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481.
Hervé Martin Francis Albarède Philippe Claeys Muriel Gargaud Bernard Marty Alessandro Morbidelli Daniele L. Pinti 《Earth, Moon, and Planets》2006,98(1-4):97-151
Except the old Jack Hills zircon crystals, it does not exit direct record of the first 500 Ma of the Earth history. Consequently, the succession of events that took place during this period is only indirectly known through geochemistry, comparison with other telluric planets, and numerical modelling. Just after planetary accretion several episodes were necessary in order to make life apparition and development possible and to make the Earth surface habitable. Among these stages are: the core differentiation, the formation of a magma ocean, the apparition of the first atmosphere, oceans and continents as well as the development of magnetic field and of plate tectonics. In the same time, Earth has been subject to extraterrestrial events such as the Late Heavy Bombardment (LHB) between 3.95 and 3.8 Ga. Since 4.4–4.3 Ga, the conditions for pre-biotic chemistry and appearance of life were already met (liquid water, continental crust, no strong meteoritic bombardment, etc...). This does not mean that life existed as early, but this demonstrates that all necessary conditions assumed for life development were already present on Earth. 相似文献
482.
EAS-TOP Collaboration M. Aglietta B. Alessandro P. Antonioli F. Arneodo L. Bergamasco M. Bertaina C. Castagnoli A. Castellina A. Chiavassa G. Cini Castagnoli B. D''Ettorre Piazzoli G. Di Sciascio W. Fulgione P. Galeotti P. L. Ghia M. Iacovacci G. Mannocchi C. Morello G. Navarra O. Saavedra G. C. Trinchero S. Valchierotti P. Vallania S. Vernetto C. Vigorito 《Astroparticle Physics》2004,21(6):583-596
The evolution of the cosmic ray primary composition in the energy range 106–107 GeV (i.e. the “knee” region) is studied by means of the e.m. and muon data of the Extensive Air Shower EAS-TOP array (Campo Imperatore, National Gran Sasso Laboratories). The measurement is performed through: (a) the correlated muon number (Nμ) and shower size (Ne) spectra, and (b) the evolution of the average muon numbers and their distributions as a function of the shower size. From analysis (a) the dominance of helium primaries at the knee, and therefore the possibility that the knee itself is due to a break in their energy spectrum (at EkHe=(3.5±0.3)×106 GeV) are deduced. Concerning analysis (b), the measurement accuracies allow the classification in terms of three mass groups: light (p,He), intermediate (CNO), and heavy (Fe). At primary energies E0≈106 GeV the results are consistent with the extrapolations of the data from direct experiments. In the knee region the obtained evolution of the energy spectra leads to: (i) an average steep spectrum of the light mass group (γp,He>3.1), (ii) a spectrum of the intermediate mass group harder than the one of the light component (γCNO2.75, possibly bending at EkCNO≈(6–7)×106 GeV), (iii) a constant slope for the spectrum of the heavy primaries (γFe2.3–2.7) consistent with the direct measurements. In the investigated energy range, the average primary mass increases from lnA=1.6–1.9 at E01.5×106 GeV to lnA=2.8–3.1 at E01.5×107 GeV. The result supports the standard acceleration and propagation models of galactic cosmic rays that predict rigidity dependent cut-offs for the primary spectra of the different nuclei. The uncertainties connected to the hadronic interaction model (QGSJET in CORSIKA) used for the interpretation are discussed. 相似文献
483.
Mauro Gemmi Marco Merlini Alessandro Pavese Nadia Curetti 《Physics and Chemistry of Minerals》2008,35(7):367-379
Phengite samples (2M
1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray
diffraction. The measured volume thermal expansion of 2M
1 phengite (<α
V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α
V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle
at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed
in 2M
1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating
of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and
by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate
phase derived from 2M
1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect
to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows
two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not
proceed to completion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
484.
485.
486.
