Dispersion and cost analysis of some finite difference schemes in one-parameter acoustic wave modeling

Given a precision threshold to be imposed on the group velocity error and a finite difference scheme for the acoustic wave equation, it is possible to determine time-step and grid-spacing in an optimal manner, i.e., so as to minimize the computational cost. Using this optimal cost as a criterion, it becomes easy to compare schemes for efficiency in homogeneous media. Heterogeneous media with constant density can be accommodated to a certain extent by minimizing the cost over a range of Courant numbers. Such analysis shows that, amongst the second-order Taylor series schemes in time, higher-order schemes are generally more efficient than lower-order schemes. However, this result does not extend to very high order schemes.     

U-Pb,Rb-Sr and Sm-Nd chronology of granitic basement,hydrothermal albitites and uranium mineralization (Lagoa Real,South-Bahia,Brazil)

The granites orthogneisses, hydrothermal albitities and rocks which have suffered uranium mineralization from the Lagoa Real District (South State of Bahia, Brazil) have been investigated by U-Pb, Rb-Sr and Sm-Nd techniques. U-Pb values on zircons from the granitic protolith give an age of 1725 Ma; U-Pb on U-mineralization dates the primary mineralization at 1395 Ma and indicates a reworking at 480 Ma, which may represent the age of the thrusting of the Lagoa Real complex over Espinhaço metasediments during the Brazilian orogeny. These two dates are given by Rb-Sr on albitites, but from sparse information and are not supported by unequivocal arguments. The uranium deposition and sodium metasomatism, however, cannot be linked either with the thermal history of the granite or with Brazilian thrusting. Sm-Nd gives scattered results which are suggestive of autochtonous reworkings of REEs. These results lead to the following suggestions (1) magmatic activity of subalkaline affinity existed within the Sao Francisco craton at about 1.7 Ga (2) such subalkaline plutons are likely sources for U-mineralization (3) at about 1.4 Ga an unknown event caused hydrothermal activity leading to U-deposition at Lagoa Real (4) Brazilian overthrusting at about 480 Ma did not play any genetic role in the hydrothermal activity.     

America-Eurasia plate boundary in eastern Asia and the opening of marginal basins

The opening of the Arctic Ocean during the past 55 Ma resulted in relative rotation of America with respect to Eurasia about a pole located in eastern Siberia, near the plate boundary. The extensional plate boundary enters deep inside the Eurasian continent up to the rotation pole. On the opposite side of the pole, on the Pacific side of the plate boundary, compressive tectonics are recorded along Sakhalin and Hokkaido. From the Oligocene to Middle Miocene, the relative movement was accommodated by strike-slip motion along Sakhalin and Hokkaido although the rotation pole was not located at a significatively different position from now. In this paper we explain this by independent motion of the southernmost tip of the American plate towards the Pacific margin which behaves as a free boundary. This oceanward motion resulted in an extension of the American plate giving rise to the wedge structure of the Okhotsk Sea. The Japan Sea opened as a pull-apart basin along the strike-slip boundary; finally the increasing extension in the Okhotsk Sea led to the opening of the oceanic Kuril Basin.     

Volcans de la chaîne des Puys (Massif central,France) : point sur la chronologie Vasset–Kilian–Pariou–Chopine

On the basis of radiocarbon ages obtained on wood burned by base surges and tephrochronological observations, it has been possible to assess that the Puy Chopine, aged ca 9700 yr is older by a few centuries than both the Vasset and Kilian volcanoes. The last two have nearly the same age, ca 9300–9400 yr. Layers of trachytic tephra, most probably originated from Kilian or Vasset, can be observed overlapping those from the Pariou. This last volcano had been previously thought to be the younger of the area on the basis of the observation of trachytic tephra beneath its own tephra. However, new observations allow us to assume that those trachytes originated from the acid phase of the Pariou itself. Therefore, it can be assessed that the four volcanoes erupted according to the following sequence: Chopine/Pariou/?Vasset?/Kilian; the chronological situation of Vasset lies on poor arguments and it should be confirmed. To cite this article: D. Miallier et al., C. R. Geoscience 336 (2004).     

