Enregistrement des fluctuations du niveau marin dans l'Holocène supérieur du lac Retba (Sénégal) par les foraminifères

A study of sea level fluctuation records based on foraminiferal assemblages was carried out for the first time in a sediment core taken at the Lac Retba edge. A total of 37 foraminifera species were collected along the core, among which most are known in the modern estuaries and lagoons of Senegal. A succession of four assemblages dominated by Ammonia parkinsoniana and A. tepida were identified; they correspond to different stages of evolution of the lake. The first association, at the base of the core, indicates a lagoon slightly opened to the sea and bordered with vegetation, under relatively humid climate. The second has the richest and most diversified microfauna with a high proportion of coastal, marine benthic and planktonic species (about 10%) that indicate a small coastal gulf bordered with mangroves. The third association is oligospecific and typical of a closed and hypersaline lagoon under a dry climate. The last association again contains coastal, marine benthic and scarce planktonic species (3%) indicating a change to a saltier and more open lagoon under dry climate. The disappearance of planktonic foraminifera at the top of the core indicates the closure of the lagoon. The biocenotic indicators are evidence for two marine intrusions that are referred to the Dakarian (3000 years B.P.) and Saint-Louisian (2140-680 years B.P.) stages of the Upper Holocene of the senegalo-mauritanian stratigraphic scale. Evidence of the Lac Retba closure since 680 years B. P. appears in the core by high reduction of foraminifers' abundance and diversity absence of planktonic species, and salts precipitated at the bottom of the lake.