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Plant remains from land, particularly sunken wood, represent significant input of organic matter into the generally oligotrophic deep sea, leading to the formation of macrofaunal assemblages around them. Following the onset of decomposition by wood-boring bivalves, microbes and fungi the wood eventually becomes an anoxic and reducing environment attracting chemosynthetic fauna. Wood falls have primarily been studied by either opportunistic sampling by trawls or artificial deployments, while in situ observations of natural systems remain scarce. Here, we report a natural late-stage wood-fall community at 5505 m depth in the Northwestern Pacific abyssal plain by a manned submersible. Our observations revealed details in faunal distribution that cannot be obtained by trawled material, including the presence of associated xenophyophores. We identified a total of 26 animal species and one xenophyophore from the wood fall including both wood-associated and chemosymbiotic taxa, although cocculiniform limpets were conspicuously missing. Living xylophagaid bivalves were also not observed, but the wood was heavily burrowed and they were clearly once present. This rare finding improves our understanding of deep abyssal wood falls at the senescence stage, and highlights the importance of direct seafloor observations. [ABSTRACT FROM AUTHOR]

Abyssal plain communities rely on the overlying water column for a settling flux of organic matter. The origin and rate of this flux as well as the controls on its fine-scale spatial distribution following seafloor settlement are largely unquantified. This is particularly true across regions where anthropogenically-induced seafloor disturbance has occurred. Here, we observed, quantified and mapped a mass deposition event of gelatinous zooplankton carcasses (pyrosomes) in July-September 2015 across one such physically disturbed region in the Peru Basin polymetallic nodule province (4150 m). Seafloor in this area was disturbed with a plough harrow in 1989 (as part of the DISCOL experiment) causing troughs in the sediment. Other parts were disturbed with an epibenthic sled (EBS) during a cruise in 2015 resulting in steep-walled, U-shaped troughs. We investigated two hypotheses: a) gelatinous food falls contribute significantly to the abyssal plain carbon pump and b) physical seafloor disturbance influences abyssal distribution of organic matter. We combined optical and bathymetric seafloor observations, to analyze pyrosome distribution on seabeds with different levels of disturbance. 2954 pyrosome colonies and associated taxa were detected in > 14,000 seafloor images. The mean regional carbon (C) deposition associated with pyrosome carcasses was significant compared to the flux of particulate organic C (182 to 1543%), and the total respired benthic C flux in the DISCOL Experimental Area (39 to 184%). EBS-disturbed seafloor tracks contained 72 times more pyrosome-associated C than an undisturbed reference site, and up to 4 times more than an area disturbed in 1989. Deposited pyrosomes collected had a higher proportion of labile fatty acids compared to the sediment. We document the temporal and spatial extent of an abyssal food fall event with unprecedented detail and show that physical seafloor disturbance results in the accumulation of detrital material. Such accumulation may reduce oxygen availability and alter benthic community structure. Understanding both the relevance of large food falls and the fine scale topography of the seafloor, is necessary for impact assessment of technologies altering seafloor integrity (e.g. as a result of bottom-trawling or deep seabed mining) and may improve their management on a global scale.

Context: Australia has the third largest exclusive economic zone in the world, but little is known about its deepest parts because, historically, research has focussed on coast regions or in the top 1000 m. Aim: We aim to identify the extent of Australia's deep sea, the deepest locations in Australian waters, and investigate how much of Australia's deep sea is located within marine parks. Methods: We use altimetry-derived water depth to calculate the total area in 1000-m bins. Key results: The total area of Australian waters, excluding Antarctic waters, is 8 914 134 km2, of which, 70.4% is deep sea greater than 1000 m and 48% is deeper than the 3000-m abyssal boundary. In total, 56% of Australian Marine Parks are deeper than 3000 m and 20 of 61 marine reserves include water deeper than 5000 m. Conclusions: The Convention on Biological Diversity calls for marine protected areas globally to increase from 7.7 to 30% by 2030: Australia has already placed over 40% of its waters under protection. Despite this, there are no long-term programs monitoring the deep sea and Australia has not produced a globally significant amount of deep-sea science. Implications: Herein lies opportunities for Australia to understand fully its largest habitat and become the global exemplar of deep-sea science and conservation. Australia has the third largest marine jurisdiction of any country but we know very little about its deepest parts. We found that almost half of Australian waters (excluding Antarctic waters) are deeper than 3000 m, of which a third is within Australian Marine Parks. Australia's deep sea may be protected, but Australian scientists and research institutes should focus on establishing long-term research programs to understand what is there and whether it is changing. [ABSTRACT FROM AUTHOR]

The abyssal plains are vast areas without large scale relief that occupy much of the ocean floor. Although long considered relatively featureless, they are now known to display substantial biological heterogeneity across different spatial scales. Ecological research in these regions benefits increasingly from non-destructive visual sampling of epifaunal organisms with imaging technology. We analysed images from ultra-high-definition towed camera transects at depths of around 3500 m across three stations (100–130 km apart) in the Bering Sea, to ask whether the density and distribution of visible epifauna indicated any substantial heterogeneity. We identified 71 different megafaunal taxa, of which 24 occurred at only one station. Measurements of the two most abundant faunal elements, the holothurian Elpidia minutissima and two xenophyophores morphotypes (the more common identifiable as Syringammina limosa), indicated significant differences in local densities and patchy aggregations that were strikingly dissimilar among stations. One station was dominated by xenophyophores, one was relatively depauperate in both target taxa as well as other identified megafauna, and the third station was dominated by Elpidia. This is an unexpected level of variation within comparable transects in a well-mixed oceanic basin, reinforcing the emerging view that abyssal habitats encompass biological heterogeneity at similar spatial scales to terrestrial continental realms.

