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71.
Jos Manuel Piarra Michel Robardet Ahmed Bourahrouh Florentin Paris Zlia Pereira Jean Le
Menn Rmy Gourvennec Toms Oliveira Hubert Lardeux 《Comptes Rendus Geoscience》2002,334(16)
A new section in the Silurian graptolitic ‘phtanites’ (black cherts) of Les Fresnaies at Chalonnes-sur-Loire (SE Armorican Massif) shows for the first time that these rocks (1) succeed conformably to Uppermost Ordovician (Hirnantian) glaciomarine deposits and (2) contain successive graptolite assemblages that characterise the base of the Silurian, the whole Rhuddanian and Aeronian stages and the lower part of the Telychian. To cite this article: J.M. Piçarra et al., C. R. Geoscience 334 (2002) 1177–1183. 相似文献
72.
J. Haines J. M. Léger C. Chateau A. S. Pereira 《Physics and Chemistry of Minerals》2000,27(8):575-582
Germanium dioxide was found to undergo a transition from the tetragonal rutile-type to the orthorhombic CaCl2-type phase above 25 GPa. The detailed structural evolution of both phases at high pressure in a diamond anvil cell has been investigated by Rietveld refinement using angle-dispersive, X-ray powder-diffraction data. The square of the spontaneous strain (a−b)/(a+b) in the orthorhombic phase was found to be a linear function of pressure and no discontinuities in the cell constants and volume were observed, indicating that the transition is second-order and proper ferroelastic. Compression of the GeO6 octahedra was found to be anisotropic, with the apical Ge-O distances decreasing to a greater extent than the equatorial distances and becoming shorter than the latter above 7 GPa. Above this pressure, the GeO6 octahedron exhibits the common type of tetragonal distortion predicted by a simple ionic model and observed for most rutile-type structures such as those of the heavier group-14 dioxides and the metal difluorides. Above the phase transition, the columns of edge-sharing octahedra tilt about their two fold axes parallel to c and the rotation angle reaches 10.2(5)° by 36(1) GPa so as to yield a hexagonal close-packed oxygen sublattice. The compressibility increases at the phase change as is expected for a second-order transition at which an additional compression mechanism becomes available. 相似文献
73.
The thermal evolution of an inter-cluster gas of relativistic electrons heated by quasars with redshifts up toz=3 and 4 is studied in the framework of a Friedmann-Robertson-Walker universe. The gas cools by Compton scattering with the microwave backgroud radiation and by adiabatic cooling due to the universe expansion. Power and exponential laws of cosmological evolution of the comoving density of sources are considered. The obtained temperatures are sensitive to the form of these laws and to the heating epochs. Compared to the nonrelativistic models, the results obtained in the case of the power law present strong differences. These differences decrease when the exponential law is considered. Thermalization times are compared to the characteristic time of variation of the universe energy density and to the time-scales of energy loss by bremsstrahlung radiation and by Compton scattering. It is shown that, in some cases, nonequilibrium effects may be important. The time delay effects in the propagation of electromagnetic waves in cosmic plasma are shown to be very important for the analysis of theoretical models. 相似文献
74.
C.B. Lopes A.I. Lilleb P. Pato J.M. Dias S.M. Rodrigues E. Pereira A.C. Duarte 《Estuarine, Coastal and Shelf Science》2008,79(4):751-757
Land/ocean boundaries constitute complex systems with active physical and biogeochemical processes that affect the global carbon cycle. An example of such a system is the mesotidal lagoon named Ria de Aveiro (Portugal, 40°38′N, 08°45′W), which is connected to the Atlantic Ocean by a single channel, 350 m wide. The objective of this study was to estimate the seasonal and inter-tidal variability of organic carbon fluxes between the coastal lagoon and the Ocean, and to assess the contribution of the organic carbon fractions (i.e. dissolved organic carbon (DOC) and particulate organic carbon (POC)) to the export of organic carbon to the Ria de Aveiro plume zone. The organic carbon fractions fluxes were estimated as the product of the appropriate fractional organic carbon concentrations and the water fluxes calculated by a two-dimensional vertically integrated hydrodynamic model (2DH). Results showed that the higher exchanges of DOC and POC fractions at the system cross-section occurred during spring tides but only resulted in a net export of organic carbon in winter, totalling 85 t per tidal cycle. Derived from the winter and summer campaigns, the annual carbon mass balance estimated corresponded to a net export of organic carbon (7957 = 6585 t yr−1 POC + 1372 t yr−1 DOC). On the basis of the spring tidal drainage area, it corresponds to an annual flux of 79 g m−2 of POC and 17 g m−2 of DOC out of the estuary. 相似文献
75.
