Spatial and Temporal Variations in the Chemical Weathering of Basaltic Pyroclastic Materials

The spatial and temporal distributions of 17 elements in basaltic pyroclastic materials originating from Mt. Fuji, Japan, were investigated. The relatedness of the profiles was established tephrochronologically, which then allowed a comparison of the chemical compositions of the various weathered pyroclastic materials in terms of their spatial (distance away from the volcano) and temporal (older to younger deposits) relations. The distributions of mobile elements (e.g., Ca, Mg, Na, and Sr) and resistant elements (e.g., Ti) are strongly affected by the extent of weathering, which increases with time and increases with distance from the volcano (the effects of the thickness and specific surface area of parent material). The application of a few weathering indices based on chemical composition was successful. Furthermore, a principal component analysis statistically extracted the expected dominance of the element mobility-resistance, which accounts for 43% of the field-scale variability of the distribution of the 17 elements. The second principal component explained 21% of variability, and discriminated soils developed from similar parent materials but different weathering-soil-formation factors (agricultural activities and the impact of a river).