−5) m2 s−1, respectively]. The Kz in the continental slope region (200–2000 m) decayed at a closed rate from the ocean bottom to the main thermocline when the measured depth D was normalized by the ocean depth H as D/H, whether in the shallow or deep oceans. The diapycnal diffusivity was parameterized as Kz = 3.3 × 10−4[1 + (1 − D/H)/0.22]−2 − 6.0 × 10−6 m2 s−1. The vertically integrated energy dissipation was nominally 15.8 mW m−2 for all data and 25.6 mW m−2 for data at stations H < 2000 m. This was about one order of magnitude higher than that in the open oceans (3.0–3.3 mW m−2), which confirmed the active mixing state in the SCS. Significance Statement: Diapycnal mixing (mixing across surfaces of constant density) is a dominant process in the ocean. It can affect ocean circulation and climate change. The South China Sea (SCS) is known as a strongly mixed marginal sea, but little research has been done on the SCS as a whole, and relatively little attention has been given to its central and southern regions. This study aimed to explore the nominal spatial variability of mixing in the entire SCS using Thorpe-scale analysis. Our study showed that the diapycnal mixing was stronger in the continental slope than in the central basin and that it followed a decay model from the bottom up in the region of the continental slope. According to our results, the averaged energy dissipation in the SCS was about one order of magnitude higher than that in the open oceans. [ABSTRACT FROM AUTHOR]

The Changning–Menglian Belt in western Yunnan, China has been widely considered as the remnants of Paleo-Tethys. However, its geological history remains waiting for further clarification. An efficient approach to this goal is to identify the tectono-sedimentary environment of different stratigraphic successions within the belt. This study examines the Nanpihe Formation, a Devonian to Early Carboniferous siliciclastics-dominated succession in the central zone of the belt. Our sedimentologic and petrographic results reveal that this formation comprises largely turbidity current deposits. Geochemical results indicate that its source area was rich in continental island arc rocks. Detrital zircon age spectra of this formation exhibit remarkable age clusters of ca. 435 Ma and ca. 950 Ma. The former is well-correlated with Silurian magmatism in the Simao and South China blocks. We thus consider that this formation was deposited in a continental slope environment along the west margin of the Simao Block, instead of the eastern margin of the Baoshan-Shan Block. We further consider that sometime in the Permian or Early Triassic, as the eastward subduction of the Paleo-Tethys progressed, the Nanpihe Formation was sheared off or collapsed from the overriding margin of the Simao Block and entered the accretionary wedge, which contained a significant amount of rock slices scraped off the subducting oceanic plate. During the closure of Paleo-Tethys in the Late Triassic, this formation and some other slices of the accretionary wedge were thrust onto the east flank of the Baoshan–Shan block. [ABSTRACT FROM AUTHOR]

A method is proposed for visualizing the inconstancy of the phytopigment composition in the water layer of the formation of backscattered solar radiation recorded by the MODIS multispectral ocean color scanner. The method relies on the fact that each pixel of the MODIS raster image of the investigated water area is assigned a wavelength reflectance minimum (WRM) index. The latter is equal to the sum of the wavelengths of minimums in the spectrum of a pixel or index 100 if they are absent. Such minimums are possible only in the presence of phytoplankton as the only carrier of plant pigments in the open sea. Pixel attributes of a MODIS raster image of the surface of water, like pixel coordinates and standard medium definitions, serve as WRM estimates. This makes it possible to visualize the variability of manifestations of the spectral diversity of phytoplankton against the background of the variability of the given medium properties. The effectiveness of the method is tested using the example of MODIS images of a spiral-eddy structure in the Caspian Sea and mesoscale variability of the bio-optical characteristics of waters over the continental slope in the Southwest Atlantic. The spectral indexing of the pixels of MODIS images of the water surface makes it possible to reveal some forms of variability of phytopigments of the aquatic environment that are inaccessible to the standard methods of ocean-color data processing. [ABSTRACT FROM AUTHOR]

Inflow of warm modified Circumpolar Deep Water (CDW) onto the Antarctic continental shelf and into ice shelf cavities is a key driver of Antarctic ice shelf mass loss. While recent research has advanced understanding of CDW heat transport onto the continental shelf, the fate of CDW on the shelf is less understood. Here, we use Lagrangian particle tracking in an ocean‐sea ice model without ice shelf cavities to map the residence time of CDW on the Antarctic continental shelf. Mean residence times vary from <1 month in the East Antarctic to >1 year in the West Antarctic. In regions of dense water formation, transformation of CDW on the shelf limits access of CDW to ice shelves, despite strong onshore CDW heat transport. Elsewhere transformation of CDW on the shelf is weak, implying that temperature on the shelf is limited by heat transport onto the shelf or the offshore heat reservoir. Plain Language Summary: Antarctic ice shelf melt is accelerating and is linked to the inflow of warm ocean waters that melt ice shelves from below. Multiple processes regulate the amount of warm water that reaches the Antarctic ice shelves, and the relative importance of these processes in different regions is not well known. Here, we conduct a virtual particle tracking experiment in a state‐of‐the‐art ocean model to trace the pathways and timescales of warm water on the Antarctic continental shelf. We find that there is a large regional variation in how long warm waters spend on the continental shelf from less than a month to well over a year. In the regions along the continental slope where very dense water formed near the coast flows off the shelf to the abyssal ocean, this is balanced by a strong inflow of warm water onto the continental shelf, which is rapidly converted to denser water, limiting the heat from reaching the ice shelves. Elsewhere warm water can remain on the continental shelf unmodified for a long time, so the heat available for ice shelf melt is instead limited by the strength of the flow of warm water onto the continental shelf. Key Points: Particle tracking in an ocean model shows Circumpolar Deep Water (CDW) residence times on the Antarctic shelf range from a month to several yearsWater mass transformation limits CDW access to ice shelves in Dense Shelf Water formation regions, despite strong onshore CDW transportIn warm shelf regions, long residence times of CDW are associated with weak water mass transformation [ABSTRACT FROM AUTHOR]

