Increased Stratification and Decreased Lower Trophic Level Productivity in the Oyashio Region of the North Pacific: A 30-Year Retrospective Study     

Sanae Chiba  Tsuneo Ono  Kazuaki Tadokoro  Takashi Midorikawa  Toshiro Saino 《Journal of Oceanography》2004,60(1):149-162

An analysis of the time series data sets collected from the 1960s to 1990s in the Oyashio Water revealed signs of alteration in the physical, chemical and biological properties of the water column in the western subarctic North Pacific. Wintertime salinity, phosphate concentration and apparent oxygen utilization (AOU) in the subsurface increased linearly over the 30 years. At the same time, salinity and phosphate in the surface mixed layer decreased. An increase in the density gradient in the surface and subsurface suggested that the water column stratification intensified, reducing the vertical exchange of water properties during the period. The Net Community Production (NCP), estimated from the phosphate consumption from February through August, also declined. Water column Chl a was approximately halved and diatoms decreased by one order of magnitude in spring, consistent with the multi-decadal decreasing trend of NCP. Zooplankton biomass was also nearly halved during the same period. In contrast, wintertime Chl a increased by 63% and diatom abundance doubled. Developmental timing became earlier in Neocalanus flemingeri, and spring occurrence of N. plumchrus increased after the 1980s. Reduced vertical water exchange might have limited nutrient supply to the level, decreasing winter-summer NCP for these three decades. It is speculated that, in the meantime, the earlier stabilization of the surface layer might have enhanced wintertime diatom production in the Oyashio's light-limited environment. This condition could allow zooplankton to effectively utilize diatoms from earlier timing, resulting in the apparent early developmental timing and abundance increase. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Boron isotopic composition of fumarolic condensates from some volcanoes in Japanese island arcs     

Masao Nomura  Tadao Kanzaki  Takejiro Ozawa  Makoto Okamoto  Hidetake Kakihana 《Geochimica et cosmochimica acta》1982,46(11):2403-2406

Boron samples from 40 fumarolic condensates from volcanoes in the Ryukyu arc (Satsuma Iwo-jima and Shiratori Iwo-yama) and the North-east Japan arc (Usu-shinzan, Showa-shinzan, Esan and Issaikyo-yama) all have ${}^{11}\text{B}{}^{10}\text{B}$ ratios close to 4.07. Higher values, from 4.09 to 4.13, were only observed in condensates from volcanoes in the southernmost end of the North-east Japan arc (Nasu-dake), the northern part of the Izu-Bonin arc (Hakone), and the North Mariana arc (Ogasawara Iwo-jima). These higher values suggest geological interaction of the magmas with sea-water enriched in ¹¹B.  相似文献   

Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2     

Vincent Renard  Kazuaki Nakamura  Jacques Angelier  Jacques Azema  Jacques Bourgois  Christine Deplus  Kantaro Fujioka  Yozo Hamano  Philippe Huchon  Hajimu Kinoshita  Pierre Labaume  Yujiro Ogawa  Tetsuzo Seno  Akira Takeuchi  Manabu Tanahashi  Akinori Uchiyama  Jean-Louis Vigneresse 《Earth and Planetary Science Letters》1987,83(1-4)

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.  相似文献   

Phononspectroscopy and lattice dynamical calculations of anhydrite and gypsum     

Kazuaki Iishi 《Physics and Chemistry of Minerals》1979,4(4):341-359

Detailed experimental and theoretical studies of the k=0 vibrational spectra of anhydrite and gypsum are reported. The dielectric constants and the infrared reflection and Raman spectra of the single crystal have been measured. The frequencies of the phonon spectrum, the contribution of potential terms to potential energy and detailed mode assignments have been determined based on a polarizable-ion model. The relative experimental intensity of the spectra, the observed crystal field effects, the rotatory lattice modes of the H₂O molecule and the mixed character of translatory and rotatory modes of the Ca, SO₄, and H₂O groups are discussed based on the theoretical vibrational modes and the potential energy distribution. The principal moments of inertia of the water molecule and the Lennard-Jones potential constants for nonbonded oxygen-oxygen interactions are presented.  相似文献   

Upper limit of heaving pressure derived by pore-water pressure measurements of partially frozen soil     

Tsutomu Takashi  Takahiro Ohrai  Hideo Yamamoto  Jun Okamoto 《Engineering Geology》1981,18(1-4):245-257

Experiments were conducted to estimate heaving pressures of saturated soil partially frozen in a closed system. Temperatures at both ends of a specimen were kept constant, i.e., positive at the top and negative at the bottom. When the overburden pressure P was maintained at a constant value, the pore-water pressure P_w, which showed a certain value before freezing, decreased gradually as freezing progressed, finally attaining a specific value, whereafter the specimen ceased taking water into it. The pressure difference between P and P_w, at this stage was defined as the upper limit of heaving pressure σ_u, which evidently depended on the temperature θ_c of the cooling end, in accordance with the relation: σ_u = −11.4 θ_c (kg/cm²)

