Stable strontium isotopes as indicators of the origin of riverine sulfate

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.岩石化学风化作用通过调控大气二氧化碳（CO₂）含量，在长期气候调控中发挥核心作用。碳酸盐风化具有双重效应：碳酸介导的风化过程对大气CO₂呈中性，而碳酸盐与硫酸的反应则会向大气-海洋系统释放CO₂。作为硫酸的主要来源之一，黄铁矿氧化风化（Oxidative Weathering of Pyrite, OWP）是全球碳循环的关键过程，亟需开展定量估算。然而，由于河流硫酸盐存在另一重要来源——石膏、硬石膏等蒸发岩矿物，对河流中黄铁矿氧化风化来源硫酸盐的溯源工作颇具挑战。本研究首次迈出探索步伐，尝试将锶（Sr）稳定同位素（δ⁸⁸/⁸⁶Sr）开发为区分河流硫酸盐不同成因的新型工具。研究为法国东部一组岩性均一的小型流域河流提供了全新的锶稳定同位素数据，其δ⁸⁸/⁸⁶Sr值介于0.17‰至0.54‰之间；同时还提供了三叠纪天然蒸发岩的对应同位素数据（0.39‰~0.54‰）。本研究构建的简易混合反应模型显示，上述河流中0~50%的锶来源于石膏。将该模型应用于全球大型河流后发现，其总锶含量中有0~70%来源于蒸发岩矿物。将上述研究结果外推至全球河流通量尺度，可得到不同地质来源硫酸盐相对输入量的一级定量估算结果。总体而言，全球河流硫酸盐中黄铁矿氧化风化来源的占比估算为16%~48%，对应的全球黄铁矿氧化风化关联碳通量约为15 TgC/年。