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11.
Kenji Kashiwaya Masayuki Ryugo Masae Horii Hideo Sakai Toshio Nakamura Takayoshi Kawai 《Journal of Paleolimnology》1999,21(2):143-150
The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around interglacial periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the glacials or 'stadials. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation. 相似文献
12.
Masayuki Umemura Jun Fukue & Shin Mineshige 《Monthly notices of the Royal Astronomical Society》1998,299(4):1123-1131
We examine the physical processes of radiatively driven mass accretion on to galactic nuclei, owing to intensive radiation from circumnuclear starbursts. The radiation from a starburst not only causes the inner gas disc to contract via radition flux force, but also extracts angular momentum owing to relativistic radiation drag, thereby inducing an avalanche of the surface layer of the disc. To analyse such a mechanism, the radiation–hydrodynamical equations are solved, including the effects of the radiation drag force as well as the radiation flux force. As a result, it is found that the mass accretion rate owing to the radiative avalanche is given by M ˙ ( r )= η ( L * / c 2 )( r / R )2 (Δ R / R )(1 − e −τ ) at radius r , where the efficiency η ranges from 0.2 up to 1, L * and R are respectively the bolometric luminosity and the radius of the starburst ring, Δ R is the extent of the emission regions, and τ is the face-on optical depth of the disc. In an optically thick regime, the rate depends upon neither the optical depth nor the surface mass density distribution of the disc. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100-pc scales down to ∼ parsec scales, and it may eventually be linked to advection-dominated viscous accretion on to a massive black hole. The radiation–hydrodynamical and self-gravitational instabilities of the disc are briefly discussed. In particular, the radiative acceleration possibly builds up a dusty wall, which 'shades' the nucleus in edge-on views. This provides another version of the model for the formation of an obscuring torus. 相似文献
13.
Masayuki Kikuchi 《Physics of the Earth and Planetary Interiors》1981,25(2):159-162
The propagation of elastic waves in a medium containing many inclusions is considered. Under the assumption that the spatial distribution of inclusions is uniform, a general equation is derived for the determination of the velocity dispersion and attenuation coefficient of the effective waves. A simple example is presented where scatterers are infinitesimally thin cracks. The calculated results show that the attenuation coefficient Q?1 takes a peak value for the wavelength nearly equal to twice the crack length. 相似文献
14.
Masayuki Kikuchi 《Physics of the Earth and Planetary Interiors》1979,19(3):255-262
Following Burg's algorithm, we develop a new mathematical method of estimating the impulse response of a linear system. Since no data-invariant time window is used in this method, the estimated impulse response gives us reliable information on both the amplitude and the phase spectra of the system.The method is applied to laboratory data to investigate the dynamical response of rock specimens. The experimental results show that the waveform distortion with travel distance may well be described by a standard viscoelastic solid model. However, they also show that there is an additional factor of attenuation which is independent of the frequency and therefore not accompanied by any velocity dispersion. 相似文献
15.
Shiho Kobayashi Eisuke Hashimoto Masayuki Nagao Yoshio Takasugi 《Journal of Oceanography》2009,65(5):657-664
We observed tidal currents, turbulent energy dissipation and water column stratification at the entrance of a narrow strait
(Neko Seto) in the Seto Inland Sea, Japan, using a free-falling turbulent microstructure profiler (TurboMAP) and acoustic
Doppler current profiler (ADCP). The variation in turbulent energy dissipation at the entrance of the strait was not at quarter-diurnal
frequency but at semi-diurnal frequency; turbulent energy dissipation was enhanced during the ebb tide, although it was moderate
during the flood tide. This result is consistent with the results of Takasugi (1993), which showed the asymmetry of tidal
energy loss during a semidiurnal tidal cycle using control volume analysis. It is suggested that significant turbulent energy
dissipation is generated in the strait, which influences the properties of water outside the strait when tidal currents flow
out from the strait. 相似文献
16.
17.
Ayaka Sakabe Ken Hamotani Yoshiko Kosugi Masahito Ueyama Kenshi Takahashi Akito Kanazawa Masayuki Itoh 《Theoretical and Applied Climatology》2012,109(1-2):39-49
Very few studies have conducted long-term observations of methane (CH4) flux over forest canopies. In this study, we continuously measured CH4 fluxes over an evergreen coniferous (Japanese cypress) forest canopy throughout 1?year, using a micrometeorological relaxed eddy accumulation (REA) system with tuneable diode laser spectroscopy (TDLS) detection. The Japanese cypress forest, which is a common forest type in warm-temperate Asian monsoon regions with a wet summer, switched seasonally between a sink and source of CH4 probably because of competition by methanogens and methanotrophs, which are both influenced by soil conditions (e.g., soil temperature and soil moisture). At hourly to daily timescales, the CH4 fluxes were sensitive to rainfall, probably because CH4 emission increased and/or absorption decreased during and after rainfall. The observed canopy-scale fluxes showed complex behaviours beyond those expected from previous plot-scale measurements and the CH4 fluxes changed from sink to source and vice versa. 相似文献
18.
