Variation of the sea surface temperature distribution across the Kuroshio in the Tokara Strait     

Yutaka Nagata  Katsuichi Takeshita 《Journal of Oceanography》1985,41(4):244-258

Distribution of the sea surface temperature (SST) across the Kuroshio has been measured in the Tokara Strait by the Kagoshima Prefectural Experimental Fishery Station, using a thermometer installed on boardEmerald-Amami, a ferry that operates regularly between Kagoshima and Naha. The data from 1 October 1978 to 30 September 1981 were analyzed in this paper.A sharp temperature front is usually formed at the northern edge of the current zone of the Kuroshio, and its position is very variable and moves north and south between Satamisaki and Nakanoshima. The northward migration of the front can easily be traced, but the southward migration is obscure in many cases. Some of the southward migrations seem to be understood as arising from the alternate appearance of two different fronts, namely a weakening of the northern front and a strengthening of the southern front, which are associated with the double structure of the Kuroshio front. The temperature contrast across the Kuroshio front is very weak in August through October, and the phase of its seasonal variation lags a few months behind that of temperature itself. Transitions between the states with and without temperature contrast occur suddenly, though the transition times differ year by year. Periodical fluctuations with a period of several tens of days are often observed in the migration region of the Kuroshio front. The fluctuations sometimes look very periodical within limited time periods, but the fluctuations are very changeable in nature from year to year.The results show that continuous observation of the SST distribution across the Tokara Strait yields a good tool for monitoring fluctuations of the Kuroshio path and the occurrence of the Ohsumi Branch Current, at least in the season when a large horizontal temperature contrast exists.  相似文献   

Geographical Variations in Prosome Length and Body Weight of Neocalanus Copepods in the North Pacific     

Toru Kobari  Kazuaki Tadokoro  Akihiro Shiomoto  Shinji Hashimoto 《Journal of Oceanography》2003,59(1):3-10

Geographical variations in prosome length and body weight of Neocalanus copepods (N. cristatus, N. plumchrus and N. flemingeri) were investigated on samples from North-South and East-West transects in the North Pacific during spring to early summer in 1998 and 1999. Southward and eastward increasing patterns were pronounced for water temperature, although no significant pattern was observed for chlorophyll a concentrations. All Neocalanus species showed large geographical variations in prosome length and body weight, being smaller in the southern and eastern waters. Comparing the relationship between prosome length and body weight, large deviations (lower body weight at a given prosome length) were evident for the eastern specimens of N. cristatus and N. plumchrus. In stepwise regression analysis, the geographical variations of prosome length and body weight revealed a significantly negative correlation with temperature variations. These results suggest that temperature is a more important environmental factor than chlorophyll a concentration in its effect on geographical variations in prosome length and body weight of Neocalanus copepods in the North Pacific. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Seasonal Variations in the Sea off Sanriku Coast, Japan     

Sachiko Oguma  Toru Suzuki  Yutaka Nagata 《Journal of Oceanography》2002,58(6):825-835

The temperature and salinity data obtained by the Iwate Fisheries Technology Center for the 25-year period from 1971 to 1995 were analyzed to clarify the seasonal variations in the sea off Sanriku, Japan. The variations of three typical waters found in this region, the Tsugaru Current water, the Oyashio water, and the Kuroshio water are discussed in terms, of a T-S scatter diagram referring to the water mass classification proposed by Hanawa and Mitsudera (1986). The mean temperature and salinity fields averaged for each month show clear seasonal variation. Distributions of the Tsugaru Current water and the Oyashio water can barely be distinguished in the fields deeper than 200 m since the Tsugaru Current has a shallow structure; however, the fields at 100 m depth indicate remarkable seasonal variation in the area of the Tsugaru Current. At 100 m depth, the temperature and salinity fronts between the Tsugaru Current water and the Oyashio water gradually disappear in January through April, appear again in May, then become clearest in September. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Brittle normal faulting in the highest-grade Sambagawa metamorphic rocks of central Shikoku, southwest Japan: Indication of the exhumation into the upper crustal level     

Abdel-Hamid El-Fakharani  Toru Takeshita   《Journal of Asian Earth Sciences》2008,33(5-6):303-322

