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391.
Denis Samyn Sean J. Fitzsimons Reginald D. Lorrain 《International Journal of Earth Sciences》2010,99(8):1849-1857
Structural analyses were conducted in the basal zone of an Antarctic glacier. The studied basal ice sequence was retrieved
from a 20-m-long subglacial tunnel dug at the margin of the glacier and is at the temperature of −17°C. For the first time,
rotating clast systems embedded within debris-rich ice were thin-sectioned using specially designed cutting techniques. The
observed structures reflect the occurrence of pervasive shearing at the base of the glacier, and can be used as shear sense
indicators. In addition, some of these structures provide evidence for the presence of thin liquid films at the time of formation
despite the marked freezing temperature of the ice. It is showed here that cautious analysis of deformation structures present
in debris-bearing ice may bring insights not only into the flow dynamics of the embedding matrix, but also into the behaviour
of the interstitial fluid network at the base of cold glaciers and ice sheets. 相似文献
392.
Geochemical study of clays used as barriers in landfills 总被引:1,自引:0,他引:1
This is a hydraulic and geochemical study on 2 materials: natural clay (AN) and a regenerated material made up of a sand-bentonite (SB) mixture. The hydraulic part allowed us to conclude that a 10% industrial bentonite content mixed with sand offers hydraulic properties that are similar to those of AN material and are lower than the required standards. The geochemical properties of both AN and SB matrixes are comparable with those of some of the synthetic leachates studied. Furthermore, the Langmuir model helped us to identify the adsorption capacities of both matrixes with the following selectivities: Pb > Cu > Cd > Zn. 相似文献
393.
This study is the result of many years of research on the ecology of the marine benthos of Russian Arctic seas. We used samples collected at various locations from the Russian continental shelf during 1993–2009 as the basis of our study. Our main aim was to analyze the spatial distribution of taxonomic and quantitative characteristics of the meiobenthos (small bottom-dwelling animals, 0.1–3.0 mm in size). Statistical analysis of the data revealed that the factors determining the spatial distribution of meiobenthic organisms under natural conditions, and conditions impacted upon by human activity, were salinity, water depth, hydrocarbons, heavy metals and radiocaesium volumetric activity. The possible use of the meiobenthos as a tool for environmental impact assessment is proposed and discussed on the level of higher taxa. 相似文献
394.
Massimo Chiaradia Othmar Müntener Bernardo Beate Denis Fontignie 《Contributions to Mineralogy and Petrology》2009,158(5):563-588
In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon
continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to
east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc),
and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted
either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional
crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb)
data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic
centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the
frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related
geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile
elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively
lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe
widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or
amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center,
rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes
are small and their values are overall rather primitive (e.g., εNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation.
We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers
characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as
by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian
rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts,
rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks
of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting
that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a
consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated
with subduction of the aseismic Carnegie Ridge. 相似文献
395.
Laurent Koechlin Denis Serre Paul Deba Roser Pelló Christelle Peillon Paul Duchon Ana Ines Gomez de Castro Margarita Karovska Jean-Michel Désert David Ehrenreich Guillaume Hebrard Alain Lecavelier des Etangs Roger Ferlet David Sing Alfred Vidal-Madjar 《Experimental Astronomy》2009,23(1):379-402
The Fresnel Interferometric Imager has been proposed to the European Space Agency (ESA) Cosmic Vision plan as a class L mission.
This mission addresses several themes of the CV Plan: Exoplanet study, Matter in extreme conditions, and The Universe taking shape. This paper is an abridged version of the original ESA proposal. We have removed most of the technical and financial issues,
to concentrate on the instrumental design and astrophysical missions. The instrument proposed is an ultra-lightweight telescope,
featuring a novel optical concept based on diffraction focussing. It yields high dynamic range images, while releasing constraints
on positioning and manufacturing of the main optical elements. This concept should open the way to very large apertures in
space. In this two spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric
array; the other spacecraft holds the field optics, focal instrumentation, and detectors. The Fresnel array proposed here
is a 3.6 ×3.6 m square opaque foil punched with 105 to 106 void “subapertures”. Focusing is achieved with no other optical element: the shape and positioning of the subapertures (holes
in the foil) is responsible for beam combining by diffraction, and 5% to 10% of the total incident light ends up into a sharp
focus. The consequence of this high number of subapertures is high dynamic range images. In addition, as it uses only a combination
of vacuum and opaque material, this focussing method is potentially efficient over a very broad wavelength domain. The focal
length of such diffractive focussing devices is wavelength dependent. However, this can be corrected. We have tested optically
the efficiency of the chromatism correction on artificial sources (500 < λ < 750 nm): the images are diffraction limited, and the dynamic range measured on an artificial double source reaches 6.2
10 − 6. We have also validated numerical simulation algorithms for larger Fresnel interferometric arrays. These simulations yield
a dynamic range (rejection factor) close to 10 − 8 for arrays such as the 3.6 m one we propose. A dynamic range of 10 − 8 allows detection of objects at contrasts as high as than 10 − 9 in most of the field. The astrophysical applications cover many objects in the IR, visible an UV domains. Examples are presented,
taking advantage of the high angular resolution and dynamic range capabilities of this concept. 相似文献
396.
