Authigenic smectite on diatom frustules in Bolivian saline lakes     

Denise Badaut  François Risacher 《Geochimica et cosmochimica acta》1983,47(3):363-375

Observation under the electron microscope of diatom frustules from Bolivian Altiplano saline lakes shows that many of these are coated with particles occurring as tiny sheets. The frustules can be found to be almost completely replaced by these sheets. Isolated sheet aggregates seem to have resulted from completely transformed frustules. Section observations of altered frustules bear out that the sheets have grown from biogenic silica through replacement. Selected area diffraction, dark field observation, microdiffraction, and elemental microanalysis show that the particles on the diatom frustules consist of a poorly crystallized MG-smectite.The unambiguous localisation of this authigenesis allows us to reconstruct its hydrochemical and sedimentological environment. Observation of the most recent lake sediments has pointed out that at least two main conditions are required for this authigenesis at 5°C: saturation with respect to amorphous silica, and a pH above 8.2. Variations in the Mg concentration have no significant effect.  相似文献   

Lithium in the interlayer space of synthetic trioctahedral micas     

J.-L. Robert  M. Volfinger  J.-N. Barrandon  M. Basutu 《Chemical Geology》1983,40(3-4):337-351

This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li⁺ can replace K⁺, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li⁺ in the interlayer is a function of the Al^IV content of the mica. It is given by the relation (Li/Li + K)_max = 2[(Al/Si + Al)^IV]². In both micas investigated — phlogopite, KMg₃(Si₃Al)O₁₀(OH)₂, and eastonite, K(Mg_2.5Al_1.5)(Si_2.5Al_1.5)O₁₀(OH)₂ — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K⁺ replaced by Li⁺.

The fixation of Li⁺ according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d₀₀₅) and a slight increase of the reticular distance d₀₆₀. Infra-red (IR) absorption spectrometry shows that vacant K⁺ sites are created when Li⁺ enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li⁺ does not provoke any modification of the IR spectra in the region 1200-300 cm⁻¹, indicating that Li⁺ is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li⁺ in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.  相似文献   

Fracturation tertiaire et évolution des contraintes en Bavière orientale: le Jura franconien et la forêt bavaroise (R.F.A.)     

Dr. Françoise Bergerat  Jacques Geyssant 《International Journal of Earth Sciences》1983,72(3):935-953

Résumé L'analyse géométrique et cinématique de la fracturation dans le Jura franconien et la bordure occidentale du Massif bohémien met en évidence les mécanismes de la tectonique cassante d'âge teriaire, essentiellement ceux des grands accidents SE-NW (lignes de Pfahl et de Franconie) qui ont présenté successivement des jeux en coulissement dextre, en faille normale et en faille inverse. Cinq états de contrainte ont été caractérisés et comparés à ceux définis antérieurement dans le Jura souabe, en liaison avec la cinématique des plaques Europe et Afrique: compression N-S, distension E-W, compression NE-SW puis E-W, distension N-S.

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The analysis of both the geometric and the kinematic cenozoic faulting in the Franconian Jura and in western Bohemia enables to characterize the mechanisms of cenozoic tectonics. More especially dextral strike-slip, normal fault and reverse fault have successively occurred along large SE-NW fault line, as the Pfahl and the Franconian fault systems. Five main stress patterns have thus been identified and then compared with those previously described in Swabian Jura. They are related to the kinematic of Europe and Africa plates, i. e.: (1) N-S compression, (2) E-W distension, (3) SW-NE compression, (4) E-W compression, (5) N-S distension.

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Zusammenfassung Durch die geometrische und kinematische Analyse der Brüche wird die tertiäre Bruchtektonik (tectonique cassante) im Fränkischen Jura und am westlichen Rand des Böhmischen Massifs belegt. Es handelt sich im wesentlichen um die großen SE-NW orientierten Störungen der Linie von Pfahl und der Fränkischen Linie, die nacheinander als dextrale Blattverschiebungen, als Normalverwerfungen und als inverse Verwerfungen wirkten. Fünf Spannungszustände können charakterisiert und mit jenen, die bereits im Schwäbischen Jura definiert wurden, verglichen werden. Sie spiegeln die verschiedenen Stadien der kinematischen Entwicklung der europäischen und afrikanischen Platte wider: Kompression N-S, Zerrung E-W, Kompression NE-SW, sodann E-W, Zerrung N-S.

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(tectonique cassante) . , , SE-NW , , , . 5 . : - , - , - , - - .

