Biotracers and geotracers of depositional events in NW Mediterranean margin over the past two centuries

A detailed sedimentological and geochemical investigation was performed on slope sediments collected from the Gulf of Lions and Catalonian margins and from the northern Balearic margin. The study was based on cores, about 50 cm long. Various physiographic sites (canyons, open slopes) were selected according to the available sediment accumulation rates obtained with the 210Pb geochronological method. At a short time scale, various parts of the slope were characterized by muddy hemipelagic sediment. Sedimentological data suggested that deposition was continuous. Total organic carbon and its hydrolysable fraction decreased downcore. Significant increase of C/N ratio downcore resulted from an early diagenetic process in response to the burial effect. However in other parts, the values of the C/N ratio were found to be irregular and maxima could frequently be related to both higher sand content and relict coastal remains derived from erosion and reworking of outcropping deposits of the last glacial maximum (lowstand system tract). Such tracts crop out over much of the shelf slope break seafloor. They provide various biological and lithological indicators: relatively well-preserved Miliolidea, coastal or lagoonal foraminifera (Ammonia beccarii, Elphidium crispum), Characea oogons, oxidized beach-rock debris, gypsum, previously mature glauconite (with cracks) and coarse vegetal debris. Throughout the sections, the degree of diagenetic overprinting of the organic matter frequently showed a positive correlation with the abundance of some coastal indicators supporting the hypothesis of sedimentation induced predominantly by gravity These downslope accumulation may correspond to 10-40 % of the total deposit. The gravity induced transfers have been shown to be negligible in the sediment record of the northern Balearic margin. It is suggested that a reduced sedimentation rate (half that observed in the Gulf of Lions) would be expressed by a higher stability of the slope. The consequence of low sedimentation would be early diagenetic solution/reprecipitation of carbonate near the redox transition layer. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS.