Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai–Khentei belt of Central Mongolia: Evidence for Middle Paleozoic subduction–accretion activity in the Central Asian Orogenic Belt     

Toshiyuki Kurihara  Kazuhiro Tsukada  Shigeru Otoh  Kenji Kashiwagi  Minjin Chuluun  Dorjsuren Byambadash  Bujinlkham Boijir  Sersmaa Gonchigdorj  Manchuk Nuramkhan  Masakazu Niwa  Tetsuya Tokiwa  Gen’ya Hikichi  Takafumi Kozuka 《Journal of Asian Earth Sciences》2009,34(2):209-225

Recent mapping projects undertaken in Central Mongolia have revealed the widespread occurrence of radiolarian chert within a Paleozoic accretionary complex. We present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the Gorkhi Formation in the Khangai–Khentei belt of the Central Asian Orogenic Belt.The Gorkhi Formation consists of sandstone shale, alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale, basalt, limestone, and clast-bearing mudstone. Radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt (sedimentary contact) and with conformably overlying siliceous shale and turbidite deposits. The tectonic stacking of basalt–chert and chert–turbidite successions is the most remarkable structural feature of the formation.The recovery of moderately well-preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest Silurian (Pridolian) to the Late Devonian (Frasnian). No age control exists for the siliceous shale, shale, and sandstone, although they are considered to be latest Devonian or slightly younger on the basis of stratigraphic relationships with underlying chert.The Gorkhi Formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed Archean–Neoproterozoic basement; however, the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate. From the lowermost to uppermost units, the stratigraphy comprises ocean floor basalt, pelagic deep-water radiolarian chert, hemipelagic siliceous shale, and terrigenous turbidite deposits. The biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep-water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years. These data, together with structural data characterized by tectonic repetition of the stratigraphy, indicate that these rocks formed as an accretionary wedge along an active continental margin, possibly that of the Angara Craton. The mid-oceanic chert was probably deposited in the Northern Hemisphere portion of the Paleo–Pacific Ocean that faced the Angara Craton and the North China–Tarim blocks. Thus, we propose that subduction–accretion processes along the Paleo–Pacific rim played an important role in the accretionary growth of the active continental margin of the Angara Craton, directly influencing the evolution of the Central Asian Orogenic Belt.  相似文献   

Horizontal and vertical distributions of <Superscript>137</Superscript>Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant     

Tadahiko Tsuruta  Hisaya Harada  Toshiharu Misonou  Toshiyuki Matsuoka  Yasuyuki Hodotsuka 《Journal of Oceanography》2017,73(5):547-558

Investigations including a bathymetric survey, sonic prospecting, and vibrocoring were performed to understand the horizontal and vertical distribution of ¹³⁷Cs in seabed sediments in shallow seas with depths less than 30 m near the Fukushima Daiichi Nuclear Power Plant. Especially, features of ¹³⁷Cs distributions in deeper sections of the seabed sediments were studied to evaluate the vertical heterogeneity of ¹³⁷Cs distribution in the seabed sediments in shallow seas. The distribution area of the seabed sediments was less than half of the investigation area, and the locations of the seabed sediments were divided into flat and terrace-like seafloors based on their topographical features. The thicknesses of the seabed sediment layers were mostly <2 m. The ¹³⁷Cs inventories in the seabed sediments varied from 13 ± 1 to 3,510 ± 26 kBq m^?2, and continuous distributions of ¹³⁷Cs at depths greater than 81 cm were observed. The ¹³⁷Cs distributions were not uniform; however, the ¹³⁷Cs inventories tended to be larger near the base of the steeper ascending slopes than in the terrace-like seafloors themselves. Based on the relationship between the ¹³⁷Cs inventories and mean shear stress, features of the seafloor topography were inferred to be significant control factors governing the horizontal and vertical distribution of ¹³⁷Cs in the seabed sediments. Rapid changes and multiple peaks in the vertical profile of the ¹³⁷Cs distributions suggest that they are related to pulse input caused by heavy-rain events. Change in the ¹³⁷Cs inventories with depth in this study are larger than those reported in previous studies, indicating earlier results of ¹³⁷Cs inventories per unit in seabed sediments in shallow seas, especially near the river mouth, which drains a radiologically highly-contaminated basin, were underestimated.  相似文献   

Biological productivity, terrigenous influence and noncrustal elements supply to the Central Indian Ocean Basin: Paleoceanography during the past ∼ 1 Ma     

J. N. Pattan  Toshiyuki Masuzawa  D. V. Borole  G. Parthiban  Pratima Jauhari  Mineko Yamamoto 《Journal of Earth System Science》2005,114(1):63-74

