Chen, Ai-TiAi-TiChenLee, Hsiao-fenHsiao-fenLeeSano, YujiYujiSanoTakahata, NaotoNaotoTakahataKagoshima, TakanoriTakanoriKagoshimaLai, Ya-chuanYa-chuanLaiLin, Li-HungLi-HungLinLin, Cheng-HorngCheng-HorngLinLo, Ching-HuaChing-HuaLo2025-12-312025-12-312025-09-26https://www.scopus.com/pages/publications/105018766295https://scholars.lib.ntu.edu.tw/handle/123456789/734887The Tatun Volcano Group (TVG) in northern Taiwan, situated at the junction of the Philippine Sea and Eurasian plates, hosts an active hydrothermal system with significant implications for volcanic hazard and geothermal resource management. From January 2016 through October 2022, we conducted monthly monitoring of major dissolved ions, dissolved gases (CO2, H2S, SO2), and helium isotope ratios at the Da-you-keng (DYK) and Geng-zi-ping fumarolic fields. In addition, in 2019 and 2024 we sampled bubbling gases from 13 hot springs and ten groundwater wells across the TVG and adjacent regions, analyzing multispecies gas compositions and stable isotopes (3He/4He, δ13C, δ15N). High-density seismic data recorded 4550 microseismic events in 2020, nearly double the long-term average, enabling correlation of geochemical fluctuations with seismic unrest. We observed pronounced periodic variations in dissolved Cl–, SO42–and cations at the DYK, and complementary patterns of CO2and H2S at both sites, reflecting episodic fluid recharge and redox shifts in the hydrothermal conduit system. Helium isotope ratios (corrected R/RA= 5.64–7.24) and CO2/3He−δ13C trends delineate mixing between mantle-derived and crustal sources, while N2–He–Ar systematics reveal sediment contributions modulated by major faults. Spatially, elevated3He/4He ratios are clustered around the DYK, with values decreasing along fault-controlled pathways, indicating focused deep degassing. Periods of heightened seismicity coincide with gas-ratio anomalies (CO2/H2S and3He/4He) and surges in magmatic helium, suggesting ascending deep fluids contribute to both degassing and earthquake generation. The integrated geochemical–seismic analysis underscores ongoing volcanic activity in the TVG and provides a framework for enhancing eruption forecasting and guiding sustainable geothermal exploitation.geochemical monitoringhelium isotopeshydrothermal systemmagmatic degassingseismicityTatun Volcano Group[SDGs]SDG13journal article10.1021/acsearthspacechem.5c00233