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41.
Takehiko Hiwatari Kunio Shirasawa Yasushi Fukamachi Ryuichi Nagata Tomoyoshi Koizumi Hiroshi Koshikawa Kunio Kohata 《Polar Science》2008,2(1):41-54
Downward material fluxes under seasonal sea ice were measured using a time-series sediment trap installed at an offshore site in the Okhotsk Sea north of Hokkaido, Japan, from 13 January to 23 March 2005. The maximum fluxes of lithogenic material (753 mg m−2 day−1) and organic matter (mainly detritus; 333 mg m−2 day−1) were recorded during the period in which sea ice drifted ashore and increased in extent, from 13 January to 9 February. Organic matter as fecal pellets (81–93 mg m−2 day−1) and opal as biosilica (51–67 mg m−2 day−1), representing diatom fluxes, were abundant in sediment trap samples obtained during the period of full sea ice coverage from 10 February to 9 March. Microscopic observations revealed that fecal pellets were largely diatom frustules, suggesting that zooplankton actively grazed on ice algae during the period of full sea ice coverage. During the period of retreating sea ice, from 10 to 23 March, the phytoplankton flux showed a rapid increase (from 9.5 to 22.5 × 106 cells m−2 day−1), reflecting their release into the water column as the sea ice melted. Our results demonstrate that the quantity and quality of sinking biogenic and lithogenic materials vary with the seasonal extent of sea ice in mid-winter. 相似文献
42.
Atsushi Nozaki Ryuichi Majima Koji Kameo Saburo Sakai Atsuro Kouda Shungo Kawagata Hideki Wada Hiroshi Kitazato 《Island Arc》2014,23(2):157-179
We present field and core observations, nannofossil biostratigraphy, and stable oxygen isotope fluctuations in foraminiferal tests to describe the geology and to construct an age model of the Lower Pleistocene Nojima, Ofuna, and Koshiba Formations (in ascending order) of the middle Kazusa Group, a forearc basin‐fill succession, exposed on the northern Miura Peninsula on the Pacific side of central Japan. In the study area, the Nojima Formation is composed of sandy mudstone and alternating sandy mudstone and mudstone, the Ofuna Formation of massive mudstone, and the Koshiba Formation of sandy mudstone, muddy sandstone, and sandstone. The Kazusa Group contains many tuff beds that are characteristic of forearc deposits. Thirty‐six of those tuff beds have characteristic lithologies and stratigraphic positions that allow them to be traced over considerable distances. Examination of calcareous nannofossils revealed three nannofossil datum planes in the sequences: datum 10 (first appearance of large Gephyrocapsa), datum 11 (first appearance of Gephyrocapsa oceanica), and datum 12 (first appearance of Gephyrocapsa caribbeanica). Stable oxygen isotope data from the tests of the planktonic foraminifer Globorotalia inflata extracted from cores were measured to identify the stratigraphic fluctuations of oxygen isotope ratios that are controlled by glacial–interglacial cycles. The observed fluctuations were assigned to marine isotope stages (MISs) 49–61 on the basis of correlations of the fluctuations with nannofossil datum planes. Using the age model obtained, we estimated the ages of 24 tuff beds. Among these, the SKT‐11 and SKT‐12 tuff beds have been correlated with the Kd25 and Kd24 tuff beds, respectively, of the Kiwada Formation on the Boso Peninsula. The Kd25 and Kd24 tuff beds are widely recognized in Pleistocene strata in Japan. We used our age model to date SKT‐11 at 1573 ka and SKT‐12 at 1543 ka. 相似文献
43.
Ryuichi Shinjo Takele Chekol Daniel Meshesha Tetsumaru Itaya Yoshiyuki Tatsumi 《Contributions to Mineralogy and Petrology》2011,162(1):209-230
Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern
Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm
three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma)
tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics
of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric
source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and 87Sr/86Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (206Pb/204Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental
lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic
evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending
from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity
of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source. 相似文献
44.
