International audienceThe 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 δ34S and δ18O 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 × 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

No abstract

This study presents a chemical protocol for the separation of Mg that is particularly adapted to alkali‐rich samples (granite, soil, plants). This protocol was based on a combination of two pre‐existing methods: transition metals were first removed from the sample using an AG‐MP1 anion‐exchange resin, followed by the separation of alkalis (Na, K) and bivalent cations (Ca2+, Mn2+ and Sr2+) using a AG50W‐X12 cation‐exchange resin. This procedure allowed Mg recovery of ∼ 10 0 ± 8%. The [Σcations]/[Mg] molar ratios in all of the final Mg fractions were lower than 0.05. The Mg isotope ratios of eleven reference materials were analysed using two different MC‐ICP‐MS instruments (Isoprobe and Nu Plasma). The long‐term reproducibility, assessed by repeated measurements of Mg standard solutions and natural reference materials, was 0.14‰. The basalt (BE‐N), limestone (Cal‐S) and seawater (BCR‐403) reference materials analysed in this study yielded δ26Mg mean values of −0.28 ± 0.08‰, −4.37 ± 0.11‰ and −0.89 ± 0.10‰ respectively, in agreement with published data. The two continental rocks analysed, diorite (DR‐N) and granite (GA), yielded δ26Mg mean values of −0.50 ± 0.08‰ and −0.75 ± 0.14‰, respectively. The weathering products, soil (TILL‐1) and river water (NIST SRM 1640), gave δ26Mg values of −0.40 ± 0.07‰ and −1.27 ± 0.14‰, respectively. We also present, for the first time, the Mg isotope composition of bulk plant and organic matter. Rye flour (BCR‐381), sea lettuce (Ulva lactuva) (BCR‐279), natural hairgrass (Deschampsia flexuosa) and lichen (BCR‐482) reference materials gave δ26Mg values of −1.10 ± 0.14‰, −0.90 ± 0.19‰, −0.50 ± 0.22‰ and −1.15 ± 0.27‰ respectively. Plant δ26Mg values fell within the range defined by published data for chlorophylls.