Sedimentological analysis of marine Pleistocene deposits of southeastern Tunisia: evidence of two positive eustatic pulsations during the marine isotopic substage 5e (Eemian, Tyrrhenian)

International audienceAbstract Detailed sedimentological and petrographic analysis of marine Pleistocene deposits along the coastal area of southeastern Tunisia allow to identify two distinct lithostratigraphic units separated by an erosion surface (fig. 2 et 5 A and B). These two commonly superposed units form a ridge parallel to the coast. The palaeocoastal morphology was more irregular than the present-day coastline with areas either more protected or more exposed than now (fig. 3). The lower unit overlies an erosion surface cutting into Mio-Pliocene and Villafranchian deposits. It consists of fine-grained bioclastic quartz-rich sands (fig. 5 C), locally overlain by thin marl layers containing benthic foraminifera and ostracods. The very fine facies (silts and clays) represent relatively protected areas while the coarser facies developed in the more exposed zones. These deposits locally display a well-developed aeolian facies that terminates the sedimentary sequence. This unit, well developed in Jerba Island and Jorf peninsula, strongly resembles the stratigraphic unit of “Khnis” as defined by Mahmoudi [1988] on the coast of Central Tunisia. The upper unit is the better developed in the studied area. It consists of carbonate deposits composed mainly by ooids and peloids (fig. 5 D) and contains also a warm Senegalese fauna, especially Strombus. Locally, in the exposed areas, it shows a coarse facies which resulted mainly from the erosion of the calcareous Villafranchian deposits. This unit displays a remarkable shallowing-upward sequence from shoreface to aeolian dunes (fig. 5 E and F). It constitutes the lateral equivalent of the stratigraphic unit of “Réjiche” as defined by Mahmoudi [1988] in Central Tunisia. These two units, called in this study “quartz-rich unit” (the lower sequence) and “carbonate unit” (the upper one), developed during two distinctive sedimentation phases associated with two sea-level highstands separated by a marine regression. During the first highstand sea-level was about 3 m higher than today whereas it was at about +5 m during the second highstand [Jedoui, 2000]. Along the Mediterranean coasts the Strombus paleobeaches, which are contemporary with the carbonate unit, are well developed and traditionally called Tyrrhenian deposits. Their radiochemical dating, using corals, gives ages of about 125 ka [Hearty et al., 1986 ; Miller et al., 1986 ; Dumas et al., 1991 ; Vai et Pasini, 1996]. We obtained the most reliable uranium/thorium dates in southeastern Tunisia on oyster shells. Results show that the two units developed during the marine isotopic substage 5e [last interglacial ; Jedoui, 2000]. This evidence suggests that substage 5e was characterised by at least two eustatic maxima separated by a lowering of sea level during a marine regression. Our results are therefore in agreement with recent palaeoclimatic reconstructions and in particular with sea level reconstructions and marine oxygen isotope records that indicate the distinct possibility of two positive eustatic pulsations during the last interglacial [Hillaire-Marcel et al., 1996 ; Kindler et al., 1997 ; Plaziat et al., 1998]. Variations in the petrographic content of the two outlined units reflect drastic palaeoclimatic fluctuations in southeastern Tunisia during the last interglacial. The establishment of wetter climatic conditions at the beginning of marine isotopic substage 5e than today was responsible for an enhanced terrigenous materiel supply from the continent as showed by siliciclastic sedimentation along the coast (lower unit). Our data are in agreement with the strong sea surface salinity lowering observed in Mediterranean basins at the beginning of the last interglacial period [Kallel et al., 2000]. The regression of these conditions during the second half of the last interglacial favoured a carbonate sedimentation (upper unit) in southeastern Tunisia

