Significant contribution of groundwater to the basalt weathering flux in an inactive volcanic field: Evidence from radiogenic uranium isotope

Abstract

Chemical weathering of basalt plays a vital role in the long-term carbon cycle and Earth's habitability. Quantifying the magnitude and controlling factors of basalt weathering requires assessing the contribution to weathering solutes from groundwater, which has remained challenging. In this work, we apply radiogenic uranium isotope activity ratios (²³⁴U/²³⁸U) to trace groundwater input to riverine weathering solutes in an inactive volcanic field from East China. Dissolved (²³⁴U/²³⁸U) is closely tied to the residence time of water bodies on the Earth's surface, allowing us to separate surface weathering solutes and groundwater characterized by distinct flow paths and residence time. We constrain the surface-weathering and groundwater end-members using hillslope-weathering solutes and spring waters, respectively. Based on the (²³⁴U/²³⁸U) ratios of the collected end-member samples, we developed a mass balance model and determined that groundwater contributes significantly to riverine solutes (∼41.5 %) but only has a minor contribution (∼10 %) to the water in man-made reservoirs in the study area. The estimated basalt weathering rates based on the alkalinity of river water may deviate from the global trend between basalt weathering rates and temperature if the contribution of groundwater is not corrected. Overall, our findings highlight the necessity to account for the influence of groundwater when estimating basalt weathering rates using river water, whereas man-made reservoirs may be less prone to groundwater inputs.