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21.
In arid mountain areas, the dating and correlation of alluvial depositional surfaces is often uncertain. Especially in regions where the geomorphology is not well known, surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages. Pleistocene wadi terraces and associated alluvial fans occur in Wadi Al-Bih, U.A.E. and Oman, for which correlations and age relationships are not known. Three age-related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships. Deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after Miocene uplift of the region. The younger two groups of terraces and fans are inset within the older group. To identify the gross effects of pavement development, comparisons have been made between terrace surface and subsurface particle-size distributions. The older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces. This reflects the degree of pavement development. Particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces. Criteria for the classification of pavements were developed based on clast fracturing and angularity, size, sorting, packing, and surface texture, from which a simple index of pavement development has been derived. Other properties, rock varnish and weathering characteristics, were also recorded; but these proved to be less discriminatory than pavement characteristics. The pavement data have been augmented by observations on soils. Detailed studies of pavements on terraces (8 sites, 12 samples covering the three age groups) and fans (5 sites, 10 samples covering the three age groups) allow differentiation between age-groups. The three terraces show three different age-related pavement types, expressed by differences in the pavement development index. Weakly-developed pavements (little fracturing, sub-rounded clasts, some modification of the depositional fabric, incipient soil development, stage I CaCO3 accumulation) occur on the youngest terrace and fan surfaces. Moderately-developed pavements (clast fracturing, sub-angular clasts, moderate sorting and packing, deeper soil development, stage II CaCO3 accumulation) occur on the middle terrace and fan surfaces. Well-developed pavements (complete clast fracturing into small angular fragments, mature sorting and packing of the pavement surface, deep soil development with strong horizonation, stage III CaCO3 accumulation) occur on the highest terrace and oldest fan surfaces. There are minor differences between the youngest pavements on terraces and fans, which reflect initial sedimentological differences. These differences become less as the pavements develop. On the basis of comparative studies, the oldest terrace is estimated to date from sometime prior to ca. 100 ka BP, the second terrace and the most extensive fan surface from the Late Pleistocene, and the youngest terrace and fan phase from the Latest Pleistocene or Early Holocene. 相似文献
22.
In this paper we examine the wave properties of a hot plasma living in a Schwarzschild magnetosphere. The 3+1 GRMHD perturbation
equations are formulated for this scenario. These equations are Fourier analyzed and then solved numerically to obtain the
dispersion relations for a non-rotating, rotating non-magnetized and rotating magnetized plasma. The wave vector is evaluated,
which is used to calculate the refractive index. These quantities are shown in graphs which are helpful to discuss the dispersive
properties of the medium near the event horizon. 相似文献
23.
A survey on the epiphytic microorganisms growing on Posidonia oceanica leaves was conducted along a depth transect along the coast of Eastern Tunisia (Mahdia). Samples were collected by SCUBA diving at depths of 3, 5, 10 and 12 m in July 2008 and January 2009. A total of 58 microepiphyte taxa were identified. Multivariate analyses revealed temporal and spatial variation of the abundance of epiphytic microalgae. Water motion, light availability, temperature and motility of species seem to be responsible of temporal and bathymetric variations of epiphytic microalgae. Unlike diatoms, dinoflagellates were more abundant in deep waters, suggesting that they are more vulnerable to hydrodynamics. The significant correlation between leaf area index and abundance of epiphytic species indicates that the phenological parameters of the host plant influence the abundance of the epiphytic microorganisms. Among the epiphytic dinoflagellates, our data showed a great number of potentially toxic species (Alexandrium minitum, Amphidinium carterae, Karenia selliformis, Coolia monatis, Karlodinium veneficum, Ostreopsis siamensis, Prorocentrum concavum, Prorocentrum minimum, Prorocentrum rathymum and Prorocentrum lima). These species were more abundant in the deep station under more sheltered conditions than found at inshore shallower stations, which are subjected to high water motion. 相似文献
24.
