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Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives

¹ Equipe de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris, Université Paris 7, Centre National de la Recherche Scientifique, 4 place Jussieu-75252 Paris cedex 05, France
² Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, 15 rue Notre-Dames des Pauvres-54501 Vandoeuvre-les-Nancy, France

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO₂. The contribution of sulfuric acid as a weathering agent is still poorly quantified; the identification of riverine sulfate sources is difficult. The use of ³⁴S and ¹⁸O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated flux of pyrite-derived sulfate is 0.13 x 10¹² mol/yr, corresponding to 20%–27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfide-derived sulfate, and thus chemical weathering with sulfuric acid, may be significantly underestimated. A strong correlation between sulfide oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfide oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO₂ in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales.

Key Words: carbonate weathering • sulfide oxidation • mechanical erosion • Mackenzie River • atmospheric CO₂