示標テフラによる黒ボク土の生成開始時期の推定 : 十和田火山テフラ分布域湯ノ台地区を例にして

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We found that the basal age of Kuroboku Soils is younger than ten thousands years and it varies at each tephra profiles in Yunodai, the area of Holocene Towada Volcanic ashes, according to opal phytolith analysis and humus analysis, utilizing some time marker tephras such as Towada-a Ash (To-a) and Chuseri Pumice (Cu). Moreover, we concluded that vegetation and human activities have strongly effected to form Kuroboku Soils. 1. Three soil units were recognized at Yunodai 1 and 2 sites. Unit III soil as brown weathered volcanic ash (BWT) soil at Yunodai 1 has formed under a vegetation of cool broad leaved forest with Bambusoideae (S as a) in the forest floor. (A) horizon of the BWT soil (III(A)b) shows yellow-brown color, low PQ value with P_<+++> type humic acid. We concluded this soil as a Brown Forest Soil. Unit II soil, named Chuseri (Cu) soil is characterized by type-A humic acid and has black mull A horizon. This II Ab horizon has formed under the grassland vegetation of Bambusoideae-dominant with Panicoideae, which formed high humic black humus. We concluded that the Cu soil as a Kuroboku soil. The Unit I , present soil as the A horizon of To-asoil (I A) has black mull humus and shows B type humic acid which is slightly similar to A type. The vegetation which offered Soil formation is a grassland of Panicoideae-dominant with fern plants. Since these facts we estimate the former humic acid as A type before artificially forested. We also concluded this soil as a Kuroboku Soil. 2. Unit III soil as A horizin (III(A)b) of BWT soil and Unit n soil as A horizon (IIAb) of Cu soil at Yunodai 2 shows dark yellow and dark brown colors, with poor humus and P_<+++> type humic acid. The vegetation type which formed these soils was cool broad leaved forest with Bambusoideae in the forest floor. We concluded it as a Brown Forest Soil. The Unit I , To-a soil has A horizon of black mull humus (I A) which is characterized by A type humic acid. The vegetation which offered soil formation has been a grassland of Panicoideae and fern plants. We concluded this soil as a Kuroboku Soil. As mentioned above, soil formation of each unit is strongly effected by vegetation. It is to say that Brown Forest Soils has formed under the forest and Kuroboku Soils has formed under the grassland. 3. We clarified the basal age and duration of soil formation utilizing time marker tephras. BWT soil of Unit IE had formed since Late Glacial to Middle Holocene (the end of Early Jomon Era), with its duration five thousands years. Cu soil of Unit n had formed between the end of Early Jomon to Heian Era with its duration 4,400 years. To-a soil of Unit I started to form 1, 000 years ago. Both Unit IE and Unit n soils of each sites are covered with Cu and To-a tephras respectively and are interpreted as fossil soils since they have not suffered form any soil formation after buried. 4. The basal age of Kuroboku soil is 5, 400 y.B.P. (Unit II, Cu) at Yunodai 1 whether 1,000 y.B.P. (Unit III To-a) at Yunodai 2. These facts shows that the basal age of Kuroboku Soils is not always the begining of Holocene, moreover it varies at every sites. 5. Vegetation have changed after Cu soil at Yunodai 1 and after To-a at Yunodai 2. Each of them change form forest to grassland of dominant Bambusoideae or Panicoideae. The appearance of grassland are estimated by deforestation by human activities rather than by natural succession. Human activities on forests in Yunodai area started too later than other areas. The reason must be a severe climate which prohibited human actibities in the area. It delayed the change of vegetation which forms the Kuroboku Soils. We gave the same reason why basal age of the Kuroboku Soil in Yunodai 2 is later than Yunodai 1. We clarified that the basal age of Kuroboku Soils in Yunodai area near Towada Volcano is far younger than 10,000 years ago and it varies at each site. We suspect that the basal age of Kuroboku Soils is not always the beginning of Holocene at every sites.
1992-03-25

示標テフラによる黒ボク土の生成開始時期の推定 : 十和田火山テフラ分布域湯ノ台地区を例にして

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