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排序方式: 共有213条查询结果,搜索用时 156 毫秒
81.
Michel Michaelovitch de Mahiques Ilana Klein Coaracy Wainer Leticia Burone Renata Nagai Silvia Helena de Mello e Sousa Rubens Cesar Lopes Figueira Ilson Carlos Almeida da Silveira Mrcia Caruso Bícego Daniel Pavani Vicente Alves
yvind Hammer 《Quaternary International》2009,206(1-2):52
A high-resolution, multi-proxy record has been used to determine the environmental changes during the Holocene on the southern Brazilian shelf. Present oceanographic conditions reveal wind and freshwater input as the determinants of short-term productivity changes in the study area. Magnetic susceptibility and grain-size variations, together with proxies of productivity (organic carbon, carbon accumulation rate, Ba, Sr, and Ca content, Ba/Al, Ba/Ti, and Al/Ti ratios) were analyzed and compared with proxies of redox condition (V/Ti ratio), terrigenous input (Fe/Ca and Ti/Ca ratios), as well as other Element/Ti ratios, to evaluate the paleoceanographic and paleoclimatic changes over the period.The core covers a time interval of about 7650 years, with sedimentation rates varying from 0.025 to 0.250 cm a−1, which represent time intervals of between 8 and 80 a per sample. There is a clear change in the sedimentation rate at about 2800 B.P.All grain-size and elemental results indicate the occurrence of conspicuous changes between 5200 and 5000 cal. B.P., as well as between 3000 and 2800 cal. B.P. A comparison of the results with the palynological information available from the adjacent continental areas suggests that the sedimentary changes in this last interval may be correlated with the onset of modern climatic conditions in South America, and especially, with the onset of the Plata Plume Water, a water mass that carries cold, less saline waters towards the north. However, minor changes are observed at ca. 1500 B.P. and are correlated with an increase in the atmospheric humidity. Furthermore, a time-series analysis undertaken using several proxies indicated the occurrence of Sub-Milankovitch cycles, which may be compared with those reported worldwide. 相似文献
82.
The processes underlying the development of the Kuroshio large meanders that occurred in 1986 and 1989 are investigated using
a satellite SST data set and hydrographic data. In both processes visible on the satellite SST images, a round-shaped, lower
SST region with a diameter of about 200 km is found to the east of the Kuroshio small “trigger” meander (Solomon, 1978) until
the region became extinguished near theEnshu Nada. The lower SST region can be interpreted as an anti cyclonic eddy, mainly because of the existence of a warm water mass in
the subsurface layer of this region. The warm water mass is characterized by a constant temperature of 18–19°C, the maximum
thickness of which is about 400 m. The satellite images show that the eddy is closely related to the Kuroshio path transforming
into a shape like the letter “S”. This means that the eddy plays an important role in the development of the Kuroshio large
meander since this, too, tends to follow an “S”-shaped path. Added to this, the subsurface layer structure of the eddy is
similar to that of the warm water mass offShikoku. This similarity, together with the eddy behavior visible on the satellite SST images, implies that the examined eddy corresponds
to the warm water mass offShikoku. In other words, the warm water mass offShikoku can be advected near to theEnshu Nada when the Kuroshio large meander occurs. 相似文献
83.
A noticeable depression of the vertical component Z of the geomagnetic field is observed in the polar cap in summer. From the statistical analysis of the equivalent overhead current patterns for daily geomagnetic variations in the summer and winter polar regions for three different conditions of IMF (interplanetary magnetic field), it was concluded that the annual variation of geomagnetic Z in the vicinity of the geomagnetic pole is attributed to the relative spatial shift of the twin-vortex current patterns over the polar cap from summer to winter. In winterthe clockwise current vortex in the dawn sector extends almost over the entire polar cap (except for the periods when the Bz component of IMF has a large positive value), and this will result in the positive variation of the Z-value at the geomagnetic pole. In summer the counter-clockwise current vortex in the dusk sector always extends over the whole polar cap even when Bz of IMF is positiveso that the variation of Z becomes negative. The persistent existence of current vortex in the dawn sector is important for the further study of magnetospheric convection when Bz is positive. 相似文献
84.
Tsugunobu Nagai Masaynki Kuwashima Makoto Kawamura Tsuyoshi Kohno 《Planetary and Space Science》1979,27(7):1035-1037
It is suggested that the drop out of the 1.2–4 MeV proton flux, observed by the geostationary satellite GMS, was due to the Earthward shift of the particle boundary in all local time. The particle boundary motions are associated with substorm activities. 相似文献
85.
Sediqi Mohammad Naser Hendrawan Vempi Satriya Adi Komori Daisuke 《Theoretical and Applied Climatology》2022,149(1-2):511-524
Theoretical and Applied Climatology - The global climate models (GCMs) of Coupled Model Intercomparison Project phase 6 (CMIP6) were used for spatiotemporal projections of precipitation and... 相似文献
86.
