Holocene hydrological and vegetation changes in southern France inferred by the study of an alluvial travertine system (Saint-Guilhem-le-Désert, Hérault)     

Adam Ahmed Ali  Paul Roiron  Lucie Chabal  Paul Ambert  Jean Gasco  Joël Andr  Jean-Frdric Terral 《Comptes Rendus Geoscience》2008,340(6):356-366

A geobotanical study of the travertine system of Saint-Guilhem-le-Désert (southern France) was carried out in order to reconstruct the local Holocene environment in a region where the postglacial vegetation history is poorly documented. The travertinisation process has started at ca. 8500 cal. BP, in a landscape dominated by Pinus sylvestris type (probably Pinus nigra sub sp. salzmannii). Around 7000 cal. BP, the travertine system recorded torrential events not evidenced at the regional scale, showing the particularity of the Verdus hydrological regime. More recently, ca. 5100 cal. BP, a lake or a marsh was filled within the Verdus plain, as attested to by sand and silt particles accumulated in the sequence. The present-day vegetation dominated by Quercus ilex, on south facing slopes, was most likely established between the Bronze Age and the Gallo-Roman period correlatively to the decline of Pinus nigra and deciduous Quercus, most probably under human influence.  相似文献   

Analysis, in a free surface steady flow, of the interstitial velocity field inside a sedimentary bed     

Mustapha Kamel Mihoubi  Michel Blorgey  Ahmed Kettab 《Comptes Rendus Geoscience》2008,340(12):858-864

The objective of our study is to exploit ultrasonic Doppler velocimetry (UDV) to analyze the velocity field within a steady flow on a porous sedimentary bottom. Velocities are measured for several sedimentary beds, as well within the fluid vein as within the sediment. Our results highlight an exponential distribution of the velocities inside the sediment and a discontinuity of the velocity at the interface water–sediment, between the velocity in the free flow and the velocity within the sediment. These results highlight the need to base the analysis of sedimentary transport on the reality of the physical processes at the interface water–sediment.  相似文献   

Two forsterite-bearing FUN inclusions in the Allende meteorite     

Robert N. Clayton  Glenn J. MacPherson  Ian D. Hutcheon  Andrew M. Davis  Lawrence Grossman  Toshiko K. Mayeda  Carol Molini-Velsko  John M. Allen  Ahmed El Goresy 《Geochimica et cosmochimica acta》1984,48(3):535-548

We have discovered two FUN inclusions, CG-14 and TE, among a group of five forsterite-rich inclusions in Allende, two of which are described for the first time herein. All five consist of euhedral forsterite and spinel crystals poikilitically enclosed by fassaite. Forsterite and spinel are usually segregated from one another, sometimes into a spinel-rich mantle and a forsterite-rich core. Some inclusions contain vesicles, indicating that they were once molten. The crystallization sequence inferred from textures is: spinel, forsterite, fassaite and, finally, Mg-rich melilite. One concentrically-zoned inclusion contains melilite in its mantle whose composition lies on the opposite side of the liquidus minimum in the melilite binary from that in its core. This suggests that segregation of forsterite from spinel in all of these inclusions could be due to volatilization of MgO and SiO₂ relative to Al₂O₃ and CaO from the outsides of droplets. CG-14 is relatively uniformly enriched in refractory elements relative to Cl chondrites by a factor similar to that for Ca-, Al-rich coarse-grained inclusions except for Ca, Al and Hf which are unusually low. No Ce anomaly such as found in FUN inclusions Cl and HAL is present in CG-14. Whole-rock samples of CG-14 and TE are more strongly mass-fractionated in oxygen relative to “normal” Allende inclusions than the FUN inclusion EK 1-4-1 and less so than Cl. Relative to bulk Allende, both inclusions have strongly massfractionated magnesium and silicon and ²⁵Mg excesses or deficits of ²⁴Mg or ²⁶Mg. CG-14 has a ²⁹Si excess or a deficit of ²⁸Si or ³⁰Si. Volatilization loss cannot be responsible for the magnesium and silicon isotope fractionations, as this would require prohibitively large mass loss from these magnesium-rich inclusions. The remarkable similarity in textures between FUN and non-FUN inclusions implies similar thermal histories, arguing against different rates of evaporative loss of major elements. Sputtering alone may be insufficient to account for the magnitude and direction of oxygen isotope fractionation in FUN inclusions.  相似文献   

Momentum flux variations at lower thermospheric levels as seen from mid-latitude meteor wind observations     

P. C. S. Devara  M. I. Ahmed 《Meteorology and Atmospheric Physics》1986,35(4):236-241

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Lead isotopes and trace elements in some Miocene Red Sea galenas     

Sayed B. El-Kholy  Ahmed H. Hashad  Ezzat T. M. Selim 《Pure and Applied Geophysics》1970,81(1):135-150

Summary Lead-zinc mineralization occurs in Miocene sediments along the Egyptian Red Sea Coast in several localities. Galena samples were collected from some of these localities for isotopic and trace elements studies. The isotopic ratios²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb,²⁰⁹Pb/²⁰⁴Pb for these samples indicate that they are contaminated with radiogenic lead (J-type anomalous lead) with different degrees. The variation in the degree of contamination is significant and indicates a non-uniform source of uranium and thorium from which radiogenic lead was deriven. The highst contaminations were observed in samples collected from sediments overlying or close to acidic basement rocks. The trace element content indicates that galenas were formed at low temperature from solutions that circulated the underlying basement picking several trace elements in addition to radiogenic lead.  相似文献   

