The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30-62°C) water springs and cold (<35°C) carbon dioxide (CO 2 ) degassing areas. From 2007 to 2011, five gas zones (GZ1-GZ5) were studied, with more than 1600 CO 2 and 850 radon flux measurements, with complementary self-potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO 2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m À2 d À1 and 38.5 ± 8.0 Bq m À2 s À1 , with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d À1 and 140 ± 30 MBq d À1 , respectively. Soil-gas radon concentration is 40 × 10 3 Bq m À3 in GZ1-GZ2 and 70 × 10 3 Bq m À3 in GZ3-GZ4. Strong relationships between CO 2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO 2 . CO 2 carbon isotopic ratios (δ 13 C from À1.7 ± 0.1 to À0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/R A < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO 2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

La Soufrière (Guadeloupe, Lesser Antilles) is one of the most active and dangerous volcanoes of the Lesser Antilles arc. The renewal of the volcanic activity at La Soufrière requires replenishment of the volatile-poor andesitic stored magma with more primitive, volatile-rich basaltic magma. This event will trigger early release of large quantities of highly volatile species, like CO 2 and helium, in the hydrothermal system. To use these precursors, first their baseline abundances and their behaviour in the hydrothermal system must be known during the current phase of dormancy. In this study, we have combined a previous systematic investigation of noble gases with carbon data of hydrothermal fluids flowing out of La Soufrière. The results for dissolved gases reveal that an intense interaction between rising magmatic volatiles and groundwaters currently takes place at La Soufrière. We show that the pristine magmatic composition, with a δ 13 C CO2 value of −3.19 ± 0.03‰, a 3 He/ 4 He ratio of 8.1 ± 0.2 R a and a CO 2 / 3 He ratio of 1.03 ± 0.18 × 10 10 , may vary during the ascent of magmatic fluids due to either fractionation or mixing processes with meteoric water and/or an organic component. Here, we provide a baseline for CO 2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios.