External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba).

45 pagesInternational audienceActive, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O2 and HS− profiles. High rates of O2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings

Late glacial to deglacial variation of coralgal assemblages in the Great Barrier Reef, Australia

Integrated Ocean Drilling Program (IODP) Expedition 325 cored submerged reefs along the shelf edge of the Great Barrier Reef (GBR) to study sea-level and environmental changes and their impacts on reef communities and reef growth since the Last Glacial Maximum (LGM). Previous work defined five reef sequences (Reef 1–5) that span the last 30,000 years. Here we examined the variation in coralgal assemblages and their paleoenvironmental settings in late glacial to deglacial sequences from 23 holes cored seaward of the modern GBR in water depths from 46 to 131 m along four transects at three localities: Hydrographers Passage (HYD-01C and HYD-02A), Noggin Pass (NOG–01B), and Ribbon Reef (RIB-02A). We identified three coralline algal assemblages and eight coral assemblages indicating a broad range of reef settings from the shallow reef crest (0–5 m) to the deep forereef slope (>20 m). We document in detail for the first time the distribution and composition of reef communities that grew in the GBR during the LGM from 22,000–19,000 years ago. They included coral taxa that are major reef builders today: Isopora, Acropora gr. humilis, Dipsastraea gr. pallida, Porites, and Montipora. Prior to the fall in sea level to the maximum extent of the LGM, late glacial reef communities developed more proximally (landward) to the modern GBR along the shelf edge. Their distribution and composition reflect influences of the older Pleistocene basement depth and possible terrigenous sediment inputs. Post-LGM deglacial reef growth was vigorous in proximal sites and characterized by the accretion of a very shallow high-energy coralgal assemblage composed of medium to robustly branching Acropora, including A. gr. humilis, and thick algal crusts of Porolithon gr. onkodes associated with vermetid gastropods. More distally, reef growth was variably impacted by terrigenous input following deglacial reflooding of antecedent reef terraces. The coralgal succession and sedimentary facies in Noggin Pass indicate that an early drowning trend was linked to increased turbidity that was likely controlled by shelf morphology (narrow shelf, steep slope) and/or proximity to a paleo-river mouth. The deglacial succession in Ribbon Reef lacks typical shallow-water indicators, which may reflect influences of the particularly steep slope of the northern GBR shelf edge on reef zonation. A major sea-level jump at the onset of the Younger Dryas displaced reef habitats further upslope, forming a barrier reef system mainly composed of robustly branching acroporids distinct from the more distal sites. Our results highlight the importance of sedimentation and shelf morphology in addition to relative sea-level changes in controlling variations in reef community over centennial to millennial timescales. © 2019 Elsevier B.V.Australian Research Council-DP109400

Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites.

12 pagesInternational audienceMicrobialites are widespread in modern and fossil hypersaline environments, where they provide a unique sedimentary archive. Authigenic mineral precipitation in modern microbialites results from a complex interplay between microbial metabolisms, organic matrices and environmental parameters. Here, we combined mineralogical and microscopic analyses with measurements of metabolic activity in order to characterise the mineralisation of microbial mats forming microbialites in the Great Salt Lake (Utah, USA). Our results show that the mineralisation process takes place in three steps progressing along geochemical gradients produced through microbial activity. First, a poorly crystallized Mg-Si phase precipitates on alveolar extracellular organic matrix due to a rise of the pH in the zone of active oxygenic photosynthesis. Second, aragonite patches nucleate in close proximity to sulfate reduction hotspots, as a result of the degradation of cyanobacteria and extracellular organic matrix mediated by, among others, sulfate reducing bacteria. A final step consists of partial replacement of aragonite by dolomite, possibly in neutral to slightly acidic porewater. This might occur due to dissolution-precipitation reactions when the most recalcitrant part of the organic matrix is degraded. The mineralisation pathways proposed here provide pivotal insight for the interpretation of microbial processes in past hypersaline environments

Evolution des plates-formes carbonatées pendant la crise de salinité messinienne (de la déformation des évaporites aux communautés microbialithiques (sud-est de l Espagne))

