The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.

Coastal areas are often contaminated by the dissemination of pathogenic bacteria from terrestrial inputs. In this study, we compared fecal conforms (FC) and Salmonella spp.loads from a coastal Mediterranean river and from the submarine outfall of a coastal wastewater treatment station. A stratified sampling strategy was used to analyze storm events. Bacterial Boxes were estimated during a 16-mo period. Salmonella spp. loads from the river were high during storm events, and the annual loads were higher than those estimated from the coastal outfall. Bacterial loads from the river represented 3.0 x 10'6 FC yr ' and 6.9 X 10" ,Salmonella yr -1 , with at least 95% occurring during high waterflow (21 % of the year). Those from the submarine outfall represented 4.3 x 10'° FC yr -1 and 4.7 x 10'° Salmonella yr -1 , with a regular temporal discharge. Bacterial loads from the river were associated with small clay particles (<2 µm), which originate from different reservoirs. These particles-sediment-trapped bacterial accumulated in the river-bed during the lowest water levels in the downstream part of the river and were resuspended timing storm events. The quantitative relationship between both bacterial parameters vary depending on the source point of contamination. Salmonella spp. loads front the river are of great sanitary concern since high loads were recorded in summer (bathing activities) and autumn (oyster production and consumption). Our results should help water quality managers to define priorities to improve the bacterial quality of river discharges.

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The sandy littoral of Sète, with its barred shoreface, is one of the three studied sites of the thematic action “Morphodynamics of sandy beaches” of the French Programme National d’Environnement Côtier (PNEC), selected because of its microtidal wave-dominated characteristics and of the long time series of available data. During the PNEC program, the principal results obtained show that the morphological evolution of the sedimentary bars can be synthesized according to two conceptual models, in reaction to hydrodynamic variability. (1) The model of “oscillation around a position of equilibrium” (O.P.E) defines the usual mode of behaviour of the bars, with an alternation of shoreward and seaward movements. The bars migrate seaward during storms and shoreward when the energy conditions decrease. This oscillation is expressed with several rhythms. (i) During the main phases of the bar evolution, which are long periods during which the bars present the same geometrical characters. The passage from one phase to the other occurs when a 4 m significant height storm happens. (ii) At the seasonal scale, which is well illustrated by the behaviour of the inner bar: during summer, when the conditions of agitation are weak, the bar aggregates punctually with the shore; when the conditions of agitation increase in autumn the inner bar is reformed and moved seaward. (2) The model of “Net Offshore Migration” (N.O.M) points to the tendency of the bars to retreat under the effect of paroxysmal events (storms with a 20 to 50 yrs return time) being a prelude to their degeneration. From the former position of equilibrium, the outer bar strongly moves seaward and drops following heavy swell. Instead of moving to the coast according to the O.P.E model, the outer bar loses material that is recovered by the inner bar and degenerates. The inner bar, exposed to the swell, then moves seaward to replace the initial outer bar, a new inner bar being created at the coast. A few years after, the standard pattern is restored. These results are compared with those described in the literature.

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