Precipitation of authigenic uranium in suboxic continental margin sediments from the Okinawa Trough   总被引：1，自引：0，他引：1  

Masatoshi Yamada  Zhong-Liang Wang  Yoshihisa Kato 《Estuarine, Coastal and Shelf Science》2006,66(3-4):570-579

Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. ²³⁸U concentrations and ²³⁸U/²³²Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, ²³⁸U and ²³⁸U/²³²Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of ²³⁰Th and ²³²Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm⁻² yr⁻¹; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink.  相似文献   

Isotope dilution sector-field inductively coupled plasma mass spectrometry combined with extraction chromatography for rapid determination of 241Am in marine sediment samples: A case study in Sagami Bay, Japan     

Jian Zheng  Masatoshi Yamada 《Journal of Oceanography》2008,64(4):541-550

²⁴¹Am is a useful tracer for understanding biogeochemical processes in the marine environment. ²⁴¹Am also poses a potential radiation threat to human health due to the continuous increase of its concentration in the global environment. We report a rapid analytical method for determining ²⁴¹Am in marine sediments using isotope dilution sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) combined with a high-efficiency sample introduction system (APEX-Q). A selective CaF₂ co-precipitation procedure followed by TRU extraction chromatography was employed to effectively remove the major sediment matrix and to pre-concentrate ²⁴¹Am. We achieved an extremely low detection limit of 0.32 fg/g or 0.041 mBq/g (for 1 gram sediment), which is better than that of alpha spectrometry, and it allowed the accurate determination of ²⁴¹Am in low-level marine sediment samples. The accuracy and precision of the developed analytical method was evaluated using a laboratory prepared Am isotope standard solution and Ocean Sediment reference material (IAEA-368). The results were satisfactory. For sediment samples, overall chemical recoveries varied from 60–90%. The developed method was applied to the study of ²⁴¹Am depth distribution in Sagami Bay, Japan, where we observed different depth profiles between ²⁴¹Am activity and ^239+240Pu activity.  相似文献   

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay,Japan     

Shuro?YoshikawaEmail author  Toshiya?Kanamatsu  Kazuhisa?Goto  Izumi?Sakamoto  Masatoshi?Yagi  Mikio?Fujimaki  Riichirou?Imura  Kenji?Nemoto  Hide?Sakaguchi 《Geo-Marine Letters》2015,35(4):315-328

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.  相似文献   

Space–time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench     

I. Kawasaki  Y. Asai  Y. Tamura 《Tectonophysics》2001,330(3-4):267-283

Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×10²⁰ N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×10²⁰ N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×10²⁰ N m, which was 20–35% of the accumulated moment (18.6–23.0)×10²⁰ N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×10²⁰ N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (10² s) for the 1968 Tokachi-oki earthquake to 10–20 min (10³ s) for the 1896 Sanriku tsunami earthquake, to one day (10⁵ s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (10⁷ s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity.  相似文献   

Middle Permian cephalopods from central Peninsular Malaysia: implications for faunal migration through the southern Tethys     

Masatoshi Sone  Mohd Shafeea Leman  Masayuki Ehiro 《Journal of Asian Earth Sciences》2001,19(6)

A Wordian (Middle Permian) cephalopod fauna consisting of four ammonoid species, Tauroceras aff. scrobiculatum (Gemmellaro), Agathiceras sp., Bamyaniceras orientale n. sp. and Pronoritidae gen. and sp. indet., and two nautiloid species, Tainoceras sp. and Orthocerida fam. indet., were recovered from the Bera South area, southern Pahang, Peninsular Malaysia. The outcrop represents the southernmost extent of the Bera Formation. Bamyaniceras and Tainoceras are recorded in Malaysia for the first time. The presence of T. aff. scrobiculatum suggests a regional correlation with an ammonoid-bearing bed of Sungai Cheroh, western Pahang, and permits global correlations with strata of northeastern Iraq, northern Oman and Sicily (Italy). A possible eastwards faunal migration through the southern Tethys is suggested.  相似文献   

Scattering of waves in elastic media containing passive and active cracks     

Masatoshi Miyazawa  Ichiro Nakanishi 《Geophysical Journal International》2006,165(2):658-667

Scattering of wavefields in a 3-D medium that includes passive and/or active structures, is numerically solved by using the boundary integral equation method (BIEM). The passive structures are velocity anomalies that generate scattered waves upon incidence, and the active structures contain endogenous fracture sources, which are dynamically triggered by the dynamic load due to the incident waves. Simple models are adopted to represent these structures: passive cracks act as scatterers and active cracks as fracture sources. We form cracks using circular boundaries, which consist of many boundary elements. Scattering of elastic waves by the boundaries of passive cracks is treated as an exterior problem in BIEM. In the case of active cracks, both the exterior and interior problems need to be solved, because elastic waves are generated by fracturing with stress drop, and the growing crack boundaries scatter the incident waves from the outside of the cracks. The passive cracks and/or active cracks are randomly distributed in an infinite homogeneous elastic medium. Calculations of the complete waveform considering a single scatter show that the active crack has weak influence on the attenuation of first arrivals but strong influence on the amplitudes of coda waves, as compared with those due to the passive crack. In the active structures, multiple scattering between cracks and the waves triggered by fracturing strongly affect the amplitudes of first arrivals and coda waves. Compared to the case of the passive structures, the attenuation of initial phase is weak and the coda amplitudes decrease slowly.  相似文献   

