Applications géodésiques du système DORIS à l'Institut géographique national

IGN is in charge of the installation and maintenance of the DORIS orbit determination network. More recently, in collaboration with JPL, precise geodetic computations were performed. The goal of this paper is to recall the various historic contributions of IGN to the DORIS system in their international context and then to describe a new estimation technique developed for a multi-satellite mode, making full profit of a better modeling for satellites and ground clocks as well as tropospheric correction parameters. Derived geodetic results demonstrate a precision in the order of 1 cm for station positions. To cite this article: P. Willis et al., C. R. Geoscience 337 (2005).     

In situ anaerobic degradation of petroleum alkanes in marine sediments: preliminary results

The degradation of acyclic petroleum hydrocarbons was studied during a 24-month experiment in Mediterranean coastal sediments (Gulf of Fos). Sediment cores entirely contaminated with oil (Arabian Light Crude Oil) were incubated in situ. The use of conservative tracers of sediment's particles reworking (luminophores) allowed the distinction of the reworked layer from the anoxic deeper sediments. Using the 17α,21βC₃₀ hopane (C₃₀H) as an inert internal reference, we could demonstrate that, after 24 months of experiment, acyclic petroleum hydrocarbons can be degraded under natural anaerobic conditions. The reactivity of individual alkanes appeared to depend on their chemical structure. To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).     

Assimilation de données dans un modèle d'écosystème marin de la mer Ligure

The objective is to explore the potentialities of sequential statistical estimation methods to assimilate observations in a primary production biological model coupled to a vertical 1D hydrodynamical model characterised by a k–l turbulent closure. The assimilation method is derived from the SEEK filter (Singular Evolutive Extended Kalman filter), which uses an error subspace represented by multivariate empirical orthogonal functions (EOFs). Real data assimilation experiments collected at sea have been realised to reconstruct the variability of the Ligurian Sea ecosystem during the FRONTAL field experiment. To cite this article: S. Magri et al., C. R. Geoscience 337 (2005).     

Amazon River discharge estimated from TOPEX/Poseidon altimetry

This paper presents an application of the TOPEX/Poseidon (T/P) satellite altimetry data to estimate river discharge at three sites along the Amazon River. We discuss the methodology to establish empirical relationships between satellite-derived water levels and daily estimations of river discharges based on rating curves and in situ level measurements at gauging stations. Three sites are chosen: Manacapuru (River Solimões), Jatuarana (nearby the confluence of the Solimões and Rio Negro rivers) and Óbidos (Amazon River). We then reconstruct the satellite-based river discharge over a 10-year time span (1992–2002). Comparison between satellite-derived and river discharge at the gauging stations shows that the T/P data can successfully be used for hydrological studies of large rivers, in providing in particular discharge estimates when in situ data are not available. To cite this article: E.A. Zakharova et al., C. R. Geoscience 338 (2006).     

Le Viséen supérieur d'Azarhare (Maroc central) : environnements de dépôt,datation et évolution diagénétique

The unexplored area of Azarhare in central Morocco is studied thanks to three sections composed of five lithological facies Main biostratigraphical and sedimentological results concern (1) the identification of Late Visean biozones, with important presence of problematic algae Ungdarella, (2) the regional extension of deposit sequences SD5, SD6 and SD7 previously defined, (3) and an analysis of the diagenetic kaolinite. To cite this article: A. Karim et al., C. R. Geoscience 337 (2005).     

Discovery of two ophiolite complexes of different ages in the Khoy area (NW Iran)

New field and laboratory studies on the ophiolite of Khoy (northwestern corner of Iran) lead to the discovery that there are not one, but two ophiolitic complexes in the Khoy area: (1) an old, poly-metamorphic ophiolite, whose oldest metamorphic amphiboles yielded a Lower Jurassic apparent ⁴⁰K–⁴⁰Ar age, and whose primary magmatic age should logically be pre-Jurassic (Upper-Triassic?); (2) a younger non metamorphic ophiolite of well dated Upper Cretaceous age. To cite this article: M. Khalatbari-Jafari et al., C. R. Geoscience 335 (2003).     

Origine du matériel altéré et hétérogénéité du matériel originel de la météorite Foum Tatahouine (Tunisie) : éléments traces et isotopes du SrOrigin of altered material and heterogeneity of original material in the Foum Tatahouine meteorite (Tunisia): Trace elements and Sr isotopes

Foum Tatahouine is one of the rare meteorites that allow studying weathering effects since their fall on Earth. The comparison between clasts collected the very day of the fall (1931) and in 1994 showed some chemical modifications. We report data obtained on samples collected in 2000. The purpose of this study is: (1) to define (REE, Sr isotopes) the terrestrial weathering origin that is to be found in soil carbonates, (2) to show (Sr, Rb, REE) that the weathering effects are small on centimetre-sized samples, and (3) to emphasise (HREE) Foum Tatahouine samples heterogeneity. To cite this article: Y. Bentahila et al., C. R. Geoscience 334 (2002) 267–272.     

