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排序方式: 共有93条查询结果,搜索用时 78 毫秒
31.
Koki Aizawa Yasuo Ogawa Masaaki Mishina Kosuke Takahashi Shintaro Nagaoka Nobumasa Takagi Shin'ya Sakanaka Takuya Miura 《Journal of Volcanology and Geothermal Research》2009,187(1-2):131-139
Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200 °C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200 °C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes. 相似文献
32.
A tripolar anomaly pattern with centers located around the Philippines, China/Japan, and East Siberia dominantly appears in climate variations of the East Asian summer monsoon. In this study, we extracted this pattern as the first mode of a singular value decomposition (SVD1) over East Asia. The squared covariance fraction of SVD1 was 59?%, indicating that this pattern can be considered a dominant pattern of climate variations. Moreover, the results of numerical experiments suggested that the structure is also a dominant pattern of linear responses, even if external forcing is distributed homogeneously over the Northern Hemisphere. Thus, the tripolar pattern can be considered an internal mode that is characterized by the internal atmospheric processes. In this pattern, the moist processes strengthen the circulation anomalies, the dynamical energy conversion supplies energy to the anomalies, and the Rossby waves propagate northward in the lower troposphere and southeastward in the upper troposphere. These processes are favorable for the pattern to have large amplitude and to influence a large area. 相似文献
33.
Assessing the role of forests in mitigating eutrophication downstream of pasture during spring snowmelt 下载免费PDF全文
Masaaki Chiwa Sachiko Inoue Naoaki Tashiro Daisuke Ohgi Yoshitoshi Uehara Hideaki Shibata Atsushi Kume 《水文研究》2015,29(4):615-623
Little research has examined whether forests reduce stream water eutrophication in agricultural areas during spring snowmelt periods. This study evaluated the role of forests in ameliorating deteriorated stream water quality in agricultural areas, including pasture, during snowmelt periods. Temporal variation in stream water quality at a mixed land‐use basin (565 ha: pasture 13%, forestry 87%), northern Japan, was monitored for 7 years. Synoptic stream water sampling was also conducted at 16 sites across a wide range of forest and agricultural areas in a basin (18.3 km2) in spring, summer and fall. Atmospheric nitrogen (N) and phosphorus (P) deposition were measured for 4 years. The results showed that concentration pulses of nitrate, organic N and total P in stream water were observed when discharge increased during spring snowmelt. Their concentrations were high when silicate concentrations were low, suggesting surface water exported from pasture largely contributed to stream water pollution during snowmelt. Atmospheric N and P deposition (4.1 kg N ha?1 y?1; 0.09 kg P ha?1 y?1, respectively) was too low to affect the background concentrations of N and P in streams from forested areas. Reduction of eutrophication caused by nutrients from pasture was mainly due to dilution by water containing low concentrations of N and P exported from forested areas, whereas in‐stream reduction was not a dominant process. Results indicate that forests have a limited capacity to reduce the concentration pulses of N and P in stream water during snowmelt in this study basin. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
34.
Basalt in the Furutobe District of the Kuroko mine area in Japan is characterized by abundant chlorite and epidote. Fluid inclusion studies indicate that chlorite is formed at lower temperatures (230–250°C) than epidote (250–280°C). The seawater/basalt mass ratio for the early chlorite-rich alteration was high (max. 40), but that for the later alteration was low (0.1–1.8). The CaO, Na2O and SiO2 of the bulk rock correlate negatively with MgO, while FeO and Σ Fe correlate positively with MgO. These changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid-ocean ridge geothermal system accompanying basalt alteration.The MgO/FeO ratios of chlorite and actinolite and the Fe2O3 concentration of epidote from the basalt are greater than those of mid-ocean ridge basalt probably owing to the differences in the Fe2O3/FeO and MgO/FeO ratios of the parent rocks. The lower CaO concentration and the higher Na2O concentration of the bulk rock compared with altered mid-ocean ridge basalt can be interpreted in terms of the difference in original bulk rock compositions.The Furutobe basalt, as well as other submarine back arc basalts, contains more vesicles filled with hydrothermal minerals (epidote, calcite, quartz, chlorite, pyrite) than do the mid-ocean ridge basalts. The abundance of vesicles plays an important role in controlling the secondary mineralogy and geochemistry of hydrothermally altered submarine back arc basin basalts. 相似文献
35.
Heavily shocked meteorites contain various types of high‐pressure polymorphs of major minerals (olivine, pyroxene, feldspar, and quartz) and accessory minerals (chromite and Ca phosphate). These high‐pressure minerals are micron to submicron sized and occur within and in the vicinity of shock‐induced melt veins and melt pockets in chondrites and lunar, howardite–eucrite–diogenite (HED), and Martian meteorites. Their occurrence suggests two types of formation mechanisms (1) solid‐state high‐pressure transformation of the host‐rock minerals into monomineralic polycrystalline aggregates, and (2) crystallization of chondritic or monomineralic melts under high pressure. Based on experimentally determined phase relations, their formation pressures are limited to the pressure range up to ~25 GPa. Textural, crystallographic, and chemical characteristics of high‐pressure minerals provide clues about the impact events of meteorite parent bodies, including their size and mutual collision velocities and about the mineralogy of deep planetary interiors. The aim of this article is to review and summarize the findings on natural high‐pressure minerals in shocked meteorites that have been reported over the past 50 years. 相似文献
36.
