Identification of multiple widespread tephras from the volcanic glass shard chemistry of muddy sediments of the Nohbi Formation, central Japan

摘要

Volcanic glass shards in muddy sediments of the late Pleistocene Nohbi Formation of central Japan are considered to be derived from multiple tephras, although the shards have petrographic properties similar to those of the Aira-Tn tephra. Hierarcmcal cluster analysis of concentration data for 58 elements obtained by laser ablation-inductively coupled plasma-mass spectrometry was used to distinguish glass populations in muddy sediment samples from the Nohbi Formation. Cluster analysis of volcanic glass shards from a sample in the lowermost horizon (Nohbi ^8-54) of the formation identified the Aira-Tn (44%), Ontake-Pml (33%), and Ebisutouge-Fukuda (23%) tephras. Glass shards from another sample in the middle horizon (Nohbi 60-2) of the formation were identified as being the Aira-Tn (35%) and Ebisutouge-Fukuda (65%) tephras. The combination of multi-element data and cluster analysis demonstrates that the muddy sediment layers contain several tens of percent of Aira-Tn glass shards, as suggested by previous studies. The Aira-Tn tephra is the youngest one within the tephras identified from the Nohbi muddy sediments. As suggested by the previous studies, this evidence indicates that the Nohbi Formation was deposited during and/or after the AT ash fall stage and before the deposition of the Nan'yo Formation.