Carbon fluxes, pCO2 and substrate weathering in a large northern river basin, Canada: carbon isotope perspectives

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Isotopic composition of dissolved inorganic carbon (δ13CDIC) in the Ottawa River basin is about −8 and −16‰ for lowland carbonate and upland silicate tributaries, respectively. This suggests that (1) the source of DIC to the Ottawa River is soil respiration and carbonate weathering, (2) exchange with the atmosphere is unidirectional or volumetrically unimportant, and (3) in-river respiration and photosynthesis are not significant influences on the river carbon budget. Accepting these constraints, chemical and isotopic data are used to reconstitute soil pCO2 for tributary catchments. Averages for upland silicate, mixed, and lowland carbonate basins are calculated to be roughly 2000, 5000, and 30,000 ppm, respectively. 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 4.3×1010 mol C/a. Utilizing carbon isotopes, 75% and 25% of the Ca2++Mg2+ flux is calculated to originate from carbonate and silicate weathering, respectively, and 61% of the DIC is calculated to originate from organic respiration. The latter represents some 6% of respired carbon in the basin, assuming an average respiration rate of 0.5 mmol C m−2 h−1. Based on a diffusion model, CO2 evasion to the atmosphere from the Ottawa River and its tributaries is estimated to be 1.3×1010 mol C/a or 30% of the DIC flux.