喜界島および南大東島の離水サンゴ礁段丘上に発達した土壌の遊離酸化鉄の結晶化指数による年代推定
2002 年 41 巻 6 号 p. 485-493
詳細
2002 年 41 巻 6 号 p. 485-493
Absolute ages of soils developed on the raised coral reef terraces in Kikai Island in the Ryukyus, Southwest Japan were estimated by using the mean rate of tectonic uplift and glacio-eustatic curve during late Quaternary in the previous paper. In order to estimate the soil age of other areas where the similar soils as those existed in Kikai Island were formed by using the index of soil age, which was highly correlated with some physico-chemical properties of soil itself, total iron (Fet), iron and aluminum extractable by dithionite-citrate (Fed, Ald) and by acid ammonium oxalate (Feo, Alo) were determined for all horizons of the six profiles in Kikai Island. The results obtained are as follows:
The crystallinity ratio of free iron oxides [(Fed-Feo)/Fet] gradually increased with the stage of soil development, while the activity ratio (Feo/Fed) decreased. There was a highly positive correlation between the soil age and (Fed-Feo)/Fet. By using this relationship, the ages of Minami-Daito Island soils, Lateritic Red soil and Lateritic Yellow soil whose age had not been determined, were estimated from the mean values of (Fed-Feo)/Fet, as 500±60 ka and 630±110ka, respectively. Therefore, it was concluded that the crystallinity ratio of free iron oxides could be a good index of the degree of soil development and age of Red-colored soils.
