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排序方式: 共有338条查询结果,搜索用时 31 毫秒
311.
Documentary Evidence on Climate in Sixteenth-Century Europe 总被引:2,自引:0,他引:2
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Massimiliano Ghinassi Andrè C. Colonese Zelia Di Giuseppe Lisa Govoni Domenico Lo Vetro Giulia Malavasi Fabio Martini Silvia Ricciardi Benedetto Sala 《第四纪科学杂志》2009,24(4):383-398
Clastic sediments deposited in caves and rock shelters bear peculiar sedimentological characteristics and have seldom been considered as a high‐resolution proxy record of climatic or environmental changes. The Romito Cave has its entrance at 275 m above sea level, about 25 km from the Tyrrhenian coast of Calabria, southern Italy. New archaeological excavation performed since 2000 has revealed a sedimentary succession spanning the record of Gravettian to Late Epigravettian cultures (Late Pleistocene). The present study focuses on the lower part (2.5 m thick) of the succession, where three main unconformity‐bounded stratigraphic units have been recognised (labelled RM1–3). Each unit consists of water‐lain deposits indicating high‐ to low‐competence flow, capped with anthropogenic deposits. The gradual deactivation and reactivation of the water drainage between 23 475 ± 190 and 16 250 ± 500 cal. a BP is correlated with regional precipitation changes due to the onset of dry climatic conditions of the Last Glacial Maximum. However, the deactivation of cave drainage after the deposition of unit RM3, around 15 400 ± 500 cal. a BP, deviates from the regional hydrological trend of progressively increasing water discharges and is attributed to the drainage cut‐off by probable cave wall collapses. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
314.
Mauro Mele Silvia Inzoli Mauro Giudici Riccardo Bersezio 《Geophysical Prospecting》2014,62(3):631-645
Electrical conductivity of alluvial sediments depends on litho‐textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho‐textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain‐size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two‐component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three‐component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid‐grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume. 相似文献
315.
Bérengère Parise Arnaud Belloche Silvia Leurini Peter Schilke 《Astrophysics and Space Science》2008,313(1-3):73-76
Highly-collimated outflows are believed to be the earliest stage in outflow evolution, so their study is essential for understanding
the processes driving outflows. The BHR71 Bok globule is known to harbour such a highly-collimated outflow, which is powered
by a protostar belonging to a protobinary system. Using the APEX telescope on Chajnantor, we mapped the BHR71 highly-collimated
outflow in CO(3-2), and observed several bright points of the outflow in the molecular transitions CO(4-3), CO(7-6), 13CO(3-2), C18O(3-2), CH3OH(7-6) and H2CO(4-3). We use an LVG code to characterise the temperature enhancements in these regions. These observations are particularly
interesting for investigating the interaction of collimated outflows with the ambient molecular cloud. In our CO(3-2) map,
the second outflow driven by IRS2, which is the second source of the binary system, is completely revealed and shown to be
bipolar. We also measure temperature enhancements in the lobes. The CO and methanol LVG modelling points to temperatures between
30 and 50 K in the two lobes. The methanol emission in the southern lobe bright knot is barely resolved with the APEX single-dish.
ALMA will thus be a central tool to study the shock chemistry in these regions. 相似文献
316.
Federico Marulli Silvia Bonoli Enzo Branchini Lauro Moscardini Volker Springel 《Monthly notices of the Royal Astronomical Society》2008,385(4):1846-1858
We model the cosmological co-evolution of galaxies and their central supermassive black holes (BHs) within a semi-analytical framework developed on the outputs of the Millennium Simulation. This model, described in detail by Croton et al. and De Lucia and Blaizot, introduces a 'radio mode' feedback from active galactic nuclei (AGN) at the centre of X-ray emitting atmospheres in galaxy groups and clusters. Thanks to this mechanism, the model can simultaneously explain: (i) the low observed mass dropout rate in cooling flows; (ii) the exponential cut-off in the bright end of the galaxy luminosity function and (iii) the bulge-dominated morphologies and old stellar ages of the most massive galaxies in clusters. This paper is the first of a series in which we investigate how well this model can also reproduce the physical properties of BHs and AGN. Here we analyse the scaling relations, the fundamental plane and the mass function of BHs, and compare them with the most recent observational data. Moreover, we extend the semi-analytic model to follow the evolution of the BH mass accretion and its conversion into radiation, and compare the derived AGN bolometric luminosity function with the observed one. While we find for the most part a very good agreement between predicted and observed BH properties, the semi-analytic model underestimates the number density of luminous AGN at high redshifts, independently of the adopted Eddington factor and accretion efficiency. However, an agreement with the observations is possible within the framework of our model, provided it is assumed that the cold gas fraction accreted by BHs at high redshifts is larger than at low redshifts. 相似文献
317.