Analysis of fault system in the high-P/T type Sambagawa metamorphic rocks of central Shikoku, southwest Japan, shows that conjugate normal faults pervasively developed in the highest-grade biotite zone (upper structural level) in three study areas (Asemi river, Oriu and Niihama areas). These conjugate normal faults consist of NE–SW to E–W striking and moderately north-dipping (set A), and NNW–SSE striking and moderately east dipping (set B) faults. The fault set A is dominant compared to the fault set B, and hence most of deformation is accommodated by the fault set A, leading to non-coaxial deformation. The sense of shear is inferred to be a top-to-the-WNW to NNW, based on the orientations of striation or quartz slickenfibre and dominant north-side down normal displacement. These transport direction by normal faulting is significantly different from that at D₁ penetrative ductile flow (i.e. top-to-the-W to WNW). It has also been found that these conjugate normal faults are openly folded during the D₃ phase about the axes trending NW–SE to E–W and plunging west at low-angles or horizontally, indicating that normal faulting occurred at the D₂ phase. D₂ normal faults, along which actinolite breccia derived from serpentinite by metasomatism sometimes occurs, perhaps formed under subgreenschist conditions (ca. 250 °C) in relation to the final exhumation of Sambagawa metamorphic rocks into the upper crustal level. The pervasive development of D₂ normal faults in the upper structural level suggests that the final exhumation of Sambagawa metamorphic rocks could be caused by “distributed extension and normal faulting (removal of overburden)” in the upper crust.  相似文献   

Medieval coastal inundation revealed by a sand layer on the Ita lowland adjacent to the Suruga Trough,central Japan     

Yuki Sawai  Koichiro Tanigawa  Toru Tamura  Yuichi Namegaya 《Natural Hazards》2016,80(1):505-519

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The emplacement of in situ greenstones in the northern Hidaka belt: The tectonic relationship between subduction of the Izanagi–Pacific ridge and Hidaka magmatic activity     

Futoshi Nanayama  Jun Tajika  Toru Yamasaki  Hiroshi Kurita  Hideki Iwano  Tohru Danhara  Takafumi Hirata 《Island Arc》2021,30(1):e12403

Greenstone bodies emplaced upon or into clastic sediments crop out ubiquitously in the Hidaka belt (early Paleogene accretionary and collisional complexes exposed in the central part of northern Hokkaido, NE Japan), but the timing and setting of their emplacement has remained poorly constrained. Here, we report new zircon U–Pb ages for the sedimentary complexes surrounding these greenstones. The Hidaka Supergroup in the northern Hidaka belt is divided into four zones from west to east: zones S, U, and R, which contain in situ greenstones; and zone Y, which does not. Detrital zircons in zones S, U, and R have early Eocene U–Pb ages (55–47 Ma) and these strata are intruded by early Eocene granites (46–45 Ma), indicating that they were deposited between 55 and 46 Ma. Therefore, in situ greenstones in the northern Hidaka belt can only be explained by the subduction of the Izanagi–Pacific Ridge during 55–47 Ma. In contrast, the deposition of zone Y (the Yubetsu Group, younging to the west) began by 73–71 Ma, indicating that the accretionary prism in front of the paleo-Kuril arc formed at the same time as that in the Idonnappu zone and grew continuously until 48 Ma. The plutonic rocks that intruded the Hidaka belt are roughly divided into three stages: (1) early Eocene granites intruded the northern Hidaka belt at 46–45 Ma, during subduction of the Izanagi–Pacific Ridge; (2) the upper sequence of the Hidaka metamorphic zone was metamorphosed by magmatism at 40–37 Ma associated with the collision of the paleo-Kuril arc and NE Asia; and (3) younger granites intruded the entire Hidaka belt at 20–17 Ma in association with asthenospheric upwelling caused by back-arc expansion.  相似文献   

Enhancement/reduction of biological pump depends on ocean circulation in the sea-ice reduction regions of the Arctic Ocean   总被引：1，自引：0，他引：1  

Shigeto Nishino  Takashi Kikuchi  Michiyo Yamamoto-Kawai  Yusuke Kawaguchi  Toru Hirawake  Motoyo Itoh 《Journal of Oceanography》2011,67(3):305-314