Benthic nitrogen cycling, including nitrification, N-loss, and other nitrogen transformations, plays a crucial role in the marine nitrogen budget. However, studies on benthic nitrogen cycling mainly focus on marginal seas, while attention to the deep ocean, which occupies the largest area of the seafloor, is severely lacking. In this study, we investigate the benthic nitrogen cycling in the Kuroshio Extension region (KE) of the northwest Pacific Ocean at water depths greater than 5,000 m through 15N enrichment slurry incubation and pore-water dissolved oxygen and inorganic nitrogen profiles. The slurry incubation indicates nitrification is the predominant process in benthic nitrogen cycling. The potential nitrification rates are nearly an order of magnitude higher than dissimilatory nitrate reduction. Nitrification and total N-loss flux estimated from pore-water nitrate and ammonium profiles are 6–42 and 5–30 μmol N m−2 d−1, respectively. Generally, anammox is the predominant N-loss process in KE sediment. The temperature gradient experiment indicates that the optimum temperature for anammox and denitrification is 13 and 41°C, respectively, partially explaining anammox as the dominant process for deep-ocean benthic N-loss. Both the low concentration of ammonium in pore-water and the discrepant results between anoxic incubation amended with 15NO3− and 15NH4++14NO3− suggest that ammonium is another limiting factor for benthic anammox. N-loss activity gradually declines with the distance from the Oyashio–Kuroshio transition zone. However, nitrification has the opposite trend roughly. This reveals that the sediment in KE transfers from nitrate sink to source from north to south. This trend is mainly caused by the variation of primary production and the supplement of active organic matter, which is the energy source for microbes and the potential source for ammonium through remineralization. Overall, our results highlight temperature and ammonium as two limiting factors for deep-ocean benthic N-loss and also exhibit a tight coupling relationship between pelagic primary production and the benthic nitrogen cycle in KE.

In the Late Triassic, a global environmental change called the Carnian Pluvial Episode (CPE) emerged, causing major biological turnover. The CPE has been recognized by siliciclastic input to sedimentary basins, multiple carbon isotope perturbations, and climate proxies for humidification. The CPE is considered to have been associated with increased atmospheric pCO2 from eruptions of large igneous provinces. However, the nature of this global environmental perturbation on the continents is still not well understood. Here we present a geochemical analysis of a pelagic deep-sea bedded chert sequence across the CPE in the Jurassic accretionary complex of Mino terrane, central Japan. Fluctuations in terrigenous material supply were reconstructed using Principal Component Analysis of major element compositions. The first principal component positively correlates with elements enriched in clay minerals such as Al2O3, whereas it negatively correlates with CaO, P2O5, and MnO, derived from apatite and manganese. A sudden increase in terrigenous supply was detected around the Julian/Tuvalian boundary, suggesting that CPE-related siliciclastic input also occurred in the abyssal plain environment. The terrigenous supply returned to the pre-CPE state in the Tuvalian. Since the terrigenous material supplied to the abyssal plain is thought to be derived from eolian dust blown from continental arid regions, the increasing terrigenous supply detected in the pelagic deep-sea chert succession may indicate extensive aridification. This result seems to conflict with the common view of the CPE as a humidification event. This contradiction possibly suggests that the extensive aridification occurred within the interior of the supercontinent Pangea, while hydrological circulation enhanced on the coastal region during the CPE.

Abyssal plains cover more than half of Earth's surface, and the main food source in these ecosystems is phytodetritus, mainly originating from primary producers in the euphotic zone of the ocean. Global climate change is influencing phytoplankton abundance, productivity, and distribution. Increasing importance of picoplankton over diatom as primary producers in surface oceans (especially projected for higher latitudes) is projected and hence altering the quantity of organic carbon supplied to the abyssal seafloor as phytodetritus, consequences of which remain largely unknown. Here, we investigated the in situ responses of abyssal biota from viruses to megafauna to different types of phytoplankton input (diatoms or cyanobacteria which were labeled with stable isotopes) at equatorial (oligotrophic) and temperate (eutrophic) benthic sites in the Pacific Ocean (1°N at 4277 m water depth and 39°N at 5260 m water depth, respectively). Our results show that meiofauna and macrofauna generally preferred diatoms as a food source and played a relatively larger role in the consumption of phytodetritus at higher latitudes (39°N). Contrarily, prokaryotes and viruses showed similar or even stronger responses to cyanobacterial than to diatom supply. Moreover, the response of prokaryotes and viruses was very rapid (within 1–2 days) at both 1°N and 39°N, with quickest responses reported in the case of cyanobacterial supply at higher latitudes. Overall, our results suggest that benthic deep‐sea eukaryotes will be negatively affected by the predicted decrease in diatoms in surface oceans, especially at higher latitudes, where benthic prokaryotes and viruses will otherwise likely increase their quantitative role and organic carbon cycling rates. In turn, such changes can contribute to decrease carbon transfer from phytodetritus to higher trophic levels, with strong potential to affect oceanic food webs, their biodiversity and consequently carbon sequestration capacity at the global scale. [ABSTRACT FROM AUTHOR]

