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

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Original article

Evolution of the sebkha Dreîaa (South-Eastern Tunisia, Gulf of Gabes) during the Late Holocene: Response of ostracod assemblages Évolution de la sebkha Dreîaa (Sud-Est de la Tunisie, Golfe de Gabès) au cours de l’Holocène supérieur : réponse des associations des ostracodes Chahira Zaïbi a,∗ , Pierre Carbonel b , Fekri Kamoun a , Michel Fontugne c , Chafai Azri d , Younès Jedoui e , Mabrouk Montacer a a

University of Sfax, Faculty of Sciences, UR GEOGLOB, B-P 802, 3018, 3018 Sfax, Tunisia b 16, Road Mégret, 33400 Talence, France c Laboratory of Sciences Climate and Environment, CNRS - Gif-sur-Yvette, rue Terrasse, bâtiment 12, 91190 Gif-sur-Yvette, France d University of Sfax, Faculty of Sciences, UR Coastal and Urbain Environment study, 3018 Sfax, Tunisia e University of Gabès, Higher Institute of Water Sciences and Techniques of Gabes, Gabès, Tunisia

Abstract 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. © 2012 Elsevier Masson SAS. All rights reserved. Keywords: Holocene; Ostracods; Evolution; Gulf of Gabes

Résumé 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. 6471–6874 ans 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’âge cal. 6471–6874 ans BP), est déduite à partir du développement des ostracodes saumâtres, des valeurs élevées des indices de la diversité et d’un pourcentage comparable des associations saumâtres, lagunaires et côtières. Elle traduit une lagune ouverte soumise aux influences estuariennes. Pendant la troisième phase (cal. 3350–3752 ans BP), un environnement marin, marqué par l’enrichissement des ostracodes marins, évolue vers la fermeture. La fermeture du milieu est liée à l’action des courants de dérive littorale édifiant des flèches sableuses. Vers

∗

Corresponding author. E-mail address: [email protected] (C. Zaïbi).

0035-1598/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.revmic.2012.03.003

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cal. 2839-3057 ans BP, les ostracodes côtiers, dominants et associés à des sables grossiers, caractérisent une lagune ouverte et probablement une transgression marine exprimée par la modification progressive dans les associations des ostracodes. Après cette transgression, la partie sud de la Sebkha Dreîaa émerge et évolue vers le présent environnement. Pendant la dernière phase, d’âge cal. 515–777 ans BP, la partie nord de la sebkha est soumise à une forte influence marine, traduite par le transport des lamellibranches et des ostracodes marins vers l’intérieure de la lagune suite à des tempêtes induisant la rupture des flèches sableuses. Ces faunes transportées sont accumulées et s’associent à des fragments de charbons et les sables grossiers. © 2012 Elsevier Masson SAS. Tous droits réservés. Mots clés : Holocène ; Ostracodes ; Évolution ; Golfe de Gabès

1. Introduction Since several decades, ostracods have been valuable tools for the reconstruction of Holocene palaeoenvironnements caused by anthropological or natural forcing. Several works established the distribution of current ostracods according to bathymetric characteristics, hydrodynamic conditions and the nature of substratum, for marine, lagoonal (Ruiz et al., 1996; Bekkali and Nachite, 1997; El Hmaidi et al., 2010) and estuarian environments (Carbonel, 1982; Laprida, 2001; Nachite et al., 2010). Frenzel and Boomer (2005) considered ostracods as an indicator of seasonal climatic variations, changes of temperature, chemical water quality and eutrophication. The anthropogenic influence in the composition of assemblages, the specific richness and the density of ostracods were evidenced by different authors (Eagar, 1999; Schornikov, 2000; Padmanabha and Belagali, 2008; Zenina, 2009). Regarding the Holocene series, the main climatic and eustatic sea level variations have been reconstructed based on quantitative and qualitative distributions of ostracods (Boomer and Godwin, 1993; Mazzini et al., 1999; Sarr et al., 2009). In Tunisia, studies dealing with ostracods are relatively limited. They date back to Gauthier (1928) who established an inventory of the recent ostracods living in the paralic and endoreic environments. Later, Carbonel and Pujos (1981) and Jouirou (1982) studied the ostracods occurring in the surface and subsurface sediments of the lagoon of Ghar El Melh, the Sebkha of Ariana and Tunis lake. Such microfauna resulted in reconstructing the different stages of the evolution of these environments during the Holocene times and providing evidence of the impact of the renewal of waters on the composition of assemblages. Lachenal (1989) described different Holocene ostracod assemblages (phytal, opportunist and ubiquitous taxa) from the cores of the Gulf of Gabes. Our works (Zaïbi et al., 2011a) allowed us, by means of particular study of ostracods and the foraminifera, to reconstruct the palaeoenvironnements in the paralic Sebkha of El-Guettiate during Holocene times. An open estuarian lagoonal environment, at cal. 7460 yr PB, evolves gradually towards a brackish lagoonal environment (cal. 5408 yr PB in age) and then towards the present sebkha associated with the settlement of a sandy spit. This brackish lagoon allowed the accumulation of bioclastic deposits (washover) during an extreme climatic event (inducing storms). In continuity with our preliminary results, which were devoted to the short period from cal. 7460 to 5408 BP, the objectives of the present work are twofold. A primary objective

