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Rapport de traitement des données issues de l'EM2040C à bord du N/O Côte de la Manche lors de la mission scientifique SABASETI.

Synthèse des traitements SMF EM2040 et sonar KLEIN 3000 des missions RESISTE à bord du N/O Thalia

Synthèse des traitements SMF EM2040 et sonar KLEIN 3000 des missions RESISTE à bord du N/O Thalia

This work presents the geomorphologic characterization of the Blanes Canyon (northwestern Mediterranean), based on high- and very-high resolution multibeam bathymetry acquired in the framework of the ABRIC project. High-resolution hullmounted multibeam data (up to 5 m grid size) were collected along the entire submarine canyon head using a Kongsberg EM710 multibeam echo sounder (MBES). Additionally, the Hybrid Remotely Operated Vehicle (H-ROV) “Ariane” was used to acquire finescale morphology of vertical rocky walls within the canyon with unprecedented detail (80 cm grid size). The vertical mapping was performed using an EM2040 MBES mounted on the front of the H-ROV with a 45° orientation. High-resolution video footage was acquired on the same canyon walls with the ROV “Liropus”. Blanes Canyon has a marked structural character, deeply incising the margin and exposing Miocene to Pliocene successions. The tectonic control is evidenced by the rectilinear trajectories and sharp bends of the canyon axis, and by the presence of vertical rocky walls that can reach up 300 m in height. The canyon is asymmetric and has a smoothed eastern flank and a western flank dominated by a dendritic lattice of gullies. H-ROV bathymetric data on vertical cliffs allowed to trace the horizontal strata of Miocene ages, which offer the substrate for the development of benthic communities. Other identified fine-scale morphological elements include terraces, thalweg incision, under-cutting, scarps and rock avalanches. The study of the different-scale morphologies found in the canyon will allow a better understanding of its evolution trough time and the erosive processes involved

To investigate the topographic constraints caused by submarine canyon walls on near-bottom currents, four moorings arrays were deployed in the Blanes Canyon (NW Mediterranean) at ~600 m water depth during winter and spring 2020. Two moorings were placed at a main canyon head and two at the head of a canyon tributary, in regions characterized by narrow gorges and the presence of vertical canyon walls. This study was carried out during the Spanish ABRIC Project aimed to investigate sediment transport mechanisms and their influence on suspension feeder benthic communities. Preliminary results reveal that near-bottom currents were aligned with the canyon wall orientations and oscillated up- and down-canyon mainly at inertial frequencies. The highest current velocities measured at both study sites were ~35 cm/s and directed up-canyon, although the residual flows in both monitored sites were predominantly downcanyon. Suspended sediment concentrations did not exceed 4 mg/L and they mainly increased during down-canyon flows, which contributed to a continuous and persistent transfer of suspended particles towards deeper parts of the canyon. These new findings reveal a complex interaction between local hydrodynamics and steep canyon morphologies, which ultimately affect the ambient suspended sediment transport throughout the canyon, potentially driving the settlement and development of benthic communities inhabiting submarine canyon walls

The Lacaze-Duthiers Canyon is located in the western Mediterranean Sea and is long known for hosting cold-water coral colonies in the canyon head region at depths ranging from 250 to 550 m. In 2019 during the CALADU cruise, three kinds of 3D-reconstructions were applied to better understand the distribution of coral colonies, their habitat and their skeleton morphologies. The canyon's flanks were mapped using a hull-mounted echosounder and an ROV multibeam echosounder. Digital terrain models were built with resolutions of 5 and 1 m and examined in three dimensions. ROV bathymetric data collected on the canyon's flanks made it possible to highlight a series of sub-parallel structures identified as lithified sedimentary strata along which coral colonies grow. Coral assemblages were explored at four locations and photographic images were assembled using structure from motion techniques to build photogrammetric models. Coral assemblages reconstructed in 3D enabled geo-localizing and recreating coral colonies on 16 models over a total area of 4370 m2. Two colonial species, Madrepora oculata and Desmophyllum pertusum were plotted and reported on bathymetric models to interpret their location at the scale of the canyon. The coordinates and depth of the colonies were used to calculate the vertical distribution (limited to our small bathymetric exploration, between 339 and 214 m depth) and density of populations (up to 4.3 colonies per m2). The spatial coverage of the 16 assemblages measured between 100 and 600 m2 each. The sizes of the colonies were measured to analyze the population structures of both species (mean sizes of 28 cm for D. pertusum and 18 cm for M. oculata, maximum sizes 1 m and 0.5 m, respectively, bushes 2.5 m long). In addition, lost fishing gears were quantified, longlines measured and their densities calculated (0.16 m/m2, up to 0.30 m/m2). An area with exuberant orange colonies of D. pertusum was discovered for the first time in the Lacaze-Duthiers Cany

