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Comportement mécanique des sols fins en lien avec leur microstructure initiale
No full textInternational audienceThe microstructure of clayey soil plays a key role in its mechanical behavior. During sedimentation, particle interactions are influenced by pH: low-pH conditions promote edge-to-face flocculation, forming card-house-like structure, while high-pH environments favor face-to-face particle stacking and denser arrangements. A specific sample preparation method enabled direct SEM observation of these structures. Flocculated samples exhibited larger pores and greater compressibility, as confirmed by oedometer tests. However, under increasing load, both flocculated and dispersed microstructures gradually aligned, leading to similar mechanical responses, particularly during unloading. SEM imaging confirmed this convergence of fabric. These findings highlight the importance of initial microstructure and its evolution under loading, providing insight into the behavior of kaolinite sediments in varying environmental conditions.La microstructure des argiles joue un rôle clé dans leur comportement mécanique. Pendant la sédimentation de la kaolinite, les interactions entre les particules sont influencées par le pH : des conditions de pH bas favorisent la floculation avec des assemblages de particules en configuration bord-à-face, formant une structure de type « château de cartes », tandis qu’un pH élevé favorise l'empilement face-à-face des particules et conduit à des arrangements plus denses. Une méthode spécifique de préparation des échantillons a permis l'observation directe de ces structures par Microscopie Électronique à Balayage (MEB). Les échantillons floculés présentent des pores plus grands et une compressibilité plus importante, comme le confirment les essais oedométriques. Toutefois, sous une contrainte unidimensionnelle croissante, les microstructures floculées et dispersées convergent progressivement, aboutissant à des réponses mécaniques similaires, notamment sur le chemin de déchargement. Les observations MEB ont confirmé cette convergence microstructurale à des niveaux de contrainte élevés. Ces résultats soulignent l'importance de la microstructure initiale et de son évolution sous l’effet du chargement mécanique, apportant un éclairage sur la compréhension du comportement des sédiments de kaolinite dans des conditions environnementales variées
Neolithic societies and landscape transformations in northwestern France: A high-resolution multi-proxy study including sedimentary DNA
No full textInternational audienceTo understand the impact of human practices on Mid-Holocene vegetation in northwestern France, more pollen records and multi-proxy studies with accurate chronologies located near excavated archaeological sites are needed. In northwestern Normandy, the Caen Plain is particularly suitable for gaining further insight into the impact of early human activity on the landscape, due to the high density of recognized and excavated archaeological sites dating back to Neolithic times. Therefore, in this area, we developed a high temporal resolution approach using multiple palaeobiological proxies (pollen, non-pollen palynomorphs (NPPs), charcoal analysis, and sedimentary ancient DNA (sedaDNA)), along with sedimentological proxies to analyze a new sequence from the Vey valley, located close to the excavated Neolithic site of Cairon, and which could be compared to the archaeobotanical and archaeozoological data from archaeological excavations. Our specific objective was to propose a reconstruction of the landscape changes under human influence at a local scale, in order to gain a precise understanding of the processes of anthropization in the Caen Plain at the end of the Mesolithic and during the Neolithic. The impact of human activity during the Mesolithic and Early Neolithic appears to have been limited. Nevertheless, the end of the Late Mesolithic (5900-5200 BCE; 7850-7150 cal. BP) showed the first increase in local fire frequency. These fires were responsible for temporary clearings of the forest along the wetland, and sedaDNA indicated that wild boar were present in the wetland. Most notably, the Cairon site offered the first opportunity to precisely determine the impact of changing land use in an area that had been inhabited since the Middle Neolithic (4400-3475 BCE; 6350-5425 cal. BP). The combined use of sedaDNA, NPPs, and charcoal analysis demonstrated the importance of pastoral practices and fires in opening up environments and transforming alluvial wetlands during this period.</div
Carbon storage of seagrass ecosystems may experience tipping points in response to anthropogenic stress - a modeling perspective
No full textInternational audienceCoastal Blue Carbon ecosystems like seagrass meadows are foundation habitats with a capacity to sequester and store organic carbon in their sediments, and their protection and restoration may thereby support climate change mitigation while also supporting biodiversity and many other ecosystem functions. However, seagrass ecosystems are being lost due to human activities, disease and, in some regions, climate change, which may trigger the release of stored carbon into the atmosphere. Yet, we do not fully understand how global change-induced seagrass loss influences sedimentary carbon dynamics. What is even less clear is whether seagrass loss may also result in tipping points, i.e., abrupt and difficultto-reverse shifts, in carbon flux dynamics turning seagrass ecosystems from net carbon sinks to net carbon sources. Here, we propose that conceptual mechanistic models of coupled ecological and biogeochemical dynamics can help to study the effects of major stressors on seagrass meadows and associated carbon fluxes. We then illustrate one case of such a conceptual model that focuses on anthropogenic induced mortality by physical stress as an example. Our perspective highlights how a modeling approach for understanding the response of carbon fluxes in seagrass ecosystems to global change stressors may be useful in informing coastal seagrass management towards climate change mitigation actions.</div
Vegetation and climate reconstructions from sedimentary archives in the Circum-Caspian region.
