The northern Kyushu-Palau Ridge (KPR), remnant conjugate arc of the Izu-Ogasawara (Bonin)-Mariana (IBM) active arc, is dominated
by basalt-andesite except for the Komahashi-Daini Seamount where acidic plutonic rocks of 38 Ma were recovered. These mafic
to intermediate volcanics are produced by the rifting volcanism in the proto-IBM arc associated with spreading of the Shikoku
Basin. The HFSE and HREE contents and ratios of these volcanics indicate enriched source mantle composition compared to recent
volcanic front. The LILE ratios exhibit similar characteristics to reararc volcanism of the recent Izu arc, and some enriched
volcanics exhibit high abundance of sediment melt inputs. Based on these observations and compilations of the published data
set, the replacement event of the wedge mantle under the IBM arc occurred two times. The first event occurred between 45 and
38 Ma, with Pacific type mantle being replaced by depleted Indian type mantle. The second event occurred between 36 and 25 Ma,
enriched mantle flowed from reararc side. The slab component during the proto-IBM arc rifting was a similar characteristic
to recent reararc volcanism of the Izu arc, and sediment melt added in a local area. 相似文献
The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma. 相似文献
Based on a laboratory study of drained direct shear tests of remoulded, pre-cut and polished specimens and drained ring shear tests of uncut and remoulded specimens of slip surface materials of five landslide soils, significant correlations of the mobilized shear strength parameters, cohesion (c, cr) and internal friction angle (Φ, Φr), are proposed. The investigated soils consisted of the slip surface material belonging to tuffaceous clay, mudstone, loess and siltstone. Most of the previous studies on residual shear strength measured by reversal direct shear and ring shear devices have obtained significantly different results using the two devices, even when pre-cut and polished specimens were used in the direct shear device. In this study, the residual shear strength values of remoulded specimens measured by a ring shear device are shown to significantly correlate with the drained large displacement shear strength values of remoulded specimens, which were measured using pre-cut and polished specimens in a direct shear device. The correlation between the cohesion measured in the two shear devices is expressed by the linear relationship, $ {c_{\text{r}}} = 0.{7394}c - {6}.{6857} $, while the correlation between the friction angle measured in the two devices is expressed by the linear relationship, $ {\Phi_{\text{r}}} = {1}.0{852}\Phi - {6}.0{247} $. The proposed linear correlations for effective cohesion (c′) and effective friction angle (Φ′) have yielded significant coefficients of determination within an effective normal stress range of 30–150?kN/m2. 相似文献
Surface conditions, such as surface roughness and soil moisture, control wind erosion and dust emission in northeast Asia.
Data on spatial and temporal changes of surface soil water content are needed for dust-modeling systems used to predict dust
events with the aim of preventing damage from them. A modified temperature-vegetation dryness index (MTVDI) was tested to
see if it could reproduce surface soil water contents measured during the dust event season in Bayan Unjuul, Mongolia, and
Shenmu, China. MTVDI was calculated from land surface temperature and aerodynamic minimum and maximum surface temperatures
estimated from meteorological data. The standard deviation of the error of estimations of soil water content from MTVDI was
±1.5% in Bayan Unjuul when soil water content was lower than 4%. This compares favorably with the observational error of ±1%
of the soil moisture sensors used. 相似文献
To determine the existence of p-nitrophenol (PNP)-lowering bacteria in intestine of Japanese coastal fish, the gastro-intestinal contents were incubated in Brain Heart Infusion (BHI) broth and minimal medium (MM) broths containing 1 mmol/L PNP at 30 °C for 7 days. Among 26 samples of 19 fish species, 17 samples showed a decrease in PNP of 0.5-0.8 mmol/L in BHI broth, but no decrease was shown in MM broth. Eighteen PNP-lowering bacterial strains were isolated from four fishes. All of the strains were identified as Lactococcus lactis subsp. lactis. Three L. lactis strains JS1-3 isolated from Japanese seabass Lateolabrax japonicus showed the highest PNP-lowering activity (0.44 mmol/L). Optimum temperature and pH for the growth and PNP decreasing corresponded with the marine environment. These results suggested that marine fishes have PNP decreasing bacteria in their intestine. These bacteria might protect host fish from toxicities of PNP and PNP related compounds. 相似文献
Illite crystallinity, K–Ar dating of illite, and fission‐track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out‐of‐sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono‐metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K–Ar ages and zircon fission‐track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years. 相似文献
