Lower crustal seismic velocity-anomalies; magmatic underplating or serpentinized peridotite? Evidence from the Vøring Margin,NE Atlantic     

Mjelde  Rolf  Kasahara  Junzo  Shimamura  Hideki  Kamimura  Aya  Kanazawa  Toshihiko  Kodaira  Shuichi  Raum  Thomas  Shiobara  Hajime 《Marine Geophysical Researches》2002,23(2):169-183

On the Vøring volcanic passive margin offshore mid-Norway, NE Atlantic, a lower crustal body with P-wave velocities in the range of 7.1–7.7 km/s has been mapped by twenty two-dimensional Ocean Bottom Seismograph (OBS) profiles. The main aim of the present paper is to evaluate to what extent the lower crust is consistent with magmatic intrusions or serpentinized peridotite. The relatively low V _p/V _s ratios of 1.75–1.78 modelled for the lower crust under the continental part of the Vøring Plateau are consistent with mafic intrusions mixed with blocks of stretched continental crust, but not with the presence of partially serpentinized peridotites. The lower crustal high-velocity body is restricted to the area of the Late Cretaceous/Early Tertiary rift that lead to continental break-up in Early Eocene. The same model can explain the observations in the northern Vøring Basin, but in the central and southern Vøring Basin the seismic velocities do not preclude a model involving serpentinized peridotite in addition to intrusions and continental remnants. On the west Iberia non-volcanic margin a similar layer is interpreted as serpentinized peridotite. The existence of Moho reflections, the observation of S-wave anisotropy but absence of P-wave anisotropy, uncertainties regarding supply of water to allow for significant serpentinization and very low stretching factors compared with the west Iberia Margin, are among factors that argue against the presence of serpentinized peridotite in the Vøring Basin.  相似文献   

Evaluation of spatial correlation between nutrient exchange rates and benthic biota in a reef-flat ecosystem by GIS-assisted flow-tracking     

Toshihiro Miyajima  Yoshiyuki Tanaka  Isao Koike  Hiroya Yamano  Hajime Kayanne 《Journal of Oceanography》2007,63(4):643-659

A Geographic Information System (GIS)-aided flow-tracking technique was adopted to investigate nutrient exchange rates between specific benthic communities and overlying seawater in a fringing reef of Ishigaki Island, subtropical Northwestern Pacific. Net exchange rates of NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, PO₄ ³⁻, Total-N and Total-P were estimated from concentration changes along the drogue trajectories, each of which was tracked by the Global Positioning System and plotted on a benthic map to determine the types of benthic habitat over which the drogue had passed. The observed nutrient exchange rates were compared between 5 typical benthic zones (branched-coral (B) and Heliopora communities (H), seaweed-reefrock zone (W), bare sand area (S), and seagrass meadow (G)). The dependence of nutrient exchange rates on nutrient concentrations, physical conditions and benthic characteristics was analyzed by multiple regression analysis with the aid of GIS. The spatial correlation between nutrient exchange rates and benthic characteristics was confirmed, especially for NO₃ ⁻ and PO₄ ³⁻, which were usually absorbed in hydrographically upstream zones B and W and regenerated in downstream zones H and G. NO₃ ⁻ uptake in zones B and W was concentration-dependent, and the uptake rate coefficient was estimated to be 0.58 and 0.67 m h⁻¹, respectively. Both nutrient uptake in zone W and regeneration in zone H were enhanced in summer. The net regeneration ratio of NO₃ ⁻/PO₄ ³⁻ in zone H in summer ranged 5.2 to 34 (mean, 17.4), which was somewhat higher than previously measured NO₃ ⁻/PO₄ ³⁻ for sediment pore waters around this zone (1.1–8.5). Nutrient exchanges in zone S were relatively small, indicating semi-closed nutrient cycling at the sediment-water interface of this zone. NH₄ ⁺ efflux from sediments was suggested in zone G. The data suggest that the spatial pattern of nutrient dynamics over the reef flat community was constrained by zonation of benthic biota, and that abiotic factors such as nutrient concentrations and flow rates, influenced nutrient exchange rates only in absorption-dominated communities such as zones B and W.  相似文献   

Fine-scale structure and mixing across the front between the Tsugaru Warm and Oyashio Currents in summer along the Sanriku Coast,east of Japan     