No full textInternational audienc
Porosity and pore and throat size distributions in carbonate-rich salt caprock of halite diapirs: effect of deformation and geochemical processes
No full textInternational audienceMicroscopic porosity of salt caprock on two salt diapirs, Karmostaj and Siah Taq in southern Iran, was studied on a range of samples that represent the major lithological types of salt caprock in the area: the gypsum matrixsupported and clast-supported dissolution breccia, the dolomite and limestone stringers of the Hormuz Formation and gypsum mylonites. Mercury intrusion porosimetry alongside the microstructural analysis with digital image analysis of resin-saturated thin-sections was employed and revealed a large variation in porosity of 2-35 % and in the median throat size of 0.1-50 μm. The highest porosity values are associated with vuggy dolomites and limestones interspersed by a dense network of gypsum veins and gypsum matrix-supported breccia with low degree of deformation. In contrast, low porosity is linked with strongly deformed gypsum matrix-supported breccias and dark micritic carbonate stringers that were not interspersed with the gypsum. Large variation in porosity and pore throat size and pore shape is attributed to 1) metasomatization of carbonate to gypsum due to reaction of H 2 SO 4 with carbonate after oxidation of H 2 S or elemental sulfur in caprock and subsequent dissolution of gypsum in metasomatized carbonates creating the abundant vuggy porosity and 2) deformation of the caprock that was responsible for recrystallization and mechanical closure of pores in the frontal, downslope margins of both reactivated diapirs. While the deformed and gypsum-rich parts of the diapir caprocks (~30-60 % of caprock on the diapirs) are associated with estimated permeabilities of ~10 -16 -10 -15 m 2 , caprocks in central parts of reactivated diapirs with less deformed caprock (undeformed gypsum-and clast-supported breccia) display higher permeabilities ranging between ~10 -15 m 2 and 10 -13 m 2 .</div
Recent aqueous alteration associated to sedimentary volcanism on Mars
No full textInternational audienceSedimentary volcanism, whereby material is brought to the surface by fluid overpressure, has been proposed to explain some of the periglacial landforms, including pitted cones, in the Northern Plains of Mars. However, in the absence of convincing mineralogical evidence, the origin for these deposits has never been conclusively determined. Here we conduct a remote sensing-based mineralogical survey to identify hydrated minerals within the Thumbprint Terrains and neighbouring Vastitas Borealis Formation. We detect several occurrences of hydrated silica along with sulfate salts in candidate mud volcano-like morphologies which likely formed during the Early Amazonian period, supporting the sedimentary volcanism origin. Buoyancy-driven analytical modelling suggests the hydrated silica and sulfate salts are sourced from reservoirs at depths of several 10 s and 100 s of metres, respectively below the Thumbprint Terrains and Vastitas Borealis Formation. The exposed sulfates may have been derived from ancient buried evaporite deposits suggesting, at least locally, a salt-rich aqueous origin for the Vastitas Borealis Formation, and would be consistent with the presence of a past northern ocean on Mars.</div
A giant virus awakens virophage-like PLVs in the green alga Tetraselmis
No full textGiant double-stranded DNA viruses infecting microalgae often interact with polinton-like viruses (PLVs), yet their dynamics within natural hosts remain poorly understood. Here, we report the isolation of a new Tetraselmis striata giant virus (Allomimiviridae) from Mediterranean coastal waters, which we designate Oceanusvirus lionense (TetV-2). From the same sample, we co-isolated a PLV, Tsa-S2b, that replicates concomitantly with TetV-2. Remarkably, TetV-2 infection also reactivates at least two additional PLVs integrated into the host genome. We show that these PLVs exert a virophage-like effect, reducing the fitness of the giant virus. Comparative genomics reveals that these virophagic PLVs are related to the autonomous lytic PLV TsV-N1, sharing structural but not replication-associated genes. This newly described pathosystem provides a unique model to investigate the evolution of PLVs and the parasitic mechanisms underlying virophage-giant virus interactions
Seeing through the haze: The Cassini/Huygens challenge of mapping Titan's surface
