The chemical weathering of minerals and elemental mobility in Andisol that has formed during the last 6,000 years were investigated based on chemical analyses of primary minerals and clay minerals, bulk composition, and dissolution kinetics-fluid flow coupling model. The order of Al-normalized values of major elements during the weathering is Na<Ca<K<Mg<P<Si<Ti=Fe<Al<Mn. Mobility of Na, Ca and Mg due to the dissolution of feldspar and olivine was larger than other elements, that was caused by higher solubility and dissolution rate of these minerals, while those of Si, Fe, and Al were smaller, that was resulted from the precipitations of these elements as allophane, halloysite and Fe-oxyhydroxide. The variation trend of Si, Al, and Fe contents of fine-grained materials less than 2μm in grain size with depth suggests that they are allophane, halloysite and Fe-oxyhydroxide. The change in the amount of Ca·Na-feldspar with depth was calculated based on the dissolution kinetics-fluid flow coupling model to estimate dissolution rate of Ca·Na-feldspar. kA/M (k: rate constant, A: surface area, M: mass of fluids) is estimated to be 10−10.3±0.5(mol/s·kgH2O) by comparing the calculation result and the observed change in the amount of Ca·Na-feldspar with depth. Experimental dissolution rate constant for Ca·Na-feldspar, k (10−11~10−12), is similar to the value of the rate constant (10−12.0±0.5) estimated from the calculation when A/M is 45 which was estimated from the grain size distribution of feldspar.
We evaluated distribution of soil thickness and soil chemistry in the western part of Tanzawa Mountains. This area is located about 50 km west of Yokohama and has been receiving atmospheric pollution and related strong acid deposition. The soil core samples (30 to 280 cm length) were collected from 89 sites within the Oomuroyama watershed (~25 km2)(elevation 525~1600 m). The bedrock of this watershed is mainly composed of Miocene tonalitic rock. Soil derived from tonalitic parent material (tonalitic soil) covers over 95% of the study area and a rest of area is blanketed by loam soil derived from the Kanto loam ejected form Mt. Fuji. The tonalitic soils show a wide range of chemical variation from bottom to topsoil layer. From C- to A0-layer, SiO2, Al2O3, CaO, K2O and Na2O systematically decrease, while higher TiO2 and P2O5 were noted at the topsoil layer. The elemental abundances of soil (normalized by Ti) suggest that up to 70 wt.% of SiO2 and 60 wt.% of Al2O3 of the parental material has leached out by chemical weathering. The GIS analysis was conducted to evaluate spatial variation of topsoil composition in the study area. The GIS data indicate that the soil developed at lower elevation has experienced higher degree of elemental leaching into surface water. This may be due to climate effect.
The Konan terrace along the middle reaches of the Arakawa River, northwestern Kanto Plain, has been believed to have formed as an alluvial fan during MIS 5e or a subsequent regressive stage. Recently, we detected a tephra layer from a tuffaceous clay bed above the terrace gravel bed in the Konan terrace. Petrographic and EPMA analyses reveal that this tephra layer is assigned to the lower part of the Iizuna-Nishiyama Tephra Group (In-Ny). The examined tephra layer is characterized by coarse- to granule-sized yellowish-white pumice grains, and contains quartz, plagioclase, cummingtonite, and iron minerals. Of them, the cummingtonite shows the refractive index of 1.663-1.670 (mode: 1.666) and the Mg value of 63.3. These characteristics coincide with those of the lower part of the In-Ny Tephra Group obtained from several localities close to the Iizuna Volcano such as Takayama, Furuma, Sugadaira, and Yokokawa. The In-Ny Tephra Group is considered to have fallen at ca. 180 ka (MIS 7-6 transition) on the basis of its stratigraphic position between the SgP.2 and Tt-D tephra layers. As the Konan tephra is intercalated in the base of the tuffaceous clay bed thought to be of eolian deposit origin, the formation age of the Konan terrace should be same as that of the In-Ny Tephra Group and older than the previous interpretation (MIS 5e). Then, the slip rate of the Konan Fault displacing the Konan terrace is reestimated at 0.06 m/ky.