Wave normal directions of VLF signals propagating through the ionosphere can be determined by measuring Doppler frequency shift of the signals by means of rocket borne receivers. Two rockets were launched to detect the NWC signal of 22.3 kHz which was transmitted from Australia and propagated on two completely different paths, one being propagated through the Earth-ionosphere waveguide and up to the rocket, the other propagated down to the rocket by the whistler mode directly from the source in the opposite hemisphere. The wave normal directions of the latter mode were almost vertically downward in the ionosphere in the northern hemisphere, although substantial error was involved in the determination of the wave normal direction for a part of the upgoing flight of the rockets, due to the relative geometry of the directions of the rocket flight and the geomagnetic field. The effect of the horizontal gradients of the ionosphere on the above results were found to be not significant. From the experimental results it is concluded that field aligned ducts stretching down to the rocket altitudes did not exist, at least, during the rocket flights. 相似文献
The Okinawa marginal basin was opened by crustal extension into the Asian continent, north of the Taiwan collision zone. It is located behind the Ryukyu Trench subduction zone and the Ryukyu active volcanic arc. If we except the Andaman Sea, the Okinawa Trough is the only example of marginal backarc basin type, opened into a continent at an early stage of evolution. Active rifting and spreading can be observed. Synthesis of siesmic reflection, seismic refraction, drilling, dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the Ryukyu-Okinawa area. The main conclusions of the reconstruction of this backarc basin/volcanic arc evolution are. (1) Backarc rifting was initiated in the volcanic arc and propagated along it during the Neogene. It is still active at both ends of the basin. Remnants of volcanic arc are found on the continental side of the basin. (2) There was synchronism between opening and subsidence of the Okinawa Trough and tilting and subsidence of the forearc terrace. The late Miocene erosional surface is now 4000 m below sea-level in the forearc terrace, above the trench slope. Retreat and subsidence of the Ryukyu trench line relative to the Asian continental plate, could be one of the causes of tilting of the forearc and extension in the backarc area. (3) A major phase of crustal spreading occurred in Pliocene times 1.9 My ago in the south and central Okinawa Trough. (4) En échelon rifting and spreading structures of the central axes of the Okinawa Trough are oblique to the general trend of the arc and trench. The Ryukyu arc sub-plate cannot be considered as a rigid plate. Rotation of 45° to 50° of the southern Ryukyu arc, since the late Miocene, is inferred. The timing and kinematic evolution of the Taiwan collision and the south Okinawa Trough opening suggest a connection between these two events. The indentation process due to the collision of the north Luzon Arc with the China margin could have provoked: lateral extrusion; clockwise rotation (45° to 50° according to palaeomagnetic data) and buckling of the south Ryukyu non-volcanic arc; tension in the weak crustal zone constituted by the south Ryukyu volcanic arc and opening of the south Okinawa Trough. Similar lateral extrusions, rotations, buckling and tensional gaps have been observed in indentation experiments. Additional phenomena such as: thermal convection, retreating trench model or anchored slab model could maintain extension in the backarc basin. Such a hypothetical collision-lateral backarc opening model could explain the initiation of opening of backarc basins such as the Mariana Trough, Bonin Trough, Parece Vela — Shikoku Basin and Sea of Japan. A new late Cenozoic palaeogeographic evolution model of the Philippine Sea plate and surrounding areas is proposed. 相似文献
Direct current measurements of the branch current of the Kuroshio intruding into Sagani Bay were carried out during 1989–1990 in order to clarify the frequency characteristics of the eddies in the lee of Izu-Oshima Island, which are well recognized as cold water mass produced by upwelling. Satellite and ADCP (Acoustic Doppler Current Profiler) data indicated that current velocity in the eddy fluctuates with periods of 2–4 days and 6–8 days.When the Kuroshio branch current intruding into Sagami Bay from the western channel is weak and its velocity at the depth of 400 m is approximately 10 cm s–1, the 6–8 day period fluctuation is dominant. On the other hand, when the branch current strongly intrudes from the western channel with a velocity of approximately 20 cm s–1, the 2–4 day period fluctuation dominates. The relationship between the periods and velocities agrees well with theory based on laboratory experiments for a flow of a homogeneous fluid past a circular obstacle. These periods correspond to the time scale of appearance of the eddy caused by the intrusion of the Kuroshio branch current into Sagami Bay and Izu-Oshima Island. 相似文献
Features of El Niño events and their biological impacts in the western North Pacific are reviewed, focusing on interactions between ENSO and the East Asian monsoon. Impacts of El Niño on the climate in the Far East become evident as ‘cool summers and warm winters’. Effects of climate regime shift on ENSO activities, western boundary currents and upper-ocean stratification, as well as their biological consequences are summarized. These have been:
1. In the western equatorial Pacific, an eastward extension of the warm pool associated with El Niño events induces an eastward shift of main fishing grounds of skip jack and big eye tunas.
