Mobility of rhenium and selenium during chemical weathering and their implication for petrogenic organic carbon oxidation

The oxidative weathering of petrogenic organic carbon (OC petro ) is an important source of carbon dioxide (CO 2 ) into the atmosphere on geological timescales and significantly influences the carbon cycle and global climate change. However, at present, a robust proxy to assess OC petro oxidation is lacking. Riverine rhenium (Re) has emerged as a potential proxy to trace OC petro oxidation, but little is known about Re sources and its relationship with OC petro oxidation. To better understand the robustness of this proxy, we analyzed a shale weathering profile from the early Cambrian Shuijingtuo Formation in Chengkou County, Chongqing City, to address the mobility of Re and selenium (Se) and their relationship with the oxidation of OC petro by analyses of the elemental variation and sequential extraction. The results showed that the chemical index of alteration (CIA) increases from the fresh bedrock to the weathered sections, indicating enhanced weathering intensity. Based on the sequential extraction and mass transfer coefficients ( τ ), Re and Se mainly exist in the organic phase of shale and are paired lost with OC petro during weathering, with no obvious correlation with sulfide minerals. Furthermore, the different proportions of Re and Se loss with OC petro loss can be attributed to their different geochemical behaviors during weathering. Our study supports Re as a robust proxy to trace OC petro oxidation, whereas Se is the potential one. In the future, combined studies on various sedimentary rock profiles with catchment basins are needed to determine the sources of riverine Re and Se and their relationships with OC petro oxidation.