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Abstract— The recent discovery of the importance of Sun-grazing phenomena dramatically changed our understanding of the dynamics of objects emerging from the asteroid belt via resonant phenomena. The typical lifetimes of such objects are now expected to be <10 Ma, thus demanding a reassessment of our general picture of the meteorite delivery process. By analysing direct numerical integrations of ～2000 test particles beginning in the v6, 3:1, and 5:2 resonances in the main belt, we have reexamined the orbital and temporal distribution of meteoroids that journey to Earth. Comparing the results with fireball data, we find that the orbital distribution of Earth-impacting chondrites is consistent with a steady-state injection of meteoroids into the 3:1 and v6, resonances. Because this is the most complete and unbiased data set concerning Earth-impacting meteoroids, the agreement leads us to believe that our model is accurate. The simulations predict a P.M. fall ratio for chondrites ～14% lower than the observed value of ～68%, which argues for a moderate bias being present in this statistic. Most interestingly, the typical meteorite transfer times predicted by our models are several factors lower than the typical chondrite exposure ages, which implies that these meteorites acquired most of their exposure in the main belt before entering the resonances. We discuss some processes that would allow such preexposure. The case of achondrites and iron meteorites is also briefly discussed. 相似文献
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Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius 总被引：4，自引：1，他引：3
CIONI RAFFAELLO; CIVETTA LUCIA; MARIANELLI PAOLA; METRICH NICOLE; SANTACROCE ROBERTO; SBRANA ALESSANDRO 《Journal of Petrology》1995,36(3):739-776
A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’Plinian eruption of Vesuvius has allowed: (1) reconstructionof the thermal, compositional and isotopic (87Sr/86Sr) pre-eruptivelayering of the shallow magma chamber; (2) quantitative definitionof the syn-eruptive mixing between the different magmas occupyingthe chamber, and its relationships with eruption dynamics; (3)recognition of the variability of mafic magma batches supplyingthe chamber. During the different phases of the eruption 25–30%of the magma was ejected as white K-phonolitic pumice, and 70–75%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850–900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonolitic‘white’ magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor ‘grey’phono-tephritic magma (mafic end-member, MEM), which was nevererupted without first being mixed with ‘white’ magma.Evidence is provided that mixing occurred within the chamberand was characterized by a transition with time from physicalmixing at a microscopic scale to chemical hybridization. TheMEM magma had a homogeneous composition and constant 87Sr86Srisotopic ratio, possibly as a result of sustained convection.No unambiguous liquidus phases were found, suggesting that theMEM magma was superheated (T = 1000–1100C); its verylow viscosity was a main cause in the establishment of a physicaldiscontinuity separating the white and the grey magmas. Thewhite-grey boundary layer possibly consisted of a multiply diffusiveinterface, periodically broken and recreated, supplying thephonolitic body through mixing of moderate amounts of fractionatedgrey melts with the overlying white magma. The presence of alarge overheated mass indicates the young, growing stage ofthe AD 79 chamber, whose main engine was the periodic arrivalof hot mafic magma batches. These were characterized by K-tephriticto K-basanitic compositions, high temperatures (>1150C),high volatile contents (20–25% H2O +Cl+F+S), low viscosities[(1+2 102 poises)] and relatively low densities (2500–2600kg/m3). The birth of the Pompei chamber followed the repeatedarrival of these batches (on average characterized by 87Sr/86Sr070729)into a reservoir containing a tephriticphonolitic, crystal-enriched,magma, a residue from the preceding ‘Avellino’ Plinianeruption (3400 BP).In fact, about half of magma ejected duringthe AD 79 eruption could have been inherited from pre-Avellinotimes. KEY WORDS: Vesuvius; magma chamber; magma mixing; compositional layering phonolites; magma supply; potassic magmas
*Correponding author 相似文献
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CLAUDIO Faccenna RENATO Funiciello† ‡ ALESSANDRO Bruni MASSIMO Mattei† LEONARDO Sagnotti‡ 《Basin Research》1994,6(1):35-46
During the Messinian—Pleistocene, the Peninsular Tyrrhenian margin underwent a NE—SW orientated stretching regime, with the formation of a NW—SE normal fault system and basins which are linked by NE—SW transfer fault zones. These fault zones border narrow and deep asymmetric basins. This paper uses geological and geophysical analysis (structural and stratigraphical data, seismic lines and anisotropy of magnetic susceptibility (AMS) data) to look at the evolution of one of these transfer-related basins, located south of Rome (Ardea basin). Comparison with other similar features indicates that the common characteristics of these transfer structures are: (i) the slip vector along the transfer fault is mostly dip-slip, which means that the local extensional direction is orthogonal to the regional extensional direction; (ii) development of a narrow and deep half-graben basin. 相似文献
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Quantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings. 相似文献
