Yoshikazu Sampei

Changes in accumulation rate of organic carbon during the last 8,000 years in sediments of Nakaumi Lagoon, Japan

Changes in the organic carbon accumulation rate (Corg A.R.) and their relation to climate changes during the last 8,000 years were investigated in a 20 m sediment core from the Nakaumi Lagoon, southwest Japan. High-resolution 14C dating was possible from fifteen shell samples. The warm climate interval 8,000-6,500 cal. yr B.P. was accompanied by an increase in the Corg A.R. (21–32 g m−2 yr−1) and the cool climate interval 3,000-2,000 cal. yr B.P. by a decrease (11 g m−2 yr−1). The accumulation of both terrestrial and planktonic organic matter increased during the warm interval. The lagoon seems to act as a more effective small sink for atmospheric CO2 following the global warming climate.

Geochemical record of the Holocene Jomon transgression and human activity in coastal lagoon sediments of the San'in district, SW Japan

Abstract Coastal lagoon muds were analyzed to evaluate changes in sedimentary environments by the Jomon transgression from the lower to middle Holocene age and human activities. Core samples from Lake Shinji, Southwest Japan were utilized, which cover the entire Holocene Nakaumi Formation (ca. 23-m thick), and comprise the Lower, Middle, Upper and Uppermost members. Total sulfur (TS) contents and Fe 2 O 3 /Al 2 O 3 ratios increase from the boundary of Middle and Upper members, which is 1 m below the Kikai-Akahoya (K-Ah) volcanic ash of 6300 years BP. This change coincides to the pollen flora zone boundary of the Pinus – Abies and the Cyclobalanopsis – Castanopsis , suggesting transition to a warming climate. Chemical index of alteration (CIA) values [(ratio of Al to Al+Ca+Na+K)×100] and Rb/K ratios also show gradual increase from the middle part of the Upper member, due to its derivation from highly weathered source material probably formed under warming and rainy condition. Al 2 O 3 /TiO 2 and SiO 2 /TiO 2 show little variation from the Lower to the Upper members, probably related to consistent feldspar composition in the source rocks, and homogeneously mixed clays. In the Uppermost member (from 1500 years), sharp increases in Ti/Zr and decreases in both Nb/Y and Zr/Y occurred, suggesting heavy mineral fractionation. This change was caused by iron sand processing called Kannanagashi and charcoal-making in the most mountainous regions. Soil erosion by these processes brought changes in mud composition, shown by the enrichment in Al 2 O 3 , and depletion in Zr and Cr. Human activities thus had severe impacts on sedimentary environments compared with the natural changes since the Jomon transgression.

Norcholestane in Miocene Onnagawa siliceous sediments, Japan

Abstract A significant amount of 24-norcholestane with 20R, 5α(H), 14α(H), 17α(H) stereochemistry is often found in marine siliceous sediments of the Middle Miocene Onnagawa Formation, Japan. Some Onnagawa siliceous sediments are abnormally abundant in 24-norcholestane. The primary production in the northeastern Japan Sea during the Middle Miocene was markedly increased due to the invasion of nutrient-rich cold seawater from the north accompanied by the tectonic opening of the Japan Sea. 24-norcholestane in high relative abundance is derived mainly from the marine diatoms which were enhanced during this event. The drastic ecological change caused by rapid tectonic opening of the Japan Sea and global climatic deterioration during Middle Miocene time is suggested to be influential in the formation of 24-norcholestane-rich Onnagawa siliceous sediments.

Geochemical characteristics of oils and source rocks in the Yamal peninsula, West Siberia, Russia

