Metamorphic CO2 fluxes offset the net geological carbon sink in the Himalayan-Tibetan orogen

Abstract

The carbon budget of the Himalayan-Tibetan orogen often neglects the poorly constrained CO₂ fluxes from the mantle and metamorphism that emerge directly from hot springs or diffusely through the soil, which can potentially warm the climate. What exists are either metamorphic phase relations, thermodynamic model-based CO₂ flux estimates, or field-based estimates in a single catchment extrapolated to the entire orogen having diverse geological and tectonics characteristics, heat flow, and hydrological regimes. Here, we apply a steady-state isotope mass balance model to observations of carbon isotope ratios to quantify the fluxes of CO₂ degassed using a global compilation of hot springs located on the fault systems of the Himalayan Tibetan orogen. We estimate that the hot springs could degas up to ∼84 % of the dissolved CO₂, releasing up to 1.5 ± 1.0 × 10¹¹ mol CO₂ yr^-1. When combined with diffused soil CO₂ emissions, the Himalayan Tibetan orogen has the potential to emit 3.0 ± 1.3 × 10¹¹ mol CO₂ yr^-1, which is similar in magnitude to CO₂ emission from continental rifts as well as the CO₂ drawdown due to silicate weathering in the orogen. We find that CO₂ released from the catchments of Himalayan-Tibetan orogen is primarily controlled by metamorphic processes $\left({79}_{-25}^{+21}\%\right)$, with secondary controls from mantle degassing $\left(7_{-7}^{+17}\%\right)$, and carbonate dissolution (${14}_{-14}^{+26}$%) and is insensitive to contributions from younger biospheric sources. Reevaluation of the geological carbon budget in Narayani Basin, Nepal, shows that metamorphic CO₂ in the collisional orogen is comparable to other carbon sources and sinks. We conclude that metamorphic CO₂ could offset the CO₂ drawdown by silicate weathering and OC_biosphere burial in the Himalayan-Tibet orogen, resulting in global warming over million-year time scales.