Uranium and Thorium Isotope Distribution in an Offshore Bottom Sediment Core of the Selenga Delta, Lake Baikal, Siberia     

Aya Sakaguchi  Masayoshi Yamamoto  Keiichi Sasaki  Kenji Kashiwaya 《Journal of Paleolimnology》2006,35(4):807-818

Distribution of uranium and thorium isotopes in a short sediment core obtained offshore of the Selenga Delta in Lake Baikal, Siberia, was investigated to establish their sedimentary behaviors and to look for a linkage to paleoenvironmental changes. The sediments were composed of dominantly fine detrital materials (70–85%) and a relatively high sedimentation rate (ca. 0.03 cm y⁻¹). The depth profile of ²³⁸U content in bulk sediment samples showed a large variation of 70–123 Bq kg⁻¹, while ²³²Th profile showed a relatively narrow range from 36 to 56 Bq kg⁻¹. The observed ²³⁴U/²³⁸U activity ratios revealed a marked disequilibrium ranging from 1.53 to 1.84 with a mean value of 1.71 ± 0.07, demonstrating the presence of 50–80% authigenic ²³⁸U in the bulk sediments. The distribution of this authigenic ²³⁸U did not display any clear correlation with variations in sediment composition (organic, carbonate, Bio-SiO₂ and mineral contents) including grain size median. The profile of terrigenous ²³⁸U showed a relatively similar pattern to that of ²³²Th. Results of sequential leaching indicate that ²³⁸U in Fe–Mn oxyhydroxides fractions were responsible for the distribution of authigenic ²³⁸U rather than in Bio-SiO₂ fraction. The distribution of authigenic ²³⁸U in the bottom sediments may be explained by the fluctuation of U adsorption capacity on particles including organic matter and Fe–Mn oxyhydroxides before they entered the lake. This study highlights the potential use of authigenic and terrigenous U (Th) signatures in sediments to trace the behavior of U (Th) and to reconstruct environmental (e.g., hydrological) changes in the lake catchment area.  相似文献   

Magnetotail response during a strong substorm as observed by GEOTAIL in the distant tail     

A. Belehaki  R. W. McEntire  S. Kokubun  T. Yamamoto 《Annales Geophysicae》1998,16(5):528-541

Simultaneous energetic particle and magnetic field observations from the GEOTAIL spacecraft in the distant tail (X_GSM -150 Re) have been analysed to study the response of the Earths magnetotail during a strong substorm (AE 680 nT). At geosynchronous altitude, LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on the ground at Kiruna Ground Observatory. The Earths magnetotail responded to this substorm with the ejection of five plasmoids, whose size decreases from one plasmoid to the next. Since the type of magnetic structure detected by a spacecraft residing the lobes, depends on the Z extent of the structure passing underneath the spacecraft, GEOTAIL is first engulfed by a plasmoid structure; six minutes later it detects a boundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estimated to range between 510 km/s and 620 km/s. The length of these individual plasmoids was calculated to be between 28 Re and 56 Re. The principal axis analysis performed on the magnetic field during the TCR encountered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first large plasmoid that engulfed GEOTAIL was much more complicated in nature probably due to the external, variable draped field lines associated with high beta plasma sheet and the PSBL flux tubes surrounding the plasmoid. From the analysis of the energetic particle angular distribution, evidence was found that ions were accelerated from the distant X-line at the onset of the burst associated with the first magnetic structure.  相似文献   

Boltzmann approach to the large-angle cosmic X-ray background fluctuations     

Y. Miura  K. Yamamoto  K. Miyauchi  & M. Hosonuma 《Monthly notices of the Royal Astronomical Society》1998,298(2):471-482

Large-angle fluctuations in the cosmic X-ray background are investigated by a new formalism with a simple model of the X-ray sources. Our method is formulated from the Boltzmann equation and a simple extension of the work by Lahav et al. to be applicable to a hyperbolic (open) universe. The low multipole fluctuations due to the source clustering are analysed in various cosmological models in both numerical and analytic ways. The fluctuations strongly depend on the X-ray source evolution model, as pointed out previously. It turns out that the nearby ( z  ≲ 0.1) sources are the dominant contributors to the large-angle fluctuations. If these nearby sources are removed in an observed X-ray map, the dipole (low multipole) moment of the fluctuation drastically decreases. In this case the Compton–Getting effect of an observer's motion can be a dominant contribution to the dipole fluctuation. This feature of fluctuation, relating to the matter power spectrum, is discussed.  相似文献   

