Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions   总被引：1，自引：0，他引：1  

SCAILLET  BRUNO; MACDONALD  RAY 《Journal of Petrology》2006,47(7):1413-1437

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan  相似文献   

A simple temperature method for the estimation of evapotranspiration          下载免费PDF全文

Temesgen Enku  Assefa M. Melesse 《水文研究》2014,28(6):2945-2960

Accurate estimation of evapotranspiration (ET) is essential in water resources management and hydrological practices. Estimation of ET in areas, where adequate meteorological data are not available, is one of the challenges faced by water resource managers. Hence, a simplified approach, which is less data intensive, is crucial. The FAO‐56 Penman–Monteith (FAO‐56 PM) is a sole global standard method, but it requires numerous weather data for the estimation of reference ET. A new simple temperature method is developed, which uses only maximum temperature data to estimate ET. Ten class I weather stations data were collected from the National Meteorological Agency of Ethiopia. This method was compared with the global standard PM method, the observed Piche evaporimeter data, and the well‐known Hargreaves (HAR) temperature method. The coefficient of determination (R²) of the new method was as high as 0.74, 0.75, and 0.91, when compared with that of PM reference evapotranspiration (ET_o), Piche evaporimeter data, and HAR methods, respectively. The annual average R² over the ten stations when compared with PM, Piche, and HAR methods were 0.65, 0.67, and 0.84, respectively. The Nash–Sutcliff efficiency of the new method compared with that of PM was as high as 0.67. The method was able to estimate daily ET with an average root mean square error and an average absolute mean error of 0.59 and 0.47 mm, respectively, from the PM ET_o method. The method was also tested in dry and wet seasons and found to perform well in both seasons. The average R² of the new method with the HAR method was 0.82 and 0.84 in dry and wet seasons, respectively. During validation, the average R² and Nash–Sutcliff values when compared with Piche evaporation were 0.67 and 0.51, respectively. The method could be used for the estimation of daily ET_o where there are insufficient data. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Time of collection and cutting sizes affect vegetative propagation of Boswellia papyrifera (Del.) Hochst through leafless branch cuttings     

G. Haile  K. GebrehiwotM. Lemenih  F. Bongers 《Journal of Arid Environments》2011,75(9):873-877

Boswellia papyrifera (Del) Hochst is a key dry land plant species in the Horn of Africa with high socio-economic significance in Ethiopia, Eritrea and Sudan. In recent decades, populations of this species are declining due to lack of recruitment through natural regeneration. This study investigated the potential of vegetative propagation using leafless branch cuttings as an alternative means of reproduction for the species. The effect of time of collection and cutting sizes (length and thickness) on shooting and rooting of the cuttings was investigated. The effects of these three factors were analysed, two-by-two, in three separate factorial experiments each arranged in a randomized complete block design. Time of collection affected cutting performance: shooting percent, shoot number, cutting vigour, leaf number, leaf length, rooting percent and root length were best when cuttings were collected in February-March, and weakest in May. Medium to long cuttings (1.0 m-2.0 m) and thicker cuttings (0.19-0.27 m) performed better than shorter or thinner cuttings. These results indicate that B. papyrifera is amenable to macro-propagation via leafless branch cuttings. Reproduction of B. papyrifera through macro-propagation may help overcome the population recruitment bottleneck of this important species and will help the long-term yield possibilities of its precious product frankincense.  相似文献   

基于植被净初级生产力和水分利用效率的埃塞俄比亚土地退化趋势及驱动因素分析     

付石林  雷加强  周仪琪  周娜  Hare Malicha Loje  Jorro Zinabu Bora 《中国沙漠》2023,43(1):128-141

土地退化对减缓和适应气候变化有重要影响,并威胁到全世界的可持续发展,造成一系列社会、经济和生态问题,是目前全球面临的最大环境挑战之一。基于2001—2020年埃塞俄比亚的归一化植被指数（NDVI）、气象数据及土地覆盖数据,通过CASA模型计算获得植被净初级生产力（NPP）和水分利用效率（WUE）,并使用Sen+MK趋势分析方法得到土地退化及其恢复趋势,同时采用多元逐步回归方法分析了土地退化及恢复的驱动因素。结果表明：2001—2020年,埃塞俄比亚土地退化整体呈现恢复趋势,恢复区域占全国面积的34.51%,主要分布在埃塞俄比亚西部以及索马里州;退化区域仅占全国面积的1.63%,主要分布在首都亚的斯亚贝巴。土地退化及恢复的主要驱动因素是人为与气候共同因素和人为单因素。土地退化的主要因素为人口的快速增长以及城市的扩张,土地恢复则与20年来实施的森林景观恢复以及可持续土地管理措施和政策有关。  相似文献   

Analysis of the hydrological response of a tropical terminal lake,Lake Abiyata (Main Ethiopian Rift Valley) to changes in climate and human activities     

Dagnachew Legesse  Christine Vallet‐Coulomb  Franoise Gasse 《水文研究》2004,18(3):487-504

