利尻山における雪崩涵養型雪渓の内部構造と氷化過程

Abstract

We performed core drilling at the Yamunai-sawa perennial snow patch (785 m a.s.l.) on Mt. Rishiri, Hokkaido, on September 9-10, 2018. The obtained core was 993 cm long. An ice layer was found at a depth of 853-993 cm, which indicates that perennial snow patches nourished by snow avalanches can have a thick ice layer, even at low altitudes. To clarify the transformation process from firn to ice, we investigated the crystal grain size, pore structure and oxygen isotope ratio of the core, and we estimated the depth to which the cold wave penetrated in early winter and the density increase in water-saturated firn in the ablation period. We found that 5-12-cm-thick transparent ice layers are superimposed ice with larger crystal grains and lower oxygen isotope ratios than in adjacent layers. The air temperature in perennial snow patches nourished by snow avalanches at low altitudes is higher than that in perennial snow patches nourished by snow drift. This higher temperature disturbs the refreezing of water-saturated firn that is due to cold wave penetration in early winter. Instead, the thick ice layer at a depth of 853-978 cm was formed by densification of water-saturated firn within two years after snow deposition. Thus, we conclude that the dominant transformation process from firn to ice in the Yamunai-sawa perennial snow patch nourished by avalanches is the same as that in temperate glaciers.