Stabilité des pentes sous-marines dans la Baie des Anges, Nice, France. Approche géotechnique.

La Baie des Anges présente des singularités qui en font un site exemplaire pour l'étude des instabilités sous-marines. L'application du modèle de glissement plan en tenant compte de l'activité sismique a permis de mettre en évidence l'instabilité potentielle d'une part importante de la baie. Deux types de glissements sont possibles : de petits réajustements locaux de la pente impliquant des sédiments sous-consolidés du haut du plateau continental et de grands glissements de flancs de crêtes, dont le volume peut permettre la transformation en courants de turbidité

Nature and rates of fine-sedimentation on a mid-shelf: ‘‘La Grande Vasie` re'' (Bay of Biscay, France)

The study area, ‘‘La Grande Vasie` re'' (LGV), stretches out on the French Atlantic continental shelf (at ca. 100m water depth), along 250km from the Gle´ nan Islands at the north to the southwest of Rochebonne at the south. Box-cores were sampled in this mid-shelf area during four cruises in June 1995, and in April, June and September 2002. They were investigated using sedimentological approaches (X-radiographs and grain-size analyses) and radionuclide studies (210Pb geochronology and excess 234Th). The main results are: (1) the surficial sediments are generally organized into a decimetrescale fining up sequence which can be the result of extreme storms; (2) an upper mixing layer of 7–20 cm reflects an important biological benthic activity and/or the impact of fishing (i.e. trawlers); (3) a thin (i.e. a few mm) surficial mud-rich layer is the result of the present-day sedimentation; (4) an apparent annual sedimentation rate of 1–3mm is recorded in several loci of the study area. Some seasonal variations appear, corresponding to the deposition of fine material from April to September, and to the reworking and the re-suspension during the winter. This fine material is the result of the decantation of estuarine plumes, mainly the Loire and the Vilaine rivers, over the study area. LGV lies (1) under the influence of a winter-to-spring thermo-haline wedge that acts as a filter for the transfer of fine river-borne material to the slope and the open sea, and (2) below water depths where the mean swell action permits sedimentation, mainly in summer. From the point of view of the nature of its sediments, LGV is not a mud-belt, but a heterolithic and patchy sandy area that is submitted to increasing silting with environmental changes, on a seasonal-time scale

Sedimentary sequences in the Gulf of Lion: A record of 100,000 years climatic cycles

The wide continental shelf of the Gulf of Lion (up to 70 km) has been the object of numerous investigations since the early days of oceanography. Yet, the question of sequences duration, the mechanisms of deposition and factors implied remained unanswered. A study of a very dense grid of Very High Resolution (VHR) seismic reflection (Sparker) data associated with surficial cores both, collected by IFREMER between 1992 and 2001 on the outer shelf and upper slope offshore of Sète in the Languedoc region gives a new insight into these issues. Analysis of the 3D geometry of the sedimentary record reveals a basic depositional pattern consisting of a pair of horizontally juxtaposed and downlapping prisms. Prism PI with low angle clinoforms (<1°) lies on the upper part of the shelf and is interpreted to be prodeltaic-offshore deposits. Prism PII with steeply dipping clinoforms (4°) lies on the outer shelf between 40 and 70 km from the present day coastline and is interpreted to be littoral deposits. Results obtained from integrating lithology, palynology, micropaleontology, seismic stratigraphy, stratigraphic simulation, support the hypothesis that the basic depositional pattern records a 100 000-years glacioeustatic (interglacial/glacial) cycle. As previously suggested by Aloïsi [Aloïsi, J.C., 1986. Sur un modèle de sédimentation deltaïque: contribution à la connaissance des marges passive, Thèse de Doctorat d'Etat. Université de Perpignan, 162 pp], prisms PI corresponds to deposition at high sea level and prisms PII to deposition during low sea level at glacial maxima. Five sequences of paired prisms capped by five major erosion surfaces have been identified and modelled showing that the corresponding glacioeustatic cycles (the last five cycles at least) are recorded on the shelf of the western part of the Gulf of Lio

Changes in northeast Atlantic hydrology during Termination 1: Insights from Celtic margin's benthic foraminifera

International audienceUsing benthic foraminiferal-based proxies in sediments from the Celtic margin, we provide a well-dated record across the last deglaciation of the Channel River dynamics and its potential impact on the hydrology of intermediate water masses along the European margin. Our results describe three main periods: 1) During the Last Glacial Maximum, and before ∼21 ka BP, the predominance of meso-oligotrophic species suggests well oxygenated water masses. After ∼21 ka BP, increasing proportions of eutrophic species related to enhanced riverine supply occurs concomitantly with early warming in Greenland air-temperatures; 2) A thick laminated deposit, occurring during a 1500-years long period of seasonal melting of the European Ice Sheet (EIS), is associated with early Heinrich Stadial 1 period (∼18.2–16.7 ka BP). The benthic proxies describe low salinity episodes, cold temperatures, severe dysoxia and eutrophic conditions on the sea floor, perhaps evidence for cascading of turbid meltwaters; 3) During late HS1 (∼16.7–14.7 ka BP), conditions on the Celtic margin's seafloor changed drastically and faunas indicate oligotrophic conditions as a result of the ceasing of EIS meltwater discharges. While surface waters were cold due to Laurentide Ice Sheet (LIS) icebergs releases, increasing benthic Mg/Ca ratios reveal a progressive warming of intermediate water masses whereas oxygen proxies indicate overall well oxygenated conditions. In addition to the well known effect of EIS meltwaters on surface waters in the Celtic margin, our benthic record documents a pronounced impact on intermediate water depths during HS1, which coincided with major AMOC disruptions.</p

Campagne Seamewe 2: Reconnaissance de trace de cable sous-marin dans l'Ocean Indien

Cruise Seamewe 2 has been devoted to the survey of the projected track of a telecommunication cable on a distance of 2300 miles between Djibouti and Colombo. Supplementary data relevant to Djibouti approaches (side scan survey) and the areas were obtained during Leg 1 of Seamewe. The survey involves the seabeam for bathymetry and a mud penetrator 3.5 KHz with a correlator for determination of the acoustic character of the sea floor. Magnetism and gravimetry have been also recorded on most of the surveyed tracks. The near-bottom water temperature had also to be checked in a number of stations