is to reconstruct the different palaeoenvironnements, for a more recent period, which succeeded from cal. 6471–6874 yr BP to 515–777 yr BP, and to deduce the control factors responsible for palaeoenvironnemental changes. A related objective is to examine the behavior of this coastal zone and the responsible climatic events within a global context. The studied material corresponds to the ostracods, which have been identified in the sebkha subsurface sediments. 2. Study area description, analysis of aerial photos and geomorphology The Sebkha of Dreîaa is located 100 km South of Sfax City, along the Gulf of Gabes coast. It is situated between the latitudes 34◦ 13 - 34◦ 07 N and the longitudes 09◦ 59 - 10◦ 01 E. The detailed morphological analysis of this area, based on the aerial photograph interpretation (Mission 2004) and the direct field observations, allowed us to distinguish the succession, from South to North, of two paralic Sebkhas (Dreîaa and El - Guettiate), separated from the sea by a system of offshore bars (Fig. 1). The Sebkha of El Guettiate is separated from the Mediterranean Sea by a narrow land strip, 2 km long and 100 to 200 m wide. This land strip, which constitutes a cliff, ten meters high, is being progressively destructed by the littoral erosion, under the influence of the waves (Zaïbi et al., 2011b). The cliff is relieved southwards by an offshore bar corresponding to a system of sandy spits, 2 km long (Fig. 2). The Sebkha of Dreîaa is also separated from the open sea, in its northern part, by a system of sandy spits, 2.5 km long, denoting a southern long shore drift (Fig. 3A). Its southern part is widely opened to the sea, allowing the individualization of a small lagoon (El Hisha lagoon) under tidal influences, especially during the high tides. The opening of the sebkha towards the sea is also allowed by a main channel, by-passing the sandy spit (Gargouri, 2011). The channel penetrates into the schorres and the slikkes situated in the northern part of the lagoon. Several fossilized sandy spits can be observed in the El Hisha Lagoon (Fig. 3B). The coast neighboring the two sebkhas is characterized by a developed strand. The storm impact on the coast morphology is well visible on the aerial photographs. Indeed, several washovers (Fig. 3C), composed of bioclastic sands and characterized by the decrease of its thickness, moving from the offshore bar, can be observed through the tidal mouths. They are characterized by the decrease of their thickness toward the sebkha (Zaïbi et al., 2011b).

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Fig. 1. Location map of Dreîaa Sebkha on satellite photo (Google earth, mission 2004). A to C location of the detailed photos of the Fig. 3.

3. Material and methods Core sediments have been obtained from two boreholes (C2 and CD2) drilled in the emerged area of Sebkha of Dreîaa. The cores C2 and CD2 are situated at 34◦ 11 19.32“N, 10◦ 2 12.05”E and 34◦ 10 20.23“N, 10◦ 0 46.49”E respectively and drilled at about 0.5 m above sea level. In the laboratory, the lithology of the cores was described and two hundred seventeen samples were obtained for macro-fossil analysis. Pelecypod and gastropod species were identified. Six datings were carried out at the Beta Analytic Laboratory (Miami, USA), the “Laboratoire des Sciences du Climat et de l’Environnement de Gif-sur-Yvette” and “Laboratoire de Mesure du Carbone 14 (ARTEMIS) de Saclay (France)” by means of radiocarbon methods applied to bivalvia, gastropoda, ostracod and foraminiferal shells (Table 1). Five-hundred and fourteen samples of approximately 5 gr. were taken for microfossil analysis completed by a sedimentologic analysis. The sampling of the central part of each core was performed at 5 cm intervals (core CD2) and at 1 cm continuous intervals (core C2), in order to obtain a more detailed

Fig. 2. Interpretative figure of Fig. 1 showing different morphological units and position of the cores C2 and CD2.

representation of faunal and palaeoenvironnemental changes. To avoid sediments pulled on the wall of the tube during the core drilling, sampling was considered only in a central part of the core (a volume of 2 cm3 ). The samples were dried at about 50 ◦ C and their dry weights were determined. Next, they were washed over 63 and 150 ␮m meshes. For sediment characterization, different fractions (< 63 ␮m, 63–150 ␮m and > 150 ␮m) have been determined. The abundance of each ostracod species and the number of individuals per 10 grams in each sample were calculated. The taxonomical identification keys were provided by Bonaduce et al. (1975), Llano (1981) and Athersuch et al. (1989). Four main groups of ostracods were distinguished (Figs. 4 and 5):

