Évidences paléomagnétiques et paléontologiques en faveur d'une position antipodale du craton Ouest-Africain et de la Chine du Nord : conséquences paléogéographiques

Reappraisal of the palaeomagnetic data previously published about the West African Craton and their comparison with the most recent information obtained on doleritic dykes and stromatolite-bearing formations allow us to propose, after a critical examination of the old geochronological data, a new path for the West African Craton between 2200 and 700 Ma. This path has been compared with that of eastern China (which can be simply divided into a northern and a southern China block) between 1400 and 700 Ma. We can observe that these two blocks show a common path before their accretion to the Rodinia super-continent. These two blocks, which were located close the equator and at the outboard of Rodinia at about 1000 Ma, were also characterized by identical stromatolitic assemblages. This result shows that West Africa and Eastern China represented the antipodal continental margins of Rodinia. One can also deduce from our data that the accretion of Rodinia mainly resulted from north–south-oriented displacements. To cite this article: J.-P. Lefort et al., C. R. Geoscience 336 (2004).     

Phase de Laves dans la première opale CT bidisperse

A Mexican opal from the state of Jalisco has a bidisperse, perfectly ordered structure with framboid silica lepispheres of two different diameters. Its structure has been determined by the observation of a vicinal section, close to a dense plane. Fresh breaks along a vicinal plane emphasized regular steps, which were studied by means of SEM. We have been able to propose unit cell models, theoretical positions of the two sets of spheres, and probable space group, P6₃/mmc. This structure arranges similarly to that of a diatomic compound having an AB₂ stoichiometry and corresponds to a cubic (MgCu₂-type) or hexagonal (MgZn₂-type) Laves phase. This last variety seems to be the most probable structure. To cite this article: J.-P. Gauthier et al., C. R. Geoscience 336 (2004).     

Sismique marine haute résolution 3D : un nouvel outil de reconnaissance à destination de la communauté scientifique

This note presents the first results of the development of 3D high-resolution marine seismic method designed for scientific application. A particular attention was paid to the realisation of an operational system to be in agreement with the expected goals in term of acquisition and processing. To cite this article: Y. Thomas et al., C. R. Geoscience 336 (2004).     

La réflectivité d'un sédiment marin poreux

Résumé Les formules établies pour les liquides ou les solides sont pratiquement applicables aux sédiments poreux saturés non-consolidés, contrairement aux grés. L'étude présentée permet de comprendre la nature de l'onde longitudinaleP2 et du couplage dynamique. Elle illustre l'extension du principe de Huyghens aux mileux poreux.

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Summary An incident longitudinal wave (P1) does not exit a longitudinal wave of the second kind (P2), when reflected on a saturated open porous sediment. Only the granular part is acoustically active. For unconsolidated marine sediments, the classical formulas for reflection and transmission can be used with a good approximation, contrarily to consolidated saturated sediments or cemented ones. This analysis clears up the concepts ofP2 wave and dynamic coupling; it also illustrates the extension of Huyghens's priciple to porous mediums.

     

Experimental study of long nonlinear internal waves in rotating fluid

Experiments were performed on the rotating platform 14 m in diameter equipped with a simple internal wave generator. Internal waves were generated for a wide range of Coriolis parameters. When the rotation is very weak, i.e., when the internal Rossby radius of deformation is much larger than the wavelength, then the stable nonlinear waves generated are solitary waves. These have a horizontal crest, as in the nonrotating case. When the rotation is strong, i.e., when the internal Rossby radius is at most comparable with the wavelength, then Sverdrup-like periodic waves can be generated, but no solitary wave can then propagate. For the intermediate case, Ostrovsky waves are generated. Their phase speed increases with increasing amplitude. Then, there are two characteristic wave lengths: one which varies with the inverse square root of the amplitude, as for the KdV wave, and the other, linked with the rotation, which varies as the square root of the amplitude. The experimental results are thus in agreement with most of the conclusions in recent analytical developments.     

Chlorinated hydrocarbons in the open mediterranean ecosystem and implications for mass balance calculations