Climate change, land use change, and China’s food security in the twenty-first century: an integrated perspective 总被引:5,自引:0,他引:5
Food security in China, the world’s most populous country, has long been a concern because of the challenges of population growth, water shortages, and loss of cropland through urbanization, soil degradation, and climate change. Here, we present an integrated analysis of China’s food demand and supply under IPCC Special Report on Emissions Scenarios A1, A2, B1, and B2 in 2020, 2050, and 2080, based on official statistics and future development scenarios. Our analysis accounts for future socioeconomic, technological, and resource developments, as well the impact of climate change. We present a covariant relationship between changes in cereal productivity due to climate change and the cereal harvest area required to satisfy China’s food demand. We also estimated the effects of changing harvested areas on the productivity required to satisfy the food demand; of productivity changes due to climate change on the harvest area required to satisfy food demand; and of productivity and land use changes on the population at risk of undernutrition. China could be able to feed herself without disturbing the global food market in the twenty-first century, but whether the government will choose self-sufficiency or increased food imports may depend on the cost of change, which remains unknown. 相似文献
19.
Hidenobu Yajima Masayuki Umemura Masao Mori Taishi Nakamoto 《Monthly notices of the Royal Astronomical Society》2009,398(2):715-721
In order to assess the contribution of Lyman break galaxies (LBGs) and Lyman α emitters (LAEs) at redshifts 3 < z < 7 to the ionization of intergalactic medium (IGM), we investigate the escape fractions of ionizing photons from supernova-dominated primordial galaxies by solving the three-dimensional (3D) radiative transfer. The model galaxy is employed from an ultra-high-resolution chemodynamic simulation of a primordial galaxy by Mori & Umemura, which well reproduces the observed properties of LAEs and LBGs. The total mass of model galaxy is 1011 M⊙ . We solve not only photoionization but also collisional ionization by shocks. In addition, according to the chemical enrichment, we incorporate the effect of dust extinction, taking the size distributions of dust into account. As a result, we find that dust extinction reduces the escape fractions by a factor of 1.5–8.5 in the LAE phase and by a factor of 2.5–11 in the LBG phase, while the collisional ionization by shocks increases the escape fractions by a factor of ≈2 . The resultant escape fractions are 0.07–0.47 in the LAE phase and 0.06–0.17 in the LBG phase. These results are well concordant with the recent estimations derived from the flux density ratio at 1500 to 900 Å of LAEs and LBGs. Combining the resultant escape fractions with the luminosity functions of LAEs and LBGs, we find that high- z LAEs and LBGs can ionize the IGM at z = 3–5 . However, ionizing radiation from LAEs as well as LBGs falls short of ionizing the IGM at z > 6 . That implies that additional ionization sources may be required at z > 6 . 相似文献
20.
Kana NAGASHIMA Masayuki NARA Jun‐ichi MATSUDA 《Meteoritics & planetary science》2012,47(11):1728-1737
Abstract– We performed micro‐Raman spectroscopic analyses of the carbon vein in five ureilites: Allan Hills (ALH) A77257, Northwest Africa (NWA) 3140, Shi?r 007, Yamato 790981 (Y‐790981), and Yamato 791538 (Y‐791538). The graphite peaks showed that the graphite structure in ureilite is well developed, especially compared with the carbonaceous material in carbonaceous chondrite. The domain sizes of the graphite were 45–110 Å. We observed shifts in the diamond peak positions to higher wave numbers with a large full width at half maximum (FWHM), especially for NWA 3140. Although the FWHM of a diamond peak is not a crucial diagnostic test for a chemical vapor deposition (CVD) origin of diamond, the shift of the diamond peaks to higher wave numbers could be a strong indicator that supports the CVD origin as these shifts have only been observed in CVD diamonds. We discuss the origin of diamond from various aspects, and confirm that the CVD model is the most plausible. We conclude that all carbon material (graphite, amorphous carbon, diamond, etc.) condensed on the early condensates in the primitive solar nebula. 相似文献