Mechanism of Nutrient Supply to Warm-Core Ring off Sanriku, Japan     

Katsuyuki Sasaki  Yutaka Hiroe 《Journal of Oceanography》2002,58(5):683-690

The mechanism by which nutrient is supplied to a warm-core ring (WCR) was investigated in order to understand the greater productivity of WCR than that of the Kuroshio, where the WCR originattes. A single WCR was observed in January and May, 1997. The thermostad (a layer of isothermal and isohaline water) of the WCR had different properties from January to May, the differences: Δwater temperature: −0.698°C, Δsalinity: −0.048, Δsigma θ: +0.072, Δnitrite+nitrate-N: +1.83 μM, Δphosphate: +0.011 μM and Δsilicate: +3.2 μM. We examined three possible mechanisms for nutrient supply to WCR in winter, namely: 1) inflow of the Oyashio surface water into WCR; 2) isopycnal mixing with Oyashio water; 3) entrainment of the water below the WCR into the WCR. The results were as follows: 1) When the decrease of salinity was due to the inflow of the Oyashio surface water, the increase of nutrients (nitrite+nitrate-N, phosphate-P and silicate-Si) was estimated to be only 17–27% of the observed increase. 2) When the decrease of salinity was due to isopycnal mixing, the increase of nutrients was estimated to be 30–42% of the observed increase. 3) When the decrease of salinity in the WCR in May was due to entrainment of the water below the WCR in winter by convection, the mixing depth was calculated be 620 m according to the salt budget. The increase of nutrients in this case was calculated to be 82–95% of the observed increase. The main mechanism of nutrient supply to WCR was concluded to be due to the entrainment of the water below the WCR by winter mixing. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations     

Masateru Ishiguro  Takahiro Hiroi  David J. Tholen  Sho Sasaki  Yuji Ueda  Tokuhiro Nimura  Masanao Abe  Beth E. Clark  Aya Yamamoto  Fumi Yoshida  Ryosuke Nakamura  Naru Hirata  Hideaki Miyamoto  Yasuhiro Yokota  Tatsuaki Hashimoto  Takashi Kubota  Akiko M. Nakamura  Robert W. Gaskell  Jun Saito 《Meteoritics & planetary science》2007,42(10):1791-1800

Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels.  相似文献   

Characterization of drying-induced deformation behaviour of Opalinus Clay and tuff in no-stress regime   总被引：1，自引：0，他引：1  

Aung Ko Ko Soe  Masahiko Osada  Manabu Takahashi  Tai Sasaki 《Environmental Geology》2009,58(6):1215-1225

The drying-induced deformation behaviour of Opalinus Clay and tuff, which are being investigated under international and local collaborative projects for nuclear waste disposal in Switzerland and Japan, was investigated under a no-stress condition in the laboratory to evaluate their generic susceptibility to the formation of excavation damaged zone. The cylindrical core samples of Opalinus Clay and tuff were prepared to a one-dimensional drying condition and submitted to an uncontrolled laboratory environment. The strain evolution, evaporative water loss and environmental entities, such as temperature and relative humidity, were recorded simultaneously and quasi-continuously. It was observed that the drying phase induced significant strain magnitude and damage in Opalinus Clay samples, which was evidenced by the formation of hairy cracks on the surface parallel to the bedding. On the contrary, the strain occurrences in tuff samples were relatively insignificant, and no tendency of cracking was observed. In addition, the quasi-continuous availability of volumetric strains was further used in poroelastic relation for the estimation of capillary suction evolution. The calculated results were validated with pore size distributions obtained from mercury intrusion porosimetry.  相似文献   

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core     

Aya Sakaguchi  Masayoshi Yamamoto  Junpei Tomita  Kenta Mino  Keiichi Sasaki  Kenji Kashiwaya  Takayoshi Kawai 《Quaternary International》2009,205(1-2):65

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ²³⁸U–²³⁴U–²³⁰Th–²³²Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The ²³⁸U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ²³⁸U peak at 23.8 m in depth, while the ²³²Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk ²³⁸U into authigenic and terrigenous ²³⁸U fractions was attempted, based on the measured ²³²Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ²³⁸U was higher in interglacials and lower in glacials. This depth profile of authigenic ²³⁸U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ²³⁸U contents were associated with the environmental change around Lake Hovsgol.  相似文献   

Editorial to OFES special issue of Ocean Dynamics     

Yukio Masumoto  Hideharu Sasaki  Jin-Song von Storch 《Ocean Dynamics》2010,60(3):631-632

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The cyclonic circulation in the Australian–Antarctic basin simulated by an eddy-resolving general circulation model     

Shigeru Aoki  Yoshikazu Sasai  Hideharu Sasaki  Humio Mitsudera  Guy D. Williams 《Ocean Dynamics》2010,60(3):743-757

