八甲田山の火山灰土壌にみられるA層の発達様式について

抄録

In the Hakkoda mountains, Aomori prefecture, forest soils derived from cumulated tephra layers are widely distributed. Two obvious tephra layers usually can be observed in the soil profiles, one of them being called Towada-a ash layer (about 1,000 years ago), and the other Chuseri pumice layer (about 4,000 years ago). A horizons 15 cm in thickness have developed on the Towada-a ash layer, and buried A horizons 20-30 cm in thickness between the two tephra layers. Morphological and mineralogical studies of Brown Forest soils, Black soils, a Podzolic soil, and a Peat soil (moor vegetation) were made to make clear forming processes of the A and buried A horizons. Tephra layers samples from the Peat soil were used as a standard. The results obtained are as follows. 1. Tephro-stratigraphy in the soil profiles suggest that parent materials of the A and buried A horizons are not composed of the upper parts of Towada-a ash layer and Chuseri pumice layer respectively, originating from other materials such as different tephra layers, eolian dusts, or reworked deposits. 2. From the mineral contents of the Peat soil, it is presumed that the parent materials of the forest soils in the area are principally composed of Towada-a ash and the upper unit of Chureri pumice. 3. Primary mineral composition and refractive index of volcanic glasses in the soil profiles indicate that parent materials of the humic horizons are transported from higher places. All the soil samples have similar characteristics and there is no difference among soil types. 4. It is concluded that the humic horizons of the forest soils of the studied area have developed from colluvial tephro-origin sediments or ash falls reworked by the wind.