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21.
Overview of the MITI Nankai Trough Wells: A Milestone in the Evaluation of Methane Hydrate Resources 总被引:4,自引:0,他引:4
Yoshihiro Tsuji Hisashi Ishida Masaru Nakamizu Ryo Matsumoto Satoshi Shimizu 《Resource Geology》2004,54(1):3-10
Abstract. Bottom-simulating reflectors suggestive of the presence of methane hydrates are widely distributed below the ocean floor around Japan. In late 1999, drilling of the MITI Nankai Trough wells was conducted to explore this potential methane hydrate resource and a Tertiary conventional structure. The wells are located in the Northwest Pacific Ocean off Central Japan at a water depth of 945 m. A total of six wells were drilled, including the main well, two pilot wells, and three post survey wells at intervals of 10–100 m. All wells except the first confirmed the occurrence of hydrates based on logging-while-drilling, wire-line logging and/or coring using a pressure and temperature coring system in addition to conventional methods. Based on the various well profiles, four methane hydrate-bearing sand-rich intervals in turbidite fan deposits were recognized. Methane hydrates fill the pore spaces in these deposits, reaching saturation of up to 80 % in some layers. The methane hydrate-bearing turbiditic sand layers are less than 1 m thick, with a total thickness of 12–14 m. The bottom depth of high hydrate concentration correlates well with the depth of the bottom-simulating reflector. Based on these exploration results, the Japanese government inaugurated a 16-year methane hydrate exploitation program in 2001. 相似文献
22.
Size-separated aerosol number concentrations and water-soluble constituents were measured in Toyama, the Hokuriku district, near the coast of the Japan Sea, during the spring and summer in 2003. The number concentrations of coarse particles were significantly high in April, which was due to Asian dust events called Kosa in Japanese. Particulate nssCa2+, which is mostly present in the coarse-mode particles, was significantly high in April. On the other hand, the concentrations of NH4+ and nssSO42−, which mainly exist as the accumulation-mode particles were not high in April. The mass-size distributions of water-soluble constituents were compared with the size-separated number concentrations of particles. Backward trajectory analysis was also employed to examine the transport process of the air mass in Toyama. 相似文献
23.
Magnetic and electric field observations during the 2000 activity of Miyake-jima volcano, Central Japan 总被引:1,自引:0,他引:1
Yoichi Sasai Makoto UyeshimaJacques Zlotnicki Hisashi UtadaTsuneomi Kagiyama Takeshi HashimotoYuji Takahashi 《Earth and Planetary Science Letters》2002,203(2):769-777
Magnetic and electric field variations associated with the 2000 eruption of Miyake-jima volcano are summarized. For about 1 week prior to the July 8 phreatic explosion, significant changes in the total intensity were observed at a few stations, which indicated uprising of a demagnetized area from a depth of 2 km towards the summit: this non-magnetic source can be regarded as a vacant space itself. Electric and magnetic field variations were observed simultaneously associated with the tilt-step event, which was the abrupt (∼50 s) inflation at a few km depth within the volcano followed by gradual recovery (∼several hours). The electric field is ascribed to the electrokinetic effect most probably due to forced injection of fluids from the source, while the magnetic field to the piezomagnetic effect due to increased pressure. Large magnetic variations amounting to a few tens of nT were observed at several stations since July 8, and they turned almost flat after the August 18 largest eruption. Magnetic changes are explained mostly by the vanishing of magnetic mass in the summit and additionally by the thermal demagnetization at a rather shallow depth. A large increase in the self-potential by 130 mV was also observed near the summit caldera associated with the August 18 eruption, which suggests that the hydrothermal circulation system sustained within the volcano for the past more than 10 years was destroyed by this eruption. 相似文献
24.
Hisashi Asanuma Yusuke Sawaki Shuhei Sakata Hideyuki Obayashi Kazue Suzuki Kouki Kitajima Takafumi Hirata Shigenori Maruyama 《Island Arc》2018,27(4)
Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a 207Pb/206Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons. 相似文献
25.
Geochemical characteristics of zircons in the Ashizuri A‐type granitoids: An additional granite topology tool for detrital zircon studies
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Yusuke Sawaki Kazue Suzuki Hisashi Asanuma Satoki Okabayashi Kentaro Hattori Takuya Saito Takafumi Hirata 《Island Arc》2017,26(6)
Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I‐, S‐, M‐, and A‐types). To clarify geochemical characteristics of zircons in A‐type granites, trace element compositions of zircons extracted from the A‐type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A‐type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A‐type granite. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A‐type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A‐type granites, implying the existence of A‐type magmatism in the Earth's earliest history. 相似文献
26.