76.
Srgio Luiz Martins Pereira Lydia Maria Lobato Juliano Efigênio Ferreira Eduardo Csar Jardim 《Ore Geology Reviews》2007,32(3-4):571
The orogenic banded iron formation (BIF)-hosted Au mineralization at São Bento is a structurally-controlled, hydrothermal deposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero region, Brazil. The deposit has reserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies São Bento BIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity of sulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainly arsenopyrite) of the Fe-rich bands of the São Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alteration zones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite within the BIF by ankerite and pyrrhotite, respectively. The São Bento BIF was intensely and extensively deformed, first into open, upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts. Mineralized horizons at São Bento result from early structural modifications imposed by major transcurrent and thrusts faults, comprising the Conceição, Barão de Cocais and São Bento shear zones. Dextral movement on the SW–NE-directed Conceição shear zone may have generated splays at a compressional side-stepping zone, such as the São Bento shear zone, which is the structural locus for the São Bento gold mineralization. Relaxation of the Conceição shear zone under more brittle conditions resulted in the development of dilatational zones where gold–sulfide–quartz veins formed. These structures are considered to have been generated in the Archean. Geochronological data are scarce, with Pb–Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened the original, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similar to Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO2 and/or CH4, with extremely variable CO2/CH4 ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the carbonic phase from 10–15% to 5%, and increase in the H2O/(CO2 + CH4) and CO2/CH4 ratios, suggesting important interaction with carbonaceous sediment. Trapping conditions indicate a temperature of 300 °C at 3.2 kbar. 相似文献
77.
H. Evangelista J. Maldonado R. H. M. Godoi E. B. Pereira D. Koch K. Tanizaki-Fonseca R. Van Grieken M. Sampaio A. Setzer A. Alencar S. C. Gonçalves 《Journal of Atmospheric Chemistry》2007,56(3):225-238
The total extent of the atmospheric impacts associated to the aerosol black carbon (BC) emissions from South America is not
completed described. This work presents results of BC monitored during three scientific expeditions (2002, 2003 and 2004)
on board of a Brazilian oceanographic vessel Ary Rongel that covered the South–West Atlantic coast between 22–62°S. This latitudinal
band encloses major urban regions of South America and the outflow region of the SACZ (South Atlantic Convergent Zone), which
is an important mechanism of advective transport of heat, moisture, minor gases and aerosols from the South America continental
land to the Southern Atlantic Ocean. Our results showed that aerosol BC enhanced concentrations from urban/industrial origin
can be transported to the South–West Atlantic Ocean due to the migration of sub-polar fronts that frequently reach tropical/subtropical
regions. Despite the decrease of aerosol BC concentrations southwards (from ∼1,200 ng m−3 at latitude 22°S to ∼10 ng m−3 at latitude 62°S), several observed peak events were attributed to regional urban activities. Most of such events could be
explained by the use of air mass back trajectories analysis. In addition, a global model simulation is presented (Goddard
Institute for Space Studies – GISS GCM BC simulation) to explore the origins of aerosol BC in the South–West Atlantic. The
model allowed isolating the biomass emissions from South America and Africa and industrial (non-biomass) pollution from other
regions of the globe. This model suggests that the apportionment of about half of the aerosol BC at the South–West Atlantic
may derive from South American biomass burning. 相似文献
78.