Naoki Nagai Kazuaki Tadokoro Kazunori Kuroda Takashige Sugimoto 《Journal of Oceanography》2006,62(5):597-606
A long-term variation in dominant species of chaetognaths in the Tsushima Warm Current was analyzed using samples collected
four times a year from 1972 to 2002 by the Maizuru Marine Observatory along the PM transect in the Japan Sea. Of 19 occurring
species, 13 subtropical species formed a major portion of the standing stock. Species richness in the study area was similar
to that in the East China Sea and the South China Sea. Four species, Sagitta minima, S. nagae, S. enflata, S. elegans dominated in the upper 150 m water column. The most dominant species per season were S. minima in both winter and autumn, S. elegans in spring and S. nagae in summer. Water temperature and salinity ranges of the most frequent occurrence for three dominant species revealed a common
tendency in the order of S. nagae > S. minima > S. enflata towards colder temperature and higher salinity. Inter-annual abundance of chaetognath species fluctuated proportionally in
response to the warm or cool phases in water temperature. Water temperature was a fundamental factor in regulating distributional
occurrence, being more decisive than salinity. Changes of both dominant species and species numbers in relation to the reduced
abundance during winter to spring and the subsequent increase during summer to autumn might be caused by specific differences
in optimum temperature. Chaetognath species are important as an indicator of water masses and significant for our understanding
of changes and evaluations of the Japan Sea ecosystem. 相似文献
87.
88.
89.
We investigated the Jurassic Daebo and Cretaceous Bulguksa granitic rocks in South Korea. The former are distributed mainly in the Gyeonggi and Yeongnam massifs and the latter are present in the Gyeongsan basin and Ogcheon belt. The Daebo granitic rocks generally are of ilmenite series and I to S type. These rocks are associated with Au–Ag hydrothermal deposits, whereas the Bulguksa granitic rocks are of magnetite series and I type, and are associated with Pb–Zn, Cu and Mo–W hydrothermal deposits, as well as Au–Ag hydrothermal deposits. The Daebo granitic rocks show adakitic signatures in their chemical compositions. They are considered to have been derived from partial melting of the thick lower continental crust. Conversely, the Bulguksa granitic rocks in the Gyeongsan basin are non‐adakitic and are considered to have been derived from partial melting of a mantle wedge. Magmas of the Daebo granitic rocks formed at relatively shallow levels, but solidified at deep levels compared with those of the Bulguksa granitic rocks. The Bulguksa granitic rocks in the central to western Ogcheon belt are considered to have been formed by fractionation of magmas derived from partial melting of continental crust. The total Al contents of biotite and hornblende in the granitic rocks increased, with the Bulguksa granitic rocks in the Gyeongsan basin < the Bulguksa granitic rocks in the Ogcheon belt and Gyeonggi and Yeongnam massifs and the Daebo granitic rocks in the Ogcheon belt < the Daebo granitic rocks in the Gyeonggi and Yeongnam massifs. This order corresponds to an increase in solidification depth. 相似文献
90.
Daisuke NAKASHIMA Takayuki USHIKUBO Rudraswami N. GOWDA Noriko T. KITA John W. VALLEY Keisuke NAGAO 《Meteoritics & planetary science》2011,46(6):857-874
Abstract– The ion microprobe is the only technique capable of determining high‐precision stable isotope ratios in individual tiny extraterrestrial particles (≤100 μm in diameter), but these small samples present special analytical challenges. We produced a new sample holder disk with multiple holes (three holes and seven holes), in which epoxy disks containing a single unknown sample and a standard grain are cast and polished. Performance tests for oxygen two‐isotope analyses using San Carlos olivine standard grains show that the new multiple‐hole disks allow accurate analysis of tiny particles if the particles are located within the 500 μm and 1 mm radius of the center of holes for seven‐hole and three‐hole disks, respectively. Using the new seven‐hole disk, oxygen three‐isotope ratios of eight magnesian cryptocrystalline chondrules (approximately 100 μm in diameter) from the Sayh al Uhaymir (SaU) 290 CH chondrite were analyzed by ion microprobe at the University of Wisconsin. Five out of eight chondrules have nearly identical oxygen isotope ratios (Δ17O = ?2.2 ± 0.6‰; 2SD), which is consistent with those of magnesian cryptocrystalline chondrules in CH/CB and CB chondrites, suggesting a genetic relationship, i.e., formation by a common (possibly impact) heating event. The other three chondrules have distinct oxygen isotope ratios (Δ17O values from ?6.4‰ to +2.2‰). Given that similar variation in Δ17O values was observed in type I porphyritic chondrules in a CH/CB chondrite, the three chondrules may have formed in the solar nebula, similar to the type I porphyritic chondrules. 相似文献