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281.
The present paper uses planktonic foraminifera and their stable isotopes to study the changes in the depth of thermocline
(DOT) in the Okinawa Trough since the last 10000 a based on the analysis of Core B-3GC in the northern Okinawa Trough, together
with that of the core in the southern Okinawa Trough. As results show, the thermocline was shallow before 6400 aBP, and deepened
afterward, then became shallow again from 4000 to 2000 aBP. The DOT fluctuations display a positive correlation with those
of sea surface temperature (SST). In addition, the changes in the northern Okinawa Trough are similar to those in the southern
trough, implying a possible connection with the variation of the Kuroshio Current. The changes of SST and DOT suggest that
the Kuroshio Current changed its intensity or main axis from 4000 to 2000 aBP and around about 6400 aBP respectively. Moreover,
the changes of DOT from 8200 to 6400 aBP may indicate a gradual intensification of the Kuroshio Current. 相似文献
282.
G. Saito K. Kazahaya H. Shinohara J. Stimac Y. Kawanabe 《Journal of Volcanology and Geothermal Research》2001,108(1-4)
Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300 yr BP to the present. Large variations in volatile concentrations of melts were observed. (1) Water concentration of rhyolitic melts decreases with time; 3–4.6 wt.% at the time of latest caldera-forming eruption of Takeshima pyroclastic flow deposit (ca. 6300 yr BP), 3 wt.% for small pyroclastic flow (ca. 1300 yr BP) of Iwodake, post-caldera rhyolitic dome, and 0.7–1.4 wt.% for submarine lava eruption (Showa-Iwojima) in 1934. (2) Rhyolitic melts of the Takeshima and Iwodake eruptions contained CO2 of less than 40 ppm, while the Showa-Iwojima melt has higher CO2 concentration of up to 140 ppm. (3) Water and CO2 concentrations of basaltic to andesitic melt of Inamuradake, a post-caldera basaltic scoria cone, are 1.2–2.8 wt.% and ≤290 ppm, respectively.Volatile evolution in the magma chamber is interpreted as follows: (1) the rhyolitic magma at the time of the latest caldera-forming eruption (ca. 6300 yr BP) was gas-saturated due to pressure variation in the magma chamber because the large variation in water concentration of the melt was attributed to exsolution of volatile in the magma prior to the eruption. Iwodake eruption (ca. 1300 yr BP) was caused by a remnant of the caldera-forming rhyolitic magma, suggested from the similarity of major element composition between these magmas. (2) Volatile composition of the Showa-Iwojima rhyolitic melt agrees with that of magmatic gases presently discharging from a summit of Iwodake, indicating the low pressure degassing condition. (3) The degassing of the magma chamber by magma convection in a conduit of Iwodake during non-eruptive but active degassing period for longer than 800 years decreased water concentration of the rhyolitic magma. (4) Geological and petrological observations indicate that a stratified magma chamber, which consists of a lower basaltic layer and an upper rhyolitic layer, might have existed during the post-caldera stage. Addition of CO2 from the underlying basaltic magma to the upper gas-undersaturated (degassed) rhyolitic magma increased CO2 concentration of the rhyolitic magma. 相似文献
283.
