アソ カルデラ ホクブ アソダニ センチョウムタ ボーリング コア ノ ショクブツ ケイサンタイ オヨビ ビリュウ タン ブンセキ

摘要

Holocene environmental changes and vegetation history are constructed using phytolith andmacroscopic-charcoal analyses of a 23-m-deep drilling core obtained at the Senchomuta marsh inAsodani Valley, northern part of Aso caldera, SW Japan. An intra-caldera lake existed in theAsodani Valley prior to approximately 9 cal ka（ calibrated 14C age）. Multiple large flood eventsoccurred during the period 8.9–8.1 cal ka and emplaced thick sandy deposits in the valley basin.Thereafter, the center of the Asodani Valley（ northern part of caldera floor） changed to swampyand fluvial environments. Sasa （cool-temperature dwarf bamboo） grasslands and/or forestswith understory Sasa covered slopes of the Asodani Valley basin between 11 and 9 cal ka. Sasaphytoliths significantly increased at ca. 7.3–6.5 cal ka, but thereafter decreased. Miscanthus（Japanese pampas grass） grasslands existed continuously on the slopes. Macroscopic-charcoalparticles were abundant during the last 6000 years, and the peak （6.1 cal ka） amount ofcharcoal particles is consistent with that of Miscanthus phytoliths. This indicates that theexistence of Miscanthus grassland might be related to fire events. Inside the Asodani Valley,Phragmites （reed） became established continuously along the shore of the intra-caldera lake（prior to ca. 9 cal ka） and in subsequent marshes. Gramineae phytoliths were detectedpredominately through all horizons of the drilling core, whereas a small amount of arborealphytolith was observed at most horizons. We, therefore, believe that forests existed on steepslopes such as the caldera wall where human impacts were small, although Sasa andMiscanthus grasslands were maintained by human activity outside Aso caldera.