Phreatomagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): eruptive style, associated deformations, and implications on CO2 release by volcanism

Oman exotics represent remnants of a Triassic carbonate platform (Misfah Formation). Within these carbonates, and coeval with the sedimentation, several basaltic magmatic events occurred mainly as intrusions, also as lava flows and projections. We describe one of these events, that produced a phreatomagmatic eruption along a volcanic fissure. The initial ascent of magma probably occurred along on a normal fault related to gravity-driven sliding of the carbonates towards the platform edge. Magma first emplaced in a saucer-shaped sill few tens of meters below the surface. This intrusion provided a decollement layer, that may have speed up the gravity-driven sliding, opening fractures that brought sea-water in contact with the magma, hence triggering the phreatomagmatic eruption. Eruption was followed by the collapse of the limestones in a megabreccia infilling the eruptive line and prohibiting further contact between sea water and magma. The main magma volume emplaced at depth in two superposed magma chambers that replaced the host sediments and uplifted the overlying eruptive line. These magma chambers fossilized the substratum of the carbonate platform, that consists in uplifted sediments from the distal Hawasina basin. Replacing limestones by magma chambers may have release huge volumes of Carbon dioxide, estimated to be two to three hundred times higher than CO2 release by volcanic gases. CO2 release by decarbonating sediments may be an important mechanism to explain climatic changes associated to some large igneous provinces such as Siberia, Central Atlantic Magmatic Province, or Karoo, where very large magmatic volumes where intruded in sedimentary basins

Impacts of stream acidification on litter breakdown: implications for assessing ecosystem functioning

1. Scientific understanding of acidification in aquatic ecosystems relies on effective assessment, which at present is mostly limited to chemical and sometimes structural biological variables. Effects on ecosystem functioning are, in contrast, largely neglected. Litter breakdown is a potentially useful, highly integrative and crucial process that could enhance such assessment programmes. 2. Breakdown rates of beech Fagus sylvatica leaves were determined in 25 woodland headwater streams along an acidification gradient in the Vosges Mountains, France. Additional data relating to micro-organisms (microbial respiration, fungal biomass and degree of conditioning measured as leaf palatability) and macroinvertebrates (shredder diversity, abundance and biomass) associated with decomposing leaves were collected to elucidate the mechanisms underlying leaf breakdown. 3. Breakdown rates varied more than 20-fold between the most acidified and circum- neutral sites (k = 0·0002–0·0055 day−1). Stream water alkalinity and total Al concen- tration together accounted for 88% of the variation in litter breakdown rates among streams. Microbial factors associated with decaying leaves, particularly microbial respiration, declined with increasing stream acidity and were significantly related to Ca2+ and total Al concentrations. 4. Total abundance, biomass and richness of leaf-shredding invertebrates associated with decomposing leaves were not related to stream acidity. However, the abundance and biomass of the amphipod Gammarus fossarum, an acid-sensitive and particularly efficient leaf-shredder, showed a strong positive relationship with leaf breakdown rate. Gammarus abundance and microbial respiration together accounted for 85% of the variation in litter breakdown rates among streams. 5. Synthesis and applications. These results indicate that leaf-litter breakdown responds strongly to stream acidification, with both microbial decomposers and invertebrate detritivores markedly affected. Measuring leaf breakdown rate may be developed into a simple, powerful and low-cost tool for assessing a critical component of ecosystem functioning. We advocate further investigation of this approach for the routine bio- monitoring of freshwaters affected by, or recovering from, other anthropogenic stresses

Recirculation cells in a wide channel

International audienceSecondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer

Effect of acidification on leaf litter decomposition in benthic and hyporheic zones of woodland streams

Anthropogenic acidification has deleterious effects on both structure and functioning of surface water ecosystems. This study examined how it may affect the leaf decomposition rate and the community structure and activity of decomposers in both benthic and hyporheic zones of five headwater streams along an acidification gradient from highly acidic (pH 4.6) to circumneutral (pH 7.4). Overall, responses to acidification in hyporheic zones were less pronounced, but followed the same pattern as in their benthic counterparts. Leaf decomposition was much faster in the circumneutral stream, both in the hyporheic and benthic zones (k = 0.0068 and 0.0534 d−1, respectively), than in the most acidic one (k = 0.0016 and 0.0055 d−1, respectively), and correlated well with the acidic gradient in both compartments. Interestingly, leaf litter decomposition was less affected by acidification in hyporheic compared to benthic compartments, likely due to the relatively low sensitivity of fungi, which were the main decomposers of buried coarse particulate organic matter. These results argue in favour of conserving hyporheic habitats in acidified streams as they can maintain matter and species fluxes that are essential to the ecosystem

