排序方式: 共有10条查询结果,搜索用时 250 毫秒
1
1.
M. Jurado Vargas P. Merchán Benítez F. Sánchez Bajo A. Astillero Vivas 《Astrophysics and Space Science》2002,279(3):261-269
In order to determine the atmospheric extinction at Physics Department Astronomical Observatory of the University of Extremadura, located at Badajoz (Spain), several stars were observed during some clear nights of atmospheric stability in the period 1998–2000, at optical wavelengths corresponding to the filters of the Kron-Cousins and Strömgren photometric systems. The determination of the extinction coefficients was made by assuming the Bouguer's law, which was shown to be a good approximation for this study. The results exhibited temporal variations and can be considered to be associated with clean atmospheres at locations of low altitude. 相似文献
2.
Diana Prado Garzón Christiano G.M. Brum Ezequiel Echer Néstor Aponte Michael P. Sulzer Sixto A. González Robert B. Kerr Lara Waldrop 《Journal of Atmospheric and Solar》2011,73(11-12):1568-1574
We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O+ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H+ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12 h. 相似文献
3.
Valentina Vanghi Andrea Borsato Silvia Frisia Daryl L. Howard Gertruida Gloy John Hellstrom Petra Bajo 《Sedimentology》2019,66(7):2661-2685
Synchrotron high‐resolution and micro‐X‐ray fluorescence elemental mapping of two coeval coralloid speleothems from Lamalunga Cave (Italy) are complemented with petrographic, morphological and microstratigraphic studies. The importance of these speleothems relies on their direct and indirect association with a complete Neanderthal skeleton (‘Altamura Man’) found inside the cave. The coralloids grew discontinuously between 64·6 ka and the Holocene and reveal exceptionally high concentrations of Mg, Sr and Si, particularly on convex surfaces, where evaporation is more intense. The incorporation of trace elements depends on several factors including location, shape and geometrical evolution during their growth, as well as climate and environmental parameters. This resulted in calcite precipitation with Sr compositions from 100 to 1200 ppm and an average concentration of 7000 ppm Mg. An unusually high Si content (up to 16%) is possibly derived from volcanic ash transported as particulate and in solution inside the cave. The most common fabrics observed consist of non‐fluorescent elongated columnar calcite forming clean isopachous bands and fluorescent fibre‐like crystals associated with laminated, lenticular bands high in Sr, Mg and Si. Variability in Sr, Mg and Si concentrations appears to induce fabric changes in the coralloids. Elongation and lattice distortion of the crystals was found to coincide with high Mg concentrations. The transition from compact elongated to open to fibre‐like, is here interpreted as due to high concentrations of Si and Sr, which are preferentially incorporated in the speleothem at crystal boundaries and intra‐laminae. It is here inferred that coralloid fabric changes and their elemental content potentially record local rainfall variations through time, with the clean compact calcite marking high infiltration and open fibre‐like and micrite fabrics recording dry periods. 相似文献
4.
Christian Ferrarin Georg Umgiesser Marco Bajo Debora Bellafiore Francesca De Pascalis Michol Ghezzo Giorgio Mattassi Isabella Scroccaro 《Estuarine, Coastal and Shelf Science》2010
The hydraulic regime-based zonation scheme of the Lagoons of Marano and Grado (Italy) has been derived by means of numerical models. A finite element modelling system has been used to describe the water circulation taking in account different forces such as tide, wind and rivers. The model has been validated by comparing the simulation results against measured water levels, salinity and water temperature data collected in several stations inside the lagoons. The analysis of water circulation, salinity and spatial distribution of passive tracers released at the inlets, led to a physically-based division of the lagoons system into six subbasins. The derived classification scheme is of crucial value for understanding the renewal capacity and pollutants distribution patterns in the lagoon. 相似文献
5.
Timothy I. Kearsey Jonathan R. Lee Andrew Finlayson Marieta Garcia‐Bajo Anthony A. M. Irving 《Boreas: An International Journal of Quaternary Research》2019,48(3):658-677
Buried palaeo‐valley systems have been identified widely beneath lowland parts of the UK including eastern England, central England, south Wales and the North Sea. In the Midland Valley of Scotland palaeo‐valleys have been identified yet the age and genesis of these enigmatic features remain poorly understood. This study utilizes a digital data set of over 100 000 boreholes that penetrate the full thickness of deposits in the Midland Valley of Scotland. It identified 18 buried palaeo‐valleys, which range from 4 to 36 km in length and 24 to 162 m in depth. Geometric analysis has revealed four distinct valley morphologies, which were formed by different subglacial and subaerial processes. Some palaeo‐valleys cross‐cut each other with the deepest features aligning east–west. These east–west features align with the reconstructed ice‐flow direction under maximum conditions of the Main Late Devensian glaciation. The shallower features appear more aligned to ice‐flow direction during ice‐sheet retreat, and were therefore probably incised under more restricted ice‐sheet configurations. The bedrock lithology influences and enhances the position and depth of palaeo‐valleys in this lowland glacial terrain. Faults have juxtaposed Palaeozoic sedimentary and igneous rocks and the deepest palaeo‐valleys occur immediately down‐ice of knick‐points in the more resistant igneous bedrock. The features are regularly reused and the fills are dominated by glacial fluvial and glacial marine deposits. This suggests that the majority of infilling of the features happened during deglaciation and may be unrelated to the processes that cut them. 相似文献
6.
