Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs

Hot springs in the Marsyandi Valley, Nepal, vent CO2 sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earthʹs surface. The δ13C of spring waters ranges up to + 13‰, while that of the coexisting free gas phase is close to − 4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests > 97 ± 1% CO2 degassing. The calculated minimum total CO2 degassing in the Marsyandi catchment is 5.4 × 109 mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO2 degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO2 flux of 0.9 × 1012 mol/yr, or ∼ 13% of solid Earth CO2 degassing. The calculated flux is a factor of three greater than the estimated CO2 drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO2 flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability.