ひがし北海道の化石周氷河現象とその古気候学的意義 (寒冷気候にまつわる諸問題(特集)) -- (寒冷気候下の現象)

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In present paper some fossil forms of frozen ground phenomena such as earth hummocks (Fig. 1), sorted patterned grounds (Figs. 3 & 4), festoon: vertical stones (Figs. 4 & 5), involutions (Figs. 7 & 8), and ice-wedge casts (Fig. 6) are described.The zonal distribution of active frozen ground phenomena and related environments are tabulated from some literatures as well as the results of our own survey on them in Japan (Table 1). Paleoclimate of the last glacial period in the esatern part of Hokkaido is considered after distribution of fossil frozen ground phenomena comparing with distribution of active ones. Volcanic ash layers and river terraces are surveyed to determine tephorochronologically the age of phenomena.The main results are as follows. All kinds of frozen ground phenomena, even earth hummocks, carry involution structures in cross section (Fig. 2). Stratified sediments such as volcanic ash layers clarify involution structures. In Japan, the southern limit of active earth hummocks coincedes approximately with +6°C mean annual isotherm. Therefore, buried earth hummocks and small-scale involutions do not indicate ancient cold climate in eastern Hokkaido where present mean annual temerature is nearly+6°C.Festoon, a kind of involution, can easily be found in fine sediments overlying river terrace gravels. Because of prevention of dense vegetation cover gravels do not reach the ground surface to form sorted patterned ground in most cases although some fossil sorted patterned grounds are found. Vegetation profited by warmth in summer sets the limit of sorted patterned grounds on nearly 0°C mean annual isotherm in central Japan and on a isotherm of somewhat lower than 0°C in Hokkaido.Some large-scale involutions (Fig. 8) may be cross sectios of palsas. Ice-wedge casts, indicators of ancient permafrost, are also found in the lowlands of eastern Hokkaido. Based on Péwé's observation decreasing of temperature at the time should be estimated as 12°-14°C.In this region ice-wedge casts indicating the maximum coldness are buried by a markable layer (Erimo Volcanic Sand: Spfa; 32, 200 14C yr.B.P.). It is, therefore, concluded from chronological studies of river terraces and volcanic ash layers that in this region the maximum decreasing of temperature precedes the maximum lowering of sea level observed all over the world in 18, 000-20, 000yr.B.P.
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ひがし北海道の化石周氷河現象とその古気候学的意義 (寒冷気候にまつわる諸問題(特集)) -- (寒冷気候下の現象)

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