Spatial scale variability in shoot density and epiphytic leaves of Posidonia oceanica on Kerkennah Island (Tunisia) in relation to current tide effects 
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下载免费PDF全文 Mounir Ben Brahim Lotfi Mabrouk Asma Hamza Mabrouka Mahfoudhi Abderahmen Bouain Lotfi Aleya 《Marine Ecology》2015,36(4):1311-1331
A study was undertaken of the patterns of spatial variability, epiphytic biomass and distribution of epiphytic fauna and flora of Posidonia oceanica. Samples were taken at four stations located approximately 4 km apart, exposed to different current conditions. Stations A and B, situated near the Oued Mimoun tidal channel with its relatively strong bi‐directional flows, were affected by high current tide. The other two stations, North Oued Mimoun (L1) and South Oued Mimoun (L2), were located further from the channel, in low current tide conditions. Sampling was conducted in the Attaya area of Kerkennah Island (Tunisia) in August 2009 at depths between 2 and 3 m, with the results indicating differences among the stations. Shoot density decreased when exposed to high levels of hydrodynamic activity generated by current tides whereas the epiphytic biomass of P. oceanica leaves decreased at sheltered stations located far from the channel. Epiphytic algae such as Heterokontophyta, Rhodophyta and Chlorophyta, and epiphytic fauna represented by Bryozoa, Hydrozoa, Annelida, Porifera and Tunicata, dominated the epiphytic assemblages and were abundant at the station most exposed to high current tide hydrodynamics. Cyanobacteria, however, were dominant in stations exposed to low current tide. 相似文献
25.
Kafeel Ahmad Zafar Iqbal Khan Asma Ashfaq Muhammad Ashraf Nudrat Aisha Akram Muhammad Sher Hazoor Ahmad Shad Vincenzo Tufarelli Antonio Lonigro Mariano Fracchiolla Eugenio Cazzato 《Environmental Earth Sciences》2017,76(8):322
Bio-concentration of elements such as Mo, As, Se, Fe, Cu, Zn, Ni and Pb was analyzed in spring onion (Allium fistulosum L.) in three different locations of central Punjab, Pakistan. At location GW, relatively low level of hazardous elements was found in spring onion, suggesting that groundwater is a safe source of water for irrigating food crops. The pH of soil at wastewater irrigation was found less acidic (pH 7.4) than the other sites. The range of concentration in the different samples of spring onion was as follows: 6.15–8.16 mg kg?1 for Mo, 2.77–4.28 mg kg?1 for As, 0.395–0.705 mg kg?1 for Se, 36.73–48.17 mg kg?1 for Fe, 10.58–16.26 mg kg?1 for Cu, 28.87–39.79 mg kg?1 for Zn, 6.66–8.75 mg kg?1 for Ni and 4.33–6.09 mg kg?1 for Pb, respectively. High bio-concentration of Zn (15.37) from soil to spring onion was found at canal water irrigated location. The estimated daily intake of metal for spring onion was less, but the health risk index was higher than 1 for Mo, As, Cu, Pb and Ni, respectively. This was due to higher proportion of spring onion in diet, which consequently increased the health risk index for metals. Therefore, it is recommended to avoid growing vegetables in untreated urban and rural wastewater containing elevated amounts of metals. 相似文献
26.
Instrumental Neutron Activation Analysis of Estuarine Sediment as a Proposed Reference Material for Environmental Studies 总被引：1，自引：0，他引：1
Shahida Waheed Mohammad Wasim Asma Rahman Shujaat Ahmad 《Geostandards and Geoanalytical Research》2001,25(1):137-143
The International Atomic Energy Agency (IAEA) Marine Environmental Studies Laboratory (MEL) organised an intercomparison exercise, through its Analytical Quality Control Services (AQCS), for the determination of trace elements in estuarine sediment IAEA-405, well suited for the characterisation of sediments. The instrumental neutron activation analysis technique (INAA) was developed using a 27 kW low power research reactor. Forty elements in the sediment were determined with a measurement precision varying from 1.8% to 12.3%. IAEA reference materials SL-1 and SD-M-2/TM were analysed throughout this work as quality assurance samples. 相似文献
27.