Studies on the biogeochemical cycling of organic contaminants in the Mediterranean have demonstrated the importance of polychlorinated biphenyls (PCBs) as relatively stable markers of recent anthropogenic influence in ocean systems. This paper presents results of hydrocarbon analyses of deep water profiles, sediments and their associated surface flocculent layers, and zooplankton samples collected in the western basin. Seawater concentrations were higher than those previously reported for the eastern basin and were consistent with the presence of industrial sources in the northwestern segment. In the water column, the percent of PCBs associated with filterable particles was related to the ambient concentrations of total suspended matter, distance from coastal input sources and on depth. The occurrence of deep water residues primarily in the dissolved phase and observations of subsurface maxima in seawater concentrations during a season of high surface productivity were consistent with the predictions of vertical transport models based on residues associated with sinking particles and equilibrium partitioning. Flocculent layers at the sea/sediment interface contained two and three orders of magnitude, respectively, more PCBs and petroleum hydrocarbons (PHCs) than their associated surface sediments. The flocculent particles also contained several biogenic hydrocarbons presumably originating from plankton and relatively soluble and labile contaminants such as hexachlorohexane isomers (HCH) and hexachlorobenzene (HCB), thus confirming that the majority of the flux of hydrocarbons to sediments is carried on rapidly settling large particles. By combining the sediment and flocculents data with published sedimentation rates for the deep basins of the Mediterranean, a yearly flux rate of PCBs to the open sea sediments was estimated as 13 μg m⁻² yr⁻¹ or less than half of the rate measured in a coastal sediment trap experiment. Analytical results are placed in the context of other distribution data for PCBs and long-term flux studies to construct a partial mass balance budget for this semi-enclosed sea. The computed coastal inventory showed that about 35% remains suspended in the water column while the majority of residues are deposited in coastal sediments. However in the open sea, the deep water column may contain up to 70% of the total inventory and may be a continually increasing reservoir of stable organic contaminants reaching the ocean. The budget shows that priority for improved research and monitoring efforts in ocean systems should be given to continued advancement in techniques for the precise measurement of deep water concentrations and for measuring current atmospheric inputs and sedimentation rates in order to develop more accurate ocean flux models.     

ERORUN-STAFOR: A collaborative observatory for the multidisciplinary study of the critical zone processes in a tropical volcanic watershed including a Tropical Montane Cloud Forest

Tropical volcanic islands are biodiversity hotspots where the Critical Zone (CZ) still remains poorly studied. In such steep topographic environments associated with extreme climatic events (cyclones), deployment and maintenance of monitoring equipment is highly challenging. While a few Critical Zone Observatories (CZOS) are located in tropical volcanic regions, none of them includes a Tropical Montane Cloud Forest (TMCF) at the watershed scale. We present here the dataset of the first observatory from the French network of critical zone observatories (OZCAR) located in an insular tropical and volcanic context, integrating a ‘Tropical Montane Cloud Forest’: The ERORUN-STAFOR observatory. This collaborative observatory is located in the northern part of La Réunion island (Indian Ocean) within the 45.0 km² watershed of Rivière des Pluies (i.e., Rainfall river) which hosts the TMCF of Plaines des Fougères, one of the best preserved natural habitats in La Réunion Island. Since 2014, the ERORUN-STAFOR monitoring in collaboration with local partners collected a multidisciplinary dataset with a constant improvement of the instrumentation over time. At the watershed scale and in its vicinity, the ERORUN-STAFOR observatory includes 10 measurement stations covering the upstream, midstream and downstream part of the watershed. The stations record a total of 48 different variables through continuous (sensors) or periodic (sampling) monitoring. The dataset consists of continuous time series variables related to (i) meteorology, including precipitation, air temperature, relative humidity, wind speed and direction, net radiation, atmospheric pressure, cloud water flux, irradiance, leaf wetness and soil temperature, (ii) hydrology, including water level and temperature, discharge and electrical conductivity (EC) of stream, (iii) hydrogeology, including (ground)water level, water temperature and EC in two piezometers and one horizontally drilled groundwater gallery completed by soil moisture measurements under the canopy. The dataset is completed by periodic time series variables related to (iv) hydrogeochemistry, including field parameters and water analysis results. The periodic sampling survey provides chemical and isotopic compositions of rainfall, groundwater, and stream water at different locations of this watershed. The ERORUN-STAFOR monitoring dataset extends from 2014 to 2022 with an acquisition frequency from 10 min to hourly for the sensor variables and from weekly to monthly frequency for the sampling. Despite the frequent maintenance of the monitoring sites, several data gaps exist due to the remote location of some sites and instrument destruction by cyclones. Preliminary results show that the Rivière des Pluies watershed is characterized by high annual precipitation (>3000 mm y⁻¹) and a fast hydrologic response to precipitation (≈2 h basin lag time). The long-term evolution of the deep groundwater recharge is mainly driven by the occurrence of cyclone events with a seasonal groundwater response. The water chemical results support existing hydrogeological conceptual models suggesting a deep infiltration of the upstream infiltrated rainfall. The TMCF of Plaine des Fougères shows a high water storage capacity (>2000% for the Bryophytes) that makes this one a significant input of water to groundwater recharge which still needs to be quantified. This observatory is a unique research site in an insular volcanic tropical environment offering three windows of observation for the study of critical zone processes through upstream-midstream-downstream measurements sites. This high-resolution dataset is valuable to assess the response of volcanic tropical watersheds and aquifers at both event and long-term scales (i.e., global change). It will also provide insights in the hydrogeological conceptual model of volcanic islands, including the significant role of the TMCFs in the recharge processes as well as the watershed hydrosedimentary responses to extreme climatic events and their respective evolution under changing climatic conditions. All data sets are available at https://doi.org/10.5281/zenodo.7983138 .