HOLOCENE TEPHRA IDENTIFICATIONS BASED ON REFRACTIVE INDICES OF DEHYDRATED GLASS AND ITS CONTRIBUTION TO QUATERNARY RESEARCH IN HOKKAIDO, NORTHERN JAPAN

The Holocene sedimentary record of Hokkaido, northern Japan, is dominated by tephras. Tephras have been commonly identified by comparing their petrographic properties and in particular their volcanic glass refractive index. The widespread availability of the electron microprobe, beginning in the late 1960s, saw a significant shift toward reliance on chemical fingerprinting for tephra correlation. The present study revives the use of glass refractive index by dehydration procedures for reversing the secondary hydration of the glasses.

The incomplete hydration in volcanic glass influences the refractive index with relatively severe extent. For example, refractive indices of hydrated Tarumae-a tephra (Ta-a: AD1739), Tarumae-b tephra (Ta-b: AD1667), and Tarumae-c tephra (Ta-c: ca.3ka) glass vary widely: their standard deviations are s=0.0034, s=0.0033, and s=0.0036, respectively. Thus, refractive indices of non-treated tephra glass are not a practical tool for identifying Holocene tephras. Heterogeneity of tephra glasses due to variable hydration increases the spread of refractive indices.

Hydration near the surface of tephra glass shards can be removed by 400 ‹C 12-hour annealing. The refractive indices of dehydrated (annealed) Ta-a, Ta-b, and Ta-c glass are 0.006-0.014 lower than those of the incompletely hydrated (non-treated) glass. Standard deviations of the refractive indices of dehydrated glass are small (s=0.0014-0.0018), in contrast with those of non-treated glass. It is strongly recommended that the refractive index of dehydrated glass be measured for Holocene tephra identification.

Most of the Holocene tephras in Hokkaido can be easily identified based on the refractive index of dehydrated glass in addition to some other petrographic properties. On the basis of the identifications, some Holocene phenomena such as the tsunami deposits in the coastal region of eastern Hokkaido, the periglacial phenomena in the alpine area of Mts. Daisetsu, and the paleo-earthquakes and the Satsumon culture horizon in the northern part of Ishikari Lowlands were examined. As a time-marker of 6500 cal yBP, Komagatake-g tephra (Ko-g) is valuable for correlating various phenomena in extensive areas of Hokkaido.