Uranium isotopes as proxy for investigating land-to-sea sediment transfer response to Late Quaternary climate changes

Studying how catchment erosion has responded to past climate change can help us to better understand not only how landscape evolution and source-to-sink processes operate, but also to predict the consequences of future climate change on soil resource availability and sediment discharge to the ocean. In this context, the aim of this PhD project was to investigate the variability of past sediment residence times in watersheds and to assess their relationships with erosion and climate change. The sediment residence times were calculated using uranium isotope activity ratios (234U/238U). First, U isotopic ratios were determined in a large number of world river sediments to improve our understanding of the environmental controls on sedimentary (234U/238U). Our results indicate that a multi-parameter approach can adequately predict the distribution of (234U/238U) in sediments, demonstrating the potential of using (234U/238U) to investigate past environments. Then, the spatial variability of sediment residence times was studied within a mountainous catchment (Var, France). Long residence times were determined in sediments from weathering-limited sub-catchments, while comparatively lower residence times were obtained in sub-catchments characterized by transport-limited weathering conditions. Finally, the temporal variability of sediment residence times in the Var Basin over the last 75,000 years was investigated, via the analysis of marine sediment cores. It revealed significant fluctuations of sediment residence times over glacial - interglacial timescales, which can be explained by the presence of LGM glaciers in the Alps, associated with enhanced erosion. A second-order cyclicity of sediment residence times was also identified during the last glacial period over the short-lived millennial events of the Bond cycle. These variations were tentatively attributed to the rapid evolution of the vegetation cover during the Dansgaard-Oeschger climatic variability

Comportement à rupture dans le sens des fibres de FRP sous chargement statique et de fatigue = Behaviour until rupture of FRP in the fibre direction under static and fatigue loads

National audienceLa rupture des matériaux composites est due à de nombreux mécanismes agissants à différentes échelles. Les modèles basés sur la mécanique de l'endommagement peuvent décrire les endommagements diffus progressifs induits par de petites fissures parallèles à la direction des fibres [1-2]. Même si la taille de ces fissures correspondent à l'épaisseur du pli, elles ne mènent pas en général à la rupture du stratifié. Par contre, la rupture d'un pli dans la direction des fibres est en général catastrophique pour le stratifié et la structure. Dans cet article, nous proposons une analyse expérimentale et un modèle à l'échelle du pli pour décrire la rupture dans le sens des fibres pour des chargements statique et de fatigue. Ce modèle repose sur une diminution de la résistance sens fibre pour des niveaux d'endommagement transverse élevés. Dans un premier temps, on rappelle le modèle d'évolution de l'endommagement associé aux fissures parallèles aux fibres, pour des chargements statiques et de fatigue [3]

Etude des micro-transitions quotidiennes et construction des repères en crèche : des professionnels en recherche

International audienceLa vie quotidienne dans une crèche est faite de multiples micro-transitions, que les enfants vont vivre différemment selon leur présence dans la structure (enfants en occasionnels ou au contraire restant sur des temps longs, plus de 50h par semaine). Au fil de ces transitions, l’enfant construit plus ou moins facilement des repères temporels, spatiaux, en lien avec les professionnels, repères qui vont donner sens à ce qu’il vit. L’objectif de la recherche que nous présentons est d’étudier l’élaboration de ces repères et surtout les conditions de cette élaboration dans les pratiques. Pour cela, nous avons impliqué des professionnels dans une démarche de recherche-action, au sein de leurs établissements. Le deuxième objectif est en effet d’accompagner la réflexion des professionnels, en mettant en place une méthodologie rigoureuse de questionnement et d’observations. Les résultats montrent que la construction des repères par les jeunes enfants et en particulier par ceux qui viennent peu régulièrement dans la structure dépend de plusieurs aspects des pratiques pédagogiques : aménagement de l’espace, disponibilité des professionnelles, anticipation dans l’organisation. Par ailleurs, l’implication des professionnelles dans la démarche de recherche-action proposée met en évidence leurs compétences en réflexivité

Endommagement de composites stratifiés verre/epoxy sous chargement statique et de fatigue

Cette étude traite de la modélisation du comportement à rupture de composites stratifiés constitués de plis tissés déséquilibrés verre/epoxy sous chargements statique et de fatigue. Des essais de traction monotone puis cycliques sur des éprouvettes plates ont permis de décrire le comportement du matériau à l'échelle du pli. Les plis tissés déséquilibrés ont un comportement élasto-endommageable dans les directions chaîne et trame, et élasto-plastique endommageable en cisaillement. Deux types de ruptures peuvent être distinguées: (i) Une rupture fragile qui correspond à une déformation maximale des fibres; (ii) une rupture par instabilité de la structure due à l'endommagement. Le modèle, basé sur un endommagement cumulé, permet de décrire l'évolution de l'endommagement sous chargements statique et de fatigue. L'évolution de l'endommagement statique dépend du maximum du chargement et prend en compte les couplages entre traction et cisaillement. L'évolution de l'endommagement de fatigue dépend du maximum du chargement mais également de l'amplitude du chargement. Le domaine de validité de ce modèle se limite à la rupture du premier pli et ne décrit pas le délaminage

