Studies on Chronology of Humified Tephras in Hokkaido

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Although many serveys and studies on the distribution and stratigraphy of tephras in Hokkaido have been done during last 50 years or more, the chronology on these tephras has not yet been correctly established. The authors have carried out the absolute chronology by dating with 14C on humified tephras in Hokkaido and made the correlation between each tephra. Moreover, they have tried to estimate the geological age of humus formation in these humified tephras. The chronology and the correlation of tephra layers in Hokkaido are as shown in Table 3, based on the 13C ages for humified tephra layers, as well as on some archeological data. Results obtained are summarized as follows;(1) In Hokkaido it seems that the age in which the remarkable humus have accumulated on tephras is not older than the earliest stage of the Holocene-about 8000 years ago.(2) Especially abundant accumulation of humus on tephras may have occured during 5000y.B.P. and 3000y, B.P. Perhaps the climate would have been rather humid and suitable for humus accumulation during this period. So called“Rodo”(humic volcanic ash soil) in Hokkaido is thought to have been farmed in this period.(3) The reddish-brown color of weathered pumices in Ta-d, Ma-k, Ma-d1 and Ko-f tephras may be due to the weathering under the past warmer climate than present. These periods are estimated about 7000 years ago for the former two and 3000-2500 years ago for the latter two.