253 research outputs found
Rheology and microstructure of experimentally deformed plagioclase suspensions
International audienceWe present the result of the first deformation experiments at high-temperatures and high-pressures on synthetic magmatic suspensions of strongly anisometric particles. The results highlight the interplay between the rheological response and the development of microstructures and they demonstrate the critical importance of the shape of crystals on the mechanical behaviour of magmas. Plagioclase suspensions with two crystal fractions (0.38 and 0.52) were deformed both in compression and in torsion in a Paterson apparatus. With increasing crystal fraction, the rheological behaviour of the magmatic suspension evolves from nearly steady-state flow to shear weakening, this change being correlated with a microstructural evolution from a pervasive strain to a strain partitioning fabric. Magmatic suspensions of plagioclase have viscosities approximately five orders of magnitude higher than suspensions of equivalent crystallinities made of isometric particles such as quartz
Cartographie de la péridotite de Ronda (Espagne) par télédétection hyperspectrale : données AVIRIS
La pĂ©ridotite de Ronda, au sud de lâAndalousie (Espagne), a Ă©tĂ© imagĂ©e par AVIRIS en 1991 et partiellement
Ă©chantillonnĂ©e par nous-mĂȘme sur le terrain Ă lâaide dâun spectromĂštre GER 3700 en 1997 dans le but dâacquĂ©rir une
expĂ©rience dans le traitement des images hyperspectrales des surfaces planĂ©taires Ă lâaide de sondes telle que ISM Phobos
(1989), OMEGA Mars Express (2003) and VIMS Cassini (2004). La haute résolution spectrale des images (224 canaux
rĂ©partis entre 400 et 2 455 nm) est nĂ©cessaire Ă la conduite dâune analyse gĂ©ologique avec identification Ă distance
des faciÚs pétrologiques. Sur Terre, il est aussi nécessaire de déterminer les espÚces végétales à cause de leur grande influence
sur la cartographie des faciĂšs pĂ©trologiques, mĂȘme dans des rĂ©gions relativement arides comme celle de la pĂ©ridotite
de Ronda. Cependant, la péridotite de Ronda reste un bon site test.
Lâimage AVIRIS de Ronda est dâabord analysĂ©e par photo-interprĂ©tation. Des compositions colorĂ©es (affichĂ©es
sur les canaux rouge, vert et bleu visibles) sont construites Ă partir de 3 canaux visibles et/ou infrarouge choisis parmi
les 150 canaux utiles (le dernier détecteur AVIRIS ne fonctionnant pas en 1991). Ces compositions colorées permettant
de visualiser les principales caractĂ©ristiques gĂ©ologiques du visible Ă lâinfrarouge, il est alors possible de les comparer Ă
des cartes géologiques, puis aux mesures de terrain de la campagne de juillet 1997. Cette analyse visuelle permet de distinguer
trÚs facilement le massif de péridotite de ses roches avoisinantes (gneiss, marbres, grÚs et calcaires) ainsi que de
mettre en évidence une nette zonation en serpentine habituellement non cartographiée.
Ce travail faisant suite Ă celui de Chabrillat et al. [2000] nous avons pris le parti dâexplorer une autre voie que
celle des analyses en composantes principales en cherchant Ă retirer couche par couche les diffĂ©rents Ă©lĂ©ments Ă lâorigine
de la rĂ©ponse spectrale de la pĂ©ridotite de Ronda. Nous avons aussi pris le parti de ne nous fier quâaux mesures de
terrain et de ne jamais avoir recours Ă des Ă©chantillons dâimage pour effectuer des classifications car notre expĂ©rience du
terrain nous a clairement montrĂ© quâaucun pixel nâĂ©tait constituĂ© dâune seule composante Ă 100 %
In vivo estimation of pigment composition and optical absorption cross-sectionby spectroradiometry in four aquatic photosynthetic micro-organisms
International audienceThe objective of the present study was to estimate in vivo pigment composition and to retrieve absorption cross-section values, aâ, of photosynthetic micro-organisms using a non-invasive technique of reflectance spectrometry. To test the methodology, organisms from different taxonomical groups and different pigment composition were used (Spirulina platensis a Cyanophyta, Porphyridium cruentum a Rhodophyta, Dunaliella tertiolecta a Chlorophyta and Entomoneis paludosa a Bacillariophyta) and photoacclimated to two different irradiance levels: 25 ÎŒmol photon mâ2 sâ1 (Low Light, LL) and 500 ÎŒmol photon mâ2 sâ1 (High Light, HL). Second derivative spectra from reflectance were used to identify pigment in vivo absorption bands that were linked to specific pigments detected by high performance liquid chromatography. Whereas some absorption bands such as those induced by Chlorophyll (Chl) a (416, 440, 625 and around 675 nm) were ubiquous, others were taxonomically specific (e.g. 636 nm for Chl c in E. paludosa) and/or photo-physiological dependent (e.g. 489 nm for zeaxanthin in the HL-acclimated S. platensis). The optical absorption cross-section, aâ, was retrieved from reflectance data using a radiative transfer model previously developed for microphytobenthos. Despite the cellular Chl a decrease observed from LL to HL (up to 88% for S. platensis), the aâ increased, except for P. cruentum. This was attributed to a âpackage effectâ and to a greater absorption by photoprotective carotenoids that did not contribute to the energy transfer to the core Chl a
Seasonality of microphytobenthos revealed by remote-sensing in a south european estuary
The spatio-temporal variation of microphytobenthos (MPB) at the scale of a large estuary (Tagus estuary,
Portugal) was studied using a combination of field and satellite remot esensing data during 2003.This is
the first attempt to use remote sensing to study MPB in an ecosystem with a Mediterranean -like climate.
