The role of sulfur in chemical weathering and atmospheric CO 2 fluxes: Evidence from major ions, δ 13C DIC, and δ 34S SO4 in rivers of the Canadian Cordillera

Abstract

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO 3 −, SO 4 2−, Cl −, Ca 2+, Mg 2+, K +, Na +), δ 13C of dissolved inorganic carbon (δ 13C DIC), and δ 34S of dissolved sulfate (δ 34S SO4) to quantify chemical weathering rates and exchanges of CO 2 between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ 34S SO4 (−8.9 to 14.1‰ CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ 13C DIC values (−9.8 to −3.7‰ VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰). δ 34S SO4 is negatively correlated with δ 13C DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ 13C DIC of rivers. The negative correlation between δ 34S SO4 and δ 13C DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas. Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10 3 mol C · km −2 · yr −1) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10 3 mol C · km −2 · yr −1). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO 2 to carbonate rocks, sulfide-carbonate weathering can liberate CO 2 from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO 2 drawdown by silicate weathering in the region.