Effet de la turbidité sur la dégradation des pigments phytoplanctoniques dans l'estuaire de la GirondeEffect of turbidity on phytoplanktonic pigments degradation in the Gironde Estuary

In the Gironde Estuary, most part of phytoplanktonic material carried by the rivers is mineralised in the maximum turbidity zone (MTZ). In order to follow the degradation of the phytoplanktonic material into the MTZ, we developed an in vitro approach based on the monitoring of phytoplanktonic pigments. Algal material from two chlorophytes (Scenedesmus suspicatus Chaudat and Chlamydomonas sp.) was incubated in the dark during 28 days into water samples from the Gironde estuary MTZ, at variable suspended solid concentrations (SPM) as well as in a sterilised turbid sample. First order decay constants of chlorophylls a and b and lutein increased by a factor 3 to 5 between SPM of 0 and $\text{3 }\text{g}\text{l}{}^{\mathrm{?1}}$. The production of pheophytin a in the presence of particles and the lack of degradation in the sterilised turbid sample confirmed the effect of attached bacteria on the particles. To cite this article: E. Lemaire et al., C. R. Geoscience 334 (2002) 251–258.     

Enrichissement en uranium authigène dans les sédiments glaciaires de l'océan AustralEnrichments in authigenic uranium in glacial sediments of the Southern Ocean

Four sediment cores from the Polar frontal zone and the Antarctic zone in the Indian sector of the Southern Ocean present an increase of authigenic uranium during glacial periods. We show that this increase in uranium is due to a combination of (i) an increase in the lateral transport of organic matter, (ii) a decrease in the oxygen in deep waters, and (iii) a process of diagenesis. It appears that uranium concentration cannot be used as a proxy of palaeoproductivity in the Southern Ocean, as previously suggested by Kumar et al. in 1995. To cite this article: L. Dezileau et al., C. R. Geoscience 334 (2002) 1039–1046.     

Natural minerals and divided solids: methodology for understanding surface phenomena related to industrial uses and environmental problems

This introductory paper aims at presenting the series of articles in this special issue of Comptes Rendus Geoscience devoted to the study and use of natural minerals. The research methodology that can be applied to the investigation of surface phenomena related to adsorption, exchange and retention onto the surface of natural minerals and divided solids is presented. A special focus is given to the retention of surfactant molecules, in view of their considerable importance in numerous industrial fields, such as, for instance, mineral flotation and enhanced oil recovery. Throughout this introductory paper, the relevance of this research approach to the field of geosciences is stressed by listing various applications where it can provide conclusive answers, especially for environmentally related issues. To cite this article: J.-M. Cases, C. R. Geoscience 334 (2002) 585–596.     

Metastable staurolite–cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M₁, while coaxial conditions prevailed during later contact metamorphic M₂ growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves conditions during M₁, estimated by AFM diagrams and P–T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite–andalusite assemblages of M₂, suggesting polyphase metamorphism and decompression along a clockwise P–T path for the staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).     

Contribution à l'étude de la vitesse critique d'érosion des sols cohésifs

The phenomena of erosion and sedimentation in rivers are treated qualitatively and quantitatively. An analytical solution of the bedload critical velocity for erosion and sedimentation is proposed, depending on material property, geometry and on flow characteristics. The critical velocity of erosion of the river bed, as defined by Hjulström in 1935, can be explained by a cohesive behaviour of the grains. Whereas the cohesive force is dominated by an 1/r² (r is the radius of grains) force when grains are small, it can be modelled by the cohesion C and the friction angle φ for larger grains. To cite this article: J. Gargani, C. R. Geoscience 336 (2004).     

Neotectonics and seismic hazard assessment in Hengchun Peninsula,southern Taiwan

Neotectonic data may indicate whether stored elastic energy will be dissipated or released destructively. The Hengchun Peninsula of Taiwan is the southern extremity of the Central Range of Taiwan and thus an emergent part of the accretionary wedge resulting from subduction of Eurasia beneath the Philippine Sea plate at the Manila Trench. Radiocarbon dating of fossil shorelines on the peninsula shows that it has been uplifted at an average rate of 3.8 mm/yr during the Holocene. About $1/3$ of the uplift is due to deformation along the Hengchun reverse fault but, in contrast with the Chelungpu and other low angle reverse faults west of the Central Range, it accommodates strain principally by aseismic creep. To cite this article: C. Vita-Finzi, J.-C. Lin, C. R. Geoscience 337 (2005).     

Rôle de la végétation dans la protection contre l'érosion hydrique de surface

This paper is a review on the current knowledge concerning the role of vegetation in protection against surface hydric erosion. It first explains the mechanisms of vegetation action in relation to processes of erosion and sedimentation. Secondly, it presents the results of researches carried out on the efficiency of different vegetation structures and the significance of the rate of vegetation cover for combating gully erosion. To cite this article: F. Rey et al., C. R. Geoscience 336 (2004).     

