Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China     

Yusuke Sawaki  Manabu Nishizawa  Takeshi Suo  Tsuyoshi Komiya  Takafumi Hirata  Naoto Takahata  Yuji Sano  Jian Han  Yoshiaki Kon  Shigenori Maruyama 《Gondwana Research》2008,14(1-2):148

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.  相似文献   

Comparison of surface current variations observed by TOPEX altimeter with TOLEX-ADCP data   总被引：1，自引：0，他引：1  

Naoto Ebuchi  Kimio Hanawa 《Journal of Oceanography》1995,51(3):351-362

Variations of surface current velocity derived by the TOPEX altimeter are compared with data from Tokyo-Ogasawara Line Experiment (TOLEX)-Acoustic Doppler Current Profiler (ADCP) monitoring for a period from October 1992 to July 1993. Since the locations of ADCP ship track and TOPEX altimeter ground tracks do not coincide with each other, and the temporal and spatial sampling are also different between the ADCP and altimeter observations, re-sampling, interpolation and smoothing in time and space are needed to the ADCP and altimeter data. First, the interpolated TOPEX sea surface height is compared with sea level data at Chichijima in the Ogasawara Islands. It is found that aliasing caused by the tidal correction error for M2 constituent in the TOPEX data is significant. Therefore, comparison of the TOPEX data with the TOLEX-ADCP data is decided to be made by using cross-track velocity components of the surface current, which are considered to be relatively less affected by the errors in the tidal correction. The cross-track velocity variations derived from the TOPEX sea surface heights agree well with those of the ADCP observations. The altimeterderived velocity deviations associated with transition of the Kuroshio paths coincide with the ADCP data. It is quantitatively confirmed that the TOPEX altimeter is reliable to observe the synoptic variations of surface currents including fluctuations of the Kuroshio axis.  相似文献   

On the wave dependence of sea-surface wind stress     

Naoto Ebuchi  Yoshiaki Toba  Hiroshi Kawamura 《Journal of Oceanography》1990,46(4):177-183

Analysis is made of wind and wave data, which were obtained during the passage of Typhoon 8013 at an Ocean Data Buoy Station south of Honshu operated by the Japan Meteorological Agency, in order to investigate the wave dependence of sea-surface roughness parameter in the situation where wind waves are dominant with less significant swells. The data fit better the wave-dependent expression of the wind stress,z _{0 p}/u*=, than to Charnock's formula,gz ₀/u*²=, wherez ₀ is the roughness length, _p the angular frequency of the spectral peak of wind waves,u* the friction velocity of air,g the acceleration of gravity, and are non-dimensional constants. The results are very similar to those of our previous study using data from an oil producing platform in the Bass Strait, Australia, although the type of observation system and the synoptic situation of the winds and wind waves were totally different.  相似文献   

Measurement of Noble Gas Solubility in Seawater Using a Quadrupole Mass Spectrometer     

Yuji?SanoEmail author  Naoto?Takahata 《Journal of Oceanography》2005,61(3):465-473

A quadrupole-based mass spectrometric system for determining noble gas (He, Ne, Ar, Kr and Xe) abundances in a liquid sample has been developed and applied to seawater equilibrated with the atmosphere at several temperatures. After known amounts of isotopic spikes (²²Ne, ³⁶Ar, ⁸⁶Kr and ¹²⁴Xe) were introduced into the preparation vacuum line, dissolved gases were extracted from seawater and well mixed with the spikes by ultrasonic vibration. Noble gases were purified using three-stage hot Ti getters and separated by two activated charcoal traps held at low temperature (liquid nitrogen and dry ice—ethanol). Noble gas abundances were measured by a relatively inexpensive quadrupole mass spectrometer based on the isotope dilution technique. Precisions obtained by repeated measurements for He, Ne, Ar, Kr and Xe are 1.5%, 0.4%, 0.4%, 0.4% and 0.6%, respectively. Noble gas abundances in air-saturated seawater at eight temperature steps (1.0°C, 5.5°C, 10.0°C, 14.4°C, 17.0°C, 21.3°C, 24.6°C, and 29.6°C) were measured using the analytical system. The overall variation of the abundances with temperature agrees well with those of NaCl brines reported in the literature. We propose here new constants to fit observed data to empirical equation of noble gas solubility in seawater with its temperature variation.  相似文献   

Anticyclonic eddy caused by the Soya Warm Current in an Okhotsk OGCM     

Keisuke Uchimoto  Humio Mitsudera  Naoto Ebuchi  Yasumasa Miyazawa 《Journal of Oceanography》2007,63(3):379-391

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Evaluation of marine surface winds observed by SeaWinds and AMSR on ADEOS-II   总被引：1，自引：0，他引：1  

Naoto Ebuchi 《Journal of Oceanography》2006,62(3):293-301

Marine surface winds observed by two microwave sensors, SeaWinds and Advanced Microwave Scanning Radiometer (AMSR), on the Advanced Earth Observing Satellite-II (ADEOS-II) are evaluated by comparison with off-shore moored buoy observations. The wind speed and direction observed by SeaWinds are in good agreement with buoy data with root-mean-squared (rms) differences of approximately 1 m s⁻¹ and 20°, respectively. No systematic biases depending on wind speed or cross-track wind vector cell location are discernible. The effects of oceanographic and atmospheric environments on the scatterometry are negligible. Though the wind speed observed by AMSR also showed agreement with buoy observations with rms difference of 1.27 m s⁻¹, the AMSR wind speed is systematically lower than the buoy data for wind speeds lower than 5 m s⁻¹. The AMSR wind seems to have a discontinuous trend relative to the buoy data at wind speeds of 5–6 m s⁻¹. Similar results have been obtained in an intercomparison of wind speeds globally observed by SeaWinds and AMSR on the same orbits. A global wind speed histogram of the AMSR wind shows skewed features in comparison with those of SeaWinds and European Centre for Medium-range Weather Forecasts (ECMWF) analyses.  相似文献   

