Use of 13C to trace origin and cycling of inorganic carbon in the Rhône river system

Dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) were sampled at 20 sites on the Rhône and its main tributaries from Switzerland to the Mediterranean, in March, September and July (flood event) 1996. Flux estimation and DIC and PIC δ13C values are used to trace watershed inputs and fluvial processes affecting the inorganic carbon budget. DIC concentration and δ13C values vary between ∼1 mM at −5‰ near the Rhôneʹs source and ∼3 mM at −10‰ near its mouth, with tributaries draining lowland, carbonate-dominated areas at ∼3.8 mM and −11‰. There is an inverse relationship between river DIC concentration and DIC δ13C. The headwaters are characterized by high DIC δ13C values and low DIC, indicating a minor input of respired CO2. DIC should be mainly produced through the reaction of carbonate with atmospheric CO2 or organic/sulfuric acids. In lowland, carbonate-dominated areas, δ13C values of ∼−11‰ are consistent with production of DIC by weathering of limestone C (δ13C≈−1‰) with CO2 derived from the oxidation of organic matter (δ13C≈−21.5‰). The evolution of the Rhôneʹs DIC concentration and δ13C appear to be largely determined by mixing between the Rhône and its main tributaries. The CO2 fluxes and DIC δ13C mass-balance, calculated for the different segments of the Rhône, point to a net, but very limited input of fluvial respired CO2 from the oxidation of organic C (<5%). The PIC represents a small but quite variable fraction of the inorganic carbon (<3%), except on Alpine catchments and during floods. In these cases, it can be up to 10% of the DIC concentration and export. The carbon and oxygen isotopic composition of PIC from the Rhône during a flood and from Alpine tributaries falls within the δ-range of limestones (δ13C≥−2‰; δ18O≥−6‰). Relatively light PIC isotopic composition indicate mixing of sedimentary rocks and carbonate precipitated in situ. Data for the Rhône system are typical of watersheds presenting high chemical weathering rates for HCO3, and Alpine/lowland conditions.