A 2m-long sediment core from the siliceous ooze domain in the Central Indian Ocean Basin (CIOB; 13‡03′S: 74‡44′E; water depth 5099m) is studied for calcium carbonate, total organic carbon, total nitrogen, biogenic opal, major and few trace elements (Al, Ti, Fe, K, Mg, Zr, Sc,V, Mn, Cu, Ni, Zn, Co, and Ba) to understand the productivity and intensity of terrigenous supply. The age model of the sediment core is based on U-Th dating, occurrence of Youngest Toba Tuff of ∼ 74 ka and Australasian microtektites of ∼ 770 ka. Low carbonate content (< 1%) of sediment core indicates deposition below the carbonate compensation depth. Organic carbon content is also very low, almost uniform (mean 0.2 wt%) and is of marine origin. This suggests a well-oxygenated bottom water environment during the past ∼ 1100ka. Our data suggest that during ∼ 1100 ka and ∼ 400 ka siliceous productivity was lower, complimented by higher supply of terrigenous material mostly derived from the metasedimentary rocks of High Himalayan crystalline. However, during the last ∼ 400 ka, siliceous productivity increased with substantial reduction in the terrigenous sediment supply. The results suggest that intensity of Himalayan weathering, erosion associated with monsoons was comparatively higher prior to 400 ka. Manganese, Ba, Cu, Ni, Zn, and Co have around 90% of their supply from noncrustal (excess) source and their burial to seafloor remained unaffected throughout the past ∼ 1100 ka.  相似文献   

Interannual-decadal variability of wintertime mixed layer depths in the North Pacific detected by an ensemble of ocean syntheses     

Toyoda  Takahiro  Fujii  Yosuke  Kuragano  Tsurane  Kosugi  Naohiro  Sasano  Daisuke  Kamachi  Masafumi  Ishikawa  Yoichi  Masuda  Shuhei  Sato  Kanako  Awaji  Toshiyuki  Hernandez  Fabrice  Ferry  Nicolas  Guinehut  Stéphanie  Martin  Matthew  Andrew Peterson  K.  Good  Simon A.  Valdivieso  Maria  Haines  Keith  Storto  Andrea  Masina  Simona  Köhl  Armin  Yin  Yonghong  Shi  Li  Alves  Oscar  Smith  Gregory  Chang  You-Soon  Vernieres  Guillaume  Wang  Xiaochun  Forget  Gael  Heimbach  Patrick  Wang  Ou  Fukumori  Ichiro  Lee  Tong  Zuo  Hao  Balmaseda  Magdalena 《Climate Dynamics》2017,49(3):891-907

Climate Dynamics - The interannual-decadal variability of the wintertime mixed layer depths (MLDs) over the North Pacific is investigated from an empirical orthogonal function (EOF) analysis of an...  相似文献   

Internal Wave Generation by Tidal Flow over a Continental Shelf Slope     

Toshiyuki Hibiya 《Journal of Oceanography》2004,60(3):637-643

The generation process of internal waves by strong tidal flow over a continental shelf slope is reproduced using a multi-level numerical model. On the basis of the numerical results, the crucial role of the tidal advection effect in the generation process of internal waves is demonstrated. The close relation between the resulting internal waveform and the strength of the tidal advection effect is also examined. The barotropic forcing on the internal wave actually works within a relatively small horizontal scale over the top of the continental shelf slope. When the maximum internal Froude number at the shelf break (Fr_m) is less than about 0.6, the amplitude of the resulting internal wave is almost proportional to F_rm. When F_rm is more than about 0.6, however, the amplitude of the resulting internal wave becomes larger than predicted by linear theory. In particular, when F_rm is more than unity, the time period during which the shoreward propagating internal wave stays in the barotropic forcing region becomes much longer. Consequently, the internal wave is significantly amplified with the horizontal scale approaching that of the barotropic forcing, which concentrates in a relatively small region over the top of the continental shelf slope. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Interstitial water chemistry in deep-sea sediments from the Japan Sea     

Masuzawa  Toshiyuki  Kitano  Yasushi 《Journal of Oceanography》1983,39(4):171-184

Vertical profiles of pH, Eh, and major and minor chemical constituents in interstitial waters have been studied in four piston cores from deep-sea basins of the Japan Sea. Sulfate concentration decreases remarkably with increasing depth in three cores, and hydrogen sulfide is observed in two cores although the overlying bottom water is highly aerated. Three types of interstitial waters are observed in the four cores: (1) alkalinity and manganese concentration increase gradually with increasing depth, and sulfate reduction is not appreciable, (2) alkalinity increases and sulfate concentration decreases remarkably, and hydrogen sulfide is not detected, and (3) alkalinity increases and sulfate concentration decreases remarkably, and hydrogen sulfide is observed. Factors controlling the chemical composition of interstitial waters are discussed.  相似文献   

