Uranium and Thorium Isotope Distribution in an Offshore Bottom Sediment Core of the Selenga Delta, Lake Baikal, Siberia

Distribution of uranium and thorium isotopes in a short sediment core obtained offshore of the Selenga Delta in Lake Baikal, Siberia, was investigated to establish their sedimentary behaviors and to look for a linkage to paleoenvironmental changes. The sediments were composed of dominantly fine detrital materials (70–85%) and a relatively high sedimentation rate (ca. 0.03 cm y⁻¹). The depth profile of ²³⁸U content in bulk sediment samples showed a large variation of 70–123 Bq kg⁻¹, while ²³²Th profile showed a relatively narrow range from 36 to 56 Bq kg⁻¹. The observed ²³⁴U/²³⁸U activity ratios revealed a marked disequilibrium ranging from 1.53 to 1.84 with a mean value of 1.71 ± 0.07, demonstrating the presence of 50–80% authigenic ²³⁸U in the bulk sediments. The distribution of this authigenic ²³⁸U did not display any clear correlation with variations in sediment composition (organic, carbonate, Bio-SiO₂ and mineral contents) including grain size median. The profile of terrigenous ²³⁸U showed a relatively similar pattern to that of ²³²Th. Results of sequential leaching indicate that ²³⁸U in Fe–Mn oxyhydroxides fractions were responsible for the distribution of authigenic ²³⁸U rather than in Bio-SiO₂ fraction. The distribution of authigenic ²³⁸U in the bottom sediments may be explained by the fluctuation of U adsorption capacity on particles including organic matter and Fe–Mn oxyhydroxides before they entered the lake. This study highlights the potential use of authigenic and terrigenous U (Th) signatures in sediments to trace the behavior of U (Th) and to reconstruct environmental (e.g., hydrological) changes in the lake catchment area.     

Soil H<Subscript>2</Subscript> and CO<Subscript>2</Subscript> Surveys at Several Active Faults in Japan

Soil H₂ and CO₂ surveys were carried out along seven active faults and around the aftershock region of the 2000 Tottori-ken Seibu earthquake in Japan. Diffuse CO₂ effluxes were also measured along one fault and around the 2000 aftershock region. The results show highly variable H₂ concentration in space and time and it seems that the maximum H₂ concentration at each active fault correlates with fault activity as exemplified by the time of the latest big earthquakes. Even though observed H₂ concentrations in four faults were markedly lower than those collected previously in the latter half of the 1970s, it is evident that the higher H₂ concentrations in this study are due to the addition of the fault gases. Comparing the chemical composition of trapped gases (H₂: 5–20% and CO₂/H₂: 0.5–12) in fractured rocks of drill cores bored at the Nojima fault, a soil gas sample with the highest H₂ concentration showed large amounts of the trapped fault gas, diluted with atmospheric component. The profile experiment across a fracture zone at the Yamasaki fault showed higher H₂ concentrations and lower CO₂/H₂ ratios as was observed in soil gas from the fracture zone. A few days after the 2000 Tottori-kei Seibu earthquake, no CO₂ effluxes related to the occurrence of earthquakes were observed at the aftershock region. However, only above the epicenter zone, relatively high H₂ concentrations in soil gases were observed.     

Formation of Starburst Galaxies in Merging Clusters

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process in cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations that show the dynamical evolution of a gas-rich, late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that the time-dependent tidal gravitational field during merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region and finally triggers a secondary starburst. This result provides a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma Cluster as one observational example indicating the global tidal effects of group–cluster merging. Our numerical results further suggest a causal link between the observed excess of blue galaxies in distant clusters and the cluster virialization process through hierarchical merging of subclusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrological processes of adsorption,sedimentation, and infiltration into the lake bed during the 2004 urban flood in Dhaka city,Bangladesh

The urban flood of Dhaka city in September 2004 is a result of heavy rainfall (341 mm a day or more than 600 mm in 5 days). In addition to the local rainfall, elevated water levels in the surrounding rivers may have affected the flood duration by bank filtration. Two lakes, Banani and Gulshan, in the mid of the city are the part of general storm water retention system in Dhaka. Flood water runs through the lakes acting as channels to the pumping station downstream. A tank model is used in this study to describe the total drainage process with the hydraulic geometry and resistance laws to derive cross-sections and friction factors of the quasi-uniform flows. Assuming the dissolved matters of the flood water was of similar composition as in the lake sediments, the process was a combination of adsorption and sedimentation. Lake sediment samples collected in March 2004 show an anomaly of chromium which may reflect the previous 1998 flood, and there may be a diffusive leakage from the source area to the adjacent zones. Further evidence of infiltration process is seen in records of diurnal thermal stratification of Lake Banani. It responds to only heavy rainfalls over 100 mm/day, and the fact means that infiltration to the groundwater is rather intermittent.     

