Local tephra as an age-determination tool: Example of 2.3 ka Yakedake volcano tephra in Nagano Prefecture, central Japan

A local tephra embedded in a hand-auger boring core drilled at the near-shore of the Kinugasanoike Pond approximately 4.6 km northeast of Mt. Yakedake, which is one of the most active volcanoes in central Japan, was found to be composed mainly of several kinds of volcanic glass shards (microlitebearing, blocky, fluted, and micro-vesicular types) and minor amounts of crystal minerals, including quartz, plagioclase, hornblende, biotite, and pyroxene. The plant remains recovered from the horizon 10 cm below the tephra layer revealed ¹⁴C ages of 2,331 – 2,295 cal years BP and 2,270– 2,155 cal years BP (19.2% and 76.2 % probability distributions, respectively). In this study, we measured the major element compositions of 241 individual glass shards using an electron probe microanalyzer, and found that they plotted on SiO₂–K₂O, SiO₂–Na₂O+K₂O, and FeO*–K₂O diagrams in different regions than those of the major regional tephras distributed in central Japan. Moreover, the observed clast and chemical compositions coincide with those of a tephra embedded in the Nakao pyroclastic flow deposit distributed approximately 2 km north-northwest of Mt. Yakedake, which is dated at approximately 2,300 cal years BP. Thus, this tephra could be used as a local marker of 2,300 cal years BP for the southern part of the Northern Japan Alps.