大雪山の永久凍土と気候環境

大雪山の事例とシベリア・アラスカ・カナダとの比較を中心としての若干の考察

抄録

In this paper, the authors report the existence of permafrost at Mt. Taisetsu, Hokkaido, and disdiscuss the climatic significance of permafrost there from a viewpoint of the Quaternary palaecolimatology. Many researchers have observed that patterned grounds, especially sorted circles well developed on many gentle slopes and small depressions at Mt. Taisetsu. And recently it was reported that the networks of fissures of the ground which are similar to large scale ploygons on permafrost in Alaska were found at some places at Mt. Taisetsu. Those sorted circles and nets of fissures suggest the existence of permafrost which is believed to be either relict or recent.
By our on-the-spot investigations at Mt. Taisetsu, permafrost was found on one of flat summits (2, 150m) by boring tests and geophysical methods. The maximum depth of the active layer was nearly 2m and the maximum depth of the lower limit of this permafrost was estimated to be 15m or larger. Its frozen soils consisted of weathered fine sands and gravels derived from an andesite bedrock, which are considered as frost-action products. Large sorted circles were form ed on the surface of permafrost.
The mean annual temperature and degree-days in this area were calculated as -3.1°C and 2, 400 degree-days by extrapolations on the basis of meteorological data in Asahikawa. The estimated temperature and degree-days suggest that this area has the same climatic conditions as those in central Alasak, where discontinuous permafrost is distributed.
Introducing the Popov's theory about the formation of an ice-wedge, we estimated the palaeo-temperature of the surface of the extended permafrost to be -7°--8°C (mean annual temperature) during a past colder period than present. Comparing the estimated chronology of the permafrost with the chronology of permafrost in Alaska and Siberia, and considering the field observations of fossil ice-wedge casts and polygons in Hokkaido, we may conclude that the permafrost at Mt. Taisetsu formed at the last ice age, after then gradually has become smaller, adapting itself to the present climatic conditions.