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1.
Solomon Tadesse 《Gondwana Research》2004,7(2):481
The Lega Dembi Primary Gold Deposit in southern Ethiopia is related to the shear zone-hosted vein in the Neoproterozoic metamorphosed volcano-sedimentary succession of greenschist- to amphibolite-facies metamorphism. The rocks consists of a sequence of biotite-feldspar-quartz schists, carbonaceous mica-schists, amphibolites and basic to ultrabasic rocks. This unit is separated from a foot wall biotite gneiss by a major shear zone. The ore bodies are hosted in the volcano-sedimentary sequence and consist of swarms of quartz veins, lenses, and stockworks that propagated along mesoscale ductile to brittle-ductile shear zones.The mineralization is defined by a complex paragenesis of gold in association with Cu-Pb-Zn-Fe sulphides, tellurides and sulphosalts. The presence of Ni-bearing minerals in amphibolites of the host sequence, together with the ore mineral association, suggests an origin related to mafic volcanism. 相似文献
2.
In this study, photospheric vector magnetograms obtained with the Synoptic Optical Long-term Investigations of the Sun (SOLIS) survey are used as boundary conditions to model three-dimensional nonlinear force-free (NLFF) coronal magnetic fields as a sequence of NLFF equilibria in spherical geometry. We study the coronal magnetic field structure inside an active region and its temporal evolution. We compare the magnetic field configuration obtained from NLFF extrapolation before and after the flaring event in active region (AR) 11117 and its surroundings observed on 27 October 2010, and we also compare the magnetic field topologies and the magnetic energy densities and study the connectivities between AR 11117 and its surroundings. During the investigated time period, we estimate the change in free magnetic energy from before to after the flare to be 1.74×1032?erg, which represents about 13.5?% of the NLFF magnetic energy before the flare. In this study, we find that electric currents from AR 11117 to its surroundings were disrupted after the flare. 相似文献
3.
Nawal Alfarrah Gebremedhin Berhane Abdelrahim Hweesh Kristine Walraevens 《Ground water》2017,55(4):593-601
The desert of eastern Libya forms one of the most arid regions of the Sahara. The Great Man‐Made River Project (GMRP) was established. It transports millions of cubic meters of water a day from desert wellfields to the coastal cities, where over 80% of the population lives. The Tazerbo Wellfield is one of the wellfields designed within the GMRP, delivering water to the eastern coast of Libya through an underground pipe network. Tazerbo Wellfield consists of 108 production wells; each well was designed to pump 100 L/s. The planned total groundwater withdrawal from all wells is 1 million m3/d. The deep sandstone aquifer (Nubian sandstone) is covered by a thick mudstone‐siltstone aquitard and is being heavily pumped. The aquifer and fine‐grained sediments of the aquitard may be compacted resulting in land subsidence as a result of high exploitation. Local sinkholes have developed in the area of Tazerbo since the start of the pumping from the wellfield in 2004. These sinkholes have been caused mainly by lowering of the piezometric heads due to the withdrawal of groundwater. In this study, a hydrogeological investigation is presented about the effect of large groundwater pumping from the Nubian sandstone aquifer in Tazerbo Wellfield, SE Libya, based on physical parameters for 108 production wells and 23 observation wells. 相似文献
4.
Tadesse Dejenie Steven A.J. Declerck Tsehaye AsmelashSarah Risch Joachim Mergeay Tom De BieLuc De Meester 《Limnologica》2012,42(2):137-143
A field survey of zooplankton communities was carried out in 32 recently established tropical semi-arid reservoirs in the highlands of Northern Ethiopia with the aim to identify to what extent environmental factors determine species composition of the cladoceran community in such isolated and young reservoirs. To address seasonal variation, the survey was carried out both at the beginning and the end of the dry season. A total of 15 species of cladocerans were identified. Daphnia was the most abundant cladoceran genus, and was present in all reservoirs. Using presence-absence data, no association between cladoceran community composition and geographic distance was found. RDA results indicate that the set of environmental variables that explained cladoceran community composition differed among seasons. Depth, altitude and fish biomass showed a significant association with cladoceran community composition during the wet season, whereas variation in cladoceran community structure was associated with phytoplankton biomass in the dry season. The relative abundance of Daphnia was much higher in the pelagic than in the littoral zone of our study systems. Two key groups of pelagic filter-feeding cladocerans, Diaphanosoma and Daphnia, showed a clear pattern, in which one or the other tended to strongly dominate the community. In addition, we observed a negative association between dominance of Daphnia in the zooplankton community and dominance of cyanobacteria in the phytoplankton community. 相似文献
5.
