Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Lithium (Li) isotopes signatures in sedimentary archives are promising proxies to reconstruct paleo-weathering rates and paleo-seawater composition. Nonetheless, the interpretation of the seawater Li isotope composition evolution over the Cenozoic is still debated. In this aim, the study of large rivers can provide constraints for continental weathering flux to the ocean. However, in the recent past, human activities like flow regulation via reservoir construction, have significantly modified the hydrodynamic of riverine systems, impacting the transfer of matter from the continent to the ocean. Hence, these effects need to be considered and investigated when unravelling the signature of natural processes from river borne materials. We report Li concentrations and Li isotope compositions (delta Li-7) for the dissolved and sediment loads of subtropical river waters from the Xijiang basin, the largest tributary of the Pearl River (also known as the Zhujiang), Southern China. These rivers display low dissolved Li concentrations. Although they are potentially impacted by carbonate dissolution in the upstream karst terrains, the Li budget of these rivers is dominated by silicate weathering. The dissolved Li isotope compositions of rivers vary over a range from +16% to +27%. These values are high compared to the estimated composition of the bedrock similar to 0%), and are explained by the preferential incorporation of the light Li-6 in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved delta Li-7 values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high delta Li-7 values in the headwaters reflect greater degree of secondary phase formations associated with the higher erosion rates in the mountainous part of the basin, whereas the lower delta Li-7 values in the lowlands characterized by low soil erosion rates and more abundant rainfall reflect a more intensive weathering. This interpretation is supported by the positive relationship between soil erosion rates and the Li dissolved fluxes. The isotope compositions of the suspended particulate material (SPM) transported by rivers in the Xijiang Basin (from -3.8% to -0.7%) are systematically lower than the composition of the bedrock. We observe that the delta Li-7 of most SPM samples collected during the wet season are lower than those of samples collected during the dry season. We interpret this seasonality as a consequence of seasonal changes in the sediment properties due to water regulation. In spite of the influence induced by river managements on the river sediments, our data also highlight that the imprint of weathering

Automated summary

The study reveals lithium concentrations and lithium isotope compositions in subtropical river waters in the Xijiang Basin, Southern China. The rivers exhibit low dissolved lithium concentrations dominated by silicate weathering, with a wide range of dissolved lithium isotope compositions. Higher δLi-7 values in the high erosion mountainous part of the basin reflect a greater degree of secondary phase formations, while lower values in the lowlands with low erosion rates and abundant rainfall reflect more intensive weathering.