Pacific Summer Water (PSW) plays a critical role in the ecosystem of the western Arctic Ocean, impacting sea‐ice melt and providing freshwater to the basin. Most of the water exits the Chukchi Sea shelf through Barrow Canyon, but the manner in which this occurs and the ultimate fate of the water remain uncertain. Using an extensive collection of historical hydrographic and velocity data, we demonstrate how the PSW outflow depends on different wind conditions, dictating whether the warm water progresses eastward or westward away from the canyon. The current carrying the water westward along the continental slope splits into different branches, influenced by the strength and extent of the Beaufort Gyre, while the eastward penetration of PSW along the shelfbreak is limited. Our results provide the first broad‐scale view of how PSW is transferred from the shelf to the basin, highlighting the role of winds, boundary currents, and eddy exchange. Plain Language Summary: Warm Pacific water that flows northward through Bering Strait significantly influences the melting of sea‐ice, the distribution of freshwater, and the ecosystem of the Arctic Ocean. North of Bering Strait, the water progresses poleward across the shallow Chukchi Sea shelf, with most of it draining from the shelf through Barrow Canyon. Presently, the manner in which the flow exits the canyon, as well as its ultimate fate in the basin, remains unclear. We analyzed historical data sets of temperature, salinity, and velocity, and found that the local wind alters the manner in which the water exits Barrow Canyon to the east and west, as well as the corresponding distribution of Pacific Summer Water. The current carrying the water westward along the continental slope subsequently splits into different branches farther downstream. This is modulated by the strength and extent of the Beaufort Gyre, a large recirculating flow in the basin. In contrast, the signature of the warm water in the current flowing to the east is limited. Here, we provide the first observational broad‐scale view of the pathways and evolution of warm Pacific‐origin water from the shelf to the basin, and clarify the effects of wind, boundary currents, and small‐scale turbulent motions. Key Points: Outflow of Pacific Summer Water (PSW) through Barrow Canyon is dictated predominantly by the local windThe westward flow carrying the water away from the canyon bifurcates downstream, modulated by the strength and extent of the Beaufort GyreThe eastward flow emanating from the canyon progresses far downstream, but the signature of the PSW is limited [ABSTRACT FROM AUTHOR]

This study presents a systematic analysis of double-frequency (DF) microseisms recorded on the unconsolidated sediments in the eastern and southeastern coasts of United States. For all recordings, the site effect parameters (predominant frequency (f0), amplification factor and unconsolidated sediment thickness (UST)) are obtained by Nakamura method and the DF spectra are classified into five groups in terms of the DF peak patterns and the recording locations relative to the coastline. The frequencies and energy levels of the DF peaks in horizontal direction and the amplification factors are associated with the UST which is resulted from seismic site effect. By polarization analysis, the primary vibration directions of the DF peaks are identified and presented as great circles passing through the recording stations intersecting mainly along the continental slope. Correlation analyses of time histories of the DF energy and the ocean wave climate observed at buoys show that the low (<0.2 Hz) and high (>0.2 Hz) frequency DF microseisms are generated in the deep ocean and the continental shelf respectively. It is concluded that the continental slope plays a significant role in the generation of DF microseisms as it causes reflection of waves from the open ocean, initiating standing waves. [ABSTRACT FROM AUTHOR]

Aim: Understanding how fish food webs emerge from planktonic and benthic energy pathways that sustain them is an important challenge for predicting fisheries production under climate change and quantifying the role of fish in carbon and nutrient cycling. We examine if a trait‐based fish community model using the fish traits of maximum body weight and vertical habitat strategy can meet this challenge by globally representing fish food web diversity. Location: Global oceans. Time period: Predictions are representative of the early 1990s. Major taxa studied: Marine teleost fish. Methods: We present a size‐ and trait‐based fish community model that explicitly resolves the dependence of fish on pelagic and benthic energy pathways to globally predict fish food web biogeography. The emergent food web structures are compared with regionally calibrated models in three different ecosystem types and used to estimate two fish ecosystem functions: potential fisheries production and benthic–pelagic coupling. Results: Variations in pelagic–benthic energy pathways and seafloor depth drive the emergent biogeography of fish food webs from shelf systems to the open ocean, and across the global ocean. Most shelf regions have high benthic production, which favours demersal fish that feed on pelagic and benthic pathways. Continental slopes also show a coupling of benthic and pelagic pathways, sustained through vertically migrating and interacting mesopelagic and deep‐sea demersal fish. Open ocean fish communities are primarily structured around the pelagic pathway. Global model results compare favourably with data‐driven regional food web models, suggesting that maximum weight and vertical behaviour can capture large‐scale variations in food web structure. Main conclusions: Mechanistically linking ocean productivity with upper trophic levels using a size‐ and trait‐based fish community model results in spatial variations in food web structure. Energy pathways vary with ocean productivity and seabed depth, thereby shaping the dominant traits and fish communities across ocean biomes. [ABSTRACT FROM AUTHOR]

Introduction: The Costa Rican northwestern coastline has a wide geological uniqueness that exhibits rocks from different environments such as abyssal plane, continental slope, continental platform, volcanic eruptions (effusive, explosive, and submarine), coastal, erosional (fluvial and gravitatory), intrusions, and upper mantle rocks. Objective: To present the geologic state of the art of the northwestern coastline of Costa Rica through the bibliographic review of geochemical, petrologic, structural, paleontological and geochronological data. Methods: A bibliographic revision was done to propose a state of the art of northwestern coastline of Costa Rica. Results: The geologic record shows eleven stages from the Jurassic to the Holocene. These stages were regrouped from petrologic, structural, geochronologic, stratigraphic and paleontological interpretations in three depositional stages and four compressive tectonic phases. Conclusions: The first depositional stage is volcanic with oceanic affinity between the Jurassic and Upper Cretaceous. The second has a sedimentary predominance with an age range between the Upper Cretaceous and the Miocene. The last depositional stage is a volcanic (effusive and explosive) from the Pliocene until the present. The tectonic phases are associate with different interactions between tectonic plates. The first phase triggered the Santa Elena Ophiolite obduction during the Early Cretaceous. The second phase occurred in the Early Upper Cretaceous - Campanian and is recognized by its current position and surrounding structures. The third phase lasted from the Upper Eocene until the Miocene and its main result was the folding of the Bahía de Salinas sedimentary rocks. Finally, the Quaternary phase created a tilt axis and the aperture for the deposition of pyroclastic density currents in front of the Guanacaste volcanic ridge. [ABSTRACT FROM AUTHOR]

