Ostracods distribution in the subsurface sediments of El-Guettiate Sebkha (Skhira, gulf of Gabes). Interest for Holocene palaeoenvironmental reconstruction

A quantitative and qualitative study of the ostracods from the SG1 and BSC2 cores of El-Guettiate Sebkha of Skhira (southeastern Tunisia) has allowed the identification of different associations and the reconstruction of the Holocene palaeoenvironments. Ostracods, rich in species, provide three associations: (i) a marine association containing Aurila prasina BARBIETO-GONZALEZ, 1971, Cushmanidea elongata (BRADY, 1868), Urocythereis oblonga (BRADY, 1866), U. favosa (ROEMER, 1838) (coastal marine), Basselerites berchoni (BRADY, 1869), Semicytherura incongruens (G.W. MÜLLER, 1894), S. paradoxa (G.W. MÜLLER, 1894), S. sella (SARS, 1866), S. ruggierii (PUCCI, 1955), Carinocythereis carinata (ROEMER, 1838) and Paracytheridea depressa (G.W. MÜLLER, 1894) (open marine); (ii) lagoonal association, of variable salinity, comprising Xestoleberis aurantia (BAIRD, 1838), Leptocythere fabaeformis (G.W. MÜLLER, 1894) and Cytherois fischeri (SARS, 1866); (iii) brackish estuarine association subject to estuarine influence as indicated by Cyprideis torosa (JONES, 1850) and Loxoconcha elliptica BRADY, 1868. The ostracods from El-Guettiate Sebkha allow us to recognize discrete palaeoenvironments that can be grouped in three phases: i) the first (6595 ± 120 - 6055 ± 30 years BP) begins with an open lagoon rich in marine ostracods followed by an estuarine lagoon (towards 6595 years BP) characterized by the richness of brackish ostracods and high energy. In the last lagoonal environment (installed towards 6055 years BP), brackish species are dominant and associated with lagoonal taxa; ii) the second phase (6055 ± 30 years BP - 5150 ± 30 years BP) is an estuarine lagoon, subject to a large marine influence in which brackish marine and lagoonal species are present in the same percentages. In this environment, littoral drift currents were responsible for the genesis of sand spits, the termination of environments and the dominance of brackish-water ostracods; iii) The last phase (5150 ± 50 years BP - recent) represents a restricted lagoon (about 5150 years BP) and finally a brackish lagoon evolved towards the sebkha environment under investigation here. This phase includes an unstable period with washover deposits resulting from an extreme climatic event.L'étude quantitative et qualitative des ostracodes des carottes SG1 et BSC2, prélevées au niveau de la Sebkha El-Guettiate de Skhira (Sud - Est Tunisien), nous a permis de reconnaître les trois associations suivantes : (1) association de milieu marin s.l. composée de Aurila prasina BARBIETO-GON-ZALEZ, 1971, Cushmanidea elongata (BRADY, 1868), Urocythereis oblonga (BRADY, 1866), U. favosa (ROEMER, 1838) (marin côtier), Basselerites berchoni (BRADY, 1869), Semicytherura incongruens (G.W. MÜLLER, 1894), S. paradoxa (G.W. MÜLLER, 1894), S. sella (SARS, 1866), S. ruggierii (PUCCI, 1955), Carinocythereis carinata (ROEMER, 1838) et Paracytheridea depressa (G.W. MÜLLER, 1894) (marin ouvert) ; (2) association de milieu lagunaire, à salinité variable, constituée, de Xestoleberis aurantia (BAIRD, 1838), Leptocythere fabaeformis (G.W. MÜLLER, 1894) et Cytherois fischeri (SARS, 1866), (3) association de milieu saumâtre estuarien où se développent Cyprideis torosa (JONES, 1850) et Loxocon-cha elliptica BRADY, 1868. Ces associations révèlent les caractéristiques des paléo-environnements qui se sont succédés, au cours de l'Holocène supérieur, à l'emplacement de la Sebkha El-Guettiate. Trois phases peuvent être reconnues : i) la première phase à caractère lagunaire estuarien dominant (6595 ± 120 à 6055 ± 30 ans B.P.) comprend un environnement lagunaire ouvert riche en ostracodes marins dominants évoluant brutalement vers un environnement lagunaire estuarien. Ce dernier est caractérisé par un évènement de haute énergie, vers 6595 ± 120 ans B.P., révélé par des apports détritiques riches en galets, synchrones à l'enrichissement des ostracodes saumâtres. Ensuite, une lagune s'instal-le vers 6055 ± 30 ans B.P. permettant le développement des ostracodes principalement saumâtres dominants mais aussi lagunaires. ii) la phase suivante (6055 ± 120 à 5150 ± 50 ans B.P.) se distingue par la réapparition des taxons marins associés aux espèces lagunaires et saumâtres traduisant une lagune estuarienne largement ouverte soumise à l'influence des courants de dérive littorale. Ces cou-rants, en édifiant des cordons littoraux, ont entraîné la fermeture du milieu et la fin de la dominance des ostracodes saumâtres. iii) la dernière phase (5150 ± 50 ans B.P. à l'actuel) comprend un milieu caractérisé par la richesse des taxons lagunaires évoluant vers un milieu laguno-saumâtre recevant le dépôt de washover suite à un événement climatique extrême. La lagune, de plus en plus fermée, évo-luera progressivement vers la sebkha actuelle

