A Lower Silurian (Aeronian) radiolarian assemblage from black cherts of the Armorican Massif (France)

International audienceThe Chalonnes-sur-Loire outcrop is the most complete Llandovery section in the Ligerian domain (Armorican Massif, NW France); it displays a ca. 10 m-thick sequence of organic-rich black bedded cherts alternating with black graptolitic shales, which were deposited above glaciomarine diamictites of the Hirnantian glaciation. It is likely the black cherts accumulated under eutrophic waters, on the outer shelf part (distal offshore) of a Gondwanan margin, situated at intermediate to high latitudes of the Southern hemisphere. Hydrofluoric acid processing allowed the extraction of radiolarians; seven species are identified in the best preserved sample discovered so far. All seven are common species in tropical assemblages known from Alaska and Nevada, which are characteristic of the Orbiculopylorum assemblage, of Aeronian to early Telychian age. This age is in good agreement with independent age control from graptolites in the Chalonnes section, suggestive of an Aeronian age for the radiolarian-yielding level

A Palaeozoic open shelf benthic assemblage in a protected marine environment

International audienceA Late Ordovician benthic assemblage containing trilobites (Colpocoryphe, Onnia) and diploporid echinoderms (Aristocystites) taxa that are usually typical of open shelf environments is preserved in protected marine settings in the lower part of the Kermeur Formation (Armorican Massif, NW France). A facies analysis carried out on the Veryac'h section (Crozon Peninsula) allows identification of four sedimentary facies (SF), ranging from the bay/lagoon domain (SF1 and SF2) to the back-barrier (SF3) and barrier (SF4) domains. A benthic fauna comprising of trilobites, brachiopods and diploporids is found in the SF1 and SF2. In SF1, fossils are randomly scattered in sediments (taphofacies 1, TF1) showing two different preservation stages: TF1a with complete articulated skeletons and TF1b with disarticulated shells. These two preservation stages can be related either to the different modes of life of the taxa (i.e., recumbent, burrowing or nektobenthic) or to a composite assemblage with both autochthonous and allochthonous taxa. In this benthic assemblage, some trilobites and echinoderms taxa are mostly preserved either as complete exoskeletons and exuviae (Colpocoryphe grandis) or complete thecae (Aristocystites sp.), indicating an autochthonous origin. These taxa are usually found in low-energy open shelf settings. Their occurrence in a protected environment results from the combination of both allocyclic factors (i.e., change in sea level) and local conditions (i.e., shelf morphology, environmental conditions). Indeed, the lower part of the Kermeur Formation was deposited during a transgressive phase following a significant sea-level fall (Sandbian-Katian boundary). During the transgressive phase, barrier-lagoon systems were formed on the flat surface inherited from the sandbian shelf. Under non-restricted ecological conditions (i.e., normal oxygenation and salinity) in the protected settings, the new ecological niches were suitable for benthic organisms accustomed to open shelf environments. The settlement of these open shelf benthic species in coastal and protected environments may represent an offshore-onshore expansion, contrary to that described in the onshore-offshore diversification model of Phanerozoic shelf communities (Jablonski et al., 1983; Sepkoski, 1991)

A Cenozoic-style scenario for the end-Ordovician glaciation

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex

Substituentenkonstanten des Pyrazol-, 1,2,3-Triazol-, Benzotriazol- und Naphthotriazol-Restes

Die Synthese von Benzoesäuren mit den im Titel genannten Resten in m- bzw. p-Position (15, 13, 11, 9) sowie ihrer Ethyl- und Methylester wird beschrieben. Über deren alkalische Verseifung in Ethanol/Wasser und Methylcellosolve/Wasser werden die Substituentenkonstanten p und m der Reste bestimmt. Sie deuten auf eine induktive Elektronenacceptor-und mesomere Elektronendonator-Wirkung dieser für die Farbstoff-Chemie wichtigen Substituenten hin

The volcaniclastic series from the luang prabang basin, Laos: A witness of a triassic magmatic arc?