A peculiar distribution of radial velocities of faint radio-galaxies with 13.0≤m corr≤15.5

A sample of 41 radio-galaxies with 13.0m _corr15.5 has been analyzed to test the angular redshift anisotropy discovered on Sc I galaxies by Rubin, Rubin and Ford. The sample does not present their anisotropy but contains an even more curious distribution of radial velocities which suggests that the Rubin-Ford effect results from an anomalous redshift of light when it travels through clusters of galaxies.     

Laboratory simulation of an oxidizing perturbation in a deep granite environment

An experiment designed to study oxidizing perturbations in deep crystalline rock, a potential host for nuclear waste disposal, was conducted. This experiment simulated a fracture surface in contact with circulating groundwater, in which dissolved oxygen was injected periodically. Major physicochemical and biological parameters were monitored during this 1-yr experiment. Modeling of the results indicates that the kinetics of oxygen uptake may be represented by a simple steady-state rate law combining enzymatic catalysis (Monod) and a first-order rate law. Combined chemical and biological data demonstrate the coupling of organic/inorganic processes during the uptake of dissolved oxygen and the progressive return to reducing conditions. Timescales for these stages are discussed. Experimental results also suggest that iron-reducing bacteria, which are robust and well-adapted microorganisms, play a key role in these interfacial processes. These results show that an operational definition of the “redox buffering capacity” in a granitic medium cannot ignore the effect of bacteria and therefore the controls on bacterial substrates (organic carbon, H₂, CH₄, CO₂).     

Water organisation at the solid–aqueous solution interfaceOrganisation de l'eau à l'interface solide–solution aqueuse

The aim of this review is to examine the present knowledge about water structure close to an interface or confined in porous spaces. First, the structure of liquid water is briefly described. Though its understanding remains incomplete, it appears that it is largely dominated by the hydrogen bond network and its dynamical evolution. The presence of any ‘foreign’ substance in water perturbs such a structure by changing at least locally the hydrogen bond network. For this reason, the presence of a solid interface significantly modifies the structure of the first adsorbed layers. Whatever the support, it is now clearly evidenced that structural perturbations are limited to distances lower than 10–15 Å from the interface. The nature, energetic heterogeneity and hydrophilicility/hydrophobicity of the solid surface influence the arrangement of water molecules. This surface organisation must definitely be considered when studying adsorption phenomena at the solid–aqueous solution interface. The relevance of such problems to geosciences is illustrated by a few situations in which water structure plays a prominent role. To cite this article: L.J. Michot et al., C. R. Geoscience 334 (2002) 611–631.     

Simulation des changements climatiques au cours du XXIe siècle incluant l'ozone stratosphériqueSimulation of climate changes during the 21st century including stratospheric ozone

Two climate simulations of 150 years, performed with a coupled ocean/sea-ice/atmosphere model including stratospheric ozone, respectively with and without heterogeneous chemistry, simulate the tropospheric warming associated with an increase of the greenhouse effect of carbon dioxide and other trace gases since 1950 and their impact on sea–ice extent, as well as the stratospheric cooling and its impact on ozone concentration. The scenario with heterogeneous chemistry reproduces the formation of the ozone hole over the South Pole from the 1970s and its deepening until the present time, and shows that the ozone hole should progressively fill during the coming decades. To cite this article: J.-F. Royer et al., C. R. Geoscience 334 (2002) 147–154.     

Argiles et zéolites dans l'altération d'un volcan bouclier en milieu tropical (Le Piton des Neiges,La Réunion)

The drilling of a gallery through the Roche-Écrite massif (Reunion Island) allowed to sample the old formations of the ‘Piton des Neiges’ volcano. The mineralogical study of the alteration products of the formations showed significant variations of the zeolites/clay minerals ratio depending on the location of the rocks in the volcano. The abundance of zeolites in the formation near the surface hydrographic network or the underground water drainage system seems to be due to the physicochemical processes favouring zeolites in more open systems in their competition with secondary clays. To cite this article: L. Bret et al., C. R. Geoscience 335 (2003).