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indicating a communication with marine water (Carbonel, 1982); • a coastal assemblage is characterized by the presence of species such as Urocythereis oblonga, U. favosa, Aurila convexa, A. prasina, Cushmanidea elongata, Loxoconcha rhomboidea, L. parallela, Neocytherideis faveolata and Cytheretta adriatica; • a marine assemblage, including phytal species such as Callistocythere discrepans, Semicytherura incongruens, S. sella, Hiltermannicythere emaciata, Basslerites berchoni, Carinocythereis carinata and Neocytherideis fasciata. Pelecypods and gastropods were identified at the species level. For each level of the cores the following parameters were determined: • the  absolute abundance or density (number of individuals) ( ), • species richness (NS), • Shannon index (H), • equitability index (E) of Pielou (1966) which completes the information supplied by the H index, • the dominance (D). Indeed, two populations, with very different numbers of species, could elucidate the same H index. Therefore, it was worthwhile to calculate in a complementary way the E index, which reflects the variety observed in the maximal theoretical variety. The values of the H, E and D indices were calculated by reporting for every value the associated bootstraps interval, of 95%, using the PAST software (Hammer et al., 2001). All the obtained values were included in the corresponding reliable intervals. To verify the results of the descriptive study and diversity indices, a Correspondence Analysis (CA) was realized based on ostracod-absolute abundance by means of the Statit cf. program (1987). The sedimentological study dealt with the count and the morphoscopic analysis of quartz grains present in the meshes 150 ␮m. 4. Results 4.1. Datings Fig. 3. A: current sandy spit oriented NE-SO (Dreîaa Sebkha), flood and ebb tidal deltas observed at outlet of tidal channel; B: fossilized sandy spits; C: washover deposits cutting the offshore bar.

• a brackish water assemblage containing Cyprideis torosa which characterizes quiet water and Loxoconcha elliptica. Preferring higher hydrodynamic conditions (Carbonel, 1982) and estuarine environments (Pascual and Carbonel, 1992); • a lagoonal assemblage, composed of Xestoleberis aurantia, Leptocythere fabaeformis, L. levis, L. pellucida and the two phytal species Xestoleberis dispar and Leptocythere castanea,

All samples (Table 1) were prepared following a standard procedure (Délibrias, 1985): shells were mechanically cleaned by adding contaminants and leached with dilute HCl to remove portions of shell matrix that might have been affected by exchange reactions and recrystallisations (Vita-Finzi and Roberts, 1984). Results of radiocarbon dating are presented as conventional 14 C ages and calibrated dates, calculated using the CALIB 6.0 program (Stuiver and Reimer, 1993). The calibrated dates were corrected for marine reservoir effects using a R correction of 35 ± 70 yr proposed by Siani et al. (2000) for the modern period and Siani et al. (2001) for the last 6000 yr in the Mediterranean Sea. Ages discussed below will

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be expressed as the total interval age of the 2 ␴ calibrated range. 4.2. Lithological units, mollusc data and radiometric dating 4.2.1. Core CD2 The core CD2, 250 cm long, consists of six lithological units (Fig. 6). The lower unit (unit UL1, 250–200 cm below surface) is characterized by blue clays without macro and microfauna. The second unit (UL2, 200–150 cm) contains brackish Bivalvia (Cerastoderma glaucum, Abra alba, Ruditapes decussatus, Hydrobia ventrosa) and Gastropoda (Cerithim vulgatum, Pirenella conica, Gibbula umbilicalis) in green silty sands. The mollusc fragments do not exceed 20%. Above this unit, sediments correspond to sands (from 150 to 90 cm, UL3) rich in lagoonal mollusca fragments (30%). The unit UL4 (90 to 75 cm) is characterized by very fine sands and contains abundant phanerogams (Posidonia and Cymodocea), marine Bivalvia (Glycymeris glycymeris, Dosina lupidus) and marine Gastropoda (Conus mediterraneus). The fine fraction varies between 40 and 50%. The interval from 75 to 20 cm (UL5) can be separated of the underlying one by the presence of coarse sands (50%) rich in marine Gastropoda and Bivalvia and in mollusca fragments (50%). A silty interval (U6) which constitutes the last 20 cm of the core is poor in macro- and microfauna. The mollusca identified in the core CD 2 allowed the dating of the different lithological units. The calibrated 2 ␴ age intervals are cal. 6471–6874 yr BP (145 cm in depth, UL3), cal. 2789–3207 yr BP (70–65 cm in depth, UL5) and cal. 2839-3057 yr BP (40 cm in depth, UL5) (Table 1). 4.2.2. Core C2 Core C2 can be divided into four lithological intervals named U1 to U4 (Fig. 6). Between 230 and 150 cm below the surface (U1), the core C2 is characterized by a sequence of green-gray laminated silty sand, rich in green algae (Halimeda) fragments, brackish and marine Bivalvia (Cerastoderma glaucum, Ruditapes decussatus, Glycymeris glycymeris) and Gastropoda (Cerithium vulgatum, Gibbula umbilicalis, Conus mediterraneus, Turritella monterosatoi). The sediment fine fraction (< 63 ␮m) varies between 10 and 30%. The second interval, from 150 to 60 cm (U2), is made up of fine sand rich in marine phanerogams and fragments of Halimeda. The fine fraction does not exceed 10% of the total sediment. This interval, marked by the importance of charcoal fragments, shows a more important Gastropoda and Bivalvia species richness than that of the underlying unit. The third interval, from 60 to 20 cm, U3 unit, corresponds to coarse sands (>90%) very rich in brackish Bivalvia (Ruditapes decussatus, Cerastoderma glaucum), brackish Gastropoda (Abra alba, Cerithium vulgatum)