Transforming Gender Norms in Rural Mali by Opening Spaces for Action

Communities in the Tommoguiné Region of Mali are actively pursuing development despite economic challenges, insecurity, and isolation from many opportunities. Historically, the marginalization of women and girls has held back some of these efforts and denied certain rights to some of the population. Currently, a transformation of norms surrounding women’s roles in public life and of practices harmful to women and girls is taking place, inspired by a Women’s Literacy, Leadership, and Enterprise program generated through a transnational collaboration between local communities, a Malian linguistic association, and The Tandana Foundation, a Dayton-based non-profit organization. Instead of reinscribing colonial dynamics, this partnership opens spaces for action that enable communities to change norms and practices on their own terms. Women who have participated in this program are making their voices heard in local decision-making fora in ways that were not accepted before, disrupting economic exploitation by exercising new literacy and numeracy skills, and leading transformations of marginalizing practices, such as early, forced marriage. By focusing on opening spaces for action, rather than attempting to fabricate social reality to fit a preconceived vision, this program avoids the dangers inherent in approaching human affairs as if they could be made, as mainstream development has so often done. It embodies an alternative approach that allows the opportunities of action--expressiveness, the remedy of forgiveness, allowance of the unexpected, inclusiveness, flourishing of human relationships, and emergence of meaning—to enable genuine and durable changes. This form of transnational collaboration offers an example of educational, economic, civic, and cultural engagement that supports locally-led transformations that increase inclusion and expand opportunities. In this panel discussion, two Malian practitioners will share their experiences supporting transformations at the grassroots level, an American practitioner-scholar will connect philosophical insights with concrete practices, and a Mexican simultaneous interpreter will facilitate immediate bilingual comprehension

Magnetoelastic polarons in the hole-doped quasi-one dimensional model system Y2-xCaxBaNiO5

October 4th, 2004Charge transport in the hole-doped quasi-1D model system Y$_{2-x}$Ca$_x$BaNiO$_5$ (x $\\leq$ 0.15) is investigated in the 50-300 K temperature range. The resistivity temperature dependence is characterized by a constant activation energy $E_{a}/k_{B}\\sim$ 1830 K at room temperature while $E_{a}$ decreases upon cooling. We suggest that $E_{a}$ measures the binding energy of the doped holes which form magneto-acoustic polarons when polarizing the neighboring Ni spins. A semi-classical model is proposed which allows to relate the electrical measurements and the bulk magnetic susceptibility. This model gives a picture of the spin-charge-lattice relation in this inhomogeneously doped quasi-1D system and explains its unusual one-particle charge excitation spectrum close to the Fermi level

Ultrasound modulated optical tomography in scattering media: flux filtering based on persistent spectral hole burning in the optical diagnosis window

Ultrasound modulated optical tomography (UOT) is a powerful imaging technique to discriminate healthy from unhealthy biological tissues based on their optical signature. Among the numerous detection techniques developed for acousto-optic imaging, only those based on spectral filtering are intrinsically immune to speckle decorrelation. This paper reports on UOT imaging based on spectral hole burning in Tm:YAG crystal under a moderate magnetic field (200G) with a well-defined orientation. The deep and long-lasting holes translate into a more efficient UOT imaging with a higher contrast and faster imaging frame rate. We demonstrate the potential of this method by imaging calibrated phantom scattering gels.Comment: 4 pages, 5 figure

Triassic alkaline magmatism of the Hawasina Nappes: Post-breakup melting of the Oman lithospheric mantle modified by the Permian Neotethyan Plume

International audienceMiddle to Late Triassic lavas were sampled within three tectonostratigraphic groups of the Hawasina Nappes in the Oman Mountains. They are predominantly alkali basalts and trachybasalts, associated with minor sub-alkaline basalts, trachyandesites, trachytes and rhyolites. Their major, trace elements and Nd-Pb isotopic compositions are very similar to those of the Permian plume-related high-Ti basalts which also occur in the Hawasina Nappes. The Triassic lavas derive from low-degree melting of an enriched OIB-type mantle source, characterized by εNdi = 0.3-5.3 and (206Pb/204Pb)i = 16.96-19.31 (for t = 230 My). With time, melting depths decreased from the garnet + spinel to the spinel lherzolite facies and the degree of melting increased. The oldest are distinguished from the others by unradiogenic Nd and Pb signatures, with εNdi = − 4.5 to − 1.2 and (206Pb/204Pb)i = 16.35-17.08, which we attribute to their contamination by Arabo-Nubian lower crust. The lavas likely derived from the Oman lithospheric mantle, the original DMM-HIMU signature of which was overprinted during its pervasive metasomatism by the Permian plume-related melts. We suggest that these lavas were emplaced during post-breakup decompression-triggered melting in the Middle Triassic during global kinematic reorganization of the Tethyan realm

Représentations de la marginalité, marginalités en représentation : Marges et pouvoir (xviie-xxe siècles)

« En marge (de la société) : sans s’intégrer au groupe social et sans se soumettre à ses normes », nous indique le dictionnaire Larousse. Cette définition suggère que la « marge », en tant que milieu, implique pour celles et ceux qui s’y trouvent une mise à distance, un écart volontaire ou non par rapport à un centre. La marginalité est en ce sens une notion relative : elle gagne à être étudiée non pas seulement pour elle-même, mais aussi dans sa relation contrariée avec le groupe ou l’instit..