Yokohama-shi, Japan
Yokohama-shi, Japan

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Kawaguchi Y.,Japan Agency for Marine - Earth Science and Technology | Nishino S.,Japan Agency for Marine - Earth Science and Technology | Inoue J.,Japan Agency for Marine - Earth Science and Technology | Inoue J.,Japan National Institute of Polar Research | And 6 more authors.
Journal of Physical Oceanography | Year: 2016

The Arctic Ocean is known to be quiescent in terms of turbulent kinetic energy (TKE) associated with internal waves. To investigate the current state of TKE in the seasonally ice-free Chukchi Plateau, Arctic Ocean, this study performed a 3-week, fixed-point observation (FPO) using repeated microstructure, hydrographic, and current measurements in September 2014. During the FPO program, the microstructure observation detected noticeable peaks of TKE dissipation rate ε during the transect of an anticyclonic eddy moving across the FPO station. Particularly, ε had a significant elevation in the lower halocline layer, near the critical level, reaching the order of 10-8 W kg-1. The ADCP-measured current displayed energetic near-inertial internal waves (NIWs) propagating via the stratification at the top and bottom of the anticyclone. According to spectral analyses of horizontal velocity, the waves had almost downward energy propagation, and its current amplitude reached ~10 cm s-1. The WKB scaling, incorporating vertical variations of relative vorticity, suggests that increased wave energy near the two pycnoclines was associated with diminishing group velocity at the corresponding depths. The finescale parameterization using observed near-inertial velocity and buoyancy frequency successfully reproduced the characteristics of observed e, supporting that the near-inertial kinetic energy can be effectively dissipated into turbulence near the critical layer. According to a mixed layer slab model, a rapidly moving storm that has passed over in the first week likely delivered the bulk of NIW kinetic energy, eventually captured by the vortex, into the surface water. © 2016 American Meteorological Society.

Higashi T.,TerraGrav LLC 1 31 | Doi K.,Japan National Institute of Polar Research | Doi K.,Graduate University for Advanced Studies | Hayakawa H.,Japan National Institute of Polar Research | And 10 more authors.
Journal of the Geodetic Society of Japan | Year: 2013

Absolute gravity measurements were carried out as a part of activities in 2011-2012 austral summer season of the 53rd Japanese Antarctic Research Expedition (JARE53) at Syowa Station, Antarctica. Obtained absolute gravity value was 982524322.7±0.1 μgal (1 μgal=10-8ms -2) using an absolute gravimeter FG5#210. Absolute gravity measurements using FG5 gravimeter have been.

Kazama T.,Kyoto University | Hayakawa H.,Japan National Institute of Polar Research | Higashi T.,TerraGrav LLC | Ohsono S.,GNSS Technologies Inc. | And 8 more authors.
Polar Science | Year: 2013

Absolute gravity values were measured with a portable absolute gravimeter A10 in East Antarctica, for the first time by the Japanese Antarctic Research Expedition. This study aims to investigate regional spatiotemporal variations of ice mass distributions and associated crustal deformations around Syowa Station by means of repeated absolute gravity measurements, and we obtained the first absolute gravity value in Southern Langhovde on the Antarctic Continent. The average absolute gravity value at the newly installed benchmark AGS01 in Langhovde (obtained on 3 February 2012) was 982535584.2±0.7μgal (1 [μgal]=1×10-8[m/s2]), which was in agreement with the gravity values obtained by the past relative gravity measurements within 1mgal. In addition, the average absolute gravity value obtained at AGSaux in Syowa Station was consistent with both previous absolute gravity values and those obtained by simultaneous measurements using an FG5 gravimeter, owing to adequate data corrections associated with tidal effects and time variations in atomic clock frequencies. In order to detect the gravity changes associated with the ice mass changes and other tectonic phenomena, we plan to conduct absolute gravity measurements at AGS01 again and at other campaign sites around Syowa Station as well in the near future, with careful attention paid to the impacts of severe environmental conditions in Antarctica on gravity data collection. © 2013 Elsevier B.V. and NIPR.