Satellite pour l'Observation de la Terre(SPOT) and Medium Resolution Imaging Spectrometer(MERIS)
images were used to map benthic microalgae through the application of a Normalized Difference
Vegetation index (NDVI).A significant relationship between in-situ benthic chlorophyll a measurements
and SPOT NDVI values was used to derive a map for biomass spatial distribution.
At the scale of the whole intertidal area, NDVI time-series from 2003 revealed that MPB showed clear
temporal variations,with lower values observed in summercompared to winter.This seasonal trend was
found both in the SPOT and MERIS images and maybe the result of extreme high temperatures that
inhibit MPB growth.Thema in MPB biofilms were spatially stable through time at a large scale.Maximum
NDVI values during the winter were found in the high shore with decreasing NDVI values towards the
low shore. MPB light limitation at the lowest bathymetries is likely to occur in winter due to the high
turbidity of Tagus estuary.
The biomass spatial distribution map,obtained for January 2003,indicated low values ranging from
0 to 20 mg Chl am-2 for the lower shores, while in the upper shore biomass varied between 60 and
80 mg Chl am-2. This study suggests striking differences in MPB seasonal patterns between the
northern and southern European estuaries and stresses the need for ecophysiological approaches to
investigate the role of thermo-andphoto-inhibition as structuring factors for MPB biomass distribution
Shergottite Northwest Africa 6963: A PyroxeneâCumulate Martian Gabbro
Northwest Africa (NWA) 6963 was found in GuelmimâEsâSemara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was resubclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original resubclassification and enable full comparison between this and other Martian samples; here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion). NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites but importantly does not contain any fineâgrained mesostasis. Consistent with the mineralogy, the reflectance (visible/nearâinfrared and thermal infrared) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface
RĂLE DE L'OCCUPATION DU SOL VIS Ă VIS DE LA MODĂLISATION DES FLUX ENERGĂTIQUES ET HYDRIQUES EN MILIEU URBAIN ET PĂRIURBAIN
National audienceLe projet Rosenhy vise Ă Ă©tudier lâimpact de lâoccupation du sol sur la modĂ©lisation mĂ©tĂ©orologique et hydrologique en termes de flux Ă©nergĂ©tiques et hydriques, en milieu urbain et pĂ©riurbain. Trois sites appartenant aux observatoires français OTHU et ONEVU sont au centre de ce projet. Le quartier urbain hĂ©tĂ©rogĂšne du Pin sec (Nantes), impermĂ©abilisĂ© Ă environ 45%, a fait lâobjet dâune campagne expĂ©rimentale durant le mois de juin 2012, visant Ă estimer les flux de chaleur sensible et latente avec une haute rĂ©solution spatiale et temporelle par rapport aux mesures rĂ©alisĂ©es en continu sur ce site depuis 5 ans. Deux bassins versant pĂ©riurbains (La ChĂ©zine Ă Nantes et lâYzeron Ă Lyon), avec un taux dâimpermĂ©abilisation moins important (environ 10%) mais grandissant depuis plusieurs dĂ©cennies, sont aussi Ă©tudiĂ©s. Ces deux derniers sites bĂ©nĂ©ficient dâun suivi hydromĂ©tĂ©orologique depuis 10 ans pour la ChĂ©zine et 15 ans pour lâYzeron. Sur ces trois sites, diffĂ©rentes sources de donnĂ©es dâoccupation du sol Ă diffĂ©rentes rĂ©solutions sont disponibles :diffĂ©rentes bases de donnĂ©es gĂ©ographiques communĂ©ment utilisĂ©es par la communautĂ© scientifique et les collectivitĂ©s et des donnĂ©es tĂ©lĂ©dĂ©tectĂ©es (multispectrales et hyperspectrales). Lâutilisation de ces donnĂ©es en entrĂ©e de diffĂ©rents modĂšles mĂ©tĂ©orologiques et hydrologiques implique un travail dâanalyse et de classification pour adapter les informations aux besoins des modĂšles. Dans ce projet, les diffĂ©rents modĂšles adaptĂ©s au milieu urbain ou pĂ©rirubain sont Ă©valuĂ©s et amĂ©liorĂ©s. Ainsi, les modĂšles hydrologiques pĂ©rirubains sont en dĂ©veloppement pour prendre en compte les diffĂ©rentes pratiques de gestion des eaux pluviales existantes (noues, toitures vĂ©gĂ©talisĂ©es, ...). Lâutilisation conjointe des donnĂ©es simulĂ©es par les diffĂ©rents modĂšles aidera Ă dĂ©terminer le rĂŽle de la part des surfaces naturelles et artificielles sur les bilans Ă©nergĂ©tique et hydrique en milieu plus ou moins urbanisĂ©. Le milieu pĂ©riurbain Ă©tant en Ă©volution, le projet sâintĂ©ressera aussi Ă des scĂ©narios dâurbanisation prospectifs en regardant dâune part lâimpact de la densification sur les scĂ©narios construits pour lâYzeron lors du projet AVuPUR (ANR-VMCS, 2008-2011) et dâautre part, en rĂ©flĂ©chissant conjointement avec Nantes MĂ©tropole, aux possibles voies dâĂ©volution sur le bassin de la ChĂ©zine
Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and <sup>40</sup>Ar/<sup>39</sup>Ar investigations
New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union
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