Geo mar. lett.

The present study investigates the history of the northern Sfax coast by means of subsurface sediments of the sebkhas El Merdessia and El Awebed through a multiproxy approach. Ostracod and mollusk assemblages, diversity index, sedimentological criteria, correspondence analysis, and radiocarbon datings together provide an overview of the development of this coast over the last 5000 years. Original data give evidence for periods of predominantly lagoonal and brackish water conditions. These data testify to the emersion of sebkha El Awebed during Holocene, while sebkha El Merdessia recorded three marine transgressions toward 4599, 2225, and 1396 years cal BP. These transgressions are indicated by the richness of sediments in lagoonal and marine ostracod assemblages coupled with marine mollusks and the high values of species richness and diversity index. Sandwiched between 2225 and 1396 years BP, a period of sea-level stability and buildup of sand barriers in front of the estuaries was evidenced. Toward 250 years cal BP, a tsunami event is evidenced by the deposition of a shelly bed containing angular and sharpened Cerithium vulgatum in coarser marine sands overlaid by silts and clays rich in charcoal particles and pottery fragments. The comparison between the proxies analyzed of the studied area and those of Skhira coast leads to the conclusion that the two coasts were subjected to the same factors. However, a time shift of sedimentation is due to the uplift of Sfax northern coast favored by the activity of the faults, unlike the southern Skhira subsidence

New insights from microfauna associations characterizing palaeoenvironments, sea level fluctuations and a tsunami event along Sfax Northern coast (Gulf of Gabes, Tunisia) during the Late Pleistocene-Holocene

Late Pleistocene to Holocene evolution of the Northern coast of Sfax (Gulf of Gabes, Tunisia) is deduced on the basis of the analyses of sediment cores and coastal outcrops. The environmental changes are established by means of microfauna and mollusk assemblages and biocenotic parameters complemented by geochemical, sedimentological and correspondence analysis (CA). An opened embayment, rich in warm Senegalese fauna, allowed the deposition of quartz-rich sands Late Pleistocene in age. During the Late Holocene, a major modification of the environmental characters is recognized. The dominance of marine ostracod and coastal foraminifera coupled with high values of species richness characterize a widely opened lagoon. This lagoon is subjected to the action of the alongshore drifts which are responsible for the build-up of sand spits and the genesis of the present sebkhas. Two transgressive events, overlying the marine Late Pleistocene quartz-rich sands, are dated at 2018-2419 and 1001-1804 a cal BP. The dominance of marine and coastal foraminifera, the recurrence of ostracods and the high values of biocenotic parameters testify these two events. Sudden changes, toward 0-502 a cal BP, in the structure of populations, geochemical element concentrations and sedimentological trend argue in favor of a tsunami event and the settlement of peculiar environment. The latter records (1) the reworking of Holocene microfauna and Tyrrhenian sands rich in mollusks eroded from the marine substrata; (2) the high concentration of poorly stratified and poorly sorted shells; (3) fining-upward and thinning landward sequences; (4) the dominance of articulated bivalves and sharpened shells; and (5) the locally extensive deposits up to 160 m inland rising in altitude of about 0.75 m. This extreme event can be correlated with the tsunami generated by the strong earthquake, 1908 AD in age, in Eastern Sicily or by one of the earthquakes recorded during the 18th Century around the Mediterranean basin such as Sfax earthquake of 1750 AD

Water level and atmospheric humidity history of Lake Ichkeul (northern Tunisia) during the last 3000 years

Changes in paleoecology and climate of northern Tunisia during the last 3000 years were reconstructed based on the flood history interpreted from a 172-cm sediment core of Lake Ichkeul (NW Tunisia). Seven wet/dry episodes were identified based on biological (ostracods, foraminifera and mollusks) and biotic indices (H and E index, species richness and abundance). These proxy-based environmental changes were supported by correspondence analyses (CAs) and ecophenotypic responses of the brackish taxon Cyprideis torosa in addition to a grain-size study. Two dry episodes were identified in the lower (EP1) and upper (EP7) parts of the core. These were marked by the dominance of the brackish ostracod C. torosa. High water salinity was indicated by the presence of the brackish ostracod Loxoconcha elliptica, the foraminifer Ammonia sp. and lagoonal mollusks. The occurrence of the freshwater ostracods Ilyocypris sp., Herpetocypris sp., Dawinula stevensoni and Limnocythere inopinata was associated with high species richness and ecophenotypic changes of C. torosa valves indicating lower salinity during three major wet episodes (EP2, EP4, and EP6). The ecological and environmental changes, occurring between 3050 and 50 cal. year BP, are most likely linked to an increase of fluvial inputs which are also recognized in several other Mediterranean lakes. The humid episodes were interrupted by two saline periods (EP3 and EP5) during which freshwater ostracod assemblages declined, diversity indices dropped to the lowest values, and fine-grained sediments became dominant. The top of the core is characterized by the remarkable dominance of L. elliptica coupled with C. torosa, the absence of freshwater ostracods, and the changes in grain-size sediment parameters which are most likely the result of anthropogenic activities. The changes in hydrochemistry and sedimentology were attributed to the deepening of the Bizerte navigation canal, main wadis damming, and Tinja sluice construction which mostly occurred in the XXth century

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)