It corresponds to the modified Clausius-Clapeyron's formula, which gives the freezingpoint depression of an ice—water system, where the pressure acting on the ice differs from that on the water. This is the same as the value obtained by Radd and Oertle (1973). It is considered, however, that, when θ_c lowers, the value of θ_u reaches finally a constant value smaller than the one obtained by the above equation. Denoted by σ_{u max}, it was defined as a maximum heaving pressure. The value of σ_{u max} depended on soil type.  相似文献   

A complex earth slide–earth flow induction by the heavy rainfall in July 2006, Okaya City, Nagano Prefecture, Japan     

Yasuhiko Okada  Hirotaka Ochiai  Takashi Okamoto  Kyoji Sassa  Hiroshi Fukuoka  Ogbonnaya Igwe 《Landslides》2007,4(2):197-203

A seasonal rain front (Baiu front) accompanied a long-term accumulation of precipitation propagated over the wide areas of the main island of Japan during 15–24 July 2006. In Okaya City, Nagano Prefecture, several flow-type landslides occurred in the early morning of 19 July 2006, claiming eight lives. Among these landslides, a most peculiar complex earth slide–earth flow occurred on a north gentle slope of the upstream portion of the Motosawagawa River. In the source area, volcanoclastic soils overlying tuffaceous rocks at about 4-m depth slid due to the prolonged precipitation that raised the water table level in the soil. Along with the travel path, the failed materials fluidized causing the liquefaction of the volcanoclastic soils underlain by volcanic black ash soils. The resulting flow spread over a wide area up to the final deposition. Constant volume box-shear tests on undisturbed volcanoclastic soil specimens taken from the source area showed effective normal stress tended to decrease during shearing. The ring shear tests on saturated disturbed specimens produced the large loss of shear resistance, which may explain the fluidized motion of the complex landslide.  相似文献   

Water transportation from the subducting slab into the mantle transition zone   总被引：4，自引：9，他引：4  

Shigenori Maruyama  Kazuaki Okamoto 《Gondwana Research》2007,11(1-2):148

Using a recently developed petrogenetic grid for MORB + H₂O, we propose a new model for the transportation of water from the subducting slab into the mantle transition zone. Depending on the geothermal gradient, two contrasting water-transportation mechanisms operate at depth in a subduction zone. If the geothermal gradient is low, lawsonite carries H₂O into mantle depths of 300 km; with further subduction down to the mantle transition depth (approximately 400 km) lawsonite is no longer stable and thereafter H₂O is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection. At this depth, hydrous β-phase olivine is stable and plays a role as a huge water reservoir. In contrast, if the geothermal gradient is high, the subducted slab may melt at 700–900 °C at depths shallower than 80 km to form felsic melt, into which water is dissolved. In this case, H₂O cannot be transported into the mantle below 80 km. Between these two end-member mechanisms, two intermediate types are present. In the high-pressure intermediate type, the hydrous phase A plays an important role to carry water into the mantle transition zone. Water liberated by the lawsonite-consuming continuous reaction moves upward to form hydrous phase A in the hanging wall, which transports water into deeper mantle. This is due to a unique character of the reaction, because Phase A can become stable through the hydration reaction of olivine. In the case of low-pressure intermediate type, the presence of a dry mantle wedge below 100 km acts as a barrier to prevent H₂O from entering into deeper mantle.  相似文献   

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source     

Takeshi Nakamura  Hiroshi Takenaka  Taro Okamoto  Yoshiyuki Kaneda 《Pure and Applied Geophysics》2014,171(7):1153-1167

We use the finite difference method to simulate seismic wavefields at broadband land and seafloor stations for a given terrestrial landslide source, where the seafloor stations are located at water depths of 1,900–4,300 m. Our simulation results for the landslide source explain observations well at the seafloor stations for a frequency range of 0.05–0.1 Hz. Assuming the epicenter to be located in the vicinity of a large submarine slump, we also model wavefields at the stations for a submarine landslide source. We detect propagation of the Airy phase with an apparent velocity of 0.7 km/s in association with the seawater layer and an accretionary prism for the vertical component of waveforms at the seafloor stations. This later phase is not detected when the structural model does not consider seawater. For the model incorporating the seawater, the amplitude of the vertical component at seafloor stations can be up to four times that for the model that excludes seawater; we attribute this to the effects of the seawater layer on the wavefields. We also find that the amplification of the waveform depends not only on the presence of the seawater layer but also on the thickness of the accretionary prism, indicating low amplitudes at the land stations and at seafloor stations located near the trough but high amplitudes at other stations, particularly those located above the thick prism off the trough. Ignoring these characteristic structures in the oceanic area and simply calculating the wavefields using the same structural model used for land areas would result in erroneous estimates of the size of the submarine landslide and the mechanisms underlying its generation. Our results highlight the importance of adopting a structural model that incorporates the 3D accretionary prism and seawater layer into the simulation in order to precisely evaluate seismic wavefields in seafloor areas.  相似文献   

Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt,central Shikoku,Japan     

Seiya Fukumura  Kazuaki Okamoto  Masaru Terabayashi 《Island Arc》2019,28(2)

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Do magnetized winds self-collimate?     

Isao Okamoto 《Monthly notices of the Royal Astronomical Society》1999,307(2):253-278

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