Radon signature of CO2 flux constrains the depth of degassing: Furnas volcano (Azores, Portugal) versus Syabru-Bensi (Nepal Himalayas).

Substantial terrestrial gas emissions, such as carbon dioxide (CO₂), are associated with active volcanoes and hydrothermal systems. However, while fundamental for the prediction of future activity, it remains difficult so far to determine the depth of the gas sources. Here we show how the combined measurement of CO₂ and radon-222 fluxes at the surface constrains the depth of degassing at two hydrothermal systems in geodynamically active contexts: Furnas Lake Fumarolic Field (FLFF, Azores, Portugal) with mantellic and volcano-magmatic CO₂, and Syabru-Bensi Hydrothermal System (SBHS, Central Nepal) with metamorphic CO₂. At both sites, radon fluxes reach exceptionally high values (> 10 Bq m⁻² s⁻¹) systematically associated with large CO₂ fluxes (> 10 kg m⁻² day⁻¹). The significant radon‒CO₂ fluxes correlation is well reproduced by an advective–diffusive model of radon transport, constrained by a thorough characterisation of radon sources. Estimates of degassing depth, 2580 ± 180 m at FLFF and 380 ± 20 m at SBHS, are compatible with known structures of both systems. Our approach demonstrates that radon‒CO₂ coupling is a powerful tool to ascertain gas sources and monitor active sites. The exceptionally high radon discharge from FLFF during quiescence (≈ 9 GBq day⁻¹) suggests significant radon output from volcanoes worldwide, potentially affecting atmosphere ionisation and climate. [ABSTRACT FROM AUTHOR]