and marine Bivalvia (Arca nœ, Dosinia lupidus) and Gastropoda (Conus mediterraneus). Mollusk fragments (50%) and the mollusk species richness (13) are the most important of the core. The last lithological unit (U4, 20 cm) is composed of a silt poor in micropalaeontologic data and without Mollusca. In the core C2, three datings were obtained on bivalvia, gastropoda, foraminifera and ostracod shells, indicating the 2 ␴ intervals ages of U1: cal. 3350–3752 yr BP (221–229 cm, depth), U3: cal. 515–777 yr BP (40 cm depth) and cal. 614–902 yr BP (20–37 cm depth) (Table 1). 4.3. Micropalaeontological analyses 4.3.1. Core zones 4.3.1.1. Core CD2. The ostracods are abundant (500 individuals 10 gr−1 of dry sediment) and diversified with 32 species and 18 genera. Cyprideis torosa is the dominant taxa (28%). It is followed by and Loxoconcha elliptica (brackish), Xestoleberis aurantia (lagoonal) and Loxoconcha parallela (coastal) which constitute 11%, 11% and 10% respectively. Other taxa do not exceed 6%. The biocenotic parameters and the vertical distribution of ostracods along the core CD2 allowed us to distinguish the following zones (Figs. 7 and 8): • Zone I: from 200 to 150 cm, marked by reduced values of species richness, density and H index. This zone includes 12 species. It is characterized by the dominance of the coastal and lagoonal assemblages (Fig. 8). The progressive impoverishment of the lagoonal assemblage, represented essentially by Xestoleberis aurantia, and marine one, towards the top of the zone, is relieved by the enrichment of the brackish species Loxoconcha elliptica, showing thereby high water energy (Ruiz et al., 1996) and estuarine environments (Pascual and Carbonel, 1992). • Zone II: from 150 to 90 cm, characterized by the increase in the density values, the species richness and the H and E indices. Shells give the 2 ␴ interval age cal. 6471–6874 yr BP. This zone differs from the previous one by the appearance of the euryhaline and eurythermic species Cyprideis torosa and the impoverishment of Loxoconcha elliptica. The percentages of the three assemblages (brackish, lagoonal and coastal) are comparable. The population structure of these assemblages reveals that the species include juvenile as well as adult forms, testifying their autochthonous origin. The coastal species, characterizing the opening of the environment, are: Loxoconcha parallela, L. rhomboidea, Neocytherideis fasciata, N. faveolata, N. subspiralis, N. subulata and Cushmanidea elongata. They are associated with the lagoonal taxa (Xestoleberis aurantia, Leptocythere fabaeformis and L. pellucida). The top of the zone is marked by the enrichment of the brack-

Fig. 4. Ostracod shells from Sebkha of Dreîaa, cores C2 and CD2. 1–5. Cyprideis torosa (Jones): 1, male carapace (Cp), left valve; 2, juv. right valve; 3, noded Cp, right valve; 4, male right valve, internal view; 5, female (Cp), left valve. 6. Loxoconcha elliptica Brady, left valve. 7. Xestoleberis aurantia (Baird), left valve. 8. Leptocythere fabaeformis (G.W. Müller), left valve. 9. Leptocythere pellucida (Baird), right valve. 10. Aurila convexa (Baird), right valve. 11. Aurila woodwardii (Brady), left valve. 12. Aurila prasina Barbeito-Gonzalez, juv. left valve. 13. Urocythereis oblonga (Brady), left valve. 14. Urocythereis favosa (Roemer), right valve. Scale bar = 250 ␮m (Figs. 1, 4, 9, 10–14); scale bar = 80 ␮m (Figs. 2, 7, 8); scale bar = 450 ␮m (Figs. 3, 5, 6).

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4.3.1.2. Core C2.

Fig. 6. Gastropoda and Bivalvia vertical repartition along the cores CD2 and C2, calendar ages, lithological units, Mollusca species number, Mollusca fragments percentages and granulometric fractions.

ish assemblage and the impoverishment of the coastal and lagoonal taxa. All these parameters indicate an open lagoonal environment, subjected to increasingly important estuarine influence. The reduction of the density, the species richness, the two diversity indices (H and E) and the number of quartz grains associated with the increase of the dominance values, observed at the top of the zone, show the closure of the lagoon (Cl.1). • Zone III: from 90 to 20 cm, marked by the reappearance and the development of marine species such as Neocytherideis subspiralis, N. subulata, Callistocythere discrepans and Carinocythereis carinata. The diversity H and E indices reach the highest values of the core. The brackish

assemblages, rich in the bottom of the zone (75%), at the levels of fine sand (U4), become gradually impoverished in the coarse sands (30%). It is relieved by coastal species that become richer. The autochthonous origin of the brackish species is proved by the presence of the juvenile and adult forms. However, the marine species are characterized by either juvenile or adult individuals, in fragments, showing their transport, corresponding thereby to a taphocœnose of high energy. • Zone IV: from 20 cm to the surface, characterized by the progressive dominance of C. torosa associated with the impoverishment of marine and coastal species.

Fig. 5. Ostracod shells from Sebkha of Dreîaa, cores C2 and CD2. 1. Cushmanidea elongata (Brady), left valve. 2. Neocytherideis foveolata (Brady), left valve. 3. Neocytherideis subspiralis (Brady, Crosskey and Robertson), right valve. 4, 5. Hiltermannicythere emaciata (Brady), 4, left valve, 5, right valve. 6. Carinocythereis carinata (Roemer), left valve. 7. Semicytherura sella, left valve. 8. Paracytheridea depressa (G.W. Müller), left valve. 9. Callistocythere discrepans (G.W. Müller), right valve. 10. Basslerites berchoni (Brady), left valve. 11. Cytherelloidea sordid (G.W. Müller), left valve. 12. Semicytherura incongruens (G.W. Müller), right valve. Scale bar = 320 ␮m (Figs. 1–5, 8, 11, 12); scale bar = 80 ␮m (Fig. 7; scale bar = 250 ␮m (Figs. 6, 9, 10).