Veronica Rossi Giulia Barbieri Stefano Claudio Vaiani Marco Cacciari Luigi Bruno Bruno Campo Marco Marchesini Silvia Marvelli Alessandro Amorosi 《第四纪科学杂志》2021,36(6):961-979
Framed into a robust stratigraphic context, multivariate analyses on the Holocene palaeobiological record (pollen, benthic foraminifers, ostracods) of the Po coastal plain (NE Italy) allowed the investigation of microtidal ecosystems variability and driving parameters along a 35-km-long land–sea transect. Millennial-scale ecosystem shifts are documented by coeval changes in the meiofauna, reflecting variations in organic matter–water depth (shallow-marine environments) and degree of confinement-salinity (back-barrier settings). In-phase shifts of vegetation communities track unsteady water-table levels and river dynamics in freshwater palustrine areas. Five environmental–ecological stages followed one another crossing four tipping points that mark changes in relative sea level (RSL), climate and/or fluvial regime. At the culmination of Mediterranean RSL rise, after the 8200 event, remarkable growth of peatlands took place in the Po estuary, while low accumulation rates typified the shelf. At the transgressive–regressive turnaround (~7000 cal a bp ), the estuary turned into a delta plain with tidally influenced interdistributary embayments. River flow regime oscillations after the Climate Optimum (post-5000 cal a bp ) favoured isolation of the bays and the development of brackish wetlands surrounded by wooded peatlands. The youngest threshold (~800 cal a bp ), which led to the establishment of the modern delta, reflects a major avulsion of the Po River. 相似文献
487.
488.
In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~ 0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~ 1 ppb). DRaphlogopite/melt was determined to 2.28 ± 0.44 and 2.84 ± 0.47 in two experiments, the ratio DRa/DBa is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of DRa and DRa/DBa for magmatic minerals containing appreciable Ra, yields DRamineral/melt ranging from ~ 2.6 for phlogopite down to 2–3 ? 10? 5 for pyroxenes, and DRa/DBamineral/melt from ~ 4 for leucite to 2 ? 10? 2 for orthopyroxene. The influence of melt composition on DRa/DBa is less than 10%. All investigated minerals have different DRa/DBa, strongly fractionating Ra from Ba. Thus, for magmatic systems, (226Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (226Ra)/Ba–(230Th)/Ba system at the time of crystallization and thus, there is no (226Ra)/Ba–(230Th)/Ba isochron at t0. 226Ra–230Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known DRa. 相似文献
489.
Shou-Hao ChiangKang-Tsung Chang Alessandro C. MondiniBor-Wen Tsai Chen-Yu Chen 《Geomorphology》2012,138(1):306-318
A coupled model has been developed to simulate, at watershed level, landslides and debris flows induced by a severe typhoon (tropical cyclone) in Taiwan. The model comprises a landslide susceptibility model to predict landslide occurrence, an empirical model to select debris-flow initiation points, and a debris flow model to simulate the transport and deposit of failed materials from the identified source areas. In raster format with a 10 m spatial resolution, the model output includes unstable cells, debris-flow initiation cells, debris-flow velocities, runout paths, and deposition zones. The model was first tested and calibrated in a small area, where the damage by landslides had been investigated and recorded. It was then applied to a watershed, and the simulation results were validated by comparing them with a landslide/debris-flow inventory map prepared from satellite images using a multiple change detection technique. Model test and validation results confirm the usefulness of the model in predicting the number and size of affected areas (landslides and runouts combined), runout path, and volume of runout deposits. It is a common practice in Taiwan to separate landslide and debris-flow inventories and to study debris flows only in select drainage basins. This study suggests that landslide and debris flow should be modeled as a sequential process for efficient watershed management. 相似文献
490.
Masaki N. Nishino Hiroshi Hasegawa Yoshifumi Saito Iannis Dandouras Henri Rème Alessandro Retinò Elizabeth Lucek 《Planetary and Space Science》2011,59(7):502-509
Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth's magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn-dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn-dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth's magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles. 相似文献