Naoki Yoshie Yasuhiro Yamanaka Michio J. Kishi Hiroaki Saito 《Journal of Oceanography》2003,59(5):563-571
A one-dimensional ecosystem model has been used to investigate the processes relevant to the spring diatom bloom which play
important roles in the biogeochemical cycle in the western subarctic Pacific. The model represents the plankton dynamics and
the nutrient cycles in the spring diatom bloom; its results show the importance of dilution by deep mixing in winter. It is
supposed that the vertically integrated biomass of phytoplankton decreases in the winter due to the decrease of photosynthesis,
because the deep mixing transports phytoplankton to a layer with a low light level. However, the observed integrated diatom
biomass increases as the mixed layer deepens. This is because the decrease of concentration due to dilution by mixing causes
the diatom grazed pressure to be less significant than diatom photosynthesis. In other words, the effect of dilution on the
grazed rate is more significant than the effect on the photosynthesis rate because the grazed rate depends on the concentrations
of both diatom and grazer, whereas the photosynthesis rate depends only diatom concentration. The average specific diatom
grazed rate, defined as grazed rate divided by diatom biomass, decreases by 35% associated with the deepening, while the average
specific photosynthesis rate of diatom decreases by 11%. As a result, the average specific net diatom growth rate during the
deep mixing is about 70% of its maximum during the spring diatom bloom. The deep mixing significantly affects the amplitude
of the spring diatom bloom not only by the supply of nutrients but also by the dilution which drastically decreases the grazed
pressure.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
284.
Modeling of spring bloom in the western subarctic Pacific (off Japan) with observed vertical density structure 总被引:1,自引:0,他引:1
A. Yoshimori J. Ishizaka T. Kono H. Kasai H. Saito M. J. Kishi S. Taguchi 《Journal of Oceanography》1995,51(4):471-488
Effects of vertical stability on spring blooms of phytoplankton were investigated for the western subarctic Pacific ocean using a one-dimensional (depth) ecosystem model. In the model, vertical stability was expressed by diffusion constants calculated from observed density distribution. Dynamics of phytoplankton in blooms was calculated by the model using the vertical diffusion. Then, the calculated results were compared with the Coastal Zone Color Scanner (CZCS) data. The comparison shows that the shallow surface mixed layer causes early start days of spring blooms at inshore (northern) stations. In addition, spring blooms continue long at inshore (northern) stations since a water column has weak stability. This is because weak stability of a water column causes large nutrient supply from a deep layer and large diffusive transport of phytoplankton biomass from the subsurface maximum. 相似文献
285.
Zhou Huaiyang Pan Jianming Chen Jianfang Wang Huaizhao Y. Saito Y. Kanai 《海洋学报(英文版)》2001,20(1):85-93
INTRODUCTIONEnvironmentalpollutionhasbecomeaveryseriousproblemwiththerapidindustrializa tion .Thepollutioncausedbypollutantssuchasnutrients ,heavymetalsandorganiccompounds(includingpesticides)etc .coulddamagethewell balancedcirculationofnaturalecosysteman… 相似文献
286.
The evolution of paleo-Changjiang incised-valley fills in relation to step-like, postglacial sea-level rises is presented, based on sedimentary facies analyses and 14C age dating on three sediment cores. Timing of rapid transgressions deduced from the succession of incised-valley fills correlate well with Barbados's very rapid, sea-level rise periods. By contrast, estuarine deposits aggraded actively, and the coastline did not retreat markedly during the Younger Dryas (YD) cooling event when the rising rate decelerated considerably relative to stages before and after the YD. 相似文献
287.
Paul J. Harrison Frank A. Whitney Atsushi Tsuda Hiroaki Saito Kazuaki Tadokoro 《Journal of Oceanography》2004,60(1):93-117
The western subarctic gyre (WSG) and the eastern Alaska Grye (AG) on each side of the subarctic North Pacific, have many similarities.
In both gyres, macronutrients are generally high and chl is low, and hence both gyres are High Nitrate, Low Chlorophyll (HNLC)
regions. Despite the general similarities between these two gyres, there are many important differences. The time series station
established at Stn KNOT on the southwest edge of the WSG and two in situ mesoscale iron enrichment experiments at each of
the gyres has provided more information on iron concentrations, the dual role of iron and silicate limitation and seasonal
cycles in the gyres. There is more seasonality in many parameters at Stn KNOT than at Stn P. There is an increase in Chl and
primary productivity at Stn KNOT in May followed by increased iron limitation in summer. Low DIC:NO3 ratios and high Si:NO3 ratios in the WSG, indicate lower calcification and higher diatom production than at Stn P. The sources of iron for these
areas are still not clear, but horizontal transport of iron rich coastal water and vertical transport could be important sources
at certain times of the year in addition to dust input. Satellite images show that chl-rich coastal waters occasionally extend
to the vicinity of Stn KNOT and therefore Stn KNOT may not always represent conditions in the main part of the WSG. This review
focuses mainly on a comparison of Stn KNOT and Stn P, two time series stations on the edge of two very large gyres. At present,
we have a limited understanding of the temporal and spatial variability within each of these large gyres.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
288.
