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Evaluating the mechanical properties of gas (primarily methane) hydrate-bearing sediments is essential for commercial production as a next-generation resource and understanding the global carbon cycle. Triaxial and uniaxial compression tests have been conducted on synthetic gas hydrate and natural core samples recovered from deep-sea beds using pressure coring techniques. The results show that four factors are vital in establishing the strength of hydrate-bearing sediments: hydrate saturation, effective confining stress, porosity, and strain rate. However, no study has evaluated these factors in a unified and quantitative manner, and even if the physical properties of the reservoir are known in detail from logging, predicting the strength has been challenging. In this study, pressure cores were drilled and recovered from the Eastern Nankai Trough in April 2018 after Japan's second offshore production test, and triaxial or uniaxial compression tests were performed on 12 pressure core samples brought back to the laboratory. The mechanical properties of the hydrate-bearing sediments were classified with previous obtained results from 53 pressure cores and 223 synthetic cores, and empirical equations for triaxial compressive strength and deformation modulus were proposed as functions of gas hydrate saturation, effective confining pressure, porosity, and strain rate. The obtained equations were found to correlate well with the experimental data and can predict the strength and deformation modulus from logging data. [ABSTRACT FROM AUTHOR]

Various aquatic organisms are suffering from alterations of estuarine environments, where human activities are generally concentrated. The commercially important but endangered Japanese eel (Anguilla japonica) is one of the species most affected by estuarine habitat loss, because it mainly uses estuarine benthic environments for growth before long migrations to an offshore spawning area. To enhance their estuarine habitats, this study aimed to explore what type of shelter structures promote medium- to long-term settlement of this species and verify whether such shelters improve their growth in a riverine estuary in Japan whose channel was heavily lined with concrete revetment. A survey of the recapture rate of elastomer-tagged eels from the fishing gear "Ishikura net (IKN)," which consists of dozens to hundreds of stones, showed that the rate was higher when their first captures were from IKNs with fine pore structures formed by approximately 10-cm-sized stones than from those with coarser pores. Furthermore, the girth length (nutritional condition index) of recaptured individuals increased during their settlement in the IKNs. The use of the IKNs may have reduced energetic costs of behaviors such as predation avoidance and feeding, considering that a large amount of prey animals (e.g., crabs) were found, especially in the IKNs with fine pores. This study showed that the establishment of artificial shelters with fine pore structures is effective not only for the temporary residence of eel individuals but also for improvement of their body conditions through promoting their settlement and perhaps gathering their prey species. [ABSTRACT FROM AUTHOR]

Episyenite is a quartz-depleted vuggy rock resulting from hydrothermal alteration of granitic rocks. This is the first report of its existence in an island arc, which is identified in a deep drill core of the Toki Cretaceous granite distributed in central Japan. In order to understand the petrographical features of the episyenite, neutron porosity measurement, geochemical analysis, microscopic observation, and X-ray computed tomography scanning were carried out. The results show remarkably high porosity (35.4 %) due to interconnecting vugs and the removal of quartz, plagioclase, and biotite. The Rb-Sr isotopic results and the paragenetic sequence of secondary minerals in the vugs suggest that the hydrothermal alteration process can be divided into an episyenitization stage and a later hydrothermal stage. At the episyenitization stage (70.6 ± 3.1 Ma) ca. 6 million years after the emplacement of the unaltered granite (76.3 ± 1.5 Ma), dissolution of quartz, biotite, and plagioclase occurred and was followed by the precipitation of albite, vermicular chlorite, and platy calcite. The episyenitization is considered as a local alteration of the Toki granite in an isotopically closed system. At the later hydrothermal stage, illite and secondary quartz precipitated from circulating meteoric-derived water in the dissolution vugs. Superimposing alteration at the later hydrothermal stage is limited, which results in the preservation of the episyenite in an almost primitive condition. [ABSTRACT FROM AUTHOR]

We investigated the relationship between variations in the thermal conductivity of surface sediments and the topography in the Nankai subduction zone off Tokai, central Japan, the easternmost part of the Nankai subduction zone, which has an accretionary prism with varied topography. We analyzed sediment thermal conductivity data obtained from the trough floor and accretionary prism. Variations in the thermal conductivity of sediments were related to the topographic features formed by accretionary prism development. Thermal conductivities of 1.1 W/m K were measured on the trough floor where thick terrigenous turbidites have been deposited. The thermal conductivity of Nankai Trough floor sediments decreases from northeast to southwest along the trough, probably because of the decreased grain size and/or changes in sediment mineral composition. High thermal conductivities (≥1.0 W/m K) were measured in fault scarps on the accretionary prism. A landward increase in these values on the prism may be explained by decreased porosity of the sediments attributable to tectonic deformation during accretionary prism development. At the base of the fault scarp of the frontal thrust, low thermal conductivities (<0.9 W/m K) were measured, likely reflecting the high porosity of the talus deposits. Low thermal conductivity (0.9 W/m K) was also measured in slope basins on the accretionary prism, likely also related to the high porosity of the sediments. Our results demonstrate that, for accurate heat flow measurement in an area of varied topography, the geothermal gradient and the thermal conductivity of the sediments must be measured within regions with similar topographic features. [ABSTRACT FROM AUTHOR]

