2019 Volume 128 Issue 3 Pages 439-452
The alkaline tephra U-Oki is widely dispersed in and around the Japanese Islands. U-2 tephra, among Holocene tephras (U-2, U-3 and U-4 tephras) on Ulleung Island, was considered to be the first candidate to be correlative with alkaline tephra. Recent studies on stratigraphy, eruption age, and chemical composition of Ulleung Island tephras and Ulleung Island-derived tephras show that U-3 and U-4 tephras, not U-2 tephra, reached the Japanese Islands. Therefore, it is necessary to confirm whether Ulleung Island-derived tephras, such as U-Oki, can be correlative with U-3 or U-4 tephras based on reliable chemical and chronological data. Such reliable correlations provide information on eruption magnitude and dispersal. Alkaline tephra (Hm-2 tephra), which is petrographically similar to U-Oki, is found as an intercalated layer along with numerous other intercalated layers of Hakusan Volcano tephra in a peat layer formed during the last 13,000 years in the summit area of Hakusan Volcano, central Japan. EPMA analyses of major element glass composition and AMS radiocarbon dating have been performed for Hm-2 tephra, showing a correlation between Hm-2 tephra and tephra on Ulleung Island. Volcanic glass shards from Hm-2 tephra have a distinctly high alkali content with high alumina and intermediate silica contents. These chemical characteristics accord with those of Ulleung Island tephra and Ulleung Island-derived tephras. AMS 14C ages obtained for peat layers just under Hm-2 tephra are 7600 BP and 8490 BP, which is consistent with the eruption age of U-3 tephra on Ulleung Island. The ages of the Hm-2 tephra indicate that Hm-2 is correlative with U-3 tephra. The correlation supports the view that tephras correlative with U-3 tephra are distributed widely in and around the Japanese Islands, as well as those correlative with U-4 tephra. A comparison of chemical compositions between Hm-2 tephra and three Units (U-3a, U-3b, and U-3c) of U-3 tephra shows that Hm-2 tephra is most similar to Unit U-3b among the three Units, indicating that a correlation is highly probable. This result suggests that Unit U-3b, in addition to Unit U-3c, also reached central Japan, which is approximately 550 km from Ulleung Island, with the eruption of U-3 tephra.