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

Impacts of Mayan land use on Laguna Tusp an watershed (Pet en, Guatemala) as seen through clay and ostracode analysis

International audienceMost of the cities built by the Mayas in the Pet en area, in the Central Yucat an Peninsula, were abandoned 1200 to 1000 years ago. The phenomenon is sometimes un-appropriately called " the collapse of the Maya civilization ". Its main causes are still debated, ranging from climatic according to the occurrence of severe or modest droughts, to societal in the form of environmental mismanagement of the environment. In both processes, it is inferred that stress triggered the formation in many Pet en lake sediments of erosional clay deposits, known as 'Maya clays'. This work presents a high resolution, multi-proxy study of 'Maya clays' in lacustrine sediments from Laguna Tusp an, near the archaeological site of La Joyanca. Micropaleontological (ostracodes), mineral-ogical (clay minerals) and geochemical (bulk elemental composition and stable isotopes in organic carbon) records reveal three different phases of soil erosion throughout the last 5300 years. The oldest phase from 5281 to 2998 cal yr BP (i.e. 3331 e 1048 BC) is characterized by successive natural and moderate soil erosion deposits which follow climatic variations recorded in the American tropical belt. The time interval between 2998 and 1281 cal yr BP (i.e. 1048 BC and AD 661) contains four distinct erosional layers which, according to clay mineralogy, are indicative of both increased erosion of the regolith and strong soil loss. The most recent, also the most massive, deposit of Maya clay ends around 1281 cal yr BP (AD 661), that is some 200 years before the so-called 'Maya collapse' in the Pet en area. Recent archeological fieldwork studies indicate that a population mobility took place into the city of La Joyanca from its hinterland by the early Late Classic Period (ca. AD 600), that is, at the end or just after this erosion episode, and well before the occurrence of the Terminal Classic-Postclassic (AD 800e1250) drastic climatic changes. Shifts in environmental management by the local society and timing of urbanization may explain environmental changes better than droughts per se

From Naples 1963 to Rome 2013 - A brief review of how the international research group on Ostracoda (IRGO) developed as a social communication system