Prediction method of sand-movement direction in rivers by geochemical elements     

K. Mizumura  T. Nishimoto  Y. Kumagai  T. Yamamoto 《Environmental Geology》1996,28(2):88-98

 The analysis of sand samples by X-ray fluorescent spectroscopy (XRF) gives the ratio of the geochemical elements to construct the sand samples. The application of the ratio matching to sand samples represents the correlation number between two sand samples with respect to geochemical elements. When the correlation number between two sand samples is low, the two samples are not geochemically similar each other. This denotes that the exchange of sand between two sampling points is scarce or the two samples are independent. When the correlation number between two sand samples is high, the two samples are geochemically similar, signifying that the exchange of sand between two sampling points is frequent or there is sand movement between two sampling points. If there exists prominent sand movement in the study area, the correlation number is almost 1 and kilo count number per second of each geochemical element per weight decreases along the flow direction. The decrease is caused by the reduction of sizes and the adherence of dirt on the surface of sand particles. Since the flow direction in rivers is usually the same as the direction of sand movement, it can be verified. This study obtains satisfactory results applying the method of prediction in sand movement to sediments in rivers. Received: 17 May 1995 · Accepted: 14 August 1995  相似文献   

On the amplification of VLF hiss     

Takashi Yamamoto 《Planetary and Space Science》1979,27(3):273-284

Based on the model calculation of VLF hiss power flux spectrum resulting from convective beam amplification of incoherent Cerenkov whistler radiation by the beam of precipitating auroral electrons, which has been developed by Maggs (1976), we examine the altitude dependence of power flux levels. Their strong altitude dependence leads us to suggest that non-linear processes are important in determining the spectrum of VLF hiss at high altitude. It is also shown that estimated power fluxes inside the electron precipitation region at low altitude might not reach as high levels as observed when the electron beam is weak. In this case, wave propagation outside of the precipitation region will account for the high power flux levels as well as significant magnetic components of VLF hiss observed especially at low altitude. In addition, we show that the transformation of the electron beam in transit to lower altitudes, determined from Liouville's theorem, may influence appreciably VLF hiss power flux spectrum. Finally, it is pointed out that two types of VLF hiss spectrum observed at the ground level can be accounted for by the difference in strength of the electron beam.  相似文献   

Petrological study of Nasu-Chausudake Volcano (ca. 16 ka to Present), northeastern Japan     

Masao Ban  Takahiro Yamamoto 《Bulletin of Volcanology》2002,64(2):100-116

Chausudake Volcano is representative of the active volcanoes in northeastern Japan, and has a record of many historical eruptions. Because its 16-ky eruptive history is well documented, Chausudake is well-suited for examining the temporal change of magma chamber processes and for assessing potential hazards. The activity of the Chausudake Volcano can be divided into six magmatic units (CH1-CH6). Most of its products have similar characteristics, but those from unit CH1 show wider variation. Most rocks are andesite and have plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides as phenocrysts, with or without olivine or quartz. Mafic inclusions, which are observed in most products, are basaltic andesites that have various combinations of the same phenocryst species. Petrographic features observed in host rocks and mafic inclusions, such as disequilibrium phenocrysts and resorbed textures, suggest magma mixing/co-mingling. Whole rock compositions of both host rocks and mafic inclusions show linear trends in variation diagrams, which suggest that the rocks are derived from the mixing/co-mingling between mafic and felsic end members. Bulk silica content of the mafic end-member magma is estimated to be ca. 52%, and contains Mg-rich olivine and An-rich plagioclase. The temperature of this end member is estimated to have been higher than 1,100 °C. Bulk silica content of the felsic end-member magma is estimated to be ~66%, and contains Mg-poor pyroxenes, An-poor plagioclase, and quartz phenocrysts, with a temperature of between 800 and 900 °C. Trace element compositions show that the end members have different origins, but have changed little over the entire 16-ky of activity. The mafic end-member magmas might come from a lower-crustal homogeneous, large magma chamber, whereas the felsic end-member magmas may be partial melts of crustal materials produced by the heat of the mafic end member. Felsic end-member magma may have accumulated in the middle crust before CH1 activity. The mixing ratio of the felsic to mafic end members was 0.5:0.5 to 0.4:0.6 for the CH1 unit, and ca. 0.4:0.6 for the other units. Considering that ca. 75% of the total volume of the eruptive products form the first unit, its wider compositional variation is attributed to more heterogeneous mixing ratios.  相似文献   

Palaeomagnetism of the Older Ontake Volcano, Japan: contributions to the palaeosecular variation for 750–400 Ka, the lower half of the Brunhes Chron     

Hidefumi Tanaka  Rieko Kamizaki  Yuhji Yamamoto 《Geophysical Journal International》2007,169(1):81-90

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Geology of the Gorny Altai subduction–accretion complex, southern Siberia: Tectonic evolution of an Ediacaran–Cambrian intra-oceanic arc-trench system   总被引：1，自引：0，他引：1  

Tsutomu Ota  Atsushi Utsunomiya  Yuko Uchio  Yukio Isozaki  Mikhail M. Buslov  Akira Ishikawa  Shigenori Maruyama  Koki Kitajima  Yoshiyuki Kaneko  Hiroshi Yamamoto  Ikuo Katayama   《Journal of Asian Earth Sciences》2007,30(5-6):666-695

