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.非喷发式火山脱气 (Noneruptive volcanic degassing) 是地球内部碳循环的重要途径，但其相关机制尚未得到充分阐释。本研究报道了2019至2022年三年间，中国台湾大屯火山群 (Tatun Volcano Group) 6个喷气孔点位的CO₂同位素组成（δ¹³C与δ¹⁸O）观测数据，同步配套γ射线 (γ-ray) 数据。δ¹³C值呈现显著的时间变异性，且在研究区域内具有空间一致性，反映了CO₂来源的动态变化。本研究共识别出三类CO₂来源：地幔源、碳酸盐源与有机质源。研究采用贝叶斯混合模型 (Bayesian mixing models) 对各来源的贡献占比进行约束，结果显示平均贡献分别为地幔源45%、有机质15%、碳酸盐源40%；其中地幔与碳酸盐组分代表深部来源，有机质源则对应浅部沉积或热液来源。各来源的阶段性变化与γ射线异常事件相吻合，表明地下裂隙发育可促进流体向地表运移。δ¹³C与γ射线活度的耦合时间变化特征表明，CO₂动力学过程主要受盖层封闭作用以及低渗透层下方流体聚集过程控制，进而引发流体增压、裂隙发育以及深部流体与热液溶质的向上运移。本研究为火山-热液系统中瞬态喷气孔CO₂动力学特征的解释提供了概念性分析框架。