90,000-year phytolith record from tephra section at the northeastern rim of Aso caldera, Japan

Abstract

Vegetation history during the last 90,000 years has been reconstructed using the phytolith record obtained from a tephra section located at the northeastern rim of Aso caldera, southwestern Japan. The phytolith assemblage from the section revealed that grassland vegetation dominated by Gramineae consistently occurred for 90,000 years. Sparse vegetation composed of Sasa sect. Crassinodi (cool-temperate dwarf bamboo) and Zoysia (lawn) was considered to be established within 1000 years after the catastrophic Aso-4 eruption (89 ka). The Sasa grassland dominated by Sasa sect. Crassinodi existed continuously at the northeastern caldera rim between 89 and 13.5 ka. The dominance of Sasa sect. Crassinodi in the grassland suggests that the Aso caldera rim during the period was under a cooler and drier climate. Plants other than Sasa dwarf bamboo declined during the period of 75–66 ka (MIS4) whereas Pleioblastus sect. Nezasa (warm-temperate dwarf bamboo), Andropogoneae (pampas grass) and Zoysia existed in the Sasa grassland between 66 and 30 ka (MIS3). During the period from 30 to 13.5 ka (MIS2) corresponding to the Last Glacial Maximum (LGM), phytoliths from plants other than Sasa dwarf bamboo and fern are only present in small numbers, suggesting that they declined in response to a cool climate during the LGM. Sasa grassland, which had continued since 89 ka, existed after 13.5 ka at the northeastern rim of Aso caldera. In the earlier stage of the Holocene, phytoliths of Sasa sect. Crassinodi and Sasa sect. Sasa dominated, but Pleioblastus dwarf bamboo became a constituent of the grassland around 8 ka. The proportion of Pleioblastus dwarf bamboo (mainly Pleioblastus sect. Nezasa) in the grassland increased and continued flourishing until the present, although Sasa gradually decreased. In addition, arboreal phytoliths were recognized at several Holocene horizons. These phenomena indicate that grassland vegetation composed mainly of Sasa and Pleioblastus dwarf bamboos with sparse trees dominated at the northeastern rim of the Aso caldera in Holocene time.