Audio frequency magneto-telluric survey of Norikura Volcano in central Japan     

Kiyoshi Fuji-ta  Yasuo Ogawa  Masahiro Ichiki  Satoru Yamaguchi  Yutaka Makino 《Journal of Volcanology and Geothermal Research》1999,90(3-4)

Norikura Volcano has not been active during the last 10,000 years in spite of the activity of the surrounding volcanic mountains. To study past volcanic activities, geological studies were carried out extensively. However, quite a few geophysical investigations were conducted to contribute to volcanology. Our objective is to detect the present subsurface structure of Norikura Volcano and to define volcanic stratifications. In the vicinity of Norikura Volcano, geothermal fields are still active. Subsurface volcanic rocks in this area have been exposed to geothermal activity and altered. To comprehend volcanic stratifications of Norikura and geothermal activity, we conducted audio frequency magneto-telluric (AMT) surveys around Norikura Volcano. AMT survey is useful in clearly defining the resistivity structure related to volcanic regions. The AMT data were acquired over a frequency range 10 Hz–10 kHz. Decomposition analysis was applied to the tensor impedance data. Subsequently, apparent resistivity and phase data were inverted using a two-dimensional magneto-telluric (MT) inversion and a model of Norikura was derived. The final model manifests that the surface resistors are in agreement with andesite lava or dacite lava. As for the deeper structure, a horizontal conductor is situated above resistive basements. The alteration of the conductor was weak, while basement rocks were strongly altered and/or heated through the thermal activity. The existence of these layers seems to indicate the degree of thermal activity of Norikura Volcano.  相似文献   

Overview of NOPACCS (Northwest Pacific Carbon Cycle Study)     

Hiroyuki Tsubota  Joji Ishizaka  Akira Nishimura  Yutaka W. Watanabe 《Journal of Oceanography》1999,55(6):645-653

The Northwest Pacific Carbon Cycle Study (NOPACCS) was a program aimed at investigating the carbon cycle of the North Pacific Ocean, which can be thought of as a large reservoir of carbon dioxide. NOPACCS was also aimed at estimating the North Pacific's capacity as a carbon sink. Project design, scientific results, and data availability, and subsequent projects resulting from this project are also described in this review. Studies of the upper ocean processes focused on the latitudinal differences in the fugacity of carbon dioxide; and on the detail of plankton community structures. Intermediate water was studied in relation to the formation of North Pacific Intermediate Water and the amount of accumulated anthropogenic carbon. The sedimentation process, past carbon cycle and coral reefs were also studied during the project. A preliminary, overall view of the carbon cycle of the North Pacific was drawn from the results of the project and compared to global values.  相似文献   

Depositional environments of well‐sorted detrital limestone from the Minatogawa Formation in the southern part of Okinawa Island,the Ryukyu Archipelago,Japan          下载免费PDF全文

Kazuhiko Fujita  Kayo Aruga  Marc Humblet  Koichi Nagai 《Island Arc》2018,27(3)

Well‐sorted detrital limestone is one of the typical lithofacies of the latest interval of the Pleistocene Ryukyu Group, which is exposed in the Ryukyu Archipelago in southwestern Japan. The depositional environments of the limestone are interpreted to be extremely shallow and to include back‐reef lagoons or moats and subaerial sand dunes. However, detailed micropaleontological analyses have not been performed on this limestone. In this study, the interpretation of the depositional environments and paleo‐water depths was improved by quantitative examination of foraminiferal assemblages for the well‐sorted detrital limestone of the Minatogawa Formation in the southern part of Okinawa Island. Thin sections of limestone collected from the Minatogawa (Horikawa) quarry were subjected to sedimentological and foraminiferal analyses. Comparison with modern foraminiferal distribution within the Ryukyu Archipelago indicates that back‐reef and fore‐reef dwelling foraminifers characterize the fossil assemblages from the well‐sorted detrital limestone (bioclastic grainstone). Three ratios of indicator foraminiferal taxa (ratios of back‐reef to fore‐reef taxa, planktonic foraminifers to Amphistegina lobifera and Amphistegina lessonii, and Calcarina gaudichaudii to other Calcarina species), as well as multivariate analyses suggest that the well‐sorted detrital limestone was deposited in fore‐reef setting shallower than 40 m in water depth. A comparable depth range was reconstructed from the coral assemblage in the associated coral limestone, suggesting that the Minatogawa Formation was deposited in a gently inclined ramp setting with patch reefs and/or fringing reefs. Stratigraphic changes in paleo‐water depth, together with evidence of several unconformities associated with paleosol layers suggest that there were repeated transgressions and regressions, with an amplitude up to several tens of meters, when the Minatogawa Formation was deposited.  相似文献   

Transport measurement with a horizontal and a vertical pipe microphone in a mountain stream: taking account of particle saltation          下载免费PDF全文

Daizo Tsutsumi  Masaharu Fujita  Michinobu Nonaka 《地球表面变化过程与地形》2018,43(5):1118-1132

