Fumarolic CO2 Dynamics: A Case Study from the Tatun Volcano Group, Taiwan

Noneruptive volcanic degassing is a major but poorly understood pathway of carbon recycling from Earth’s interior. Here we present a three-year (2019–2022) record of fumarolic CO2 isotopic compositions (δ13C and δ18O) from six fumarolic sites within the Tatun Volcano Group, Taiwan, along with γ-ray data. The δ13C values showed pronounced temporal variability with spatial coherence in the region, reflecting changes in the CO2 sources. Three CO2 sources were identified: mantle, carbonate, and organic. Bayesian mixing models were used to constrain the contributions of these sources. The average contributions are approximately mantle 45%, organic matter 15%, and carbonate 40%, with the mantle and carbonate components representing deep sources while the organic matter reflects shallow sedimentary or hydrothermal sources. Episodic changes in the sources coincide with γ-ray anomalies, indicating subsurface fracturing that facilitates fluid migration to the surface. The coupled temporal variation in δ13C and γ-ray activity suggests that the CO2 dynamics is largely controlled by caprock sealing and fluid accumulation beneath a low-permeability layer, leading to pressurization, fracturing, and the ascent of deep fluids and hydrothermal solutes. This study provides a conceptual framework for interpreting transient fumarolic CO2 dynamics in volcanic–hydrothermal systems.