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The stratospheric version of LMDz: dynamical climatologies, arctic oscillation, and impact on the surface climate     

François Lott  Laurent Fairhead  Frederic Hourdin  Phu Levan 《Climate Dynamics》2005,25(7-8):851-868

A climatology of the stratosphere is determined from a 20-year integration with the stratospheric version of the Atmospheric General Circulation Model LMDz. The model has an upper boundary at near 65 km, uses a Doppler spread non-orographic gravity waves drag parameterization and a subgrid-scale orography parameterization. It also has a Rayleigh damping layer for resolved waves only (not the zonal mean flow) over the top 5 km. This paper describes the basic features of the model and some aspects of its radiative-dynamical climatology. Standard first order diagnostics are presented but some emphasis is given to the model’s ability to reproduce the low frequency variability of the stratosphere in the winter northern hemisphere. In this model, the stratospheric variability is dominated at each altitudes by patterns which have some similarities with the arctic oscillation (AO). For those patterns, the signal sometimes descends from the stratosphere to the troposphere. In an experiment where the parameterized orographic gravity waves that reach the stratosphere are exaggerated, the model stratosphere in the NH presents much less variability. Although the stratospheric variability is still dominated by patterns that resemble to the AO, the downward influence of the stratosphere along these patterns is near entirely lost. In the same time, the persistence of the surface AO decreases, which is consistent with the picture that this persistence is linked to the descent of the AO signal from the stratosphere to the troposphere. A comparison between the stratospheric version of the model, and its routinely used tropospheric version is also done. It shows that the introduction of the stratosphere in a model that already has a realistic AO persistence can lead to overestimate the actual influence of the stratospheric dynamics onto the surface AO. Although this result is certainly model dependent, it suggests that the introduction of the stratosphere in a GCM also call for a new adjustment of the model parameters that affect the tropospheric variability.  相似文献   

North Atlantic Oscillation sensitivity to the El Niño/Southern Oscillation polarity in a large-ensemble simulation     

P. Melo-Gonçalves  A. C. Rocha  J. M. Castanheira  J. A. Ferreira 《Climate Dynamics》2005,24(6):599-606

A large ensemble modeling experiment with the Melbourne University General Circulation Model is presented. Thirty 17-year-long independent simulations were performed. All integrations were forced by the same observed sea surface temperatures, obtained from the Atmospheric Model Intercomparison Project II. The simulations were analyzed to assess the sensitivity of the North Atlantic Oscillation (NAO) to the El Niño/Southern Oscillation (ENSO) polarity. The results show signals of the ENSO phases both in the mean strength of the NAO as well as in its internal variability. During the cold ENSO phase, the probability density function of the NAO index presents a small but positive mean value, whereas it is negative during the warm ENSO phase. Also, the NAO variability associated with each ENSO phase shows a different behavior: during the warm phase the probability density function of the NAO index presents a larger variance and suggests a bimodality, whereas no bimodality is suggested in the cold phase.  相似文献   

High-Frequency Hydroacoustic Monitoring in an Underground Iron Mine     

Séverine Bernardie  Jean-Pascal Gilbert  François Lebert  Hubert Fabriol 《Pure and Applied Geophysics》2007,164(1):177-197

The monitoring of the stability of old mines constitutes an important research objective for our institution, BRGM. The study reported here shows the contribution of high-frequency (>30 kHz) acoustic emissions to the detection of the damage within a rock mass, during an experiment within a pilot site of an old flooded iron mine. The experiment consisted of recording all the hydroacoustic events in a broad frequency band (between 30 Hz and 180 kHz), during 18 months. The monitoring network has been calibrated by a triggered block fall that made it possible to highlight a relationship between the occurrence of high-frequency/low-frequency hydroacoustic emissions and rock falls. The events recorded have been associated with the micro-failure of the rock mass near the roof, prior to the detachment of the blocks. This monitoring showed important high-frequency hydroacoustic activity, which may be associated with mechanical instabilities generated by the evolution of water pressure during the experiment. In conclusion, the high-frequency hydroacoustic activity appears to be a good indicator of instability and, therefore, this new technique constitutes a promising tool for monitoring abandoned underground cavities.  相似文献   

Ophiolite emplacement by strike-slip tectonics between the Pontide Zone and the Sakarya Zone in northwestern Anatolia, Turkey     

Ali Elmas  Erdinç Yiğitbaş 《International Journal of Earth Sciences》2001,90(2):257-269