A few thousand (2612) seeps are releasing microbial methane bubbles from the seafloor at the Aquitaine Shelf edge (Bay of Biscay) at shallow water depths (140‐220 m). This methane contributes to the formation of meter‐scale subcircular carbonate structures, which are (sub‐)outcropping over 375 km2. Based on in situ flow rate measurements and acoustic data, and assuming steady and continuous fluxes over time, the methane entering the water column is estimated to 144 Mg/yr. Microbial methane circulation has been on‐going for at least a few thousand years. This discovery highlights the importance of microbial methane generation, disconnected from deep thermogenic sources and gas hydrates, at continental shelves. The shelf edge may be viewed as a focus area for methane circulation and release and related diagenesis, all having an impact on the shaping of continental shelves and potentially on the oceanic and atmospheric carbon budget. Plain Language Summary At the Aquitaine Shelf of the Bay of Biscay (Northeast Atlantic Ocean), the recent acoustic, chemical, and visual investigations of microbial methane release at the seafloor have led to the discovery of a vast fluid system. This methane escapes as bubbles from the seafloor into the seawater at 2612 sites, all located at shallow water depths (140‐220 m) along the edge of the continental shelf. Methane‐derived authigenic carbonates that are by‐products of gas seepage cover the (sub‐)seafloor over a large area of 375 km2. These carbonates form subcircular meter‐scale pavements and mounds, less than 2 m in height above the surrounding seafloor. Based on the growth rate of authigenic carbonates, it can be inferred that methane circulation has occurred for at least a few thousand years. The amount of methane released from the Aquitaine Shelf seafloor into the water column, estimated at 144 t/yr, questions the fate of the methane in the ocean and its possible passage to the atmosphere with therefore consequent potential con

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. On the other hand, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record.

The upper slopes of continental margins are very energetic areas where nepheloid layers are often observed. Multibeam bathymetry, sub-bottom profiler and multi-channel highresolution seismic reflection data acquired during the PAMELA-MOZ04 survey in the Mozambique Channel revealed the presence of sand dunes on the upper slope at 120-250 m water depth. The dunes migrate upslope and their crests are oblique to the contours. They are medium to large dunes, with wavelengths between 20 and 150 m and heights between 0.15 and 1.50 m, and their size decreases upslope. Seismic reflection data of the water column show internal solitary waves travelling offshore in the depth range of the dune field. The formation of the dune field could be related to the interaction of the barotropic tide with the upper slope that results in the generation of internal tides.

Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0.9 to 9.0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec−1 and maximum speeds of 57 cm sec−1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning and the impact of bottom currents on sediment distribution and sea floor morphology.

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100 km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geophysical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that submarine flat-top seamounts correspond to karstified and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well-constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical environments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typified by the establishment of wide lava flow complexes, and (2) to extensional tectonic deformation associated with high-offset normal faults dividing the flat-top seamounts into distinctive structural blocks. Explosive volcanic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impac

The protective activity of a non-bacteriocinogenic Lactococcus piscium CNCM I-4031 strain against Listeria monocytogenes was investigated in tropical cooked peeled shrimp stored at 8°C in modified atmosphere packaging (50% N2-50% CO2). When inoculated alone (L. piscium 107 CFU g-1 and L. monocytogenes 104 CFU g-1), protective culture and target strain grew very well on shrimp reaching a maximum cell number of 109 CFU g-1 after 7 and 14 days, respectively. In the presence of L. piscium, growth of L. monocytogenes was totally prevented after 3 days of storage. The count was 3.4 log CFU g-1 lower than in the control after 10 days and until the end of storage (31 days). Using the Seafood Spoilage and Safety Predictor Software (http://sssp.dtuaqua.dk), it was shown that pH decrease from 6.58 to 5.94 and lactic acid concentration of 89.65 mM measured in the co-inoculated batch did not fully explain the inhibition observed.