• NEANIAS Underwater Research Community
• FR close

International audience; A new conodont species, Siphonodella leiosa, is described from the lower Carboniferous pelagic limestones of the Montagne Noire (France), deposited in North Gondwana on a outer platform environment. Specimens were obtained from one level dated to the Siphonodella jii conodont Zone. The major difference from other siphonodellid conodonts known in this area is that the elements of this new species have a practically entirely smooth and unornamented platform, apart from the development of one or two low rostral ridge-like nodes. Similar morphologies were generally observed in shallow marine deposits of the same time frame from China, Russia and East and Central European areas. The new discovery reinforces the idea that ornamentation of siphonodellids is not only related to bathymetry, but that temperature could play an important role in the diversification and radiation of unornamented species during the Siphonodella jii conodont Zone.

A quantitative sampling was made on the non-cultivated molluscs of the soft bottom macrofauna in the bay of Marennes-Oleron. A stratified random sampling, including an optimal allocation strategy. provided adequate data to estimate abundance and biomass (total weight. dry weight and ash-free-dry weight) of each molluscan species. The main stratification factors were respectively the bathymetric levels. the different types of sediment and the differences between natural, oyster-free mud flats and cultivated areas. A Smith Mc-Intyre grab was used to sample the subtidal, shallow areas and the tidal flats as well. The total number of sampling stations was 370. The final estimation of the total weight of benthic molluscs was 14,400 ± 3,000 tons (at 95 % confidence limits). The main trophic competitors for oysters were : Cerastoderma edule (4,500 ± 2,200 tons), Mytilus edulis (2,600 ±. 2,000 tons), Crepidula fornicata (1,800 ±. 900 tons), Solen marginatus (800 ±. 600 tons) and Macoma balthica (800 ± 400 tons). La stratégie d'échantillonnage retenue pour l'estimation des stocks de mollusques benthiques non cultivés du bassin de Marennes-Oléron consiste en un type d'échantillonnage aléatoire stratifié, après allocation optimale de l'effort d'échantillonnage au sein des strates. L'engin de prélèvement, une benne Smith Mc-Intyre, a été utilisée sur l'ensemble de la zone soit 180 km. Les principaux stratificateurs sont la bathymétrie, les types biosédimentaires et la présence ou l'absence de culture d'huîtres. Un ensemble de 370 stations a été effectué. Le stock global de mollusques non cultivés est évalué en biomasse fraîche à 14 400 ± 3 000 tonnes (au seuil d'erreur de 5 %). Les principales espèces compétitrices sur le plan trophique des huîtres sont : Cerastoderma edule (4 500 ± 2 000 tonnes), Mytilus edulis (2 600 ±. 2 000 tonnes), Crepidula fornicata (1 800 ± 900 tonnes), Solen marginatus (800 ± 600 tonnes) et Macoma balthica (800 ±. 400 tonnes).

International audience; A unique late Glacial–Preboreal record of changes in sea-level and sediment fluxes originating from the Alps is recorded in the Rhône subaqueous delta in the Western Mediterranean Sea. The compilation of detailed bathymetric charts, together with high-resolution seismic profiles and long cores, reveals the detailed architecture of several sediment lobes, related to periods of decreased sea-level rise and/or increased sediment flux. They are situated along the retreat path of the Rhône distributaries, from the shelf edge and canyon heads up to the modern coastline. They form transgressive backstepping parasequences across the shelf, the late Holocene (highstand) deltas being confined to the inner shelf. The most prominent feature is an elongated paleo-shoreface/deltaic system, with an uppermost sandy fraction remolded into subaqueous dunes. A long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system. In seismic data, it displays aggradation, starting at not, vert, similar 15 cal kyr BP, followed by progradation initiated during the first phase of the Younger Dryas, a period of reduced sea-level rise or stillstand. The delta kept pace with resumed sea-level rise during the Preboreal (which is estimated at about 1 cm/yr), as a result of increased sediment supply from the Alps (melting of glaciers and more humid climate “flushing” the sediment down to the sea). Abandonment of the delta occurred around 10,500 cal yr BP, that is to say about 1000 yr after the end of the Younger Dryas, probably because of decreased sediment flux.

