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排序方式: 共有208条查询结果,搜索用时 31 毫秒
81.
Taketoshi Kodama Akihiko Morimoto Tetsutaro Takikawa Masashi Ito Yosuke Igeta Shoko Abe Ken-Ichi Fukudome Naoto Honda Osamu Katoh 《Journal of Oceanography》2017,73(6):759-769
To identify water with an excess nitrate concentration to phosphate ratio and its potential source, the nutrient concentrations in the Tsushima Strait (TSS) were investigated over ten cruises in August and September 2007–2014, excluding 2010. On the basis of the Redfield ratio, water with an excess nitrate concentration of >1 μM (positive ExNOx water) was identified below the surface mixed layer during four cruises in 2011–2013. Positive ExNOx water was present mainly in less-saline (<34) waters with a density of 22–25 σ θ , and 25–75 m depth. However, in August 2012, positive ExNOx was detected in dense (25–25.5 σ θ ) and deep (50–110 m depth) waters near the salinity maximum, although the salinity during this period was significantly lower than that in other years. The horizontal length of positive ExNOx water was >100 km across the TSS during two cruises in August 2012 and September 2013, respectively. According to multi-regression analysis conducted on the silicate concentration, temperature, and salinity, the silicate concentration was increased in the less-saline subsurface water. The required amount of original freshwater was 108–9 m3 day?1 based on the excess nitrate concentration. This evidence indicates that the positive ExNOx water originated from large river waters such as the Changjiang. Thus, discharged water from the rivers of the East Asia is contributing to the increased N:P ratio in the Tsushima Warm Current, southern Japan Sea. 相似文献
82.
Kazuhisa Goto Tetsuya Shinozaki Koji Minoura Kiyohiro Okada Daisuke Sugawara Fumihiko Imamura 《Island Arc》2010,19(3):412-426
This study investigates the distribution of boulders at Miyara Bay of Ishigaki Island, Japan. These boulders were deposited on a reef flat extending approximately 400–1300 m in width. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef and coral rock fragments estimated as <335 m3 (<633 t). Locally in the bay, the relationship between the boulder weight and position shows that boulders of a given weight have a clear limit on seaward distribution on the reef flat. For example, more than 1, 10, and 100 tons of boulders were deposited, respectively, more than 500, 300, and 100 m from the reef edge. The line is consistent with the possible landward transport limit by maximum storm waves at the Ryukyu Islands, suggesting that the line was formed by the reworking of some boulders by maximally strong storm waves, although we can not exclude the possibility that the line was formed by tsunamis. Furthermore, 68% of boulders at the bay are deposited beyond this line. Therefore, the presence of these boulders at their present positions is difficult to explain solely by storm waves, implying the possible tsunami origin of these boulders. The boulders are characteristically concentrated along the high‐tide line, suggesting the drastic reduction of the tsunami hydraulic force along the line. Previous studies using radiocarbon age dating, as well as our study, imply that at least 69 boulders at Miyara Bay were probably deposited at their present positions by the 1771 Meiwa tsunami, although some of these boulders might have been emplaced and displaced on the reef flat by prior tsunami or storm surges. 相似文献
83.
Sedimentary facies of the Tsagayan Formation is distributed in the eastern Zeya-Bureya Basin has been analyzed. The formation is of the Maastrichtian to Danian in age and characterized by the cyclicity of the fining-upwards successions. Analysis of environmental changes during the K/T boundary is the focus of this study. Five facies have been identified: Facies A, thick and laterally extensive coarse-grained to medium-grained sandstone units, interpreted as channelfill deposits; Facies B, parallel-laminated to massive mudstone units interpreted as interchannel lakes and flood plain deposits; Facies C, sheet-like medium-grained to fine-grained sandstones interpreted as crevasse splay deposits;Facies D, coal to coaly mudstone beds interpreted as deposits ofpeatlands; Facies E, very poorly sorted sandy mudstone beds interpreted as debris flow deposits. Fluvial environments with the low-relief flat topography was inferred. A channel transported large volumes of clasts, and a flood basin with interchannel lakes and peatlands was deciphered. Any distinct change of sedimentary environments has not been identified throughout the Tsagayan Formation (including the K/T boundary). However, two beds of debris flow deposits were identified. The one occurs at the uppermost part of the lower Tsagayan Subformation and contains dinosaur fossils. The other is intercalated in the upper Tsagayan Subformation. 相似文献
84.
