Geochemical approach to the sedimentary evolution of the Bay of Nice (NW Mediterranean sea)

Abstract During the 1960s and 1970s, the coastline of the Bay of Nice receded. As remedial measures, breakwaters were built in the western part of the Bay, and a beach replenishment policy was adopted. This resulted in the beach broadening from 10 or 15 to about 25 m. The main aim of this study was to determine the impact of these works and other urban changes on the sedimentation in the circalittoral zone along the Bay, using a geochemical approach. The concentrations of Pb, Zn and Cu and the activity of 210 Pb were analysed in 8 cores collected at a water depth of 30 m along the coast, and in another one collected more offshore, at a depth of 756 m. The isotopic ratio 206 Pb/ 207 Pb was measured in 5 cores; the ratios in surface sediments range from 1.163 to 1.195. The isotopic ratios indicated that most of the lead contamination is from industrial and domestic sources, rather than gasoline. The average sediment accumulation rates, calculated assuming constant 210 Pb supply to bottom sediment, range from 0.2 to 1.5 g/cm 2 /yr. Metal concentration profiles as well as the 210 Pb profiles at the 30 m sites reflect the evolution of Nice since 1920, in particular the expansion of the town to the West on the Plio-Quaternary hills. At that time, expansion works furnished the materials for beach replenishments with uncontaminated sediments, whereas from 1985 sediments used were those of the Paillon bed (the small river that crosses the town), characterised nowadays by a moderate metal pollution. The high sediment accumulation rate in the centre of the Bay suggests the convergence of drifting from the western beaches toward the east, and from the Paillon embouchure toward the west. The sediment accumulation rate appears lowered during the last 10 yr in some 30 m sites. This can be related to the fact that, in the recent years, the sedimentary material discharged on the beaches for replenishment purpose originated chiefly from the Paillon bed, but such material would anyway have been transported to the Bay, and its use for the replenishment seems therefore unable to compensate the sedimentary load loss due to the Nice airport expansion on the Var prodelta area.