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排序方式: 共有598条查询结果,搜索用时 546 毫秒
331.
Queenie H. S. Chan Aiko Nakato Yoko Kebukawa Michael E. Zolensky Tomoki Nakamura Jessica A. Maisano Matthew W. Colbert James E. Martinez A. L. David Kilcoyne Hiroki Suga Yoshio Takahashi Yasuo Takeichi Kazuhiko Mase Ian P. Wright 《Meteoritics & planetary science》2019,54(1):104-125
We present in this study the effects of short‐term heating on organics in the Tagish Lake meteorite and how the difference in the heating conditions can modify the organic matter (OM) in a way that complicates the interpretation of a parent body's heating extent with common cosmothermometers. The kinetics of short‐term heating and its influence on the organic structure are not well understood, and any study of OM is further complicated by the complex alteration processes of the thermally metamorphosed carbonaceous chondrites—potential analogues of the target asteroid Ryugu of the Hayabusa2 mission—which had experienced posthydration, short‐duration local heating. In an attempt to understand the effects of short‐term heating on chondritic OM, we investigated the change in the OM contents of the experimentally heated Tagish Lake meteorite samples using Raman spectroscopy, scanning transmission X‐ray microscopy utilizing X‐ray absorption near edge structure spectroscopy, and ultraperformance liquid chromatography fluorescence detection and quadrupole time of flight hybrid mass spectrometry. Our experiment suggests that graphitization of OM did not take place despite the samples being heated to 900 °C for 96 h, as the OM maturity trend was influenced by the heating conditions, kinetics, and the nature of the OM precursor, such as the presence of abundant oxygenated moieties. Although both the intensity of the 1s?σ* exciton cannot be used to accurately interpret the peak metamorphic temperature of the experimentally heated Tagish Lake sample, the Raman graphite band widths of the heated products significantly differ from that of chondritic OM modified by long‐term internal heating. 相似文献
332.
Fumiaki Nakata Kazuhiro Shimasaku Mamoru Doi Nobunari Kashikawa Wataru Kawasaki Yutaka Komiyama Sadanori Okamura Maki Sekiguchi Masafumi Yagi Naoki Yasuda 《Monthly notices of the Royal Astronomical Society》1999,309(4):L25-L29
We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e. no number density change or morphological transition), and examine whether these models are consistent with observed number counts in the B , I and K bands, and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time-scale τ SF and the time t gw when the galactic wind starts to blow, in addition to several other conventional parameters. We find that the single-burst model ( τ SF =0.1 Gyr and t gw =0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with the redshift distributions of early-type galaxies in the field environment, owing to overpredictions of the number of galaxies at z ≳1.4 even with strong extinction which is at work until t gw . In order for dust extinction to be more effective, we treat τ SF and t gw as free parameters, and find that models with τ SF ≳0.5 Gyr and t gw >1.0 Gyr can be made consistent with both the observed redshift distributions and the number counts, if we introduce strong extinction [ E ( B − V )≥1 as a peak value]. These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model. 相似文献
333.
Simon F. Portegies Zwart Koji Takahashi 《Celestial Mechanics and Dynamical Astronomy》1999,73(1-4):179-186
Recent N-body simulations have shown that there is a serious discrepancy between the results of N-body simulations and the
results of Fokker-Planck simulations for the evolution of globular and rich open clusters under the influence of the galactic
tidal field. In some cases, the lifetime obtained from Fokker-Planck calculations is more than an order of magnitude smaller
than those from N-body simulations. In this paper we show that the principal cause for this discrepancy is an over-simplified
treatment of the tidal field used in previous Fokker-Planck simulations. We performed new Fokker-Planck calculations using
a more appropriate implementation for the boundary condition of the tidal field. The implementation is only possible with
anisotropic Fokker-Planck models, while all previous Fokker-Planck calculations rely on the assumption of isotropy. Our new
Fokker-Planck results agree well with N-body results. Comparison of the two types of simulations gives a better understanding
of the evolution of such clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
334.
Wuyi Wang Shigeho Sueno Eiichi Takahashi Hisayoshi Yurimoto Tibor Gasparik 《Contributions to Mineralogy and Petrology》2000,139(6):720-733
Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined
using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast
to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements
(REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the
lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations
were observed with their major element compositions. Analyses of randomly selected two to three points within every grain
of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity,
and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the
trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian
diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets
become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in
high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism
by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by
carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature
are the main reasons for the large variations of REE patterns and other trace element concentrations.
Received: 27 April 1999 / Accepted: 1 March 2000 相似文献
335.