The 1st International Symposiumon Ostracoda (ISO) was held in Naples (1963). The philosophy behind this symposiumand the logical outcome of what is nowknown as the International Research Group on Ostracoda (IRGO) are here reviewed, namely ostracodology over the last 50 years is sociologically analysed. Three different and important historic moments for the scientific achievements of this domain are recognised. The first one, between about 1963 and 1983, is related to applied research for the oil industry aswell as to the great interest in the better description of the marine environment by both zoologists and palaeontologists. Another important aspect during this period was thework by researchers dealing with Palaeozoic ostracods,who had their own discussion group, IRGPO. Gradually, the merger of this latter group with those dealing with post-Palaeozoic ostracods at various meetings improved the communication between the two groups of specialists. A second period was approximately delineated between 1983 and 2003. During this time-slice, more emphasis was addressed to environmental research with topics such as the study of global events and long-term climate change. Ostracodologists profited also from the research "politics" within national and international programmes. Large international research teams emerged using new research methods. During the third period (2003-2013), communication and collaborative research reached a global dimension. Amongst the topics of research we cite the reconstruction of palaeoclimate using transfer functions, the building of large datasets of ostracod distributions for regional and intercontinental studies, and the implementation of actions that should lead to taxonomic harmonisation. Projects within which molecular biological techniques are routinely used, combined with sophisticated morphological information, expanded now in their importance. The documentation of the ostracod description improved through new techniques to visualise morphological details, which stimulated also communication between ostracodologists. Efforts of making available ostracod information through newsletters and electronic media are evoked

Le tuf de Caours (Somme, France) : mise en évidence d'une séquence eemienne et d'un site paléolithique associé

Les nouvelles investigations menées sur les tufs du bassin de la Somme dans le cadre du programme SITEP (CNRS) ont permis de mettre en évidence une formation tufacée reposant sur une nappe alluviale en position de très basse terrasse à Caours (Scardon). La séquence de tufs est séparée de la nappe alluviale périglaciaire sous-jacente par des limons fluviatiles calcaires se terminant par un petit sol de marais coiffé par un liseré de tourbe compactée. La formation tufacée proprement dite comporte essentiellement des faciès palustres à nombreux restes végétaux encroûtés en place et des concrétions travertineuses de type stromatolithes. En direction de la vallée actuelle l’ensemble passe rapidement à des faciès nettement fluviatiles à oncolithes structurés en grandes lentilles à stratifications obliques. La séquence de tuf ainsi que les limons fluviatiles ont livré une abondante faune malacologique qui a permis de décrire une évolution climatique contemporaine des phases initiales d’un interglaciaire, suivi d’un optimum climatique, puis d’une phase de réouverture du milieu exprimant le déclin des conditions tempérées. La base du tuf comporte plusieurs horizons organiques qui ont livré des restes de grands mammifères et de rongeurs contemporains de l’optimum interglaciaire déterminé par les assemblages malacologiques. Dans ces horizons, plusieurs niveaux du Paléolithique moyen ont été découverts en place, en association avec des restes de grande faune interglaciaire fortement fracturés par l’action de l’homme et présentant des traces de découpe. Compte tenu de sa position dans le système de la Somme, des datations par U/Th obtenues sur le tuf (moyenne : ± 120 ka BP) et des conclusions des études bioclimatiques, la séquence de Caours représente le premier témoin de l’interglaciaire Eemien en contexte fluviatile dans le bassin de la Somme. Enfin, les niveaux archéologiques découverts à Caours constituent un exemple unique d’occupation humaine contemporaine du dernier interglaciaire dans la France septentrionale.The new investigation lead on the tufa deposits of the River Somme basin, within the SITEP (CNRS) Program, have allowed the discovery of a new tufa sequence overlying an alluvial formation in low terrace position at Caours (Scardon valley). The tufa sequence is separated from the underlying periglacial alluvial gravels by fluvial calcareous silts overlain by a thin marshy soil and a thin peat layer. The tufa formation is mainly composed by paludal (marshy) soil facies showing numerous in situ incrusted vegetal remains and travertine concretions (stromatoliths). Downslope, looking to the present day valley the whole tufa formation quickly evolves toward typical fluvial facies including oncolithic sands and large scale cross beddings. The tufa sequence and the fluvial silts have provided abundant malacological faunas that have allowed describing a climatic evolution contemporaneous of the initial phases of an interglacial, followed by a climatic optimum and then a decline of temperate conditions. The lower part of the tufa includes several organic horizons that have yielded numerous large mammals and rodent remains contemporaneous of the interglacial optimum previously evidenced by malacology. Within these horizons, several Palaeolithic layers have been discovered in situ in association with interglacial large mammal remains showing evidences of human operation (systematic breaking of long bones and cut marks). Taking into account its relative position within the Somme terraces system, the U/Th ages (average ± 120 ka BP) and the results of the various bioclimatic studies, the Caours sequence represents the first record of the Eemian interglacial in the Somme basin. In addition, the archaeological levels discovered at Caours are a unique example of Human occupation during the Last Interglacial in Northern France

