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International audience; Major eruptions in the Andes are mainly characterized by the emission of large volumes of gas and volcanic ash. The plume may reach the stratosphere and be transported by winds. In Ecuador, the prevailing winds are westward, and the volcanic ash, also called tephra, is transported towards the Pacific Ocean. Vallejo (2011) studied tephra layers present in marine terraces and proposed a first draft of the identification of their source in the Cordillera. However tephras deposited onshore may have been remobilized, eroded or covered by younger deposits. There are therefore several interests in studying tephras recorded in marine sediments : it allows to (1) know the age and source of major eruptions whose products reached the coast, (2) know the distribution of fallout from each major eruption, (3) estimate the volume of largest events, (4) participate to better assess the current volcanic hazard, and (5) provide temporal constraints on continental or marine sediments that cannot be dated by radiocarbon. To study the tephra layers recorded in marine sediments I have described the glass morphology and the mineralogy of tephra beds. To determine the source of the eruptions, I have used data published onshore (e.g., Hidalgo et al., 2008; Hall et Mothes, 2008; Robin et al., 2010). As each volcano has its own geochemical signature when we combine major, trace and isotope data, I compared them with geochemical analyses performed on distal tephra recorded in marine sediments. To determine the age of distal tephra beds, we have performed 14C ages on foraminifera present above and below the bed for tephra emitted during the past 50 ka, and d18O and biostratigraphy for the older deposits. In this presentation, I first presented our results obtained on cores collected during Amadeus (2005) and Atacames (2012) oceanic campaigns along the Ecuadorian margin, and recently published (Bablon et al., 2022). We have sampled 28 tephra layers, from coring sites that cover 5 degrees of latitude, from the southern half of Ecuador to southern Colombia. We observed four main lithofacies: isolated lenses that typical of bioturbation, layers with sharp and sometimes diffuse contacts in the upper part that correspond to primary deposits of tephra fallout, and successions of thin laminated layers that correspond to tephra layers reworked by turbidity currents, and that have not been sampled. Volcanic glass shards have various morphologies depending on the density of vesicles and their deformation, such as block-shaped glass without bubbles, pumice-shaped glass, or glass with completely elongated bubbles. The glass morphology of each tephra allows us to propose a first correlation of layers between each marine core. Concerning the geochemistry, glasses are mostly rhyolitic and belong to the low potassic series typical of the volcanic front, and to high potassic series typical of the eastern cordillera. This distinction between the eastern and western cordillera is also found in their trace element signature. To identify the source volcano, we used Sr and Pb isotopes. On The compositional fields of the volcanoes products overlap little and thus allows us to refine the correlations. We show that distal marine tephra come from the Cerro Machin in Colombia, and from Pichincha, Atacazo, and Cotopaxi volcanoes in Ecuador. Together with the determination of the sources, the radiocarbon dating of sediments allowed us to show that the oldest tephras belonged to the about 8 ka eruption of Cotopaxi, and the youngest correspond to the 10th century eruption of Pichincha. Using the spatial distribution of tephra, we made isopach maps of the fourth major eruptions of Pichincha, Atacazo and Cotopaxi, and we estimated their volumes. They vary between 1.3 and 6 km3, which corresponds to volcanic explosivity indexes of 5, thus eruptions which would be particularly destructive today. A perspective for our work is to study of turbidite beds present in the Holocene cores along the coast. As such deposits are emplaced during major earthquakes, and we can therefore use their correlation to identify past seiscally active areas.Unfortunately, some major Holocene eruptions described in the Cordillera and constitute stratigraphic markers have not been recorded in marine sediments. Tephra layers may have been destructed during drilling such as the very young 700 BP eruption of Quilotoa, they may have been dispersed by ocean currents, or tephra were not present in the cores du to a restricted distribution of deposits, for example for the 3000 BP eruption of Cuicocha (Vallejo, 2011).In the second part of my presentation, I focused on another case study, performed at ODP site 1239, above the Carnegie Ridge. This core is much deeper as it reaches 500 m, and sediments deposited about 10 Myr ago. It contains 24 tephra layers, and we focused on the thickest, 18 cm-thick at 7 m deep (Schipboard Scientific Party, 2003; Bablon et al., 2020). The main volcanic structure that could be the source of such a thick deposit is the Chalupas caldera, located in the Eastern Cordillera, near Cotopaxi volcano. About 50 km southwest of the caldera, we sampled the ignimbrite and dated the glass shards at 216 +/- 5 ka using the K-Ar dating method applied on glass shards. In order to verify if the ignimbrite and marine tephra of ODP Site 1239 belong to the same eruption, we have compared their geochemistry, and we have shown that their major and trace element contents are very close. To check the reliability of this land-sea correlation, we have also compared their ages. Variations of d18O are related to climate changes linked to the Earth's orbital forcing. We then used d18O data available to know the age of sediments as a function of depth. The stratigraphic position of the tephra layer corresponds to the 7d isotopic stage that occured at 220 ka, in agreement with our K-Ar age obtained onland. We then have shown that the 216 +/- 5 ka eruption of Chalupas is the largest of the Quaternary in northern Andes, with products that reached more than 1000 km from their source. Our land-sea correlation also allow to provide an independant temporal constraint to the regional d18O records.

