Isotopic tracers for water masses in the coastal region of eastern Hokkaido     

Sachiko Oguma  Tsuneo Ono  Akira Kusaka  Hiromi Kasai  Yasuhiro Kawasaki  Tomonori Azumaya 《Journal of Oceanography》2008,64(4):525-539

In this study we used two stable isotopes, δ¹³C and δ¹⁸O, for water mass classification in the coastal region off eastern Hokkaido. δ¹³C* values, which were corrected for the biological effect, and δ ¹⁸O values up to 300 m depth suggested that the isotopic character of the onshore and offshore water in the southern Okhotsk Sea, the Nemuro Strait and the western North Pacific could be explained by the mixing of three source waters: the Oyashio water (OYW), Soya Warm Current water (SWCW) and East Sakhalin Current water (ESCW). In summer, δ ¹³C*-δ ¹⁸O plots indicated mixing between SWCW from the southern Okhotsk Sea and OYW in the Pacific coast of southeastern Hokkaido, while temperature-salinity plots of the onshore water showed minimal difference from the offshore OYW. In winter, on the other hand, the mixed water of ESCW and OYW (or SWCW) appeared in the Pacific coastal region, distributed as cold, low salinity onshore water. Finally, we estimated mixing ratios of OYW, SWCW and ESCW in the coastal region of western North Pacific using their mean values of δ ¹³C* and δ ¹⁸O as endmembers. These results suggest seasonal and yearly changes of water mass combination en route from the southern Okhotsk Sea to the western North Pacific.  相似文献   

Velocity of finer fragments from impact     

Akiko M. Nakamura  Akira Fujiwara  Toshihiko Kadono   《Planetary and Space Science》1994,42(12):1043-1052

We describe the method and the result of a new experiment to obtain velocity distribution of fine ejecta fragments, from a few to a hundred microns in size, produced from basalt targets by impacts of nylon projectiles at a velocity of 3.7 km s⁻¹. The size distribution of holes perforated by the ejecta fragments on thin films and foils placed around the targets was investigated, and the size-velocity relation was determined with the aid of an empirical formula for threshold penetration (McDonnell and Sullivan, Hypervelocity Impacts in Space, Unit for Space Sciences, University of Kent, 1992). The velocity of the fastest fragments, at a given size, is from the extrapolation of the size-velocity relation for 1–100 mm fragments (Nakamura and Fujiwara, Icarus92, 132–146, 1991; Nakamura et al, Icarus100, 127–135, 1992). The laboratory results are also compared with those obtained from the study of secondary craters around large lunar craters (Vickery, Icarus67, 224–236, 1986, Geophys. Res. Lett. 14, 726–729, 1987). All these data provide a smooth size-velocity relationship in the normalized fragment size range of four orders of magnitude.  相似文献   

Textural variations and impact history of the Millbillillie eucrite     

Akira Yamaguchi  Hiroshi Takeda  Donald D. Bogard  Daniel Garrison 《Meteoritics & planetary science》1994,29(2):237-245

Abstract— We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microanalyser and scanning electron microscope. Although originally described as having fine-grained texture, the new specimens show diversity of texture. The compositions (Mg/Fe ratios) of the host pigeonites and augite lamellae are homogeneous, respectively, in spite of the textural variation. In addition to their chemical homogeneity, pyroxenes in coarse and fine-grained clasts are partly inverted to orthopyroxene. Chemical zoning of plagioclase during crystal growth is preserved. This eucrite includes areas of granulitic breccias and impact melts. Large scale textures show a subparallel layering suggesting incomplete mixing and deposition of impact melt and lithic fragments. An ³⁹Ar-⁴⁰Ar age determination for a coarse-grained clast indicates a strong degassing event at 3.55 ± 0.02 Ga. We conclude that Millbillillie is among the most equilibrated eucrites produced by thermal annealing after impact brecciation. According to the classification of impact breccias, Millbillillie can be classified as a mixture of granulitic breccias and impact melts. The last significant thermal event is characterized by network-like glassy veins that run through clasts and matrices. Consideration of textural observations and requirements for Ar-degassing suggests that the ³⁹Ar-⁴⁰Ar age could in principle date either the earilier brecciation and annealing event or the event which produced the veins.  相似文献   

Plume-type Ophiolites, Large Igneous Provinces and Geology of the Earth-like Planets     

ISHIWATARI Akira 《《地质学报》英文版》2015,89(Z2):33-34

<正>Plume-type is a new branch of ophiolite classification introduced by Dilek and Furnes(2011;GSAB,123,387-).Its most typical example is the komatiite-basalt-gabbrowehrlite assemblage that is exposed on the Gorgona Island off Colombia,South America and is interpreted as a part of the Caribbean large igneous province(LIP).Analogous  相似文献   

An anomalous eucrite,Dhofar 007, and a possible genetic relationship with mesosiderites     

Akira Yamaguchi  Takehiko Setoyanagi  Mitsuru Ebihara 《Meteoritics & planetary science》2006,41(6):863-874

