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251.
Damien Pas Anne-Christine Da Silva Thomas Suttner Erika Kido Pierre Bultynck Monica Pondrelli Carlo Corradini David De Vleeschouwer Claudia Dojen Frédéric Boulvain 《International Journal of Earth Sciences》2014,103(2):519-538
The development and behavior of million year-scaled depositional sequences recorded within Palaeozoic carbonate platform has remained poorly examined. Therefore, the understanding of palaeoenvironmental changes that occur in geological past is still limited. We herein undertake a multi-disciplinary approach (sedimentology, conodont biostratigraphy, magnetic susceptibility (MS), and geochemistry) of a long-term succession in the Carnic Alps, which offers new insights into the peculiar evolution of one of the best example of Palaeozoic carbonate platform in Europe. The Freikofel section, located in the central part of the Carnic Alps, represents an outstanding succession in a fore-reef setting, extending from the Latest Givetian (indet. falsiovalis conodont zones) to the Early Famennian (Lower crepida conodont zone). Sedimentological analysis allowed to propose a sedimentary model dominated by distal slope and fore-reef-slope deposits. The most distal setting is characterized by an autochthonous pelagic sedimentation showing local occurrence of thin-bedded turbiditic deposits. In the fore-reef slope, in a more proximal setting, there is an accumulation of various autochthonous and allochthonous fine- to coarse-grained sediments originated from the interplay of gravity-flow currents derived from the shallow-water and deepwater area. The temporal evolution of microfacies in the Freikofel section evolves in two main steps corresponding to the Freikofel (Unit 1) and the Pal (Unit 2) limestones. Distal slope to fore-reef lithologies and associate changes are from base to top of the section: (U1) thick bedded litho- and bioclastic breccia beds with local fining upward sequence and fine-grained mudstone intercalations corresponding, in the fore-reef setting, to the dismantlement of the Eifelian–Frasnian carbonate platform during the Early to Late Frasnian time (falsiovalis to rhenana superzones) with one of the causes being the Late Givetian major rift pulse; (U2) occurrence of thin-bedded red nodular and cephalopod-bearing limestones with local lithoclastic grainstone intercalations corresponding to a significant deepening of the area and the progressive withdrawal of sedimentary influxes toward the basin, in relation with Late Frasnian sea-level rise. MS and geochemical analyses were also performed along the Freikofel section and demonstrate the inherent parallel link existing between variation in MS values and proxy for terrestrial input. Interpretation of MS in terms of palaeoenvironmental processes reflects that even though distality remains the major parameter influencing MS values, carbonate production and water agitation also play an important role. 相似文献
252.
F. Fröhlich G. Poupeau A. Badou F. X. Le Bourdonnec Y. Sacquin S. Dubernet J. M. Bardintzeff M. Véran D. C. Smith E. Diemer 《Meteoritics & planetary science》2013,48(12):2517-2530
Results are presented of new geological observations and laboratory analyses on Libyan Desert Glass (LDG), a unique kind of impact glass found in Egypt, probably 28.5–29.4 million years in age. A new LDG occurrence has been discovered some 50 km southward of the main LDG occurrences in the Great Sand Sea. From Fourier transform infrared (FTIR) analysis, the molecular structure of LDG is refined and significant differences are shown between LDG specimens and other pure silica glasses (fulgurite, industrial fused quartz, and amorphous biogenic silica) that are related to differences in their structures. The slight variations observed here for the mean Si‐O‐Si angle between the different glasses are attributed to their thermal histories. With regard to the other glasses analyzed, the LDG infrared spectral parameters point to a higher ratio of discontinuities and defects in the tetrahedral (SiO4) network. The quantitative mineralogical constitutions of sandstones and quartzites from the LDG geological setting were analyzed by FTIR. Cretaceous sandstones have a specific composition (about 90 wt% quartz, 10% dickite), clearly different from the Paleozoic ones (about 90 wt% quartz, but ≥7% kaolinite). It is shown that the reddish silts bearing the LDG are constituted mainly of microquartz enriched with dickite, whose particle size distribution is characteristic of fluvio‐lacustrine deposits, probably Oligocene to Miocene in age. The target rocks, most probably quartz sand, resulted from the weathering (loss of the cementing microquartz) of the Cretaceous sandstones from the Gilf Khebir Plateau with deposition in a high‐energy environment. 相似文献
253.
254.
Rock joint constitutive modelling is discussed through two new rock joint constitutive relations and a discrete numerical model. Regarding the constitutive relations, we emphasise the number of ‘tensorial zones’, that is, domains of constitutive incremental linearity; they involve four zones for the first (called ‘quadrilinear’) and an infinite number for the second one (called ‘incrementally nonlinear’). Using these formulations, a large class of loading paths can be considered. Hardening through shearing and relations between the normal and tangential directions of the joint (e.g., dilatancy) can be described. Their predictive abilities are checked. Plastic features are included even if the relations are defined outside the elasto‐plastic formalism. These relations obey, hence, the physical evidence as the plastic limit criterion and flow rule. The flow rule is nonassociated, and the corresponding link with the nonsymmetry of the constitutive matrix is examined. Comparisons between the two relations and the discrete numerical model, that is, a direct numerical simulation, which is fundamentally different, also are discussed within the context of infilled rock joints. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
255.
