Fusao Arai

Extensive ash falls in and around the sea of Japan from large late quaternary eruptions

Abstract Five widespread ashes of late Quaternary age have been recognized on land and also in deep-sea sediments in and around the Sea of Japan. All are favorable for determining the magnitude and type of the great eruptions, and for establishing a Quaternary chronology around Japan. Accurate determination of refractive indices of volcanic glass shards and phenocrysts, together with chemical data, have enabled successful characterization of the tephra layers and correlation between land and sea. Of the five tephras two are alkalic and are the products of major eruptions of two Korean Volcanoes: the Baegdusan—Tomakomai ash (B-Tm) and the Ulreung-Oki ash (U-Oki), The B-Tm ash is recognized at the top of most cores in the northern part of the Sea of Japan and also on Hokkaido and the northern part of Honshu, where a reliable archeological age is given around the 11th century. The U-Oki ash occurs between the Kikai-Akahoya and Aira-Tn ashes in cores from adjacent to the Ulreung-do to central Honshu, where three radiocarbon ages of around 9300 yr B.P. were obtained. The other three tephras are subalkalic and are correlated with large-scale eruptions producing voluminous pyroclastic-flows and calderas in Kyushu: the Kikai-Akahoya (K-Ah), Aira-Tn (AT) and Aso-4 ashes. The former two have been well described already but their fallout areas are partly revised in this manuscript. The AT ash is the most prominent marker, recognized not only on land of Japan but also in cores from the whole area of the Sea of Japan and part of the Pacific Ocean. The airfall Aso-4 ash, simultaneous with the Aso-4 pyroclastic-flow eruption is also described. It mantles very extensive areas and serves as an important time marker about 70,000 years ago.

Widespread submarine tephras around Japan — Petrographic and chemical properties

Abstract In deep-sea sediments around the Japanese Islands, tephra layers are frequently recognized, and some tephras can be identified by petrographic and chemical features and correlated to source volcanoes of both Japan and Korea. Most tephras distributed over more than several hundred kilometers from the source volcanoes were presumably carried away by the dominant westerly wind. Six characteristic marker tephras in deep-sea sediments can be identified and correlated according to their distribution and grain size, petrography, and chemistry. They play an important role in compiling the Late Quaternary chronology of the deep-sea sediments distributed around the Japanese Islands. Petrographic and geochemical studies reveal that the refractive index of volcanic glass shards in tephras is controlled by the amount of total transition metal oxides rather than the amount of SiO 2 . We propose a new equation relating refractive index to glass shard chemistry.