Dynamic finite-element simulations reveal early origin of complex human birth pattern

Human infants are born neurologically immature, potentially owing to conflicting selection pressures between bipedal locomotion and encephalization as suggested by the obstetrical dilemma hypothesis. Australopithecines are ideal for investigating this trade-off, having a bipedally adapted pelvis, yet relatively small brains. Our finite-element birth simulations indicate that rotational birth cannot be inferred from bony morphology alone. Based on a range of pelvic reconstructions and fetal head sizes, our simulations further imply that australopithecines, like humans, gave birth to immature, secondary altricial newborns with head sizes smaller than those predicted for non-human primates of the same body size especially when soft tissue thickness is adequately approximated. We conclude that australopithecines required cooperative breeding to care for their secondary altricial infants. These prerequisites for advanced cognitive development therefore seem to have been corollary to skeletal adaptations for bipedal locomotion that preceded the appearance of the genus Homo and the increase in encephalization

Author Correction: Dynamic finite-element simulations reveal early origin of complex human birth pattern

Human infants are born neurologically immature, potentially owing to conflicting selection pressures between bipedal locomotion and encephalization as suggested by the obstetrical dilemma hypothesis. Australopithecines are ideal for investigating this trade-off, having a bipedally adapted pelvis, yet relatively small brains. Our finite-element birth simulations indicate that rotational birth cannot be inferred from bony morphology alone. Based on a range of pelvic reconstructions and fetal head sizes, our simulations further imply that australopithecines, like humans, gave birth to immature, secondary altricial newborns with head sizes smaller than those predicted for non-human primates of the same body size especially when soft tissue thickness is adequately approximated. We conclude that australopithecines required cooperative breeding to care for their secondary altricial infants. These prerequisites for advanced cognitive development therefore seem to have been corollary to skeletal adaptations for bipedal locomotion that preceded the appearance of the genus Homo and the increase in encephalization

Modélisation et expertise de la rupture de structures composites

La mécanique de l’endommagement continu peut décrire les endommagements diffus induits par de petites fissures parallèles à la direction des fibres. Nous proposons ici un modèle pour décrire la rupture dans le sens des fibres en statique et fatigue. Ce modèle repose sur une diminution de la résistance sens fibre pour des niveaux d’endommagement transverse élevés. Ce type de modélisation peut être intégré dans un code de calcul EF et utilisé pour l’expertise de structures composites rompues

New Insights in the Contribution of Voltage-Gated Nav Channels to Rat Aorta Contraction

BACKGROUND: Despite increasing evidence for the presence of voltage-gated Na(+) channels (Na(v)) isoforms and measurements of Na(v) channel currents with the patch-clamp technique in arterial myocytes, no information is available to date as to whether or not Na(v) channels play a functional role in arteries. The aim of the present work was to look for a physiological role of Na(v) channels in the control of rat aortic contraction. METHODOLOGY/PRINCIPAL FINDINGS: Na(v) channels were detected in the aortic media by Western blot analysis and double immunofluorescence labeling for Na(v) channels and smooth muscle alpha-actin using specific antibodies. In parallel, using real time RT-PCR, we identified three Na(v) transcripts: Na(v)1.2, Na(v)1.3, and Na(v)1.5. Only the Na(v)1.2 isoform was found in the intact media and in freshly isolated myocytes excluding contamination by other cell types. Using the specific Na(v) channel agonist veratridine and antagonist tetrodotoxin (TTX), we unmasked a contribution of these channels in the response to the depolarizing agent KCl on rat aortic isometric tension recorded from endothelium-denuded aortic rings. Experimental conditions excluded a contribution of Na(v) channels from the perivascular sympathetic nerve terminals. Addition of low concentrations of KCl (2-10 mM), which induced moderate membrane depolarization (e.g., from -55.9+/-1.4 mV to -45.9+/-1.2 mV at 10 mmol/L as measured with microelectrodes), triggered a contraction potentiated by veratridine (100 microM) and blocked by TTX (1 microM). KB-R7943, an inhibitor of the reverse mode of the Na(+)/Ca(2+) exchanger, mimicked the effect of TTX and had no additive effect in presence of TTX. CONCLUSIONS/SIGNIFICANCE: These results define a new role for Na(v) channels in arterial physiology, and suggest that the TTX-sensitive Na(v)1.2 isoform, together with the Na(+)/Ca(2+) exchanger, contributes to the contractile response of aortic myocytes at physiological range of membrane depolarization

Non-Destructive Techniques Based on Eddy Current Testing

Non-destructive techniques are used widely in the metal industry in order to control the quality of materials. Eddy current testing is one of the most extensively used non-destructive techniques for inspecting electrically conductive materials at very high speeds that does not require any contact between the test piece and the sensor. This paper includes an overview of the fundamentals and main variables of eddy current testing. It also describes the state-of-the-art sensors and modern techniques such as multi-frequency and pulsed systems. Recent advances in complex models towards solving crack-sensor interaction, developments in instrumentation due to advances in electronic devices, and the evolution of data processing suggest that eddy current testing systems will be increasingly used in the future