Distance-dependence from volcano for Asian dust inclusions in Andosols: A key to control soil ability to retain radiocesium

Abstract

Deposition of Asian dust, i.e. mica-bearing aeolian particles, throughout geological times has contributed greatly to the increased ability of soil to specifically retain radiocesium (137Cs), particularly in soils derived from mica-deficient parent materials. Allophanic Andosols, volcanic mica-deficient soils, in Japan were hypothesized to contain fewer micaceous minerals proximal to volcanoes due to the dilution of Asian dust by thicker volcanic ash depositions. To test this hypothesis, we collected soils from stratified horizons mainly composed of mafic volcanic ash at four sites near the volcanic crater of Mt. Aso, Japan. The profiles were in volcanic ash soil dominated by poorly ordered clay minerals, and classified as Allophanic Andosols. Radiocesium interception potential (RIP) and mineral content were determined on soil particles fractionated to < 20 µm effective spherical diameter. The RIP for these particles ranged widely from < 0.1 to 4.5 mol kg soil−1, showed a strong-positive correlation with mica and quartz content, and increased with distance away from the crater. Isolated quartz grains, ranging from 2 to 20 µm, were analyzed for oxygen isotopic ratio (δ18O) as an indicator of Asian dust contribution. The average δ18O was 16.0 ± 0.4‰, very close to those of fine quartz in the Gobi Desert, while clearly differentiated from high temperature formation environments of SiO2 grains from volcanic origins, δ18O < 10‰. Results showed that RIP of Allophanic Andosols is primarily controlled by the proportion of Asian dust incorporated into volcanic ashes, which increased with distance from the main volcanoes. These findings improve predictions of the spatial distribution patterns soil radio cesium (RC) and the soil-to-plant transfer of RCs in Andosols in Japan.