Seasonal variations of the upper ocean in the western north pacific observed by an argo float     

Naoto Iwasaka  Fumiaki Kobashi  Yosuke Kinoshita  Yuko Ohno 《Journal of Oceanography》2006,62(4):481-492

A seasonal evolution of surface mixed layer in the western North Pacific around 24°N between 143°E and 150°E was observed by using an Argo float for more than 9 months, from December 2001 through August 2002. The result showed that the mixed layer deepened gradually in the first two months. It reached its maximum depth of about 130 m at the end of January, after which the mixed layer varied largely and sometimes the pycnocline below the mixed layer was much weakened until the summer mixed layer formed in late April. The thin surface mixed layer was maintained during the rest of the observation period. Heat budget analysis suggests that the vertical and horizontal temperature advections are the two most dominant terms in the heat balance in the upper layer on time scales from a few days to a month. The vertical motions that are possibly responsible for the vertical temperature advection are discussed.  相似文献   

Sea-surface roughness length fluctuating in concert with wind and waves   总被引：2，自引：0，他引：2  

Yoshiaki Toba  Naoto Ebuchi 《Journal of Oceanography》1991,47(3):63-79

When the nondimensional aerodynamic roughness parameter for the sea surface (gz ₀/u _* ²,g being the acceleration of gravity,u _* the air friction velocity) is plotted as a function of the wave age, the data points in the diagram are distributed mostly in a triangle area between the Charnock formula and the Toba-Koga formula; the nondimensional roughness perameter is not expressed as a unique function of the wave age, but rather there seem to be multiple regimes. In order to investigate the cause of the data point scattering, a reanalysis was made of the 4.5-hour time series of the wind profile and wind-wave statistics which were obtained at an oceanographic tower station under the conditions of a winter monsoon wind having slightly fluctuating speed and steadily growing wind waves.It is concluded that the averaged variation ofz ₀ is given by the Toba-Koga formula with a constant of value 0.015. However, as a result of the wind fluctuation on the time scales ranging from several minutes to an hour, data points show a conspicuous fluctuation on the nondimensional roughness parameter-wave age diagram in the direction transverse to the averaged variation. The variation inz ₀ directly reflects the degree of over- or under-saturation in the high-frequency range of the wind-wave spectra. Physical interpretation of these variations is also presented.  相似文献   

Direct ascent to the surface of asthenospheric magma in a region of convex lithospheric flexure     

Yuki Sato  Naoto Hirano  Shiki Machida  Junji Yamamoto  Masao Nakanishi  Teruaki Ishii 《International Geology Review》2018,60(10):1231-1243

The stress field of oceanic lithosphere controls the distribution of submarine petit-spot volcanoes. However, the eruption sites of these petit-spot volcanoes are considered to be limited to concavely flexed regions of lithosphere off the outer rise. Here, we present new data for a recently identified petit-spot lava field on a convexly flexed section of the lithosphere adjacent to the subduction zone offshore of northeast Japan in an area containing more than 80 volcanoes. This area is marked by strongly alkaline lavas that were erupted on the convexly flexed region. As for the concavely flexed region where the petit-spots previously reported, the base of the lithosphere beneath the eruption sites is under extension, whereas the upper part of the lithosphere is under compression. This change in the stress field, from the lower to upper lithosphere, causes ascending dikes to stall in the mid-lithosphere, leading to metasomatic interaction with the surrounding peridotite. The new geochemical data of rocks and xenocrysts presented in this study indicate that strongly alkaline magmas erupted on the convexly flexed region would have ascended more rapidly through the mid-depth of lithosphere because of the extensional regime of the upper lithosphere and decreasing the degree of metasomatic reaction with the surrounding mantle peridotite. The results indicate that the degree of metasomatism and the compositional variations of petit-spot magmas are controlled mainly by the stress field of the lithosphere.  相似文献   

Growth of Wind Waves with Fetch in the Sea of Japan under Winter Monsoon Investigated Using Data from Satellite Altimeters and Scatterometer     

Naoto Ebuchi 《Journal of Oceanography》1999,55(5):575-584

By using wind vector fields observed by the NASA Scatterometer (NSCAT) and significant wave heights observed by the TOPEX/POSEIDON and European Remote Sensing Satellite-2 (ERS-2) altimeters, one-dimensional fetch growth of wind waves has been investigated under conditions of strong wind and high waves caused by the East Asian winter monsoon in the Sea of Japan. The evolution of fetch-limited wind waves can be observed by the altimeters along their ground tracks. The fetch is estimated by using vector wind fields observed by NSCAT. The derived growth characteristics of wind waves are compared with empirical relationships between the non-dimensional fetch and significant wave height proposed by previous studies. Good agreement is discernible with Toba's fetch graph formula normalized by the friction velocity, while Wilson's well-known formula normalized by the wind speed at a height of 10 m tends to underestimate the wave height under such severe conditions of high wind and very long fetch. This discrepancy is explained by the wind-speed dependence of the drag coefficient. A simple correction to Wilson's formula for the high wind conditions is proposed and compared with the observed data.  相似文献