Abstract Eight oil and condensate samples and four samples of possible source rocks from the Yamal peninsula, northernmost West Siberia, have been characterized geochemically in order to determine source rocks and their maturity level. The high content of C29 steranes (m/z = 217) in extracts combined with geological data indicate nearshore marine and continental depositional environments for Cretaceous argillites. Jurassic source rocks, with a predominance of C27 steranes and Pr/Ph ratios of 2.4–2.6, have been deposited in the deeper marine basin and contain oil-prone type II kerogen. A number of parameters 20S/20S + 20R, ββ/ββ + αα steranes (m/z = 217), 22S/22S + 22R, hopane/hopane + moretane (m/z = 191), ratios of tri- and monoaromatic steroids (m/z = 231, m/z = 253), and methylphenanthrene ratios show an increase in maturity with depth, from immature Cretaceous source rocks to moderately mature Jurassic sources (Bazhenov and Tyumen Formations). Oil-oil and oil-source rock correlations based on genetic parameters and maturity indicators have demonstrated that fluids found in Jurassic and Cretaceous reservoirs, including heavy biodegraded oil from the Cenomanian, belong to a single family and could be derived jointly from the Bazhenov (J3) and Tyumen (J1–2) Formations. The origin of naphthenic condensate found at shallow depth is still controversial. Although earlier work has suggested that biodegradation has been a dominant factor in the formation of naphthenic condensates, their formation from Cretaceous source rocks with type III kerogen could also be possible.

Abnormally abundant alkenone-derived C37 and C38 n-alkanes in Miocene Onnagawa siliceous mudstones, northeast Japan

Abstract Abnormally abundant n-C37 and n-C38 alkanes, together with a significant amount of the n-C39 alkane, were found in the Miocene Onnagawa siliceous mudstones. The relative abundance of these alkanes resembles that of ΣC37, ΣC38 and ΣC39 alkenones from Emiliania huxleyi and Gephyrocapsa oceanica. The stable carbon isotope ratio of the n-C37 alkane is −23.5‰, supporting their planktonic origin. The n-C37-C39 alkanes are likely to be derived from alkenones produced by the family Gephyrocapsaceae during the Middle to Late Miocene period. Total organic carbon concentration of the samples rich in the alkenone-derived n-alkanes is in the range of 1.5–2.3%, and the samples are characterized by a low pristane/phytane ratio and a high homohopane index, suggesting anoxic depositional conditions. These sediments are relatively immature (Ro=0.33–0.45%), suggesting that the generation of the long-chain n-alkanes from alkenones took place during early diagenesis. The mudstones rich in n-C37-C39 alkanes are generally poor in CaO (

Geochemical constraints on marine invasion and provenance change related to the opening of the Japan Sea: an example from the Lower Miocene shales in the Hoda section, Shimane Peninsula, SW Japan

Abstract Geochemical changes related to the opening of the Japan Sea are described in the Miocene shale sequence across the boundary of the nonmarine Koura and the marine Josoji formations in southwest Japan. Typical shales of the Koura Formation were derived from source material of acid to intermediate compositions. The Josoji Formation consists of black shales and is more homogeneous in composition than the Koura Formation, with gradual change from intermediate to basic source rock compositions. CIA values [chemical index of alteration; ratio of (Al to Al+Ca+Na+K)×100] for the upper Koura and Josoji formations show higher values (70–80) than those of the underlying lower and middle Koura Formation (40–60). This indicates that highly weathered detritus was supplied to the basin after transgression of the Japan Sea. Both Koura and Josoji shales are depleted in Nb compared with the average composition of cratonic shales, suggesting a volcanic arc signature. Total organic carbon and nitrogen contents increase gradually upward in the two formations, while total sulfur contents show oscillatory change. This relationship suggests a signal of fluctuating sea water invasion into the sedimentary basin during the deposition of the upper units of the Koura Formation (probably between 20–23 Ma) and a regression dominated by fluvial conditions for the lower Josoji Formation.

Spatial distribution of recent ostracode assemblages and depositional environments in Jakarta Bay, Indonesia, with relation to environmental factors