The pipe microphone has been shown to be an effective means for monitoring bedload transport in mountain streams. It is commonly installed perpendicular to the flow direction on a stable river bed, such as that of a check dam. Acoustic pulses caused by bedload collisions with the pipe are detected by a microphone. However, bedload particles saltating over the pipe remain undetected. To overcome this disadvantage, we installed a horizontal as well as a vertical pipe microphone in the Ashi‐arai‐dani supercritical channel located in the Hodaka mountain range, Japan. The vertical pipe was installed on the wall of the channel and the horizontal pipe was installed on the channel bed. The acoustic response of the horizontal pipe is expected to be larger than that of the vertical pipe, because the bedload concentration decreases with increasing height above the bed. However, at high amplifications, the peak pulse value from the vertical pipe is higher than that from the horizontal pipe. We explain this observation as follows: under high bedload discharge conditions, the pulses of the horizontal pipe are saturated but those of the vertical pipe are not. We proposed a ratio (R_hv) between the pulses detected by these sensors, and applied this ratio for calibrating the contemporaneous pulses detected by a microphone located immediately upstream of a bedload slot sampler. Indeed the R_hv‐corrected pulses correlated well with the bedload discharge calculated from the sampler, supporting our explanation. We conclude that bedload monitoring using concomitant vertical and horizontal pipe microphones can be used to calibrate centrally located pipe microphones when the bedload concentration is approximately homogeneous laterally across the width of the channel cross‐section, and thereby represent bedload discharges more accurately than with only a single pipe microphone. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

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  

Masahito Shigemitsu  Yutaka W. Watanabe  Hisashi Narita 《Journal of Oceanography》2009,65(4):541-548

We have measured inorganic nitrogen (IN) content and the isotope ratio of IN (δ¹⁵N_IN) 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. δ¹⁵N_IN 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 δ¹⁵N_IN 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 δ¹⁵N_IN to be used as a proxy for climate change on land, and is consistent with other published explanations of the spatial distribution of δ¹⁵N_IN in modern sea-floor sediments.  相似文献   

Skewed Water Temperature Occurrence Frequency in the Sea off Sanriku, Japan, and Intrusion of the Pure Kuroshio Water     

Sachiko Oguma  Yutaka Nagata 《Journal of Oceanography》2002,58(6):787-796

Temperature and salinity data obtained by the Iwate Fisheries Technology Center were analyzed for the period 1971–1995. It was found that occurrence frequency distributions of temperature and salinity are very skewed at depths deeper than 200 m and that temperature sometimes exceeds m + 5σ (m: mean and σ: standard deviation. If such abnormally high temperatures are real the 3σ criterion cannot be adopted. Oceanic conditions were surveyed in 1972 and 1979, when temperatures exceeding m + 5σ were observed. It was found that the abnormally high temperature (and high salinity) water was the pure Kuroshio Water introduced into the region due to some special conditions such as abnormal approach of large warm-core ring to the Sanriku Coast or abnormal northward extension of the Kuroshio along the coast. These events are very rare, occurring only twice in the 25-year period analyzed, but the abnormally high temperature observed is real. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Discovery of a large-scale clumpy structure around the Lynx supercluster at z∼ 1.27     

Fumiaki Nakata  Tadayuki Kodama  Kazuhiro Shimasaku  Mamoru Doi  Hisanori Furusawa  Masaru Hamabe  Masahiko Kimura  Yutaka Komiyama  Satoshi Miyazaki  Sadanori Okamura  Masami Ouchi †  Maki Sekiguchi  Yoshihiro Ueda  Masafumi Yagi  Naoki Yasuda 《Monthly notices of the Royal Astronomical Society》2005,357(4):1357-1362

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Survival of a proto-atmosphere through the stage of giant impacts: the mechanical aspects   总被引：2，自引：0，他引：2  

Hidenori Genda  Yutaka Abe 《Icarus》2003,164(1):149-162

When a giant impact occurs, atmosphere loss may occur due to global ground motion excited by a strong shock wave traveling in the planetary interior. Here, the relations between the ground motion and the amount of the lost atmosphere are systematically investigated through calculations of a spherically one-dimensional atmospheric motion for various initial atmospheric conditions. The fraction of the lost atmosphere to the total mass of the atmosphere is found to be controlled only by the ground velocity and, insensitive to the initial atmospheric conditions. Unlike the previous studies (Ahrens, 1990, Origin of the Earth, H.E. Newson, J.H. Jones (Eds.), pp. 211-227; Ahrens, 1993, Annu. Rev. Earth Planet. Sci. 21, 525-555; Chen and Ahrens, 1997, Phys. Earth Planet. Inter. 100, 21-26); the estimated loss fraction for the giant impact is only 20%. Significant escape occurs only when the ground velocity is close to the escape velocity. Thus, most of the atmosphere should survive the giant impact. The cause of the difference from previous estimates is discussed from energetic and dynamic points of view. Moreover, if our estimates are applied to the atmosphere of the impactor planet, a significant fraction of it is carried to the target planet. Survival of the proto-atmosphere has very important effects on the origin and evolution of the terrestrial planets' volatile budget.  相似文献   

Comments on “The significance of coral reefs as global carbon sinks—response to greenhouse” by D.W. Kinsey and D. Hopley     

Stuart R Gaffin  Rodney M Fujita 《Global and Planetary Change》1992,5(4)

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New age constraints on counter-clockwise rotation of NE Japan     

Ayumi K. Baba  Takaaki Matsuda  Tetsumaru Itaya  Yutaka Wada  Noriyuki Hori  Masahiko Yokoyama  Nobuaki Eto  Rieko Kamei  Haider Zaman  Tesfaye Kidane  Yo-Ichiro Otofuji 《Geophysical Journal International》2007,171(3):1325-1341

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