Submarine landslides are a major geohazard among worldwide continental slopes, posing significant threats to offshore infrastructure, marine animal habitats and coastal urban centres. This study establishes an original numerical package for time-efficient modelling of the entire submarine landslide evolution covering the pre-failure shear band propagation, slab failure and post-failure dynamics. The numerical scheme is based on the conservation of mass and the conservation of momentum and combines the shear band propagation theory and the depth-integrated method, with the consideration of the drag force from the ambient water. Shear band propagation in the weak layer and slab failure in the sliding layer are controlled by the strain softening and rate dependency of the corresponding undrained strength parameters. The post-failure behaviour in the sliding layer, such as retrogression upslope and frontally confined and frontally emergent mechanisms downslope, is also simulated. The numerical results from the proposed method are comparable to the analytical solutions and the large deformation finite element analysis. Application of this method to a back analysis of the St. Niklausen slide in Lake Lucerne reproduced the observed shape of the mass transport deposits, the position of the main scar and the travel distance. Because of its easy implementation and efficiency, the proposed numerical method for modelling of submarine landslides seems promising for practical applications. [ABSTRACT FROM AUTHOR]

The possibility of applying ocean circulation modeling data to forecast the effective velocities of pulse acoustic signal propagation from the continental slope to the deep ocean is studied. Prediction of these velocities is crucial for reliable operation of acoustic navigation and ranging systems, while typical lengths of paths and the prediction accuracy requirements almost completely exclude the use of direct measurements for this purpose. Analysis of experimental data obtained on a 200-km-long acoustic path shows that the NEMO ocean circulation model used for reconstruction of the sound speed distribution along this path allows one to calculate with sufficiently high accuracy the effective velocities of propagation of pulse acoustic signals transmitted by a source on the shelf and received in the deep-water part of the Sea of Japan. The proposed method for effective velocities calculation is based on the adiabatic mode theory of sound propagation on the shelf, as well as on the fact that in the deep-water part of the path, the group velocities of low-number modes are very close to each other. [ABSTRACT FROM AUTHOR]

In this paper we present geomorphological hazard mapping of the San Vito Peninsula offshore (Sicilian margin of Tyrrhenian Sea), characterised by a very narrow continental shelf and a very dipping, tectonically active continental slope, no far from a seismogenic belt. The data set consists of morpho-bathymetric models achieved by means of multibeam survey and scattered high resolution seismic profiles. The morpho-bathymetric study highlighted some potential sites of geomorphological hazards, the most representative of which are: (i) two canyons developing from the upper slope off San Vito Cape down to the Erice basin; (ii) a dense network of small incisions across the upper slope; (iii) a very active landslide, developed onland and extending in the continental shelf close to the Scopello village. The geomorphological features that can generate submarine hazard are represented by: unstable sediment packages into canyon heads or along the upper slope; submarine canyons with regressive erosion; coastal landslides. [ABSTRACT FROM AUTHOR]

The values and direction of carbon dioxide flux in the area of the continental slope in the north of the Kara Sea (St. Anna Trough) are calculated based on field studies in 2020 within the Siberian Arctic Sea Ecosystems program. The existence of a stable frontal zone in this area has been confirmed, which is formed by an alongslope current and limits the northward spread of surface waters freshened by the continental runoff. The simultaneous analysis of the carbonate system in the upper sea layer and the CO2 concentration in the surface air layer shows the CO2 flux with a rate of 0.2 to 22 mmol/m2 day to be directed from the atmosphere into the water in the area of the outer shelf, which is affected by the river runoff, and in the area of the continental slope, which is beyond this effect. The highest rates of CO2 absorption by the sea surface layer are localized above the continental slope. Local processes in the area of the slope frontal zone determine the CO2 emission into the atmosphere with a rate of 0.34 mmol/m2 day. [ABSTRACT FROM AUTHOR]

Clay minerals preserved in marine sediments carry important information about sediment transport processes from sources to sinks. To reveal the dynamic transport processes of clay minerals and related controlling factors in the southern South China Sea, spatiotemporal variations of clay mineral assemblages from previous studies (Cores MD05-2893, CG2, SO18287-3, MD01-2393, SO18383-3, and MD97-2150) are integrated to assess their controlling factors over the past 30 ka. Based on the smectite/(illite + chlorite) ratio in six cores, we found that the clay mineral variations on the continental slope off Sunda Shelf (Cores MD05-2893, CG2, and SO18287-3) have been controlled by the sea-level changes during the last glacial sea-level lowstand and by the East Asian monsoon during the Holocene sea-level highstand periods. Meanwhile, the clay mineral variations on the continental slope off the Mekong River (Cores MD01-2393, SO18383-3, and MD97-2150) have been driven by the chemical weathering in the Mekong River basin that associates with the East Asian summer monsoon evolution. Accordingly, we conclude that the glacial-interglacial variation of clay minerals in an integrated picture of the southern South China Sea region since the Last Glacial Maximum has been mainly controlled by sea-level change and East Asian monsoon evolution. However, these controlling factors play independent roles in different parts of the southern South China Sea, which appears more heterogeneous than generally expected. [ABSTRACT FROM AUTHOR]

The northern margin of the South China Sea (SCS) is often regarded as a magmapoor passive continental margin. Magmatic activities occurred after the cessation of seafloor spreading were founded mainly over the Continent-Ocean Transition (COT) zone. Intrusive rocks are observed in the Luzon-Ryukyu Transform Plate Boundary (LRTPB) dividing Southwest Taiwan Basin (SWTB) and Pearl River Mouth Basin (PRMB) north of the COT and their evolution mechanisms are not very well-studied. Here, detailed structural and geophysical features of a large-scale anomaly (LSA) is revealed from high-resolution multi-channel seismic (MCS) profiles over the LRTPB dividing the SWTB and the PRMB. After velocity estimation, AVO analysis, and geological interpretation for the LSA, we suggest that the LSA is an intrusive igneous rock and further classified as a laccolith. The coexistence of the laccolith and surrounding sills over the LRTPB shows that the north limit of magmatism is further north than the COT zone. The elongated shape of the laccolith is accordant to the NW directional LRTPB, indicating that magmatic activities of study region maybe controlled by the faulting. [ABSTRACT FROM AUTHOR]

Synchronous interaction between bottom currents and turbidity currents has been reported often in channel–levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along‐slope and down‐slope sedimentary processes is one of the mechanisms by which 'mixed turbidite–contourite systems' can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along‐slope and down‐slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep‐sea late Quaternary deposit that sits adjacent to the north‐west flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south‐west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water, which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer‐grained fraction of turbidity currents out of the canyon at the boundary between major water masses. [ABSTRACT FROM AUTHOR]