Igneous History of the Aubrite Parent Asteroid: Evidence from the Norton County Enstatite Achondrite
Akihiko Okada Klaus Keil G. Jeffrey Taylor Horton Newsom 《Meteoritics & planetary science》1988,23(1):59-74
Abstract— We studied numerous specimens of the Norton County enstatite achondrite (aubrite) by optical microscopy, electron microprobe, and neutron-activation analysis. Our main conclusions are the following: 1. Norton County is a fragmental impact breccia, consisting of a clastic matrix made mostly of crushed enstatite, into which are embedded a variety of mineral and lithic clasts of both igneous and impact melt origin. 2. The Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, not grains formed by condensation from the solar nebula. 3. The Mg-silicate-rich aubrite parent body experienced extensive melting and igneous differentiation, causing formation of diverse lithologies, some of which have not been described previously. These lithologies include dunites (represented by forsterite crystals), plutonic orthopyroxenites (represented by most enstatite crystals in the matrix), plutonic pyroxenites (the pyroxenitic clasts), and plagioclase-silica rocks (like the feldspathic clasts). Presence of impact melt breccias (the microporphyritic clasts and the diopside-plagioclase-silica clast) of still different compositions further attest to the lithologic diversity of the aubrite parent body. 相似文献
85.
Sun G M.Akhmetiev Dong Z.M A.R.Ashraf Sun Y.W E.Bugdaeva D.L.Dilcher L.Golovneva I.Harding K.Johnson T.Kezina T.Kodrul LU J.S V.Markevich H.Nishida H.Okada Park S.O Sun C.L Xiong X.Z Xing Y.L Zhou Z.L 《世界地质(英文版)》2002,5(2)
Abstract A new geological section recognized in Baishantou of Jiayin, Heilongjiang Province, China, was studied preliminarily in searching for the K/T boundary. An early Paleocene florule characrerized by abundant Tiliaephyllum tsagajanicum leaves and its associated plants, was collected from the Baishantou Member (new member representing the lower part of the Paleocene Wuyun Formation) overlying the possible K/T boundary. A tuff bed was found from the upper part of the Furao Formation (uppermost Cretaceous) in this section. The new findings imply that the K/T boundary is probable lying in the interval between the base of the Baishantou Member and the tuff bed, based on the correlation of the present strata with the stratotype of the Tsagajan Group spanning the K/T boundary in "Belaya Gara" of the Bureya Basin, Russia. The early Paleocene florule from the Baishantou Member and the early-middle Maastrichtian dinosaurs found from the Yuliangzi Formation underlying the Furao Formation suggest that the Baishantou section is more promising for searching for the K/T boundary in this area. Further studies of the section is needed, integrating paleontological, biostratigraphic, geochemical, paleomagnetic, and sedimentary research would benefit our understanding the mass extinction event and biotic recovery in the Heilongjiang River area, China, and even all of northeast Asia during the K/T boundary interval. 相似文献
86.
Akihiko Morimoto Shoichiro Kojima Sen Jan Daisuke Takahashi 《Estuarine, Coastal and Shelf Science》2009
We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s−1. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf. 相似文献
87.
Ayako?ShinozakiEmail author Hisako?Hirai Hiroyuki?Kagi Tadashi?Kondo Taku?Okada Daisuke?Nishio-Hamane Shin-ichi?Machida Tetsuo?Irifune Takumi?Kikegawa Takehiko?Yagi 《Physics and Chemistry of Minerals》2012,39(2):123-129
High pressure and temperature reactions of a mixture of forsterite and hydrogen molecules have been carried out using a laser
heated diamond anvil cell at 9.8–13.2 GPa and ~1,000 K. In situ X-ray diffraction measurements showed no sign of decomposition
or phase transitions of the forsterite under these experimental conditions, indicating that the olivine structure was maintained
throughout all runs. However, a substantial expansion of the unit cell volume of the forsterite was observed for samples down
to ~3 GPa upon quenching to ambient pressure at room temperature. The Raman spectroscopy measurements under pressure showed
significant shifts of the Raman peaks of the Si–O vibration modes for forsterite and of the intramolecular vibration mode
for H2 molecules toward a lower frequency after heating. Additionally, no OH vibration modes were observed by Raman and FT-IR spectroscopic
measurements. These lines of evidence show that the observed volume expansion in forsterite is not explained by the incorporation
of hydrogen atoms as hydroxyl, but suggest the presence of hydrogen as molecules in the forsterite structure under these high
pressure and temperature conditions. 相似文献
88.