Nadja Zupan Hajna Roman Skála Asma Al-Farraj Martin Šťastný Pavel Bosák 《Arabian Journal of Geosciences》2016,9(17):689
The x-ray powder diffraction identification of clay minerals both in bulk samples and in separated clay fraction confirmed the presence of palygorskite in samples of cave sediments from Wadi Haqil (the western slopes of Musandam Mountains; Ras Al-Khaimah Emirate, UAE). Samples contain quartz, gypsum, smectite, kaolinite, calcite, and palygorskite, some of them chlorite, illite, feldspars, and goethite. Calcite dominates in most samples; smectite prevails in clay fraction. After heating, the 001 reflection of chlorite shifts to higher diffraction angles and its intensity decreases; these features indicate that the chlorite represent a Fe-dominant species. Unit-cell dimensions of major phases as refined by the Rietveld method are in agreement with literature data. Chemical composition of palygorskite was derived from unit-cell dimensions as follows: MgO content is 11–14 wt% and Al2O3 10–13 wt%. Clay mineralogy is only hard to ascertain from the scanning electron microscope (SEM) images even after being combined with the energy-dispersive spectrometer data. The SEM was also used to characterize gypsum grains; they often display flow deformation features. Studied cave sediments represent palygorskite-bearing weathering products and desert soils re-deposited from the cave surroundings by slope processes and wind and/or surface runoff. The mixture with other clay minerals, quartz, feldspars, etc. supports this interpretation. Fine-grained quartz fraction is probably wind-blown. Gypsum and calcite are the precipitates (crusts and/or cements), although gypsum can also be re-deposited from omnipresent gypsum-cemented surface sediments. 相似文献
28.
Mehrorang Ghaedi Gholam Reza Ghezelbash Farzaneh Marahel Shiva Ehsanipour Asma Najibi Mustafa Soylak 《洁净——土壤、空气、水》2010,38(9):877-885
This paper presents a biosorption procedure for the preconcentration of Pb2+ ions using Saccharomyces cerevisiae biomass. The influence of several factors including pH, biomass dosage, contact time, and temperature on biosorption efficiency were optimized. At optimum value of all the equilibrium, thermodynamic, and kinetic parameters of Pb2+ ion biosorption was investigated by testing the Langmuir and Freundlich models and first and second order kinetic models were applied. The biosorption capacity of S. cerevisiae biomass was determined 89.6 mg/g, while the retained Pb2+ ions by S. cerevisiae were reversibly eluted using 5 mol/L HNO3. Due to the high stability of S. cerevisiae the applied biomass can be used successively ten times with a slightly decrease (about 20%) in the recovery of Pb2+ ions. The calculated thermodynamic parameters, ΔG°, ΔH°, and ΔS° showed that the biosorption of Pb2+ ion onto S. cerevisiae biomass was feasible, spontaneous, and endothermic under examined conditions. The results of kinetic analysis showed that the biosorption processes of Pb2+ ions onto S. cerevisiae biomass followed pseudo second order kinetics. 相似文献
29.
Lotfi Mabrouk Taher Guarred Asma Hamza Imed Messaoudi Ahmed Noureddine Hellal 《Marine pollution bulletin》2014
Fluctuating asymmetry, a random deviation of bilateral traits, is assessed on grass goby Zosterisessor ophiocephalus Pallas, 1811 living in one disturbed site and in one unpolluted site Gulf of Gabes (Tunisia). Statistical analysis showed that assessed asymmetries are fluctuating. FA of orbital diameter, sub-orbital distance, pectoral fin and post-orbital length are higher in polluted site compared to control. Chemical pollution seems to be responsible for this high FA. 相似文献
30.