2. The surface salinity front in the North Equatorial Current region retreats southward, associated with El Niño events. This leads to a southward shift of the spawning ground of Japanese eel, which is responsible for a reduction in the transport of the larval eels to the Kuroshio and Japanese coastal region, causing poor recruitment.
3. Intensification of winter cooling and vertical mixing associated with La Niña (El Niño) events in the northern subtropical region of the western (central) North Pacific reduces surface chlorophyll concentration levels and larval feeding condition for both Japanese sardines and the autumn cohort of Neon squid during winter–early spring. The semi-decadal scale calm winter that occurred during the early 1970s triggered the first sharp increase of sardine stock around Japan.
4. A remarkable weakening of southward intrusion of the Oyashio off the east coast of Japan during 1988–91, resulted in a decrease in chlorophyll concentrations and mesozooplankton biomass in late spring–early summer of the Kuroshio-Oyashio transition region. Changes occurred in the dominant species of small pelagic fish, through successive recruitment failures of Japanese sardine.
Article Outline
1. Introduction
2. Linkage between Asian monsoon and ENSO
2.1. Features of Asian monsoon and its role in ENSO
2.2. Influence of ENSO events on summer and winter climate and hydrographic conditions in the western North Pacific
3. Evidence of biotic impacts of ENSO events in the western and central North Pacific
3.1. Eastward shift or spread of fishing grounds of skipjack, bigeye and albacore
3.2. Decrease of recruitment rate of neon squid and Japanese eel
3.3. Increase of plankton biomass in El Niño winters in the northern subtropical gyre south of Japan
3.4. Bleaching phenomena of corals around the Okinawa Islands
4. Discussion
4.1. Modulation of extra-tropical effect of ENSO by inter-decadal variations
4.2. Effects of ENSO and ocean/climate regime shifts on plankton biomass and population variation of small pelagic fish
5. Conclusion
Acknowledgements
References
1. Introduction
During El Niño events the climate in Northeast Asia is generally cool and wet in summer, and warm and calm in winter (Kurihara and Kimura). In the 1998 summer, near the end of 1997/98 El Niño, the East China Sea and southern part of the Japan Sea were covered with abnormally low saline water. This was the result of the huge amounts of fresh water that were discharge from the Yangtze River and caused poor year classes of Japanese common squid.During the recent cold regime that persisted between 1976/77 and 1987/88 in the North Pacific, Japanese sardine, Sardinops melanostictus, maintained a higher stock level, whereas stocks of anchovy, Engraulis spp., remained low (Kasai; Yasuda and Nakata).To clarify the features of this biological response associated with El Niño events and climate regime shifts, in this paper we provide evidence of several environmental and biological responses in the western and central North Pacific. First, we review the linkage between ENSO and the Asian Monsoon. Second, we present data on the extra-tropical effects of El Niño and La Niña on marine ecosystems and the ocean environment. Finally, we describe the modification of extra-tropical effects of ENSO by interdecadal variations in the ocean and the atmosphere.
2. Linkage between Asian monsoon and ENSO
2.1. Features of Asian monsoon and its role in ENSO
Climate of the western North Pacific is dominated by monsoon winds and precipitation. In summer, the southeast monsoon develops between the Tibetan Low and the North Pacific Subtropical High (Fig. 1a). When the summer monsoon encounters the Japanese mountain range, it produces a considerable amount of precipitation on the Pacific side of Japan. In winter, however, the northwesterly monsoon develops between the Siberian High and the Aleutian Low superimposed on the westerly wind (Fig. 1b). 相似文献
