New paleomagnetic result from the Ethiopian flood basalts in the Abbay (Blue Nile) and Kessem gorges     

Tesfaye Kidane  Bekele AbebeVincent Courtillot  Emilio Herrero 《Earth and Planetary Science Letters》2002,203(1):353-367

New paleomagnetic investigations on the Ethiopian trap series have been undertaken at the Abbay and Kessem gorges in an attempt to better constrain the 30 Ma paleomagnetic pole of Africa. We sampled six thick massive basaltic lava flows, totaling 230 m, from Abbay Gorge and 10 lava flows, 180 m in thickness, from Kessem Gorge. Detailed paleomagnetic analyses disclosed that the carriers of the characteristic remanent magnetization (ChRM) are different in different lava flows. These are mostly titanomagnetites, titanomaghemites, and magnetite minerals with a broad range of coercive force and blocking temperatures. The heating and cooling susceptibility vs. temperature curves, many of which are irreversible, may indicate chemical remagnetization, notably low temperature maghemitization. Only one flow (KS04) with a clear 580°C Curie temperature was apparently unaffected by chemical remagnetization. The ChRM direction of this flow is identical to that in other flows, which suggests that if and when remagnetization occurred, this was shortly after emplacement of the lava flows. All of the flows sampled have normal polarity. However, a reversed component of low to medium coercive force and low to medium unblocking temperature occurs in flow KS01 at Kessem Gorge. The ChRM directions for the 16 sites are D=3.1°, I=5.8° (α₉₅=12.7°). The paleomagnetic pole obtained from these is at λ=83.0°N, φ=193.3°E (A₉₅=9.0°). Comparison with three previous studies of the traps shows remarkable consistency and a number of means are derived and discussed. Two final preferred poles for the traps are at λ=79.0°N, φ=196.9°E (A₉₅=2.8°) when all 112 published flows are used, and λ=78.7°N, φ=209.4°E (A₉₅=3.4°) when only the 76 flows from the four more recently analyzed sections are included. Both are compatible with the recent reference synthetic pole for Africa of Courtillot and Besse [J. Geophys. Res. (2002) in press]. In that sense, the Ethiopian trap pole is not anomalous and does not require more of a non-dipolar contribution than indicated by analyses of the global paleomagnetic data base covering the last few million years.  相似文献   

OH in the tropical upper troposphere and its relationships to solar radiation and reactive nitrogen     

R. S. Gao  K. H. Rosenlof  D. W. Fahey  P. O. Wennberg  E. J. Hintsa  T. F. Hanisco 《Journal of Atmospheric Chemistry》2014,71(1):55-64

In situ measurements of [OH], [HO₂] (square brackets denote species concentrations), and other chemical species were made in the tropical upper troposphere (TUT). [OH] showed a robust correlation with solar zenith angle. Beyond this dependence, however, [OH] did not correlate to its primary source, the product of [O₃] and [H₂O] ([O₃]?[H₂O]), or its sink [NO_y]. This suggests that [OH] is heavily buffered in the TUT. One important exception to this result is found in regions with very low [O₃], [NO], and [NO_y]. Under these conditions, [OH] is highly suppressed, pointing to the critical role of NO in sustaining OH in the TUT and the possibility of low [OH] over the western Pacific warm pool due to strong marine convections bringing NO-poor air to the TUT. In contrast to [OH], [HO_x] ([OH] + [HO₂]) correlated reasonably well with [O₃]?[H₂O]/[NO_y], suggesting that [O₃]?[H₂O] and [NO_y] are the significant source and sink, respectively, of [HO_x].  相似文献   

Evaluation of Global Gridded Precipitation and Temperature Datasets against Gauged Observations over the Upper Tekeze River Basin,Ethiopia     

Kidane Welde REDA  Xingcai LIU  Qiuhong TANG  Tesfay Gebretsadkan GEBREMICAEL 《Journal of Meteorological Research》2021,35(4):673-689

