Abstract
The weathering of carbonate rocks with sulfuric acid releases carbon dioxide (CO2) to the atmosphere, offsetting the CO2 drawdown from carbonic acid weathering of silicates thought to regulate global climate. Quantifying CO2 release from sulfuric acid weathering requires the partitioning of riverine sulfate between its two main sources: sedimentary sulfate and sulfide. Although the sulfur (δ34SSO4) and oxygen (δ18OSO4) isotope ratios of sedimentary sulfates (gypsum and anhydrite) of different ages are well constrained, the δ34S of sulfide minerals is highly variable, restricting the utility of δ34S for partitioning sulfur sources. Here, we use oxygen isotope ratios in the river water (δ18OH2O) and sulfate molecules (δ18OSO4) to partition the fraction of sulfate and associated uncertainty delivered by the oxidative weathering of pyrite (fpyr). The partitioning is illustrated using the Mekong River, one of the world's largest river basins, presenting new δ18OSO4, δ18OH2O and δ34SSO4 data collected on 18 tributaries and 6 mainstem sites over two field seasons at peak flux. The geological, geomorphological and climatic diversity of the Mekong River basin make it an ideal field site to quantify the role of sulfuric acid weathering and its implications for the carbon cycle. There is a 12‰ range in both the difference between δ18OSO4 and δ18OH2O (Δ18OSO4−H2O) and δ34S in the river waters of the basin. In the Mekong tributaries, sources of sulfate are highly variable with the fraction of sulfate derived from pyrite oxidation (fpyr) ranging from 0.19 to 0.84. In the mainstem, fpyr reflects the flux-weighted mean of these tributary inputs, with 56±7% (1σ) of the sulfate delivered to the ocean at the Mekong mouth being derived from the oxidative weathering of pyrite. As a result, we estimate that ∼70% of CO2 consumed through silicate weathering in the Mekong basin is offset by the release of CO2 via the dissolution of carbonates by sulfuric acid.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.