Volcanic CO2 as a non-linear geochemical transfer signal

Introduction Volcanic carbon dioxide (CO 2 ) is widely used to infer deep magmatic degassing, but the flux measured at the surface is filtered by hydrothermal storage, chemical reactions, pathway permeability, and environmental forcing. Methods In this study, a low-dimensional geochemical transfer theory links dissolved magmatic carbon, mobile exsolved CO2, hydrothermal pressure, aqueous and mineral carbon storage, and connected-fracture permeability. Local linearization quantifies the frequency-dependent gain and delay of a mobile-gas perturbation transmitted to the surface, together with barometric admittance. A reduced nonlinear realization provides synthetic checks of direct, buffered, delayed, and pathway-controlled responses; frequency-specific recovery, parameter-sensitivity, and time-step analyses show how these responses depend on storage and pathway time scales. A published-data test using 30 Solfatara surveys evaluates whether the sampled flux distributions can be explained using a single multiplicative amplitude factor. Results The direct-proxy interpretation is recovered only when hydrothermal retention is weak, pathway conductance is approximately stationary, and atmospheric forcing is small. Both ordinary least-squares and robust Theil–Sen estimates show upper-quantile scaling shallower than the unit slope required by that null model (for Q 95 , slopes of 0.28 and 0.46, with Theil–Sen 95% interval 0.10–0.88). Discussion The result indicates non-uniform scaling of the sampled flux distribution, consistent with changes in spatial distribution and pathway contribution in addition to bulk amplitude. The theory therefore converts a qualitative source-versus-pathway ambiguity into explicit transfer regimes, local observation degeneracies, and measurement requirements within one physically interpretable geochemical structure.