Sachihiko Itoh  Hitoshi Kaneko  Miho Ishizu  Daigo Yanagimoto  Takeshi Okunishi  Hajime Nishigaki  Kiyoshi Tanaka 《Journal of Oceanography》2016,72(1):23-37

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The Role of Suspended Oyster Culture on Nitrogen Cycle in Hiroshima Bay   总被引：1，自引：0，他引：1  

Putth Songsangjinda  Osamu Matsuda  Tamiji Yamamoto  Narasimmalu Rajendran  Hajime Maeda 《Journal of Oceanography》2000,56(2):223-231

The predominance of bivalves affects the cycle of materials in the coastal ecosystem. In the present study, the role of suspended oyster culture on the nitrogen cycle was demonstrated for the northern Hiroshima Bay. The nitrogen cycle was considered as two systems, (1) the primary production (PP) system and (2) the oyster culture (OC) system. The results show that about 26% of N productivity was supplied to process by cultured oysters in the OC system. This process varies seasonally due to the seasonal variations of PON, physiological activities and biomass of oysters. The N processing rates were found to be high in summer and low in winter. The biodeposition and excretion of N in the OC system are 3.0 and 2.1 ton N d^–1, while the natural sedimentation rate and N regeneration in the PP system are 8.3 and 18.0 ton N d^–1, which indicates that the PP system is a major system regenerating N in the water column. The release of total dissolved N from the bottom to the water column is about 8.3 ton N d^–1. The amount of N harvested as oyster product was about 1.3 ton N d^–1, which is about 10% of daily N loading in north Hiroshima Bay. According to the N cycle developed in the present study, the results suggest the significant role of suspended oyster culture on the nitrogen cycle in Hiroshima Bay. In addition, our results indicate that oyster production was efficiently harvested, suggesting that oyster culture could probably be used as a tool to remove N from Hiroshima Bay.  相似文献   

Collapse of low-mass clouds in the presence of a UV radiation field     

Hajime Susa  Tetsu Kitayama 《Monthly notices of the Royal Astronomical Society》2000,317(1):175-178

The collapse of marginally Jeans-unstable primordial gas clouds in the presence of a UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of the collapsing central core until H₂ cooling dominates photoheating and the temperature drops to below 10⁴ K. Consequently, the mass of the cooled core is evaluated as M _cool=3.6×10⁶ M_⊙ ( I ₂₁/1)^−0.32. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field.  相似文献   

The grade index model as a rationale for autogenic nonequilibrium responses of deltaic clinoform to relative sea-level rise     

Junhui Wang  Hajime Naruse  Tetsuji Muto 《Basin Research》2020,32(2):378-387

Grade index (G_index) is a dimensionless number given as the volume-in-unit-time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the G_index, using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive-to-transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (A_t*) encounters a critical value that is expressed in terms of G_index: regression and transgression are sustained when A_t* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both A_t* and G_index decrease as the relative sea-level rises. Eventually, the depositional system experiences autodrowning when A_t* = G_index = 0. If γ < β; on the other hand, both A_t* and G_index increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which A_t* = G_index = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.  相似文献   

POC fluxes estimated from 234Th in late spring–early summer in the western subarctic North Pacific     

Hajime Kawakami  Makio C. Honda  Kazuhiko Matsumoto  Masahide Wakita  Minoru Kitamura  Tetsuichi Fujiki  Shuichi Watanabe 《Journal of Oceanography》2015,71(3):311-324

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Iron, manganese and aluminum in upper waters of the western South Pacific Ocean and its adjacent seas   总被引：1，自引：0，他引：1  

Hajime Obata  Kiminori Shitashima  Kenji Isshiki  Eiichiro Nakayama 《Journal of Oceanography》2008,64(2):233-245

Total dissolvable iron, manganese and aluminum distributions in upper waters were determined in the western South Pacific, Solomon Sea, Coral Sea, and Tasman Sea. In these oceanic regions, the surface aluminum distributions well reflect the atmospheric deposition pattern of mineral dust in the western South Pacific reported previously. Surface manganese distributions derive mainly from lateral transportation from the coastal sediments of western tropical islands. Compared to Mn and Al, the Fe distributions reflect the nutrient cycle in upper waters. Iron limitation over the vast South Pacific, as revealed by physiological features of phytoplankton, seems to be caused by low atmospheric dust deposition and low Fe:N ratios in deep waters. In the western South Pacific, with its unique geographic and oceanographic settings, the local sources of trace metals might considerably affect their biogeochemical cycles.  相似文献   

