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排序方式: 共有103条查询结果,搜索用时 15 毫秒
91.
Manon Fourrire Hctor Reyes‐Bonilla Cristian Moiss Galvn‐Villa Arturo Ayala Bocos Fabin Alejandro Rodríguez‐Zaragoza 《Marine Ecology》2019,40(3)
Revillagigedo Archipelago and Clipperton atoll are oceanic islands located in the eastern tropical Pacific. The composition and trophic structure of their fish assemblages have never been characterized in detail, and the aim of this study was therefore to analyse the community structure and assess the similarity of fish assemblages between these two areas. Underwater visual fish censuses were conducted using band transects at two depth levels in three islands of the Revillagigedo Archipelago (Socorro, San Benedicto, and Roca Partida) and at Clipperton atoll. We calculated ecological indices (density, richness, Shannon diversity, Pielou evenness, and taxonomic distinctiveness) for each transect. These were compared in order to determine quantitative differences among the islands and bathymetric levels. Qualitative analyses (ANOSIM, nMDS, and SIMPER) were also performed. Finally, we analysed the trophic structure of the assemblages and the functional diversity of the four studied sites. The results showed significant differences for all indices, except diversity (H′). Roca Partida was the site with greatest abundance and richness due to the presence of pelagic species in large numbers. No significant differences were found in functional diversity, suggesting a high degree of ecological redundancy. Lower‐level carnivores with mean trophic level values of between 3.3 and 3.8 dominated both areas. The ordination analysis did not show a clear difference in fish composition among the islands, although Clipperton atoll has a particular fauna characterized by endemic species (Thalassoma robertsoni and Stegastes baldwini). In conclusion, the fish communities of the studied areas seem quite similar, but their assemblage structure differs. In particular, Clipperton is differentiated by the influence of endemic species. 相似文献
92.
A one-dimensional global climate model is constructed from both the basic energy balance equation for the earth-atmosphere system and the planetary boundary layer equations of motion. The theoretically based model accurately simulates many features of the present climate system and appears useful in evaluating various climatic change mechanisms. The simplicity of this process-response model increases its educational and research applicability for climatologists in geography. 相似文献
93.
F. Barberi F. Innocenti P. Landi U. Rossi M. Saitta R. Santacroce I. M. Villa 《Bulletin of Volcanology》1984,47(1):125-141
Subsurface geothermal exploration has considerably added to our knowledge of the Latera volcanic complex. A syenitic body is located about 2 km below the present-day surface; K-Ar data point a 0.9 Ma age. The primary magma was a silica-saturated trachyte; undersaturated, hauyne-bearing products are found near the carbonatic wall-rocks and have been interpreted as reaction products. Subsurface data from deep drilling and geophysical surveys suggest that the Latera caldera resulted from three main successive collapse phases: (i) formation of an old caldera, now buried, related to the eruption of ignimbrites from the syenitic magma chamber; (ii) sinking of the eastern sector as a consequence of the formation of the nearby Bolsena caldera (0.3 Ma); (iii) multistage formation of the present Latera caldera (0.16 Ma). 相似文献
94.
M. R. Sims R. D. Andresen E. A. Leimann P. Lamb J. Raymont J. Ives S. Kellock G. Boella G. Villa B. Falconi G. Manzo S. Re R. M. Robba 《Astrophysics and Space Science》1985,116(1):61-79
The First Spacelab mission, launched on Space ShuttleFlight STS-9 in November 1983 carried a multidisciplinary payload which was intended to demonstrate that valuable scientific results can be achieved from such short duration missions. The payload complement included a spectrometer to undertake observations of the brighter cosmic X-ray sources. The primary scientific objectives of this experiment were the study of detailed spectral features in cosmic X-ray sources and their associated temporal variations over a wide energy range from about 2 up to 30 keV. The instrument based on the gas scintillation proportional counter had an effective area of some 180 cm2 with an energy resolution of 9% at 7 keV.The instrument parameters and the performance, using data from the flight and ground calibration, are discussed. 相似文献
95.
L. Bassani R. C. Butler A. J. Dean G. Di Cocco F. Perotti G. Villa 《Astrophysics and Space Science》1982,82(1):199-207
Three active galaxies, generally classified as Seyferts, have been discovered recently to be powerful, low energy -ray sources. The similarity between their spectral characteristics and those of the cosmic background at -ray energies suggests that these objects could make a significant contribution to this diffuse flux. This contribution has been assessed using two different number densities of -ray-emitting Seyfert galaxies based on optical and X-ray data. The comparison of the estimated and measured diffuse -ray background intensities is used to gain a deeper understanding of the metabolism of Seyfert galaxies. 相似文献
96.
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98.
The conversion of pyrite to pyrrhotite is identified in this study as a new source of isotopically heavy sulphur in thermal domes. Contact metamorphism of a sequence of pyrite-rich shales by the Penedela blind intrusion (NW Spain) causes the conversion of diagenetic pyrite to pyrrhotite at a temperature of 150 °C. This process occurred in a closed system with total consumption of the diagenetic pyrite and production of H2 S-rich fluids. The H2 S liberated in this unidirectional reaction shows a relatively heavy δ34 S, in agreement with a batch volatilization process. The mixture of this H2 S-rich fluids with derived sulphur from Lower Cambrian carbonates produced gold-bearing pyrite at a temperature of 340 °C. The contribution from these two different sulphur sources was 86% and 14%, respectively. The metamorphic fluids migrated through permeable zones such as fractures and shear zones, producing a lode-gold deposit. 相似文献
99.
The eruptive history of Monte Vulture has been the subject of several geochronological investigations during the past decades,
which reliably dated only a small number of eruptions. Understanding the causes of sub-optimum data yield in the past requires
an interdisciplinary approach. We re-analyzed samples from previous works and present new data on samples from the main volcano-stratigraphic
units of Monte Vulture, so as to provide an improved, consistent chronostratigraphic database. Imaging of minerals by cathodoluminescence
and backscattered electrons reveals that heterochemical, high-temperature deuteric reaction textures are ubiquitous. Such
observations are common in metamorphic rocks but had not frequently been reported from volcanic rocks. In view of the mineralogical
complexity, we base our chronological interpretation on isochemical steps, defined as steps for which the Cl/K and/or the
Ca/K ratios are constant. Isochemical steps carry the isotopic signature of chemically homogeneous mineral phases and therefore
allow a well-constrained age interpretation. Comparison of old and new 39Ar-40Ar data proves the reproducibility of age spectra and their shapes. This quantifies the analytical reliability of the irradiation
and mass-spectrometric analyses. Anomalous age spectra are a reproducible property of some specific samples and correlate
with mineralogical anomalies. The present data allow us to fine-tune the age of the volcanostratigraphic units of Monte Vulture
during the known interval of main volcanic activity from ca. 740 to 610 ka. After a very long stasis, the volcanic activity
in the Monte Vulture area resumed with diatremic eruptions, one of which (Lago Piccolo di Monticchio, the site of a palynological-paleoclimatological
drilling) was dated at ca. 140 ka. 相似文献
100.