Sea level and climate changes during OIS 5e in the Western Mediterranean

Palaeontological, geomorphological and sedimentological data supported by isotopic dating on Oxygen Isotopic Stage (OIS) 5e deposits from the Spanish Mediterranean coast, are interpreted with the aim of reconstructing climatic instability in the Northern Hemisphere. Data point to marked climatic instability during the Last Interglacial (OIS 5e), with a change in meteorological conditions and, consequently, in the sedimentary environment. The oolitic facies generated during the first part of OIS 5e (ca. 135 kyr) shift into reddish conglomeratic facies during the second part (ca. 117 kyr). Sea surface Temperature (SST) and salinity are interpreted mainly on the basis of warm Senegalese fauna, which show chronological and spatial differential distribution throughout the Western Mediterranean. Present hydrological and meteorological conditions are used also as modern analogues to reconstruct climatic variability throughout the Last Interglacial, and this variability is interpreted within the wider framework of the North Atlantic record. All the available data indicate an increase in storminess induced by an increase in the influence of northwesterlies, a slight drop of SST in the northern Western Mediterranean, and an important change in meteorological conditions at the end of OIS 5e (117 kyr). These changes correlate well with the decrease in summer insolation and with the climatic instability recorded in North Atlantic high latitudes

Hydrogeochemical and stable isotope data of groundwater of a multi-aquifer system: Northern Gafsa basin – Central Tunisia

The hydrodynamic of the multi-aquifer system (the Continental Intercalaire “ C.I ” and the Complex Terminal “ C.T ”) of the North Gafsa basin is largely determined by tectonics (Tebessa - Gafsa fault). The composition of groundwater is controlled by complex reactions at gas-liquid-solid “mineralogical composition of associated rocks” interfaces, which depend on the natural surrounding and potential anthropogenic impact. The hydrochemical data (major ion geochemistry) indicate that these groundwaters are characterized by the dominance a Ca-Mg-HCO3/SO4 and Na-Cl-NO3 water types. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of carbonate minerals. The pH of these samples range from 6.54 to 8.89, supporting the conclusion that the H2CO3/HCO3 couple control pH buffering. Oxygen-18 (δ18O‰SMOW) and deuterium (dD‰SMOW) isotopic data show the exchange between the groundwater and the rock (water-rock interaction) and the evaporation effect. The isotopic content of the boreholes waters is of mixed Mediterranean - Atlantic origin and is opposite to the quantity of rainwater distribution, both in space and time in the study area. This is due to its geographical situation in the southern and south-western of the Mediterranean Sea and between the Atlas area and the Sahara Platform. The concentrations of the isotopic composition of the groundwater are significantly higher than the rainwater. This is indicative of the dissolution of salts and other processes modifying the rainwater geochemical composition during infiltration into the vadose zone. The hydraulic interconnection of these components of the system has led to the evolution of these interesting groundwater types

Late Quaternary sea-level change and early human societies in the central and eastern Mediterranean Basin : an interdisciplinary review

This article reviews key data and debates focused on relative sea-level changes since the Last Interglacial (approximately the last 132,000 years) in the Mediterranean Basin, and their implications for past human populations. Geological and geomorphological landscape studies are critical to archaeology. Coastal regions provide a wide range of resources to the populations that inhabit them. Coastal landscapes are increasingly the focus of scholarly discussions from the earliest exploitation of littoral resources and early hominin cognition, to the inundation of the earliest permanently settled fishing villages and eventually, formative centres of urbanisation. In the Mediterranean, these would become hubs of maritime transportation that gave rise to the roots of modern seaborne trade. As such, this article represents an original review of both the geo-scientific and archaeological data that specifically relate to sea-level changes and resulting impacts on both physical and cultural landscapes from the Palaeolithic until the emergence of the Classical periods. Our review highlights that the interdisciplinary links between coastal archaeology, geomorphology and sea-level changes are important to explain environmental impacts on coastal human societies and human migration. We review geological indicators of sea level and outline how archaeological features are commonly used as proxies for measuring past sea levels, both gradual changes and catastrophic events. We argue that coastal archaeologists should, as a part of their analyses, incorporate important sea-level concepts, such as indicative meaning. The interpretation of the indicative meaning of Roman fishtanks, for example, plays a critical role in reconstructions of late Holocene Mediterranean sea levels. We identify avenues for future work, which include the consideration of glacial isostatic adjustment (GIA) in addition to coastal tectonics to explain vertical movements of coastlines, more research on Palaeolithic island colonisation, broadening of Palaeolithic studies to include materials from the entire coastal landscape and not just coastal resources, a focus on rescue of archaeological sites under threat by coastal change, and expansion of underwater archaeological explorations in combination with submarine geomorphology. This article presents a collaborative synthesis of data, some of which have been collected and analysed by the authors, as the MEDFLOOD (MEDiterranean sea-level change and projection for future FLOODing) community, and highlights key sites, data, concepts and ongoing debates

