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311.
312.
Tomoki Nakamura Aiko Nakato Hatsumi Ishida Shigeru Wakita Takaaki Noguchi Michael E. Zolensky Masahiko Tanaka Makoto Kimura Akira Tshuchiyama Toshihiro Ogami Takahito Hashimoto Mitsuru Konno Masayuki Uesugi Toru Yada Kei Shirai Akio Fujimura Ryuji Okazaki Scott. A. Sandford Yukihiro Ishibashi Masanao Abe Tatsuaki Okada Munetaka Ueno Junichiro Kawaguchi 《Meteoritics & planetary science》2014,49(2):215-227
The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed. 相似文献
313.
Takaaki Noguchi Makoto Kimura Takahito Hashimoto Mitsuru Konno Tomoki Nakamura Michael E. Zolensky Ryuji Okazaki Masahiko Tanaka Akira Tsuchiyama Aiko Nakato Toshinori Ogami Hatsumi Ishida Ryosuke Sagae Shinichi Tsujimoto Toru Matsumoto Junya Matsuno Akio Fujimura Masanao Abe Toru Yada Toshifumi Mukai Munetaka Ueno Tatsuaki Okada Kei Shirai Yukihiro Ishibashi 《Meteoritics & planetary science》2014,49(2):188-214
On the basis of observations using Cs‐corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2–3 nm), (2) composite rims (30–60 nm), and (3) composite vesicular rims (60–80 nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two‐layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation‐induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa. 相似文献
314.
Abstract We investigated global axisymmetric (m = 0) and non-axisymmetric (m = 1) modes of magnetic fields generated by the galactic dynamo including the α2-dynamo. The α2-dynamo is responsible for the field generation in the central region of galaxies where the shear of galactic rotation is weak (e.g. M51). The highest growth rate of m = 1 modes is always smaller than that of m = 0 modes; thus m = 1 modes of the standard galactic dynamo cannot explain the dominance of the bisymmetric fields in spiral galaxies. Radial extent of each m = 1 mode is too narrow to reproduce the observed bisymmetric structure extending over a disk. 相似文献
315.
Takako Masuda Ken Furuya Naoko Kohashi Mitsuhide Sato Shigenobu Takeda Makoto Uchiyama Naho Horimoto Takashi Ishimaru 《Journal of Oceanography》2010,66(6):801-813
Phytoplankton dynamics in the lower euphotic zone were observed by tracking a subsurface water released at 20-m depth from
Takumi, an artificial upwelling device. Takumi continually discharged seawater pumped up from a depth of 205 m: this water was mixed with 5-m depth water to adjust the
density to that of 20-m depth water of Sagami Bay, Japan. The discharged water was pulse-labeled at Takumi with uranine and tracked for 63.9 h with a drifting buoy equipped with a drogue at 20-m depth. We present a simple model
to estimate in situ phytoplankton net growth rates from temporal changes in phytoplankton abundance in the discharged water with correction for
the influence of water exchange between the discharged water and neighboring layers. Lagrangian observation showed active
growth of pico- and nanophytoplankton, especially cryptophytes and Synechococcus (Cyanobacteria), in the subsurface layer. In contrast, diatoms grew little in spite of micromolar concentrations of nutrients.
The active growth of pico- and nanophytoplankton was in good agreement with shipboard serial dilution culture experiments.
The low growth activity of diatoms was suggested to be related to low light availability in the subsurface layer. 相似文献
316.
Simon T. Belt Guillaume Mass Lindsay L. Vare Steven J. Rowland Michel Poulin Marie-Alexandrine Sicre Makoto Sampei Louis Fortier 《Marine Chemistry》2008,112(3-4):158-167
A C25 highly branched isoprenoid (HBI) monoene hydrocarbon, designated IP25, has been proposed previously to originate from diatoms living in Arctic sea ice, while the presence of IP25 in sediments has been suggested to be a proxy for the occurrence of former Arctic sea ice. Here, we show that the 13C isotopic composition of IP25 in sea ice, in sediment trap material collected under sea ice, and in high latitude northern sediments, is distinctive (isotopically ‘heavy’) and distinguishable from that of organic matter of planktonic or terrigenous origin. Mean δ13C values for IP25 were − 22.3 ± 0.4‰ (sea ice), − 19.6 ± 1.1‰ (sediment traps) and − 19.3 ± 2.3‰ (sediments). These measurements, therefore, support further the proposed use of IP25 as an Arctic sea ice proxy. 相似文献
317.
