Ion microprobe U–Pb dating of zircon with a 15 micrometer spatial resolution using NanoSIMS     

Naoto Takahata  Yukiyasu Tsutsumi  Yuji Sano 《Gondwana Research》2008,14(4):587

We developed a ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb zircon dating method using a Cameca NanoSIMS NS50 ion microprobe. A 7-to 9-nA O⁻ primary beam was used to sputter a 15-μm crater, and secondary positive ions were extracted for mass analysis using the Mattauch–Herzog geometry. The multicollector system was modified to detect ⁹⁰Zr⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²³⁸U¹⁶O⁺, and ²³⁸U¹⁶O₂⁺ ions simultaneously. A mass resolution of about 4000 at 10% peak height and with a flat peak top was attained, and the sensitivity of Pb was about 4 cps·nA^− 1·ppm^− 1. A multicrystal zircon standard (QGNG) from South Australia with a U–Pb age of 1842 Ma was used as a reference for Pb⁺/UO⁺–UO₂⁺/UO⁺ calibration, and on the basis of the positive correlation between these ratios, we determined the sample ²⁰⁶Pb/²³⁸U ratios. ²⁰⁷Pb/²⁰⁶Pb ratios were measured by magnetic scanning in single-collector mode. The standard zircons 91500, from Canada, and SL13, from Sri Lanka, were analyzed against QGNG. Observed ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb ages agreed well with published ages within experimental error. Then, 16 zircon grains in a metamorphic rock from Nagasaki, Japan, were analyzed. Observed ages were compatible with SHRIMP ages, suggesting that the NanoSIMS with a 15-μm probe diameter is suitable for ion microprobe U–Pb zircon dating.  相似文献   

Geographical distribution of helium isotope ratios in northeastern Japan   总被引：1，自引：0，他引：1  

Keika  Horiguchi  Sadato  Ueki  Yuji  Sano  Naoto  Takahata  Akira  Hasegawa  George  Igarashi 《Island Arc》2010,19(1):60-70

In order to study the precise geographical distribution of helium isotope ratios in northeastern Japan and compare it with geophysical data, we collected 43 gas and water samples from hot and mineral springs in the region where the ratio had never been reported, and measured the ³He/⁴He and ⁴He/²⁰Ne ratios of these samples. It was found that the ³He/⁴He ratios show clear contrasts between the forearc and the back-arc regions in the Tohoku district in northeastern Japan. In the forearc region, the ratios are smaller than 1 R_A (1 R_A = 1.4 × 10⁻⁶; R_A means the ³He/⁴He ratio of the atmosphere). On the other hand, those along the volcanic front and in the back-arc region are apparently higher. Moreover, we found a variation in the ³He/⁴He ratios along the volcanic front. In Miyagi Prefecture (38–39°N), the ratios range from 2 to 5 R_A. On the other hand, the ratios are less than 1 R_A in and around the southern border between Iwate and Akita Prefectures (39–39.5°N). Comparing the distribution of helium isotope ratios to results of recent geophysical studies, we found that the features in geographical distribution of helium isotope ratios are similar to those of seismic low-velocity zone distributions and high Qp⁻¹ distributions in the uppermost mantle. These observations strongly suggest that the helium isotope ratios reflect the distribution of melts in the uppermost mantle and are a useful tool for investigating the origin, behavior, and distribution of deep fluids and melts.  相似文献   

Response of a summertime anticyclonic eddy to wind forcing in Funka Bay,Hokkaido, Japan     

Daisuke Takahashi  Hideo Miyake  Takeyasu Nakayama  Naoto Kobayashi  Kazuo Kido  Yoshinori Nishida 《Continental Shelf Research》2010

Flow fluctuations inside an anticyclonic eddy in summertime Funka Bay were examined using three moorings and hydrographic data. The flow pattern above a sharp pycnocline with a concave isopycnal structure during the mooring period was characterized by high mean kinetic energy and relatively low eddy kinetic energy. The ratios of eddy to mean kinetic energy were equal to or less than one, and the mean flow field and isopycnal structure indicated the existence of a stable anticyclonic eddy above the sharp pycnocline under approximate geostrophic balance. Larger flow fluctuations with periods longer than 7 days were dominant inside the eddy. The low-frequency flow fluctuations are accompanied by north to northeastward movement of the eddy with deepening of the pycnocline and spin-up of the anticyclonic circulation. The wind field over Funka Bay is characterized by bay-scale wind fluctuations. The bay-scale winds are greatly influenced by the land topography around Funka Bay, resulting in non-uniform structure with significant wind stress curl. The bay-scale wind fluctuations are termed “locally modified wind” in the present study. The locally modified wind has a negative (positive) wind stress curl in the central–northeastern (southwestern) region of Funka Bay. The north to northeastward movement of the eddy is caused by horizontal non-uniform supply of vorticity from the locally modified wind forcing by the Ekman pumping process. Through this process, the anticyclonic circulation is enhanced (weakened) in the central–northeastern (southwestern) part of the eddy, resulting in the eddy moving north to northeastward with the pycnocline deepening and spin-up of the anticyclonic circulation. The locally modified wind forcing induces low-frequency flow fluctuations through the movement of the eddy in summertime Funka Bay.  相似文献   

Mountain-building Movement in Japan and its Vicinity     

Naoto Kawai 《Geophysical Journal International》1961,6(1):1-8

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Chemical evidence for the origin of the cold water belt along the northeastern coast of Hokkaido     

