Revised stratigraphy of Holocene tephras on Ulleung Island, South Korea, and possible correlatives for the U-Oki tephra

Detailed field surveys and measurements of major oxide compositions of volcanic glass shards, using electron microprobe analyses, revealed the stratigraphy of Holocene tephras U-4, U-3, and U-2 on Ulleung Island, South Korea. The bubble-wall-type glass shards of Kikai-Akahoya (K-Ah) tephra, detected in soil intercalated with tephra layers, indicate that the depositional horizon of the tephra is situated between the U-3 and U-2 tephras. This study has established a revised stratigraphy, in ascending order, of U-4, U-3, K-Ah, and U-2 tephras. Comparisons of the major chemical compositions and eruption ages of the U-4 and U-3 tephras on Ulleung Island and those of Ulleung tephras detected in and around the Japanese Islands suggest that these tephras are distributed widely and potentially can serve as useful and important time markers of the early to middle Holocene. Radiocarbon dates and stratigraphic relationships between the K-Ah and Ulleung tephras on the island suggest that the Ulleung-Oki tephra (U-Oki), which has been used as an essential time marker of the early Holocene, is most likely correlated with the U-4 tephra. Therefore, detailed analyses of chemical compositions of glass shards with dating of these tephras are necessary to identify U-Oki in and around Japanese Islands.