La longue séquence de Marchésieux: reconstitution de paléoenvironnements marins durant le premier cycle glaciaire de l'hémisphère nord

International audienceA l'échelle des cinq derniers millions d'années, les enregistrements paléoclimatiques restitués par les sédiments océaniques illustrent le contrôle des paramètres orbitaux (précession, obliquité et excentricité) sur le climat global. La tendance au refroidissement qui accompagne la fin du Néogène est ainsi marquée, vers -2,4 Ma, par le développement rapide des calottes de glace de l'hémisphère nord (Shackleton et al., 1984). Alors que l'histoire du climat global restituée par les enregistrements océaniques est sans cesse précisée, les données concernant l'évolution des environnements continentaux et côtiers au cours de ces changements restent essentiellement fragmentaires. Le forage effectué à Marchésieux (Manche; Normandie) a permis de réaliser une étude pluridisciplinaire de la signature de ce premier cycle glaciaire de l'hémisphère nord (Prétiglien). Les premiers résultats stratigraphiques, paléoenvironnementaux (eustatisme, température, paléobathymétrie) sont présentés

Le tuf calcaire de La Celle-sur-Seine (Seine et Marne) : nouvelles données sur un site clé du stade 11 dans le Nord de la France

De nouvelles études morphostratigraphiques et biostratigraphiques ainsi que des datations ont été entreprises sur le célèbre dépôt de tuf calcaire de La Celle-sur-Seine afin d'obtenir une reconstruction détaillée des successions paléoenvironnementales et climatiques enregistrées dans cette formation interglaciaire du Pléistocène moyen. L'approche développée a privilégié l'aspect pluridisciplinaire afin d'exploiter la richesse paléontologique du gisement, de mieux appréhender ses caractères morphostratigraphiques et de préciser sa position chronologique. Les premiers résultats obtenus à partir des études malacologiques montrent que le dépôt, composé de tufs et de niveaux limono-argileux, s'est construit en progradant le long du versant. Ainsi l'épaisseur totale de la formation atteint près de 9 mètres. Les malacofaunes permettent de reconstituer une évolution paléoenvironnementale correspondant au début puis à l'optimum d'une phase climatique interglaciaire. Les niveaux sommitaux sont, eux, caractérisés par un net recul de la couverture forestière.La création de nouveaux profils stratigraphiques a permis la découverte dans un niveau de limon gris tufacé d'une abondante faune de mammifères accompagnée par quelques silex taillés. Cet horizon appartient à la malacozone la plus riche en taxons thermophiles qui est interprétée comme la phase optimum de l'Interglaciaire. La signification environnementale et climatique du cortège mammalogique, qui comprend en particulier de l'hippopotame et du macaque, corrobore les résultats malacologiques. L'industrie lithique est de type bifaciale en cohérence avec les récoltes de bifaces acheuléens effectuées à la fin du 19ème siècle. La composition du cortège de mollusques permet de rapprocher la faune de La Celle de "l'assemblage à Lyrodiscus", caractéristique des tufs du stade 11 dans le nord-ouest de l'Europe. Cette attribution chronologique est confirmée par les premières mesures radiométriques obtenus par U-Th sur des échantillons de tuf induré et par RPE/U-Th sur une dent de cheval. Enfin la révision des collections paléontologiques d'empreintes foliaires permet, en conjonction avec les données malacologiques, d'initier la discussion sur l'interprétation paléoclimatique du biome à Lyrodiscus. L'association floristique et faunique de La Celle qui comprend quelques taxons arboréens méditerranéens et nombre de gastéropodes éteints ou de répartition actuelle centre-européenne et océanique rend compte d'un environnement tempéré de forêt humide. La présence de ces espèces allochtones dans le nord de la France est mise en parallèle avec la durée exceptionnellement longue du Stade Isotopique Marin (SIM) 11 et de faibles amplitudes de températures saisonnières, qui auraient permis à plusieurs taxons méridionaux de coloniser cette zone septentrionale, plutôt qu'avec un climat plus tempéré que l'actuel.New morpho and biostratigraphical studies have been undertaken at the famous calcareous tufa deposit of La Celle-sur-Seine in order to obtain a detailed paleoenvironmental and climatic reconstruction from this Middle Pleistocene Interglacial formation. Multiproxy approach allows to study palaeontological contain of the tufa as well as its morphostratigraphical aspects and chronological allocation. First results from malacological study show that the tufa edificated downwards the slope reaching a total thickness of 9 metres high. Moreover molluscs show palaeoenvironmental succession corresponding to the early part of an Interglacial period followed by an optimum phase. Finally, at the top of the deposit faunas clearly indicate decrease of forest biotopes.Excavation of new profiles have allowed to discover a grey silty tufa horizon (lgt) yielding abundant mammal remains and a few lithic artefacts. This level is part of the molluscs zone 3 corresponding to the climatic optimum. Palaeoenvironmental and climatic conditions inferred from mammal assemblage (including Hippopotamus and Macaca) strengthened the malacological interpretation. The lithic industry is identified as Acheulean and appears in agreement with earliest handaxes discoveries during the late 19th century.La Celle malacological fauna belongs to the well-known MIS 11 "Lyrodiscus assemblage" in north-west Europe. This chronological allocation is confirmed by first U-series radiometric measures. Finally reappraisal of leaf print collections allows together with malacological data to discuss palaeoclimatic interpretation of the "Lyrodiscus biome". Flora and fauna from La Celle include a few mediterranean trees and several extinct gastropods together with few molluscs of central Europe modern range, allowing to describe a forest and humid lanscape. Presence of these peculiar species in northern France is thought to be related with the long length of MIS 11 and lower seasonal temperature contrast, that have allowed northern colonisation by southern species, rather than with occurrence of temperature higher than today

