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排序方式: 共有147条查询结果,搜索用时 109 毫秒
21.
C. Fourdrin E. Balan T. Allard C. Boukari G. Calas 《Physics and Chemistry of Minerals》2009,36(5):291-299
Radiation effects on kaolinite were investigated using He+ ions of 1.5 MeV at radiation doses up to 4.3 × 108 Gy, which are comparable to the doses expected for clay barriers in high-level nuclear waste repositories. The concentration
of paramagnetic radiation-induced defects in kaolinite reaches 2 × 1016 spins/mg (400 at. ppm), as determined by electron paramagnetic resonance spectroscopy. The broadening of X-ray diffraction
patterns and transmission infrared (IR) absorption bands is mostly related to the structural strain induced by radiation-induced
point defects. The broadening of IR absorption spectra is analyzed using an autocorrelation approach and is related to a change
in the distribution of vibrational frequencies due to crystal heterogeneities. We theoretically analyze how the effective
dielectric properties of kaolinite samples depend on macroscopic parameters and how irradiation can modify some of them. Irradiation
leads to an increase in the electronic polarizability of kaolinite particles, related to the accumulation of radiation-induced
electronic point defects. 相似文献
22.
J. Schou P. H. Scherrer R. I. Bush R. Wachter S. Couvidat M. C. Rabello-Soares R. S. Bogart J. T. Hoeksema Y. Liu T. L. Duvall Jr. D. J. Akin B. A. Allard J. W. Miles R. Rairden R. A. Shine T. D. Tarbell A. M. Title C. J. Wolfson D. F. Elmore A. A. Norton S. Tomczyk 《Solar physics》2012,275(1-2):229-259
23.
Clara Lemos France Floc’h Marissa Yates Nicolas Le Dantec Vincent Marieu Klervi Hamon Véronique Cuq Serge Suanez Christophe Delacourt 《Ocean Dynamics》2018,68(9):1207-1220
Eleven-year long time series of monthly beach profile surveys and hourly incident wave conditions are analyzed for a macrotidal Low Tide Terrace beach. The lower intertidal zone of the beach has a pluriannual cycle, whereas the upper beach profile has a predominantly seasonal cycle. An equilibrium model is applied to study the variation of the contour elevation positions in the intertidal zone as a function of the wave energy, wave power, and water level. When forcing the model with wave energy, the predictive ability of the equilibrium model is around 60% in the upper intertidal zone but decreases to 40% in the lower intertidal zone. Using wave power increases the predictive ability up to 70% in both the upper and lower intertidal zones. However, changes around the inflection point are not well predicted. The equilibrium model is then extended to take into account the effects of the tide level. The initial results do not show an increase in the predictive capacity of the model, but do allow the model free parameters to represent more accurately the values expected in a macrotidal environment. This allows comparing the empirical model calibration in different tidal environment. The interpretation of the model free parameter variation across the intertidal zone highlights the behavior of the different zones along the intertidal beach profile. This contributes to a global interpretation of the four model parameters for beaches with different tidal ranges, and therefore to a global model applicable at a wide variety sites. 相似文献
24.
Jessica L. Till Yohan Guyodo France Lagroix Guillaume Morin Nicolas Menguy Georges Ona-Nguema 《Comptes Rendus Geoscience》2017,349(2):63-70
The oriented chains of nanoscale Fe-oxide particles produced by magnetotactic bacteria are a striking example of biomineralization. Several distinguishing features of magnetite particles that comprise bacterial magnetosomes have been proposed to collectively constitute a biosignature of magnetotactic bacteria (Thomas-Keprta et al., 2001). These features include high crystallinity, chemical purity, a single-domain magnetic structure, well-defined crystal morphology, and arrangement of particles in chain structures. Here, we show that magnetite derived from the inorganic breakdown of nanocrystalline goethite exhibits magnetic properties and morphologies remarkably similar to those of biogenic magnetite from magnetosomes. During heating in reducing conditions, oriented nanogoethite aggregates undergo dehydroxylation and transform into stoichiometric magnetite. We demonstrate that highly crystalline single-domain magnetite with euhedral grain morphologies produced abiogenically from goethite meets several of the biogenicity criteria commonly used for the identi?cation of magnetofossils. Furthermore, the suboxic conditions necessary for magnetofossil preservation in sediments are conducive to the reductive alteration of nanogoethite, as well as the preservation of detrital magnetite originally formed from goethite. The findings of this study have potential implications for the identification of biogenic magnetite, particularly in older sediments where diagenesis commonly disrupts the chain structure of magnetosomes. Our results indicate that isolated magnetofossils cannot be positively distinguished from inorganic magnetite on the basis of their magnetic properties and morphology, and that intact chain structures remain the only reliable distinguishing feature of fossil magnetosomes. 相似文献
25.
