Journal of the Japanese Society of Snow and Ice

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This study investigates the firn–ice transition age and the transformation processes of firn to ice in a glacier on the southeastern slope of Mt. Shakushi in the northern Japanese Alps. We base our analyses on two ice cores excavated from the Shakushizawa Glacier. The ice cores, measuring 706 cm and 330 cm, were extracted, respectively, on October 14-15, 2022, and September 18, 2023. Pollen analysis and oxygen isotope ratio measurements reveal that the peaks in total pollen concentration closely match the low d-excess layers and indicate summer layers. These results suggest that pollen peaks can be used to delineate annual layer boundaries. Further analysis shows that dirt layers, which have traditionally been used as annual markers, do not coincide with pollen peaks. This mismatch implies that relying on dirt layers as annual markers may underestimate snow accumulation. To better understand the firn–ice transition, we use digital surface models (DSMs) to estimate snow depths during peak accumulation periods, from which we calculate overburden pressure. The resulting pressures support the feasibility of densification-driven ice formation. Yet observations of accumulation layers and surface mass balance reveal that the ice formation process depends on the depth of the firn-to-ice transition boundary during the late melt season. In particular, when the boundary is deeper, densification alone formed ice; when shallower, both melt-refreeze and densification contribute. The resulting firn–ice transition age is estimated to be 1-3 years when melt-refreeze is involved and 4 years or more under densification alone.

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