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Fig. 7. Ostracods vertical repartition along the cores CD2 and C2, calendar ages, lithological units and columns, vertical distribution of zones. Table 1 ages obtained for the lithologic units of the cores C2, CD2 of the Sebkha of Dreîaa (calibrated dates were obtained with CALIB.06 calibration program with R = 35 ± 70).

14 C

Lithologic units

Cores

Depth (cm)

Studied material

14 C

(UL5) Coarse sands (UL5) Coarse sands (UL3) Sands (U1) Silty sands (U3) Coarse sands (U3) Coarse sands

CD2 CD2 CD2 C2 C2 C2

40 65–70 145 221–229 59–60 20–37

Gastropoda and Bivalvia Gastropoda and Bivalvia Gastropoda Foraminifera and ostracods Gastropoda and Bivalvia Gastropoda and Bivalvia

3110 3235 6260 3670 1115 1215

The ostracods in core C2 are less abundant than in core CD2 (100 individuals 10/gr. of dry sediment) and diversified (24 species and 15 genera). Cyprideis torosa (brackish) and Xestoleberis aurantia (lagoonal) are the dominant taxa and constitute 20% and 19% respectively, of the total ostracod population. They are followed by the coastal Aurila convexa (9%) and the marine Semicytherura incongruens (7%). Other taxa do not exceed 5%. Ostracod assemblages, species richness, number of individuals and diversity indices allowed the recognition of four successive zones in this core (Figs. 7 and 8). • Zone IA (230 to 170 cm), which is characterized by its richness in green Algae (Halimeda) fragments, presents a rich ostracod assemblage mostly diversified within the entire core. Twenty four species, belonging to four

age (Years B.P.) ± ± ± ± ± ±

30 30 40 40 30 30

Calibrated age 2 Sigma (Years B.P.)

Median age (Years BP)

Laboratory

2839–3057 2789–3207 6471–6874 3350–3752 515–777 614–902

2857 2998 6672 3539 644 735

Sac A12050/Gif-12355 Sac A12051/Gif-12356 Beta-282579 Beta-282581 Sac A12307/Gif-12331 Sac A12306/Gif-12330

assemblages, are inventoried. The marine taxa (50%) are the phytal species: Semicytherura incongruens, S. sella, Hiltermannicythere emaciata, Paracytheridea depressa, Basselerites berchoni, Callistocythere discrepans and Carinocythereis carinata. They are associated with phytal coastal species (20%), such as Aurila convexa, Loxoconcha rhomboidea, Cushmanidea elongata and Urocythereis oblonga and the lagoonal taxa (20%) Xestoleberis aurantia, X. dispar, Leptocythere fabaeformis, L. levis and L. macella. This zone is also marked by the highest values of two diversity indices (H) and (E), associated with the development of the marine assemblage. E index varies between 0.75 and 1, giving evidence of a population in which the individuals are fairly distributed between the species. H index varies between 1.5 and 2, and is positively correlated to the high values of species richness.

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Fig. 8. Evolution of ostracod and Mollusk assemblages, ostracod biocenotic parameters (diversity indices (H and E), dominance, species and individuals number), number of quartz grains, Mollusk fragments percentages, palaeoenvironnements and closure episodes along CD2 and C2 cores.

• Zone IB (from 170 to 130 cm) is marked by the development of the levels of fine sands, rich in Posidonia and Cymodocea and the reduction of the proportion of the Halimeda fragments. From 170 to 160 cm, the ostracod density and species richness decrease (15 individuals/10 gr. and 4 species), coinciding with the reduction of the proportions of lagoonal and coastal assemblages. This thin interval is also evidenced by the increase of Loxoconcha elliptica abundance and the values of the dominance and the reduction of H index. These parameters explain the onset of a restricted lagoon during short-time episode (Cl. 2). • Zone II (from 130 to 60 cm), more loaded with Posidonia and Cymodocea, is marked by a radical change in the ostracod assemblages dominance. Indeed, this zone is characterized by its three-time enrichment in euryhalin species. The interval (130–100 cm) shows a decrease in the values of the H index, the development of the brackish and the lagoonal assemblages and an impoverishment of the coastal and marine species. From 100 to 80 cm, the marine species are rare, the coastal taxa become impoverished even more and the brackish taxa Cyprideis torosa grows rich (50%) at the expense of Loxoconcha elliptica, which disappears at the top. The positive correlation, in these two intervals, between the brackish

species richness and the increase in the abundance of quartz grains suggests the continental origin of these latter. Fluvial channels are responsible for the transport of these grains and would explain the euryhalinity of lagoon waters. In the second interval (100–80 cm), the decrease of H index, species richness and density and the increase of dominance explain the closure of the environment and the limited communication of marine waters. From 80 cm to the top of the zone, we note the development of the Cymodocea and Posidonia, the reduction in the abundance of quartz grains, the decrease of the species richness and the increase of the number of individuals of ostracods. C. torosa dominates gradually the population and constitutes 90% at the top of the zone. H and E indices supply the lowest values of the core. All these parameters reveal a probable closure episode (Cl. 3) and the installation of a brackish estuarian lagoon. • Zone III (from 60 to 20 cm) is formed by levels of sands, enclosing lumachellic accumulations. Bivalvia and Gastropoda provide the most important species richness of the core. This zone is marked by the reappearance of the marine ostracods (essentially within its first centimeters), the development of coastal and lagoonal species and a decrease in ostracod density. C. torosa generally characterizes low energy