To investigate the nature of gas hydrates in the Makran area, new high-resolution geophysical data were acquired between 2018–2019. The data collected comprise multibeam and two-dimensional multi-channel seismic reflection data. The multibeam bathymetry data show East-North-East (ENE) ridges, piggy-back basins, canyon and channel systems, and the morphology of the abyssal plain. Continuous and discontinuous bottom simulating reflectors (BSRs) occur in the piggy-back basins on most of the seismic profiles available. The BSRs cut the dipping layers with strong amplitude and reversed polarity. Discontinuous BSRs indicate a transition along a dipping high-permeable sand layers from gas-rich segment to the gas hydrate-bearing segment and suggest alternating sediments of fine and relatively coarse grain size. Double BSRs are highly dynamic and attributed to slumps occurring in the study area. The BSRs induced by slumps are located both at deep and shallow depths, responding to the temperature or pressure variation. For the first time, BSRs are observed in the abyssal plain of the Makran area, being associated with anticline structures, which do not show large spatial continuity and are strongly conditioned by structural conditions such as anticlines and fluid migration pathways, including deep fault, gas chimney, and high-permeable sedimentary layer. Our results may help to assess the gas hydrate potential within the piggy-back basins and to determine the most promising target areas. Moreover, results about the abyssal plain BSR may help to locate hydrocarbon reservoirs in the deep ocean. [ABSTRACT FROM AUTHOR]

The Clarion Clipperton Zone (CCZ) is a vast area of the central Pacific Ocean where the abyssal seabed is a focus for future polymetallic nodule mining. Broad-scale environmental gradients occur east-to-west across the CCZ seabed, including organic matter supply and nodule abundance, factors that influence benthic faunal community structure and function. A network of protected areas across the CCZ, called Areas of Particular Environmental Interest (APEIs), has been designated to cover this variation. Most previous studies of the benthic environment and megafaunal communities have focussed on the eastern CCZ, leaving the impact of these large-scale gradients unexamined and the network design untested. Seamounts are a further source of heterogeneity in the region. We examined the benthic megafaunal ecology of three APEIs in the western CCZ, spanning a range of environmental conditions. We used a combination of seabed photography and direct sampling to assess the environment and megafauna on the soft sediment habitats on the abyssal plain in three APEIs, and seamounts in two of those APEIs. We found that environmental conditions on abyssal plains differed between the three APEIs in terms of water depth, nodule abundance and coverage, sediment particle size distribution, and estimated organic matter flux. Megafauna were low density and high diversity, with few common morphotypes between sites and many morphotypes being observed only once. Xenophyophores dominated the assemblages. The density and diversity of invertebrates were greater at the sites with lower organic matter inputs and greater nodule abundance. Seamounts in the same APEIs were nodule-free and had coarser sediments than on the plain. Invertebrate megafaunal diversity was lower on the seamounts than on the plains, and most morphotypes recorded on the seamounts were only found on seamounts. Low morphotype overlap also suggests little connectivity between APEIs, and between seamounts and adjacent abyssal plains. Our results provide the first evaluation of the seabed habitats and megafaunal ecology in the western CCZ, highlighting environmental gradients that influence benthic communities, and are important for evaluating the design of the network of protected areas.

Basaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole-rock major and trace element abundances, Sr-Nd-Mo-Os isotopes, water contents and He isotopes of volcanic glasses, U-Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid-ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine-dominated (peridotitic) mantle source. There is currently no evidence for a high-³He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes. [ABSTRACT FROM AUTHOR]

Trophic ecology of detrital-based food webs is still poorly understood. Abyssal plains depend entirely on detritus and are among the most understudied ecosystems, with deposit feeders dominating megafaunal communities. We used compound-specific stable isotope ratios of amino acids (CSIA-AA) to estimate the trophic position of three abundant species of deposit feeders collected from the abyssal plain of the Northeast Pacific (Station M; ~ 4000 m depth), and compared it to the trophic position of their gut contents and the surrounding sediments. Our results suggest that detritus forms the base of the food web and gut contents of deposit feeders have a trophic position consistent with primary consumers and are largely composed of a living biomass of heterotrophic prokaryotes. Subsequently, deposit feeders are a trophic level above their gut contents making them secondary consumers of detritus on the abyssal plain. Based on δ13C values of essential amino acids, we found that gut contents of deposit feeders are distinct from the surrounding surface detritus and form a unique food source, which was assimilated by the deposit feeders primarily in periods of low food supply. Overall, our results show that the guts of deposit feeders constitute hotspots of organic matter on the abyssal plain that occupy one trophic level above detritus, increasing the food-chain length in this detritus-based ecosystem. [ABSTRACT FROM AUTHOR]