Akihiko Morimoto Shoichiro Kojima Sen Jan Daisuke Takahashi 《Estuarine, Coastal and Shelf Science》2009
We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s−1. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf. 相似文献
336.
Tetsuya Takahashi Hideaki Nakata Keiji Hirano Kazumi Matsuoka Mitsunori Iwataki Hitoshi Yamaguchi Tomoyuki Kasuya 《Journal of Oceanography》2009,65(1):113-120
Wind-induced circulation and the distribution of hypoxia corresponding to the upwelling of oxygen-depleted water (called “Aoshio”
in Japan and “Sumishio” locally in Omura Bay) in Omura Bay, Japan, was examined field observations and by three-dimensional
modeling. During the calm weather in summer, well-mixed strait water, in rich oxygen at the mouth of the bay intruded into
the middle layer of the bay, southward and northward along the west and east coast, respectively, forming basin-scale cyclonic
circulation. A stagnant water mass was formed below the center of this cyclonic circulation, and it became hypoxic water.
As a result of the prevailing strong southeast (northeast) wind, the bottom hypoxia moved in a southeasterly (northeasterly)
direction. This induces the upwelling of hypoxic water, accompanied by mass mortality of marine organisms. 相似文献
337.
Kiyoshi Tanaka Yutaka Michida Teruhisa Komatsu Kenji Ishigami 《Journal of Oceanography》2009,65(2):165-177
We have investigated the spreading of river water in Suruga Bay by performing numerical experiments and conducting field surveys
with drifting buoys. There are clear seasonal variations in the large river discharges into the bay: increased discharge in
the rainy summer season and decreased discharge in the dry winter season. The numerical model reproduces the main feature
that has been observed in the actual sea: the river water extends gradually from the northwestern to the southeastern regions
in the bay, especially in summer. The river water spreading is greatly influenced by the bottom topography of the bay: the
Fuji River water spreads over a deep continental slope as a surface-advected plume and extends well offshore, since a large
bulge (anticyclonic eddy at the river mouth) extends well offshore and effectively transports the river water offshore. On
the other hand, the Oi River water tends to flow parallel to isobaths (along a coastline) on a shallow continental shelf as
a bottomadvected plume. Moreover, the influences of seasonal variations in the stratification and a bay-scale, wind-driven
circulation are also investigated. Trajectories of the drifting buoys, which were released around the Fuji River mouth, certainly
suggest that the bulge exists there. 相似文献
338.
Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk 总被引:1,自引:1,他引:0
Akihiro Hachikubo Alexey Krylov Hirotoshi Sakagami Hirotsugu Minami Yutaka Nunokawa Hitoshi Shoji Tatiana Matveeva Young K. Jin Anatoly Obzhirov 《Geo-Marine Letters》2010,30(3-4):313-319
Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites. 相似文献
339.
The possible influences of the Emperor Seamounts (ESs) upon the subarctic gyre of the North Pacific (NPSAG) were investigated
by a series of numerical experiments. In the experiments, a two-layer ocean with a meridional mountain ridge was forced by
seasonally varying wind stress. We focused on how the return ratio, the ratio of the boundary transport along the eastern
side of the ridge to the interior Sverdrup transport, changes with ridge height, width and density stratification. It was
found that the return ratio can be large if the ridge width is greater than the width of the viscous boundary layer. In this
case, the bottom pressure torque determines the return ratio; the return ratio is almost proportional to the ridge height
when the ridge height is small and some contours of planetary potential vorticity pass over the ridge. However, the return
ratio is independent of the ridge height and decreases with the stratification when the ridge height is large and all the
contours of planetary potential vorticity are closed. These dependences of the return ratio were understood in terms of barotropic
and baroclinic components of the bottom pressure torque. Implications for the bathymetric influences of ESs on the actual
NPSAG are also discussed. 相似文献
340.
The dynamics of the wind-driven circulations and surface transport processes in Suruga Bay have been examined by performing
numerical experiments. While strong winds exist outside the bay, the winds inside the bays are greatly reduced, which generates
a strong wind stress curl in winter and autumn. In particular, in winter, a strong positive curl region is located across
the bay mouth, and a strong surface circulation with counterclockwise rotation is generated beneath it. The circulation is
nearly geostrophic, but is not affected by the bottom topography in the deep bay. It is suggested that intense surface water
exchange through the bay mouth occurs in winter, whereas it is not active in the other seasons when no significant vorticity
is supplied on the bay mouth from the atmosphere. Moreover, we propose a hypothesis that the atmospheric wind stress curl
will cause the frequent appearance of the counterclockwise circulation in winter in the real ocean. 相似文献