International audienceThe paleogeographic evolution of South East Asia (SEA) during the early Mesozoic is still poorly understood and a number of models have recently been put forward to account for the geodynamic evolution of SEA. The Luang Prabang Basin (north Laos), located in the core of a “paleogeographic jigsaw” in SEA, recorded a long lasting volcanism that spanned for ∼ 35 my from the earliest Triassic up to Late Triassic as evidenced by combined stratigraphic and geochronological (U-Pb/zircon) analyses performed on both volcanic and volcaniclastic series. The volcanic rocks are arc tholeiites and calk-alkaline andesites to dacites. The volcaniclastic rocks contain, in part, volcaniclasts produced contemporaneously with sedimentation. Both the volcanic and volcaniclastic series display geochemical features characteristic of a subduction related volcanism. Therefore, the Luang Prabang Basin documents a magmatic arc in a good agreement with the recent recognition of neighboring ophiolitic rocks in the Luang Prabang area. Following a passive margin setting that prevailed from the late Carboniferous to the late Permian, an active margin then initiated along the western margin of the Indochina Block. This active magmatic arc developed as the result of an east-dipping subduction below the Indochina Block during most of the Triassic, at least from ca. 250 to 215 Ma. Subsequently, this oceanic subduction episode must have been followed by a continental collision of the Indochina Block with the eastern Simao Block, at a period that remains to be defined

Histoire Géologique du massif Armoricain : Actualité de la recherche

National audienceUne part essentielle de l'histoire géologique de la France (et même d'Europe occidentale, avec des roches ayant environ 2000 Ma) est déchiffrable dans le Massif armoricain. Si celui-ci est réputé pour ses excellentes qualités d'affleurement sur le littoral (Armor, ou pays de la mer), certains objets ou structures ne peuvent être observés que dans le bocage (Argoat, ou pays des arbres), où leur lisibilité est souvent problématique. En Armor comme en Argoat, de nombreux sites constituent un réel patrimoine géologique (l'intérêt de certaines localités sera mis en exergue dans le texte), dont la valeur ne peut être jaugée qu'au regard de son intérêt scientifique. Ainsi ce travail - une mise en perspective de nos connaissances scientifiques sur l'évolution géologique du Massif armoricain - est-il basé sur plusieurs synthèses antérieures (par ex. Le Corre et al., 1991 ; Ballèvre et al., 2009), qu'il complète en intégrant les nouvelles données disponibles. Plus qu'un exposé complet des faits, nous visons à clarifier certains débats, et montrer en quoi les recherches en cours changent notre image globale du Massif armoricain. Nous restreindrons notre analyse à la période qui couvre la fin du Protérozoïque (Ediacarien : 635-540 Ma), le Paléozoïque (540-250 Ma) et le début du Mésozoïque (Trias : 250-200 Ma)

Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission

Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dual-color, power tunable, emission can be obtained through an electrical excitation

Elektronenreiche Heterocyclen als Donorgruppen in Fluoreszenzfarbstoffen

Die Darstellung von N-Methylphtalimiden wird beschrieben, die in 4-Stellung die Donorgruppen Pyrazol, Triazol, Benzotriazol und Naphthotriazol tragen (17, 18, 14, 12). über die alkalische Hydrolyse des Imids werden die Substituentenkonstanten der Reste bestimmt. Die Solvatochromie in Absorption und Fluoreszenz gibt Information über die Ladungsverteilung in Grund- und angeregtem Zustand

Social interaction, noise and antibiotic-mediated switches in the intestinal microbiota

The intestinal microbiota plays important roles in digestion and resistance against entero-pathogens. As with other ecosystems, its species composition is resilient against small disturbances but strong perturbations such as antibiotics can affect the consortium dramatically. Antibiotic cessation does not necessarily restore pre-treatment conditions and disturbed microbiota are often susceptible to pathogen invasion. Here we propose a mathematical model to explain how antibiotic-mediated switches in the microbiota composition can result from simple social interactions between antibiotic-tolerant and antibiotic-sensitive bacterial groups. We build a two-species (e.g. two functional-groups) model and identify regions of domination by antibiotic-sensitive or antibiotic-tolerant bacteria, as well as a region of multistability where domination by either group is possible. Using a new framework that we derived from statistical physics, we calculate the duration of each microbiota composition state. This is shown to depend on the balance between random fluctuations in the bacterial densities and the strength of microbial interactions. The singular value decomposition of recent metagenomic data confirms our assumption of grouping microbes as antibiotic-tolerant or antibiotic-sensitive in response to a single antibiotic. Our methodology can be extended to multiple bacterial groups and thus it provides an ecological formalism to help interpret the present surge in microbiome data.Comment: 20 pages, 5 figures accepted for publication in Plos Comp Bio. Supplementary video and information availabl