Costa Rica has been affected by several local and distant tsunamis in the past, but the historical information is scarce and incomplete. Its Pacific coast stretches for over a thousand kilometers, and tsunami hazard has never been evaluated for its full extent. Numerical modeling of tsunami propagation and inundation is a useful tool to assess tsunami hazard, particularly in cases with limited historical information available. Here, we perform a first estimation of tsunami hazard for the Pacific coast of Costa Rica from seismogenic sources, by numerical propagation of 57 local and distant tsunamis to a depth of 20 m. The results of our study identified tsunami sources that are particularly threatening for Costa Rica and determined locations with higher tsunami hazard. For the analysis, the Pacific coast of Costa Rica was divided into segments and subsegments based on differences in continental slope morphology. Subsegments with higher tsunami heights were Southwest Nicoya Peninsula and West Osa Peninsula, and in a lesser extent North Guanacaste, North Nicoya Peninsula, and Central Pacific. Regions with long and gentle slopes and narrow continental shelf were affected by higher tsunami waves, due to more efficient tsunami energy transmission to the shelf and reduced energy loss while traveling through a narrow shelf. On the opposite, steeper continental slopes reflected most of the tsunami energy, causing smaller tsunami heights nearshore, regardless of the shelf width. Nevertheless, other effects played a major role, like curved coastlines that focused tsunami energy, wave refraction, interference, and trapped edge waves. Distant tsunamis dominated the threat, with tsunamis coming from the Tonga-Kermadec and the Colombia-Ecuador Trenches causing the greatest heights due to directivity, and arrival times of about 15 h and 75 min, respectively. Local tsunamis had short arrival times but a localized impact, mainly at the shoreline in front of the generation region but were also affected by tsunami focusing, wave refraction, and edge waves. Outer rise and Osa sources caused the lowest impact within local sources. These results provide a guide for emergency planners to prioritize coastal locations and tsunami sources for tsunami preparedness actions and warning protocols. [ABSTRACT FROM AUTHOR]

Most previous Black Sea palynology studies have used pollen-spore assemblages as proxies for climate and landscape changes, and dinocyst assemblages as proxies for surface water salinity. However, there are few data on within-region variations in these assemblages using large sets of surface samples and a full suite of palynomorphs, including terrigenous pollen + spores, freshwater algal spores, and organic remains of marine microplankton, micro- and meiobenthos. Here we fill this knowledge gap, using results from a palynological study of 43 surface samples from water depths of 71–905 mbsl on the Ukrainian Shelf and adjoining continental slope, NW Black Sea. The palynology samples were collected in conjunction with water and sediment chemistry data, grain-size, micropaleontological and meiobenthos data from the EU-FP6 project HERMES "Hotspot Ecosystems Research on the Margins of European Seas". This dataset uniquely covers shelf and upper slope areas east and west of the Odessa-Sinop Fault Zone (OSFZ) that delimits areas of lower and higher hydrocarbon gas contents, respectively. The new data show that on the wide shelf with a shallow (ca. 80 m) oxycline, pollen assemblages differ from classical models for marine pollen transport. On the Ukrainian Shelf, pollen concentrations are extraordinarily high far offshore and the grains are often pyrite-infilled. These features apparently reflect high pollen preservation potential in low oxygen sediments, probably enhanced by cross-shelf transport of land-derived particulate organic matter. The new data have important consequences for accurate modelling of carbon burial and storage in epicontinental seas, and they provide a baseline for evaluating marine pollen concentrations expected to reflect the early Neolithic farming on the Ukrainian shelf. Furthermore, although large concentrations of dinocysts comprise more than half of the total Non-Pollen Palynomorphs (NPP), the high pollen concentrations skew the P:D index (pollen:dinocyst ratio) commonly used to evaluate changes in paleo-shoreline (proxy sea-level) positions. Despite the discharge of several large rivers to the Ukrainian Shelf, there are negligible traces of freshwater algae in the HERMES samples except at Sta. 15 where Pediastrum is abundant. Overall, palynomorphs of terrestrial origin make up about half (52%) of the acid-resistant organic-walled particles on the outer NW Black Sea Shelf and adjacent slope. The abundances of the major micro- and meiobenthic palynomorph groups show the same trends east and west of the OSFZ as the micropaleontological data reported for the HERMES samples taken at the same sites. The combined microzooplankton and zoobenthos surface assemblage data provide an important new baseline for evaluating historical and past changes in biodiversity and aquatic foodweb structure in the now polluted NW Black Sea. Comparison can also be made with foodweb reconstructions for Permian black shales. [ABSTRACT FROM AUTHOR]

Arctic Ocean Observing System Simulation Experiments (OSSEs) were performed with a pan‐Arctic coupled ocean–sea ice data assimilation system to assess the impacts of assimilating available observations on the Arctic ocean–sea ice state. To this end, the adjoint method with a 3‐year assimilation window was used to assimilate synthetic observations sampled from a 4 km model simulation at the spatio‐temporal distribution of the existing observing system. After data assimilation, the sea ice state, including sea ice concentration (SIC), sea ice thickness (SIT), and sea ice volume (SIV), were significantly improved, benefiting from the high spatio‐temporal coverage of SIC observations and the wintertime SIT observations. In contrast, the ocean state is not very well constrained with the existing hydrographic observing system. An additional 1° × 1° ocean profiling array with a 10‐day sampling frequency was seen to substantially improve the estimated ocean temperature and freshwater content. Data from additional moorings deployed in the Fram Strait and continental slope of the Laptev Sea could also improve the pathway of Atlantic inflow to the Arctic Ocean and the temperature of the Atlantic inflow but degrade the mean volume transport through the Fram Strait. Moreover, estimated exchanges between the Arctic Ocean and the Atlantic Ocean through the Fram Strait, Davis Strait and the St. Anna Trough were found to benefit from the high‐density profiling array. [ABSTRACT FROM AUTHOR]

The pencil cardinal Epigonus denticulatus is a small deep‐water fish inhabiting continental slopes usually between 300 and 600 m depth. We report the first record of E. denticulatus in the Azores archipelago, where one specimen was found floating by fisherman off Faial island. Meristic and morphometric characters are in accordance with those reported for the species and molecular analyses further supported species identity. The record of E. denticulatus as a native species in the Azores increases the number of Epigonus species in the region to a total of three. [ABSTRACT FROM AUTHOR]