The epi-benthic dinoflagellate Ostreopsis cf. ovata Fukuyo has an increasingly global distribution. In Brazil there are reports of O. cf. ovata along the coast from 8°S to 27°S latitude and blooms have been registered on the Rio de Janeiro coastline. In the current study, an O. cf. ovata bloom is reported at Saint Paul's Rocks (0°55'10″N; 29°20'33″W), between the southern and northern hemispheres. The Archipelago is not inhabited and not subjected to eutrophication, due to isolation, and sustains a number of endemic species. Therefore, blooms of O. cf. ovata may potentially cause demise to trophic chains by affecting marine invertebrates and vertebrates. 相似文献
318.
In the first decades of the 20th century, the Ebro River was the Iberian channel with the most active fluvial dynamics and the most remarkable spatial‐temporal evolution. Its meandering typology, the dimensions of its floodplain, and the singularities of its flow regime produced an especially interesting set of river functions. The largest dynamics of the Ebro River are concentrated along the meandering profile of the central sector. During the 20th century, this sector experienced a large alteration of its geomorphological structure. We present here an analysis of this evolution through the cartographic study of a long segment of the river (~250 km) in 1927, 1956 and 2003. The results show a large reduction in bank sinuosity, a progressive loss of fluvial territory, and a large decrease in channel width. These changes are especially clear in the areas previously most ecologically connected with the active channel. The fluvial territory of the river in 2003 was approximately half that found during the first decades of the 20th century. Forest plantations, which were non‐existent in 1927, occupied more than 1500 ha of the study area in the last decade. This intense geomorphological transformation becomes ecologically visible in (i) a 35% reduction of the area occupied by riparian vegetation; (ii) a loss of the heterogeneity of riparian forest spots, which were formerly structured in an irregular mosaic far from the river thalweg; and (iii) a modification of the riparian forest structure, which is currently linear, uniform, thin and very close to the river axis. The ecomorphological alteration was intensified by the remarkable reduction in bank length (13%) and the reduced dynamism of the present river system, indicated by an increase in the percentage of fluvial territory occupied by riparian forests and a reduction in the area occupied by the active channel. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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320.
Silvia Maßmann 《Ocean Dynamics》2010,60(6):1463-1477
Unstructured mesh models can resolve the model domain with a variable and very fine mesh resolution. Nevertheless, tuning
the model setup is still required (for example because of parametrized sub-grid processes). Adjoint models are commonly used
to calculate sensitivities of ocean models and optimize their parameters so that better agreement is achieved between model
simulations and observations. One major obstacle in developing an adjoint model is the need to update the reverse code after
each modification of the forward code, which is not always straightforward. Automatic differentiation is a tool to generate
the adjoint model code without user input. So far this method has mainly been used for structured mesh ocean models. We present
here an unstructured mesh, adjoint, tidal model using this technique, and discuss the sensitivities of the misfit between
simulated and observed elevations with respect to open boundary values, the bottom friction coefficient and the bottom topography.
The forward model simulates tides on the European Continental Shelf and we show that the tidal model dynamics in the adjoint
simulations can be used to define regions, where parameters or mesh has to be optimized. We analyze the dependence of the
sensitivities on the wave type and mesh resolution to specify whether the model misfit originates from physical or numerical
model deficiencies. In the sensitivity patterns, it is possible to identify islands not resolved in the mesh. We propose to
refine the mesh prior to the parameter optimization. 相似文献