Bulk DNAs were extracted from 79 deep-sea sediment core samples of the Nankai Trough, off-central Japan. Total 429 clones of the genes encoding ammonia monooxygenase, amoA (β), and particulate methane monooxygenase, pmoA, yielded ten and two operational amoA (β) and pmoA units (OAUs and OPUs), respectively. Patchy distributions of OAUs and OPUs were not correlated to and explained by geological and geochemical features of the sediments.
Modes of occurrence of fossil barnacle reflecting transgression are discovered from the basal part of the Miocene Natori Group in the Moniwa-Goishi area. The group exhibits the transgressive sequence from subaerial lavas and volcaniclastics to shoreface-shelf deposits in ascending order. Fossil barnacles show autochthonous (in situ individuals on gravel surface) to allochthonous (disarticulated fragments) occurrences. Autochthonous barnacle species, Balanus bisulcatus and Concavus sendaicus are found from bay deposits of the early transgressive stage, and the overlying ravinement deposits formed during marine flooding, respectively. Discoveries of autochthonous barnacles make possible reconstruction of depositional environments and paleoecolgy of extinct barnacles.
The Tertiary Taishu Group in the Tsushima Islands, southwestern Japan is characterized by deltaic systems. The group is subdivided into the Lower, Middle, and Upper Formations. The Lower Formation (Early Eocene) represents a tide-influenced, fluvial-dominated deltaic system, and the Upper Formation (Early Miocene) represents a wave-dominated deltaic system. During the deposition of the Upper Formation, the Japan Sea is interpreted to have started opening and a shallow marine environment appears to have expanded around the Tsushima Islands area. This paleogeographic change is interpreted to have resulted in changes in major depositional processes for the delta formation.
Some turbidite beds of the Middle Pleistocene Kokumoto Formation on the Boso Peninsula of Japan exhibit different types of vertical and lateral changes in grain size and sedimentary structures compared with the Bouma model, which is generally interpreted to represent turbidites deposited from surge-type, ephemeral turbidity currents. Some turbidite beds examined by this study are characterized by several repetitions of coarsening-to-fining upward patterns in association with climbing ripple lamination and parallel lamination. Furthermore, these turbidites are not necessarily laterally continuous in the two-dimensional cross section. Two thickest parts are separately observed within the same stratigraphic level. The geometry and internal organization of these turbidites are interpreted to be a product of hyperpycnal flows generated from a multi-source system associated with a shelf-margin delta in an upslope area during falling- and lowstand stages.
Siltstone clasts are commonly known from turbidites in deep-water successions and have been interpreted to document information regarding depositional processes of host sandstones. Here we studied formative processes of siltstone clasts on the basis of size and fabric of siltstone clasts and of host sandstones. Although siltstone clasts generally exhibit imbrication dipping to upslope directions regardless of depositional features of host sandstones, their size, density, and position in a turbidite bed are variable in response to vertical variations in grain size and grain fabric of host sediments. Therefore, the variation in size, density, and position of siltstone clasts in a single turbidite bed is interpreted to be controlled by temporal variations in the relative rates of suspended-load fallout during a single depositional process of sandstone beds from turbidity currents.
Distribution and diversity of the genes for the enzyme, dissimilatory sulfite reductase (Dsr), which serve as a genetic marker for microbial sulfate reduction, in the Nankai Trough sediment of off central Japan were surveyed. The genetic material, genomic DNA, was extracted from five piston-cored samples with 41 vertical sections, and full-length Dsr gene sequences (ca. 1.9 kb) were PCR amplified for phylogenetic analysis. A total of 382 Dsr gene sequences yielded three Dsr gene groups : Group I, sulfate-reducing Deltaproteobacterial Dsr ; Group II, Nankai Trough novel Dsr ; and, Group III, previously unknown Dsr.
Pliocene Dainenji Formation is distributed in the Pacific Ocean side of the Fukushima Prefecture. The depositional age is from 4.18 to 2.6 Ma. We carried out facies analysis and geochemical analysis, such as total organic carbon (TOC), total sulfur (TS), and stable carbon isotope ratio of organic matter (δ13Corg). Our results are as follows : (1) Abundant land-derived organic matter was transported to the shelf and was preserved in the sediment in warm period (3.75-3.5 Ma), (2) Productivity of marine organism increased in the early cooling period (3.5-3.35 Ma), (3) The considerable variations in TS, TOC and δ13Corg are seen in cooling stage after 3 Ma, which succeeded to the Quaternary ice age. Theses considerable variation are shown in δ18O curve of the same age.