Distribution of sardine eggs and larvae related to the surface circulation in Sagami Bay     

Hideaki Nakata  Keiichi Hasunuma  Toshiyuki Hirano 《Journal of Oceanography》1989,45(1):11-23

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Oceanographic analysis of the North Pacific,with special reference to the Kuroshio     

Jotaro Masuzawa 《Journal of Oceanography》1976,32(4):187-193

I have been concerned with oceanographic observations made since 1948 by the oceanographic group of the Japan Meteorological Agency. My primary interest has been to describe physical aspects of the Kuroshio current system and its adjacent regions. Much efforts have been devoted to observing current and water characteristics of the Kuroshio south of Japan and the Kuroshio extension east of Japan and to describing various kinds of fluctuations ranging from diurnal to seasonal or year-to-year variations. I have also attempted to study the equatorial current system particularly in the western Pacific and those characteristic water-types in the central region of the North Pacific.  相似文献   

Strong tidal currents observed near the bottom in the Suruga Trough,central Japan     

Masaji Matsuyama  Suguru Ohta  Toshiyuki Hibiya  Haruya Yamada 《Journal of Oceanography》1993,49(6):683-696

Current measurements carried out at the depth of 4 m above the sea bottom near the northern edge of the Suruga Trough in the early fall of 1985 indicated the existence of strong semidiurnal tidal currents, which were considered to be associated with internal tides. In order to examine the spatial structure of the bottom intensified tidal flow, more detailed current observations were carried out at three or four depths at two stations along the main axis of the Suruga Trough during about 70 days from August to October 1988. We obtained the following results: (1) the variations of the current velocity caused by the semidiurnal and diurnal internal tides are evident in all of the records, and the orientation of the major axis of each tidal ellipse nearly coincides with that of the main axis of the trough; (2) the semidiurnal internal tide is dominant over the diurnal internal tide at 4 m above the sea bottom at both stations; (3) at the northern station the semidiurnal internal tide is dominant over the diurnal internal tide, whereas they are nearly equal at the southern station except at 4 m above the sea bottom; (4) the biharmonic internal tides with 1/3 day and 1/4 day periods, are found near the sea bottom and the major axis of the tidal ellipse is perpendicular to the orientation of the main axis of the Suruga Trough.  相似文献   

Temporal variation in mass fluxes and the major components of sinking particles in Sagami Bay, off Japan     

Toshiyuki Masuzawa  Tie LiYun Duan  Mineko YamamotoYuriko Hibi  Takeshi NakatsukaHiroshi Kitazato  Yoshihisa Kato 《Progress in Oceanography》2003,57(1):59-75

Sinking particles were collected using time-series sediment traps deployed at 350 and 20 mab at Site SB (34° 58.5’N, 139° 20.9’E, 1544 m depth) near the center of Sagami Bay, off Japan with high time resolutions of 5-8 days (March 1997 to August 1998) and 3-4.5 days (March 1998 to August 1998), respectively. The major components (CaCO₃, OM, opal, and clay) of these sinking particles and surface bottom sediments were determined using a stepwise leaching method combined with gravimetry. Average total mass fluxes were 1480, 5560 and 3068 mg/m²/year at 350 mab, at 20 mab, and in the surface sediments, respectively, indicating an enhanced collection of sinking particles at 20 mab. Clay was the dominant component and biogenic components (opal+OM+CaCO₃) were dominated mainly by opal and secondly by OM. On average, opal and CaCO₃ contents decreased gradually as clay content increased with increasing depth from 350 mab-20 mab and in the surface sediments, indicating dissolution of opal and CaCO₃ through sinking, rebound, resuspension or sedimentation processes. Thirteen total mass flux peaks at 17--40-day intervals were observed at 350 mab during the period from March 1997 to August 1998 except for winter, while eight peaks were observed at 20 mab for the period from March 1998 to August 1998. Two types of total mass peaks can be distinguished: one with a clear increase in biogenic flux (opal+OM+CaCO₃) and little or no increase in clay flux and termed a bloom type (B-type), and the other with a clear increase in clay flux, little increase in biogenic flux and termed a resuspension type (R-type). Some R-type peaks, but not all, coincided with total mass flux peaks observed at the mouth of Tokyo Bay and suggested the possibility of the effect of particulate materials transported from Tokyo Bay to site SB. The enormously large peak observed at 20 mab in late May 1998 and that at 350 mab in early June 1998 were considered to be due to some physical perturbations from an earthquake swarm near site SB during the period from April to June 1998. The 17--40-day periodicity was associated clearly with the change in biogenic flux dominated by opal flux and is thought to reflect the periodicity of biological productivity dominated by diatoms in the euphotic zone of Sagami Bay.  相似文献