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| | | Dissolved carbon in large boral catchment basins and hydroelectric reservoirs: isotopic clues
JEAN-FRANCOIS HÉLIE1*, ÉRIC ROSA1, ANNIE LALONDE1 AND CLAUDE HILLAIRE-MARCEL1
1 GEOTOP – Université du Québec à Montréal, C.P. 8888 succursale Centre-ville, Montréal (QC), Canada, H3C 3P8
* correspondence: helie.jean-francois@uqam.ca
Carbon sources and cycling in boreal environments are currently the focus of a growing number of investigations in Eastern Canada, mainly beacause of the importance of hydropower and its potential in this region. Here, we investigate dissolved inorganic and organic carbon fluxes and sources in 5 impounded (La Grande 3400 m3/s, Eastmain, St. Lawrence 12 100 m3/s, Ottawa 1950 m3/s and Nelson 2370 m3/s) as well as 2 pristine (Great Whales 680 m3/s and Koksoak) river systems. These major rivers were sampled monthly at their outlet for at least one year. The St. Lawrence River has been under investigation since June of 1997 on a bi-weekly basis. Also, synoptic surveys were undertaken in August 2008 on the La Grande and Great Whales Rivers. At each visit, water temperature, pH, Gran alkalinity and specific conductivity were measured. Samples were also collected for the analysis of i) major ions concentrations; ii) δ13C of dissolved inorganic and organic carbon as well as particulate organic carbon (respectively DIC, DOC and POC); iii) δ18O and δ2H of the water molecule; and iv) radio-isotopes (Uranium series and Strontium - not discussed here). In all the sampled river systems, POC concentrations were at least an order of magnitude smaller than the dissolved forms. As a result, the dissolved forms will be the focus of this investigation. Rivers draining carbonates (St.Lawrence and Nelson Rivers) present higher DIC concentrations and higher 13C-contents in dissolved inorganic carbon (DIC), in response to the dissolution of soil carbonates. DOC/DIC ratios above 2.4 are observed in rivers draining silicates; their lower 13C-DIC content directly reflects the organic matter oxidation in soils. However, the DIC isotopic composition of impounded rivers draining silicates reflects both the organic oxidation origin of DIC and CO2 degassing along the reservoir. The striking feature of this boreal data set is the homogeneity of the isotopic composition of DOC (-27.4±0.2‰ vs V-PDB). Moreover, C:N ratios and 14C activity (A14C) of bulk dissolved organic matter (MOD) measured in the La Grande River reveal that this DOC is relatively fresh and young (A14C higher than 100% MC).
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