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environments. It develops at present in the Gulf of Gabes in lagoonal environments with muddy substratum, rich in Phanerogams (Posidonia and Cymodocea) and organic matter. Dominating the population of ostracods, the presence of this taxa in the sands of the core C2 (zone III) evidences its probable allochtonous character. Besides, its transport is confirmed by the association of this species with L. elliptica, which is characteristic of the high hydrodynamic environments, and the marine (at the basis of the zone), lagoonal and coastal taxa. The lagoonal Mollusca, Cerastoderma glaucum and Cerithium vulgatum, typical of quiet environments (Picard, 1965), mixed with marine taxa could also be transported. The percentage of the Mollusk fragment (50%) and the number of the quartz grains are the most important of the core. • Zone IV (from 20 to the top), is characterized by the absence of macro and microfauna. 4.3.2. Correspondence Analysis (CA) 4.3.2.1. Core CD2. Correspondence Analysis applied to data, ostracod absolute abundances and sampling levels, reveals on the axis 1-axis 2 factorial plans with the maximum of inertia (62%) three groups (Fig. 9). The first group G1, positioned on the negative pole of the axis F1, contains coastal taxa such as Cytherois fisheri, Basslerites berchoni, Aurila convexa and A. prasina, the brackish species Loxoconcha elliptica and the lagoonal species Leptocythere levis. This group corresponds to an open lagoon environment, represented by the unit U2 (zone I). The second group G2 is composed of the coastal species Loxoconcha rhomboidea and Aurila woodwardii, the marine species Semicytherura sulcata, Carinocythereis carinata, Hemicytherura diaforei and Callistocythere discrepeans and the lagoonal species Leptocythere fabaeformis, Leptocythere sp., Xestoleberis dispar and X. aurantia. This group corresponds to the units U3 and U4 (zone II) and it represents an open lagoonal environment subjected to estuarian influences. The last group G3, situated on the positive pole of axis F1, contains as variables: the brackish species Cyprideis torosa, the marine species Neocytherideis subulata, N. fasciata, N. faveolata, N. subspiralis, Semicytherura incongruens, Cytherelloidea sordida, Cytheretta adriatica, C. vulgata, Hiltermannicythere rubra, H. emaciata, Triebelina raripila and Paracytheridea depressa, and the coastal species Urocythereis oblonga, U. favosa, Cushmanidea elongata and Loxoconcha parallela. This group, corresponding to the units U5 and U6 (zones III and IV), indicates a widely open lagoon environment. Therefore, the axis F1 shows an evolution from an open lagoonal environment (group G1), on its negative pole, towards a widely open lagoon (group G3), on its positive pole, passing by intermediate open lagoon subjected to estuarian influences (group G2). The axis F1 behaves as the energy factor. 4.3.2.2. Core C2. Species and sample representation on the factorial plan (F1 x F2) (Fig. 9) shows clouds of points representing a characteristic parabolic form which shows an environmental gradient, connected to the energy of the environment. Three groups can be distinguished. The first group G1 is placed on the negative pole of the axis F1 and on the positive pole of the axis F2, and contains marine and

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coastal species Leptocythere fabaeformis, Semicytherura sella, Xestoleberis dispar, Urocythereis oblonga, Paracytheridea depressa, Carinocythereis carinata, Semicytherura incongruens, Basselerites berchoni and Callistocythere discripans. This group corresponds to a marine environment, represented by the bottom part of the unit U1 (zone IA). The second group G2 is located on the negative poles of the axes F1 and F2 and contains variables as Xestoleberis aurantia (lagoonal), Loxoconcha elliptica (brackish) and the coastal species Aurila convexa, Loxoconcha rhomboidea, Neocytherideis faveolata and N. fasciata. It is representative of an open lagoon environment, corresponding to the upper part of U1 and the lower part of U2 (zone IB). Group G3 is situated on the positive pole of axis F1 and includes the species Cyprideis torosa (brackish), Leptocythere pellucida (lagoonal) and the coastal taxa (Loxoconcha parallela, Cushmanidea elongata and Cytheretta adriatica). It evidences a brackish estuarian lagoon and corresponds to the upper part of the unit U2 and the unit U3 (zones II and III). Indeed, on the axis F1, there is a clear opposition between group G1, characterizing a marine environment, with species belonging to the zone IA, and group G3, characterizing a lagoonal-brackish environment of high energy, where lived laguno-brackish species. Group G3 shows the micropalaeontological similarity of the zones II and III, which was proved in the micropalaeontologic descriptive study. Both groups G1 and G3, constituting branches of the parabola, are separated in the center by an intermediate group G2. The latter contains the variables of the zone IB which are characteristic of an open lagoon. Therefore, the axis F1, which acts as the factor energy of the environment, shows an evolution from a marine environment on its negative pole towards a laguno-brackish environment in high energy, on its positive pole. The transition between the two environments is made up by an intermediate lagoonal environment, subjected to coastal influences. 5. Palaeoenvironnemental evolution determined from the ostracod assemblages and the diversity index Five phases may be distinguished in the Late Holocene evolution of the Sebkha of Dreîaa and described below (Fig. 10): • Phase 1 (>cal. 6471–6874 yr BP). The subsurface sediments of the Sebkha of Dreîaa record a transgressive episode, with the deposition of silty sands overlying the Holocene continental azoic clays. This phase is marked by the important species richness and the weak density of ostracods. Sediments, characterized by the dominance of marine ostracods (50%), reveal an open lagoon and indicate the importance of the transgressive episode recorded in the Sebkha of ElGuettiate (Zaïbi et al., 2011a). The impoverishment of the lagoonal assemblage, represented essentially by Xestoleberis aurantia, and marine one is relieved by the enrichment of the brackish species Loxoconcha elliptica, showing thereby high water energy (Ruiz et al., 1996) and estuarine environments (Pascual and Carbonel, 1992). This evolution indicates the installation of an open lagoonal environment progressively subjected to estuarian influences. During the Holocene times,