Abraha Gebrekidan Hannah Nicolai Liese Vincken Mekone Teferi Tsehaye Asmelash Tadesse Dejenie Samuel Zerabruk Kindeya Gebrehiwet Hans Bauer Josef Deckers Patricia Luis Luc De Meester Bart van der Bruggen 《洁净——土壤、空气、水》2013,41(3):235-243
This study aims to examine the efficiency of Opuntia ficus‐indica for removing organochlorine pesticides from surface waters. Adsorption properties such as size, dose, and time of O. ficus‐indica for aldrin, dieldrin, and dichlorodiphenyltrichloroethane (DDT) were studied through stirring and column methods. Because of their high affinity and swelling characteristics, dried O. ficus‐indica was studied in stirring while fresh unpeeled O. ficus‐indica was applied in both stirring and column experiments and proved to be well‐suited to column application. Before removing pesticides, the column was flashed with distilled water eliminate the turbidity and smell from fresh unpeeled cactus. The removal of pesticides increased with an increasing adsorbent dose and decreased with adsorbent particle sizes. The optimum adsorbent dose is 10 g for dried and 15 g for fresh unpeeled O. ficus‐indica. The experimental results show that O. ficus‐indica possesses strong adsorption ability for aldrin, dieldrin, and DDT, and the adsorption isotherm data obeyed the Freundlich model. The results of our small‐scale experiments suggest a strong potential to develop local small‐scale water treatment units that can be used at the level of individual households or local communities, using a widely available adsorbent. 相似文献
6.
Neoproterozoic rocks constitute the Kenticha, Alghe and Bulbul litho-tectonic domains in the Negele area of southern Ethiopia. Structural features and fabrics in these rocks were developed during north-south folding (D1), thrusting (D2) and shearing (D3) deformation. From micro-structural inferences and fabric relationships in semi-pelitic schists/gneisses of the Kenticha and Alghe domains, three episodes of metamorphic mineral growths (M1, M2 and M3) are inferred to have accompanied the deformational events. Pressure-Temperature estimates on equilibrium garnet-plagioclase-biotite and garnet-biotite assemblages from semi-pelitic schists/gneisses of the two domains indicate metamorphic recrystallization at temperatures of 520–580°C and 590–640°C, and pressures of 4–5 kb and 6–7 kb in the Kenticha and Alghe domains, respectively. These results correspond to regional metamorphism at a depth of 16–20 km for the Kenticha and 22–25 km for the Alghe domains. The P-T results suggest that the protoliths to the rocks of the Kenticha and Alghe domains were subjected to metamorphism at different crustal depths. This implies exhumation of the Alghe gneissic rocks from intermediate crustal level (ca. 25 km) before juxtaposition with the Kenticha sequence along a north-south trending thrust at the present crustal level during the Neoproterozoic. The combined deformation, fabric and mineral growth data suggest that rocks in the Kenticha and Alghe domains evolved under similar tectono-metamorphic conditions, which resulted from crustal thickening and uplift followed by extension and orogenic collapse, exhumation and cooling before litho-tectonic domains coalesced and cratonized in the Neoproterozoic southern Ethiopian segment of the East African Orogen. 相似文献
7.