On the atmospheric tidal wind variations at meteor heights     

Dr. P. C. S. Devara  Dr. M. I. Ahmed 《Meteorology and Atmospheric Physics》1982,31(1-2):35-47

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A Participatory GIS Approach to Identify Critical Land Degradation Areas and Prioritize Soil Conservation for Mountainous Olive Groves （Case Study）   总被引：1，自引：0，他引：1  

Ahmed AL-WADAEY  ;Feras ZIADAT 《山地科学学报》2014,11(3):782-791

Climate change scenarios predict an increase in the frequency of heavy rainfall events in some areas. This will increase runoff and soil erosion, and reduce agricultural productivity, particularly on vulnerable mountainous agricultural lands that is already exhibiting high rates of soil erosion. Haphazard implementation of soil and water conservation （SWC） interventions on scattered fields is inefficient in reducing soil erosion. The objective of this study was to identify areas at high risk of erosion to aid the design and implementation of sustainable SWC using GIS analysis and farmers＇ participation approach. A 25 m digital elevation model （DEM） was used to derive layers of flow accumulation, slope steepness and land curvature, which were used to derive an erosion-risk （priority） map for the whole watershed. Boundaries of farmers＇ fields were mapped and verified by the community and each field was classified into high, moderate or low erosion risk. Fields with low flow accumulation （top of hill） and/or steep slope and/or convex slope were assigned high erosion risk and therefore high implementation priority. The study showed that more than 54% of the fields were classified into high erosion risk areas. Accordingly, a community-watershed plan was established, revised and approved by the community.Incentive loans to implement SWC measures were distributed to 100 farmers based on the priorities of their fields. Judged by local farmers and using 16 randomly selected fields, 90% of the targeted areas were correctly identified using the erosion risk map. After two years, the conservation measures had led to marked improvement of soil conservation. The approach is simple and easy to comprehend by the community and provides scientific basis to prioritize the implementation of SWC and to target the most degraded areas, which amplify the impact of these in reducing the vulnerability to land degradation.  相似文献   

Spatio-temporal Assessment of Urban Heat Island Effects in Kuala Lumpur Metropolitan City Using Landsat Images     

Yusuf Ahmed Yusuf  Biswajeet Pradhan  Mohammed O. Idrees 《Journal of the Indian Society of Remote Sensing》2014,42(4):829-837

Alteration in climatic pattern has resulted to a steady decline in quality of life and the environment, especially in and around urbanized areas. These areas are faced with increasing surface temperature arising mostly from human activities and other natural sources; hence land surface temperature has become an important variable in global climate change studies. In this paper, Landsat TM/ETM imagery acquired between 1997 and 2013 were used to extract ground brightness temperature and land use/land cover change in Kuala Lumpur metropolis. The main objective of this paper is to examine the effectiveness of quantifying UHI effects, in space and time, using remote sensing data and, also, to find the relationship between UHI and land use change. Four land use types (forest, farmland, built-up area and water) were classified from the Landsat images using maximum likelihood classification technique. The result reveals that Greater KL experienced an increase in average temperature from 312.641°K to 321.112°K which was quite eminent with an average gain in surface temperature of 8.4717°K. During the period of investigation (1997–2013), generally high temperature is been experienced mostly in concentrated built-up areas, the less concentrated have a moderate to intermediate temperature. Again, the study also shows that low and intermediate temperature classes loss more spatial extent from 2,246.89 Km² to 1,164.53 Km² and 6,102.42 Km² to 3,013.63 Km² and a gain of 4,165.963 Km² and 307.098 Km² in moderate and high temperature respectively from 1997 to 2013. The results of this study may assist planners, scientists, engineers, demographers and other social scientists concerned about urban heat island to make decisions that will enhance sustainable environmental practices.  相似文献   

Urban surface temperature behaviour and heat island effect in a tropical planned city     

Adeb Qaid Ahmed  Dilshan Remaz Ossen  Elmira Jamei  Norhashima Abd Manaf  Ismail Said  Mohd Hamdan Ahmad 《Theoretical and Applied Climatology》2015,119(3-4):493-514

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Long-term rainfall characteristics in the Mzingwane catchment of south-western Zimbabwe     

S. Sibanda  S.W. Grab  F. Ahmed 《Theoretical and Applied Climatology》2020,139(3):935-948

Rainfall characteristics during the annual rainy season are explored for the Mzingwane catchment of south-western Zimbabwe, for both historic period (1886–1906) and more recent times (1950–2015), based on available daily and monthly precipitation series. Annual and seasonal rainfall trends are determined using the modified Mann-Kendall test, magnitude of trends test and Sen’s slope estimator. Rainfall variability is quantified using the coefficient of variation (CV), precipitation concentration index (PCI) and standard precipitation index (SPI). Results suggest that contemporary mean annual rainfall may not have changed from that measured during the historic period of 1886–1906. However, the number of rainy days (≥ 1 mm) has decreased by 34%, thus suggesting much more concentrated and increased rainfall intensity. A notable shift in both the onset and cessation dates of the rainy season is recorded, particularly during the twenty-first century, which has resulted in a significantly reduced (p < 0.05) length of the rainy season. The combination of a reduced number of rainy days (≥ 1 mm) and a shortened rainy season suggests that long intra-season dry spells have become more common through time and have considerable negative consequences for agriculture and wetland ecosystem in the region. In addition, high spatio-temporal rainfall variability and seasonal PCI values indicate strong seasonality in the rainy season. Based on the SPI results, the El Niño Southern Oscillation (ENSO) strongly influences rainfall variability. The results further suggest high uncertainty in rain season characteristics, which requires effective planning for water needs.  相似文献