Evaluating susceptibility of karst dolines (sinkholes) for collapse in Sango, Tennessee, USA

Dolines or sinkholes are earth depressions that develop in soluble rocks complexes such as limestone, dolomite, gypsum, anhydrite, and halite; dolines appear in a variety of shapes from nearly circular to complex structures with highly curved perimeters. The occurrence of dolines in the studied karst area is not random; they are the results of geomorphic, hydrologic and chemical processes that have caused partial subsidence, even total collapse of the land surface, when voids and caves are present in the bedrock and the regolith arch overbridging these voids is unstable. In the study area, the majority of collapses occur in the regolith (bedrock cover) that bridges voids in the bedrock. Because these collapsing dolines can damage property and cause even the loss of lives, there is a need to develop methods for evaluating karst hazards; such methods can be used by planners and practitioners for urban and economic development, especially in regions with a growing population. The purpose of this project is threefold: 1) to develop a karst feature database, 2) to investigate critical indicators associated with doline collapse, and 3) to design a doline susceptibility model for potential doline collapse based on external morphometric data. The study revealed the presence of short range spatial dependence in the distribution of the dolines’ morphometric parameters such as circularity, geographic orientation of the main doline axes and the length-to-width doline ratios; therefore, geostatistics can be used to spatially evaluate the susceptibility of the karst area for doline collapse using the probability of occurrence of these critical parameters. The partial susceptibility estimates were combined into final spatial probabilities enabling the identification of areas where undetected dolines may cause significant hazards

Le polymorphisme chez Lindisfarnia guttata (Ostracoda) : repérage des phénomènes hydrologiques sur le plateau continental marocain

Carbonel Perre, Hamoudi Mohamed. Le polymorphisme chez Lindisfarnia guttata (Ostracoda) : repérage des phénomènes hydrologiques sur le plateau continental marocain. In: Géologie Méditerranéenne. Tome 14, numéro 1, 1987. Xe colloque africain de micropaléontologie. A – Le Cenozoique. Rabat – Université Mohamed V – Maroc. 29 septembre-1er octobre 1987. p. 81