This study presents a detailed reconstruction of the state of stress in the crust of the Japan Benioff zone that followed the Tohoku earthquake of March 11, 2011. The stress parameters were found using the method of cataclastic analysis applied to discontinuous displacements based on the identification of a homogeneous selection of earthquake mechanisms around a point of interest. The criteria used to make the selection rely on principles of plasticity theory as extended to cover the case of strain evolving in a cracked medium. The reconstruction used data on earthquake mechanisms from the NIED f-net catalog for the period of observation until June 1, 2018. An analysis of the patterns in the spatial stress distribution derived for six depth levels in the crust showed substantial changes in the state of stress observed in the aftershock area of the Tohoku earthquake in the upper lithospheric layers of the continental slope (0–30 km). The horizontal tensile stresses that appeared immediately after the earthquake are best observed in the uppermost crustal layer (0–10 km). As we go deeper into the earth, this state of stress becomes increasingly less representative. Estimates of the stress magnitudes based on a model of horizontal tension with pure shear showed that the change in the type of geodynamic regime in the upper crustal layer was accompanied by a large change in the level of shear stresses that are directed along the axis of the Benioff zone (the drop was about 90%) and a decrease in the level of maximum shear stresses by nearly 65% relative to its mean value. These results can be explained by a larger contribution into the change of the state of stress due to a quasi-homogeneous horizontal tension compared with that due to pure shear caused by the drop of tangential stresses in the Tohoku source. [ABSTRACT FROM AUTHOR]

A brief account of the achievements of the AUSCAN cruise is presented together with an extensive list of published papers resulting from this cruise. Many of these papers provide considerable detail about the link between marine and continental records, many of which span the entire last glacial cycle (MIS 5e - MIS 1). The second part of the paper argues that a large estuary to the north of the River Murray mouth during the early Holocene, postulated by Helfensdorfer et al. (2019, 2020) requires scrutiny because their postulated sealevel reconstruction is flawed. In addition, the extensive deposition of sediments dammed near the mouth of the River Murray thought by these authors to have prevented the dispersal of fluvial sediments to the continental shelf and slope is unfounded. [ABSTRACT FROM AUTHOR]

The implementation of satellite geological investigations within the continent–Sea of Japan transition zone in Southern Primorye has made it possible to establish series, connected with faults, of rows (chains) of Miocene and Miocene–Pliocene volcanic structures and large fracture zones, which extended over the continental slope from the shelf to Central Basin of the Sea of Japan. They are structural markers, the steady strike of which calls into question tectonic formation models of the Sea of Japan Basin, based on large-scale horizontal movements of crustal blocks. The combination of linear systems of small monogenic volcanic structures and fissure eruptions of basalt lava reflects the multioutlet nature of Central Basin volcanism, which occurred due to the rather shallow location of asthenospheric highs or mantle diapirs. It is suggested that the formation and relatively rapid subsidence of this basin is related to active mantle defluidization, with large-scale basalt volcanism and injections of deep mafic and ultramafic material that replaced rocks of the continental crust. [ABSTRACT FROM AUTHOR]

The article is devoted to studing of the features of sedimentation on the bottom of the northwestern shelf and western deep-water area of the Black Sea based on an analysis of our own and published data obtained by bottom sediment radioisotope geochronology. It was determined that the relationship between the mass accumulation rate and primary production was linear, due to the aggregate biogeographic, hydrological, and hydrochemical conditions. The features of bottom sediment formation in the open part of the sea were associated with the presence of hydrogen sulfide (H2S) in deep waters (deeper than 100–200 m). It was shown that 20 (on the continental slope) and 17 g/(m2 year) of particulate organic matter (in the deep-sea basin) entered from the oxygen-containing zone. In this case, the predominant fractions in bottom sediments were mineralized biogenic matter: 54 and 68.5%, respectively, in these areas. The contribution of organic, mineralized biogenic, and lithogenic sedimentary matter to the formation of the mass accumulation rate was estimated, and changes in their ratio with distance from the coast and estuaries were shown. Based on the nature of the formation of the mass accumulation rate, three groups of sea areas were identified in which the ratio of biogenic and lithogenic components in absolute masses reflected a change in their role of these components in bottom sediment formation. [ABSTRACT FROM AUTHOR]

Climatic changes have decreased the stability of the Gulf Stream (GS), increasing the frequency at which its meanders interact with the Mid-Atlantic Bight (MAB) continental shelf and slope region. These intrusions are thought to suppress biological productivity by transporting low-nutrient water to the otherwise productive shelf edge region. Here we present evidence of widespread, anomalously intense subsurface diatom hotspots in the MAB slope sea that likely resulted from a GS intrusion in July 2019. The hotspots (at ~50 m) were associated with water mass properties characteristic of GS water (~100 m); it is probable that the hotspots resulted from the upwelling of GS water during its transport into the slope sea, likely by a GS meander directly intruding onto the continental slope east of where the hotspots were observed. Further work is required to unravel how increasingly frequent direct GS intrusions could influence MAB marine ecosystems. Plain Language Summary As the climate has warmed, the changing large-scale circulation of the northwest Atlantic has resulted in increasing western boundary current instability. As a consequence, onshore intrusions of Gulf Stream (GS) water into the Northeast U.S. continental shelf have become increasingly frequent. The impacts of this shift on marine ecosystems have yet to be resolved. While these intrusions of low-nutrient GS water have been thought to potentially diminish biological productivity, we present evidence of an unexpectedly productive subsurface diatom bloom resulting from the direct intrusion of a GS meander toward the continental shelf. These results suggest that changing large-scale circulation has consequences for regional productivity that are not detectable by satellites by virtue of their occurrence well below the surface. [ABSTRACT FROM AUTHOR]

Large scale studies of underwater noise during rain are important for assessing the ocean environment and enabling remote sensing of rain rates over the open ocean. In this study, approximately 3.5 yrs of acoustical and meteorological data recorded at the northeast Pacific continental margin are evaluated. The acoustic data are recorded at a sampling rate of 64 kHz and depths of 81 and 581 m at the continental shelf and slope, respectively. Rain rates and wind speeds are provided by surface buoys located in the vicinity of each hydrophone. Average power spectra have been computed for different rain rates and wind speeds, and linear and nonlinear regression have been performed. The main findings are (1) the linear regression slopes highly depends on the frequency range, rain rate, wind speed, and measurement depth; (2) noise levels during rain between 200 Hz and 10 kHz significantly increase with increasing wind speed; and (3) the highest correlation between the spectral level and rain rate occurs at 13 kHz, thus, coinciding with the spectral peak due to small raindrops. The results of this study indicate that previously proposed algorithms for estimating rain rates from acoustic data are not universally applicable but rather have to be adapted for different locations. [ABSTRACT FROM AUTHOR]