Gerard W. Hazeu Arnold K. Bregt Allard J.W. de Wit Jan G.P.W. Clevers 《International Journal of Applied Earth Observation and Geoinformation》2011
Land cover and land use are important information sources for environmental issues. One of the most important changes at the Earth's surface concerns land cover and land use. Knowledge about the location and type of these changes is essential for environmental modeling and management. Remote sensing data in combination with additional spatial data are recognized as an important source of information to detect these land cover and land use changes. 相似文献
26.
T. Allard T. Weber C. Bellot C. Damblans M. Bardy G. Bueno N.R. Nascimento E. Fritsch M.F. Benedetti 《Chemical Geology》2011,280(1-2):79-88
Suspended solids found in porewaters obtained in waterlogged soil sequences that included representative laterite–podzol transitions, associated brooks and major rivers of the Rio Negro watershed (Brazil) were studied using electron paramagnetic resonance (EPR) and Fourier-transform infrared spectroscopies. The main goal was to ascertain sources and track the evolution of suspended matter using a ubiquitous chemical species, FeIII complexed to organic matter (FeOM). Three size fractions were separated by tangential-flow (ultra)filtration: particulate (> 0.2 μm), dense (P) and light (Ps), and colloidal (5 kD < Col. < 0.2 μm) fractions. Quantitative results were acquired for Col. and Ps fractions which are predominantly organic in nature.FeOM concentration (in ‰ dry weight) was determined to be relatively low in suspended solids found in black waters from podzol porewaters and brooks whereas in the main rivers it was several times higher. FeOM concentrations were also correlated with Fe(II)/Fe(III) ratios in solution; these ratios were high in podzol porewaters and low in the rivers. Considering that organic complexation of Fe(II) is minor when compared to that of Fe(III), two interpretations were proposed to account for the above observation. First, [FeOM] was assumed to be distributed along a mixing line, with the clear waters from laterites and the black waters from podzols being its end-members. Consequently, [FeOM] can be used to trace the source of suspended material. Second, dissolved Fe(II) from podzol areas was considered to be progressively oxidized as pore waters move towards the mainstream. According to this mechanism, iron is complexed by organic matter or precipitated as oxides, thus producing an evolution of colloidal matter. As a result of these mechanisms' action, both the high production of Fe(II) and organic matter at the waterlogged podzol–laterite transition areas are major factors affecting iron export in the Rio Negro watershed. 相似文献
27.
Béatrice Allard Michaël Danger Loïc Ten-Hage Gérard Lacroix 《Aquatic Sciences - Research Across Boundaries》2011,73(1):113-126
The effects of food web structure on the quantity and biochemical composition of seston, zooplankton and recently deposited
sediment in experimental freshwater mesocosms were examined. Food web structure was manipulated by addition of zooplanktivorous
fish. Biochemical characterisations were carried out using lipid biomarkers (sterols, fatty acids, chlorophyll-derived compounds
and long-chain alkanediols). Fish addition decreased zooplankton biomass and increased seston biomass and deposited sediment
through a trophic cascade. Fish presence strongly influenced the biochemical characteristics of seston and sediment. In contrast,
food web structure had a minor impact on the lipid biomarker composition of zooplankton. Although the relative abundance of
sterols in the different compartments did not differ strongly between treatments, sterol profiles in seston and sediment depended
on food web structure. The predominance of Δ7-sterols in seston and sediment in the fish treatment indicated a major contribution of Chlorophyceae. In contrast, the distribution
of sterols in seston and sediment in the fishless treatment, dominated by cholesterol, indicated a major zooplanktonic input.
The distribution of fatty acids and the relative abundance of chlorophyll-derived compounds and long-chain alkanediols agreed
with the predominant contribution of phytoplankton or zooplankton to seston and sediment in the two treatments. The relative
abundance of bacterial biomarkers suggested that the contribution of bacteria was rather low. The high relative abundance
of polyunsaturated fatty acids (PUFAs) and the absence of stanols in sediments suggested low microbial reworking of organic
matter in the recently accumulated sediments. The trophic cascade, generated by the addition of fish, increased the relative
abundance of PUFAs in deposited organic matter, thus enhancing sediment quality and potential degradability. 相似文献
28.