K. Slezakova M. C. Pereira M. A. Reis M. C. Alvim-Ferraz 《Journal of Atmospheric Chemistry》2007,58(1):55-68
Epidemiological studies initially considered the impact of total solid particles on human health, but according to the acquired
knowledge about the worse effect of smaller particles, those studies turned to consider the impact of PM10. However, for the last decade PM2.5 began to be more important, once as they are smaller they can penetrate deeper in the lungs, being possible their trapping
in alveoli and worse effects on human health. Therefore, more information on PM2.5 should be provided namely concerning the levels and elemental composition. Considering the relevance of traffic on the emission
of particles of small sizes, this work included the detailed characterization of PM10 and PM2.5, sampled at two sites directly influenced by traffic, as well as at two reference sites, aiming a further evaluation of the
influence of PM10 and PM2.5 on public health. The specific objectives were to study the influence of traffic emission on PM10 and PM2.5 characteristics, considering concentration, size distribution and elemental composition. PM10 and PM2.5 samples were collected using low-volume samplers; the element analyses were performed by particle induced X-ray emission
(PIXE). At the sites influenced by traffic emissions PM10 and PM2.5 concentrations were 7–9 and 6–7 times higher than at the background sites. The presence of 17 elements (Mg, Al, Si, P, S,
Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn and Pb) was determined in both PM fractions; particle metal contents were 3–44 and
3–27 times higher for PM10 and PM2.5, respectively, than at the backgrounds sites. The elements originated mostly from anthropogenic activities (S, K, V, Mn,
Ni, Zn and Pb) were predominantly present in PM2.5, while the elements mostly originated from crust (Mg, Al, Si and Ca) predominantly occurred in PM2.5–10. The results also showed that in coastal areas sea salt spray is an important source of particles, influencing PM concentration
and distributions (PM10 increased by 46%, PM2.5/PM10 decreased by 26%), as well as PM compositions (Cl in PM10 was 11 times higher). 相似文献
79.
Carlos Roberto de Souza Filho Alfredo Rossetto Nunes Emilson Pereira Leite Lena Virgínia Soares Monteiro Roberto Perez Xavier 《Surveys in Geophysics》2007,28(5-6):377-405
The Serra Leste region, located in the northeast portion of Carajás Mineral Province (PA), hosts important chrome-PGE, Au–Pd–Pt,
Cu–Mo–Au, iron, and manganese deposits and occurrences related to different geological settings. We present a regional prospective
model with focus on PGE mineralizations based on airborne geophysics (magnetometry, gamma-ray spectrometry and time domain
electromagnetometry) and geological data. Geological and geophysics data analysis indicate a plausible correspondence between
areas with meta-mafic–ultramafic rocks and low values in gamma spectrometry, mainly for the thorium channel. The meta-ultramafic
units of the complexes, the main host rock of the PGE mineralization, are well defined by analytic signal from the anomalous
magnetic field and by the apparent conductance anomalies derived from the GEOTEM system. The combination of the derivative
products with updated geological data of the Serra Leste region and PGE-Luanga deposit enable to establish a regional prospective
model for PGE and the identification of new targets within the studied area. 相似文献
80.
Ronald?GoldbergEmail author Beth?Phelan Jose?Pereira Stacy?Hagan Paul?Clark Allen?Bejda Anthony?Calabrese Anne?Studholme Kenneth?W.?Able 《Estuaries and Coasts》2002,25(2):215-226
We compared distribution and abundance by habitat for age-0, young-of-the-year (YOY) winter flounder,Pseudopleuronectes americanus, in three estuaries (Hammonasset River, Navesink River, and Great Bay-Little Egg Harbor) in the northeastern United States to better define essential fish habitat (EFH). Two replicates of five representative habitats were sampled in most estuaries: eelgrass (Zostera marina), unvegetated areas adjacent to eelgrass, macroalgae, (primarilyUlva lactuca), unvegetated areas adjacent to macroalgae, and tidal marsh creeks. Fish were sampled every two weeks, May through October 1995 and 1996, with a beam-trawl (1-m width, 3-mm mesh net). Abundance of YOY winter flounder was highest in the Navesink River estuary and similar between years, but was significantly lower and differed between years in the Great Bay-Little Egg Harbor and Hammonasset River estuaries. Annual temperature differences appear to influence estuary use by YOY. In the years and estuaries studied, where habitat-related differences in abundance were significant, YOY were found in higher densities in unvegetated areas adjacent to eelgrass. The exception was in the Hammonasset River in 1995 when densities were higher in eelgrass. We conclude that the type of habitat most important to YOY winter flounder varies among estuaries and as a result, care should be taken in defining EFH, based only on limited spatial and temporal sampling. 相似文献