Marine pleistocene sediments in Hergla (Sahel - Tunisia): record of two high sea level phases during the last interglacial period

The aim of the research was to assess importance of two high sea level phases during the last interglacial period in Tunisia. The research was based on analysis of structure and petrography of marine sediments in a vicinity of Hergla by the Hammamet bay. The article describes consecutive sediment facies deposited during the upper Pleistocene, which allowed to reconstruct former sea limit. Two overlapping recessional sequences of a different origin (from subtidal to aeolian) were discovered. Sediments constituting these sequences consist of usually well rounded quartz grains and calcareous cement. These two sequences, separated by an erosional surface, were developed during two different phases of high sea level (much higher than today). The sediments were cemented in a fresh-water environment after their deposition, during progradation of the sea-shore. The researched sediments in the vicinity of Hergla are similar to coastal sediments found in the vicinity of Mahdia and also in other places along the southern part of Tunisian coast. This justifies the obtained conclusion. Lack of petrographical differentiation between the two sediment sequences in the vicinity of Hergla (which is not the case in southern Tunisia) is explained by an intensive supply of terrigenous material to the sea coast by the El Medjini Wadi during the upper Pleistocene

Interaction morphodynamique d’une plage sableuse dans une conjoncture d’élévation du niveau marin ; exemple du littoral de Trab el Makhadha dans le golfe de Gabes-Tunisie

Le littoral de Trab el Makhadha (golfe de Gabès) est caractérisé par une évolution morphologique rapide et réversible à l’échelle saisonnière et évènementielle. Un suivi quotidien de l’évolution du profil transversal durant deux années (septembre 2007-novembre 2009) a mis en évidence les facteurs hydrodynamiques à l’origine du transport sédimentaire ainsi que le temps d’ajustement morphologique. Le changement morphologique du littoral de Trab el Makhadha, à l’échelle événementielle, est contrôlé par l’action du vent et des courants locaux ayant le temps d’ajustement morphologique le plus court. La cyclicité saisonnière entre accrétion estival et érosion hivernale du profil transversal de la plage est dictée par l’action de la houle. Le bilan sédimentaire, au cours des trente années précédentes, est excédentaire. La progradation du littoral est matérialisée par la mise en place, depuis les années 1980, d’un cordon sableux de 2 m de hauteur. La genèse du cordon côtier est contemporaine à l’accélération de l’élévation récente du niveau marin

Stromatolite and serpulid bioherms in a holocene restricted lagoon (Sabkha el Melah, Southeastern Tunisia)

The Sebkha el Melah, located near the Libyan-Tunisian border, is fringed by a relict stromatolite-serpulid belt. These bioherms developed 5500 years ago during the Flandrian sea-level highstand when the area of the present-day sebkha was covered by a wide restricted lagoon. Relict lagoonal beaches and spits, mainly composed of cerebroid ooids and cerithid gastropods, are flanked by mushroom-shaped stromatolites reaching up to 50 cm in height. These stromatolites grew on top of small serpulid bioherms as well as on top of beachrock and beachrock blocks in a shallow subtidal pre-evaporitic realm. The primary porosity of these bioherms is obliterated by botryoidal and spherulitic aragonite, probably of microbial origin. This sub-Recent association of stromatolites, serpulid bioherms and cerebroid ooids provides a useful additional model for the interpretation of upper Paleozoic deposits, where similar facies have been described