Aeolian-dust-derived micaceous minerals control radiocesium retention in Andosols in Japan

Factors controlling the abilities of Allophanic Andosols (representative of Japanese volcanic ash soils) to retain radiocesium (RCs) were assessed. The hypothesis was that retention is largely controlled by traces of micaceous minerals deposited as components of aeolian dust. The radiocesium interception potentials (RIPs), mica K contents were determined in the 2- to 20- and <2.0-μm particle fractions of 23 soil samples from agricultural fields in areas representative of Japanese Allophanic Andosols. Quartz contents were determined in the 2- to 20-μm particle fractions of the same soil samples. The oxygen isotope ratio (δ¹⁸0) in the quartz isolated from each 2- to 20-μm particle fraction was determined to identify the origin of the quartz in each soil sample. The mean RIP for the 2- to 20- and <2-μm particles was 1.7 ± 0.8 and 2.6 ± 1.3 mol kg^-1, respectively, and the mica K contents were 3.2 ± 1.3 and 3.4 ± 1.7 g kg^-1, respectively, corresponding to micaceous mineral contents of <5% by mass in each size fraction. In spite of their rather low values, the RIPs and mica K contents in each fraction positively correlated (p > 0.01), indicating that the abilities of the soils to retain RCs were mainly controlled by the micaceous mineral contents. The mica K content was proportional to the quartz content in the <20-μm particles, and the mean δ¹⁸0 in the quartz was + 14.8%, similar to those in fine quartz in Chinese loess. These results strongly indicate that the abilities of Allophanic Andosols to retain RCs are largely controlled by micaceous minerals probably originating in aeolian dust.