[A-HW24] Isotope Hydrology 2024

[AHW24-P07] Recharge area specification for hot springs in Shiretokoiozan Volcano by water isotope analysis with application to process interpretation of melted sulfur eruption

*Mutsunori Yamamoto¹, Koki Kashiwaya², Yohei Tada², Tada-nori Goto³, Katsuaki Koike² (1.Osaka Museum of Natural History, 2.Laboratory of Environmental Geosphere Engineering Department of Urban Management Graduate School of Engineering, Kyoto University, 3.Earth-Science Laboratory, Graduate School of Science, University of Hyogo )

Keywords:Shiretokoiozan Volcano, Molten Sulfur Eruption, Water Isotope Analysis, Altitude effect of water isotope, Kamuiwakka Hot Springs

Shiretokoiozan Volcano, located in the central part of the Shiretoko Peninsula in Hokkaido, is an active volcano widely known for ejecting large quantities of molten sulfur. At the 600-meter elevation on the northwest slope of the mountain lies the Crater I (New Erupting Crater), which has recorded four molten sulfur eruptions since the 19th Century. Particularly, the last eruption in 1936 saw activities such as sulfur mining resulting in a reported production of 116,523 tons of crude ore and an equivalent of 96,208 tons of sulfur content between 1936 and 1943 (Geological Survey of the Agency of Industrial Science and Technology, 1967). Previous studies have conducted Self Potential surveys around the crater to identify regions of vigorous volcanic activity (Yamamoto et al., 2017). Furthermore, through DC resistivity surveys, field inspections, and comparison with descriptions by Watanabe and Shimotomai (1937), it was revealed that groundwater is involved in sulfur generation. However, the pathways of groundwater have not been evident with confirmation of hot spring discharges only at Crater I and the Kamuiwakka Creek. Therefore, this study aims to elucidate the distribution and flow patterns of groundwater related to molten sulfur eruptions through geochemical analysis of water samples. To achieve this, water samples were collected from Kamuiwakka hot springs, hot springs at Crater I and surrounding creeks. Hydrogen-and oxygen isotope ratios and concentrations of dominant dissolved ions, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, and SO4^2-, were determined with cavity ring-down spectroscopy and ion chromatography. Based on these characteristics, it was estimated that the aquifer beneath the Crater I identified by DC resistivity surveys and the aquifer of Kamuiwakka hot springs are connected to the volcanic gas ascent zone considered as the sulfur generation site. Furthermore, by estimating the elevation of the recharge area of Kamuiwakka hot springs water from isotopic analysis results of surrounding creek water and elucidating the groundwater flow system around the crater, the mechanism of molten sulfur eruption is discussed.

References:
Geological Survey of the Agency of Industrial Science and Technology. 1967. Overview of Metal and Non-metal Mineral Deposits in Hokkaido.

Watanabe T and Shimotomai T. 1937. Report of the Hokkaido Geological Survey No. 9 Shiretokoiozan Volcano Activities in 1936: 1-91.

Yamamoto, M., Goto, T-n., Kiji, M., 2017. Possible mechanism of molten sulfur eruption: Implications from near-surface structures around of a crater on a flank of Mt. Shiretokoiozan, Hokkaido, Japan. J. Volcanol. Geoth. Res. 346, 212–222.