Review of the research on formation of andosols and paleoredsoils in South Kyusyu, Japan

In the present review, the outline of studies on the weathering and the forming process of "Akahoya" ash layer, andosols and paleoredsoils 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 important stratigraphic key beds for studies of late Quaternary geology and tephrochronology in Japan. Akahoya ash layer was grouped in to three types (type I, 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 physicochemical properties and clay mineral compositions. 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. Imogolite was 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 only in 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 formation from 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 : (1) Organic 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. (2) From 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. (3) The 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.