Determination of Sr/Sr mass-dependent isotopic fractionation and radiogenic isotope variation of Sr/Sr in the Neoproterozoic Doushantuo Formation     

Takeshi Ohno  Tsuyoshi Komiya  Yuichiro Ueno  Takafumi Hirata  Shigenori Maruyama 《Gondwana Research》2008,14(1-2):126

We measured both mass-dependent isotope fractionation of δ⁸⁸Sr (⁸⁸Sr/⁸⁶Sr) and radiogenic isotopic variation of Sr (⁸⁷Sr/⁸⁶Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr compositions showed three remarkable characteristics: (1) high radiogenic ⁸⁷Sr/⁸⁶Sr values and gradual decrease in the ⁸⁷Sr/⁸⁶Sr ratios, (2) anomalously low δ⁸⁸Sr values at the lower part cap carbonate, and (3) a clear correlation between ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO₂ content caused sudden precipitation of cap carbonate from the surface seawater with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ⁸⁸Sr ratios, probably caused gradual decrease of the ⁸⁷Sr/⁸⁶Sr ratios of the seawater and deposition of carbonate with normal δ⁸⁸Sr ratios. The combination of ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.  相似文献   

Solar flares observed in microwaves: Recent results from solar radio groups in Japan     

Kosugi  Takeo 《Solar physics》1982,113(1-2):327-332

Recent observational studies on solar flares made by solar radio groups in Japan during the period around the maximum of Cycle 21 are briefly reviewed. Much attention is paid especially to comparison studies of microwave observations with hard X-ray and -ray observations.  相似文献   

Radio and X-ray observations of a multiple impulsive solar burst with high time resolution     

Takeo Kosugi 《Solar physics》1981,71(1):91-105

A well-developed multiple impulsive microwave burst occurred on February 17, 1979 simultaneously with a hard X-ray burst and a large group of type III bursts at metric wavelengths. The whole event is composed of several subgroups of elementary spike bursts. Detailed comparisons between these three classes of emissions with high time resolution of 0.5 s reveal that individual type III bursts coincide in time with corresponding elementary X-ray and microwave spike bursts. It suggests that a non-thermal electron pulse generating a type III spike burst is produced simultaneously with those responsible for the corresponding hard X-ray and microwave spike bursts. The rise and decay characteristic time scales of the elementary spike burst are 1 s, 1 s and 3 s for type III, hard X-ray and microwave emissions respectively. Radio interferometric observations made at 17 GHz reveal that the spatial structure varies from one subgroup to others while it remains unchanged in a subgroup. Spectral evolution of the microwave burst seems to be closely related to the spatial evolution. The spatial evolution together with the spectral evolution suggests that the electron-accelerating region shifts to a different location after it stays at one location for several tens of seconds, duration of a subgroup of elementary spike bursts. We discuss several requirements for a model of the impulsive burst which come out from these observational results, and propose a migrating double-source model.  相似文献   

Carbon isotopes and petrography of kerogens in ∼3.5-Ga hydrothermal silica dikes in the North Pole area, Western Australia     

Yuichiro Ueno  Hideyoshi Yoshioka  Yukio Isozaki 《Geochimica et cosmochimica acta》2004,68(3):573-589

More than 600 specimens of ∼3.5 Ga-old hydrothermal silica dikes from the North Pole area, Pilbara craton, Western Australia, have been studied petrographically. The kerogens in 44 samples have been analyzed isotopically (C and N) and chemically (C, N, and H). The silica dikes are composed mainly of fine-grained silica (modal abundance: >97%) and are classified into two types by minor mineral assemblages: B(black)-type and G(gray)-type. The B-type silica dikes contain kerogen (0.37 to 6.72 mgC/g; average 2.44 mgC/g, n = 21) and disseminated sulfides, dominantly pyrite and Fe-poor sphalerite. In some cases, carbonate and apatite are also present. Their silica-dominated and sulfide-poor mineral assemblages suggest precipitation from low-temperature reducing hydrothermal fluid (likely 100-200°C). On the other hand, the G-type silica dikes are sulfide-free and concentrations of kerogen are relatively low (0.05 to 0.41 mgC/g; average 0.17 mgC/g, n = 13). They typically contain Fe-oxide (mainly hematite) which commonly replaces cubic pyrite and rhombic carbonate. Some G-types occur along secondary quartz veins. These textures indicate that the G-type silica dikes were formed by postdepositional metasomatism (oxidation) of the B-types, and that the B-types probably possess premetasomatic signatures. The δ¹³C values of kerogen in the B-types are −38.1 to −33.1‰ (average −35.9‰, n = 21), which are ∼4‰ lower than those of the G-types (−34.5 to −30.0‰; average −32.2‰, n = 19), and ∼6‰ lower than bedded chert (−31.2 to −29.4‰; average −30.5‰, n = 4). This indicates the preferential loss of ¹²C during the metasomatism (estimated fractionation factor: 0.9985). Considering the metasomatic effect on carbon isotopes with probably minor diagenetic and metamorphic overprints, we conclude that the original δ¹³C values of the kerogen in the silica dikes would have been heterogeneous (∼5‰) and at least some material had initial δ¹³C values of ≤ −38‰. The inferred ¹³C-depletions of organic carbon could have been produced by anaerobic chemoautotrophs such as methanogen, but not by aerobic photoautotrophs. This is consistent with the estimated physical and chemical condition of the hydrothermal fluid, which was probably habitable for anaerobic and thermophilic/hyperthermophilic chemoautotrophs. Alternatively, the organic matter may have been possibly produced by abiological reaction such as Fischer-Tropsch Type (FTT) synthesis under the hydrothermal condition. However, the estimated condition is inconsistent with the presence of the effective catalysts for the FTT reaction (i.e., Fe-Ni alloy, magnetite, and hematite). These lines of evidence suggest the possible existence of biosphere in the ∼3.5 Ga sub-seafloor hydrothermal system.  相似文献   

