南九州における黒ボク土および古赤色土の生成に関する研究

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In the present review, the outline of studies on the weathering and the forming process of"Akahoya" ash layer, Andosols and Pareoredsoils in south Kyushu were described.The source volcano of Akahoya ash layer was determined to be Kikai calderas by using chemical composition of ferromagnetic minerals. The Akahoya ash layer is a glassy decitic ash-fall deposit which is widely distributed in Japan from south Kyushu, through Shikoku, to Kanto districts, as one of the most importantstratigraphic key beds for studies of late Quaternary geology and tephrochronology in Japan. Akahoya ash layer was grouped in to three types(typeI, type II, type III), based mainly on the depth of it from the land surface. They were studied in more detail with regard to their physico-chemical properties and clay mineralcompositions. Results showed that the thicker the tephra deposits overlying the Akahoya ash layer(type II,type III) were, the more bases as well as silica relative to aluminum in clay fractions were enriched. Imogolitewas dominant under the strong leaching conditions of shallowly buried Akahoya ash layer(type I) whereas allophane was the main clay mineral in deeply buried ones(type II and type III). Halloysite was present onlyin deeply buried A horizon of Akahoya ash layer of type II. Consequently, environmental changes induced by subsequent overlying of tephra deposits may have a greet influence on the weathering of and the soil formationfrom pre-existing tephras.Several tephra deposits in the cumulative Andosols profile in Miyakonojo basin in south Kyushu were identified by using chemical composition of ferromagnetic minerals of them to estimate the genetical period of the Andosols based on tephrochromology. It made possible to investigate the forming process of Andosols.Obtained results were as follows : ①Organic carbon content was changed successively from lower to upper layer in the cumulative Andosols profiles. In this fact, it was suggested that various changing patterns of organic carbon content have been based on the volume and condition of parent material, climate, and thickness of the vegetation. ②From phytolith analysis, different grass species were detected to be dominant in each horizon. This result suggested that soil organic matter came mainly from grass species, which have changed with the change of the time. ③The development of the Brown Loam layer has been formed during　about 26,00-13,000 year B.P. in the season of Last Glacial Stage, and the humus layers in the upper part of the profile have been formed under condition of thick vegetation during in warming age which periods of Last Glacial Stage. To estimate the genetical period of the pareoredsoils based on tephrochronology, some tephra deposits overlying and/or underlying the pareoredsoils in south Kyushu area were identified by using chemical composition of ferromagnetic minerals of them. Consequently, the tephra deposit overlying pareoredsoils onChausubaru higher terrace in Miyazaki Plain and the tephra deposit in the gravel bed under the pareoredsoils in the profile of Obaru in Izumi Plain, respectively, was identified Kakuto tephra(340ka)and Ata-Torihama tephra(200-250 ka). And also, Kikai-Tozurahara tephra(90-95ka)and Ata tephra(95-110ka)covered the pareoredsoils in the profile of Tsubakiyama in Tanegashima. These results have suggested that the genesis of pareoredsoils began before 340 ka and lasted till 95-110 ka in south Kyushu.
宮崎大学の論文
2006-03-23

南九州における黒ボク土および古赤色土の生成に関する研究

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