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71.
Hidekazu Yoshida Koshi Yamamoto Yuki Murakami Nagayoshi Katsuta Toru Hayashi Takeshi Naganuma 《Environmental Geology》2008,55(6):1363-1374
Takashikozo is a phenomenon of Quaternary sediments in Japan. They are cylindrical Fe-oxyhydroxide nodules that form as plaques round
plant roots, where Fe is preferentially concentrated to develop a solid wall. Structural features suggest that after the roots
have decayed, the central space where the roots were situated acts as a flow path for oxidized water. Analysis of microbial
16S rDNA extracted from the nodules identified iron-oxidizing bacteria encrusted round the roots where they are the likely
initiators of nodule formation. Direct microscopic observation revealed an accumulation of Fe-oxyhydroxides that fill the
pore spaces and is also likely to be linked with the encrusting microbial colonies. Geological history and nanofossil evidence
suggest that these Fe-nodules may have been buried at a depth of up to several tens of meters for at least 105 years in reducing Quaternary sediments. Thus Fe-oxyhydroxide nodules that have formed in a geological environment at the
interfaces between water and rock by microbial mediation can persist under reducing conditions. If this is the case, the phenomenon
is significant as an analogue of post-closure conditions in radioactive waste repositories, since it could influence nuclide
migration. 相似文献
72.
73.
The two-dimensional problem of wave transformation by, and motions of, moored floating objects is solved numerically as a boundary value problem by direct use of Green's identity formula for a potential function. The cross-sectional shape of the floating object, the bottom configuration and the mooring arrangements may be all arbitrary. For a given incident wave, the three modes of body motion, the wave system and mooring forces are all solved at the same time. A laboratory experiment is conducted to verify the theory. Generally good agreements between the theory and experiments are obtained as long as the viscous damping due to flow separation is small. A numerical experiment indicates that a conventional sluck mooring is to worsen the wave attenuation by a floating breakwater and that a properly arranged elastic mooring can considerably improve the wave attenuation by a floating breakwater. 相似文献
74.
The Algal Growth Potential (AGP) of water samples collected off Gamagori in Mikawa Bay was measured from May 1978 through February 1979, and the limiting nutrient was determined using regression analysis and enrichment bioassays. The surface and bottom water samples had AGP that produced increments of chemical oxygen demand (COD) of 2.1 mg l–1 and 3.1 mg l–1, respectively, on average. These values ofCOD correspond to 46% and 97% of the average COD values of the raw water samples at the surface and bottom, respectively. Seasonal changes of AGP showed a close correlation with those of dissolved inorganic nitrogen (DIN) concentration. Enrichment bioassays showed that DIN was the most deficient nutrient. The DIN:phosphate-phosphorus (PO4
3–-P) ratios and DIN: dissolved phosphorus (DP) ratios in the water samples were below the cellular N:P ratios of the natural algal populations. These results suggest that AGP was mainly limited by DIN concentration. 相似文献
75.
Osamu Shida Tomonori Hamatsu Akira Nishimura Akifumi Suzaki Jun Yamamoto Kazushi Miyashita Yasunori Sakurai 《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2822
The Japanese Pacific walleye pollock (Theragra chalcogramma) stock is the largest stock of this species in Japanese waters. It is a key component of the Oyashio ecosystem. In southern Hokkaido waters, these fish spawn mainly during January and February near the mouth of Funka Bay (FB), and most eggs and larvae are transported into FB. During midsummer juvenile pollock migrate along the southern coast of Hokkaido to a nursery ground on the continental shelf off eastern Hokkaido (Doto area). However, some eggs and larvae are transported southward to the Tohoku region (TR). Transport depends largely on the Oyashio, which generally flows southward along the eastern coasts of Hokkaido and Tohoku. Thus, this stock has two different recruitment routes: FB–Doto and FB–TR. In the 1980s, when the southward flow of the Oyashio was strong, the number of age-2 pollock estimated from a virtual population analysis (VPA) indicated that recruitment to the entire stock remained at a medium level. In the 1990s, when the Oyashio weakened, strong year-classes occurred in 1991, 1994, and 1995, but not in the latter half of the 1990s. Juvenile catches in the TR by commercial fisheries, which can be taken as indices of recruitment level via FB–TR, were high during the 1980s and decreased in the 1990s. Although there was no significant difference in the average number of recruits between the 1980s and the 1990s as estimated from a VPA, the recruitment patterns differed between the two decades. Here, we propose that recruitment routes of this stock shifted in response to environmental changes. 相似文献
76.
