Toyosaburo Sakai

Paleoenvironmental Changes in the Japan Sea During the Last 85,000 Years

Five distinct changes in the paleoenvironment of the Japan Sea within the last 85,000 years are revealed from the sedimentary record of a piston core recovered from the Oki Ridge. Changes in both surface and deepwater conditions are registered by changes in lithology, calcium carbonate content, organic carbon content, oxygen and carbon isotope ratios, and microfossil assemblages including calcareous nannoplankton, diatoms, radiolaria, and foraminifera. Between 85 and 27 ka the warm Tsushima Current did not flow into the Japan Sea, and cold surface water conditions prevailed. Environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions. Between 27 and 20 ka, freshwater input to the Japan Sea, probably from the Huang Ho River in China, stratified the water column, and the severe anoxic conditions eliminated most benthic fauna. Between 20 and 10 ka the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Shallow water benthic assemblages of the North Pacific Ocean subsequently colonized the Japan Sea and occupied the vacant niches of the deep basins. Between 10 and 8 ka the foraminifer compensation level (FCL) gradually rose to a depth shallower than 1000 m, and bottom conditions changed from dysaerobic to oxic. At 10 ka the warm Tsushima Current started to flow into the Japan Sea through the Tsushima Strait to establish the modern oceanographic regime which has existed since 8 ka. The eustatic sea level during the last glacial maximum was above the sill depths (130 m) of the Tsushima and Tsugaru straits, assuming that tectonic movements at these straits were negligible for the last 20 ka.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Postglacial environmental change of the Pacific Ocean off the coasts of central Japan

Abstract Downcore changes in microfossil assemblages and oxygen isotope ratios in three piston cores recovered from the Northwestern Pacific, off central Japan, show that the subtropical Kuroshio front was located to the south of C-4 core site (Lat. 33° N) during the last glacial. The front then advanced northward, passing over the C-4 site and the C-6 site (34.6° N) at about 13 ka and 10 ka, respectively, and reached the C-1 core site (36° N) at about 7 ka. After 5.5 ka it retreated to the area between the C-1 and C-6 sites. A brief but significant cold event, the readvance of the cold Oyashio Current, is recognized between 11 and 10 ka in the two northern cores, but the current did not reach the southern C-4 site. A contemporaneous cold event is known in the North Atlantic, and the cooling was probably a global phenomenon likely to be associated with lowering of sea level. Contamination of isotopically light water is apparent between 14 and 11 ka in the marked change in isotopic composition of benthic foraminifers. Oxygen isotope ratios of planktonic foraminifers show that prior to the advance of the Kuroshio front, the surface water at these core sites was isotopically lighter than the Kuroshio water at that time.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Hydrogeochemistry in the barbados accretionary complex: Leg 110 ODP

Abstract Detailed studies of the chemical composition of interstitial waters in the sediments obtained along a drill hole transect across the accretionary prism of the Northern Barbados Ridge have revealed a complex set of processes: (1) In Plio-Pleistocene volcanic ash-bearing sediments increased concentrations of dissolved calcium and decreases in dissolved magnesium are the result of volcanic ash alteration; (2) below the decollement large concentration increases in dissolved calcium, accompanied by large decreases in sodium as well as a granual decrease in dissolved magnesium, suggest exchange of these constituents with the deeper seated rocks of layer 2 of the oceanic crust; (3) low chloride concentrations, particularly associated with the decollement zone separating the converging plates and underthrusted sandstones, and also with recent faults in the accretionary complex, suggest advective transport of low chlorinity waters from deeper within the accretionary complex. In and below the decollement zone elevated concentrations of methane of thermogenic origin have been measured.

Triassic radiolarians from the ocean‐floor sequence of the Waipapa Terrane at Arrow Rocks, Northland, New Zealand

Abstract Triassic radiolarians are reported from two horizons in Waipapa Terrane at Arrow Rocks, Whangaroa area, Northland. This relatively undisturbed succession represents an ocean‐floor sequence, consisting (in ascending order) of basalt with limestone layers, bedded chert, black shale, and red, maroon and green siliceous mudstone. The age of the lower part of the section is Middle‐Late Permian. The radiolarian assemblages reported here indicate Early or Middle Triassic and Middle Triassic (Anisian) ages for maroon siliceous mudstones in the upper part of the section. Between strata of known Permian and Triassic age there is a thin potential Permian/Triassic boundary interval consisting of alternating black shale and grey chert. Radiolarian paleogeography indicates that the Arrow Rocks sequence experienced long distance plate tectonic displacement from a position of relatively low latitude in the Middle Permian and to a high latitude in mid‐Triassic time.

The Ehrenberg type species of flat-shaped radiolarian genera (Spongodiscidae and Stylodictyidae, Spumellaria, Polycystina)

Synopsis Original material for almost all of the polycystine radiolarian species described by Christian Gottfried Ehrenberg (∼8o genera, ∼530 species) are housed in the Museum für Naturkunde, Berlin. Eleven type specimens of 10 spongodiscid and stylodictyid radiolarian genera were found in Ehrenbergs original mica strips. These micas are from six different sample locations and range in geological age from (approximately) late Eocene to Holocene. Ten specimens are confirmed as those sketched by Ehrenberg and are designated as lectotypes. Consequently, Dictyocoryne profunda is the senior valid synonym of Euchitonia für cata and Rhopalodictyum abyssorum. The Ehrenberg collection contains several specimens identified as F. concentrica, the type species of the genus Flustrella. After careful examination, the best preserved specimen was defined as the lectotype and, as a result, Flustrella (type species: F. concentrica) is confirmed as a valid name, while Porodiscus (Trematodiscus orbiculatus) and Trematodiscus (T. orbiculatus) are junior synonyms of Flustrella. The type species of Spongodiscus (S. resurgens), Spongaster (S. tetras) and Dictyocoryne (D. profunda) possess concentric internal structures, suggesting close phylogenetic relationships with Flustrella. Lectotypes of Stylodictya (S. gracile), Perichlamydium (Flustrella praetexta), and Stephanastrum (S. rhombus) are designated.

Mesozoic phosphatic and calcareous nodules containing well-preserved radiolarian fauna from the North Island, New Zealand

ニュージーランドには日本とよく似たチャート・砕屑岩を含む中・古生代の地質体が広く分布するが, 放散虫化石による生層序学的研究はようやく端緒に着いたばかりある. 我々は1995, 96年の2年にわたり, 国際学術研究 (代表: 酒井豊三郎)による, ニュージーランドの中生界を中心とした含放散虫岩の調査を行った. その結果のうち, ワイパパ, ムリヒク両テレーンの, 中生代南半球の極めて保存のよい放散虫化ヨを産するノジュールの産状を紹介する.