T. Toutain T. Appourchaux F. Baudin C. Fröhlich A. Gabriel P. Scherrer B. N. Andersen R. Bogart R. Bush W. Finsterle R. A. García G. Grec C. J. Henney J. T. Hoeksema A. Jiménez A. Kosovichev T. Roca Cortés S. Turck-Chièze R. Ulrich C. Wehrli 《Solar physics》1997,175(2):311-328
The three helioseismology instruments aboard SOHO observe solar p modes in velocity (GOLF and MDI) and in intensity (VIRGO
and MDI). Time series of two months duration are compared and confirm that the instruments indeed observe the same Sun to
a high degree of precision. Power spectra of 108 days are compared showing systematic differences between mode frequencies
measured in intensity and in velocity. Data coverage exceeds 97% for all the instruments during this interval. The weighted
mean differences (V-I) are −0.1 μHz for l=0, and −0.16 μHz for l=1. The source of this systematic difference may be due to
an asymmetry effect that is stronger for modes seen in intensity. Wavelet analysis is also used to compare the shape of the
forcing functions. In these data sets nearly all of the variations in mode amplitude are of solar origin. Some implications
for structure inversions are discussed. 相似文献
256.
Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. This results in a strong correlation between sunspot area and TSI. During the growth phase of a sunspot other physics may affect this correlation; if the physical growth of the sunspot resulted in surface flows affecting the temperature, for example, we might expect to see an anomalous variation in TSI. In this paper we study NOAA active region 8179, in which large sunspots suddenly appeared near disk center, at a time (March 1998) when few competing sunspots or plage regions were present on the visible hemisphere. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with a large thermal conductivity in solar convection zone. In addition we have searched for a smaller-scale effect by analyzing white-light images from MDI (the Michelson Doppler Imager) on SOHO. A representative upper-limit energy consistent with the images is on the order of 3×1031 ergs, assuming the time scale of the actual spot area growth. This is of the same order of magnitude as the buoyant energy of the spot emergence even if it is shallow. We suggest that detailed image analyses of sunspot growth may therefore show `transient bright rings' at a detectable level. 相似文献
257.
Appourchaux Thierry Andersen Bo N. Fröhlich Claus Jiménez Antonio Telljohann Udo Wehrli Christoph 《Solar physics》1997,170(1):27-41
The Luminosity Oscillations Imager (LOI) is a part of the VIRGO instrument aboard the Solar and Heliospheric Observatory (SOHO). The scientific objective of the LOI experiment is to identify and characterize pressure and internal gravity oscillations of the Sun by observing the radiance variations. The LOI is a low-resolution imager with 12 pixels, for the measurement of the radiance distribution over the solar disk at 500 nm. The low resolution capability of the instrument allows the identification of individual azimuthal orders for l = 0 to 7, without suffering the mixing that affects integrated solar disk instruments. The performance, calibrations and instrumental effects of the LOI are described together with the procedures for extracting the solar p modes. 相似文献
258.
Elena A. Zakharova Sara Fleury Kévin Guerreiro Sascha Willmes Frédérique Rémy Alexei V. Kouraev 《Marine Geodesy》2015,38(3):522-533
Sea ice leads play an essential role in ocean-ice-atmosphere exchange, in ocean circulation, geochemistry, and in ice dynamics. Their precise detection is crucial for altimetric estimations of sea ice thickness and volume. This study evaluates the performance of the SARAL/AltiKa (Satellite with ARgos and ALtiKa) altimeter to detect leads and to monitor their spatio-temporal dynamics. We show that a pulse peakiness parameter (PP) used to detect leads by Envisat RA-2 and ERS-1,-2 altimeters is not suitable because of saturation of AltiKa return echoes over the leads. The signal saturation results in loss of 6–10% of PP data over sea ice. We propose a different parameter—maximal power of waveform—and define the threshold to discriminate the leads. Our algorithm can be applied from December until May. It detects well the leads of small and medium size from 200 m to 3–4 km. So the combination of the high-resolution altimetric estimates with low-resolution thermal infra-red or radiometric lead fraction products could enhance the capability of remote sensing to monitor sea ice fracturing. 相似文献
259.
Alexei V. Kouraev Elena A. Zakharova Frédérique Rémy Andrey Ya. Suknev 《Marine Geodesy》2015,38(3):477-486
Comparison of ENVISAT and SARAL missions data shows that AltiKa can be successfully used for ice discrimination methodology and extension of ice conditions time series. Due to shorter wavelength and large bandwidth (480 MHz) which leads to a higher sensitivity to different surface conditions, AltiKa shows more clearly the separation between open water and various ice types. We observe significant decrease of backscatter (25–30 dB) in late spring for both ENVISAT and SARAL and discuss it in the context of ice metamorphism. There is a clear need to continue and expand our dedicated field studies of lake Baikal ice cover to better assess influence of ice structure on altimetric signal. 相似文献
260.
In view of the construction of new sunspot-based activity indices and proxies, we conducted a comprehensive survey of all existing catalogs providing detailed parameters of photospheric features over long time intervals. Although there are a fair number of such catalogs, a global evaluation showed that they suffer from multiple limitations: finite or fragmented time coverage, limited temporal overlap between catalogs, and, more importantly, a mismatch in contents and conventions. Starting from the existing material, we demonstrate how the information from parallel catalogs can be merged to form a much more comprehensive record of sunspots and sunspot groups. To do this, we use the uniquely detailed Debrecen Photoheliographic Data (DPD), which is already a composite of several ground-based observatories and of SOHO data, and the USAF/Mount Wilson catalog from the Solar Observing Optical Network (SOON). We also outline our cross-identification method, which was needed to match the non-overlapping solar active-region nomenclature. This proved to be the most critical and subtle step when working with multiple catalogs. This effort, focused here first on the last two solar cycles, should lead to a better central database that collects all available sunspot group parameters to address future solar-cycle studies beyond the traditional sunspot-index time series [R i]. 相似文献