Polymetallic nodule fields provide hard substrate for sessile organisms on the abyssal seafloor between 3000 and 6000 m water depth. Deep-seabed mining targets these mineral-rich nodules and will likely modify the consumer-resource (trophic) and substrate-providing (non-trophic) interactions within the abyssal food web. However, the importance of nodules and their associated sessile fauna in supporting food-web integrity remains unclear. Here, we use seafloor imagery and published literature to develop highly-resolved trophic and non-trophic interaction webs for the Clarion-Clipperton Fracture Zone (CCZ, central Pacific Ocean) and the Peru Basin (PB, South-East Pacific Ocean) and to assess how nodule removal may modify these networks. The CCZ interaction web included 1028 compartments connected with 59,793 links and the PB interaction web consisted of 342 compartments and 8044 links. We show that knock-down effects of nodule removal resulted in a 17.9% (CCZ) to 20.8% (PB) loss of all taxa and 22.8% (PB) to 30.6% (CCZ) loss of network links. Subsequent analysis identified stalked glass sponges living attached to the nodules as key structural species that supported a high diversity of associated fauna. We conclude that polymetallic nodules are critical for food-web integrity and that their absence will likely result in reduced local benthic biodiversity. [ABSTRACT FROM AUTHOR]

The elpidiid holothurian Peniagone leander Pawson and Foell, 1986 is recorded for the first time from the Mariana Trench area at a depth of 5 571 m. The type description of this species was based on only in-situ photographs. P. leander is re-described and illustrated based on a preserved material and in-situ photographs taken on the seabed. The 16S rRNA was also obtained from the specimen and submitted to GenBank. This is the fourth discovery of this species, extending its bathymetric range in the western Pacific Ocean. [ABSTRACT FROM AUTHOR]

Highlights • Seafloor geomorphology was important in the structuring of abyssal megafauna. • Differences in megafaunal community ecology were found between all landscape types. • Lower megafauna density & diversity in a bathymetric valley than flat and ridge areas. • Large samples, collected by AUV, were required to make robust ecological conclusions. Abstract The potential for imminent polymetallic nodule mining in the Clarion Clipperton Fracture Zone (CCZ) has attracted considerable scientific and public attention. This concern stems from both the extremely large seafloor areas that may be impacted by mining, and the very limited knowledge of the fauna and ecology of this region. The environmental factors regulating seafloor ecology are still very poorly understood. In this study, we focus on megafaunal ecology in the proposed conservation zone 'Area of Particular Environmental Interest 6′ (study area centred 17°16′N, 122°55′W). We employ bathymetric data to objectively define three landscape types in the area (a level bottom Flat, an elevated Ridge, a depressed Trough; water depth 3950–4250 m) that are characteristic of the wider CCZ. We use direct seabed sampling to characterise the sedimentary environment in each landscape, detecting no statistically significant differences in particle size distributions or organic matter content. Additional seafloor characteristics and data on both the metazoan and xenophyophore components of the megafauna were derived by extensive photographic survey from an autonomous underwater vehicle. Image data revealed that there were statistically significant differences in seafloor cover by nodules and in the occurrence of other hard substrata habitat between landscapes. Statistically significant differences in megafauna standing stock, functional structuring, diversity, and faunal composition were detected between landscapes. The Flat and Ridge areas exhibited a significantly higher standing stock and a distinct assemblage composition compared to the Trough. Geomorphological variations, presumably regulating local bottom water flows and the occurrence of nodule and xenophyophore test substrata, between study areas may be the mechanism driving these assemblage differences. We also used these data to assess the influence of sampling unit size on the estimation of ecological parameters. We discuss these results in the contexts of regional benthic ecology and the appropriate management of potential mining activities in the CCZ and elsewhere in the deep ocean. [ABSTRACT FROM AUTHOR]

Abstract The α-diversity in two large-scale, systematic macrobenthos surveys (1983–1985 and 2000–2001) in the deep northern Gulf of Mexico (GoM) has been compared under strong vertical and horizontal productivity gradients to examine underlying factors that control the distribution of deep-sea diversity. We calculated the effective numbers of equally abundant species (Hill numbers) from 100 randomly selected individuals to examine spatial patterns of species richness and evenness. Macrofauna α-diversity was a negative parabolic function of depth; the minima occurred at high export POC flux on the upper continental slope and on the abyssal plain with extremely low input of POC; the maximum occurred at intermediate levels of macrofauna biomass and input of POC. The overall parabolic pattern of α-diversity was stable over the 20-year period and not statistically different between meso-scale depressions (e.g., basins and canyons) and their adjacent slope habitats; however, the diversity was significantly higher in the NE than the NW slope habitats. The observed Mid-Depth-Maximum (MDM) in α-diversity, we suggest, was likely a result of dynamic equilibrium between productivity and competitive exclusion. The higher diversity and productivity in the NE than the NW GoM at similar depth ranges suggests that, for the first time, the productivity-diversity relationship was observed independent of other depth-related factors. The findings provide a foundation for predicting the possible responses of diversity to climate change and anthropogenic impacts in the deep sea. Highlights • Macrofauna α-diversity was a negative parabolic function of depth in N GoM. • Minimum diversity occurred at highest and lowest input of POC. • Maximum diversity occurred at intermediate levels of input of POC. • The high export POC flux on the NE GoM slope drove higher macrofauna diversity. • Productivity and biological interactions shape the observed diversity gradient. [ABSTRACT FROM AUTHOR]

