Identification of the late Quaternary tephra layers in the Ulleung Basin of the East Sea using geochemical and statistical methods

The late Quaternary tephra layers discovered in piston cores from the Ulleung Basin of the East Sea (Sea of Japan) were studied using X-ray fluorescence spectrometry (XRF), inductively coupled plasma atomic emission (ICP-AES) and inductively coupled plasma mass spectrometer (ICP-MS) in order to identify and differentiate the tephra layers and to characterize their major and trace element compositions. The morphology and geochemistry of glass shards, combined with 14C dating and statistical methods, confirm the Ulleung and Aira-Tanzawa eruptions as the origin of the tephra layers, respectively. The so-called Ulleung-Oki (U-Oki; 10.1 cal. ka) and Ulleung-Yamato (U-Ym; 30.9 cal. ka) tephra layers consist predominantly of pumice-type glass shards. In contrast, the Aira-Tanzawa (AT; 23 cal. ka) tephra layer consists mainly of plane-type and/or bubble-wall glass shards. The U-Oki and U-Ym tephra layers contain K-rich glasses (R = K2O/SiO2 > 0.09) ranging from phonolitic to trachytic alkali compositions, whereas the AT tephra layer which contains K-poor glasses (R = ca. 0.04) is rhyolitic. According to trace element content, the relative amounts of the elements Ti, Zr, Y, Nb and P from the tephra layers evidently indicate the different tectonic zones between the Ulleung (back-arc) and Kyushu (island-arc) islands. Especially, the relatively depleted concentrations of FeO, V, Co, and Ni in the U-Oki tephra layer most likely reflect more fractional crystallization of olivine, pyroxene, hornblende, and magnetite than those in the U-Ym tephra layer.