The availability of satellite and reanalysis climate datasets and their applicability have been greatly promoted in hydro-climatic studies. However, such climatic products are still subject to considerable uncertainties and an evaluation of the products is necessary for applications in specific regions. This study aims to evaluate the reliability of nine gridded precipitation and temperature datasets against ground-based observations in the upper Tekeze River basin(UTB) of Ethiopia from 1982 to 2016. Precipitation, maximum temperature(T_(max)), minimum temperature(T_(min)), and mean temperature(T_(mean)) were evaluated at daily and monthly timescales. The results show that the best estimates of precipitation are from the Eart H2 Observe, WFDEI, and ERA-Interim reanalysis data Merged and Bias-corrected for the Inter-Sectoral Impact Model Intercomparison Project(EWEMBI), and the Climate Hazards Group Infra Red Precipitation with Station data(CHIRPS) datasets. The percentage biases and correlation coefficients(CCs) are within ±15% and 0.5, respectively, for both EWEMBI and CHIRPS at the two timescales. All products underestimate the drought conditions indicated by the standardized precipitation index(SPI), while the EWEMBI and CHIRPS datasets show higher agreement with the observations than other datasets. The Tmean estimates produced by the ECMWF ReAnalysis version 5(ERA5) and the Climate Hazards Group Infra Red Temperature with Station data(CHIRTS) are the closest to the observations, with CCs of 0.65 and 0.55, respectively, at the daily timescale. The CHIRTS and EWEMBI datasets show better representations of Tmax(T_(min)), with CCs of 0.69(0.72) and 0.62(0.68), respectively,at the monthly timescale. The temperature extremes are better captured by the ERA5(T_(mean)), CHIRTS(T_(max)), and EWEMBI(T_(min)) datasets. The findings of this study provide useful information to select the most appropriate dataset for hydrometeorological studies in the UTB and could help to improve the regional representation of global datasets.  相似文献   

Re‐orientation of the extension direction and pure extensional faulting at oblique rift margins: comparison between the Main Ethiopian Rift and laboratory experiments     

Giacomo Corti  Melody Philippon  Federico Sani  Derek Keir  Tesfaye Kidane 《地学学报》2013,25(5):396-404

In this study, we draw on a unique combination of well‐resolved fault‐slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip‐slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep‐seated, pre‐existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift.  相似文献   

New age constraints on counter-clockwise rotation of NE Japan     

Ayumi K. Baba  Takaaki Matsuda  Tetsumaru Itaya  Yutaka Wada  Noriyuki Hori  Masahiko Yokoyama  Nobuaki Eto  Rieko Kamei  Haider Zaman  Tesfaye Kidane  Yo-Ichiro Otofuji 《Geophysical Journal International》2007,171(3):1325-1341

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Palaeomagnetism and K-Ar and ⁴⁰Ar/³⁹Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr     

L. Audin †  X. Quidelleur  E. Coulié  V. Courtillot  S. Gilder  I. Manighetti  P.-Y. Gillot  P. Tapponnier  T. Kidane 《Geophysical Journal International》2004,158(1):327-345

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Age estimation of the deposition of the cretaceous Upper Sandstones and a later remagnetization event,from Hirna area,SE Ethiopia     

Balemwal Atnafu  Tesfaye Kidane 《Physics of the Earth and Planetary Interiors》2012

Outcrops of the Cretaceous Upper sandstone formation some 375 km to the East of Addis Ababa on the motor Highway to Harar was paleomagnetically investigated. About seventy core samples were collected at various stratigraphic levels from 250–300 meters thick sedimentary formation. After standard sample preparations in the laboratory the resulting specimens were subjected to routine paleomagnetic demagnetization protocol. In the first steps of demagnetizations process the recent and viscous magnetizations were removed by heating until a temperature of level of 300 °C. Further demagnetization of the samples resulted in the isolation of the final magnetization with stable line segments that is directed towards the origin, which is interpreted as Characteristic Remanent Magnetization (ChRM). Rock – magnetic experiments have identified goethite (αFeOOH), hematite (αFe₂O₃), detritial hematite, and magnetite as the magnetic mineral phases carrying the remanence. The ChRM identified resulted in an average value of (D_s = 0.5°, I_s = ?0.7°, α₉₅ = 4.3°, N = 34) for the red sandstones while an average value of (D_s = 335.8°, I_s = ?31.8°, α₉₅ = 4.7°, N = 14) for the limestone intercalations. The former ChRM in the red sandstone is determined to be secondary while the latter ChRM is known to be primary. Comparison of these directional results and their pole equivalents with the African plate Apparent Polar Wander Path curve established by Besse and Courtillot (2003) give ages of between 115–130 Million years for the limestone intercalation and ages of 30 million years for red sandstone unit. These are interpreted respectively as estimates of the age of deposition and a later remagnetization respectively.  相似文献   

Patterns of Changes to Woody Vegetation near Resettlement Sites in Semi-arid Northwestern Ethiopia     

