2006 Volume 60 Issue 2 Pages 147-163
The formation of melanic horizons so called "Kurobokudo" was chronologically studied in the tephra-soil sequence of the Ashitaka Loam at the foot area of Ashitaka Volcano, central Japan to discuss the history of Kurobokudo formation by examining soil chemical properties and paleo-phytolith assemblages. The black humic soils intermitted between tephra layers were recognized continuously from the lowermost layer of Upper Loam to the surface soil. The pH(NaF) value exceeded 9.5 through the tephra-soil sequence from the uppermost part of middle Loam. Based on the criteria of andic property which is given by pH(NaF)≧9.5, the analytical results implied that the increasing humus accumulation was recognized not only in Upper Loam but also in the uppermost part of Middle Loam. Paleo-phytolith assemblages were dominated by phytoliths of dwarf bamboo such as Pleioblastus and Sasa, and phytoliths of Panicoideae. This indicated that the warm-cool condition, under which Melanic horizons could be sufficiently formed, had continued since the age of the uppermost part of Middle Loam. Melanic Index (MI) values in Upper Loam were generally under 1.7, which implied the existence of the A type humic acid. While Middle Loam was determined as non-A type humic acid due to its MI value exceeding 1.7. These facts suggested that vegetation had transformed from forest to grassland around the age between Upper Loam and Middle Loam and then grassland dominantly succeeded. Since this vegetation change occurred almost simultaneously with the start of human activity in this area, supported by the archeological findings in the lowermost layer of Upper Loam and many archeological sites discovered throughout Upper Loam to Holocene soils, it was inferred that human activity had affected to generate melanic horizons associated with changing the vegetation from forest into grassland. The Yasumiba bed, the uppermost layer of the Upper Loam including a famous archeological site (Yasumiba-iseki), and Kuri-iro bed are the two definite non-melanic horizons formed in the tephra-soil sequence above the base of Upper Loam. It was estimated that the Yasumiba bed was formed without melanic facieses during vegetation decline in Glacial age, which might had been induced by the excessive movement of people from the higher elevated area towards the foot area of Ahitaka volcano under milder climate conditions. On the other hand, the Kuri-iro bed, a Holocene soil developed above the definite melanic bed named as "Fuji-kuro", could be recognized as a forest soil classified as Yellow Brown Forest soils due to its rich phytolith assemblages derived from evergreen broad-leaved trees. It was concluded that the following two boundaries existed in the history of melanic horizon since the last Glacial age 1) The later boundary at the age of ca.10ka corresponding to global warm period between Pleistocene and Holocene, and 2) the earlier boundary at the age of ca.32ka corresponding to the beginning of human activity in Japan Islands.