The N–S trending Tuludimtu Belt in the extreme west of Ethiopia has been subdivided into five lithotectonic domains, from east to west, the Didesa, Kemashi, Dengi, Sirkole and Daka domains. The Kemashi, Dengi and Sirkole Domains, forming the core of the belt, contain volcano-sedimentary successions, whilst the Didesa and Daka Domains are gneiss terranes, interpreted to represent the eastern and western forelands of the Tuludimtu Belt. The Kemashi Domain, which consists of an ophiolitic sequence of ultramafic and mafic volcanic and plutonic rocks together with sedimentary rocks of oceanic affinity, is interpreted as oceanic crust and is considered to represent an arc-continent suture zone. The Dengi Domain, composed of mafic to felsic volcanic and plutonic rocks, and a sequence of volcanoclastic, volcanogenic, and carbonate sediments, is interpreted as a volcanic arc. The Sirkole Domain consists of alternating gneiss and volcano-sedimentary sequences, interpreted as an imbricated basement-cover thrust-nappe complex. All the domains are intruded by syn- and post-kinematic Neoproterozoic granitoids. Structural analysis within the Didesa and Daka Domains indicate the presence of pre-Pan African structures, upon which Neoproterozoic deformation has been superimposed. The gneissic rocks of these two domains are regarded as pre-Pan African continental fragments amalgamated to West Gondwana during Neoproterozoic collision events. Unconformably overlying all of the above are a series of tilted but internally undeformed conglomerate–sandstone–shale sequences, regarded as post-accretionary molasse-type deposits, formed during gravitational collapse of the Tuludimtu Belt. The Tuludimtu Belt is interpreted as a collision orogenic belt formed during the assembly of West Gondwana prior to final closure of the Mozambique Ocean. 相似文献
8.
Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been done sporadically documenting a change in zooplankton composition including the disappearance of cladocerans, hypothesizing the stocked planktivorous fish could be a cause. Nevertheless, no detailed research was conducted to identify potential effects of fish stocking predominantly due to its remote location. The article presents data about zooplankton composition, abundance and biomass done between October 2007 and January 2009 on short-time intervals including the underlying limnological variables. The zooplankton community was depauperate comprising two copepods, three cladocerans, and six rotifers taxa, as typical for tropical lakes. Total mean standing biomass of all crustacean zooplankton was 237 mg dry mass m−3, which gave Lake Hayq an intermediate position when compared with other tropical lakes. Of copepods, Thermocyclops ethiopiensis was almost an exclusive species, and its temporal variation was influenced by food supply and water temperature. We refute the hypothesis that Tilapia was the cause for the seasonal disappearance of cladocerans, and attribute it to the adverse effect of episodic mixing. Nevertheless, the planktivorous fish probably plays a key role in structuring the cladocerans in particular the large-sized Daphnia magna. In January 2008, we observed a massive planktivorous fish mortality that triggered high algal biomass, which was later grazed by large-sized D. magna demonstrating the trophic cascade hypothesis in a natural ecosystem. 相似文献
9.
Solomon Tadesse 《Gondwana Research》2001,4(1):97
Trace element distribution patterns are reported for whole rocks (granites, aplites, greisens, pegmatites, alaskites) and minerals from the Kenticha pegmatite field. The data shed light on the evolution, regional and local zonal pattern of the granitepegmatites and associated mineralization in the Kenticha belt. The complex mineralization of commercial concentrations of Ta, Nb, Hf, Zr, REE, U and Th is related to Be, Li, Cs, Rbbearing zones of pegmatites and is structurally controlled. Whole rock chemical signatures of the suite of felsic rocks of the Kenticha belt are predominantly similar to those generated by subduction in modern magmatic arcs and indicate a mantle derivation. Columbotantalite concentrates extracted from the pegmatitic ores represent the basic raw materials from which a number of possible byproducts can be recovered. 相似文献
10.
Treelines are widely studied worldwide in relation to climate changes because they are hypothesized to be sensitive climate proxies. However, forest treeline expansion toward higher altitudes may be influenced both by a warming climate and by other factors, such as surface morphology and, in the European Alps, the decline in alpine farming. Our results from five valleys in the inner and peripheral regions of the Alps show that present-day treeline altitudes mostly depend on anthropogenic and orographicgeomorphologic factors. Climatic treelines are limited to steep and inaccessible slopes, and occur at higher altitudes and farther from mountain peaks in the inner regions than in the peripheral regions of the mountain range. Looking for sites in which to study treeline responses to climate change, we recommend investigating the inner regions of the Alps where treelines are farther from human disturbances and from geomorphologic constraints, potentially resulting in freer upward shifts under warmer temperature conditions. We also found that, in the valleys selected, human disturbance is mainly concentrated about 165 m below non-climatic treelines, suggesting a homogeneous influence on treelines, regardless of geographic position. 相似文献