Aoyama Y.,Japan National Institute of Polar Research | Doi K.,Japan National Institute of Polar Research | Shibuya K.,Japan National Institute of Polar Research | Ohta H.,51St Japanese Antarctic Research Expedition | And 3 more authors.
Earth, Planets and Space | Year: 2013

The horizontal velocity vector of ice flow on the floating ice tongue of the Shirase Glacier, East Antarctica, was determined using two GPS buoys located on its east and west sides. The GPS buoys consisted of a singlefrequency GPS receiver module and an Iridium satellite communication system. The instantaneous horizontal position of each GPS buoy was automatically obtained every 30 minutes, and the data were immediately transmitted to the National Institute of Polar Research (NIPR), Tokyo, Japan, via a satellite link. The location data demonstrated that the floating ice tongue moved primarily in a linear manner during the monitoring period between February and April, 2010. The speed and azimuth of the eastern buoy were (5.779 ± 0.004 m/day, N1.4°E ± 0.5°), respectively, while for the western buoy the speed and azimuth were (7.005 ± 0.006 m/day, N13.1°W ± 0.6°), respectively. Short-term variations about the mean speed and azimuth of the ice flow, with a period of 3-10 days, were also identified. © The Society of Geomagnetism and Earth Planetary and Space Sciences (SGEPSS) The Seismological Society of Japan The Volcanological Society of Japan The Geodetic Society of Japan The Japanese Society for Planetary Sciences TERRAPUB.

Goto S.,Japan National Institute of Advanced Industrial Science and Technology | Mizoguchi T.,System Intech Co. | Kimura R.,Global Ocean Development Inc. | Kinoshita M.,Japan Agency for Marine - Earth Science and Technology | And 2 more authors.
Marine Geophysical Research | Year: 2012

We investigated the relationship between variations in the thermal conductivity of surface sediments and the topography in the Nankai subduction zone off Tokai, central Japan, the easternmost part of the Nankai subduction zone, which has an accretionary prism with varied topography. We analyzed sediment thermal conductivity data obtained from the trough floor and accretionary prism. Variations in the thermal conductivity of sediments were related to the topographic features formed by accretionary prism development. Thermal conductivities of 1.1 W/m K were measured on the trough floor where thick terrigenous turbidites have been deposited. The thermal conductivity of Nankai Trough floor sediments decreases from northeast to southwest along the trough, probably because of the decreased grain size and/or changes in sediment mineral composition. High thermal conductivities (≥1.0 W/m K) were measured in fault scarps on the accretionary prism. A landward increase in these values on the prism may be explained by decreased porosity of the sediments attributable to tectonic deformation during accretionary prism development. At the base of the fault scarp of the frontal thrust, low thermal conductivities (<0.9 W/m K) were measured, likely reflecting the high porosity of the talus deposits. Low thermal conductivity (0.9 W/m K) was also measured in slope basins on the accretionary prism, likely also related to the high porosity of the sediments. Our results demonstrate that, for accurate heat flow measurement in an area of varied topography, the geothermal gradient and the thermal conductivity of the sediments must be measured within regions with similar topographic features. © 2012 Springer Science+Business Media B.V.

Matsumoto T.,University of Ryukyus | Mori A.,Global Ocean Development Inc. | Kise S.,University of Ryukyus | Abe N.,Japan Agency for Marine - Earth Science and Technology
Geochemical Journal | Year: 2013

The Chile Triple Junction (CTJ), an RTT-type triple junction located at 46° 13' S, 75°48' W off the western coast of Chile, is characterized by the subducting Chile Ridge, which is the constructive plate boundary that generates both the Nazca Plate and the Antarctic Plate. The ridge subduction mechanism and the regional tectonics around the CTJ were investigated primarily using marine geophysical data (topography, gravity, geomagnetic field and single-channel seismics) collected during the SORA2009 cruise (Cruise ID = MR08-06) by R/V MIRAI together with other cruise data from the National Geophysical Data Center. The segment of the ridge axis just before the subduction around the CTJ is associated with an axial deep covered with thick sediment unlike that seen in typical ridge crests. The profiles of both topography and the free air anomaly around the CTJ show quite different patterns from those of ordinary subduction zones. However, topographic features typical of a slow-spreading type ridge, including a median valley and both flanks, remain in the seaward side of the trench. Even after the subduction of the eastern flank, the topographic features of the western flank remain. A slight Outer Rise and an Outer Gravity High, which are common in the western Pacific area, were observed in an area far away from the CTJ on both Nazca and Antarctic plate sides. The geomagnetic anomaly pattern around the Chile Ridge near the CTJ shows that the estimated spreading rate decreases gradually towards the ridge crest. This suggests that volcanic activity diminishes gradually towards the subducting ridge axis. The lithosphere under the Chile Ridge might have amalgamated with the surrounding oceanic lithosphere due to heat loss after the cessation of volcanic activity. The oceanic lithosphere towards the trench also thickens rapidly due to heat loss. Consequently, shallow-angle subduction of the youngest and most immature oceanic plate occurs smoothly via slab-pull force without any resistance along the interface between the South American continental plates. © 2013 by the Geochemical Society of Japan.