Late Cretaceous uplift history of the Cretaceous volcanic arc in Southwest Japan: Provenance analysis of the Yuasa–Aridagawa basin based on U–Pb zircon ages 下载免费PDF全文
The Ryoke Metamorphic complex has undergone low‐P/T metamorphism and was intruded by granitic magmas around 100 Ma. Subsequently, the belt was uplifted and exposed by the time deposition of the Izumi Group began. The tectonic history of uplift, such as the timing and processes, are poorly known despite being important for understanding the spatiotemporal evolution of the Ryoke Metamorphic Belt. U–Pb zircon ages from sedimentary rocks in the forearc and backarc basins are useful for constraining uplift and magmatism in the provenance. U–Pb dating of detrital zircons from 12 samples (four sandstones and eight granitic clasts) in the Yuasa–Aridagawa basin, a Cretaceous forearc basin in the Chichibu Belt of Southwest Japan, gave mostly ages of 60–110 Ma. Granitic clasts contained in conglomerate suggest that granitic intrusions predate the formation of Coniacian and Maastrichtian conglomerate. Emplacement ages of granitic bodies originated from granitic clasts in Coniacian conglomerate are (110.2 ±1.3) Ma, (106.1 ±1.8) Ma, (101.8+5.8–3.8) Ma, and (95.3 ±1.4) Ma; for granitic clasts in Maastrichtian conglomerate, (89.6 ±1.8) Ma, (87.3+2.4–1.8) Ma, (85.7 ±1.2) Ma, and (82.7 ±1.2) Ma. The results suggest that detrital zircons in the sandstones were mainly derived from volcanic eruptions contemporaneous with depositional age, and plutonic rocks of the Ryoke Metamorphic Belt. Zircon ages of the granitic clast samples also indicate that uplift in the provenance began after Albian and occurred at least during the Coniacian to Maastrichtian. Our results, together with the difference of provenance between backarc and forearc basins suggest that the southern marginal zone of the Ryoke Metamorphic Belt was uplifted and supplied a large amount of clastic materials to the forearc basins during the Late Cretaceous. 相似文献
318.
Temporal stress variations along a seismogenic megasplay fault in the subduction zone: An example from the Nobeoka Thrust,southwestern Japan 下载免费PDF全文
Ryoji Kawasaki Yoshitaka Hashimoto Makoto Otsubo Asuka Yamaguchi Yujin Kitamura Jun Kameda Yohei Hamada Rina Fukuchi Gaku Kimura 《Island Arc》2017,26(3)
The Nobeoka Thrust, an ancient megasplay fault in the Shimanto Belt, southwestern Japan, contains fault rocks from the seismogenic zone, providing an accessible analog of active megasplay faults in deep subduction settings. In this study, the paleostress along the Nobeoka Thrust was analyzed using multiple inversion techniques, including k‐means clustering of fault datasets acquired from drillcores that intersected the thrust. The six resultant stress orientation clusters can be divided into two general groups: stress solutions with north–south‐trending σ1 axes, and those with east–west‐trending σ1 axes. These groups are characterized by the temporal changes for the orientations of the σ1 and σ3 principal stress axes that involve alternation between horizontal and vertical. The findings are probably due to a change in stress state before and after earthquakes that occurred on the fault; similar changes have been observed in active tectonic settings, such as the 2011 Tohoku‐Oki earthquake (Japan). 相似文献
319.
Evolution of crustal deformation in the northeast–central Japanese island arc: Insights from fault activity 下载免费PDF全文
We evaluated fault activity in northeast–central Japan based on fault orientation, regional stress field, and slip tendency analysis for active and non‐active faults (i.e. faults for which Quaternary activity has not been identified). Slip tendency is generally higher along active faults than non‐active faults, although a high slip tendency was observed along some non‐active faults, indicating their potential to become active. The potential for fault activity along non‐active faults can be modeled using the temporal evolution from non‐active to active during long‐term crustal deformation. The density of potentially active faults varies spatially across the study areas and reflects the temporal evolution of crustal deformation in northeast–central Japan. 相似文献
320.
Stemflow was evaluated in a water balance and its contribution to groundwater recharge determined. Gross precipitation, throughfall and stemflow were measured for one year in a pine forest (Tsukuba, Japan) to determine each component of the water balance in the forest. Groundwater recharge rates by stemflow and throughfall were calculated from a mass balance method using chloride in subsurface waters. The stemflow in the water balance was relatively small when estimated as a value per canopy projected area of the tree in the forest. However, the results for the mass balance of chloride in subsurface waters indicated that it was impossible to disregard the stemflow in determining groundwater recharge. Although the ratio of stemflow to the net precipitation was small in the water balance, the effect of stemflow on groundwater recharge was relatively large. 相似文献