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Development of hydrological models for seasonal and real-time runoff forecast in rivers of high alpine catchments is useful for management of water resources. The conceptual models for this purpose are based on a temperature index and/or energy budget and can be either lumped or distributed over the catchment area. Remote sensing satellite data are most useful to acquire near real-time geophysical parameters in order to input to the distributed forecasting models. In the present study, integration of optical satellite remote sensing-derived information was made with ground meteorological and hydrological data, and predetermined catchment morphological parameters, to study the feasibility of application of a distributed temperature index snowmelt runoff model to one of the high mountainous catchments in the Italian Alps, known as Cordevole River Basin. Five sets of Landsat Multispectral Scanning System (MSS) and Thematic Mapper (TM) computer-compatible tapes (CCTs) were processed using digital image processing techniques in order to evaluate the snow cover variation quantitatively. Digital elevation model, slope and aspect parameters were developed and used during satellite data processing. The satellite scenes were classified as snow, snow under transition and snow free areas. A second-order polynomial fit has been attempted to approximate the snow depletion and to estimate daily snow cover areal extent for three elevation zones of the catchment separately. Model performance evaluation based on correlation coefficient, Nash–Sutcliffe coefficient and percentage volume deviation indicated very good simulation between measured and computed discharges for the entire snowmelt period. The use of average temperature values computed from the maximum and minimum temperatures into the model was studied and a suitable algorithm was proposed. © 1997 John Wiley & Sons, Ltd. 相似文献
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R. VITOLO P. M. RUTI A. DELL'AQUILA M. FELICI V. LUCARINI A. SPERANZA 《地球，A辑:动力气象学与海洋学》2009,61(1):35-49
A statistical methodology is proposed and tested for the analysis of extreme values of atmospheric wave activity at mid-latitudes. The adopted methods are the classical block-maximum and peak over threshold, respectively based on the generalized extreme value (GEV) distribution and the generalized Pareto distribution (GPD). Time-series of the 'Wave Activity Index' (WAI) and the 'Baroclinic Activity Index' (BAI) are computed from simulations of the General Circulation Model ECHAM4.6, which is run under perpetual January conditions. Both the GEV and the GPD analyses indicate that the extremes of WAI and BAI are Weibull distributed, this corresponds to distributions with an upper bound. However, a remarkably large variability is found in the tails of such distributions; distinct simulations carried out under the same experimental setup provide sensibly different estimates of the 200-yr WAI return level. The consequences of this phenomenon in applications of the methodology to climate change studies are discussed. The atmospheric configurations characteristic of the maxima and minima of WAI and BAI are also examined. 相似文献
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ERIC LAJEUNESSE LUCE MALVERTI PIERRE LANCIEN LAWRENCE ARMSTRONG FRANCOIS MÉTIVIER STEPHEN COLEMAN CHARLES E. SMITH TIMOTHY DAVIES ALESSANDRO CANTELLI GARY PARKER 《Sedimentology》2010,57(1):1-26
The interaction of flow with an erodible bed in alluvial rivers and deep‐sea channels gives rise to a wide range of self‐formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple‐row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near‐laminar flow analogues of morphologies observed in the field. Laminar‐flow analogues of turbulent‐flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent‐flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows. 相似文献
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ISABELLA BORDI ALESSANDRO DELL'AQUILA ANTONIO SPERANZA† ALFONSO SUTERA 《地球，A辑:动力气象学与海洋学》2002,54(3):260-272
Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has been proposed. However, GCM results have shown that the dependence of the theory on external parameters is not consistent with that displayed by these numerical experiments. In the present paper we suggest an analytic formula for baroclinic adjustment to the neutrality of Eady waves through tropopause modification. This formula extends considerably the abovementioned theory by taking into account both a simple representation of the stratosphere and the topography. These modifications alter the tropopause condition for a baroclinically neutral state and its sensitivity to the external parameters. In particular, the topography introduces a dependence on the tropospheric vertical wind shear of the neutrality condition. This feature is not present in other models that assume a background state with a zero potential vorticity gradient in the troposphere. We show, furthermore, that the modified neutrality condition has sensitivities that may resemble those displayed by GCM simulations, with respect to the parameters defining the background flow. 相似文献
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RAFFAELE DI CUIA ALBERTO RIVA ANDREA SCIFONI ALESSANDRO MORETTI CHRISTOPH SPÖTL BRUNO CALINE 《Sedimentology》2011,58(6):1347-1369
A multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements. 相似文献
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