HOLOCENE ENVIRONMENT CHANGES IN THE HACHICHINA WETLAND (GULF OF GABES, TUNISIA) EVIDENCED BY FORAMINIFERA AND OSTRACODA, GEOCHEMICAL PROXIES AND SEDIMENTOLOGICAL ANALYSES

Four sediment cores, drilled at Younga and Ouadrane Sebkhas (Hachichina wetland), were selected for micropalaeontological, sedimentological, geochronological and geochemical investigation coupled to hierarchical cluster and correspondence analysis. Our research aimed at reconstructing the past biodiversity, the Holocene palaeoenvironmental evolution and the responsible forcing. Subsurface sediments record four major phases: i) the first one is characterized by poorly to very poorly sorted silts, the dominance of the lagoonal/estuarine ostracod and the lagoonal/marine foraminifera and high values of terrestrial geochemical element ratios, such as K/Al and Fe/Al, which indicate an estuarine lagoon; ii) two major marine transgressions, TR1 and TR2, allowing the settlement of widely opened lagoons towards 7160-8680 and 5070-5520 cal yr BP, are marked by the enhancement of the marine/brackish ostracods and marine foraminifera, the improvement of species number, individuals number and H and E index values and of the marine element ratios such as Cl/Al and Sr/Al; iii) the opening of the lagoon is hampered by the action of drift currents allowing the genesis of sand spit and the settlement of a closed lagoon, towards 2610-3610 cal yr BP, characterized by the enhancement of lagoonal/estuarine ostracods and lagoonal/marine foraminifera and the increase of the individuals number; iv) the dominance of the lagoonal/estuarine ostracods and the lagoonal/marine foraminifera and the decline of the species number and density mark a brackish lagoon, also characterized by very poorly sorted silts transported by uniform suspension, which evolves to the actual sebkha. This evolution, within a global climate alteration context, from an estuarine environment to the present sebkha passing by an opened lagoon, is linked to the complex hydrographic setting of the sebkhas and of the Ouadrane Wadi outlet discharging in the Hachichina wetland

Environmental evolution of the Acholla coast (Gulf of Gabes, Tunisia) during the past 2000 years as inferred from palaeontological and sedimentological proxies

International audienceA multi-proxy approach to infer the environmental evolution and coastal dynamics was conducted on four sediment cores with the aim to decipher forcing factors shaping the coastline of Acholla (Tunisia) over the last 2000 years. The data recorded from palaeontological and sedimentological studies combined with 14C dating suggest a progression of four successive phases: (1) the first Holocene marine transgression (183 A.D.), overlying Upper Pleistocene marine sandstones, resulted in a brackish shallow-water coastal environment characterized by a mixture of lagoonal/estuarine, marine/brackish and marine/lagoonal ostracod assemblages with high species richness, (2) a high-energy event marked by the deposition of coarse bioclastic sands, rich in molluscs, charcoal particles and Tyrrhenian lithoclasts (towards 417 A.D.), (3) progradation of the coast (between 417 and 1577 A.D.), where longshore current drifts and a period of flooding/detrital input resulted in the build-up of a sandspit and the deposition of coastal sand bars, as indicated by increasing abundances of predominantly lagoonal/estuarine ostracods and molluscs. The natural sedimentation pattern was impacted by the construction of a Roman pier that accelerated the formation of the sandspit from 1577 A.D. onward (4), ultimately resulting in the complete closure of the lagoon as recorded by the expansion of saltmarshes containing a monospecific ostracod fauna (Cyprideis torosa)