Geochemical identification of cryptotephras using INAA from Late Quaternary hemipelagic sediments, southwest Japan

Abstract

Abstract We present new compositional data on glass shards from cryptotephra layers in five marine sediment cores from the southern Japan/East Sea. The hand-picked glass shards were extracted from cryptotephra horizons that were distinguished by SEM/EDS for major elements and INAA for trace elements on individual shards. Most of the glass shards have alkaline and trachyte–phonolite compositions, and have major and trace element compositions similar to eruptive units on Ulleung Island. Stratigraphic inferences indicate the presence of at least four alkaline tephras, U-3, Post-U–Oki, U–Oki, and Pre-U–Oki in the interval between the K–Ah rhyolitic tephra (7.3 cal ka BP) and the TL1 regional dark sediment layer (11.8 cal ka BP). The new trace elements on glass shards indicate minor compositional distinctions from one visible tephra and four cryptotephras that help to infer fine-scale regional correlations. The development of a more comprehensive alkaline tephrostratigraphical framework for this region will greatly aid in the reconstruction and chronology of regional stratigraphic variations. The recognition of new alkaline cryptotephras in the marine sediments of the Japan/East Sea also contributes towards a better understanding of the volcanic history of Ulleung Island and nearby seamounts.