Kenshi Kuma  Ryohei Sasayama  Nanako Hioki  Yuichiroh Morita  Yutaka Isoda  Tohru Hirawake  Keiri Imai  Takafumi Aramaki  Tomohiro Nakamura  Jun Nishioka  Naoto Ebuchi 《Journal of Oceanography》2014,70(4):377-387

In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature–salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO₃ + NO₂)/PO₄ relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water.  相似文献   

Interannual variations in the Hawaiian Lee Countercurrent     

Hiroto Abe  Kimio Hanawa  Naoto Ebuchi 《Journal of Oceanography》2013,69(2):191-202

The Hawaiian Lee Countercurrent (HLCC) is an eastward surface current flowing against the broad westward flow of the North Pacific subtropical circulation. Analyses of satellite altimeter data over 16 years revealed that the HLCC is characterized by strong interannual variations. The strength and meridional location of the HLCC axis varied significantly year by year. The eastward velocity of the HLCC was higher when the location of the axis was stable. Mechanisms for the interannual variations were explored by analyses of the altimeter data and results from a simple baroclinic model. The interannual variations in the strength of the HLCC did not correlate with those of the wind stress curl (WSC) dipole formed on the leeward side of the Hawaii Islands, although the WSC dipole has been recognized as the generation mechanism of the HLCC. Meridional gradients of the sea surface height anomaly (SSHA) across the HLCC generated by baroclinic Rossby waves propagating westward from the east of the Hawaii Islands were suggested as a possible mechanism for the interannual variations in the HLCC. The spatial patterns in the observed SSHAs were reproduced by a linear baroclinic Rossby wave model forced by wind fields from a numerical weather prediction model. Further analysis of the wind data suggested that positive and negative anomalies of WSC associated with changes in the trade winds in the area east of the Hawaii Islands are a major forcing for generating SSHAs that lead to the HLCC variations with a time lag of about 1 year.  相似文献   

Particle-tracking simulation for the drift/diffusion of spilled oils in the Sea of Okhotsk with a three-dimensional, high-resolution model     

Jun Ono  Kay I. Ohshima  Keisuke Uchimoto  Naoto Ebuchi  Humio Mitsudera  Hajime Yamaguchi 《Journal of Oceanography》2013,69(4):413-428

To conduct the simulation of oil spills in the Sea of Okhotsk, we developed a three-dimensional, high-resolution ocean circulation model. The model particularly improved the reproducibility of velocity field during the strong stratification period. Particle-tracking experiments with the effects of evaporation and biodegradation were performed using the combined data of daily ocean currents from the present model and the hourly diurnal tidal currents from the tidal model. The results are shown by the relative concentration of the particles averaged over the 8 years of 1998–2005 based on the ensemble forecast idea. For the case of particles released from the Sakhalin II oil field, the particles deployed in September–January are carried southward by the East Sakhalin Current, finally arriving at the Hokkaido coast, after 60–90 days. The particles deployed in March–August are diffused offshore by the synoptic wind drift, and hardly transported to regions south of Sakhalin. For the case of particles released from the region off Prigorodnoye, the oil export terminal, after the diffusion by the synoptic wind drift, a part of them are carried offshore of Hokkaido by the Soya Warm Current. The particles released in November–April flow out to the Japan Sea through the Soya Strait, mainly by the synoptic wind drift and secondly by the diffusion due to strong tidal currents around the Soya Strait. By considering the effects of evaporation and biodegradation, the relative concentration of the particles is considerably decreased before arriving at the Hokkaido coast, particularly in the case of drift from the Sakhalin II oil field.  相似文献   

Evaluation of Vector Winds Observed by NSCAT in the Seas around Japan     

Harunobu Masuko  Kohei Arai  Naoto Ebuchi  Masanori Konda  Masahisa Kubota  Kunio Kutsuwada  Teruko Manabe  Akira Mukaida  Tetsuo Nakazawa  Atsushi Nomura  Akira Shibata  Yoshihiko Tahara 《Journal of Oceanography》2000,56(5):495-505

In order to validate wind vectors derived from the NASA Scatterometer (NSCAT), two NSCAT wind products of different spatial resolutions are compared with observations by buoys and research vessels in the seas around Japan. In general, the NSCAT winds agree well with the wind data from the buoys and vessels. It is shown that the root-mean-square (rms) difference between NSCAT-derived wind speeds and the buoy observations is 1.7 ms^–1, which satisfies the mission requirement of accuracy, 2 ms^–1. However, the rms difference of wind directions is slightly larger than the mission requirement, 20°. This result does not agree with those of previous studies on validation of the NSCAT-derived wind vectors using buoy observations, and is considered to be due to differences in the buoy observation systems. It is also shown that there are no significant systematic trends of the NSCAT wind speed and direction depending on the wind speed and incidence angle. Comparison with ship winds shows that the NSCAT wind speeds are lower than those observed by the research vessels by about 0.7 ms^–1 and this bias is twice as large for data observed by moving ships than by stationary ships. This result suggests that the ship winds may be influenced by errors caused by ship's motion, such as pitching and rolling.  相似文献   

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination     

Takahiro Katagiri  Hajime Naruse  Naoto Ishikawa  Takafumi Hirata 《Island Arc》2020,29(1)

The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (?9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.  相似文献