Gas discharges have been identified at the Syabru–Bensi hot springs, located at the front of the High Himalaya in Central Nepal, in the Main Central Thrust zone. The hot spring waters are characterized by a temperature reaching 61 °C, high salinity, high alkalinity and δ 13 C varying from + 0.7‰ to + 4.8‰. The gas is mainly dry carbon dioxide, with a δ 13 C of − 0.8‰. The diffuse carbon dioxide flux, mapped by the accumulation chamber method, reached a value of 19 000 g m − 2 day − 1 , which is comparable with values measured on active volcanoes. Similar values have been observed over a two-year time interval and the integral around the main gas discharge amounts to 0.25 ± 0.07 mol s − 1 , or 350 ± 100 ton a − 1 . The mean radon-222 concentration in spring water did not exceed 2.5 Bq L − 1 , exponentially decreasing with water temperature. In contrast, in gas bubbles collected in the water or in the dry gas discharges, the radon concentration varied from 16 000 to 41 000 Bq m − 3 . In the soil, radon concentration varied from 25 000 to more than 50 000 Bq m − 3 . Radon flux, measured at more than fifty points, reached extreme values, larger than 2 Bq m − 2 s − 1 , correlated to the larger values of the carbon dioxide flux. Our direct observation confirms previous studies which indicated large degassing in the Himalaya. The proposed understanding is that carbon dioxide is released at mid-crustal depth by metamorphic reactions within the Indian basement, transported along pre-existing faults by meteoric hot water circulation, and degassed before reaching surface. This work, first, confirms that further studies should be undertaken to better constrain the carbon budget of the Himalaya, and, more generally, the contribution of mountain building to the global carbon balance. Furthermore, the evidenced gas discharges provide a unique natural laboratory for methodological studies, and appear particularly important to study as a function of time, especially in relation to the seismic activity. For this purpose, the observed high radon-222 flux is a particularly interesting asset. Indeed, while the relationship between radon and carbon dioxide needs to be better understood, radon measurements, using the available radon sensors, constitute a powerful tool for robust and cost effective long term monitoring.

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