Abstract
Hot springs in the Marsyandi Valley, Nepal, vent CO 2 sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth's surface. The δ 13C 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% CO 2 degassing. The calculated minimum total CO 2 degassing in the Marsyandi catchment is 5.4 × 10 9 mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO 2 degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO 2 flux of 0.9 × 10 12 mol/yr, or ∼ 13% of solid Earth CO 2 degassing. The calculated flux is a factor of three greater than the estimated CO 2 drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO 2 flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability.
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