Mulubrhan Balehegn  Kidane Hintsa 《资源与生态学报(英文版)》2018,9(3):317-329

Communal rangelands provide diverse ecosystem services to millions of pastoralists and agro-pastoralists. Resettling destitute communities into hitherto uninhabited communal rangelands and forests, a common practice throughout Sub-Saharan Africa, is a threat to the sustainable use of range and forest land resources. In order to understand the effect of resettlement on a semi-arid woodland in northwestern Ethiopia, satellite imagery of 23 resettlement villages taken over a period of fourteen years, and woody vegetation floristic data for three old resettlements, three new resettlements, two refugee camps and one protected area were analyzed using ANOVA and canonical correspondence analysis (CCA). The normalized differential vegetation index (NDVI) and canopy cover around all village types decreased with disturbance gradients, while the magnitude of change varied according to the type of settlement. Limited canopy cover was observed in refugee camps and new resettlements, compared to old resettlements and protected areas. Woody vegetation height class showed a J shaped distribution in all sites except refugee camps (RC), indicating a decline in vegetation. CCA showed that variables like site type, altitude and disturbance gradient significantly affected the diversity of woody species at the different sites. Comparison of individual species responses to disturbances indicated that low fodder value invaders like Dichrostachys cinerea, and many Acacia species were increasing in proportion and coverage at the expense of some multipurpose species including Tamarindus indica, Diospyros mespiliformis, and Pterocarpus lucens. In the absence of regulated vegetation use, resettlements result in a decline in overall vegetation cover and a shift in floristic diversity in favor of invasive species.  相似文献   

埃塞俄比亚西北部半干旱地区居民安置点附近木本植被变化规律（英文）     

Mulubrhan Balehegn  Kidane Hintsa 《资源与生态学报(英文版)》2018,(3)

公共牧场为数百万牧民和农牧民提供多样化的生态系统服务。将受贫穷困扰的社区居民重新安置到迄今无人居住的社区牧场和森林,是整个非洲撒哈拉以南地区的一种普遍做法,但这对牧场和林地资源的可持续利用构成了威胁。为了解居民的迁移行为对埃塞俄比亚西北部半干旱林地的影响,采用方差分析和典范对应分析(CCA)方法,对埃塞俄比亚西北部半干旱林地的23个安置村14年的卫星影像和3个旧移民点、3个新移民点、2个难民营和1个保护区的木本植物区系数据进行了分析。各类型居民点的归一化植被指数(NDVI)和冠层盖度均随干扰梯度的减小而减小,但变化幅度则因聚落类型而异。与旧的安置区和保护区相比,难民营和新安置区的树冠覆盖有限。除难民营外,所有地点的木本植被高度等级均呈J形分布,表明植被呈下降趋势。CCA结果表明,居民点类型、海拔高度和干扰梯度等变量对不同居民地的木本植物多样性有显著影响。个别物种对干扰的反应比较表明,低饲料价值的入侵物种,如色穗花(Dichrostachys cinerea),许多金合欢属植物种的比例和盖度都在增加,而以牺牲一些多用途的物种为代价,包括罗望子(Tamarindus indica)、非洲黑檀(Diospyros mespiliformis)和光亮紫檀(Pterocarpus lucens)。在没有植被使用管控的情况下,居民的重新定居安置会导致总体植被盖度下降,植物区系多样性发生变化,将为物种的入侵提供机会。  相似文献   

What can we learn from the projections of changes of flow patterns? Results from Polish case studies   总被引：1，自引：1，他引：0  

Mikołaj Piniewski  Hadush Kidane Meresa  Renata Romanowicz  Marzena Osuch  Mateusz Szcześniak  Ignacy Kardel  Tomasz Okruszko  Abdelkader Mezghani  Zbigniew W. Kundzewicz 《Acta Geophysica》2017,65(4):809-827

River flow projections for two future time horizons and RCP 8.5 scenario, generated by two projects (CHASE-PL and CHIHE) in the Polish-Norwegian Research Programme, were compared. The projects employed different hydrological models over different spatial domains. The semi-distributed, process-based, SWAT model was used in the CHASE-PL project for the entire Vistula and Odra basins area, whilst the lumped, conceptual, HBV model was used in the CHIHE project for eight Polish catchments, for which the comparison study was made. Climate projections in both studies originated from the common EURO-CORDEX dataset, but they were different, e.g. due to different bias correction approaches. Increases in mean annual and seasonal flows were projected in both studies, yet the magnitudes of changes were largely different, in particular for the lowland catchments in the far future. The HBV-based increases were significantly higher in the latter case than the SWAT-based increases in all seasons except winter. Uncertainty in projections is high and creates a problem for practitioners.  相似文献