Deep-Sea Research Part I-Oceanographic Research Papers最新文献

The classification of the ocean in water masses with similar physical and/or biogeochemical characteristics provides an ideal framework for an efficient monitoring of the change in ocean properties. Particularly, the combination of the seawater temperature and salinity set the stratification of the water column and impacts ocean circulation, with important implications for the surface-to-interior propagation of climate signals. The objective of this study is to find spatially coherent thermohaline structures in different regions of the global ocean, as well as their link with regional dynamics. To this end, we apply clustering techniques to identify water masses delimited by coherent thermohaline structures at different spatial scales over the global ocean. The clustering technique known as K-mean was used with a wide range of $k$ values for conservative temperature and absolute salinity profiles of the entire ocean. Our analysis revealed the impact of the main dynamical oceanic structures (such as ocean fronts, currents, or regions of sluggish circulation) on the vertical thermohaline component. We present three cases of study, the California Current System, the Southern Ocean, and the Eastern Tropical Latitudes, where we could identify regions with common thermohaline characteristics (despite being from different ocean basins), as well as to identify seasonal changes and anomalous profiles. This method makes it possible to reduce the dimensionality of the water column, and allows for the establishment of regional limits driven by their vertical thermohaline structure instead of more rigid, and not always appropriate geographical borders. This has potential important applications for the monitoring and prediction of ocean variability in the context of a rapidly changing climate.

The first multidisciplinary characterisation of Bowditch Seamount in the Sargasso Sea was conducted to provide new baseline knowledge of the biodiversity, geomorphology, oceanography and glacial history of this seamount. A dropframe camera transect 1483–1562 m deep on the seamount documented 77 megafaunal taxa including Vulnerable Marine Ecosystem indicator taxa such as sponges, cold-water corals, and stalked crinoids. Seabed terrain analysis of multibeam echosounder data showed species varied significantly along this transect in response to local geomorphological variability (R²_adj = 31%, p < 0.0001), with changes in seafloor relief and substrata driving species composition over the seamount. ¹⁴C-calibrated and ²³⁰Th-ages of fossil corals (Desmophyllum dianthus) collected by Van Veen grabs 1517 m deep showed corals thrived on the seamount ∼24 ka BP and ∼17 ka BP. Abrupt excursions between higher and lower radiogenic ε_Nd-composition values of the skeletons suggested that D. dianthus persisted on the seamount over times of southern source water input and detrital sediments from the melting Laurentide Ice Sheet, respectively. In agreement with other studies from the western North Atlantic, living D. dianthus were absent in the contemporary setting at these depths, and suggest a significant re-organisation of the seamount community since the deglacial when ice-rafted debris of carbonates likely resulted in a lower aragonite compensation depth allowing D. dianthus to proliferate at deeper depths. New conductivity-depth-temperature profiling revealed the seamount at these depths is now bathed by highly oxygenated Labrador Sea Water (LSW) formed at high latitudes. Co-analysis of a newly constructed 70-year long time series of temperature and salinity for the Labrador Sea and Bermuda regions revealed a 10-year transit time from high latitudes to Bowditch Seamount. This multidisciplinary approach shows how geomorphology drives local biodiversity patterns, but also how upstream climatic forcing in subpolar regions may influence Bermuda's subtropical seamount ecosystem.

The NW Moroccan Margin has a complex geological evolution, being located close to the transition zone between the Azores – Gibraltar Fracture Zone and the western front of the Betic–Rif collisional orogen. The interaction between tectonic, halokinetic and fluid flow processes with bottom-current activity shapes the seafloor and influences the distribution of seafloor biological communities (such as the cold-water coral mounds) and deep-water sedimentation. The aims of this work are to study the interaction of the paleo-oceanographic and morpho-tectonic processes that generated the various seafloor features of the NW Moroccan Margin. To achieve this, high-resolution multibeam bathymetry and parasound data acquired in the “ALBOCA II” cruise have been used, complemented by high-resolution 2D seismic reflection data and the EMODnet bathymetric compilation.

Several morphological features were identified in the margin, which are related to different processes of sedimentary (contourites and sediment waves), structural (faults and diapirs), gravitational (slide scars, mass transport deposits), fluid migration (mud volcanoes and pockmarks) and biogenic (exposed and buried coral mounds) nature. The structural features (e.g., strike-slip faults) have a major control on the seafloor morphology and, consequently, on the development and evolution of the sedimentary systems in the study area.

The evolution of the NW Moroccan Margin during the late Quaternary has been controlled by climatic variations and tectonic activity. The action of these factors has been dominant in distinct parts of the study area where: i) contourite terraces developed when climatic and oceanographic changes were the prevalent factor, ii) mounded and confined contourite drifts and local mass transport deposits formed when the major control was tectonic activity.

