δ13C pattern of dissolved inorganic carbon in a small granitic catchment:: the Strengbach case study (Vosges mountains, France)

The transfers and origins of dissolved inorganic carbon (DIC) were studied for a year in a soil-spring-stream system in the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce (80%). Brown acid and podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO, generated by oxidation of soil organic matter. The (delta(13)C(DIC)) in catchment waters is highly variable, from about -22 parts per thousand in the springs and piezometers to about - 12 parts per thousand in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of delta(13)C(DIC) exist, with the DIC in isotopic equilibrium with the soil CO2 that has estimated delta(13)C of about - 24 parts per thousand in winter and -20 parts per thousand in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC (-12 parts per thousand on average) is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2. (C) 1999 Elsevier Science B.V. All rights reserved.