排序方式： 共有125条查询结果，搜索用时 0 毫秒
121.
Khaled A. Al-Wosabi Abdulwahab S. Alaug Ali A. A. Khudeir 《Arabian Journal of Geosciences》2014,7(7):2933-2940
The Pan-African (Neoproterozoic) low-grade ophiolitic fragment occurring to the south of Ataq City, Shabwah Province, southeastern central part of Yemen is positioned tectonically between the underlying Pre-Pan-African syntectonic granite infrastructure and the overlying Mesozoic–Cenozoic sedimentary successions. It is incomplete and differentiated in the field into (a) NW plunging nappes, namely a lower metagabbro nappe, and (b) the upper metavolcanic nappe. The sedimentary successions separated from each other by eastward dipping normal faults. These successions can be subdivided into three main rock units: Amran, Tawilah, and Hadramawt groups. The Amran Group is represented in the study area by Shuqra and Madbi formations. The Shuqra Formation consists mainly of highly fossiliferous carbonate facies yielding several terebratulids and rhynchonellids. It belongs to the Toracian–Oxfordian (or probably extend to Early Kimmeridgian) age. The Madbi Formation consists of sand–marl intercalations of Kimmeridgian–Early Tithonian age. The Tawilah Group is mainly composed of variegated unfossiliferous continental sandstones with few siltstone intercalations, and on the basis of its stratigraphic position, it is dated as Cretaceous (probably Early Cretaceous). The Hadramawt Group in the study area is represented by Umm er Radhuma Formation, which is widely distributed in the Arabian Gulf countries. 相似文献
122.
Contribution of shallow groundwater rapid fluctuation to soil salinization under arid and semiarid climate 总被引：4，自引：0，他引：4
Mohamed Khaled Ibrahimi Tsuyoshi Miyazaki Taku Nishimura Hiromi Imoto 《Arabian Journal of Geosciences》2014,7(9):3901-3911
Rising saline shallow groundwater and associated soil salinization problems are widespread especially in arid and semiarid areas. There have been numerous studies on groundwater-associated salinity, but more information is required on the effects of groundwater frequent and high fluctuations on soil salinization. In the present study, laboratory experiments and numerical simulations using HYDRUS-1D model were carried out for this purpose. The experimental and modeling results showed that groundwater fluctuation caused not only the accumulation of more salt in the soil profile compared to stable groundwater, but also an enhancement of the mechanism. Water table fluctuation induced a much greater spreading of the bromide (Br) tracer within the column than the constant water table. The Br content was on average five orders of magnitude greater under a fluctuating water table than under a constant one. Further, the numerical simulations showed that an increase in the groundwater fluctuation frequency brought about an increase in soil surface salinization under the same evaporation boundary conditions. Additional simulations with HYDRUS-1D were used to study the effects of various management strategies on soil salinization induced by shallow groundwater. Hence, by reducing the evaporation rate through the application of surface mulching, a significant reduction of salt concentration at the soil surface was observed. Moreover, frequent irrigations with small quantities were effective to reduce soil surface salt accumulation induced by saline shallow groundwater. 相似文献
123.
The western part of Yemen is largely covered by Tertiary volcanics and is bounded by volcanic margins to the west (Red Sea) and the south (Gulf of Aden). The Oligo–Miocene evolution of Yemen results from the interaction between the emplacement of the Afar plume, the opening of the Red Sea, and the westward propagation of the Gulf of Aden. Structural and microtectonic analyses of fault slip data collected in the field reveal that the volcanic margins of Yemen are affected by three main extensional tectonic events. The chronological order of these events is as follows: first E–W extension was associated with the emplacement of volcanic traps of Yemen, then NE–SW extension was related to the Red Sea rifting, and finally, the volcanic margin was submitted to N160°E extension, perpendicular to the overall trend of the Gulf of Aden, which we interpret as induced by the westward propagation of the oceanic ridge of the Gulf of Aden. 相似文献
124.
Temporal species distributions of planktonic protist communities in semi-enclosed mariculture waters and responses to environmental stress 总被引：1，自引：0，他引：1
In order to evaluate the relationships between temporal species succession of planktonic protists and physical-chemical parameters in semi-enclosed mariculture waters, species distributions in response to environmental stress were investigated in a shrimp-farming pond near Qingdao, China during a complete shrimp-culture cycle (May to October 2002). A clear temporal succession in species distribution was found over the complete farming cycle. For example, before the introduction of the shrimp larvae there was low-variability of species distribution in the protist communities whereas during the stages immediately after, there was higher variability in species composition. Multiple linear/logistic regression analyses demonstrate that 12 protist taxa (e.g., Gyrodinium spirale, Teleaulax acuta, Prorocentrum spp. and Mesodinium pupula) were related to the nutrients, in particular ammonia and phosphates, alone or in combination with water temperature. These results suggest that temporal variations in species distribution of planktonic protist communities might be used in assessing water quality of semi-enclosed mariculture waters. 相似文献
125.
In order to clarify the phylogeny and relationships of the most confused hypotrichous ciliates, Holosticha-complex, four closely related holostichids (five populations), Holosticha bradburyae, H. diademata, Anteholosticha sp., and A. manca, were compared and analyzed using ITS2 secondary structures, ITS1-5.8S-ITS2 region and SSrRNA gene sequences. The ITS1-5.8S-ITS2 region sequences of these four species were first sequenced, and they shared sequence identities ranging from 68.0% to 90.1%, while two populations of Anteholosticha sp. differed in three nucleotides (sequence identity 99.8%). There were several minor differences among ITS2 secondary structures of these species, while two populations of Anteholosticha sp. had the identical secondary structure. Phylogenetic trees inferred from the ITS1-5.8S-ITS2 region sequences of stichotrichs using multiple algorithms (Neighbor-Joining, Maximum Parsimony and Bayesian) revealed similar topologies. The results show that:(1) Holosticha bradburyae and H. diademata firmly clustered together with strong bootstrap supports, forming a sister clade with Anteholosticha sp., (2) Anteholosticha appeared to be a paraphyletic assemblage, in which the morphotype A. manca was more closely related to Diaxonella trimarginata than to its congener Anteholosticha sp. Phylogenetic analyses based on the SSrRNA gene and the combined sequences of SSrRNA gene and ITS1-5.8S-ITS2 region revealed the similar relationships between Holosticha and Anteholosticha, nevertheless their positions within the subclass Stichotrichia differed from each other inferred from different genes. 相似文献