抄録
We clarified the relationship among the basal age of Kuroboku Soils, the humus properties of volcanic ash soils and the vegetation environment, according to the opal phytolith analysis and the humus analysis utilizing time marker tephras on the cumulative volcanic ash soils in Akasaka, Sannohe and in Kawamukai, Takko, the area of Towada volcanic ashes. Moreover, we examined the formation process of volcanic ash soils. 1. Kuroboku soils have been formed from Nb (Nanbu pumice) soil of Unit III in Kawamukai and from NA (Ninokura volcanic ash) soil of Unit IV in Akasaka. According to the ages of marker tephras, the basal age of Kuroboku soils is between 8,600 y. B. P. and 5,400 y. B.P. in Kawamukai and between 9,500 y. B. P. and 8,600 y. B. P. in Akasaka. Moreover, these ages corresponded with the time when the vegetation changed from forest to grassland. The basal ages of Kuroboku soils in Kawamukai and Akasaka are older than in Yunodai, Towadako (Hosono et al., 1992), where the relationship between the basal age of Kuroboku Soils and the change of vegetation was recognized also. We estimated that Kuroboku Soils formation following the formation of grassland had relation to human activities on forests and the older basal age of Kuroboku Soils in Kawamukai and Akasaka than in Yunodai accounted for earlier human activities on forests in the former than the latter, according to lower height and genialer climate of Kawamukai and Akasaka. Besides, the difference of the basal age between Kawamukai and Akasaka has relation to other factors, because both climate conditions are almost equal. 2. Both the Nb soil, Unit IK in Kawamukai and the NA soil, Unit TV in Akasaka had the sections of A/BC/C. The BC horizons of both soils were recognized the (A) horizons of Brown Forest Soils, according to vegetational career of broadleaved forest with Bambusoideae (Sasa). Therefore, both of soils cumulated (A) horizons of Brown Forest Soils and A horizons of Kuroboku Soils. We estimated that these cumulus were formed when atmospheric dust composed mainly from volcanic ash had been cumulating slowly under each other vegetation. We propose to call such soils as have cumulated plural soil units while soil forming factor changed on the process of soil formation and slow cumulus of parent material progressing side by side "Accumulatin Polygenic Soil".