Co-auteur étranger; International audience; Deep-sea submarine eruptions are the least known type of volcanic activity, due to the difficulty of detecting, monitoring, and sampling them. Following an intense seismic crisis in May 2018, a large submarine effusive eruption offshore the island of Mayotte (Indian Ocean) has extruded at least 6.5 km3 of magma to date, making it the largest monitored submarine eruption as well as the largest effusive eruption on Earth since Iceland's 1783 Laki eruption. This volcano is located along a WNW-ESE volcanic ridge, extending from the island of Petite Terre (east side of Mayotte) to about 3,500 m of water depth. We present a detailed petrological and geochemical description of the erupted lavas sampled by the MAYOBS 1, 2, and 4 cruises between May and July 2019 and use these to infer characteristics and changes through time for the whole magmatic system and its dynamics from the source to the surface. These cruises provide an exceptional time-series of bathymetric, textural, petrological, and geochemical data for the 2018-2019 eruptive period, and hence bring an invaluable opportunity to better constrain the evolution of magma storage and transfer processes during a long-lived submarine eruption. Integrating the petrological signatures of dredged lavas with geophysical data, we show that the crystal-poor and gas-rich evolved basanitic magma was stored at mantle depth (>37 km) in a large (≥10 km3) reservoir and that the eruption was tectonically triggered. As the eruption proceeded, a decrease in ascent rate and/or a pathway change resulted in the incorporation of preexisting differentiated magma stored at a shallower level. Magma transfer from the deep mantle reservoir is syn-eruptive, as indicated by transfer times estimated from diffusion in zoned olivine crystals that are much shorter than the total eruption duration. Our petrological model has important hazard implications concerning the rapid and stealthy awakening of a deep gas-rich magma reservoirs that can produce unusually high output rates and long-lived eruption. Sudden tapping of large crystal poor reservoirs may be the trigger mechanism for other rarely witnessed high-volume (>1 km3) effusive events.

Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.

During swath mapping cruises various geophysical data were collected. Bathymetry, imagery and other geophysical information require specialised post-processing. Dedicated software enables post-processing and visualisation of each type of data. A graphic interface collects the files and exports them to a CAD system for cleaning and for adding extra information. The huge amount of soundings from multibeam echo sounder systems and the very particular sampling along and across the ship's track demand powerful software such as TRISMUS provides. Major steps in the processing are the merging of raw soundings with navigation corrected if necessary, the cleaning of soundings with a band-pass filter and the gridding of the data to obtain a Digital Terrain Model. Despite care and real time filtering during the acquisition phase, some errors persist and appear as abnormal patterns on contoured maps. It is often difficult to distinguish the origins of the errors and consequently to correct them in a deterministic way. But the analysis and description of typical patterns allows errors to be classified into five groups — sounding, profile, overlap area, surveyed area or gridding artefact — and in some cases to adopt specific processing techniques to reduce or cancel undesirable effects. Merging bathymetric maps and sonar mosaics can be achieved with IMAGEM or with the high level graphic interface MFEDIT. Since dedicated software splits the information up into different layers, the operator can manage the layers according to the information he wants on the final document. Additional information such as seismic reflection data and interpretation, geological logs and legends, can be imported or created as extra layers.

Smooth Particle Hydrodynamics is a Lagrangian meshless numerical method with substantially improved capabilities in simulation of both fluid dynamics and solid mechanics due to its meshless nature. GPUSPH is an implementation of Smoothed Particle Hydrodynamics (SPH) on Nvidia CUDA-enabled (graphics) cards. In this paper the GPUSPH is applied to runup and overtopping applications and compared with experimental results from Roos and Battjes for a plane slope and Oaks, Edge and Lynett for complex bathymetry representing a complex levee transition. Results for both models show good comparison with experimental data and suggest GPUSPH as a reasonable tool for complex runup and overtopping problems.

The waters of the Weddell-Scotia Confluence (WSC) lie above the rugged topography of the South Scotia Ridge in the Southern Ocean. Meridional exchanges across the WSC transfer water and tracers between the Antarctic Circumpolar Current (ACC) to the north and the subpolar Weddell Gyre to the south. Here, we examine the role of topographic interactions in mediating these exchanges, and in modifying the waters transferred. A case study is presented using data from a free-drifting, intermediate-depth float, which circulated anticyclonically over Discovery Bank on the South Scotia Ridge for close to 4 years. Dimensional analysis indicates that the local conditions are conducive to the formation of Taylor columns. Contemporaneous ship-derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column, with values of O(1000 m2/s) obtained. Although necessarily coarse, this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the ACC. A picture emerges of the Taylor column acting as a slow, steady blender, retaining the waters in the vicinity of the WSC for lengthy periods during which they can be subject to significant modification. A full regional float data set, bathymetric data, and a Southern Ocean state estimate are used to identify other potential sites for Taylor column formation. We find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the ACC in this sector of the Southern Ocean.

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