Age of the SKP-I and SKP-II tephras from the southern East Sea/Japan Sea: Implications for interstadial events recorded in sediment from marine isotope stages 3 and 4

The ages of two newly identified tephras in marine oxygen isotope stages (MIS) 3 and 4 from the southern East Sea/Japan Sea (ES/JS) have been determined. Analysis of chemical composition, grain-size distribution, sediment texture, and pumice micromorphology of material from 24 marine sediment cores was combined with a high-resolution age model established for core MD01–2407. The marine sedimentary record of the ES/JS is characterized by a drastic, millennium-scale variation in paleoceanographic conditions that is closely linked to Dansgaard–Oeschger (D–O) events. The chemical composition of the two tephras, which are stratigraphically located between previously dated AT (29.24 ka) and Aso-4 (88 ka) tephras, varies from phonolitic to trachytic (57.11–62.87 wt% SiO2 with 5.03–6.81 wt% K2O and 6.32–9.87 wt% Na2O). They exhibit a conspicuously bimodal grain-size distribution, and have randomly oriented vesicles with thick walls. The dispersal patterns of these tephras differ markedly from that of the U-Oki tephra, which is originated from a subaerial eruption on Ulleung Island. The two newly identified tephras, named SKP-I and SKP-II, reach their maximum thickness and grain-size at a location near the South Korea Plateau (SKP). An age model for a core from the Oki Ridge (MD01–2407) yields eruptive ages of about 40–41 ka for the SKP-I tephra and about 60–61 ka for the SKP-II tephra. A comparison of sediment lightness (L⁎) with GISP2 oxygen isotope data suggests that the SKP-I tephra is stratigraphically between D–O Interstadial (IS) 9 and 10, and the SKP-II tephra is between D–O IS 17 and 18. The SKP-I and SKP-II tephras are equivalent to the U-Ym tephra described by Arai et al. [Arai, F., Oba, T., Kitazato, H., Horibe, Y., Machida, H., 1981. Late Quaternary tephrochronology and paleo-oceanography of the sediments of the Japan Sea. Quat. Res. (Daiyonki-kenkyu) 20, 209–230 (in Japanese, with English Abstr.)] and that described by Ikehara et al. [Ikehara, K., Kikkawa, K., Chun, J.-H., 2004. Origin and correlation of three tephras that erupted during oxygen isotope stage 3 found in cores from the Yamato Basin, central Japan Sea. Quat. Res. (Daiyonki-kenkyu) 43, 201–212 (in Japanese, with English Abstr.)], respectively. They not only provide marker horizons within the stratigraphic interval corresponding to MIS 3 and 4 but also can be useful to identify D–O events for marine cores from the southern part of the ES/JS.