La crise de salinité messinienne (CSM) est un évènement majeur de l histoire méditerranéenne, marqué par le dépôt de plusieurs milliers de mètres d évaporites dans le centre du bassin méditerranéen. Cette étude pluridisciplinaire (sédimentologie, stratigraphie, géochimie, etc.) explore l enregistrement de cette crise par les plates-formes carbonatées des bassins marginaux ouest méditerranéens. Les deux unités carbonatées pré-CSM unité à heterozoan puis unité récifale présentent des bioconstructions à coraux, dont la diversité diminue à l approche de la CSM. Les derniers niveaux de l unité récifale sont dominés par l algue verte Halimeda, alors que les coraux ont cessé de se développer. Ce changement de communautés est interprété comme la première conséquence d un refroidissement global, associé à une augmentation de salinité liée à la restriction de la Méditerranée. La seconde conséquence de l isolement tectonique des bassins marginaux est une chute du niveau d eau estimée entre 150 et 200 m. Dans le centre des bassins marginaux, les niveaux à Halimeda passent sans discontinuité majeure aux évaporites autour de 5.96 Ma, alors que les marges sont érodées et karstifiées. L unité carbonatée syn- à post-évaporitique, appelée Terminal Carbonate Complex (TCC), est caractérisée par une grande variété d environnements de dépôt et par une production carbonatée dominée par les ooïdes et les microbialithes. Le TCC montre une première phase à salinité normale caractérisée par une production oobioclastique, attribuée à un ré-ennoiement des marges ouest méditerranéennes pendant une période de haut niveau marin global débutant à 5.57 Ma. Une phase de déformation/dissolution des évaporites syn-TCC se déroule à l échelle du sud-est de l Espagne en parallèle d une nouvelle restriction des bassins marginaux qui sont alors de de vastes lac salés. Une remise en solution par dissolution des évaporites messiniennes et un climat contrasté aride/humide, pourraient avoir induit de fortes variations de salinité entraînant un bloom des microbialithes à la fin du TCC. Ces microbialithes, contenant de potentiels fossiles de bactéries, ont induit une précipitation carbonatée en majorité composée de dolomite dont l origine est encore incertaine. Le Lago Mare, caractérisé par des environnements d eau douce et un climat régional plus humide, succède au TCC. Enfin, le Bassin Méditerranéen est intégralement ré-ennoyé par les eaux Atlantiques au début du Pliocène (5,3 Ma), lors de la rupture du seuil de Gibraltar.The Messinian Salinity Crisis (MSC), characterized by the deposition of thousands-ofmetres thick evaporites in the centre of the Mediterranean Basin, is one of the most important event in the Mediterranean history. A multidisciplinary study (sedimentology, stratigraphy, geochemistry, etc.) has been carried out to explore the recording of this crisis by the carbonate platforms of western mediterranean marginal basins. The two pre-MSC units the Heterozoan Unit, followed by the Reef Unit show the development of coral build-ups displaying a decreasing coral diversity prior to the MSC. The last beds of the Reef Unit are dominated by the green alga Halimeda, while corals have ceased their development. These community changes are interpreted as the first consequence of a global cooling, associated with an increasing salinity linked to the restriction of the Mediterranean. The second consequence of the tectonic isolation of the marginal basins is a water level fall estimated between 150 and 200 m. The Halimeda beds are overlain by the first evaporite deposits without a major discontinuity around 5.96 Ma. The syn- to post-evaporitic carbonate unit, called the Terminal Carbonate Complex (TCC), is characterized by a high variablility of depositional environments and by a carbonate production dominated by microbialites and ooids. The TCC shows a first phase with a close to normal marine salinity dominated by oobioclastic facies, linked to a reflooding of the western mediterranean margins during a global sea level highstand beginning at 5.57 Ma. A syn-TCC deformation/dissolution phase occurs at the South-East Spain scale at the same time as a new restriction of the marginal basins, corresponding to vast salt lakes at this time. Evaporative brines seepage, associated with a contrasted arid/humid climate, could have produced strong salinity variations inducing the microbialite bloom at the end of the TCC. These microbialites, containing potential fossils of bacteria, have mediated the precipitation of carbonate mineral, mostly dolomite, which origin remains undetermined. The TCC is followed by the Lago Mare, depositing in freshwater during a regional humid climate phase. Finally the whole Mediterranean Basin is reflooded by Atlantic seawater at the beginning of the Pliocene (5.3 Ma) due to the opening of the Gibraltar Strait.DIJON-BU Sciences Economie (212312102) / SudocSudocFranceF

Early–middle Jurassic lytoceratid ammonites with constrictions from Morocco: palaeobiogeographical and evolutionary implications.

13 pagesInternational audienceThe ammonite genus Alocolytoceras Hyatt, 1900 is an uncommon lytoceratid with distinctive shell ornament. A set of 58 specimens, recently collected at Amellago in the central High Atlas (Morocco), has enabled us to trace a succession of three species over eight biozones from the Toarcian to the Aalenian. Two specimens from the Lusitanian Basin are added for comparison. Following a review of the genus, based on original specimens and data from the literature, seven species are considered valid. A palaeobiogeographical synthesis of 13 regions demonstrates irregular distribution patterns over time, with a constant presence in the south-west Tethys and an instance of rapid diversification of an endemic fauna in north-west Europe. Our data challenge the conventional view that lytoceratid ammonite evolution was ‘conservative'

The stratigraphic record of the Middle-Late Jurassic Transition events on the Peri-Tethyan carbonate platforms

