Abstract
The piston cored sediment KT96-17 P-2 from the southeastern marginal part of the Oki Trough in the southern Japan Sea contained five volcanic ash layers. We studied the morphologic feature and chemical composition of volcanic glass shards and mineral composition in each ash layer. Between the Aira-Tn Ash (AT) and Kikai-Akahoya Ash (K-Ah) layers in the core, we recognized two ash fall deposits derived from the Ulleung Island. The glass shards from these two ash fall deposits have different chemical composition. This fact suggests that the Ulleung-Oki Ash (U-Oki) which has been regarded as one of important time-marker tephras in the paleoceanographic study of the Japan Sea might not be a unique layer but comprised of these two ash fall deposits from the Ulleung Island. It also implies that the U-Oki Ash might possibly be correlatable not only to U-2 but also to U-3 or U-4, which are three thick pumice layers between the AT and K-Ah layers in the Ulleung Island. We also recognized the Daisen-Kusadanihara Pumice (KsP) layer between the AT and K-Ah Ash layers in the core. AMS 14C datings were conducted on planktic foraminiferal tests picked from the horizons above and below the KsP layer in the core. The AMS 14C ages were 18, 070±50 yrBP (GX-26361) and 18, 470±160 yrBP (GX-25558), respectively. The eruption age of KsP was estimated to be 20, 260-22, 130 cal yrBP (2 sigma range) by calibration to calendar ages using CALIB 4.3 program.