The article discusses the isotopic data of the first Neodymium (Nd)-Strontium (Sr) of microgranitoid inclusions in the granitoides of Eastern Sikhote-Alin in Japan Sea coast, as well as their genetic significance. It says that the isotope data of Samarium (Sm) and Nd for inclusion rock pairs possess isotope characteristics that are typical for acid rocks. It states that the inclusions occur in various types of rocks and are characterized to have higher magnetic susceptibility and porosity compared to their host rocks. Moreover, data analysis of rubidium (Rb)-Sr and Sm-Nd isotopes reveals a simultaneous increase of 87Sr/86Sr and 87Rb/86Sr ratios in the series of diorites to granites.

We investigated the distribution patterns of Rhyacophila larvae at 2 habitat scales in mountain streams in central Honshu. One was at a rapid glide within a stream reach and the other was in microvertical layers within each rapid and glide. Cobbles were predominant in rapids, whereas sand and gravels were predominant in glides. Mean sediment particle size was smaller in glides, but evenness of sediment size and porosity were greater. A total of 9 species of Rhyacophila larvae were collected from rapids. On the other hand, in glides, only 4 species, R. nipponica, R. shikotsuensis, R. kawamurae, and R. transquilla, were collected, but they were more abundant than in rapids. The larvae of R. transquilla were found mainly in upper layers of the stream bed, whereas the other 3 species, R. nipponica, R. shikotsuensis, and R. kawamurae, occurred both in upper and lower layers. The latter 3 species belong to R. nigrocephala species group, the larvae of which share several common morphological and behavioral characters adapted to hyporheic habitat, such as slender body shape and vermicular movement. It is suggested that higher porosity in glides provides more microhabitat for them, and small-sized sediment particles are appropriate for their burrowing and creeping movement in hyporheic habitat. [ABSTRACT FROM AUTHOR]

To clarify the reason for the higher CH4 uptake rate in Japanese forest soils, twenty-seven sites were established for CH4 flux measurement. The first order rate constant for CH4 uptake was also determined using soil core incubation at 14 sites. The CH4 uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH4 uptake rates ranged from 2.7 to 24.8 kg CH4 ha−1 y−1 (the average of these rates was 9.7 or 10.9 kg CH4 ha−1 y−1, depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH4 uptake rate correlated closely with the CH4 oxidation rate of the topsoil (0–5 cm) in the study sites. The CH4 oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio ( p < 0.05, R2 = 0.684). This result and comparison with literature data suggest that the high CH4 uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH4 uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH4 uptake rate in temperate forests was estimated to be 5.4 Tg y−1 (1 SE is 1.1 Tg y−1) on a continental basis. [ABSTRACT FROM AUTHOR]

An accurate determination of the hydrological characteristics of porous media, such as the values of the porosity and effective porosity, are essential. This is important to understand the transport processes of infiltration and the movement of water and contaminants in the porous media. In this study, a laboratory soil column experiment to estimate the porosity and effective porosity of Toyoura standard sand samples, using a dielectric method termed the frequency domain reflectometry with vector network analyzer (FDR-V), was performed. The FDR-V device uses high-frequency microwaves, ranging from 0.1 to 3 GHz, to measure the complex dielectric constants of the sample. From the measured complex dielectric constant, the two parameters of the soil samples then were derived, using a proposed dielectric mixture model and tracer concentration model. The effective porosity of the soil sample is measured at 0.311 with the FDR-V dielectric method and 0.345 by the soil tracer column test. Comparing this with the calculated porosities of the soil sample, the ratio of effective porosity is approximately 78% for the dielectric method, and approximately 86% for the tracer concentration method. These different values can be explained to be within the measurement range with regard to the measurement volume for the soil column. These results indicate that measurement by the dielectric method using an FDR-V device is an efficient and useful tool for estimating the hydrological parameters of porous media because of the dielectric response of earth materials. [ABSTRACT FROM AUTHOR]

Several factors controlling the soil radon level in the present site were found to be changing air-filled porosity caused by fluctuations in moisture content, differences between the atmospheric and soil temperatures as well as volumetric 226Ra content of the soil. The radon activity increased significantly in early October, especially at point 1, possibly as a result of a magnitude 8.0 earthquake which occurred on September 26, 2003, with epicenter located offshore near Tokachi, Hokkaido. [ABSTRACT FROM AUTHOR]