Widespread late Quaternary tephras in Japan with special reference to archaeology

Abstract

Detailed description of specific features of tephra has recently disclosed a number of tephras spread over extensive areas in and around Japan. The present paper gives an overview of such widespread tephras of late Quaternary in Japan and discusses their significances in several problems regarding the Japanese archaeology.
1) Widespread tephras can be classified into three types: large-scale plinian airfall pumice, large-scale ignimbrite and coignimbrite ash falls. The third type of tephra should be the dominant class of widespread tephra. Fine-grained vitric ashes of this type, represented by the Kikai-Akahoya (K-Ah), Aira-Tn (AT) and Aso-4 ashes, cover most of the Japanese islands and adjacent areas, forming important time-markers in the upper Quaternary sequences.
2) The characterization of tephras for identification is made from various viewpoints. However, accurate determination of refractive indices of volcanic glass shards and phenocrysts, together with chemical data and other criteria, have enabled most successful characterization of the tephra layers and interregional correlation.
3) Of the widespread tephra occurring in and around Japan two are alkalic in componitin and are the products of the two Korean volcanoes of the Holocene age: the Baegdusan-Tomakomai (B-Tm) and Ulreung-Oki (U-Oki) ashes. The B-Tm is recognized on Hokkaido and the northern part of Honshu, where a reliable archaeological age is given around the 11th century. The U-Oki occurs on central Honshu, where five radiocarbon ages of around 9.300y.B.P. were obtained.
4) Recognition of the widespread tephras in archaeological sites, especially the AT and K-Ah ashes, has compelled us to revise some former views on archaeological chronology, bh in the neolithic and paleolithic ages. A number of archaeological assemblages were reacovered from below AT (21, 000-22, 000y.B.P., determined from 24 radiocarbon dates). Diversity in technological traditions and rapid cultural change, however, appear to begin immediately after the eruption of AT. The neolithic Jomon ceramic culture began about 10, 000y.B.P., during which K-Ah ash (6, 300y.B.P., determined from 41 radiocarbon dates) provides the definite datum plane in the archaeological sequence. The K-Ah eruption would have given heavy impacts on the Japanese Jomon world. Some K-Ah event and was reoccupied several hundred years ago by bearers of the different cultural tradition.