BSMILES - a balloon-borne superconducting submillimeter-wave limb-emission sounder for stratospheric measurements     

Irimajiri  Y. Manabe  T. Ochiai  S. Masuko  H. Yamagami  T. Saito  Y. Izutsu  N. Kawasaki  T. Namiki  M. Murata  I. 《Geoscience and Remote Sensing Letters, IEEE》2006,3(1):88-92

A balloon-borne superconducting submillimeter-wave limb-emission sounder (BSMILES) was developed to observe thermal emission lines from stratospheric minor constituents. BSMILES carries a 300-mm-diameter offset parabolic antenna, a 624-639-GHz superconductor-insulator-superconductor (SIS) receiver, a three-axis fiber-optical gyroscope, and an acousto-optical spectrometer. BSMILES was launched from the Pacific Coast of Japan. All systems operated properly and emission line spectra of stratospheric gases, such as O/sub 3/, HCl, HO/sub 2/, and O/sub 3/ isotopes were measured. The system noise temperature in double sideband (DSB) during the flight was less than 460 K over the observing bandwidth with a best value of 330 K that is 11 times as large as the quantum limit (11h/spl nu//k/sub B/). After the observation, the gondola splashed down in the Pacific Ocean and was retrieved. Almost all instruments were waterproofed, and it has been proved that they are reusable.  相似文献   

Eccentric instability of a two-dimensional fluid disc with a central massive object     

Masatoshi Taga  & Masanori Iye 《Monthly notices of the Royal Astronomical Society》1998,299(4):1132-1138

In this paper we investigate, by linear modal analysis, the one-armed dynamical instability of a two-dimensional fluid disc that has a massive object at its centre. The model of the disc is chosen to avoid the artificial instabilities that originate from the unrealistic disc configurations that have been adopted in previous studies. We find a one-armed instability for which the central massive object is displaced from the centre, which is generally called the 'eccentric instability'. However, to excite the eccentric instability, the mass of the central object should be appreciably smaller than that of the disc, and this mass ratio is far smaller than what was originally proposed. The instability shown in this paper is likely to be excited in a stellar system with a central massive object, e.g. a galactic nucleus harbouring a massive black hole, and further studies are desirable via techniques such as numerical simulations.  相似文献   

Experimental Study of Fe-Mg Exchange Reaction Between Orthopyroxene and Sapphirine and its Calibration as a Geothermometer     

Toshisuke Kawasaki  Kei Sato 《Gondwana Research》2002,5(4):741-747

The exchange of Fe²⁺ and Mg²⁺ between orthopyroxene and sapphirine has been investigated at pressures 7-16 kbar and temperatures 850-1300°C using a piston cylinder apparatus for the synthetic and natural systems. This reaction is temperature-dependent and would be a good geothermometer. The equilibration temperature T is approximately expressed by the distribution coefficient as follows: T(°C)=1515/(ln K_D+0.943)-273.

This empirical equation was applied to some Antarctic granulites and associated rocks. The new orthopyroxene-sapphirine geothermometer gives consistent results with those estimated from the Fe-Mg orthopyroxene-garnet geothermometer.  相似文献   

Climatology of yellow sand (Asian sand, Asian dust or Kosa)in East Asia     

Masatoshi Yoshino 《中国科学D辑(英文版)》2002,45(Z1)

In order to study climatology of yellow sand (Asian sand, Asian dust or Kosa) in EastAsia, secular fluctuation in China, Korea and Japan in the recent 30 years was presented. Thenumber of days with sand-dust storm at five stations in China-Hotan, Zhangye, Minqin, Jurhand Beijing, decreases a lot at the former three stations, but changed little at the latter two stations.Suggesting that the recent global warming is more evident in Xinjiang and Gansu, where the fre-quency of cold air invasions from the higher latitudes is decreasing. But, the eastern parts ofMongolia, inner Mongolia, and North China encounter stronger cyclones in early spring as a resultof global warming. These cyclones bring cold air from higher latitudes, causing severe duststorms. Secular variation in the annual days with sand-dust storms in China and Kosa days in Ko-rea and Japan show a parallel change with higher frequency from 1975 to 1985. This may be re-lated to the higher frequency of La Nina years. However, different tendency was shown in theperiod from 1986 to 1996. Since 1996 or 1997, a sharp increase is clear, which may be caused bythe developed cyclones in East Asia as well as human activities, and stronger land degradationunder La Nina conditions. Anomalies of the total number of stations with Kosa days were dis-cussed in accordance with some synoptic meteorological conditions such as the differences be-tween Siberian anticyclone and Aleutian cyclone center at 500 hPa level during the previous winter.  相似文献