秋田県男鹿半島で発見された姶良Tn火山灰

概要

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A thin volcanic ash intercalation was found in a 0.2 to 1.4m thick peat layer in the basal part of the sedimentary layer, provisionally named the Hakoi Formation, unconformably overlying the Katanishi Formation; this formation yielded warm, near-shore marine and brackish water fauna and flora and has been correlated with the Last Interglaciation in the glaciated regions. The refractive index of glass shards of the ash (1.499-1.501) coincides with the Aira-Tn ash (AT), and the glass has almost the same major element composition as the AT detected in Dekizima, western Aomori Prefecture. The carbon-14 age of fossil wood from a horizon slightly above the ash is 15, 470±620y.B.P. (I-14, 646). These characteristics indicate that the ash is Aira-Tn ash, ejected from the Aira Caldera, southern Kyushu, the most prominent marker tephra of the Late Pleistocene in Japan. The pollen assemblage of the peat indicates a cold climate during the time of deposition of the ash.