野尻ローム層・仲町層中の腐植炭素量および全窒素量から見た堆積環境の変化
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概要
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It appears that high contents of humic material in soils are probably indicative of a high degree of humification and suggest a land surface covered by grass during the course of deposition of the parental soil materials. This paper gives an assessment of the depositional environments of volcanic ash soils based on the results of a quantitative analysis of their organic carbon and total nitrogen. The Nojiri Loam Formation consists mainly of aeolian volcanic ash soils and can be divided into four members indescending order as follows: the Kashiwabara Black Volcanic Ash, and the Upper, Middle and Lower Nojiri Loam Members. The Kashiwabara Black volcanic Ash Member is composed of humic volcanic ash soil, the so-called "Kuroboku", while the other members consist mainly of brown volcanic ash soils intercalated by scoria, pumice and black bands thought to represent ancient weathering surfaces. The Nakamachi Formation is composed of brown clay in the upper part and of greyish sand and silt in the lower part. The former may have been deposited in an upland environment, and the later in the ancient lake. The quantitative analysis of organic carbon was carried out by a modification of Tyurin's method, and total nitrogen by the improved method of Honda. As shown in Fig. I, the contents of organic carbon and total nitrogen in the soils were high in the Kashiwabara Black volcanic Ash Member, and in the black bands and crack zones, both of which are related to uncomform able planes. They were poor in the scoria and pumice layers. Comparison with the clay mineral components of the Nojiri Loam Formation revealed that the layers rich in humic carbon and total nitrogen agreed roughly with the layers of complicated clay mineral components and high contents of clay minerals, indicating a progressive weathering process. Fig. 2 shows that the organic carbon and total nitrogen contents in the Nakamachi Formation were high in the layers of dark brown clay in the upper part of this formation, and poor in the layers of sand and silt deposited in the ancient lake. Based on these results, the following conclusions are drawn. (1) The volcanic ash soils with abundant organic carbon and total nitrogen probably accumulated on the land surface during pauses in volcanic activity. (2) Generally speaking, the variation in organic carbon and total nitrogen contents in the volcanic ash soils may be utilized for assessing their depositional environments.
- 日本地質学会の論文
- 1980-03-29