In order to examine temporal variations of the surface oceanic and atmospheric fCO2 and the DIC concentration, we analyzed air and seawater samples collected during the period May 1992–June 1996 in the northwestern
North Pacific, about 30 km off the coast of the main island of Japan. The atmospheric CO2 concentration has increased secularly at a rate of 1.9 ppmv yr−1, and it showed a clear seasonal cycle with a maximum in spring and a minimum late in summer, produced mainly by seasonally-dependent
terrestrial biospheric activities. DIC also showed a prominent seasonal cycle in the surface ocean; the minimum and maximum
values of the cycle appeared in early fall and in early spring, respectively, due primarily to the seasonally-dependent activities
of marine biota and partly to the vertical mixing of seawater and the coastal upwelling. The oceanic fCO2 values were almost always lower than those of the atmospheric fCO2, suggesting that this area of the ocean acts as a sink for atmospheric CO2. Values varied seasonally, mainly reflecting seasonal changes of SST and DIC, with a secular increase at a rate of 3.7 μatm
yr−1. The average values of the annual net CO2 flux between the ocean and the atmosphere calculated by using the different bulk equations ranged between −0.8 and −1.7 mol
m−2yr−1, and its magnitude was enhanced and reduced late in spring and mid-summer, respectively, due mainly to the seasonally varying
oceanic fCO2. 相似文献
37.
Volume transport of the Soya Warm Current revealed by bottom-mounted ADCP and ocean-radar measurement 总被引:2,自引:2,他引:0
Yasushi Fukamachi Iori Tanaka Kay I. Ohshima Naoto Ebuchi Genta Mizuta Hideo Yoshida Shiro Takayanagi Masaaki Wakatsuchi 《Journal of Oceanography》2008,64(3):385-392
The vertical structure of the Soya Warm Current (SWC) was observed by a bottom-mounted acoustic Doppler current profiler (ADCP)
in the region of the SWC axis near the Soya Strait during a 1-year period from May 2004. The ADCP data revealed a marked seasonal
variability in the vertical structure, with positive (negative) vertical shear in summer and fall (winter and spring). The
volume transport of the SWC is estimated on the basis of both the vertical structure observed by the ADCP and horizontal structure
observed by the ocean radars near the strait. The transport estimates have a minimum in winter and a maximum in fall, with
the yearly-averaged values in the range of 0.94–1.04 Sv (1 Sv = 106 m3 s−1). These lie within a reasonable range in comparison to those through other straits in the Japan Sea. 相似文献
38.
Multivariate analyses applied to Pleistocene bivalve assemblages from the Oga Peninsula (northern Japan) discriminate three distinct assemblages. The assemblages and their taphonomy were used to recognize environmental settings and changes. The Astarte–Cyclocardia–Glycymeris assemblage indicates shelf environment (below the storm wave base) where gravels and shells are transported from shallower settings. Supply of the exotic coarse sediment probably enabled epifaunal bivalves to inhabit the sea floor. The Glycymeris assemblage is characterized by dominance of G. yessoensis and represents current-swept shoreface environment (above the storm wave base). The Moerella assemblage is characterized by bivalves inhabiting bay to open-marine conditions and diverse deposit-feeders, indicating a moderately land-locked environment, such as an open bay or a bay mouth. Fine-grained substrata rich in organic matters in the bay were probably suitable for the deposit-feeders. Ordination also shows the assemblages along two environmental gradients, a bathymetrical one and the other related to open-marine and bay conditions. The environmental changes are explained mainly by glacio-eustatic sea-level changes and alternation of coastal geomorphology caused by local crustal movements. This study also suggests that fossil assemblages can be a powerful tool to reconstruct environments and depositional dynamics even in intensely bioturbated sedimentary facies. 相似文献
39.
Shun Yukawa Yustiawati M. Suhaemi Syawal Kenichi Kobayashi Toshiyuki Hosokawa Takeshi Saito Shunitz Tanaka Masaaki Kurasaki 《Environmental Earth Sciences》2014,71(4):1945-1952
The pollution of water by heavy metals is one of the most serious problems in the developing countries, where watercourses play important roles in transport and economic activities. The aim of this study was to examine whether Hyposarcus pardalis, a fish species widespread in the freshwater environment in Indonesia, could be used as a biomarker for environmental pollution by metals. To this effect, the concentrations of metallothioneins and metals in the livers and kidneys of H. pardalis were measured. In addition, to clarify the relationship between metallothionein concentrations and metal exposure, the concentrations of metallothioneins and metals were determined in the liver and the kidney of fish exposed to 50 and 500 ppb Cu and 500 ppb Mn, compared with those kept in clean water. Sufficient concentrations of metallothionein were detected in fish captured from Lake Rawakalong located in an industrial area in the suburbs of Jakarta. The results of exposure experiments suggested that H. pardalis retained a history of pollution in its organs for a long duration, and the metals bound to metallothioneins in the liver and kidney could be replaced with Cu following exposure. In conclusion, the hepatic and renal metallothioneins in H. pardalis are a useful candidate biomarker for monitoring heavy metal contamination. 相似文献
40.