[S-VC28] Hydrothermal systems of volcanoes

[SVC28-07] Dissolved polythionates in hot water pool at Mt. Iwoyama, Kirishima Volcanoes

*Yoshikazu Kikawada¹, Emily Suzuki², Natsumi Shibano², CHEN YIFAN² (1.Faculty of Science and Technology, Sophia University, 2.Graduate School of Science and Technology, Sophia University)

Keywords:Kirishima Volcanoes, Ebino-Iwoyama, ploythionates, sulfur species, hot water pool, crater lake

The composition of dissolved constituents in active crater lakes is one of the most critical observations in assessing hydrothermal-dominated volcanic activity. In some volcanoes, the formation and loss of dissolved polythionate ions in crater lake waters are known to show a good correlation with volcanic activity.
Mt. Iowoyama of the Kirishima volcanoes had a phreatic eruption in 2018, and its vent formed a hot water pool. The vent continues to emit hydrothermal water and volcanic gases, making it a highly active crater lake. The chemical composition of the hot water pool at Mt. Iwoyama has fluctuated significantly with the rise and fall of volcanic activity. However, the presence of polythionate ions in the pool has not been determined. In this study, we re-determined the chemical species of dissolved sulfur in 21 water samples collected from the hot water pool from 2018 to 2023 to confirm the presence of polythionate ions. The results confirmed the presence of polythionate ions in the water samples collected in November 2018, November 2022, and May 2023. The percentages of polythionate ions in the total dissolved sulfur species content were estimated to be approximately 4%, 23%, and 7%, respectively, regarding the molar ratio of sulfur. The high and low percentages of polythionate ions appear to correspond to that of the Cl/SO₄ concentration ratio, suggesting that they correspond to changes in the composition of the volcanic gases supplied to the vent at the hot water pool.