Growth history and formation environments of ferromanganese deposits on the Philippine Sea Plate, northwest Pacific Ocean     

Akira  Usui  Ian J.  Graham  Robert G.  Ditchburn  Albert  Zondervan  Hiroshi  Shibasaki  Hajime  Hishida 《Island Arc》2007,16(3):420-430

Abstract   Hydrogenetic ferromanganese crusts are widespread on the floor of the northwestern Pacific Ocean, south and east of the Japanese Islands, despite vigorous tectonic activity, such as subduction and back-arc spreading, since at least the Mid-Paleogene over the Philippine Sea Plate region and nearby. The crusts occur mainly at water depths shallower than 3000 m, but also at greater depths of up to 6000 m. Fine-scale ¹⁰Be/⁹Be dating was undertaken on several 5–10 cm thick hydrogenetic ferromanganese crusts sampled from different geological environments, including inactive submarine volcanoes, tectonic escarpments and abandoned rifts. The results indicate that the crusts have grown at relatively constant rates of 4–7 mm/my without any significant time breaks. These uniform and constant growth rates suggest that the basins have been exposed constantly to oxygenated bottom waters since their formation in the Middle Miocene ( ca 15 Ma) or earlier. Local geological or oceanographic environmental changes might have slowed or increased some of the growth rates resulting in correlation of some internal structures. The Philippine Sea Plate region could have economic potential in areas of thick hydrogenetic ferromanganese crusts over a wide range of water depths.  相似文献   

An atomic level study of rhenium and radiogenic osmium in molybdenite   总被引：1，自引：0，他引：1  

Yoshio Takahashi  Tomoya Uruga  Hajime Tanida  Keiko H. Hattori 《Geochimica et cosmochimica acta》2007,71(21):5180-5190

Local atomic structures of Re and radiogenic Os in molybdenite from the Onganja mine, Namibia, were examined using X-ray absorption fine structure (XAFS). Rhenium L_III-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) show that the oxidation state of Re, the interatomic distances between Re and the neighboring atoms, and the coordination number of Re to S are very similar to those of Mo in molybdenite. The results confirm that Re is present as Re(IV) in the Mo site in molybdenite.Measurement of L_III-edge XANES and EXAFS of a minor concentration (8.55 ppm) of radiogenic Os was accomplished in fluorescence mode by removing the interfering X-rays from Re and other elements using a crystal analyzer system. The data indicate that the oxidation state of radiogenic Os is Os(III) and Os(IV) and clearly different from Os(II) in natural sulfide minerals, such as OsS₂ (erlichmanite). XANES data also suggest that radiogenic Os does not form a secondary Os phase, such as OsS₂ or Os metal, in molybdenite.EXAFS of radiogenic Os was successfully simulated assuming that Os is present in the Mo site in molybdenite. The data are consistent with the XANES data; Os does not form Os phases in molybdenite. The EXAFS simulation showed that the interatomic distance between Os and S is 2.27 Å, which is 0.12 Å smaller than the distances of Re-S and Mo-S (2.39 Å) in molybdenite. Similar valences and ionic sizes of Re and Mo in molybdenite support the fact that large amounts of Re can be incorporated into the Mo site as has been observed in previous studies, whereas the different properties of Os compared to Mo and Re suggested here support much lower abundance of common Os in molybdenite. This makes molybdenite an ideal mineral for the Re-Os geochronometer as shown in many studies. However, the shorter distance between radiogenic Os and S compared to those of Re-S and Mo-S in molybdenite suggests that the radiogenic Os has a smaller ionic size than Re(IV) and Mo(IV). Furthermore, Os may be partly present as Os(III). Smaller and lower charge Os can diffuse faster than larger and higher charge Re in molybdenite at a given set of conditions. Hence, our study provides an atomic-level explanation for the high mobility of Os compared to Re, which has been suggested by earlier workers using laser ablation ICP-MS.  相似文献