Evolution of the sebkha Dreîaa (South-Eastern Tunisia, Gulf of Gabes) during the Late Holocene: Response of ostracod assemblages

The quantitative and qualitative study of the ostracod assemblages supported by the correspondence analysis (CA) allowed the reconstruction of the palaeoenvironnemental change during the Holocene in the sebkha Dreîaa of Skhira (Gulf of Gabes, SE Tunisia). Five phases were distinguished: the first phase (>cal. 6471–6874 yr BP) coincides with the first Holocene marine transgression following the deposition of the continental Holocene series. It induces the setting of an open lagoon where numerous Bivalvia, Gastropoda and marine ostracods lived. The second one (cal. 6471–6874 yr BP) is characterized by the development of brackish ostracods, high diversity index values and comparable percentages of the brackish, lagoonal and coastal assemblages. It corresponds to an open lagoon subjected to estuarine influences. During the third phase (cal. 3350–3752 yr BP), a marine environment, marked by the enrichment of marine ostracods, is evolving toward the closing. The settlement of the restricted lagoonal environments is linked to the building-up of sandy spit. At cal. 2839–3057 yr BP, the dominance of coastal ostracods, associated with coarse sands, show an open lagoon and probably a marine transgression evidenced by the progressive modification in the ostracods assemblages. After this transgression, the southern part of Sebkha of Dreîaa emerged and evolved towards the present state. The last phase, cal. 515–777 yr BP, is marked by a strong marine influence, in the northern part of the sebkha, with transport of marine Bivalvia, Gastropoda and ostracods towards the inner lagoon by means of storms. The rupture of sandy spits induced the introduction of marine macrofauna and microfauna which were accumulated and associated with charcoals and coarse sands. L’étude quantitative et qualitative des assemblages d’ostracodes soutenue par l’analyse factorielle des correspondances (AFC) permet de reconstituer l’évolution des paléoenvironnements holocènes à l’emplacement de la Sebkha Dreîaa de Skhira (golfe de Gabès, S-E tunisien). Cinq phases sont reconnues : La première (>cal. 6471–6874ans BP), coïncide avec la première transgression marine recouvrant les dépôts continentaux holocènes. Elle permet l’installation d’une lagune ouverte où vivaient de nombreux lamellibranches, gastéropodes et ostracodes marins. La deuxième phase (d’âgecal. 6471–6874ans BP), est déduite à partir du développement des ostracodes saumâtres, des valeurs élevées des indices de la diversité et d’un pourc