排序方式： 共有37条查询结果，搜索用时 15 毫秒
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Martin C. Smith Gregory R. Ruchti Amina Helmi Rosemary F. G. Wyse J. P. Fulbright K. C. Freeman J. F. Navarro G. M. Seabroke M. Steinmetz M. Williams O. Bienaymé J. Binney J. Bland-Hawthorn W. Dehnen B. K. Gibson G. Gilmore E. K. Grebel U. Munari Q. A. Parker R.-D. Scholz A. Siebert F. G. Watson T. Zwitter 《Monthly notices of the Royal Astronomical Society》2007,379(2):755-772
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Christian Coulon M'hamed Megartsi Serge Fourcade Ren C. Maury Herv Bellon Amina Louni-Hacini Joseph Cotten Alain Coutelle Daniel Hermitte 《Lithos》2002,62(3-4):87-110
During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle. 相似文献
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Karima Haddou Amina Bendaoud Nouria Belaidi Amina Taleb 《Environmental Earth Sciences》2018,77(13):520
Very few studies have evaluated the role of the hyporheic zone (HZ) in aquatic ecosystem functioning or the factors driving hyporheic exchange flows on a large scale, especially in a semi-arid environment such as the Tafna watershed in Algeria. To understand this role through time and space, hydrogeochemical parameters, particularly nitrate concentrations (NO3–N), were measured monthly between February 2013 and April 2014 in surface water (SW), interstitial water (IW) and groundwater (GW) along a 170-km stretch from the river’s source to the outlet into the main stream of the Tafna River and its Isser and Chouly tributaries. The significant longitudinal evolution of NO3–N in the HZ was related to stream order and distance from the source. Moreover, the study indicated a significant difference between nitrate concentrations in the riffles (R) and pools (P) of the Tafna wadi. Principal component analysis (PCA) revealed the considerable impact of agriculture on nitrate concentrations. This study indicated on a large scale that nitrate-rich HZ contributed to increasing surface nitrate concentrations in upwelling sites and could be an important nitrate source for downstream SW, particularly during low-water (LW) periods when the Tafna can run dry on the surface. Thus, these results underline the importance of hyporheic zones functioning to the water quality of the watershed (process of enrichment and retention of nitrogen). 相似文献
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阿米娜.麦图尔迪 《沙漠与绿洲气象(新疆气象)》2013,7(5):44-50
长白山温带针阔叶混交林作为典型的温带森林生态系统,对气候变化的反应比较敏感,其生长季及积温的变化是准确评价该我国东北林区气候变化的重要依据。本研究采用中国科学院长白山森林生态系统定位站获取的1982-2010年气温目值观测资料,分析了近20a该地区生长季长度及积温的变化,结果标明长白山温带针阔叶混交林29a的平均温度为3．6℃,29a中该地区气温有较弱的上升趋势;长白山温带针阔叶混交林1990-2010年生长季长度的平均值为174d,近20a中生长季长度呈弱增长趋势;长白山温带针阔叶混交林1990～2010年生长季起始日期变化较稳定,生长季终止日期有推后的趋势;长白山温带针阔叶混交林1990-2010年≥5℃的活动积温和有效积温平均值分别为2616．9℃、1744．8℃。近20a中,≥5℃活动积温和有效积温都有着缓慢升高的趋势。 相似文献
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Mark Vogelsberger Simon D. M. White Amina Helmi Volker Springel 《Monthly notices of the Royal Astronomical Society》2008,385(1):236-254
We present a new and completely general technique for calculating the fine-grained phase-space structure of dark matter (DM) throughout the Galactic halo. Our goal is to understand this structure on the scales relevant for direct and indirect detection experiments. Our method is based on evaluating the geodesic deviation equation along the trajectories of individual DM particles. It requires no assumptions about the symmetry or stationarity of the halo formation process. In this paper we study general static potentials which exhibit more complex behaviour than the separable potentials studied previously. For ellipsoidal logarithmic potentials with a core, phase mixing is sensitive to the resonance structure, as indicated by the number of independent orbital frequencies. Regions of chaotic mixing can be identified by the very rapid decrease in the real-space density of the associated DM streams. We also study the evolution of stream-density in ellipsoidal NFW haloes with radially varying isopotential shape, showing that if such a model is applied to the Galactic halo, at least 105 streams are expected near the Sun. The most novel aspect of our approach is that general non-static systems can be studied through implementation in a cosmological N -body code. Such an implementation allows a robust and accurate evaluation of the enhancements in annihilation radiation due to fine-scale structure such as caustics. We embed the scheme in the current state-of-the-art code gadget -3 and present tests which demonstrate that N -body discreteness effects can be kept under control in realistic configurations. 相似文献
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Pressure and shear stress caused by raindrop impact at the soil surface: Scaling laws depending on the water depth 
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下载免费PDF全文 Amina Nouhou Bako Frédéric Darboux François James Christophe Josserand Carine Lucas 《地球表面变化过程与地形》2016,41(9):1199-1210
Raindrop impact is an important process in soil erosion. Through its pressure and shear stress, raindrop impact causes a significant detachment of the soil material, making this material available for transport by sheet flow. Thanks to the accurate Navier–Stokes equations solver Gerris, we simulate the impact of a single raindrop of diameter D, at terminal velocity, on water layers of different thickness h: , , D, 2D, in order to study pressures and shear stresses involved in raindrop erosion. These complex numerical simulations help in understanding precisely the dynamics of the raindrop impact, quantifying in particular the pressure and the shear stress fields. A detailed analysis of these fields is performed and self‐similar structures are identified for the pressure and the shear stress on the soil surface. The evolution of these self‐similar structures are investigated as the aspect ratio h/D varies. We find that the pressure and the shear stress have a specific dependence on the ratio between the drop diameter and the water layer thickness, and that the scaling laws recently proposed in fluid mechanics are also applicable to raindrops, paving the road to obtain effective models of soil erosion by raindrops. In particular, we obtain a scaling law formula for the dependence of the maximum shear stress on the soil on the water depth, a quantity that is crucial for quantifying erosion materials. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Mohamed Amine Boukhedimi Amina Louni-Hacini Youcef Bouhadad Jean-François Ritz Djamel Machane Azzeddine Benhamouche Hamid Bourenane 《Journal of Seismology》2017,21(3):539-549
Coastal Quaternary deposits of western Oranie show typical soft-sediment deformations including sedimentary dykes, sand volcanoes, sismoslumps, thixotropic bowls, thixotropic wedges, diapir-like structures, and faults grading. Field observations indicate that these deformations exist at several levels of the studied deposits along the west Oranian coast. This study demonstrated that these structures are earthquakes-related, by analysis of potential trigger sources. Several arguments demonstrate the seismic origin: the depositional environment rich in water that located in an active tectonic region, the thixotropic nature of deformations and their large vertical and horizontal diffusion in the Quaternary series consistent with a seismic recurrence. This allows characterizing these Quaternary soft-sediment deformations as seismites that were triggered by earthquakes. 相似文献