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Zusammenfassung:
Kusatsu-Shirane volcano is a famous active volcano in Japan. The volcano has an active crater lake, Yugama, having subaqueous fumaroles. Yugama is a vital observation object in the chemical monitoring of the volcano. The Yugama water contains polythionate occasionally, and the variation in their concentrations probably connects to the volcano's activity. During the past half-century, the volcano went through three phreatic eruption activities in 1976, 1982/1983, and 2018. In this study, we constructed precise time-series data for the contents of dissolved components in Yugama since the 1960s. We then discussed the variation in the contents of sulfur species in Yugama and its relation to those of the other dissolved components and the volcanic activity of the volcano.Even during the unrest period that did not lead to eruptions, changes in the dissolved composition of Yugama were observed. When volcanic activity increases, the dissolved contents of fluoride, chloride, and sulfur oxyanions (sulfate and polythionate) increase. Although the content ratio of chloride to sulfate did not change significantly, those of polythionate to chloride and polythionate to total sulfur species increased significantly. However, the details of the variability vary from active period to active period.The polythionate content in Yugama water varied synchronized with the ORP value, probably reflecting the SO2/H2S ratio in volcanic gasses supplied from subaqueous fumaroles into the lake water. The higher SO2/H2S ratio in the gasses might induce the production of polythionate in the shallow hydrothermal system and stabilize them in the lake water.