Gas hydrates from the continental margin settings are mostly confined to continental slopes. The free gas occurring below the gas hydrate laden sediments play a crucial role in the submarine slope failures and can potentially trigger the submarine landslides. The present study is carried out to investigate such a scenario in the shale dominated Krishna–Godavari (KG) basin having proven gas hydrate deposits. Shale reservoirs are less permeable compared to sandstone reservoirs and as such, the free gas that is locked in the pore spaces of shales would eventually create hydro fractures or faults to escape from the pore spaces. This overpressure from the gas zone will find a pathway to reach the shallower depths. In the present study, two faults are identified of which, one is acting as a potential pathway for overpressures to travel through it, while the other is found to be critically pressurized due to the gas zone below it. The calculated gas column height is ~51 m and any further increase in the gas column height would create a fault slip and cause the mechanical failure of the sediments, creating another potential pathway. We infer that these faults are acting as a migration pathway in our study area and the advection of fluid/gas from the gas zone via the pathway might have played a major role in creating a slope break above the fault. Research highlights: Gas migration and role of free gas on slope stability. Seismic attributes for characterization of free gas zones and migration pathways Gas column height and critical gas pressure necessary to initiate fault activation. Active and passive faults and their role as migration pathways. [ABSTRACT FROM AUTHOR]

Mesoscale eddy mixing profoundly modulates the ocean tracer budgets, and is typically parameterized via the isopycnal eddy diffusivity in ocean climate models. However, relatively little is known about the magnitude/structure of isopycnal eddy diffusivity across continental slopes, which hinders the understanding and prediction of shelf‐open ocean exchanges. In this study, we quantify the isopycnal eddy diffusivity in a suite of eddy‐resolving, process‐oriented simulations of mesoscale turbulence over continental slopes under upwelling‐favorable winds, a configuration that commonly arises around the margins of subtropical gyres. Cross‐shore eddy diffusivity is found to be suppressed in the upper open ocean occupied by strong alongshore flows, but enhanced at depths where alongshore flows are weakened, a finding that is consistent with the enhancement of eddy mixing near the steering level. Over continental slopes, eddy diffusivity also strengthens at mid‐depths, but almost vanishes near the seafloor. To theoretically constrain the simulated eddy fluxes, we examine the scaling of eddy diffusivity proposed by Ferrari and Nikurashin (2010, https://doi.org/10.1175/2010JPO4278.1), which accounts for the suppression of eddy mixing induced by the relative propagation of eddies to the mean flow. We show that, apart from the mean‐flow suppression effect, the eddy anisotropy effect induced by steep topography shapes both the horizontal and vertical structures of cross‐shore eddy diffusivity. Finally, we propose prospective closures of the eddy propagation speed and eddy anisotropy effect over continental slopes using the large‐scale flow and bathymetric quantities. This work offers a basis upon which a "slope‐aware" parameterization of mesoscale eddy mixing can be developed. Plain Language Summary: The ocean is replete with turbulent eddies that are highly efficient in mixing oceanic tracers. Across continental margins, turbulent eddies drive the exchanges between shelf seas and open ocean, which modulate the coastal water properties and sea level, upon which human habitation depends the most. Understanding the impact of eddies on shelf‐ocean exchanges in a changing climate is therefore critical, and would typically rely on predictive ocean climate models. However, owing to limited computer powers, even state‐of‐the‐art ocean climate models must adopt a grid spacing that is too coarse to resolve turbulent eddies across continental margins. Instead, these models rely on parameterization schemes that infer eddy effects from the large‐scale, explicitly resolved flow properties. This work investigated the utility of a parameterization developed based on open ocean turbulent properties in quantifying eddy tracer fluxes across continental margins, following which further refinements of the parameterization have been made. Our results show that, apart from previously considered ingredients, the tendency of steep topography across continental margins to "squeeze" the eddy shapes must be incorporated into this parameterization. This work provides a basis upon which a parameterization scheme for eddy tracer fluxes across both continental margins and open ocean environments can be devised. Key Points: The scaling of isopycnal eddy diffusivity proposed by Ferrari and Nikurashin (2010) is adapted to continental slopesTopographically induced eddy anisotropy effect modulates both the horizontal and vertical structures of isopycnal eddy diffusivityA basis for parameterizing isopycnal eddy mixing across continental slopes is presented [ABSTRACT FROM AUTHOR]

Manganese (Mn) is an abundant element in the Earth's crust. However, its concentrations in open ocean seawater are low, where external inputs are scarce. In this study, we report the dissolved Mn and particulate Mn distributions in the Southern Ocean, measured along the GEOTRACES—SR3 transect, from Tasmania (Australia) to Antarctica in the Southern Ocean, during the austral summer 2018. Both dissolved Mn and particulate Mn concentrations were generally low away from localized sources (< 0.3 nmol L−1 and < 0.1 nmol L−1, respectively) along the transect. Our observations of a lower labile particulate fraction than previously measured suggest the Southern Ocean has a unique particulate Mn composition. Low surface dissolved Mn concentrations were attributed to biological uptake and few external sources. Our results suggest biological control of the Mn cycle was higher above the Antarctic continental slope, compared to the rest of the section, and our particulate Mn : P ratios indicated the presence of iron‐stressed diatoms south of the Sub‐Antarctic Front. We suggest low dissolved Mn surface concentrations may (co‐)limit phytoplankton growth in this region. Localized higher dissolved Mn concentrations were observed due to external sources, such as sedimentary and hydrothermal inputs. The presence of an eddy at the same latitude as a hydrothermal plume induced an upwelling of hydrothermally enriched waters up to 1000 m; however, no dissolved Mn inputs to the surface waters were observed. We suggest previous dissolved Mn inputs to the surface layer may be obscured by biological uptake. [ABSTRACT FROM AUTHOR]