Jost Eikenberg Guido Vezzu Irene Zumsteg Sixto Bajo Max Ruethi Georg Wyssling 《Quaternary Science Reviews》2001,20(18):129
In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e.226Raex/226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/234U chronometer. Analytical solutions of the coupled 234U/230Th/226Ra radionuclide system predicted that the 226Raex/226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Raex/226Ra(0) ages agreed well with those derived from 230Th/234U, (ii) all data plot within uncertainty on the 226Raex/226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Raex/226Ra(0) should be applicable to materials which are suitable for 230Th/234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/234U for samples that have been formed a few thousand years ago. 相似文献
7.
Sixto R. Fernández-López Giulio Pavia Elisabetta Erba Myett Guiomar Maria H. Henriques Roberto Lanza Charles Mangold Davide Olivero Daniele Tiraboschi 《Swiss Journal of Geoscience》2009,102(2):271-295
The Bathonian Global Stratotype Section and Point (GSSP) is proposed at the base of limestone bed RB071 (bed 23 in Sturani
1967) in the Ravin du Bès Section (43° 57′ 38′′ N, 6° 18′ 55′′ E), Bas-Auran area, “Alpes de Haute Provence” French department.
The Ravin du Bès Section, as formal candidate GSSP for the base of the Bathonian Stage, satisfies most of the requirements
recommended by the International Commission on Stratigraphy: 1) The exposure extends over 13 m in thickness. At the Bajocian-Bathonian
transition, no vertical (bio-, ichno- or tapho-) facies changes, condensation, stratigraphic gaps or hiatuses have been recorded.
Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant
constraints. 2) There is a well-preserved, abundant and diverse fossil record across the boundary interval, with key markers
(ammonites and nannofossils) for worldwide correlation. The base of the Bathonian Stage and Zigzag Zone in Bas-Auran corresponds
to the first occurrence level of Gonolkites convergens Buckman, which coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils, as secondary global marker, are present in all beds and allow characterizing the Bajocian-Bathonian
transition. 3) Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray
data corroborate an Early Bathonian deepening half-cycle of second order. 4) The criteria of accessibility, conservation and
protection are assured by the “Réserve Naturelle Géologique de Haute Provence”. The Cabo Mondego Section (Portugal) is suggested
as the Bathonian auxiliary section and point (ASSP) within this GSSP proposal. 相似文献
8.
Large biases and inconsistent climate change signals in ENSEMBLES regional projections 总被引:1,自引:1,他引:0
Marco Turco Antonella Sanna Sixto Herrera Maria-Carmen Llasat José Manuel Gutiérrez 《Climatic change》2013,120(4):859-869
In this paper we analyze some caveats found in the state-of-the-art ENSEMBLES regional projections dataset focusing on precipitation over Spain, and highlight the need of a task-oriented validation of the GCM-driven control runs. In particular, we compare the performance of the GCM-driven control runs (20C3M scenario) with the ERA40-driven ones (“perfect” boundary conditions) in a common period (1961–2000). Large deviations between the results indicate a large uncertainty/bias for the particular RCM-GCM combinations and, hence, a small confidence for the corresponding transient simulations due to the potential nonlinear amplification of biases. Specifically, we found large biases for some RCM-GCM combinations attributable to RCM in-house problems with the particular GCM coupling. These biases are shown to distort the corresponding climate change signal, or “delta”, in the last decades of the 21st century, considering the A1B scenario. Moreover, we analyze how to best combine the available RCMs to obtain more reliable projections. 相似文献
9.
Christian Ferrarin Luca Zaggia Elio Paschini Tommaso Scirocco Giuliano Lorenzetti Marco Bajo Pierluigi Penna Matteo Francavilla Raffaele D’Adamo Stefano Guerzoni 《Estuaries and Coasts》2014,37(1):79-93
A multidisciplinary approach that combines field measurements, artificial neural networks, water balance analyses and hydrodynamic modelling was developed to investigate the water budget and renewal capacity of semi-closed coastal systems. The method was applied to the Lesina Lagoon, a micro-tidal lagoon in the southern Adriatic Sea (Italy). Surface water flux between the lagoon and the sea was determined by neural network prediction and used as input in the analysis. Strong seasonal variations in the water budget equation were predicted. Fresh water inputs estimated by the water balance analysis were used as forcing by a calibrated finite element model to describe the water circulation and transport time scale of the lagoon’s surface waters. The model highlighted the spatial heterogeneity of the renewal behaviour of the system, with a strong east–west water renewal time gradient. Knowledge of spatial distribution of water renewal times is crucial for understanding the lagoon’s renewal capacity and explaining the high spatial variability of the biogeochemistry of the Lesina Lagoon. 相似文献
10.
F. Snchez‐Bajo A. Garrigs‐Snchez J.M. Gmez‐Forrellad J. Nomen‐Torres W. Kleikamp P. Frank W. Moschner W. Quester E. García‐Melendo 《Astronomische Nachrichten》2003,324(6):511-515
Results of the photometric analysis by means of the Wilson‐Devinney method, combined with an empirical massradius relation for LC IV stars support the hypothesis that GX Gem is a well detached evolved system of subgiant stars. The solution is consistent with the known spectroscopic parameters obtained by Popper (1996) and the photometric data of Lacy (2002a). (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1