Abstract. Jakarta Bay is a semi-enclosed bay, located on the western side of the northern part of Java Island, Indonesia. This study reports the spatial distribution of recent ostracode assemblages from the top of 19 core samples collected in 1994 and the relationship between the assemblages and environmental factors. This study is the first of the ostracode species of Jakarta Bay, and succeeded in identifying 94 species living there. Ostracodes are common in the East Indian Province. The dominant species found were Keijella carriei Dewi, Hemicytheridea reticulata Kingma, Loxoconcha wrighti Dewi, and Hemicytheridea ornata Mostafawi. Species belonging to the genera Cytherella, Cytherelloidea, Neomonoceratina, and Pistocythereis were also abundant. According to Q-mode cluster analysis, three biofacies (I, II, and III) were recognized, clearly distributed from the inner to the outer parts of the bay. Biofacies I is distributed in the muddy bottoms of the inner to the middle parts of the bay and is composed mainly of K. carriei, L. wrighti, and H. reticulata. Biofacies II is distributed in the muddy bottoms of the outer part of the bay and is characterized by the abundance of H. reticulata, H. ornata, and Cytherella spp. Biofacies III is distributed in the sandy mud bottoms of the outer part of the bay, and is characterized by high-diversity assemblages composed of Atjehella kingmai Keij, Foveoleberis cypraeoides (Brady), Neomonoceratina bataviana (Brady), and Pistocythereis cribriformis (Brady). This study correlated relationships between dominant species and bottom environment factors such as total organic carbon (TOC), total sulfur (TS), total nitrogen (TN), total organic carbon/ total nitrogen ratio (C/N ratio), and total organic carbon/total sulfur ratio (C/S ratio). The results show that K. carriei and L. wrighti are common in areas with high TOC and TN contents, even when they are anoxic, while H. ornata and H. reticulata preferably thrive in deeper areas with low TOC and TN contents. Thus, because of these environmental factors, species diversity and density are low in near-shore sites where the TOC content of mud is relatively high and the bottom is anoxic or oxygen-poor even though the water is shallow.

Current status of hydrocarbon exploration in Sri Lanka

The offshore investigations in Sri Lanka have identified three oceanic basins (i.e., the Mannar, Cauvery and Southern Basins) for possible oil and gas generation. These sedimentary basins recorded thick sedimentary successions from the Upper Jurassic to recent in age. The Cauvery and Mannar Basins contain a complex sequence of potential source rock beds from the Upper Jurassic to Neogene. An extensive kitchen source can be expected during the Upper Cretaceous. In addition, the basin modelling suggests that the Upper Cretaceous rock entered the maximum gas generation window around 20 Ma ago. The excellence reservoir potential can be expected by channel-levee complexes and slope fans in the Cretaceous and Tertiary sequences. The presence of combined stratigraphic and structural traps such as the Upper Cretaceous stratigraphic pinch-outs, horsts and tilted fault blocks and deepwater turbidites give positive force to the hydrocarbon exploration in Sri Lanka. [Received: October 3, 2015; Accepted: January 28, 2017]

Development of aquatic environment measurement systems of estuaries and coastal lagoons

We have developed several new measurement systems for wide area and long-term measurement of the brackish water area (estuaries and coastal lagoons) using ultrasonic waves, optical fiber temperature sensor high-precision salinity and temperature sensors. They are (1) an underwater acoustic reflection profiling system for survey of the halocline, (2) an underwater acoustic reflection measurement system for long-term observation of halocline behavior, (3) a thermometry system using optical fiber distributed temperature sensor and (4) a CT multi-sensor temperature salinity measurement system. Long-term observations using these systems were successfully carried out in the estuaries and coastal lagoons. We are acquiring basic data for preservation and creation of the hydrosphere environment.

Seasonal and tidal influence for water quality changes in coastal Bolgoda Lake system, Sri Lanka

The variation in physicochemical properties is a notable feature to identify hydrodynamic and biogeochemical processes in coastal aquatic systems. In this study, the spatial and temporal changes of water quality parameters were measured with respect to spring and neap tides, covering both single and different monsoon periods. Bulk organic geochemical proxies such as total organic carbon to total nitrogen (C/N ratio) and total organic carbon to total sulfur (C/S ratio) were also considered to evaluate organic sources and depositional environments, respectively. The conductivity was decreased with distance from the sea-mouth to stream during the neap tide under the influence of weak precipitation. No significant variation in conductivity can be observed during the spring tide under the influence of strong precipitation. In addition, the conductivity values in North Bolgoda Lake show homogeneous distribution during spring and neap tides within single monsoon period. Therefore, the reduction of freshwater supply (precipitation) had played a significant role in developing salinity stratification along this coastal water body. The seasonal stratification can control organic matter preservation, as oxygen dissolution is decreased with increasing salinity. In contrast, tidal variation and precipitation had no direct connection to change salinity in surrounding groundwater aquifers. Organic geochemical proxies indicate possible oligotrophic to mesotrophic levels of productivity in Bolgoda Lake system. The prominent fluvial process during rainy periods and seawater invasion during dry periods can diminish the accumulation of nutrients and anthropogenic deteriorations in Bolgoda Lake system under sea-mouth open nature.