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排序方式: 共有170条查询结果,搜索用时 171 毫秒
31.
Luca Bindi Ekaterina A. Sirotkina Andrey V. Bobrov Fabrizio Nestola Tetsuo Irifune 《Physics and Chemistry of Minerals》2016,43(2):103-110
The crystal structure and chemical composition of a crystal of (Mg14?x Cr x )(Si5?x Cr x )O24 (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å3 and Z = 2. The structure was refined to R 1 = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments. 相似文献
32.
Wave normal directions of VLF signals propagating through the ionosphere can be determined by measuring Doppler frequency shift of the signals by means of rocket borne receivers. Two rockets were launched to detect the NWC signal of 22.3 kHz which was transmitted from Australia and propagated on two completely different paths, one being propagated through the Earth-ionosphere waveguide and up to the rocket, the other propagated down to the rocket by the whistler mode directly from the source in the opposite hemisphere. The wave normal directions of the latter mode were almost vertically downward in the ionosphere in the northern hemisphere, although substantial error was involved in the determination of the wave normal direction for a part of the upgoing flight of the rockets, due to the relative geometry of the directions of the rocket flight and the geomagnetic field. The effect of the horizontal gradients of the ionosphere on the above results were found to be not significant. From the experimental results it is concluded that field aligned ducts stretching down to the rocket altitudes did not exist, at least, during the rocket flights. 相似文献
33.
Tetsuo Yanagi Suhendar I Sachoemar Toshiyuki Takao Shunji Fujiwara 《Journal of Oceanography》2001,57(4):461-470
Intensive hydrographic observations were carried out in the western part of the Gulf of Thailand and the east coastal sea
of Peninsular Malaysia in September 1995 and April–May 1996. The characteristics of seasonal variation of oceanic condition
in that area are discussed basis of an analysis of observed water temperature, salinity and density distributions in these
cruises and NAGA cruises (Yanagi and Takao, 1998a). Stratification is most developed in March–May mainly due to large sea
surface heating and weak sea surface wind, which weakened until September–October, vanishing in December–January. The horizontal
distribution of bottom cold, saline and heavy water masses, which are found during the stratified season, is governed by the
tidal mixing and the water depth. Water exchange between the Gulf of Thailand and the South China Sea becomes large in March–May
due to a coupled effect of the intensified estuarine circulation and the Ekman transport by the southwest monsoon.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
34.
Two new types of mechanism for the generation of tidal residual flow are revealed with the use of a hydraulic model experiment. A remarkable anticlockwise tidal residual circulation is formed in a model bay due to the presence of a tidal current, the Coriolis force and a horizontal boundary. A similar circulation is also formed due to the presence of a bottom slope, a horizontal boundary and a tidal current which flows normal to the inclination of the bottom slope. The residual circulation in the Sea of Iyo in the Seto Inland Sea is considered to be due to a combination of the effects of the Coriolis force, a bottom slope, a horizontal boundary and the tidal current. We classified some of the generation mechanisms of tidal residual flow which have been described to date into seven types on the basis of vorticity considerations. 相似文献
35.
Baroclinic eddies south of Cheju Island in the East China Sea 总被引:3,自引:0,他引:3
The detailed structures of baroclinic eddies, that is, an anticlockwise circulation in the upper layer and a clockwise one in the lower layer south of Cheju Island in the East China Sea have been revealed by intensive field observation. Such baroclinic eddies are accompanied by the southward spreading of the Yellow Sea Bottom Cold Water. 相似文献
36.
A numerical experiment on the sedimentation processes in the Yellow Sea and the East China Sea 总被引:6,自引:0,他引:6
Sedimentation processes of suspended matter supplied from the Huanghe (Yellow River) and the Changjiang are investigated with the use of a three-dimensional numerical model of the Yellow Sea and the East China Sea which includes the tidal current, residual flow and wind waves. Suspended matter supplied from the Huanghe mainly deposits in the Bohai Sea and that from the old river mouth of the Huanghe and from the Changjiang in the central part of the Yellow Sea or south of Cheju Island. The calculated results well reproduce the observed sedimentation pattern qualitatively except for the offshore area of the southeastern coast of China. 相似文献
37.
The generation, variation, and disappearance of the thermohaline front at the mouth of Tokyo Bay were investigated using water
temperature data obtained by a commercial ferry boat in 1987. The thermohaline front was generated south of Kan-non-zaki in
mid November 1987 after the beginning of sea-surface cooling. It moved northward to Kan-non-zaki until the end of the year.
The thermohaline front was most intense in February and March and disappeared in late March 1987 when sea-surface warming
began. 相似文献
38.
39.
This study aims to investigate the spatiotemporal trends in snow depth (SD) and snow cover extent (SCE) for Arctic lands, excluding Greenland, for the period 1948–2006. The investigation not only delineates how the Arctic regions are manifesting significant annual trends in both SD and SCE, but also provides a comprehensive understanding of their historical context. To achieve these objectives, the combined resources of the hydrological and biogeochemical model (CHANGE), National Oceanic and Atmospheric Administration (NOAA) weekly SCE data, and in situ observations of SD were used. Most regions in the Arctic exhibited a significant negative trend in SD over the 59 years of study. The magnitude of the negative trend was stronger in North America than in Eurasia, where the decrease became more significant, starting from the late 1980s, coinciding well with the temperature rise during that time. During the same period, the warming temperature caused a prominent decrease in deeper SDs (i.e., >35 cm), so that the corresponding SCEs exhibited negative anomalies, with the greatest declines being observed at SDs > 55 cm. In contrast, the SCEs for SD ≤ 35 cm showed increased anomalies during the most recent two decades. The increased anomalies signify a sequential result induced by the decrease in the SCEs with deeper SDs, rather than the expansion of snow to snow-free regions. These changes resulted in a northward shift of the shallow SD line, which took place to a highly significant degree in North America. These results suggest that the Arctic SCE and SD will undergo more intense changes in response to the future climate warming. 相似文献
40.