Four ichthyoplankton cruises and backward tracking experiments were conducted to study the connectivity of coastal and neritic fish larvae over the continental slope and to the oceanic deep‐water region of the western Gulf of Mexico. Distribution patterns of larval abundance at oceanic stations showed higher abundance and the presence of larvae at oceanic stations during two cruises. Larval transport was simulated using outputs of a data assimilation model that represented the flow conditions during each cruise. Higher abundances of larvae of coastal and neritic species at oceanic stations agreed with offshore transport inferred from numerical experiments seeding particles over different spatial scales (stations vs. transects). Satellite images of surface chlorophyll were consistent with the circulation patterns indicated by the model, indicating filaments of shelf waters were transported toward the transects with higher larval abundances. Particle tracking experiments indicated that the northwestern shelf provinces of Perdido, Tamaulipas, and Texas were the main source of propagules to the oceanic region, while shelf provinces of northern Veracruz, Campeche, Yucatan, Louisiana, and Mississippi‐Alabama contributed much less. The length and intensity of the shelf front limited ichthyoplankton cross‐shelf exchange during some cruises, and mesoscale anticyclonic and cyclonic eddies advected larvae to the deep‐water region during others. The agreement between the spatial distribution of fish larvae and the simulated larval transport confirm that circulation models are a valuable tool for examining potential dispersal pathways of neritic species, as long as similar spatial and temporal scales as the ones used in this study are considered. [ABSTRACT FROM AUTHOR]

Background and aims: Planktonic dinoflagellates have a great ecological significance in marine environments. While some dinoflagellate species commonly reach bloom concentrations and are thus conspicuous components of marine phytoplankton, others occur in very low abundances which make them difficult to detect in field studies. Here we analyzed dinoflagellate composition and abundance in five oceanographic expeditions carried out in continental shelf and slope waters of the Argentine Sea. M&M: Plankton abundance was estimated by the Utermöhl method, using inverted microscopy, whereas further optical and scanning electron microscopy was applied for the identification of dinoflagellate species. Results: We focused on the occurrence of seven dinoflagellates that have been previously poorly documented or overlooked in marine environments worldwide: Dinophysis microstrigiliformis; Gyrodinium sp.; Karlodinium elegans; Oxytoxum laticeps; Peridiniella danica; Peridiniella globosa and Prorocentrum nux. The latest and K. elegans are observed for the first time in field conditions after their original descriptions based on cell cultures. While most species were detected in low or moderate abundances, P. nux, which is the smallest Prorocentrum species, reached 82,000 cells L-1 in slope waters. Very small Gyrodinium cells (11.5 μm long; 8.7 μm wide) with a distribution restricted to slope waters during spring, were not possible to be accurately assigned to a species. Conclusions: This study contributes to the understanding of dinoflagellate diversity in the Argentine Sea and the worldwide distribution of little known species. [ABSTRACT FROM AUTHOR]

The narrow, short-lived Taconic-Grampian Orogen occurs along the north-western margin of the Appalachian-Caledonian Belt from, at least, Alabama to Scotland, a result of the collision of a series of early Ordovician oceanic island arcs with the rifted margin of Laurentia. The present distribution of Taconian-Grampian ophiolites is unlikely to represent a single fore-arc from Alabama to Scotland colliding at the same time with the continental margin along its whole length; more likely is that there were several Ordovician arcs with separate ophiolites. The collision suture is at the thrust base of obducted fore-arc ophiolite complexes, and obduction distance was about two hundred kilometres. Footwalls to the ophiolites are, sequentially towards the continent, continental margin rift sediments and volcanics and overlying rise sediments, continental shelf slope carbonates, and sediments of foreland flexural basins. The regionally-flat obduction thrust complex between the ophiolite and the rifted Laurentian margin is the collision suture between arc and continent. A particular problem in drawing tectonic profiles across the Taconic-Grampian Zone is several orogen-parallel major strike-slip faults, both sinistral and dextral, of unknown displacements, which may juxtapose portions of different segments. In western Newfoundland, most of the Grenville basement beneath the Fleur-de-Lys metamorphic complex (Neoproterozoic to early Ordovician meta-sediments) was eclogitised during the Taconic Orogeny and separated by a massive shear zone from the overlying Fleur-de-Lys, which was metamorphosed at the same time but in the amphibolite facies. The shear zone continued either to a distal intracontinental "subduction zone" or to the main, sub-fore-arc, subduction zone beneath which the basement slipped down to depths of up to seventy kilometres at the same time as the ophiolite sheet and its previously-subcreted metamorphic sole were being obducted above. Subsequently, the eclogitised basement was returned to contact with the amphibolite-facies cover by extensional detachment eduction, possibly enhanced by subduction channel flow, which may have been caused by slab break-off and extension during subduction polarity flip. Although the basal ophiolite obduction thrust complex and the Fleur-de-Lys-basement subduction-eduction surfaces must have been initially gently-dipping to sub-horizontal, they were folded and broken by thrusts during late Taconian, late Ordovician Salinic-Mayoian, and Acadian shortening. [ABSTRACT FROM AUTHOR]

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment. Plain Language Summary: In the Arctic Ocean deep basins, only a tiny fraction of the algae that grows in the surface layer sinks down to the sea floor. Most of the particles reaching the sea floor originate from the shallower regions closer to the coast. These particles have already settled on the sea floor once, and originate from rivers or algae that grew, died and sank down in shallow regions. Later, these particles are lifted off the ground again by strong turbulent motions and transported toward deeper regions in the middle of the water column. These lift‐off and transport events happen only occasionally, and have not been directly observed in the Eurasian part of the Arctic Ocean yet. Also, we present a new mechanism for the creation of turbulence, which is necessary to lift particles off the sea floor. This mechanism happens mostly during the summer season, when less sea ice is present. Based on this seasonality, it is likely that sediment transport events will become more frequent in the future, when the Arctic sea ice is further declining. Key Points: First direct observation of an intermediate nepheloid layer in the Eurasian part of the Arctic OceanCoinciding strong midwater turbulence is likely caused by a down‐slope current displacing isopycnalsSimilar downslope flow events exhibit a strong seasonality toward the ice free season [ABSTRACT FROM AUTHOR]