Observations of current and mixing around the shelf break in Pribilof Canyon in the Bering Sea     

T. Tanaka  I. Yasuda  H. Onishi  H. Ueno  M. Masujima 《Journal of Oceanography》2015,71(1):1-17

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A study on the impact of asymmetric components around tropical cyclone center on the accuracy of bogus data and the track forecast   总被引：6，自引：0，他引：6  

M. Ueno 《Meteorology and Atmospheric Physics》1995,56(1-2):125-134

Summary The Japan Meteorological Agency (JMA) has used a tropical cyclone bogus insertion procedure to produce correctlypositioned, cyclone-like vortices within the initial analyses and to track the vortices throughout the model forecasts. The TC bogus soundings are constructed from a standard axisymmetric vortices for well developed tropical cyclones based on a few manually-analyzed parameters such as storm position, central pressure and radius of gale force wind. Mainly because of such an axi-symmetric property of JMA TC bogus data, which is likely to remove the steering flow from the central core region of TC, all the JMA models have a noticeable slow-start bias error and also northward drifting blas error in TC movement. In order to investigate the impact of asymmetric wind components on the TC track forecast, an experimental analysis-forecast cycle is conducted using the JMA global spectral model, in which asymmetric components extracted from the first guess fields are added to the axisymmetric TC bogus. It is found from the experiment that both the slow-start bias error and northward bias error can be reduced by introducing the asymmetric components into the TC bogus. Besides the impact study, a statistical verification study of the bogus data was also made against real data such as sonde data and superiority of the preparation method of asymmetric components was proved.With 9 Figures  相似文献   

Crustal structure in and around the region of the 1995 Kobe Earthquake deduced from a wide-angle and refraction seismic exploration     

Takeshi Ohmura  Takeo Moriya  Chengshi Piao  Takaya Iwasaki  Toshikastu Yoshi  Shin'ichi Sakai  Tetsuya Takeda  Kaoru Miyashita  Humihito Yamazaki  Kiyoshi Ito  Akira Yamazaki  Yoji Shimada  Katsuya Tashiro  Hiroki Miyamachi 《Island Arc》2001,10(3-4):215-227

Abstract The 1995 Kobe (Hyogo-ken Nanbu) earthquake (M_JMA 7.2, M_w 6.9) occurred on Jan. 17, 1995, at a depth of 17 km, beneath the areas of southern part of Hyogo prefecture and Awaji Island. To investigate P-wave velocity distribution and seismological characteristics in the aftershock area of this great earthquake, a wide-angle and refraction seismic exploration was carried out by the Research Group for Explosion Seismology (RGES) . The profile including 6 shot points and 205 observations was 135 km in length, extending from Keihoku, Northern Kyoto prefecture, through Kobe, to Seidan on Awaji Island. The charge of each shot was 350–700 kg. The P-wave velocity structure model showed a complicated sedimentary layer which is shallower than 2.5 km, a 2.5 km-thick basement layer whose velocity is 5.5 km/s, overlying the crystalline upper crust, and the boundary between the upper and lower crust.
Almost all aftershock hypocenters were located in the upper crust. However, the structure model suggests that the hypocenters of the main shock and some aftershock clusters were situated deeper than the boundary between the upper and lower crust. We found that the P-velocity in the upper crust beneath the northern part of Awaji Island is 5.64 km/s which is 3% lower than that of the surrounding area. The low-velocity zone coincides with the region where the high stress moment release was observed.  相似文献