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431.
Pascal Richet 《Physics and Chemistry of Minerals》1990,17(1):79-88
Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2. The glass transition is very sensitive to the impurity content, with a T g ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative C p change at T g of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two C p cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO2 and SiO2 the coordination state markedly affects C p and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S 298 = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO2 and SiO2, respectively. The high-temperature C p's of coesite and stishovite are likely similar to those of the low-pressure SiO2 forms. Finally, these results, low-temperature C p data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO2 modifications. 相似文献
432.
Gwénaëlle Chaillou Pierre Anschutz Carole Dubrulle Pascal Lecroart 《Aquatic Geochemistry》2007,13(2):157-172
Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment.
Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O2, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic
species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose
relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O2 from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation.
After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn2+ and then Fe2+, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical
diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the
transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases
of Fe2+. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides
buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification
processes led to the formation of a NO3−/NO2− rich horizon at the new oxic horizon. Over the experiment period, NO3− concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production. 相似文献
433.
Osmar O. Möller Patrice Castaing Elisa Helena Leão Fernandes Pascal Lazure 《Estuaries and Coasts》2007,30(2):311-320
The Patos Lagoon is a choked, microtidal coastal laggon situated in southern Brazil between 30°S and 32°S. The response of
the lagoon to tidal oscillations is studied through data analysis and numerical modeling experiments. Two types of high frequency
oscillations are observed in the tidal frequency band: mixed tides, predominantly diurnal; and forced oscillations having
a period of 24 h occurring in the inner lagoon. In the southern portion of the lagoon, tides are selectively filtered by the
entrance channel. The main diurnal constituent O1 is linearly attenuated as it progresses landwards. In the inner parts of the lagoon, 24-h oscillations are mainly forced
by the combined effect of diurnal tides and sea breeze action. They are tied with a natural period of oscillation of 24 h.
Results also indicate that these are not inertial frequency oscillations, despite the lagoon being placed in a critical 30°S
area. The interaction between astronomical tides and meteorological effects produces a complicated picture for tidal forecasting
derived from data collected inside the lagoon. 相似文献
434.
435.
Pascal Audet Michael G. Bostock Jean-Philippe Mercier 《Geophysical Journal International》2007,171(3):1212-1225
It is now widely accepted that elastic properties of the continental lithosphere and the underlying sublithospheric mantle are both anisotropic and laterally heterogeneous at a range of scales. To fully exploit modern three-component broad-band array data sets requires the use of comprehensive modelling tools. In this work, we investigate the use of a wide-angle, one-way wave equation to model variations in teleseismic 3-D waveforms due to 2-D elastic heterogeneity and anisotropy. The one-way operators are derived based on a high-frequency approximation of the square-root operator and include the effects of wave propagation as well as multiple scattering. Computational cost is reduced through a number of physically motivated approximations. We present synthetic results from simple 1-D (layer over a half-space) and 2-D (subduction zone) models that are compared with reference solutions. The algorithm is then used to model data from an array of broad-band seismograph stations deployed in northwestern Canada as part of the IRIS-PASSCAL/LITHOPROBE CANOE experiment. In this region radial-component receiver functions show a clear continental Moho and the presence of crustal material dipping into the mantle at the suture of two Palaeo-Proterozoic terranes. The geometry of the suture is better defined on the transverse component where subduction is associated with a ∼10 km thick layer exhibiting strong elastic anisotropy. The modelling reproduces the main features of the receiver functions, including the effects of anisotropy, heterogeneity and finite-frequency scattering. 相似文献
436.
437.
From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin 总被引:1,自引:0,他引:1
Sylvie Leroy Pascal Gente Marc Fournier Elia d'Acremont Philippe Patriat Marie-Odile Beslier Nicolas Bellahsen Marcia Maia Angelina Blais Julie Perrot Ali Al-Kathiri Serge Merkouriev Jean-Marc Fleury Pierre-Yves Ruellan Claude Lepvrier Philippe Huchon 《地学学报》2004,16(4):185-192
A geophysical survey in the eastern Gulf of Aden, between the Alula–Fartak (52°E) and the Socotra (55°E) transform faults, was carried out during the Encens–Sheba cruise. The conjugate margins of the Gulf are steep, narrow and asymmetric. Asymmetry of the rifting process is highlighted by the conjugate margins (horst and graben in the north and deep basin in the south). Two transfer fault zones separate the margins into three segments, whereas the present‐day Sheba Ridge is divided into two segments by a transform discontinuity. Therefore segmentation of the Sheba Ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading. Extensive magma production is evidenced in the central part of the western segment. Anomaly 5d was identified in the northern and southern parts of the oceanic basin, thus confirming that seafloor spreading in this part of Gulf of Aden started at least 17.6 Ma ago. 相似文献
438.
439.
The rock-avalanche of February 1995 at Claix (French Alps) 总被引:3,自引:0,他引:3
Pascal Bertran 《Geomorphology》2003,54(3-4):339-346
The sedimentology of a rock-avalanche involving ca. 7·103 m3, that happened on February 1995 at Claix, the French Alps, is analysed here. The talus shows a proximal depression due to the impact of the debris, then a grooved area with few, mainly sandy silt deposits and a downslope accumulation of coarse debris that reaches up to 3 m in thickness. The material underwent mass movement as demonstrated by the steep distal front, the strong fabric strength and the thrust planes. The greatest part of the deposits consists of a clast-supported to partially openwork debris with angular clasts over a sheared matrix-supported basal layer. The largest blocks, often more than 1 m in length, are concentrated at the surface and the front. The overall sedimentological characteristics are similar to those of the small dry grain flows that occur usually on rockfall talus, except for the abundance of fine-grained particles that results from intense cataclasis during flow. It is suggested that the deposits of such medium-scale rock-avalanches, probably not uncommon in seismic regions, may have been confused in previous studies with other slope mechanisms such as debris flow and rockfall. 相似文献
440.
Maurice Reille Valrie Andrieu Jacques-Louis de Beaulieu Pascal Guenet Claude Goeury 《Quaternary Science Reviews》1998,17(12):1107-1123
Pollen analysis was carried out on sediments older than the Würm pleniglacial (OIS 4), in two new sequences (H and I) derived from the centre of Lac du Bouchet, Massif Central. The inferred vegetation history enables, for the first time in France, five temperate episodes to be defined which pre-date the last interglacial. These temperate episodes alternate with episodes during which the changes in vegetation are indicative of glacial climates. Comparison of these climatic episodes with the oceanic isotope record shows that the pollen record of sequences H and I from Lac du Bouchet spans the time interval from OIS 9c (Ussel interstadial) to OIS 5e (Ribains/Eemian interglacial). In the organic sediments from the Amargiers interstadial (OIS 9a), a trachytic layer, Ar/Ar dated to ca 275 ka, enables a correlation to be established with the upper part of a sequence derived from the nearby Praclaux crater, the lower part of this sequence being of Holsteinian age (OIS 11c). The cross-dating of the pollen sequences from Lac du Bouchet (cores H, I and D) and from Praclaux provides a complete record from the Massif Central, southern France, of successive glacial and interglacial episodes that span the last ca. 400 ka, that is the interval from the Holsteinian to the Holocene. 相似文献