Re-examination of Marker-tephra Layers in the 200m Biwa-Lake Core

Drill core samples from Lake Biwa provide important data for reconstructing Quaternary environmental changes. Of the many components of the cores, the tephra layers should give the best data for chronological studies. The present paper deals with the re-examination of these tephra layers, based on petrographic analyses and recent tephrochronological results for Japan.Determinations of the refractive indices of volcanic glass and orthopyroxene and hornblende phenocrysts, together with other criteria show that approximately half of the tephra layers are widespread tephras originating from large caldera volcanoes in Kyushu, and the remainder are possibly from Mt. Daisen and other volcanoes in Honshu. Almost all tephras identified here have been dated by a number of radiometric and stratigraphic methods and hence give more reliable, and younger, ages for the Biwa Lake sediments than previously obtained.