Rock denudation rates and organic carbon exports along a latitudinal gradient in the Hudson, James, and Ungava bays watershed

This study documents chemical denudation rates (CDR) in the Canadian Shield and Interior Platform. It focuses on the dissolved chemistry of rivers flowing into the Hudson, James, and Ungava bays (HJUB). Major ions, strontium, neody- mium, and dissolved organic carbon (DOC) concentrations were monitored in four rivers (Koksoak, Great Whale, La Grande, and Nelson). Six other rivers flowing into the HJUB were sampled during baseflow and snowmelt conditions. The rivers of the Canadian Shield exhibit major cation concentrations ranging between 62 and 360 µmol/L, (Nd) of 0.57-4.72 nmol/L, and vari- able (DOC) of 241-1777 µmol/L. In comparison, the Nelson River (Interior Platform) shows higher major cation concentrations (1200-2276 µmol/L), lower (Nd) (0.14-0.45 nmol/L), and intermediate (DOC) (753-928 µmol/L). Altogether, the studied riv- ers export 8 × 10 6 t/year of dissolved major cations and 50 t/year of dissolved Nd towards the HJUB. Basin-scale rock denuda- tion rates (RDR) range from 1.0 to 5.3 t·km -2 ·year -1 and are essentially controlled by lithology, as illustrated by the relationship established between RDR and the proportion of sedimentary and volcanic rocks (%S + %V) within the basins: RDR = 0.08(%S + %V) + 0.9. In contrast, dissolved Nd exports (and likely other insoluble elements) seem to be dependent upon organic matter leaching, as illustrated by the empirical coupling between Nd and DOC exports. These fluxes decrease north- wards, likely in response to the hydroclimatic gradient. The CDR evaluated within the Canadian Shield are among the lowest on the planet, and the alkalinity generated by rock weathering remains small with respect to DOC exports.

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