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Fig. 9. Correspondence Analysis. Projection of paleontological data, samples and number of quartz grains in the 1 × 2 factorial plane (C2 (gray groups) and CD2 cores). Cytherois fisheri (CF), Basslerites berchoni (BB), Aurila convexa (AC), A. prasina (AP), A. woodwardii (AW), Loxoconcha elliptica (LE), L. rhomboidea (LR), L. parallela (LP), Leptocythere levis (LLV), L. fabaeformis (LF), L. pellucida (LPE), Leptocythere sp. (Lsp), Semicytherura sulcata (Ssul), S. sella (SSL), S. incongruens (SIN), Carinocythereis carinata (CCR), Hemicytherura diaforei (HD), Callistocythere discrepeans (CD), Xestoleberis dispar (XD), X. aurantia (XA), Cyprideis torosa (CT), Neocytherideis subulata (Nubl), N. fasciata (NF), N. faveolata (NVT), N. subspiralis (Nsb), Cytherelloidea sordida (Csord), Cytheretta adriatica (CAD), C. vulgata (Cul), Hiltermannicythere rubra (HR), H. emaciata (HE), Triebelina raripila (TR), Paracytheridea depressa (PD), Urocythereis oblonga (UO), U. favosa (UF), Cushmanidea elongata (CE), quartz grains abundance (DET).

the global sea level rises about 30 m between 9000 and 6000 yr BP (Kidson, 1986; Fairbanks, 1989). This transgressive phase has been observed in several sites of Great Britain between 8500–6500 yr BP (Spencer et al., 1998); 7000–4500 yr BP (Huddart et al., 1999), and 7000–5500 yr BP (Devoy, 1979). It has been evidenced along the Algarve Coast (Portugal) between cal. 9000-7000 yr BP (Teixeira et al., 2005); along the Lebanese coast towards 7800 yr BP, in France (Provansal et al., 1998), where it reaches its maximum towards 6000 yr BP and in the South of Spain, where a fast rise of the marine level until cal. 6500 yr BP is quoted by Zazo et al. (2008). • Phase 2 (cal. 6471–6874 yr BP). This phase is characterized by an increase of the density and the species richness of ostracods and the development of Gastropoda and Bivalvia. Phase 2 is also marked by the appearance of euryhaline and eurytherm species Cyprideis torosa and by the relative abundance of the three assemblages (brackish, lagoonal and marine) which become comparable denoting an open lagoon environment subjected to estuarian influences. A phase of humidity increase and an improvement of environmental conditions were detected by different studies in several locations. Ouda et al. (1998), Zarai (2006) and Amami (2010) proved the occurrence of humid period in Central Tunisia during the periods (cal. 7500–7200 yr BP), (cal. 7000–10,000 yr B.P) and (cal. 8000–7500 yr BP) respectively. Furthermore Ritchie et al. (1985) and Ritchie and Haynes (1987) showed evidence of humid phases in the Eastern part of the Sahara between 1,0000–9500 and 7000–6000 yr B.P. Essallemi et al. (2007) detected four humid phases 9500 yr; 8500–7000 yr; 5500 and 4700 yr BP in the Siculo - Tunisian Strait. All of these findings support our interpretation for the cores of Sebkha of Dreîaa evidencing a humid period at cal. 6471–6874 yr BP.