This paper presents an overview of the composition and abundance of pelagic fish communities collected during the 43rd cruise of the R/V ′Akademik Sergey Vavilov' on the transect in the central Atlantic, from the Azores to the Equator, in autumn 2016. Samples were taken only at night with a Samyshev-Aseev modified Isaacs-Kidd non-closing midwater pelagic trawl (SAMT) at 18 stations from two depth of 0–200 and 0–700 m. The study recorded 112 species of juvenile and adult mesopelagic fishes belonging to 20 families at the sampled stations. The statistical analysis indicated notable disparities in the fish communities that were caught at depths of 0–200 and 0–700 m. Our study suggests that there are notable differences in pelagic fish associations between the North Atlantic anticyclonic gyre zone and the equatorial jets region, at both depths analyzed. These differences are most likely the result of variations in hydro-physical and other environmental parameters. It has been observed that deep-sea fish communities in the equatorial and central North Atlantic are capable of inhabiting at the same time multiple habitats that exhibit varying physical and chemical structures of water masses. This phenomenon highlights the remarkable ecological adaptability of the species that make up mesopelagic deep-sea communities, as well as their wide geographical distribution. The higher abundance and diversity of mesopelagic fishes observed at stations located above the Mid-Atlantic Ridge may be due to a general decrease in depth over the ridge compared to the surrounding regions and/or local circulations occurring over its crests. Thus, the tops and slopes of seamounts may become unique concentrations of mesopelagic fishes for a limited time due to their topography.

Seamounts can have strong influence on the dynamics and production of waters surrounding them, sometimes creating a kind of oasis effect. To assess to what depths and under what conditions this effect may persist on seamounts, we analysed the fauna and recent history of the Valencia Seamount (VS), a deep, small seamount (summit depth: 1056 m) from an oligotrophic zone of the Mediterranean Sea. The (living) epibenthic fauna of the VS summit (up to 1300 m) was composed of filter feeders, surface deposit feeders and carnivores (63 species), structured in a trophic chain as indicated by the high positive relationship (r² 0.502) between the δN¹⁵and δC¹³ of the fauna analysed. This trophic linearisation (compared with Catalan and NW Mallorca slope sites at similar depths) may optimise the energy flux reaching the seamount, with rather high %TOC (0.93–1.19%) and C/N (5.0–5.5) in July. The Valencia Seamount can also act as a reservoir zone for fish (grenadiers: Nezumia aequalis, and Hymenocephalus italicus) and shrimps (Plesionika martia), which are distributed at shallower depths on adjacent slopes, where they are exploited. The specimens caught were mainly juveniles, suggesting that these species may rather form pseudopopulations. Tuna and cetaceans were regularly sighted on the surface of VS during a specific period of the year (June–July). Reconstructing the recent history of Valencia Seamount (MC2 sediment core taken at 1151 m in 2021) we evidenced a decline of zooplankton pteropods (primary consumers) since ca. the 1960s (MC2 dating after ²¹⁰Pb, metals and microplastic analyses), linked to a decline of Si in sediments suggesting a decrease in primary production due to a decrease in rainfall and river runoff. An increase of vorticity (ω⁺ values), i.e. of cyclonic gyres, and currents after the 1970s over the summit probably increased resuspension of organic matter, which in turn may enhance zooplankton aggegations at VS summit. We hypothesise that this would explain the parallel increase in mesopelagic myctophids in MC2 (from otolith records). Sediments of the Valencia seamount recorded several historical changes. Reconstruction studies carried out in oceanic areas can therefore help to understand the history and improve the management of fragile systems such as seamounts.

The three-dimensional (3-D) sound speed structure of the ocean is a fundamental environmental element in studying underwater sound propagation modeling and forecasting. The accurate classification of the sound speed fields (SSFs) provides a comprehensive understanding and analysis of the sound propagation pattern. To take advantage of the 3-D structure of the ocean SSFs, this paper presents a quick method based on tensor decomposition for classifying the ocean 3-D SSFs. Utilizing the WOA18 dataset, High Order Iterative Orthogonal (HOOI) decomposition of the 3-D SSFs is executed so as to accurately extract the characteristic information of the SSFs. The Fuzzy C-Means clustering (FCM) method is applied to classify the feature tensors, partitioning of regional categories in different seasons and revealing the typical SSFs structures. By combining the BELLHOP model with analysis of the characteristics of the first convergence zone of each category, it is concluded that there are six categories of SSFs in the Western Pacific Ocean. The SSFs across all categories are primarily latitudinally distributed, featuring distinct sound channel axes and surface sound speed variations.