Submarine fans are archives of Earth‐surface processes and change, recording information about the turbidity currents that construct and sculpt them. The volume and recurrence of turbidity currents are of great interest for geohazard assessment, source‐to‐sink modelling, and hydrocarbon reservoir characterization. Yet, such dynamics are poorly constrained. This study integrates data from four Quaternary submarine fans to reconstruct the volume and recurrence of the formative turbidity currents. Calculated event volumes vary over four orders of magnitude (105 to 109 m3), whereas recurrence intervals vary less, from 50 to 650 years. The calculated turbidity‐current‐event volume magnitudes appear to be related to slope position and basin confinement. Intraslope‐fan deposits have small event volumes (ca 106 m3) while ponded‐fan deposits have very large event volumes (108 to 109 m3). Deposits in non‐ponded, base‐of‐slope environments have intermediate values (107 to 108 m3). Sediment bypass in intraslope settings and flow trapping in ponded basins likely account for these differences. There seems to be no clear relationship between event recurrence and basin confinement. Weak scaling exists between event volume and source‐area characteristics, but sediment storage in fluvial and/or intraslope transfer zones likely complicates these relationships. The methodology and results presented here are also applied to reconstruct the time of deposition of ancient submarine‐fan deposits of the Tanqua Karoo basin, South Africa. The volume and recurrence of submarine‐fan‐building turbidity currents form intermediate values between values measured in submarine canyons and channels (<105 m3 and <101 years) and on abyssal plains (>108 m3 and >103 years), indicating that small, frequent flows originating in submarine canyons often die out prior to reaching the fan, while rare and very large flows mostly bypass the fan and deposit sediment on the abyssal plain. This partitioning of flow volume and recurrence along the submarine sediment‐routing system provides valuable insights for better constraining geohazards, hydrocarbon resources and the completeness of the stratigraphic record. We provide a methodology to estimate the volume and recurrence of turbidity currents that deliver sediment to submarine fans. We apply this method to four modern fan systems from around the world and also to ancient fan deposits. Finally, we compare these event volume and frequency values to those derived from submarine canyons and abyssal plains, providing valuable data for geohazard assessment. [ABSTRACT FROM AUTHOR]

The acoustic facies analyses have provided an important basis for sedimentary processes in the deep-sea environments. The echo-character mapping, through the interpretation and correlation of very high-resolution seismic profiles, is a very useful tool for the characterization of the recent sedimentary processes and their distribution. This work presents the first echo-character map at 1:800,000 scale of the Galicia Continental Margin and the adjacent abyssal plains elaborated by the Geological Survey of Spain. The map was carried out on the basis of the analysis and interpretation of the bathymetry and reflectivity data from the SIMRAD EM12, EM120 and EM1002 echosounders, and the high-resolution seismic profiles from the SIMRAD TOPAS PS18 parametric echosounder. On the basis of seafloor morphology, surface bedforms, backscatter and sub-bottom acoustic echo-characters, 26 echo-types were identified in the uppermost sedimentary sequence. These echo-types have been grouped into four main echoes: Distinct, Irregular, Hyperbolic and Undulated, according to their main acoustic characteristics. This information has been acquired in the framework of the 'Scientific Research Program of the Economic Exclusive Zone of Spain', which is coordinated and leaded by the Defense Ministry of Spain, during the oceanographic cruises carried out on board of the R/V Hesperides in 2001-2003 and 2006-2009. [ABSTRACT FROM AUTHOR]