High resolution organic carbon isotopic compositions (δ13Corg) were analyzed for the samples from Wangjiawan section, Yichang, south China. Three negative shifts have been recognized in the uppermost Ordovician. The results indicate that the shifts were related to environmental changes during the development and the decline of the Gondwana ice sheet. Carbonate deposited in the ice period records, inorganic carbon isotope (δ13Ccarb) curve showing a different trend from δ13Corg : δ13Ccarb shows a sharp and negative shift while δ13Corg shifts positive. The δ13Ccarb curve is a local trend in the restricted Yangtze basin. The in-situ oxidation of organic matter enhanced by glacier induced oceanic circulation in narrow and shallow Yangtze basin, and decreased δ13Ccarb in the carbonate bed.
The Umitaka (UT) spur in Japan Sea is characterized by methane-induced features such as Bottom-simulating reflectors (BSRs), pockmarks, mounds and ∼600 m high plumes in the water column (Aoyama et al., 2004). These features seem to indicate active fluid and/or gas venting on the UT spur. In order to estimate methane flux and gas hydrate existence, we measured sulfate and chloride ion concentration of pore water. Sulfate ion concentration has revealed much stronger methane flux than Brake Ridge and chloride anomaly seems to result of recent gas hydrate formation and past dissociation.
This study aims to reconstruct shelf environment of the Lower Triassic Osawa Formation in the Inai Group, the South Kitakami Terrane (SKT), on the basis of the vertical variations of chemical composition and ichnofacies. The Osawa Formation in the Ogatsu area is divided into the lower part (OL ; intensely bioturbated calcareous siltstones) and upper part (OU ; parallel laminated mudstones). The redox proxies (Zn/TiO2 and Ni/TiO2 ratios) and the benthic activity proxies (ichnofabric index and burrow diameter) suggest that the OL was deposited in a shelf where benthic activity was very high, whereas the OU was deposited in a relatively reductive environment that prevented benthic activity.
The Dainenji Formation is deposited on shelf environment and consists mainly of sandy siltstone and sandstone. It includes a number of volcanic ash layers, which serve as good key beds useful for precise correlation. Mappable units of submarine canyon-fill sediment are recognized in this area. A valley has a maximum depth of 100 m and is considered to be formed by submarine landslide, which took place after 2.7 Ma. Submarine canyon-fill sediments are composed of siltstone interbedded with thin sandstone layers, thick sandstone beds and slump deposits. Formation and filling of the submarine canyon might be controlled by climate cooling of latest Pliocene.
The Izu Peninsula and its surrounding area, central Japan represent an arc-arc collision zone. This area is the region of ongoing arc-arc collision between the Izu arc, (=intra-oceanic island) and Honshu arc since the middle Miocene. The arc-arc collision zone provides a lot of significant information to understand the process of an arc-arc collision. We described a framework composition of modern beach sand in the arc-arc collision zone based on the characteristics of petrofacies. The provenance compositional field astriding the boundary line between undissected arc and lithic recycled represent a particular tectonic setting of the arc-arc collision between the Izu Peninsula and the Shimanto accretional prism.
The effects of sedimentation rate on the three dimensional grain fabric were examined using X-ray microtomography. Samples are cut out from plane bed sediments which were aggradated under the four different supply rates with the small recirculating flume. Samples were imaged at SR X-ray microtomogaphic system BL20B2 of SPring-8, Japan. The grain identification program was used to get the information about their axes' direction and length. We analyzed 12 samples by two methods. One is projection of long axes onto equal-area Schmidt net. The other is newly developed method of five dimensional cluster analysis. Grains which long axes are horizontal and parallel to the flow are dominant. Concentration of axes' direction is less in the largest amount of sediment supply on the Schmidt net. Sediment supply affects dominant combination of orientation and shapes of grains.
Strong and unusual BSRs (e.g. High-Amplitude reflections below the BSR) and gas chimneys have been revealed by seismic profiling survey off Shimokita peninsula. These observations suggest anomalously high flux of upward migrating methane and subsurface methane production. This study aims to examine the methane activity in shallow sediments and methane concentration of seawater in this unique area. Piston cores and seawater samples were collected at this area during KT05-7 cruise, April 2005. After cruise, sulfate and chloride concentrations in pore-water, methane concentration and carbon isotopic composition of pore-water and seawater were determined. High methane concentration and 13C-depleted methane (δ13C∼δ-90‰) proved that the study area are characterized by high methane flux and active microbial methane production in shallow sediments. Moreover, seawater has high concentration methane suggesting the possibility of methane release at sea floor.