No full textInternational audienceLike Venus, Titan is one of the toughest bodies in the Solar System to map due to its very thick atmosphere. Only a very limited number of wavelengths can see though the haze and reveal its surface. In this talk, we will start from our very limited knowledge from Pionner/Voyager era, that show how little we knew about Titan's surface prior to Huygens landing. Then we will go though the Cassini instruments (VIMS, ISS, RADAR, ...) that allowed us to build a complete coverage of this moon after 13 years of mission. Finally we will present the latest geological maps that are derived from these datasets which are an essential key for the future exploration of Titan
Harnessing thermal spring water and hydrophobic eutectic solvent for galactoglucomannans, glucans and phenolics recovery from Evernia prunastri and their potential biological applications
No full textInternational audienceLichens produce polysaccharides scarcely studied, in addition to secondary metabolites, that can exhibit high bioactivity. The aim of this work is to evaluate the efficiency of subcritical water extraction using thermal spring water and hydrophobic eutectic solvent for the extraction of these compounds. Subcritical water extraction of E. prunastri compounds was performed between 150 and 190 • C with distilled or with thermal spring water, combined or not with eutectic solvent men:lau as modifier. The biochemical composition of the polysaccharide extracts from each process (P-D for distilled water, P-T for thermal spring water, P-DM and P-TM when the eutectic was added) and of the soluble liquid extracts free from the precipitated polysaccharides (E-D, E-T, E-DM and E-TM) was analyzed as well as their antioxidant capacity and biological activity against herpes viruses. After purification through resin of E-T170 and E-TM150, extracts with an antioxidant activity of 1.38 and 1.78 g Trolox eq./ g of extract were obtained. Polysaccharides were identified as galactoglucomannans and glucans by NMR analysis. All polysaccharides were formulated in films, however, those extracted with thermal water presented much better viscoelastic properties than those extracted with distilled water. P-T-170 • C and P-TM-150 • C were non-irritating according to Skinethic® test but did not exhibit protective activity against HSV-1 and HSV-2 viruses. E-T170 • C presented a slightly higher antiviral activity (around 40 % protection) with low cytotoxicity. In addition, the films' physical and mechanical properties suggest potential for biomedical applications, especially in topical or mucosal delivery systems
Assessing multivariate bias corrections of climate simulations on various impact models under climate change
No full textInternational audienceAtmospheric variables simulated from climate models often present biases relative to the same variables calculated by reanalysis in the past. In order to use these models to assess the impact of climate change on processes of interest, it is necessary to correct these biases. Currently, the bias correction methods used operationally correct one-dimensional time series and are therefore applied separately, physical variable by physical variable and site by site. Multivariate bias correction methods have been developed to better take into account dependencies between variables and in space. Although the performance of multivariate bias correction methods for adjusting the statistical properties of simulated climate variables has already been evaluated, their effectiveness for different impact models has been little investigated. In this work, we propose a comparison between two multivariate bias correction methods (R2D2 and dOTC) in three different configurations (intervariable, spatial and spatial-intervariable) and a univariate correction (CDF-t) through several highly multivariate impact models (phenological stage, reference evapotranspiration, soil water content, fire weather index) integrating the weather conditions over a whole season. Our results show that CDF-t does a fair job in most situations and that there is no single best MBC method. The performances of multivariate bias correction methods depend both on some characteristics of the studied process and on the configuration of the chosen bias correction method. When all characteristics are important (multivariate, time cumulative and spatial) it is found that dOTC in its spatial-intervariable configuration brings improvements in most cases and no significant improvement in some rare cases. We did not find any multivariate cases where the spatial-intervariable configuration for dOTC performs less well than CDF-