Central canyons are considered to be important conduits for the transfer of sediment to the abyssal plains. High‐resolution 3D seismic data and logging data from the east of Lingshui Depression are used to investigate the depositional elements, patterns, and filling processes of central canyon in the Qiongdongnan Basin, northern South China Sea. The Central Canyon in this segment is 30 km wide and more than 1,000 m deep. It was found to be characterized by being subparallel to the continental shelf. In this study, 6 types of deep‐water gravity flow depositional elements were identified, including erosion surfaces, basal lags, turbidite channel complexes, mass transport deposits, lobe complex, and pelagic deposits. Each type of these depositional elements was found to have distinct external features, internal structures, and lateral characteristics in the seismic profiles. Meanwhile, these different assemblages of depositional elements could be composed of 5 depositional units. And in a vertical direction, the evolutionary history could be divided into 5 stages as follows: (a) a repeated cut and fill stage; (b) high net‐to‐gross channel complex dominated stage; (c) low high net‐to‐gross channel and pelagic shale dominated stage; (d) lobe complex deposition stage; and (e) mass transport deposit dominated stage. During Stage 3, the development of small‐scale turbidite channels is the indicators of northern sediment supply. During Stage 4, 7 lobes have been identified in the lobe complexes. The evolution and depositional processes in the study area were most likely controlled by the negative relief induced by the palaeo‐seafloor morphology, as well as the structural inversions of the Red River Fault and the basement faults. Additionally, the sediment supply, sea‐level fluctuations, and tectonic activities also controlled and influenced the depositional processes, along with the internal architecture of the canyon. The results of this study have potentially important implications for the improved understanding the vertical evolution of submarine canyons and also shed a new light on the reservoir potential of the Lingshui section of the Central Canyon. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Geochemical data of Co-rich Fe-Mn crusts from the Dirck Hartog Ridge, are presented. • Two types and two subtypes of the colloforms were distinguished. • The mean concentration of Cu + Ni + Co is lower than in crusts from the Atlantic. • The mean ΣREY contents are moderate compared to crusts from the global ocean. • Discrimination diagrams show the crusts to be of hydrogenetic origin. • Individual layers could be regarded as formed hydrothermally and/or diagenetically. Abstract Geochemistry of cobalt-rich ferromanganese crusts from the as-yet-unexplored Dirck Hartog Ridge (DHR), a linear bathymetric feature of poorly understood origin, located in the centre of the Perth Abyssal Plain (PAP), is described for the first time, based on 3 samples analyzed with XRF, ICP-MS and EPMA. Mean bulk concentrations of Fe and Mn were 20.8 and 18.4 wt%, respectively; individual crust layers, however, proved chemically variable. The mean concentration of metals of the highest economic potential (Cu + Ni + Co) was 0.69 wt%. The mean total rare earth elements (REE) and yttrium (ΣREY) contents in the samples (1908 mg/kg) proved moderate. The crusts showed strong positive Ce anomalies, negative Y anomalies and a low Y SN /Ho SN ratio (mean 0.56). Observations on the texture and chemical composition of the crust samples studied allowed us to distinguish two major and two subtypes of representative colloform structures built of laminae. The Type I colloform structures showed the Mn/Fe ratio to range from 0.86 to 5.69, the ranges in Type II colloform structures being 0.49–2.47 (Subtype II-1) and 6.91–221.42 (Subtype II-2). The Subtype II-2 colloform structures were enriched in Ni (up to 7.31 wt%). The Co and Cu concentrations showed a high variability, 0.03–1.64 and 0.03–0.22 wt%, respectively. According to the current genetic models and discrimination diagrams, the crusts studied were classified as hydrogenetic, although individual laminae and groups of laminae could be regarded as diagenetic or hydrothermal. [ABSTRACT FROM AUTHOR]

We have studied the rock magnetic characteristics of 31 multicores and 7 gravity cores from deep-sea sediments of the distal Amazon Fan and Demerara Abyssal Plain. This study has permitted us to delineate the space/time pattern of Holocene physical sedimentation along the northeastern margin of South America associated with the Amazon River sediment plume. The rock magnetic variability is correlatable over the entire study region. This degree of correlation has permitted us to develop a composite picture of Holocene rock magnetic (and overall physical clastic) sedimentation. 26 radiocarbon dates have been combined with the rock magnetic correlations to provide a composite Holocene chronostratigraphy for that variability. We have built isopach maps of overall sedimentation for three windows in the Holocene (3000, 4500, and 7000 calendar years BP). The overall sedimentation in the center of our region is ~5 cm/kyr. But, sedimentation rates are faster closer to the coastline and associated with the distal clastic flux from the Amazon Plume. There is a secondary high in sediment accumulation under the North Brazil Current retroflection. Our maps suggest that the retroflection has been active at the millennial scale for at least the last 7000 years. We estimate that the mid-Holocene (~4.5–9000 calendar years BP) pattern of sedimentation was distinctly different from that of the last few thousand years. The clastic sediments were ~25% reduced in concentration (with the biogenic sediments similarly increased by ~25%), they were finer–grained, and significantly less rich in ferric iron phases (hematite/goethite). We think these three covarying clastic parameters are due to an interval of reduced clastic flux from the Amazon River Plume with a reduced proportion of Amazon Basin sediments (as opposed to Andes-derived sediments). We hypothesize this was due to significantly drier Amazon Basin environmental conditions during that time interval. [ABSTRACT FROM AUTHOR]