For the “Yellow beds” occur in the Member 1 of the Lower Cambrian Mila Formation in northern Iran, which consists mainly of stromatolitic and algal dolostone. The beds include conspicuous sedimentary structures including crusts formed in an evaporating condition. Based on the lithology, mineral composition, carbon, oxygen, and Sr isotopes, the “Yellow beds” are regarded as the terrestrial deposits on an arid land. Their heavy δ18O indicates evaporation and the light δ13C may imply a characteristic of terrestrial carbonate. The “Yellow beds” represent subaerial deposits, likely paleo-soil profile, and were deposited in periods of sea-level low during early Cambrian.
Lake Qinghai has thick layers of the sediment over the range of 10,000 years. In order to reconstruct the palaeoenvironment around the Qinghai-Tibet plateau, water content, TOC, OC isotopes of the sediment and ion concentration of Na+ and Cl- of the pore water were examined. They show fluctuations which can be interpreted as variations of the past primary products and productivity, salinity change, and perhaps humidity.
The littoral zone of Miho Peninsula, Shizuoka Prefecture shows a typical example of the coastal erosion which is a serious environmental problem in Japan. This area has been suffered from intensive retrogression of the coastline during last 20 years. This study examines the relation between the coastal erosion and clastic sediment transport from beach to shelf and also describes bottom materials and distributions of bedforms in relation to sediment migration and redeposition.
Structural evidences in the Koshiki Islands and northern Okiwana trough are very important to understand the opening history of the back arc basin. Based on the fault orientation, fault character, cross-cutting relationship and moved direction, NNE-trending fault system of each area is identified as the rifting phase deformation of the Northern Okinawa-trough. The youngest is the northern Okinawa Trough which formed E-W trend sedimentary basin with ENE-trending fault are related in recent stress field of the Kyushu area.
A number of piston cores have been recovered from the active methane seep area off Joetsu, eastern margin of Japan Sea. The sediment cores, approximately 7.0 m long, are typically composed of intensively bioturbated unit, thinly laminated unit, and bioturbated unit, in descending order. Carbon 14 age determination has revealed that the recovered sediments span the last 28,000 years, and the middle laminated unit corresponds to the low stand stage of the Last Glacial Maximum (LGM). Detailed foraminiferal study has revealed that (1) the abundance of benthic foraminifers dropped in the laminated unit whereas the planktonic ones increased for the interval, and (2) Stainforthia rotundata, which is characteristic in methane dominated environment, occurs only in the lower part of the laminated unit. Carbon isotopic composition of the benthic foraminifers shows remarkable negative shift from -1.5‰ to -3.0‰, for approximately 24,000 to 20,000 BP. These lines of evidence strongly suggest that methane seeps in the study area were enhanced for the period of about 24,000 to 20,000BP in LGM.
The Koshiki Islands in western Kyushu consist of the Upper Cretaceous to Paleogene sedimentary rocks and some igneous rocks. This area is one of the best places to understand the stratigraphy and structures of western Kyushu area. These structural data enable us to reconstruct the tectonic history of this area. Based on the characteristics of faults and their crosscutting relationship, we identified two fault systems in the study area. One is the NW-SE trending normal fault system (F1 faults) and the other is the NNE-SSW trending oblique-slip normal fault system (F2 faults). K-Ar ages from one of F1 faults range 17.9-24.8 Ma.
The Lower to early Middle Miocene Goto Group is well exposed in Nakadori-jima and Wakamatsu-jima, central part of the Goto Islands, and is divided lithologically into three formations ; Takasaki, Michidoi and Inoseto formations in ascending order. The Goto Gruop exceeds to 3600 m and represents a stratigraphic evolution beginning with thick pyroclastics deposits in lacustrine environment adjacent to volcanoes (Takasaki Formation), passing into fluvial-lacustrine environments (Michidoi Formation), and ending with deltaic environment (Inoseto Formation). The Takasaki and Michidoi formations are identified as continent marginal lake environment of southern portion of rifting of the Japan Sea. The Inoseto Formation is characterized by a dominance of northeast paleocurrent pattern and quartz-rich sandstone. This stratigraphy indicate that the Goto Group identified as the continental river-delta environments on the southern part of rifted Japan Sea. The floral assemblage found at Atotsugi in the Michidoi Formation, middle part of the Goto Group, suggests cooler climate than that of the Daijima-type flora of late Early to earliest Middle Miocene reported from the upper part of the Goto Group.