Apports des phyllosilicates dans la différenciation entre altération hypogène et altération supergène dans le basalte triasique du Moyen Atlas (Maroc)Contribution of phyllosilicates to distinguish between hypogene alteration and weathering in Triassic basalt from Middle Atlas (Morocco)

Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: (i) metamorphic basalt, where iron occurs mainly as the ferrous form; (ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; (iii) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884.     

Les paléoenvironnements éocènes à crinoïdes pédonculés des marnes de Miretrain (Angoumé, SW France) : interprétations paléobathymétriques

A well-diversified fauna of stalked crinoids was found in the Eocene series exposed in the quarry of Miretrain at Angoumé (Landes, southwestern Aquitaine, northern edge of the Tercis diapir structure). The most frequent taxa belong to the genera Isselicrinus and Conocrinus. Adaptive stalk characters related to depth in the pentacrinid genus Isselicrinus allow us to document the bathymetric zonation of stalked crinoids during the Eocene. Both stalked crinoids, benthic foraminifera and additional organisms give convergent information on palaeodepths throughout the progradation of sedimentary bathyal slope between the two major tectonic events providing Pyrenean foreland shortening. The depth changes from more than 700 m (maybe about 1000 m) at the base of the series (Middle Lutetian) to about 50 m at the top (Late Bartonian). To cite this article: M. Roux et al., C. R. Geoscience 338 (2006).     

Les sondages électromagnétiques temporels comme outil de reconnaissance du gisement phosphaté de Sidi Chennane (Maroc) : apport à la résolution d'un problème d'exploitation

Exploitation of the phosphatic layers in Sidi Chennane deposit (Morocco) collides frequently with problems bound to the existence, in the phosphatic series, of sterile bodies qualified as derangements. Our study shows that these bodies, masked by the Quaternary cover can be mapped using the Time-Domain ElectroMagnetic Soundings method (TDEM). It is based on the acquisition and the interpretation of a series of tests carried out above a visible derangement in an old trench of exploitation and on 2500 TDEM soundings carried out in a virgin area of the deposit. The article concerns to the analysis of the results and of the proceeding for a possible large geophysics survey. To cite this article: A. Kchikach et al., C. R. Geoscience 338 (2006).     

Mise en évidence d'une discontinuité émersive majeure au sein de la plate-forme récifale plio-pléistocène de l'avant-arc des Petites Antilles

A major emersion surface (S1) reveals at least a 60-m relative sea-level fall within the ‘Calcaires supérieurs’ of Grande-Terre (Lesser Antilles fore-arc), an Upper Pliocene–Lower Pleistocene reefal unit. We thus divide the ‘Calcaires supérieurs’ into 2 units. S1, together with the transgresssive calcarenitic and the Acropora-rich reefal units on top of it, corresponding to the record of a complete eustatic cycle by the carbonate platform of this active margin. A comparison with the carbonate platform of the Bahama Bank stable margin, allows us to relate S1 to the major emersive event of 1.66 Ma and to infer that its origin is glacio-eustatic. To cite this article: J.-L. Léticée et al., C. R. Geoscience 337 (2005).     

Caractérisation par résonance paramagnétique électronique (RPE) de quartz naturels issus de différentes sources

Natural quartz crystals coming from Madagascar, Angola, Brazil, and Spain were studied by EPR after β irradiation in order to characterise the paramagnetic centres due to impurities, and examine in which way their relative proportion could constitute a useful criterion for origin researches. In all our samples, EPR experiments revealed germanium centres [GeO₄/M⁺]° with M = Li or Na, aluminium ones [AlO₄/h]° and two titanium centres ([TiO₄/H⁺]° and [TiO₄/M⁺]°) also appeared, except for samples coming from Spain. The comparison of the relative concentrations of paramagnetic species led us on to draw diagrams that allowed distinguishing the origins of the studied samples using only EPR spectra. To cite this article: M. Duttine et al., C. R. Geoscience 334 (2002) 949–955.     

Importance du choix des échelles de temps pertinentes pour l'étude du fonctionnement d'écosystèmes marins littoraux

Semi-continuous (1/30 min) records of physicochemical parameters (T,S, DO and pH) have been measured at 1.50 m in Mediterranean coastal waters (Marseilles) from 1998 to 2000, together with meteorological parameters. Physicochemical data were studied in order to synthesise the information at different time scales (inter-annual, season, day, hour), particularly the semi-conservative DO and pH response to hydroclimatic variables. At a daily time scale, important circadian dynamic has been measured under all circumstances with a strong reactivity to irradiance change under the influence of biological activity. This work may contribute to the definition of new tools for coastal waters observation. To cite this article: N. Bensoussan et al., C. R. Geoscience 336 (2004).