Carbon fluxes, pCO(2) and substrate weathering in a large northern river basin, Canada: carbon isotope perspectives

Isotopic composition of dissolved inorganic carbon (delta(13)C(DIC)) in the Ottawa River basin is about -8 and - 16 parts per thousand for lowland car bonate and upland silicate tributaries, respectively. This suggests that (1 ) the source of DIC to the Ottawa River is soil respiration and carbonate w eathering, (2) exchange with the atmosphere is unidirectional or volumetric ally unimportant, and (3) in-river respiration and photosynthesis are not s ignificant influences on the river carbon budget. Accepting these constrain ts, chemical and isotopic data are used to reconstitute soil pCO(2) for tri butary catchments. Averages for upland silicate, mixed, and lowland carbona te basins are calculated to be roughly 2000, 5000, and 30,000 ppm, respecti vely. These values are used as input to model the pathway of carbon through the watershed-rain water to soil water to river water. The flux of carbon from the Ottawa River as DIC is calculated to be 4.3 x 10(10) mol C/a. Util izing carbon isotopes, 75% and 25% of the Ca2+ + Mg2+ flux is calculated to originate from carbonate and silicate weathering:, respectively, and 61% o f the DIC is calculated to originate from organic respiration. The latter r epresents some 6% of respired carbon in the basin, assuming an average resp iration rate of 0.5 mmol C m(-2) h(-1). Based on a diffusion model, CO2 eva sion to the atmosphere from the Ottawa River and its tributaries is estimat ed to be 1.3 x 10(10) mol C/a or 30% of the DIC flux. (C) 1999 Elsevier Sci ence B.V. All rights reserved.