Integrated dynamic water and chloride balance models with a catchment‐scale hydrological model (PRMS) are used to investigate the response of a terminal tropical lake, Lake Abiyata, to climate variability and water use practices in its catchment. The hydrological model is used to investigate the response of the catchment to different climate and land‐use change scenarios that are incorporated into the lake model. Lake depth–area–volume relationships were established from lake bathymetries. Missing data in the time series were filled using statistical regression techniques. Based on mean monthly data, the lake water balance model produced a good agreement between the simulated and observed levels of Lake Abiyata for the period 1968–83. From 1984 onwards the simulated lake level is overestimated with respect to the observed one, while the chloride concentration is largely underestimated. This discrepancy is attributed to human use of water from the influent rivers or directly from the lake. The simulated lake level and chloride concentration are in better agreement with observed values (r² = 0·96) when human water use for irrigation and salt exploitation are included in the model. A comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level. Sensitivity analysis based on a mean climatic year showed that, after prolonged mean climatic conditions, Lake Abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions. This study shows the significant sensitivity of the level and salinity of the terminal Lake Abiyata to small changes in climate or land use, making it a very good ‘recorder’ of environmental changes that may occur in the catchment at different time scales. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Reconstructing pre-agricultural expansion vegetation cover of Ethiopia     

《Applied geography (Sevenoaks, England)》2015

Landscape reconstructions can be used to define a reference condition from which to assess the magnitude of land changes caused by human influence. Since the beginning of the last century, the population of Ethiopia has increased drastically with large effects on the natural vegetation and biodiversity. However, the original land cover patterns in Ethiopia have not been precisely mapped, which hinder the identification of the biophysical and socio-economic factors that contributed to the current landscape patterns. The objective of this study was to reconstruct the past century vegetation landscapes of Ethiopia (i.e. vegetation cover before agricultural expansion) and identify which ecosystems have been most affected by land changes. First, the net primary productivity (NPP) was modelled based on the climatic constraints of natural vegetation growth (water availability, solar radiation and minimum temperature) derived from remote sensing and climate data. This analysis showed that water availability is the most critical constraint for vegetation growth for all regions and land cover types in Ethiopia. Then, the past vegetation was mapped based on predicted NPP. Our results show that i) the extent of broadleaved evergreen or semi-deciduous forest, open broadleaved deciduous forest, closed to open shrubland, mosaic forest-shrubland/grassland, sparse vegetation and grassland was 18.8%, 12.4%, 20.6%, 31.5%, and 16.8%, respectively, and ii) current agricultural landscapes were previously covered mainly by broadleaved evergreen and deciduous forest, which encompassed 38.9%. The least affected by agricultural expansion were sparse vegetation and grassland. Our study provides novel insights on pre-agricultural expansion landscapes in Ethiopia with critical information for scientists and other stakeholders working on the restoration and rehabilitation of degraded areas.  相似文献   

Crustal velocity structure across the Main Ethiopian Rift: results from two-dimensional wide-angle seismic modelling   总被引：1，自引：0，他引：1  

G. D. Mackenzie  H. Thybo  P. K. H. Maguire 《Geophysical Journal International》2005,162(3):994-1006

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Wetland sustainability and the evolution of indigenous knowledge in Ethiopia     

ALAN B DIXON 《The Geographical journal》2005,171(4):306-323

Much research in recent years has addressed the contribution of indigenous knowledge (IK) to development initiatives in developing countries. An IK system that continuously evolves and adapts in response to environmental and socio-economic change is often considered to be at the core of sustainable natural resource management practices and rural livelihoods. In the context of indigenous wetland management in western Ethiopia, this paper examines the relationship between IK and wetland sustainability, focusing on the mechanisms through which IK evolves and how local adaptive capacity is built up. A series of participatory research activities undertaken in four wetland communities revealed spatial variations in the degree of innovation and communication taking place. The paper argues that these mechanisms are key factors influencing adaptive capacity, suggesting a key link between wetland sustainability and the occurrence of innovation and communication among communities.  相似文献   

Specific sediment yield in Tigray-Northern Ethiopia: Assessment and semi-quantitative modelling   总被引：2，自引：0，他引：2  

Nigussie Haregeweyn  Jean Poesen  Jan Nyssen  Gert Verstraeten  Joris de Vente  Gerard Govers  Seppe Deckers  Jan Moeyersons 《Geomorphology》2005,69(1-4):315-331

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Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen   总被引：1，自引：0，他引：1  

Ingrid Ukstins Peate  Joel A. Baker  Mohamed Al-Kadasi  Abdulkarim Al-Subbary  Kim B. Knight  Peter Riisager  Matthew F. Thirlwall  David W. Peate  Paul R. Renne  Martin A. Menzies 《Bulletin of Volcanology》2005,68(2):135-156

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km³. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×10⁷ km² and estimated maximum total volumes of up to 5,000 km³, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km³) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at  相似文献