S. Calabrese A. Aiuppa P. Allard S. Bellomo W. D’Alessandro 《Geochimica et cosmochimica acta》2011,75(23):7401-7425
This study reports on the first quantitative assessment of the geochemical cycling of volcanogenic elements, from their atmospheric release to their deposition back to the ground. Etna’s emissions and atmospheric depositions were characterised for more than 2 years, providing data on major and trace element abundance in both volcanic aerosols and bulk depositions. Volcanic aerosols were collected from 2004 to 2007, at the summit vents by conventional filtration techniques. Precipitation was collected, from 2006 to 2007, in five rain gauges, at various altitudes around the summit craters. Analytical results for volcanic aerosols showed that the dominant anions were S, Cl, and F, and that the most abundant metals were K, Ca, Mg, Al, Fe, and Ti (1.5-50 μg m−3). Minor and trace element concentrations ranged from about 0.001 to 1 μg m−3. From such analysis, we derived an aerosol mass flux ranging from 3000 to 8000 t a−1. Most analysed elements had higher concentrations close to the emission vent, confirming the prevailing volcanic contribution to bulk deposition. Calculated deposition rates were integrated over the whole Etna area, to provide a first estimate of the total deposition fluxes for several major and trace elements. These calculated deposition fluxes ranged from 20 to 80 t a−1 (Al, Fe, Si) to 0.01-0.1 t a−1 (Bi, Cs, Sc, Th, Tl, and U). Comparison between volcanic emissions and atmospheric deposition showed that the amount of trace elements scavenged from the plume in the surrounding of the volcano ranged from 0.1% to 1% for volatile elements such as As, Bi, Cd, Cs, Cu, Tl, and from 1% to 5% for refractory elements such as Al, Ba, Co, Fe, Ti, Th, U, and V. Consequently, more than 90% of volcanogenic trace elements were dispersed further away, and may cause a regional scale impact. Such a large difference between deposition and emission fluxes at Mt. Etna pointed to relatively high stability and long residence time of aerosols in the plume. 相似文献
29.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen France Lagroix François Demory José M. Nieto Reinaldo Sáez M. Ángeles Capitán 《Mineralium Deposita》2011,46(8):981-999
In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps,
i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans.
To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci:
(1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive
sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in
all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal
foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation
interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be
inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates
and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic
remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF)
direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable
high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities.
No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans,
the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal
test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in
the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older
age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the
lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These
two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion. 相似文献
30.
Johanna Lomax Markus Fuchs Pierre Antoine Denis‐Didier Rousseau France Lagroix Christine Hatt Samuel N. Taylor Jessica L. Till Maxime Debret Olivier Moine Diana Jordanova 《Boreas: An International Journal of Quaternary Research》2019,48(1):179-194
The Harletz loess‐palaeosol sequence is located in northwestern Bulgaria and represents an important link between well‐studied loess sequences in eastern Romania and further sites to the west of the Carpathians (e.g. Serbia and Hungary). The aim of this study was to establish a chronostratigraphy of the deposits, using various methods of luminescence dating, together with basic stratigraphical field observations as well as magnetic properties. Luminescence dating was carried out using the quartz fine grain fraction and a SAR protocol, and the feldspar coarse grain fraction, applying the MET‐pIRIR protocol. Due to underestimation of the quartz fine grain fraction in the lower parts of the sequence, the resulting chronology is mainly based on the feldspar ages, which are derived from the stimulation temperature at 150 °C. A comparison with nearby sequences from Serbia, Hungary and Romania, and interpretations obtained through the stratigraphical and sedimentological signature of the sequence, supports the established chronology. Our data suggest that the prominent palaeosol (soil complex) in the upper quarter of the sequence was formed during MIS 5. It would follow that large parts of the Last Glacial loess overlying this palaeosol were probably eroded, and that the thick loess accumulation underlying this soil complex can be allocated to the penultimate glacial (MIS 6). A prominent MIS 6 tephra, which has been reported from other sequences in the area, is also present at Harletz. 相似文献