Dissolved inorganic carbon export from carbonate and silicate catchments estimated from carbonate chemistry and δ¹³C_DIC

Abstract. This work presents a study of the dissolved inorganic carbon (DIC) exchange associated with groundwater discharge and stream flow from two upstream catchments with distinct basement lithologies (silicate vs. carbonate). The effects of catchment lithology were evident in the spring waters showing lower δ¹³C_DIC and alkalinity (−16.2 ± 2.7 ‰ and 0.09 ± 0.03 meq l⁻¹, respectively) in the silicate and higher values (−9.7 ± 1.5 ‰ and 2.0 ± 0.2 meq l⁻¹) in the carbonate catchment. The streams exhibited relatively high δ¹³C_DIC, −6.9 ± 1.6 ‰ and −7.8 ± 1.5 ‰, in silicate and carbonate catchments, respectively, indicating CO₂ degassing during groundwater discharge and stream flow. The catchment lithology affected the pattern of DIC export. The CO₂ degassing from stream and groundwater could be responsible for 8–55 % of the total DIC export in the silicate catchment, whereas the proportion is comparatively low (0.4–5.6 %) in the carbonate catchment. Therefore, the dynamic carbon exchange occurring at headwater regions and its possible variability with catchment lithology need to be examined for a more reliable carbon budget in river systems.