The Bay of Biscay is a structurally complex region, exhibiting high biodiversity and productivity and hosting a wide variety of benthic habitats. Unfortunately, current scientific knowledge of deep-sea ecosystems is limited. Our investigation provides an updated vision of the existing knowledge on key aspects of the deep-sea ecosystem and identifies research priorities for the definition of a research roadmap in support of sustainable management, including the protection and conservation of deep-sea ecosystem of the Bay of Biscay. To achieve this, firstly a revision of projects, surveys and studies conducted in the region, was carried out resulting in the identification of 62 publicly available scientific documents. Secondly, an online survey was carried out, receiving 51 responses from 24 research institutions in 9 countries. Finally, a workshop was attended by 39 scientists to identify and discuss ways forward on deep-sea research in the Bay of Biscay. Results indicate that knowledge of benthic habitats, and vulnerable and sensitive habitats/species is good or very good for 45% of the respondents, while limited knowledge was reported for deep pelagic habitats. Almost 75% of the scientists agreed that deep-sea processes affect shallower areas. Funding (71%) and research vessel availability (57%) were considered the most constraining aspects limiting deep-sea research. More than 50% of the scientists perceived a short- or medium-term threat from human activities and pressures, while over than 40% of the respondents considered that there are no, or limited, data available to assess the environmental status. Regarding climate change, 55% of scientists partially or fully agreed with the existence of evidence of climate change effects, but 47% of them totally or partially considered that climate change monitoring is insufficient to assess eventual changes and effects. Most respondents (63%) considered that deep-sea ecosystems are not properly represented in the current Marine Protected Areas network. In terms of future needs, the development of monitoring strategies to reduce costs and guarantee an optimal design, data sharing and an increase in transnational collaboration are most cited. Finally, a proposal for a roadmap to fill in knowledge gaps for a better understating of the ecosystem processes of the deep sea of the Bay of Biscay is delivered.

A novel deep-sea in-situ anion analyzer (DIAA) is proposed and developed for long-term and continuous observation of marine biogeochemistry and ecology. The DIAA is designed to analyze the concentrations of various anions in seawater, such as F⁻, Cl⁻, Br⁻, NO₃⁻, SO₄²⁻, and PO₄³⁻, based on ion chromatography (IC) technology. The DIAA consists of a control circuit module, a conductivity signal acquisition module, and a flow analysis module. The control circuit module is installed in a sealed pressure-resistant vessel, while the conductivity signal acquisition module and the flow analysis module are housed in a sealed pressure-balanced vessel. The two vessels are capable of functioning at a water depth of up to 4000 m. A seawater in-situ dilution device is designed to solve the issue that the concentrations of Cl⁻ and SO₄²⁻ in seawater exceeds the IC detection limit. Moreover, an in-situ calibration device enables the DIAA to operate subsea for a long time without recovery. In addition, the DIAA can diagnose and isolate faults automatically. The operation status can be monitored remotely on the shore, and the scientific data can be acquired in real-time. During the sea trial, the DIAA was connected to Monterey Accelerated Research System (MARS) seafloor observatory at a water depth of approximately 900m and operated for six months successfully. The measurement accuracy and the long-term stability of the instrumentation have been verified.

Rapid rise of salinity is observed in the Black Sea in recent years, with the largest positive trend (0.07 psμ per 10 years) detected in the pycnocline. We use long-term hydrological measurements for 1985–2019 to show that salinity of pycnocline has intense seasonal and interannual variability modulated by the mechanical and convective mixing. In the warm period of a year, shear turbulence driven by strong winds and intense geostrophic currents causes the penetration of warm waters into the lower density layers. This is accompanied by the rise in their salinity, the source of which is the deep saline waters situated below the halocline. This process is most intense in the areas of downwelling and intensifies in the autumn period, when thermal stratification is relatively weak. Another important reason is the entrainment of salty Mediterranean waters in the upper part of the Black Sea halocline, which is modulated by the deepening of the seasonal thermocline near the Bosphorus strait and mechanical mixing.

The increase of salinity is compensated during cold winters, when convective mixing transports fresher water influenced by river discharge into lower density layers of the basin and causes a decrease in pycnocline salinity. This process is most intense in the center of the cyclonic gyres, where pycnocline is located closer to the surface and winter temperature reaches minimal values.

Due to the long-term warming of the Black Sea, the process of freshening of deep layers now is observed only in rare cold years. At the same time, an intensification of wind speed, vorticity, and geostrophic circulation processes promote the blurring of the halocline and the rise of the salinity of the Black Sea upper layers. Such rise begins after 2007 in the upper part of Black Sea halocline (depth 50–100 m) and is traced down to 250 m by 2020.

The occurrence of cold-water coral (CWC) reefs off Northwest Africa that has a broad oxygen minimum zone (OMZ) is poorly studied. A 400 km long almost continuous coral mound chain off Mauritania that was investigated by earlier expeditions revealed mainly dead corals. In 2021, the EAF-Nansen Programme conducted a survey at the border between Mauritania and Senegal with the main objective to map CWCs. Acoustic mapping with multibeam echosounder was used to target mounds at 450–650 m depth and 14 video lines were conducted using a remotely operated vehicle (ROV). The occurrence and health status of CWC reefs along video transects were annotated using the software “CampodLogger”, oceanographic variables were measured using a CTD sonde, and terrain and backscatter analysis were conducted.

Here we present the environment and health status of 13 Lophelia reefs established in the study area, six of them were large and healthy reefs with areas having 15–50% cover of live colonies. Oxygen concentrations were measured to be as low as 1 ml L⁻¹ and temperature ranged from 8.8 to 11.6° C. We compare reef health with the environmental variables: temp, salinity, oxygen, and particle load. A GIS based model was developed to predict the occurrence of live reefs in the study area based on the observed average range of a set of terrain descriptors measured where live Lophelia reefs occurred.

Our findings of healthy Lophelia reefs are unexpected and further north in the OMZ reefs have been described as dormant. There is an urgent need for visual seafloor mapping to aid the development of spatial management plans in these understudied waters.