Tran T.H.,University of Shizuoka | Kato K.,Yamagata University | Wada H.,University of Shizuoka | Fujioka K.,Global Ocean Development Inc. | Matsuzaki H.,Tokyo University of Science
Journal of Geochemical Exploration | Year: 2014

Samples were collected from two carbonate chimneys on the summit of Conical Seamount, a serpentine diapir in the Mariana Forearc, by JAMSTEC's Shinkai 6500 submersible. The chimneys are composed of calcite, aragonite and an amorphous phase. The calcite and aragonite were precipitated under conditions of carbonate saturation, but the processes are complex in detail. Stained thin sections from samples of both chimneys demonstrate that the aragonite formed during the initial stage of growth, mainly in the presence of seawater with relatively high concentrations of Mg, while the calcite formed later in fractures in the aragonite framework, where rising vent fluids effectively diluted any seawater present, thus reducing the amount of Mg available during carbonate growth. A staining technique was used to differentiate between calcite and aragonite prior to analysis of various isotopes (δ13C, δ18O, δ14C, and 87Sr/86Sr) in each mineral, which provide clues to the processes involved in their formation. Radiocarbon activities (δ14C) and strontium isotope ratios (87Sr/86Sr) are significantly different between the aragonite and calcite. These results suggest that the calcite and aragonite were precipitated at different stages in the formation of the chimney. Since the δ14C in each mineral is strongly depleted compared with ambient seawater, both calcite and aragonite precipitation must have been affected by vent fluids (cold seepage) that contained "dead carbon" derived from the serpentinization that is taking place under Conical Seamount. These isotopic analyses (δ14C and 87Sr/86Sr) enabled us to estimate the amount of mixing of ambient seawater and vent fluids during precipitation of the calcite and aragonite. However, the calculated amounts of mixing for aragonite precipitation are not in good agreement, and this is probably due to the low concentrations of Sr in the vent fluid. The low abundance of Sr within the vent fluid suggests that the amount of mixing calculated with δ14C provides a better indication of the true mixing ratios of seawater to cold seepage during calcite and aragonite precipitation. © 2013 Elsevier B.V.

Nakano I.,Global Ocean Development Inc. | Ishida H.,Kobe University
2013 IEEE International Underwater Technology Symposium, UT 2013 | Year: 2013

In order to measure the velocity of rip current, we have developed an acoustic monitoring system consisting of two transducers and several sound reflectors. The two transducers are connected to a modified two-frequency fish-finder which can alternatively transmit and receive the preset sound signals of 50kHz or 200kHz at an interval of several seconds. Sound reflectors were composed of commercial steel rods. The sea trial of this system was carried out in the Uradome Beach in early September 2012. The transducers were placed at a height of 0.5m over the sea floor at a depth of 1m to 1.5m and horizontally separated at a distance of 30m offshore. Six reflectors were placed at a distance of 15m, 30m, 45m, 60m, 75m and 90m along the shoreline. The sound signal of 50 kHz burst waves were transmitted by one transducer and received by the other, and vice versa. The SNR's of the signals through the direct base line and through reflective paths of the 30m, 45m and 60m reflectors were sufficient for the analysis as expected. The preliminary result suggested to be able to reconstruct a current velocity distribution in the beach based on passive reciprocal sound transmissions. © 2013 IEEE.

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