Northwestern Anatolia contains three main tectonic units: (a) the Pontide Zone in the north which consists mainly of the Gstanbul-Zonguldak Unit in the west and the BallLda<-Küre Unit in the east; (b) the Sakarya Zone (or Continent) in the south, which is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys prior to Late Jurassic time; and (c) the Armutlu-OvacLk Zone which appears to represent a tectonic mixture of both zones. These three major tectonic zones are presently bounded by the two branches of the North Anatolian Transform Fault. The two tectonic contacts follow older tectonic lineaments (the Western Pontide Fault) which formed during the development of the Armutlu-OvacLk Zone. Since the earliest Cretaceous, an overall extensional regime dominated the region. A transpressional tectonic regime of Coniacian/Santonian to Campanian age caused the welding of the Gstanbul-Zonguldak Unit to the Sakarya Zone by an oblique collision. In the Late Campanian, a transtensional tectonic regime developed, forming a new basin within the amalgamated tectonic mosaic. The different tectonic regimes in the region were caused by activity of the Western Pontide Fault. Most of the ophiolites within the Armutlu-OvacLk Zone belong to the Paleo-Tethyan and/or pre-Ordovician ophiolitic core of the Gstanbul-Zonguldak Unit. The Late Cretaceous ophiolites in the eastern parts of the Armutlu-OvacLk Zone were transported from Neo-Tethyan ophiolites farther east by left-lateral strike-slip faults along the Western Pontide Fault. There is insufficient evidence to indicate the presence of an ocean (Intra-Pontide Ocean) between the Gstanbul-Zonguldak Unit and the Sakarya Zone during Late Cretaceous time.  相似文献   

Quantification of salt weathering in porous stones using an experimental continuous partial immersion method   总被引：1，自引：0，他引：1  

D. Benavente  M. A. Garcí  a del Cura  A. Bernab  u  S. Ord    ez 《Engineering Geology》2001,59(3-4):313-325

In this study, an experimental salt weathering simulation and porous stone durability classification are proposed. There are many laboratory tests that quantify durability against salt crystallisation weathering action. These are usually based on the total immersion of samples into a saline solution, which is not representative of the salt weathering mechanism. An experimental test based on partial immersion is suggested. This is a comparable study of weight loss and degradation of visual appearance due to salt crystallisation using, on the one hand, a standard durability test (UNE), and, on the other, the proposed durability test. The weight loss and visual appearance in our test is comparable to the degradation of building stone. The differences between weight loss data in both tests depend on the petrophysical properties: porous media and degree of coherence.

From this testing, a new durability classification as a function of dry weight loss in the partial immersion test is proposed. Four divisions of different types of materials can be made in this classification, which quantifies salt weathering action mainly in environments and mild climatic conditions.  相似文献   

Spatiotemporal changes in cadmium contamination in the Seine estuary (France)     

Jean-François Chiffoleau  Dominique Auger  Emmanuelle Chartier  Pierre Michel  Isabelle Truquet  André Ficht  Jean-Louis Gonzalez  Louis-Alexandre Romana 《Estuaries and Coasts》2001,24(6):1029-1040

Cadmium (Cd) is among the major contaminants in the Seine estuary. In the biota, the RNO (Réseau National d’Observation, the French Mussel-Watch) has shown that Cd concentrations in mussels living at the mouth of the estuary are related to changes in inputs to this area of phosphogypsum, a calcium sulphate that is a by-product of the phosphoric acid naturally enriched with Cd. In the water column, Cd concentrations at several key estuary sites show a very marked trend toward decreased contamination in the particles as well as in the dissolved phase. The behavior of Cd in the estuary has been studied between 1991 and 1998 in the framework of the scientific program Seine-Aval. This program has highlighted punctual Cd inputs in the estuarine water column. The partition of Cd between the dissolved and the particulate phase, previously described in various estuaries, is characterized by an intense phenomenon of solubilization in the mixing zone freshwater-seawater, but the colloidal Cd fraction remains low along the whole salinity gradient, about 5% to 10% of the apparent dissolved fraction. Although the decrease of inputs induced a fall of Cd concentrations in the water column, laboratory experiments show that the estuarine particles are far from being exhausted in Cd. Despite continuous efforts to reduce the urban and industrial inputs into the estuarine and coastal waters, the Seine estuary still remains very contaminated by Cd.  相似文献   

Pore fluid pressure effects in anisotropic rocks: Mechanisms of induced seismicity and weak faults     

Qiang Chen  Amos Nur 《Pure and Applied Geophysics》1992,139(3-4):463-479

Many observations and studies indicate that pore fluid pressure in the crustal rocks plays an important role in deformation, faulting, and earthquake processes. Conventional models of pore pressure effects often assume isotropic porous rocks and yield the nondeviatoric pressure effects which seem insufficient to explain diverse phenomena related to pore pressure variation, such as fluid-extraction induced seismicity and crustal weak faults. We derive the anisotropic effective stress law especially for transversely-isotropic and orthotropic rocks, and propose that the deviatoric effects of pore fluid pressure in anisotropic rocks not only affect rock effective strength but also cause variation of shear stresses. Such shear stress variations induced by either pore pressure buildup or pore pressure decline may lead to faulting instability and trigger earthquakes, and provide mechanisms for the failure of crustal weak faults with low level of shear stresses. We believe that the deviatoric effects of pore fluid pressure in anisotropic rocks are of wide application in studies of earthquake precursors and aftershocks, oil and gas reservoir characterization, enhanced oil recovery, and hydraulic fracturing.  相似文献