抄録
It is very important for reconstructing volcano eruptive history to identify paleosols interbedded between tephra layers because they indicate dormant or gentle periods of volcanoes. However, it is difficult to recognize paleosols around volcanoes dominating long-term small ash-emitting activity for a long time. Gain-size, total carbon content and phytolith analyses and measurement of soil hardness of paleosols and fine-grained tephra layers (ash-fall deposits) were undertaken at a proximal (4 km) and a distal (11 km) sites of active Nakadake crater, Aso Volcano (southwest Japan) whose activity is characterized by small ash eruption, to discuss effective discrimination between paleosols and tephras using their physical and chemical properties. Paleosols were finer grained than tephra layers at the proximal site whereas there was no distinct difference in grain size between them at the distal site. Since Holocene tephras tended to be more consolidated than the paleosols, hardness may be an effective indicator to distinguish paleosols from tephras only in the Holocene. Although Holocene paleosols had higher total carbon contents than the tephras, both paleosols and tephras in the late Pleistocene contained extremely low carbon. Phytolith concentrations of paleosols were significantly higher than those of tephras both in the Holocene and late Pleistocene. Therefore, phytolith analysis is a useful method to divide into paleosols and tephras even at distal sites although the analysis is needed a practiced technique.