Abstract— We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse‐grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg‐rich pyroxene. The coarse‐grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse‐grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two‐stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 °C/yr at ˜1000 °C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite‐taenite boundaries indicates that the cooling rate was very slow at lower temperatures (˜1–10 °C/Myr at <600–700 °C). The slow cooling rate is comparable to those in mesosiderites and pallasites. The two‐stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that Dhofar 007 is a large inclusion of mesosiderite. However, we cannot rule out a possibility that Dhofar 007 is an anomalous eucrite.  相似文献   

The role of sticky interstellar organic material in the formation of asteroids     

T. Kudo  A. Kouchi  M. Arakawa  H. Nakano 《Meteoritics & planetary science》2002,37(12):1975-1983

Abstract— Collision experiments and measurements of viscoelastic properties were performed involving an interstellar organic material analogue to investigate the growth of organic grains in the protosolar nebula. The organic material was found to be stickiest at a radius of between 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m s^?1 for millimeter‐size organic grains. This stickiness is considered to have resulted in the very rapid coagulation of organic grain aggregates and subsequent formation of planetesimals in the early stage of the turbulent accretion disk. The planetesimals formed in this region appear to be represent achondrite parent bodies. In contrast, the formation of planetesimals at <2.1 and >3.0 AU begins with the establishment of a passive disk because silicate and ice grains are not as sticky as organic grains.  相似文献   

Eagle Nebula Pillars: From Models to Observations     

Marc W. Pound  Jave O. Kane  Bruce A. Remington  Dmitri D. Ryutov  Akira Mizuta  Hideaki Takabe 《Astrophysics and Space Science》2005,298(1-2):177-181

Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars. The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one.  相似文献   

Periodic ab initio Hartree-Fock study of trigonal and orthorhombic phases of boric oxides     

Akira Takada  C. R. A. Catlow  G. D. Price  C. L. Hayward 《Physics and Chemistry of Minerals》1997,24(6):423-431

The structure and electronic properties of trigonal and orthorhombic boric oxide (B₂O₃) are studied using periodic ab initio Hartree-Fock method. The optimised structural parameters for two B₂O₃ polymorphs are in good agreement with experimental data. The analyses of their electronic structures provide insights into the chemical nature of the B–O bond and the way in which it changes with the coordination number around boron and oxygen. Our quantum-chemical study suggests that the orthorhombic form is more ionic than the trigonal form and that the coordination number of boron around oxygen plays a more dominant role than that of oxygen around boron in B₂O₃ crystals.  相似文献   

Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H,and its bearing on redox states of chondrites     

Akira Tsuchiyama  Masao Kitamura 《Meteoritics & planetary science》1995,30(4):423-429

Abstract— Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H under H-rich conditions (～700–2000 K and 10^?2–10^?20 atm of Ph ₂), including solar nebula conditions, were constructed based on thermochemical calculations. Boundaries of vaporous phases, which are the first phases to condense from a gas, can be obtained without calculating condensation temperatures of individual gas compositions because the numbers of major gaseous species are the same as those of components in the concerned systems. Fractionations by condensation and/or evaporation can be discussed easily in such phase diagrams. A thermal divide, which is a barrier that vapors cannot cross by a single cooling process, was recognized in the phase diagrams. This is present on the Fe-MgO-SiO₂-CO-H plane at high temperatures (≥500–700 K) and plays an important role in fractionations. Oxidizing states of ordinary chondrites and carbonaceous chondrites before aqueous alteration are located at the O-rich side of the thermal divide. Such oxidizing states can be formed from the solar gas by fractionation in the primordial solar nebula because the solar composition is located on the O-rich side. On the other hand, the reducing states of enstatite chondrites, located at the O-poor side, cannot be formed as long as the thermal divide is present. The reducing states can be obtained by CO to CH₄ molecular reaction at low temperatures (≤500–700 K), where the high-temperature thermal divide is absent. Addition of H₂O-rich and CH₄-rich ice can explain establishment of the redox states of ordinary and enstatite chondrites, respectively.  相似文献   

Microbial responses using denaturing gradient gel electrophoresis to oil and chemical dispersant in enclosed ecosystems     

Yoshida A  Nomura H  Toyoda K  Nishino T  Seo Y  Yamada M  Nishimura M  Wada M  Okamoto K  Shibata A  Takada H  Kogure K  Ohwada K 《Marine pollution bulletin》2006,52(1):89-95

Microbial responses to the addition of oil with or without a chemical dispersant were examined in mesocosm and microcosm experiments by using denaturing gradient gel electrophoresis of bacterial ribosomal DNA and direct cell counting. When a water-soluble fraction of oil was added to seawater, increases in cell density were observed in the first 24h, followed by a decrease in abundance and a change in bacterial species composition. After addition of an oil-dispersant mixture, increases in cell density and changes in community structure coincided, and the amount of bacteria remained high. These phenomena also occurred in response to addition of only dispersant. Our results suggest that the chemical dispersant may be used as a nutrient source by some bacterial groups and may directly or indirectly prevent the growth of other bacterial groups.  相似文献