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. 相似文献
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. 相似文献
In Japan, many major cities are located on tectonic basins which are surrounded by faults and underlain by soft alluvial materials. Because these areas are subject to earthquake damages, it is important to determine their seismic engineering characteristics. Geotechnical databases which contain many borehole logs are useful information sources for this type of analysis. Each datum stored in the database has a value or an attribute, and its location is irregular in both horizontal and vertical directions. A new interpolation method based on the optimization principle is proposed here to deal with such three-dimensionally distributed data. Susceptibility of unconsolidated ground to liquefaction is known to be related to the content of loose and saturated sand. The mixture ratio of several soil types in a deposit, i.e., granular composition, is strongly influenced by the sedimentary environment. There are two numerical methods: the optimization principle method (OPM) used to determine three-dimensional distribution of granular composition and the model used to evaluate liquefaction. The application of the proposed methods to two locations in Japan indicated that the zones with high susceptibility to liquefaction were indeed those that had suffered from liquefaction during past earthquakes.相似文献
Abstract The lateral (along trench axis) variation in the mode of large earthquake occurrence near the northern Japan Trench is explained by the variation in surface roughness of the subducting plate. The surface roughness of the ocean bottom near the trench is well correlated with the large-earthquake occurrence. The region where the ocean bottom is smooth is correlated with'typical'large underthrust earthquakes (e.g. the 1968 Tokachioki event) in the deeper part of the seismogenic plate interface, and there are no earthquakes in the shallow part (aseismic zone). The region where the ocean bottom is rough (well-developed horst and graben structure) is correlated with large normal faulting earthquakes (e.g. the 1933 Sanriku event) in the outer-rise region, and large tsunami earthquakes (e.g. the 1896 Sanriku event) in the shallow region of the plate interface zone. In the smooth surface region, the coherent metamorphosed sediments form a homogeneous, large and strong contact zone between the plates. The rupture of this large strong contact causes great under-thrust earthquakes. In the rough surface region, large outer-rise earthquakes enhance the well-developed horst and grabens. As these structure are subducted with sediments in the graben part, the horsts create enough contact with the overriding block to cause an earthquake in the shallow part of the interface zone, and this earthquake is likely to be a tsunami earthquake. When the horst and graben structure is further subducted, many small strong contacts between the plates are formed, and they can cause only small underthrust earthquakes. 相似文献
The Daiichi-Kashima Seamount subducting in the Japan Trench has two reef-capped flat tops with different depths, which are bounded by a nearly straight scarp. The western (inboard) crest is 5300–5450 m deep, and the eastern (outboard) one is 3880–4000 m deep.
A variety of shallow-water reefoid limestones studied by the Kaiko “Nautile” dives not only confirms the similarity in thickness and lithologies, but also establishes an approximate biostratigraphic correlation between the two reefoid caps, based for the first time on the orbitolinid foraminifers (Orbitolina (Mesorbitolina) parva from the western block and O. (M.) texana from the eastern block).
This conclusion supports the interpretation that the present topography of the seamount has resulted from a subduction-induced faulting of a once single reef-capped guyot. A plausible scenario of the fate of a subducting Early Cretaceous guyot in the Western Pacific is outlined as exemplified by the Daiichi-Kashima Seamount. 相似文献
A theoretical equation was developed to express the time variation of drainage density in a basin or geomorphic surface: Di(t, T) is the drainage density at time T on the i-th basin or geomorphic surface, which was formed at time t; β(τ) is a factor related to the erosional force causing the development of the rivers of the basin or surface at time τ; δi is the maximum drainage density; and Di is the initial drainage density on the i-th geomorphic surface or basin. The equation is based on the assumption that the drainage density increases with time until it reaches a specific upper limit δi(t)), the maximum drainage density, which is related to certain physical properties of the basin. The equations for various dated basins or geomorphic surfaces can be combined into one modified equation if the same relative erosional forces have acted on those basins or surfaces (β(t) = β(t) and if the basins or surfaces have the same physical properties δi(t) = δi(t), (Di = D0). The application of this equation to coastal terraces and glacial tills shows that the model is compatible with observed drainage densities on various dated basins or surfaces. 相似文献
In order to understand the role of aqueous fluid on the differentiation of the mantle, the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system MgO-SiO2-H2O at pressures of 3 to 10 GPa and temperatures of 1000 to 1500°C with an MA8-type multianvil apparatus. Phase boundaries between the stability fields of forsterite + aqueous fluid, forsterite + enstatite + aqueous fluid, and enstatite + aqueous fluid were determined by varying the bulk composition at constant temperature and pressure. The composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries. The solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa, from about 30 wt% at 3 GPa to about 70 wt% at 8 GPa. It becomes almost constant above 8 GPa. The Mg/Si weight ratio of these aqueous fluids is much higher than at low pressure (0.2 at 1.5 GPa) and almost constant (1.2) at pressures between 3 and 8 GPa. At 10 GPa, it becomes about 1.4. Aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates, leaving modified Mg/Si ratios of residual materials. It is suggested that the chemical stratification of Mg/Si in the Earth may have been formed as a result of aqueous fluid migration. 相似文献