The biological pump is a central process in the ocean carbon cycle, and is a key factor controlling atmospheric carbon dioxide (CO₂). However, whether the Arctic biological pump is enhanced or reduced by the recent loss of sea ice is still unclear. We examined if the effect was dependent on ocean circulation. Melting of sea ice can both enhance and reduce the biological pump in the Arctic Ocean, depending on ocean circulation. The biological pump is reduced within the Beaufort Gyre in the Canada Basin because freshwater accumulation within the gyre limits nutrient supply from deep layers and shelves hence inhibits the growth of large-bodied phytoplankton. Conversely, the biological pump is enhanced outside the Beaufort Gyre in the western Arctic Ocean because of nutrient supply from shelves and greater light penetration, enhancing photosynthesis, caused by the sea ice loss. The biological pump could also be enhanced by sea ice loss in the Eurasian Basin, where uplifted isohaline surfaces associated with the Transpolar Drift supply nutrients upwards from deep layers. New data on nitrate uptake rates are consistent with the pattern of enhancement and reduction of the Arctic biological pump. Our estimates indicate that the enhanced biological pump can be as large as that in other oceans when the sea ice disappears. Contrary to a recent conclusion based on data from the Canada Basin alone, our study suggests that the biological CO₂ drawdown is important for the Arctic Ocean carbon sink under ice-free conditions.  相似文献   

Non-destructive elemental analysis of lunar meteorites using a negative muon beam     

I-Huan Chiu  Kentaro Terada  Takahito Osawa  Changkun Park  Soshi Takeshita  Yasuhiro Miyake  Kazuhiko Ninomiya 《Meteoritics & planetary science》2023,58(9):1333-1344

We report the result of a non-destructive elemental analysis of lunar meteorites using a negative muon beam at J-PARC. An experimental system of six Ge semiconductor detectors and a newly designed He analysis chamber (to enable quantitative analysis of Al) was used to provide a high signal-to-noise ratio for the detection of major elements from lunar rocks (Mg, Si, Fe, O, Ca, and Al). We performed a Monte Carlo simulation to determine the chemical compositions at two sides and the center of a sample (at depths of 0.33 and 0.96 mm below the sample surface, respectively) of the lunar meteorite DEW 12007. These results indicate that the three interior regions of DEW 12007 are likely to be 55.8:44.2, 51.4:48.6, and 54.4:45.6 wt% mixtures of anorthositic and basaltic clasts, respectively. This study is the first quantitative analysis of a heterogeneous meteorite interior using a negative muon beam. As elemental analysis using a muon beam is non-destructive and highly sensitive to light elements, including C, N, and O, the protocols established in this study are applicable to initial characterization of returned samples from the South Pole of the Moon.  相似文献   

Miocene provenance change in Himalayan foreland basin and Bengal Fan sediments,with special reference to detrital garnet chemistry     

Kohki Yoshida  Toru Nakajima  Yuki Matsumoto  Ai Osaki  Lalit Kumar Rai  Jarrett W. Cruz  Harutaka Sakai 《Island Arc》2021,30(1):e12408

Bengal Fan Miocene sediments were collected during International Ocean Discovery Program Expedition 354 and investigated using petrographic and detrital garnet chemistry analyses. The Miocene Siwalik Group, which is composed of sediments deposited in the Himalayan foreland basin, was also analyzed for comparison with the Bengal Fan data for the provenance change during the Miocene. Our petrographic analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy; most of the garnet is rich in almandine and poor in spessartine and pyrope. However, pyrope-rich garnet, which is considered to originate from the HHC core (granulite facies), was found in the lower to upper Miocene deposits. The deposition of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast, indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion of the HHC zone since the Early Miocene that appears in the Siwalik sediments. Furthermore, we found that the HHC-derived inputs decreased from the late Middle Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus is probably the result of dilution by Lesser Himalayan detritus, which suggests that the Lesser Himalayan zone, which is composed of metamorphosed and unmetamorphosed sedimentary rocks, was uplifted.  相似文献   

Development of the core‐suspended isolation system     

Yutaka Nakamura  Masaaki Saruta  Akira Wada  Toru Takeuchi  Shigeru Hikone  Teiichi Takahashi 《地震工程与结构动力学》2011,40(4):429-447

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