On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

Recent observational studies of intermediate-age star clusters (SCs) in the Large Magellanic Cloud (LMC) have reported that a significant number of these objects show double main-sequence turn-offs (DMSTOs) in their colour-magnitude diagrams (CMDs). One plausible explanation for the origin of these DMSTOs is that the SCs are composed of two different stellar populations with age differences of ∼300 Myr. Based on analytical methods and numerical simulations, we explore a new scenario in which SCs interact and merge with star-forming giant molecular clouds (GMCs) to form new composite SCs with two distinct component populations. In this new scenario, the possible age differences between the two different stellar populations responsible for the DMSTOs are due largely to secondary star formation within GMCs interacting and merging with already-existing SCs in the LMC disc. The total gas masses being converted into new stars (i.e. the second generation of stars) during GMC-SC interaction and merging can be comparable to or larger than the masses of the original SCs (i.e. the first generation of stars) in this scenario. Our simulations show that the spatial distributions of new stars in composite SCs formed from GMC-SC merging are more compact than those of stars initially in the SCs. We discuss both advantages and disadvantages of the new scenario in explaining fundamental properties of SCs with DMSTOs in the LMC and in the Small Magellanic Cloud (SMC). We also discuss the merits of various alternative scenarios for the origin of the DMSTOs.     

Some aspects of the biology of the micronektonic fishCyclothone pallida andC. acclinidens (Pisces: Gonostomatidae) in Sagami Bay,Central Japan

Some aspects of the biology of the micronektonic fishesCyclothone pallida andC. acclinidens are described on the basis of samples taken during a series of 20 cruises from December 1982 to November 1985 at a fixed station near the center of Sagami Bay, Central Japan.C. pallida is a regular component of theCyclothone population in Sagami Bay, being found in more than 90% of the samples. On the other hand,C. acclinidens was encountered sporadically, being found in less than 25% of the samples. The depth range ofC. pallida is estimated to be about 400–1,000 m. It spawns mainly during the spring and summer in Sagami Bay.C. pallida releases about 1,000–3,000 eggs and may spawn several times during its life span. On the average, it reaches 18.5 mm standard length (SL) in one year, 24 mm SL in two years and 29.5 mm SL in three years during its subadult stage. Extrapolation of the growth curve suggests that males and females attain first sexual maturity in three to four years at 30–35 mm and five to six years at 40–45 mm SL, respectively.Cyclothone pallida is concluded to have a regular life cycle in Sagami Bay. It remains uncertain whether or notC. acclinidens reproduces in this area.     

Middle Miocene to Quaternary primary basalt and high magnesian andesite magmas of North Hokkaido, Japan: Source mantle characteristics and degrees of partial melting

Abstract   Middle Miocene to Quaternary primitive basalts and high magnesian andesite (HMA) in North Hokkaido resulted from three periods of intense volcanism; early-stage (12–10 Ma), middle-stage (9–7 Ma) and late-stage (3–0 Ma). Based on the chemical compositions of olivines and chromian spinels and bulk chemistry of the primitive rocks, we examined depths of segregation of the calculated primary magmas and the degrees of partial melting of the source mantle. In the context of asthenospheric mantle upwelling, petrological data from the present study can be accounted for by the secular change in the depth of magma segregation from the upwelled asthenospheric mantle, which is composed of fertile peridotite. Thus, the early-stage primary magmas were generated by higher degrees of partial melting of the shallower part of hot asthenospheric mantle, whereas the middle- and late-stage primary magmas resulted from lower degrees of partial melting of a deeper part of the asthenospheric mantle. The early-stage HMA magma was generated by partial melting of the remnant subcontinental lithospheric mantle composed of refractory peridotite. This melting might have resulted from an increased geothermal gradient caused by upwelling of hot asthenosphere.