The influence of aeolian dust in non-allophanic Andosols on Yakushima Island

The oxygen isotope (δ¹⁸O) composition of quartz and the d₍₀₆₀₎ values of clay minerals were determined from four pedons of non-allophanic Andosols derived mainly from the Holocene volcanic ash on Yakushima Island. These soils contained considerable amounts of aerosol-sized (1-10 μm) and coarse (>53μm) quartz. The δ¹⁸O values for the aerosol-sized quartz ranged from 14.7‰ to 17.4‰, which was comparable to or slightly lower than known values for loess-derived Red and Yellow soils on Tanegashima Island located approximately 20 km east of Yakushima Island. The abundance and δ¹⁸O values of the aerosol-sized quartz indicated that non-allophanic Andosols on Yakushima Island were strongly influenced by aeolian dust. However, the presence of coarse quartz implied that granite-derived materials were also incorporated into non-allophanic Andosols. X-ray diffraction patterns for most clay minerals showed two broad peaks around 0.154 and 0.150 nm, respectively. The d₍₀₆₀₎ values confirmed that 2:1-2:1:1 clay minerals consisted of dioctahedral and trioctahedral clay minerals. Since aeolian dust contains little or no coarse quartz and trioctahedral clay minerals, the abundance of coarse quartz and trioctahedral minerals confirmed that the occurrence of non-allophanic Andosols on Yakushima was influenced by biotite-granite, in addition to aeolian dust.