Abstract Mercury concentrations in surface sediments collected from the Northwestern Pacific were analyzed by mercury Zeeman atomic absorption spectrometer with high frequency modulation of light polarization and a pyrolysis attachment. The range of total Hg concentrations in sediments was 19–158 µg kg−1, with a mean of 77 µg kg−1 (n = 50). The variation in mercury concentrations in sandy deposits of the slopes was 19–79 µg kg−1 Hg; in clayey deposits from the Kuril Basin was 84–130 µg kg−1 Hg; in clayey deposits from slopes of the Kuril Basin was 44–84 µg kg−1 Hg; in clayey deposits from the abyssal area was 46–116 µg kg−1 Hg; and in clayey deposits from the slopes and bottom of the Kuril-Kamchatka Trench was 42–143 µg kg−1 Hg. Within the distribution of mercury across the study area, high mercury concentrations were observed in clayey sediments, which are enriched in organic matter and the remains of silicate phytoplankton. The Hg content of sandy deposits was minimal. The level of mercury in our deep-water sediments is somewhat overestimated by comparison to representative background values in clayey sediments from the impact areas (subject to anthropogenic pollution). Anomalies of mercury in bottom sediments near a hydrothermal source (Piip Volcano) have not been observed. The enrichment factor values at the stations range between 0.3 and 4.3. EF values of the most samples were generally more than 1, indicating that enrichment was by biogeochemical processes. Based upon four guideline values (ERL, ERM, ISQG and PEL) we can assume a minimal toxic mercury effect on the benthic marine biota. [ABSTRACT FROM AUTHOR]

Abstract Macrofauna (46,343 invertebrates from 41 taxa) were collected by means of a camera-epibenthic sledge (C-EBS) during the expedition SokhoBio (Sea of Okhotsk Biodiversity Studies) on board of the RV Akademik M.A. Lavrentyev in 2015. In total 11 sites and 22 stations were sampled at bathyal and abyssal depths ranging from 1696 m to 4798 m in the Kuril Basin of the Sea of Okhotsk (16 stations), the Bussol Strait (two stations) and western slope of the Kuril-Kamchatka Trench (KKT) (four stations). Polychaetes occurred most frequently in the samples with 17,546 individuals, followed by Peracarida with 14,099 individuals (5625 isopods, 3887 amphipods, 3225 tanaids, 1269 cumaceans, and 90 mysids), Copepoda (4679 ind.) and Bivalvia (3999 ind.). Numbers of individuals ranged between 140 at station 3–10–6383 at station 7–3. The numbers of taxa per station were smallest at station 3–10 (4 taxa) and highest at station 2–7 (31 taxa). Peracarida were the most common malacostracan crustaceans recorded, with Isopoda comprising 40% of all peracarids within this fraction, followed by Amphipoda with 27%, Tanaidacea with 23%, Cumacea with 9% and Mysida with 1%. At station 3–10, not a single peracarid was sampled. Isopoda dominated most of the stations, especially in the abyssal sites of the Kuril Basin. Amphipoda occurred in higher numbers in the Bussol Strait at site 8 as well as at Pacific station 9-7. In the SokhoBio area, studied macrofauna was distributed very patchily. In terms of diversity and composition the abyssal macrofauna of the Kuril Basin shows an intermediate state between the adjacent studied abyssal areas: the semi-isolated Sea of Japan and the open abyssal plain of the Kuril-Kamchatka Trench (KKT) area. [ABSTRACT FROM AUTHOR]

A 400m long array with 201 high-resolution NIOZ temperature sensors was deployed above a north-east equatorial Pacific hilly abyssal plain for 2.5 months. The sensors sampled at a rate of 1 Hz. The lowest sensor was at 7m above the bottom (m a.b.). The aim was to study internal waves and turbulent overturning away from large-scale ocean topography. Topography consisted of moderately elevated hills (a few hundred metres), providing a mean bottom slope of one-third of that found at the Mid-Atlantic Ridge (on 2 km horizontal scales). In contrast with observations over large-scale topography like guyots, ridges and continental slopes, the present data showed a well-defined nearhomogeneous "bottom boundary layer". However, its thickness varied strongly with time between <7 and 100ma.b. with a mean around 65ma.b. The average thickness exceeded tidal current bottom-frictional heights so that internal wave breaking dominated over bottom friction. Near-bottom fronts also varied in time (and thus space). Occasional coupling was observed between the interior internal wave breaking and the near-bottom overturning, with varying up- and down- phase propagation. In contrast with currents that were dominated by the semidiurnal tide, 200m shear was dominant at (sub-)inertial frequencies. The shear was so large that it provided a background of marginal stability for the straining high-frequency internal wave field in the interior. Daily averaged turbulence dissipation rate estimates were between 10-10 and 10-9 m2 s-3, increasing with depth, while eddy diffusivities were of the order of 10-4 m2 s-1. This most intense "near-bottom" internal-wave-induced turbulence will affect the resuspension of sediments. [ABSTRACT FROM AUTHOR]