Euphausiids are a keystone species in coastal food webs due to their high lipid content and seasonally high biomass. Understanding the habitat and environmental drivers that lead to areas of high biomass, or 'hotspots', and their seasonal persistence, will support the identification of important foraging regions for mid- and upper- trophic level predators. We quantify the distribution of hotspots of the two dominant species of euphausiid in the north-east Pacific Ocean: Euphausia pacifica and Thysanoessa spinifera, as well as euphausiid larvae (mixed species). The Canadian coast encompasses the northern California Current Ecosystem and the transition zone to the Alaska current, and is a highly productive region for fisheries, marine mammals, and seabirds. We used spatiotemporal modelling to predict the distribution of these three euphausiid groups in relation to geomorphic and environmental variables during the important spring-summer months (April through September) when euphausiid biomass is highest. We quantified the area, intensity, and persistence of biomass hotspots across months according to specific oceanographic ecosections developed for marine spatial planning purposes. Persistent hotspots of both adult species were predicted to occur along the 200 m depth contour of the continental slope; however, differences were predicted on the shallower Dixon shelf, which was a key area for T. spinifera, and within the Juan de Fuca Eddy system where E. pacifica hotspots occurred. The continental slope along the west coast of Vancouver Island was the only persistent hotspot region common between both adult species and euphausiid larvae. Larval distribution was more correlated with T. spinifera than E. pacifica biomass. Hotspots of adults were more persistent across months than hotspots of euphausiid larvae, which were seasonally patchy. The persistence of biomass hotspots of forage species through periods of low overall biomass could maintain trophic connectivity through perturbation events and increase ecosystem resilience to climate change. [ABSTRACT FROM AUTHOR]

Intra-annual variability in the East Auckland Current (EAuC) was studied using a year-long timeseries of in situ and remotely-sensed velocity, temperature and salinity observations. Satellite-derived velocities correlated well ( r > 0.75 ) with in situ observations and well-represent the long-term ( > 30 days) variability of the upper ocean circulation. Four mesoscale eddies were observed during the year (for 260 days) which generated distinct flows between the continental slope and rise. The EAuC dominated the circulation in the continental shelf break, slope and rise for 110 days and generated the most energetic events associated with wind forcing. Current variability on the continental slope was coherent with along-slope wind stress (wind stress curl) at periods between 4 and 12 days (16 and 32 days). We suggest that along-slope winds generated offshore Ekman transport, uplift on the shelf-break, and a downwind geostrophic jet on the slope. In contrast, positive wind stress curl caused convergence of water, downwelling, and increased the current speed in the region. Bottom Ekman transport, generated by the EAuC, was suggested to have caused the largest temperature anomaly ( - 1. 5 ∘ C ) at the continental shelf-break. [ABSTRACT FROM AUTHOR]

Methane hydrates are an attractive source of future clean energy and are abundant in voids of sediments, in permafrost regions, and along continental slopes. Understanding the mechanical behavior of methane hydrate-bearing sediment (MHBS) is essential for commercial gas production from hydrate reservoirs. In this paper, the effects of the intermediate principal stress are studied on the microscopic and macroscopic mechanical behavior of MHBS-filled porous media. Using discrete element method (DEM) simulations, methane hydrates are modeled as cemented agglomerates filling the pores of soil at a specific hydration level. The simulated sample is sheared under strain-controlled conditions to different values of the intermediate principal stress ratio b. The results suggest evolution rules of the principal stresses that depend on the parameter b. The friction angle increases with b, reaching a peak value at a certain value, after which it decreases. Monitoring local variables, including principal strong fabric, contact rose diagram, coordination number, and damage parameter, the effects on the macroscopic bulk behavior are demonstrated. In particular, the changes in the fabric tensor are strongly correlated with response to changes in the principal stresses, the parameter b, and the strain tensor. [ABSTRACT FROM AUTHOR]

The European eel hatches in the Sargasso Sea and migrates across the Atlantic Ocean toward Europe. At the continental shelf, larvae metamorphose into glass eels and then recruit to coastal habitats and estuaries. Among other cues, glass eels orient in situ using lunar cues, but what role this lunar compass plays in their recruitment to the coast is unknown. To assess this, we incorporated empirical in situ observations of glass eel swimming and lunar‐driven orientation into a biophysical advection model. We simulated dispersal of glass eels drifting with the North Atlantic Current to test the hypothesis that lunar‐driven swimming and orientation behavior affects recruitment to North Sea coasts. Particles were released from the continental slope north of Scotland, an obligate passage for migrating eel larvae. Four numerical experiments were conducted: one with passive drift and three including glass eel swimming speeds (ranging from 3 to 12 cm/s) and lunar‐driven orientation. With a speed of 3 cm/s, the lunar compass increased recruitment to the North Sea coasts of Southwestern Norway and Scotland by 34%–40%. Conversely, orientation behavior decreased recruitment to northern areas like Iceland (−46%), the Faroe Islands (−39%) and Northern Norway (−49%). Behavior affected the timing of recruitment to Southwestern Norway, causing peaks of abundance in May–June, but not to other regions. These results show that lunar‐driven orientation and swimming behavior observed in glass eels substantially increases their recruitment to North Sea coasts. Results agree with the distribution of eel in northern Europe, which decreases in abundance with increasing latitude. [ABSTRACT FROM AUTHOR]

The Northern Chilean subduction zone is characterized by long-term subduction erosion with very little sediment input at the trench and the lack of an accretionary prism. Here, multichannel seismic reflection (MCS) data were acquired as part of the CINCA (Crustal Investigations off- and onshore Nazca Plate/Central Andes) project in 1995. These lines cover among others the central part of the M W 8.1 Iquique earthquake rupture zone before the earthquake occurred on 1 April 2014. We have re-processed one of the lines crossing the updip parts of this earthquake at 19°40′S, close to its hypocentre. After careful data processing and data enhancement, we applied a coherency-based pre-stack depth migration algorithm, yielding a detailed depth image. The resulting depth image shows the subduction interface prior to the Iquique megathrust earthquake down to a depth of approximately 16 km and gives detailed insight into the characteristics of the seismogenic coupling zone. We found significantly varying interplate reflectivity along the plate interface which we interpret to be caused by the comparably strong reflectivity of subducted fluid-rich sediments within the grabens and half-grabens that are predominant in this area due to the subduction-related bending of the oceanic plate. No evidence was found for a subducted seamount associated to the Iquique Ridge along the slab interface at this latitude as interpreted earlier from the same data set. By comparing relocated fore- and aftershock seismicity of the Iquique earthquake with the resulting depth image, we can divide the continental wedge into two domains. First, a frontal unit beneath the lower slope with several eastward dipping back-rotated splay faults but no seismicity in the upper plate as well as along the plate interface. Secondly, a landward unit beneath the middle slope with differing reflectivity that shows significant seismicity in the upper plate as well as along the plate interface. Both units are separated by a large eastward dipping mega splay fault, the root zone of which shows diffuse seismicity, both in the upper plate and at the interface. The identification of a well-defined nearly aseismic frontal unit sheds new light on the interplate locking beneath the lower continental slope and its controls. [ABSTRACT FROM AUTHOR]