• Phase 3 (cal. 3350–3752 yr BP). The important proportion of the marine assemblage, the reduced percentage of the brackish taxa (10%) and the high values of the diversity indices all imply a marine environment deduced from the core C2. This environment is subjected to an important pelagic influence evidenced by the enrichment of marine ostracods. It evolves, gradually, towards the closure illustrated by the decrease of species richness, the reduction in the richness of marine ostracods and the ostracods density. The values of H and E indices are reduced, implying less structured populations. During this phase, the number of the quartz grains decrease. The correlation between the reduction of the abundance of the coastal ostracods and the decrease of the number of the quartz grains indicate their probable marine origin. Littoral drift currents would have transported the quartz grains and built sand spits. It is worthwhile to mention the presence of several fossilized sandy spits in Sebkha of Dreîaa, which testifies the occurrence of coastal drift currents during the Late Holocene. Concerning the morphology of the Tunisian coastline, the development, during the Late Holocene, of island gates and sandy spit inducing the genesis of lagoonal environments has been investigated by numerous authors: Mansouri (1979) showed evidence of closure of the lake Ghar El Melah and the Sebkha of Ariana towards 6000 years; Lakhdar et al. (2006) dated the genesis of sediments rich in organic matter in the Sebkha of Boujmel between cal. 6800 and 4000 yr BP and Masmoudi (2005) characterized the individualization of the lagoon of Bin El Oudiane of Djerba island towards cal. 5000 yr BP. All these data allow considering the presence of marine environments rich in quartz grains during the Holocene times. These quartz grains would be transported by the coastal drift currents, permitting the genesis of coastal

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Fig. 10. Late Holocene evolution of the sebkha of Dreîaa deduced from the ostracod assemblages. NS (species number), index) and E (equitability index).

sandy spits, which are, to-day, visible inside the Sebkha of Dreîaa. The different phases of the closure of the inshore fringe, of the studied cores site, would be in relation with the action of coastal drift currents causing the genesis of sandy spits and the closure of the environment. In the same period, the episodes of the genesis of the littoral barrier are known in Spain, where two coastal progradation phases are recognized after the Holocene transgressive period towards 6500 yr BP (Zazo et al., 1994). These phases include four systems of offshore bare towards cal. 6500–4400, 4200–2550, 2300–800 and 500 until the present. They are limited by episodes of erosion towards cal. 4500–4200, 2600–2300

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(individuals number), H (Shannon

and 1100–1000 yr BP (Zazo et al., 1994; Dabrio et al., 2000). • Phase 4 (cal. 2739-3207 yr BP). The development of coastal and marine species, contained in the coarse sands of the core CD2, indicate a lagoon even more opened towards the sea. This evolution could be a consequence of storms or a marine transgression. However, the progressive change in the microfauna, also revealed by sediments present in the core C2 (unit U2), would rather show a marine transgression. These coarse sediments are characterized by coastal ostracods, indicating a lagoon environment opened to coastal influences. After this transgressive episode, the southern part of the Sebkha of

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Dreîaa, where the core CD2 is located, emerged. The emersion is evidenced by the progressive dominance of Cyprideis torosa, associated with the impoverishment of marine and coastal species and the reduction of the density and species richness. • Phase 5 (cal. 515–777 yr BP). The environment in the site of the core C2, in the northern part of the Sebkha of Dreîaa, is marked by the deposition of high energy sediments following an extreme climatic event. This event is revealed by the reappearance of marine ostracods and the development of coastal and lagoonal species. The presence, in coarse sands, of the brackish species C. torosa, which generally characterizes the low energy environments rich in organic matter, testifies its probable allochtonous character. Marine bivalvia and gastropods fragments, associated with lagoonal ostracods, corroborate the high energy deposits (probably washover). This extreme event, preceded by the genesis of sandy spits, occurred at the limit between the medieval climatic optimum and the Little Ice Age. It may correspond to one of the storms of the Little Ice Age. These events are 10 times more frequent than during the medieval climatic optimum (Sabatier et al., 2008). Marquer et al. (2008) identified four humidity/dry cycles between cal. 1400 and 650 yr BP in the Sebkha of El Mhabeul (Gulf of Gabes). The three last cycles are characterized by the strongest hydrologic instability, constituting the upper part of the medieval climatic optimum. One of these three cycles could correspond to the enrichment of the lagoon of Dreîaa in brackish ostracods. Humid pulsations toward cal. 1100 yr BP have been also mentioned by Zarai (2006) in the area of Kasserine (Central Tunisia), which allows the development of paleosoils, rich in organic matter and Gastropods such as Helicidea. By means of the palynologic study of the Gulf of Gabes cores, Brun (1992) showed the extension of Artemisia and Quercus regression and the dominance of Olea over Pistacia between the end of the Roman time and the tenth century. 6. Conclusions A Late Holocene evolution of the Sebkha of Dreîaa is established from the micropalaeontological and sedimentological study of the sediments present in two collected drill cores. This study allowed the reconstruction of the palaeoenvironmental changes during the Holocene times. Four typical assemblages of ostracods (marine, coastal, lagoonal and estuarine/brackish) have been distinguished. Based on such evidence, the microfauna present in the subsurface sediment of the Sebkha of Dreîaa are linked to two transgressive phases (> cal. 6471–6874 yr BP and cal. 2739–3207 yr BP in ages). After the last transgressive episode, the sediments of the core CD2 reveal the dominance of brackish species, indicating there by the emersion of the southern part of the Sebkha of Dreîaa. Sedimentation continued during this period in the northern part of the sebkha (core C2). The installation of the brackish/estuarine lagoon environments is related to the building of sand spits. This lagoonal sedimentation was interrupted by high-energy events, at cal. 515–777 yr BP, causing the breakthrough of the sandy spits.

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