The Middle and Upper Jurassic peri-Tethyan carbonate platforms host some of the largest hydrocarbon reserves in the world. Over the past decades, extensive research has led to the construction of high-resolution stratigraphic frameworks on the northwestern (e.g., Paris Basin) and southern (e.g., Arabian Platform) Tethyan margins. These studies and others show that although being a period of elevated carbonate production, the Callovian-Oxfordian interval is marked by major changes in carbonate production and platform architecture, stratigraphic gaps and condensed series. These changes have commonly been linked to regional to global climatic and tectonic events associated with the so-called Middle-Late Jurassic Transition (MLJT). The stratigraphy of the northern Tethyan margin (Central Asia) was comparatively less documented. A vast carbonate platform developing on the northern margin of the Amu Darya Basin was recently investigated. A high-resolution stratigraphic framework of the platform was built based on facies analyses, sequence stratigraphy, chemostratigraphy and biostratigraphy. This presentation reviews the key stratigraphic features of the Callovian-Oxfordian carbonate platforms in several areas of the northern, western and southern Tethyan margins. Our comparison shows striking similarities in the stratigraphic records of carbonate platforms which developed at the periphery of the Tethys, several thousands of kilometers apart and reinforce the hypothesis that global-scale events strongly affected the shallow water domain of the Jurassic ocean. The impact of climate, tectonics and major paleoenvironmental events, e.g. the Upper Callovian-Lower Oxfordian cold episode, the sea level fall possibly linked to glacio-eustatism and the tectonic events are finally discussed

Les reptiles et synapsides fossiles de Bourgogne

65 pagesNational audienceL'inventaire actuel des Amniotes (Synapsida et Reptiles) fossiles à partir des collections bourguignonnes est présenté dans l'ordre stratigraphique. Il montre que les Synapsida (Reptiles Mammaliens) sont représentés au Carbonifère et au Permien inférieur par quelques squelettes. Dans le reste du Permien (moyen ou moyen à supérieur) ) puis au Trias, c'est la palichnofaune qui prend le relai avec quelques empreintes de pas permiennes attribuées à des Capthorinomorpha et avec celles beaucoup plus nombreuses du Trias laissées par une faune très variée. Celle-ci comportait le groupe souche des Crurotarsi d'où sont issus les Crocodylomorpha et les Dinosauria via les Dinosauriformes qui ont laissé des petites traces de pied tridactyles, fréquentes dans le Trias moyen. A partir du Trias supérieur (Rhétien), apparaissent des restes osseux d'Ichthyosaures et de Plésiosaures qui vont être rencontrés jusqu'à la base du Crétacé supérieur. Ces Reptiles marins carnivores ont donc évolué pendant près de 100 millions d'années dans la mer bourguignonne en compagnie de Crocodiliens dont on perd la trace à la fi n du Jurassique dans notre région. La mer du Crétacé supérieur dite de la craie est riche en restes osseux de Mosasaures, grands lézards prédateurs qui semblent avoir pris le relai des Ichthyosaures et des Plésiosaures qui n'ont pas été trouvés dans le reste du Crétacé supérieur bourguignon. Quelques Dinosaures herbivores ont été recueillis dans l'Yonne. Les données paléogéographiques de cette période suggèrent la possibilité d'en trouver d'autres dans ce département. Tous les groupes de Reptiles évoqués précédemment disparaissent au cours du Crétacé supérieur et, peut-être, de manière brutale pour les Dinosaures à la limite du Crétacé et du Tertiaire (crise K-T). Mais les Reptiles n'ont pas tous disparu car nombreuses sont les familles de Chelonia, Crocodilia, Lacertilia, , Ophidia qui vont survivre à cette crise et continuer leur évolution pendant le Tertiaire. Par suite de modifi cations paléogéographiques et climatiques, en France et en Bourgogne en particulier, hormis celles de Crocodilia, les autres vont devenir résiduelles. L'herpétofaune moderne est globalement mise en place, il y a une centaine de millénaires (Pléistocène). En Bourgogne, il y a actuellement 5 espèces de couleuvres, 1 de vipère (peut-être 2), 5 de lézards et 1 de tortue dont le suivi est assuré par des spécialistes. Face à certaines activités humaines destructrices, quelques unes de ces espèces sont menacées mais depuis une dizaine d'années plusieurs associations et organismes publiques bourguignons, français et européens ont uni leurs efforts pour que cette herpétofaune soit protégée

Les Amphibiens fossiles de Bourgogne (Temnospondyles et Lissamphibiens).

23 pagesNational audienceL'inventaire actuel des Amphibiens fossiles des collections bourguignonnes montre que les Temnospondyles (Branchiosaures, Eryopidés), Lépospondyles (Nectridiens, Aïstopodes) et Discosauridés du Carbonifère et du Permien sont bien représentés ; ainsi que leurs traces de pas. Par contre, les Lissamphibiens n'ont pas été rencontrés dans les séries du Mésozoïque et du Cénozoïque hormis plusieurs espèces modernes qui ont été récoltées à la faveur des recherches préhistoriques dans le Quaternaire (Pléistocène, Holocène, Actuel)

Evaporite control on mixed carbonated-clastic platform architecture and carbonate production: A Late Messinian case study (Sorbas Basin, SE Spain).

International audienc