Geochemical identification of late Quaternary cryptotephra layers from marine sediments, southwest Japan, using INAA

Abstract

New compositional data were obtained from glass shards from cryptotephra layers in five marine sediment cores from the Ulleung Basin in the East (Japan) Sea. The shards were extracted from cryptotephra layers that were distinguished in a previous study by INAA core scanning, and analysed by EDS-SEM for major elements and INAA for trace elements. Most of the glass shards have alkaline, trachyte–phonolite, compositions, and have similar major and trace element compositions to eruptive units on Ulleung Island to the west. Stratigraphic inferences indicate the presence of at least four alkaline tephras (referred to in order of increasing age as: U-3, Post U-Oki, U-Oki, Pre-U-Oki) in the interval between the K–Ah rhyolitic tephra (7.3 cal ka BP) and the TL1 regional dark sediment layer (12.4 cal ka BP). The new geochemical data indicate minor compositional distinctions between these tephras that help to infer fine-scale regional correlations. Paleoceanographic conditions in the East (Japan) Sea changed drastically during the late Quaternary, linked to global environmental changes, and 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 cryptotephra in the marine sediments of the Ulleung Basin also contributes towards a better understanding of the volcanic history of Ulleung Island and nearby seamounts.