Abstract
A total of 240 alluvial soils collected from the southern to northeastern Kanto district, Japan, were analyzed for major and minor elements to characterize the geochemical map of soil. It has been considered that these alluvial soils originated mainly from elastic materials derived from basement rocks and altered volcanic ashes. The chemical composition change of elastic materials from basement rocks in alluvial soils reflects the geology of catchment areas, whereas the change of altered volcanic ashes depends mainly on erupted volcanoes. The vertical variations of the elemental concentrations in alluvial soils are generally smaller than volcanic ash soils. Several sand layers derived from basement rocks are found in the cores. Compared to the mud layers, sand layers are poor in Al2O3, TiO2 and heavy metals, but rich in alkali and alkali earth metals in most cases. The elution and/or deposition of chemical elements result from the decreasing grain size during weathering. Compared to the volcanic ash soils, the alluvial soils are rich in alkali and alkali earth metals, and depleted in Al2O3, TiO2 and heavy metals. The sedimentation rate of alluvial soils is estimated about 0.3 to 1.2 mm/yr at some places. It is less than or equal to the rate of recent lake sediments, but much higher than volcanic ash soils (0.03-0.08 mm/yr).