Abstract Thirty-six fragments of meteorite falls, never touched by water or other possible sources of B contamination, were analyzed for B by prompt gamma-ray neutron activation analysis at McMaster University and at the U.S. National Institute of Standards and Technology. Boron concentrations are close to the sensitivity limit in both laboratories. Results between the two laboratories agree well but with slight systematic differences attributable to blank and background correction factors. The mean B concentrations in different meteorites are similar, from 0.5 to 1.1 ppm. The ranges in different CC, OC and ACH classes overlap, mostly from 0.4 to 1.4 ppm, which is similar to previous measurements on falls. The H, L and LL meteorites overlap in B content with Antarctic chondrites. The solar system abundance, taken as the mean B content of the matrix in all CC, was calculated from seven samples and is 0.69 ± 0.09 ppm. When normalized, this abundance is 16.9 ± 2.2 (atoms/106Si). Normalized B and S concentrations show a linear relationship in the CC; the average OC lies on the same line, but individual OCs are dispersed. 相似文献
397.
398.
Kris Vanneste Koen Verbeeck Thierry Camelbeeck Etienne Paulissen Mustapha Meghraoui François Renardy Denis Jongmans Manfred Frechen 《Journal of Seismology》2001,5(3):329-359
Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically best-defined segment of the southwestern border fault of the Roer Valley graben in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three surface displacements since about 40 ka BP. The most recent eventprobably occurred between 1000 and 1350 yr cal BP. Geophysical and trenching studies at a new site near the southeastern end of the fault scarp reconfirmed the coincidence of the frontal escarpment with a shallow normal fault, which displaces the Middle Pleistocene `Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two discrete faulting events, but primary evidence for the coseismic nature of these events is sparse. Radiocarbon and optically stimulated luminescence dating constrainthe age of these events to the Holocene and between 14.0 ± 2.3 ka BP and 15.8 ± 2.9 ka BP, respectively. In addition, four older surface-rupturing events are inferred from the presence of four wedge-shaped units of reworked Main Terrace deposits that are interbedded with coversand units in the hanging wall of the trench and in shallow boreholes. These wedges are interpreted as colluvial wedges, produced by accelerated slope processes in response torejuvenation of the fault scarp, most probably in a periglacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 ± 17.6 ka. The antepenultimate event was the largest faulting event, associated with a total fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates the viability of the paleoseismological approach to identify past large earthquakes in areas of present-day moderate to low seismic activity. 相似文献
399.
Florence Le Hébel Olivier Vidal Jean-Robert Kienast Denis Gapais 《Comptes Rendus Geoscience》2002,334(3):205-211
The so-called ‘Porphyro??des’ Unit of South Brittany is located below units marked by an early HP–LT event (blueschists from the Île de Groix) and above units marked by Upper Carboniferous HT metamorphism. P–T estimates in the Porphyro??des (at Belle-Île-en-Mer), using the phengite–chlorite thermobarometer, indicate pressures around 8 kbar and temperatures of 350–400 °C. These new estimates suggest that the early tectonic history of the ‘Porphyro??des’ should be linked to that of the overlying Blueschists, and imply a much larger areal extension of the HP–LT domain in this part of the Hercynian Belt. To cite this article: F. Le Hébel et al., C. R. Geoscience 334 (2002) 205–211. 相似文献
400.
Numerical convergence of the dynamics of a GCM 总被引:1,自引:0,他引:1
Atmospheric general circulation models (GCMs) are characterized by many features but especially by: (1) the manner of discretizing the governing equations and of representing the variables involved at a given resolution, and (2) the manner of parameterizing unresolved physical processes in terms of those resolved variables. These two aspects of model formulation are not independent
and it is difficult to untangle their intertwined effects when assessing model performance. The attempt here is to separate
these aspects of GCM behaviour and to ask, “Given a perfect parameterization of the physical processes in a model, what resolution is needed to capture the dominant dynamical aspects
of the atmospheric climate?” Alternatively, “At what resolution do the dynamics of a GCM converge”? The perfect parameterization
approach assumes that the calculation of the physical terms returns the “correct” result at all resolutions. In the idealized
case, a time-independent forcing is one of the simplest that satisfies this condition. However, experiments show that it is
difficult for the dynamics of a GCM to balance a time-independent forcing with atmosphere-like flows and structures. The model
requires, and the atmosphere presumably includes, physical feedback mechanisms which act so as to maintain the kinds of flows
and structures that are observed. Resolution experiments are performed with a simplified forcing function for the thermodynamic
equation which combines a dominant time-independent specified forcing with a weak linear relaxation feedback. These experiments
show that the dynamics of the GCM have essentially converged at T32 and certainly by T63 which is the next resolution considered.
This is shown by the constancy of structures, variances, covariances, transports and energy budgets with increasing resolution.
Experiments with an alternative forcing proposed by Held and Suarez, which has the form of a linear relaxation, show somewhat
less evidence of convergence at these resolutions. In both cases the “physics” are known by assumption. However, the form
and nature of the forcing is different, as is the behaviour with resolution. The implication for the real system is that the
resolution required for simulating the dynamical aspects of climate is rather modest. The simulated climate does, however, apparently depend on the ability to correctly and
consistently parameterize the physical processes in a GCM, involving both forcing and feedback mechanisms, as a function of
resolution.
Received 19 January 1996/Accepted 22 August 1996 相似文献