International audience; Les algorithmes d’apprentissage profond pour détecter automatiquement des objets à partir d’imagerie donnent, dans plusieurs domaines, des résultats prometteurs, et sont de plus en plus utilisés et accessibles sans expertise pointue. Nous souhaitons donc voir si leur utilisation sur de l’imagerie sous-marine comme la bathymétrie et ses dérivés, ou encore la réflectivité du fond sous-marin, pourrait nous permettre de réaliser une détection automatique de structures géomorphologiques comme des sorties de fluides, des glissements de terrain… En géosciences marines, nous disposons de jeux de données manuels de ces structures, et nous comptons les utiliser comme données d’apprentissage sur des composites d’imagerie adaptés.

International audience

National audience; La mer ligure : une région de montagnes sous-marines Le relief sous-marin de la région niçoise est particulièrement pentu, creusés par des canyons, et certains évènements, tels que des séismes, peuvent le déstabiliser jusqu'à provoquer des glissements de terrain sous-marins. Glissement de terrain meurtrier et tsunami au large de Nice Le 16 octobre 1979, un glissement de terrain se produit sur le chantier de l'agrandissement de l'aéroport de Nice et provoque la mort de 10 ouvriers ainsi qu'une vague de 2 m qui inonde le quartier de la Salis à Antibes et provoque le décès d'une promeneuse.

International audience; The Nile deep-sea fan is the largest Plio-Quaternary depocentre in the Mediterranean Sea, extending over an area of >150,000 km2 within which rapid deposition drives syn-sedimentary collapse tectonics, as well as widespread seafloor venting of mixed thermogenic and biogenic gases. It is thus an ideal setting for the near-seabed accumulation of gas hydrates, which are stable below water depths of about 1100 m in the warm waters of the eastern Mediterranean. Gas hydrates have yet to be sampled on the Nile fan, although their presence has been suggested from semi-published industry seismic and well log data. Here we integrate available industry data with a regional grid of academic geophysical data (seismic and multibeam) acquired by Géoazur, in order to identify a BSR on the central Nile fan, invert it to geothermal gradients, and examine its relationship to fluid venting. The BSR is observed on several intersecting seismic profiles of varying frequency content and offset, as a discontinuous reflection of negative polarity that lies 220-330 ms below seafloor, deepening downslope in water depths of 2000-2500 m. The BSR occurs within a relatively small area (2500 km2), among a larger system of slope-parallel extensional faults. Inversion of BSR depth to temperature gradients was performed using a velocity-depth function for deep-sea sediments, a phase boundary for methane hydrate in equilibrium with seawater of 3.86% salinity (Mediterranean average), and water temperatures from MEDATLAS. The results indicate spatially varying gradients of 27-42˚C/km, up to twice background values reported in the geothermally cool eastern Mediterranean offshore. ‘Shallow’ BSRs yielding elevated thermal gradients within the Nile fan can be explained in terms of advective heat transfer by upward fluid flux. Upward fluid migration is supported by evidence of widespread seafloor fluid venting, but seafloor data indicate few or no vents in the area of the BSR. In contrast, in an area with many gas vents over 50 km from the BSR, subsurface gas hydrate accumulations are indicated by resistivity log data at two well-sites. If gas and gas hydrates are present across wider areas, why is the BSR so restricted? We hypothesise that these observations can be explained in terms of spatially varying fluid flux, such that away from the BSR a greater flux of fluids rich in dissolved gas leads to rapid gas hydrate formation and the growth of gas chimneys. We intend to test this hypothesis during a forthcoming Franco-Brazilian campaign, focused on understanding the dynamics of the fluid vents. Acknowledgement : this work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 656821 (SEAGAS project).