From the viewpoint of dynamical topology, planetary magnetospheres are classified into three: Types 1, 2 and 3. When the rotation vector and dipole moment of a planet and the velocity vector of the solar wind are denoted as Ω, M, and V, respectively, the planetary magnetosphere with Ω∥M⊥V is called Type 1. The magnetospheres of the present Earth, Jupiter, and Uranus at its equinoctial points belong to this type. The magnetosphere with Ω∥M∥V is called Type 2, which includes the Uranian magnetosphere at its solstitial points. The magnetosphere with Ω⊥M and Ω⊥V is called Type 3. The Earth's palaeomagnetosphere is considered to have experienced Type 3 during excursions and transition stages of palaeomagnetic polarity reversals. In the Type 3 magnetosphere, drastic diurnal variations are expected in configurations of the dayside cusps, tail axis, neutral sheet, polar caps, and so on. A possible relation between the Type 3 palaeomagnetosphere and palaeoclimate of the Earth during polarity reversals and geomagnetic excursions is suggested. It is also suggested that the heliomagnetosphere during polarity reversals of the general field of the Sun exhibits a drastic configuration change similar to the Type 3 palaeomagnetosphere of the Earth. A relation between the perpendicular condition Ω⊥M and magnetic variable stars and pulsars is briefly discussed. 相似文献
289.
The auroragraph is a device for the analysis of any movements of patterns on movie films. Its application to the PPI record from an auroral radar is described, and examples from Syowa Station, Antarctica are given. The radar auroragraph has advantages over other more conventional methods of deriving the speed and direction of movement of auroral radar echoes in space. 相似文献
290.
Masaaki Okuda Hiroomi Nakazato Norio Miyoshi Takeshi Nakagawa Hiroko Okazaki Saneatsu Saito Asahiko Taira 《Island Arc》2006,15(3):338-354
Abstract The 250-m Choshi core (CHOSHI-1), drilled from hemipelagic muds of the Inubo Group, has been physically, geochemically and tephrochronologically analyzed back to 1 Ma. We provide pollen results for the 19–169 m section of the core (400–780 ka) bracketed by the marker tephra Ty1 (equivalent to J4) and the Brunhes–Matuyama paleomagnetic boundary. The results show good agreement with the corresponding oxygen isotope (δ18 O) profile, with high δ18 O intervals dominated by boreal conifers Picea , Abies , Pinus (subgen. Haploxylon ) and Tsuga ( diversifolia ), whereas low δ18 O intervals are dominated by temperate conifers Cryptomeria , Taxaceae-Cephalotaxaceae-Cupressaceae and Sciadopitys . In order to confirm pollen-climate relations for the relevant taxa, a modern surface pollen dataset for the Japanese archipelago was consulted. In this analysis, the ratios of Cryptomeria / Picea and temperate/boreal conifers serve as proxies for the 100-kyr glacial/interglacial cycle during the Middle Pleistocene. Distinct signals for marine isotope stages (MIS) 11, 12, 13–15, 16, 17 and 18–19 are recognized in accordance with the tephrochronology and δ18 O of the same core. Application of the criteria to an independent pollen record from Lake Biwa provides an integrated pollen stratigraphy for mid-latitude Japan during the past 800 ky. Some degree of uncertainty remains in the chronology of the MIS13–15 interval, relating to the uncertainty in the eruption age of widespread tephra Ks11. 相似文献