The offshore region of northwestern Iberia offers an opportunity to study the impacts of along-slope processes on the morphology of a glacially influenced continental margin, which has traditionally been conceptually characterised by predominant down-slope sedimentary processes. High-resolution multibeam bathymetry, acoustic backscatter and ultrahigh-resolution seismic reflection profile data are integrated and analysed to describe the present-day and recent geomorphological features and to interpret their associated sedimentary processes. Seventeen large-scale seafloor morphologies and sixteen individual echo types, interpreted as structural features (escarpments, marginal platforms and related fluid escape structures) and depositional and erosional bedforms developed either by the influence of bottom currents (moats, abraded surfaces, sediment waves, contourite drifts and ridges) or by gravitational features (gullies, canyons, slides, channel-levee complexes and submarine fans), are identified for the first time in the study area (spanning ~90,000 km 2 and water depths of 300 m to 5 km). Different types of slope failures and turbidity currents are mainly observed on the upper and lower slopes and along submarine canyons and deep-sea channels. The middle slope morphologies are mostly determined by the actions of bottom currents (North Atlantic Central Water, Mediterranean Outflow Water, Labrador Sea Water and North Atlantic Deep Water), which thereby define the margin morphologies and favour the reworking and deposition of sediments. The abyssal plains (Biscay and Iberian) are characterised by pelagic deposits and channel-lobe systems (the Cantabrian and Charcot), although several contourite features are also observed at the foot of the slope due to the influence of the deepest water masses (i.e., the North Atlantic Deep Water and Lower Deep Water). This work shows that the study area is the result of Mesozoic to present-day tectonics (e.g. the marginal platforms and structural highs). Therefore, tectonism constitutes a long-term controlling factor, whereas the climate, sediment supply and bottom currents play key roles in the recent short-term architecture and dynamics. Moreover, the recent predominant along-slope sedimentary processes observed in the studied northwestern Iberian Margin represent snapshots of the progressive stages and mixed deep-water system developments of the marginal platforms on passive margins and may provide information for a predictive model of the evolution of other similar margins. [ABSTRACT FROM AUTHOR]

Future deep-sea mining for polymetallic nodules in abyssal plains will negatively impact the benthic ecosystem, but it is largely unclear whether this ecosystem will be able to recover from mining disturbance and if so, to what extent and at what timescale. During the "DISturbance and reCOLonization" (DISCOL) experiment, a total of 22% of the seafloor within a 10.8 km² circular area of the nodulerich seafloor in the Peru Basin (SE Pacific) was ploughed in 1989 to bury nodules and mix the surface sediment. This area was revisited 0.1, 0.5, 3, 7, and 26 years after the disturbance to assess macrofauna, invertebrate megafauna and fish density and diversity. We used this unique abyssal faunal time series to develop carbon-based food web models for each point in the time series using the linear inverse modeling approach for sediments subjected to two disturbance levels: (1) outside the plough tracks; not directly disturbed by plough, but probably suffered from additional sedimentation; and (2) inside the plough tracks. Total faunal carbon stock was always higher outside plough tracks compared with inside plough tracks. After 26 years, the carbon stock inside the plough tracks was 54% of the carbon stock outside plough tracks. Deposit feeders were least affected by the disturbance, with modeled respiration, external predation, and excretion rates being reduced by only 2.6% inside plough tracks compared with outside plough tracks after 26 years. In contrast, the respiration rate of filter and suspension feeders was 79.5% lower in the plough tracks after 26 years. The "total system throughput" (T ..), i.e., the total sum of modeled carbon flows in the food web, was higher throughout the time series outside plough tracks compared with the corresponding inside plough tracks area and was lowest inside plough tracks directly after the disturbance (8.63 x 10-3 ± 1.58 x 10-5 mmol Cm-2 d-1). Even 26 years after the DISCOL disturbance, the discrepancy of T .. between outside and inside plough tracks was still 56 %. Hence, C cycling within the faunal compartments of an abyssal plain ecosystem remains reduced 26 years after physical disturbance, and a longer period is required for the system to recover from such a small-scale sediment disturbance experiment. [ABSTRACT FROM AUTHOR]

The identification of recent major tectonic structures in the Galicia continental margin and adjacent abyssal plains was carried out by means of a quantitative analysis of the linear structures having bathymetric expression on the seabed. It was possible to identify about 5800 lineaments throughout the entire study area, of approximately 271,500 km2. Most lineaments are located in the Charcot and Coruña highs, in the western sector of the Galicia Bank, in the area of the Marginal Platforms and in the northern sector of the margin. Analysis of the lineament orientations shows a predominant NE-SW direction and three relative maximum directions: NW-SE, E-W and N-S. The total length of the lineaments identified is over 44,000 km, with a mode around 5000 m and an average length of about 7800 m. In light of different tectonic studies undertaken in the northwestern margin of the Iberian Peninsula, we establish that the lineaments obtained from analysis of the digital bathymetric model of the Galicia continental margin and adjacent abyssal plains would correspond to fracture systems. In general, the orientation of lineaments corresponds to main faults, tectonic structures following the directions of ancient faults that resulted from late stages of the Variscan orogeny and Mesozoic extension phases related to Triassic rifting and Upper Jurassic to Early Cretaceous opening of the North Atlantic Ocean. The N-S convergence between Eurasian and African plates since Palaeogene times until the Miocene, and NW-SE convergence from Neogene to present, reactivated the Variscan and Mesozoic fault systems and related physiography. [ABSTRACT FROM AUTHOR]