Tokuo Yamamoto 《Geo-Marine Letters》1982,2(3-4):205-208
The physical processes of instability and soil mass transport in a soft clay bed by waves were quantitatively reproduced and
measured in a laboratory soil-wave tank. Soft clays behave like plasto-elastic materials and dissipate wave energy quickly.
Waves destroy clay fabrics gradually and soften clay. Waves induce mass transport of clay in the clay bed. The clay mass transport
increases with wave energy. The Froude-Mach similitude is applied to the experimental data to hindcast the soil mass transport
caused by Hurricane Camille in 1969. 相似文献
77.
78.
Jutarat Kittiwanich Tamiji Yamamoto Toshiya Hashimoto Keiko Tsuji Osamu Kawaguchi 《Journal of Oceanography》2006,62(4):493-509
A numerical ecosystem model expressing both phosphorus and nitrogen cyclings in the pelagic system of Hiroshima Bay, Japan,
was developed in order to investigate the seasonal variations of these elements and their annual budgets. Based on the geophysical
and chemical structures of the bay, Hiroshima Bay was divided into northern and southern boxes, themselves divided into two
layers of an upper 5 m and a lower layer according to the stratification of the water column. The model consists of equations
representing all relevant physical and biological processes. The results revealed that the internal regeneration of materials
was an important source of bioavailable nutrients for phytoplankton growth in the water column. The incorporation of phytoplankton
aggregation improved the accuracy of the outputs in comparison to the observed data, especially during the stratified summer
season. The results also indicated that phosphorus limitation of phytoplankton growth occurs in the upper layer during summer
while light limitation occurs in the lower layer. In addition, physical processes such as diffusion and advection were also
important as they ensured that most of regenerated nutrients in the lower layer were transported to the upper layer. Thus,
these processes might support the high primary production and the production of oysters that are extensively cultured in this
bay. Considering the informative results obtained, the model used in this study provides a sound basis and tool to describe
the dynamics of phosphorus and nitrogen cyclings in Hiroshima Bay. 相似文献
79.
The major elements and the minor elements cadmium, chromium and vanadium in 12 samples of shallow-water deposits collected
in Tokyo Bay were studied. Average silica content of the deposits is 53.93%, which is approximate to the average of 54.15%
of red clay. Iron (av. 5.67% Fe2O3), titanium (av. 0.71% TiO2), especially manganese (av. 0.87% MnO), are more abundant in the deposits than in the deposits along the entire sea-coasts
of Honshu and of Nanao Bay, Japan. The deposits show higher contents of cadmium (av. 0.61 ppm), the chromium (av. 86 ppm)
and vanadium (av. 79ppm). About 3 to 17 times cadmium and manganese contents compared with those in the deposits along the
sea-coasts of Honshu and of Nanao Bay, Japan was found. Such accumulation, of cadmium and manganese in the deposits is probably
caused by artificial contamination rather than of natural origin. No relation was found between the contents of cadmium, chromium
and vanadium and those of major elements. 相似文献
80.
A theory is proposed to evaluate the loosening earth pressure (vertical earth pressure after excavation) acting on a shallow tunnel in unsaturated ground with an arbitrary groundwater level. The theory is developed based on the limit equilibrium theory, combining soil–water characteristic curves, Mohr–Coulomb failure criteria, and effective stress for unsaturated soils. The proposed theory is applied to predict the vertical distribution of loosening earth pressure in unsaturated ground, which shows a significant difference from that in saturated ground. In unsaturated ground, suction contributes to the increase in effective loosening earth pressure and shear resistance. The remarkable effects of groundwater depth, soil type, and scale of overburden height and trapdoor width on loosening earth pressure are also revealed. Based on the soil–water characteristic curve, the degree of saturation decreases, which causes wet density to decrease and the total and effective loosening earth pressures to have contrary tendencies. Moreover, effective loosening earth pressures vary with soil type as the degree of saturation varies. The total loosening earth pressures are, however, very similar regardless of soil type, because wet density and shear resistance have similar tendencies. The proposed theory provides a valid model for loosening earth pressure in unsaturated ground that will be useful for shallow tunnel excavations. 相似文献