The Gorny Altai region in southern Siberia is one of the key areas in reconstructing the tectonic evolution of the western segment of the Central Asian Orogenic Belt (CAOB). This region features various orogenic elements of Late Neoproterozoic–Early Paleozoic age, such as an accretionary complex (AC), high-P/T metamorphic (HP) rocks, and ophiolite (OP), all formed by ancient subduction–accretion processes. This study investigated the detailed geology of the Upper Neoproterozoic to Lower Paleozoic rocks in a traverse between Gorno-Altaisk city and Lake Teletskoy in the northern part of the region, and in the Kurai to Chagan-Uzun area in the southern part. The tectonic units of the studied areas consist of (1) the Ediacaran (=Vendian)–Early Cambrian AC, (2) ca. 630 Ma HP complex, (3) the Ediacaran–Early Cambrian OP complex, (4) the Cryogenian–Cambrian island arc complex, and (5) the Middle Paleozoic fore-arc sedimentary rocks. The AC consists mostly of paleo-atoll limestone and underlying oceanic island basalt with minor amount of chert and serpentinite. The basaltic lavas show petrochemistry similar to modern oceanic plateau basalt. The 630 Ma HP complex records a maximum peak metamorphism at 660 °C and 2.0 GPa that corresponds to 60 km-deep burial in a subduction zone, and exhumation at ca. 570 Ma. The Cryogenian island arc complex includes boninitic rocks that suggest an incipient stage of arc development. The Upper Neoproterozoic–Lower Paleozoic complexes in the Gorno-Altaisk city to Lake Teletskoy and the Kurai to Chagan-Uzun areas are totally involved in a subhorizontal piled-nappe structure, and overprinted by Late Paleozoic strike-slip faulting. The HP complex occurs as a nappe tectonically sandwiched between the non- to weakly metamorphosed AC and the OP complex. These lithologic assemblages and geologic structure newly documented in the Gorny Altai region are essentially similar to those of the circum-Pacific (Miyashiro-type) orogenic belts, such as the Japan Islands in East Asia and the Cordillera in western North America. The Cryogenian boninite-bearing arc volcanism indicates that the initial stage of arc development occurred in a transient setting from a transform zone to an incipient subduction zone. The less abundant of terrigenous clastics from mature continental crust and thick deep-sea chert in the Ediacaran–Early Cambrian AC may suggest that the southern Gorny Altai region evolved in an intra-oceanic arc-trench setting like the modern Mariana arc, rather than along the continental arc of a major continental margin. Based on geological, petrochemical, and geochronological data, we synthesize the Late Neoproterozoic to Early Paleozoic tectonic history of the Gorny Altai region in the western CAOB.  相似文献   

Observations of C2 Molecules in the Coma of Comets at Mitaka     

Sekiguchi  T.  Watanabe  J.  Fukushima  H.  Yamamoto  T.  Yamamoto  N. 《Earth, Moon, and Planets》1997,78(1-3):143-148

In order to monitor the various phenomena of comets, we continuously made the near-nucleus imaging observations of comets at National Astronomical Observatory, Mitaka. Here we report on the results of the observations of two spectacular comets. One is the analysis of photometry of C₂ emission images in the coma of comet Hyakutake (C/1996 B2). By comparing the production rates of C₂ radicals with C₂H₂ molecules, we conclude that the most C₂ radicals were originated from C₂H₂, while some of them might be produced from other parents. The second item is the analysis of the morphology of C₂ emission images in the coma of comet Hale-Bopp(C/1995 O1). By applying a ring masking technique, we detected asymmetrical distribution of the C₂ molecules. The results of these two comets might suggest that a portion of C₂ radicals were formed by the disintegration of the small organic dust grains, like the so-called CHON particles. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Estimation of nearshore groundwater discharge and its potential effects on a fringing coral reef     

Blanco AC  Watanabe A  Nadaoka K  Motooka S  Herrera EC  Yamamoto T 《Marine pollution bulletin》2011,62(4):770-785

Radon (²²²Rn) measurements were conducted in Shiraho Reef (Okinawa, Japan) to investigate nearshore submarine groundwater discharge (SGD_nearshore) dynamics. Estimated average groundwater flux was 2-3 cm/h (maximum 7-8 cm/h). End-member radon concentration and gas transfer coefficient were identified as major factors influencing flux estimation accuracy. For the 7-km long reef, SGD_nearshore was 0.39-0.58 m³/s, less than 30% of Todoroki River’s baseflow discharge. SGD_nearshore was spatially and temporally variable, reflecting the strong influence of subsurface geology, tidal pumping, groundwater recharge, and hydraulic gradient. SGD_nearshore elevated nearshore nitrate concentrations (0.8-2.2 mg/l) to half of Todoroki River’s baseflow -N (2-4 mg/L). This increased nearshore Chl-a from 0.5-2 μg/l compared to the typically low Chl-a (<0.1-0.4 μg/l) in the moat. Diatoms and cyanobacteria concentrations exhibited an increasing trend. However, the percentage contributions of diatoms and cyanobacteria significantly decreased and increased, respectively. SGD may significantly induce the proliferation of cyanobacteria in nearshore reef areas.  相似文献