Flow structure in the Australian–Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80–110° E) and a series of eddies in the east (120–150° E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres.  相似文献   

Surface photometry of simpleHii regions     

S. Yoshida  M. Nakano  T. Kogure  T. Sasaki  S. D. Wiramihardja  S. Mizuno  K. Sakka 《Astrophysics and Space Science》1986,119(1):131-133

We have carried out surface photometry of six simpleHii regions which are characterized by the existence of an exciting star earlier than B0, and a nearly round shape. On the basis of on balibrated image data and adopting a spherical model, we derive the distributions of gas and dust densities in theHii regions.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.On leave from the Bosscha Observatory, Institute of Technology Bandung, Indonesia.  相似文献   

Sulfur isotopic composition of the magnetite-series and ilmenite-series granitoids in Japan   总被引：1，自引：0，他引：1  

Akira Sasaki  Shunso Ishihara 《Contributions to Mineralogy and Petrology》1979,68(2):107-115

Sulfur isotopic composition has been measured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan. The two series of granitoids, the magnetite-series and ilmenite-series, defined by Ishihara (1977), show two specific isotope trends. The magnetite-series granioids all have positive (su34)S (CDT) values from +1 to +9, while the ilmenite-series rocks are dominated by negative values between –11 and –1. The trend in the ilmenite-series is consistent with the thesis that the magma has been influenced by light biogenic sulfur from the continental crust. The inferred large scale magma-crust interaction in the ilmenite-series granitoids indicates that the emplacement of this series of magma has been governed by a stoping mechanism.In contrast, the magnetite-series granitoids have little if any evidence for significant magma-crust interaction, indicating that the intrusion of this series of magma may have been more or less of fissure-filling type. Their trend towards positive (su3 4)S values (average +5) argues for the introduction of some heavy sulfur, probably of seawater origin, into the mantle derived sulfur. This is most likely to occur in an arctrench system by the subduction of an oceanic plate which accompanies the sulfate-bearing pelagic sediments.The isotopic data of gabbroids, mostly between –1 and +3, are close to the commonly assumed value for mantle sulfur. Nevertheless, the gabbroids from the magnetite-series granitic terranes tend to have higher (su34)S value than those from the ilmenite-series belts. It is inferred that the factors controlling the isotope characteristics of the granitoid sulfur have also been operative in these grabbroids at least to some extent.  相似文献   

SELENE project status     

Konishi Hisahiro  Manabu Kato  Susumu Sasaki  Yoshisada Takizawa  Hitoshi Mizutani 《Journal of Earth System Science》2005,114(6):771-775

SELENE (Selenological and Engineering Explorer) project started as a joint mission of the former ISAS (Institute of Space and Astronautical Science) and the former NASDA (National Space Development Agency: the two organizations were merged into JAXA in 2002) of Japan in 1998. The launch target is rescheduled for 2006 due to delay of completion of launch vehicle, H-IIA. The SELENE project is now under a sustained design phase. The flight model components were manufactured, and the interface tests between the bus-system and the mission instruments were completed by the end of March 2004. The functional checks and calibration for the flight model components are being carried out at present. From the beginning of 2005, the final assembly tests will start.  相似文献   

Observation of shear zone development in ring-shear apparatus with a transparent shear box   总被引：1，自引：1，他引：1  

Hiroshi Fukuoka  Kyoji Sassa  Gonghui Wang  Ryo Sasaki 《Landslides》2006,3(3):239-251

Using a new ring-shear apparatus with a transparent shear box and video image analysis system, drained and undrained speed-controlled tests were conducted on coarse-grained silica sands to study the shear-zone formation process in granular materials. Velocity distribution profiles of grains under shear at various stages in the ring shear tests were observed through processing the video image by the Particle Image Velocimetry (PIV) program. Shear-zone thickness and type of shear mode (slide-like or flow-like) during shear were observed. Before reaching peak strength in low-speed and drained condition test, a comparatively major part of the sample in the upper shear box showed a velocity distribution profile of structural deformation and dilatancy behavior. After peak strength, the velocity profile changed into a slide-like mode and thereafter showed almost no change. In higher speed tests with drained and undrained conditions, an almost slide-like mode was observed, compared to low-speed test. Apparent shear-zone thicknesses of high-speed tests are thinner than low-speed tests. Unexpectedly, almost no difference was observed in the shear-zone thickness and mode of shear (slide or flow-like) between drained and undrained tests. This study was conducted as part of the International Programme on Landslides (IPL) M101 “Areal prediction of earthquake and rain induced rapid and long-traveling flow phenomena (APERITIF)” of the International Consortium on Landslides (ICL). These results will contribute to understanding the mechanism of shear-zone development in granular materials as a basic knowledge for disaster risk mitigation of rapid long run-out landslides.  相似文献