Stable strontium isotopes as indicators of the origin of riverine sulfate
Description
Chemical weathering of rocks plays a fundamental role in long-term climate control through its impact on atmospheric CO2. Carbonate weathering has a dual effect: weathering by carbonic acid is CO2-neutral, while reactions between carbonates and sulfuric acid release CO2 into the atmosphere-ocean system. The oxidative weathering of pyrite (OWP), as a primary source of sulfuric acid, is therefore an important process for the global carbon cycle, which requires quantitative estimation. However, tracing OWP-derived sulfate in rivers is challenging due to another prominent source of riverine sulfate: evaporites, such as gypsum and anhydrite. This study makes the first step toward testing stable isotopes of Sr (δ88/86Sr) as a new tool for distinguishing between different origins of riverine sulfate. The study contributes new stable Sr data for a set of rivers with small, lithologically homogeneous catchments in eastern France (δ88/86Sr ranges between 0.17‰ and 0.54‰), along with the data for natural evaporites of the Triassic age (0.39–0.54‰). A developed simple mixing-reaction model shows that 0–50% of the Sr in these rivers originates from gypsum. Applying this model to large rivers across the world suggests 0–70% of their Sr content comes from evaporites. Upscaling these results to the global riverine flux provides first-order quantitative estimates of the relative inputs of sulfate from different geological sources. Overall, the global contribution of sulfate from OWP is estimated to be 16–48%, with the corresponding global OWP-associated carbon flux of around 15 TgC/yr.