Soil CO2 emissions as a proxy for heat and mass flow assessment, Taupō Volcanic Zone, New Zealand

The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high-resolution measurement of carbon dioxide flux (φCO₂) and heat flow at the surface to characterize the mass (CO₂ and steam) and heat released to the atmosphere from two magma-hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3000 direct measurements of φCO₂ and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO₂ emission rates (ΣCO₂) of 441 ± 84 t d⁻¹ and 124 ± 18 t d⁻¹were calculated for Rotokawa (2.9 km²) and for the crater floor at White Island (0.3 km²), respectively. The total CO₂ emissions differ from previously published values by +386 t d⁻¹ at Rotokawa and +25 t d⁻¹ at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO₂ emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal-sourced CO₂ emission (constrained using stable isotopes) with reservoir H₂O:CO₂mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MW_t) is 10 times greater than the White Island crater floor (22.5 MW_t).