Evaluation of ADEOS-II GLI ocean color atmospheric correction using SIMBADA handheld radiometer data
Hajime Fukushima Mitsuhiro Toratani Hiroshi Murakami Pierre-Yves Deschamps Robert Frouin Akihiko Tanaka 《Journal of Oceanography》2007,63(3):533-543
The performance of the “version 2” Global Imager (GLI) standard atmospheric correction algorithm, which includes empirical
absorptive aerosol correction and sun glint correction, was evaluated using data collected with handheld above-water SIMBADA
radiometers during 23 cruises of opportunity (research vessels, merchant ships), mostly in the North Atlantic and European
seas. A number of 100 match-up data sets of GLI-derived and SIMBADA-measured normalized water-leaving radiance (nL
W
) and aerosol optical thickness (AOT) were sorted out, using objective selection criteria, and analyzed. The Root-Mean-Square
(RMS) difference between GLI and SIMBADA nL
W
was about 0.32 μW/cm2/nm/sr for the 412 nm band, showing improvement by 30% in RMS difference with respect to the conventional “version 1” GLI
atmospheric correction algorithm, and the mean difference (or bias) was reduced significantly. For AOT, the RMS difference
was 0.1 between GLI estimates and SIMBADA measurements and the bias was small (a few 0.01), but the ?ngstr?m exponent was
systematically underestimated, by 0.4 on average, suggesting a potential GLI calibration offset in the near infrared. The
nL
W
differences were not correlated to AOT, although performance was best in very clear conditions (AOT less than 0.05 in the
865 nm band). Despite the relatively large scatter between estimated and measured nL
W
, the derived chlorophyll-a concentration estimates, applying the same ratio algorithm (GLI OC4V4) to GLI and SIMBADA, were
consistent and highly correlated in the range of 0.05–2 μg/l. The large variability in chlorophyll-a concentration estimate
for clear clean water areas (e.g. with the concentration range lower than about 0.05 μg/l) turns out to be due to the nature
of the “band ratio” based in-water algorithm. 相似文献
89.
Koki Aizawa Wataru Kanda Yasuo Ogawa Masato Iguchi Akihiko Yokoo Hiroshi Yakiwara Takayuki Sugano 《Journal of Volcanology and Geothermal Research》2011,199(1-2):165-175
Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ± 2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change. 相似文献
90.
T. Noguchi M. Kimura T. Hashimoto M. Konno T. Nakamura M. E. Zolensky A. Tsuchiyama T. Matsumoto J. Matsuno R. Okazaki M. Uesugi Y. Karouji T. Yada Y. Ishibashi K. Shirai M. Abe T. Okada 《Meteoritics & planetary science》2014,49(7):1305-1314
We observed cross sectional ultra‐thin sections near the surface of 12 particles recovered from the S‐type asteroid Itokawa by the Hayabusa spacecraft in 2010, using spherical aberration–corrected STEM and conventional TEM. Although their mineralogy is almost identical to the equilibrated LL chondrites and therefore basically anhydrous, micrometer‐to‐submicrometer‐sized sylvite was identified on the surface of Itokawa particle RA‐QD02‐0034. Separately, micrometer‐sized halite was also identified on the surface of Itokawa particle RA‐QD02‐0129. Detailed inspection of the sample processing procedures at the JAXA's Planetary Materials Sample Curation Facility and textural observation of the sylvite and halite indicate that they were clearly present on two Itokawa particles before they were removed from Clean Chamber #2 at JAXA. However, there is no direct evidence for their extraterrestrial origin at present. If the sylvite and halite are extraterrestrial, their presence suggests that they may be more abundant on the surface of S‐type asteroids than previously thought. 相似文献