Widespread tephras in sediments from lake Ichi-no-Megata in northern Japan: Their description, correlation and significance

Lake Ichi-no-Megata is a maar in the Oga Peninsula, which is connected with the western side of Honshu Island, NE Japan. A 37-m long sediment core (IMG06) obtained from the center of the lake contains five fine ash layers. Stratigraphic positions in the core provide ages of five ashes to be 30, 21, 17, 7.3, and 1.0 cal ka BP, respectively. These layers are correlated with Aira-Tn (AT), Daisen-Kusatanihara (D-KsP), Asama-Kusatsu (As-K), Kikai-Akahoya (K-Ah) and Baitoushan-Tomakomai (B-Tm) tephras in ascending order, based on stratigraphy and petrographic characteristics such as grain assemblages, refractive indices, and major element compositions of glass shards. Although the AT, K-Ah and B-Tm tephras have been well known as widespread tephras, this study finds that the D-KsP, As-K and K-Ah tephras have more extensive distributions than the previous studies estimated. The D-KsP and As-K tephras have the potential to become useful time-markers in and around the Sea of Japan between the AT and K-Ah tephras.