Emily S. Antonio Akihide Kasai Masahiro Ueno Nam-il Won Yuka Ishihi Hisashi Yokoyama Yoh Yamashita 《Estuarine, Coastal and Shelf Science》2010
We investigated the distribution of δ13C and δ15N of organic matter among benthic communities from the upper estuary of Yura River to offshore of Tango Sea, Japan, to determine spatial variation in utilization of organic matter by benthic communities. The δ13C values of benthic animals ranged from −27 to −15‰ in the upper estuary, −21 to −15‰ in the lower estuary, −20 to −16‰ in the shallow coast (5–10 m depths), −18 to −16‰ in the deep coast (30–60 m depths) and −19 to −15‰ in offshore (100–150 m depths) stations. Adapting the dual isotope values to mixing models, we estimated the relative contributions of potential food sources to the benthos diet. Phytoplankton and macroalgae that intruded the estuary in summer were utilized as alternative food aside from the terrestrial-origin organic matter assimilated by the estuarine benthic consumers. Resuspended benthic microalgae were important source of energy in the shallow coastal stations, while abundant supply of phytodetritus fueled the deep coastal and offshore benthic food webs. Spatial difference in the diet of benthic communities depends largely on the shifts in the primary carbon source. Thus, benthic communities are important link of autochthonous/allochthonous production and secondary production in the continuous river–estuary–marine system. 相似文献
27.
Sedimentary inorganic nitrogen and its isotope ratio in the western subarctic Pacific over the last 145 kyr 总被引:1,自引:0,他引:1
We have measured inorganic nitrogen (IN) content and the isotope ratio of IN (δ15NIN) in a sediment core covering the last 145 kyr in the western subarctic Pacific (WSAP). IN content was generally high during
glacial periods and shows positive correlations with both eolian dust content and the ratio of organic carbon (C) to organic
nitrogen (ON) (C/ON) found in our previous studies. This means that IN was transported from continental areas to the WSAP
together with eolian dust and that the IN was not contaminated by volcanic materials, because the eolian dust content was
reconstructed using metal components to remove contaminating volcanic materials. Therefore, IN content in the WSAP sediments,
the clay fraction of which is not greatly affected by drift deposits seen at the other sites in this region, may potentially
be an effective proxy for eolian dust, without the need to consider contamination by volcanic materials. δ15NIN was generally low during glacial periods and shows negative correlations with IN, eolian dust, and C/ON. The possible causes
of the observed variations in δ15NIN are as follows: (1) authigenic fixation of NH4
+ in water-column and pore water of sea-floor sediments to clay minerals; (2) contamination of measured IN by highly resistant
organic matter; or (3) variations in the continental source region of the eolian dust supplied to the WSAP and climatically
induced changes in δ15N of soil organic matter there. The last mechanism shows the potential for δ15NIN to be used as a proxy for climate change on land, and is consistent with other published explanations of the spatial distribution
of δ15NIN in modern sea-floor sediments. 相似文献
28.
29.
Satoshi Tonai Yusuke Suganuma Juichiro Ashi Tetsumaru Itaya Hisashi Oiwane Shoichi Kiyokawa 《Tectonophysics》2011,497(1-4):71-84
Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7°, I = 51.8°, α95 = 9.6°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1°, I = 57.7°, α95 = 7.1°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F1 faults) without vertical-axis block rotation, (2) strike-slip reactivation of F1 faults and oblique-normal displacement on NE–SW-trending faults (F2 faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F2 faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks. 相似文献
30.
Calcium-alkalinity relationship in the North Pacific 总被引:3,自引:0,他引:3
The dissolution of calcium carbonate in deep ocean water causes variation in calcium concentration (Ca) and alkalinity (TA) in the ratio of one to two. The decomposition of organic matter generates nitric acid, phosphoric acid and sulfuric acid. A proton flux which is derived from this process also changes alkalinity. Using the variation in nitrate concentration (NO3) as an index of the proton flux, the relationship betweenCa,TA andNO3 is expressed asCa=0.5TA+0.63NO3 The values of Ca obtained from direct measurements in the North Pacific are in good agreement with the values estimated from this equation. 相似文献