We performed numerical simulations to determine the contemporary maximum horizontal compressive stress (σHmax) in the northeast India region, the Bengal basin (Bangladesh), and the adjoining Indo-Burma Ranges, with different boundary conditions. The regional tectonic stress was simulated using the finite element method (FEM) under the plane stress condition. Most of the study areas show NE–SW regional stress orientation, which is consistent with other stress indicators, such as earthquake focal mechanism solutions. The E–W trending Dauki fault, which separates the Shillong plateau to the north from the Bengal basin to the south, plays a major role in the stress distribution and regional deformation. This fault alone accommodates ～25% of the regional surface displacement rate of the study area. The fault pattern of the study area was also simulated using rheological parameters and the Mohr–Coulomb failure criterion. The simulated results reproduce the observed tectonic state of the area, including a strike-slip regime along the Dauki fault, in the southwestern part of the Bengal basin, and in the Tripura fold belt areas. The modeling indicates that the Brahmaputra valley to the north of the Shillong plateau and to the south of the Himalayan frontal thrust exhibits thrust/reverse faulting with a strike-slip component, and in the Indo-Burma Ranges, strike-slip faulting is predominant with a reverse fault component. 相似文献
45.
Partitioning Eddy-Covariance Methane Fluxes from a Shallow Lake into Diffusive and Ebullitive Fluxes 总被引：1，自引：0，他引：1
Hiroki Iwata Ryuichi Hirata Yoshiyuki Takahashi Yuichi Miyabara Masayuki Itoh Kotaro Iizuka 《Boundary-Layer Meteorology》2018,169(3):413-428
Methane (\(\mathrm {CH}_{4}\)) is known to be emitted from lakes to the atmosphere via processes such as diffusion and ebullition (i.e., bubble emission). We developed a practical method for partitioning eddy-covariance \(\mathrm {CH}_{4}\) fluxes from a shallow lake into diffusive and ebullitive fluxes using a wavelet analysis based on local scalar similarity between the \(\mathrm {CH}_{4}\) concentration and other reference scalars, such as the air temperature or water vapour concentration, in the wavelet time-scale domain, with the hypothesis that similar and dissimilar fluctuation components are related to diffusive and ebullitive \(\mathrm {CH}_{4}\) fluxes, respectively. Our method is applied to approximately two weeks of data obtained at a shallow mid-latitude lake. The partitioned diffusive flux has a physically sound relationship with wind speed, supporting the validity of the method. The ratio of the diffusive flux to the total flux is typically 0.11 with flow from an area of steady bubble emission, but otherwise 0.36. Further validation is required using a larger dataset and data from other lakes. The proposed method can be easily applied to historical data because it requires only 10-Hz data of \(\mathrm {CH}_{4}\) concentration and other reference scalars, along with an empirical parameter. 相似文献
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48.
Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin 总被引：23，自引：0，他引：23
The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have 87Sr/86Sr (0.7040–0.7042) and 143Nd/144Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher 87Sr/86Sr (0.7044–0.7047) but similar 143Nd/144Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (Ma/Mc) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current. 相似文献
49.
Relation between hydrogen emission and seismic activities 总被引：2，自引：0，他引：2
Ryuichi Sugisaki 《Pure and Applied Geophysics》1984,122(2-4):175-184
Measurements of chemical composition of bubbles from a mineral spring at Yuya Spa situated close to the Median Tectonic Line, the longest active fault in Japan, showed that the periods of increased H2 emission coincided with occurrences of the Ohno earthquake swarm nearby. Four cases of the coincidence without exception were observed in the last three years. The fluctuation of H2 concentration ranges between <0.5 and 200 ppm, whereas other gases such as He, Ar, N2, and CH4 do not fluctuate much. The H2 concentration is correlated with the energy released by the seismic activity. This field evidence, together with the results of laboratory experiments conducted bySugisaki
et al. (1983), leads to the conclusion that H2 observed at the mineral spring was produced by the reaction between groundwater and rock fractured in the seismic activities. The observation that H2 in the mineral spring tends to appear prior to an earthquake suggests that microcracks may occur in rocks prior to earthquakes. The precursory emission of H2 may be useful for earthquake prediction. 相似文献
50.
Yasuyuki Nakagawa Kazuo Nadaoka Hiroshi Yagi Ryuichi Ariji Haruo Yoneyama Kazuhiro Shirai 《Ocean Dynamics》2012,62(10-12):1535-1544
Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300?%) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied. 相似文献