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• The riverine dissolved Li budget is mainly controlled by silicate weathering. • The variations of dissolved δ7Li values reflect the change of weathering intensity. • River management by damming insert a control on the river sediment geochemistry. • The Li isotopic fingerprint of weathering is preserved in river sediments. 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 (δ7Li) 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 (~0‰), and are explained by the preferential incorporation of the light 6Li in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved δ7Li values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high δ7Li 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 δ7Li 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 δ7Li 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 and erosion processes is still preserved in the sedimentary load of the Xijiang as suggested by the relationship between the SPM δ7Li values and the weathering intensity defined as the ratio between the chemical weathering rates and the total denudation rates. Altogether, our study confirms the robustness of Li isotopes as a proxy of weathering and erosion processes, even in this heavily-managed river system. [ABSTRACT FROM AUTHOR]

Large rivers integrate the products of physical erosion and chemical weathering in different forms (dissolved, fine to coarse particles). The geochemical composition of these products carry important informations on the mechanisms controlling erosion and weathering at the basin scale : sources of the products, vegetation, climate, anthropogenic and tectonic factors, timescale involed. In the framework of a large franco-chinese project, the four largest rivers in China (Changjiang, Huanghe, Zhujiang, Heilongjiang) were sampled : vertical profiles, surface tributaries, bedload sediments and surface monthly time series over a year were collected at different locations along the stream of these rivers. We report major and trace elements concentrations and Nd-Sr isotopes in the suspended particulate matter and bedloads. The four river basins cover a large diversity of climatic, lithologic, tectonic and anthropic settings, that our results put in perspective as controls on erosion-weathering mechanism and its associated timescales. [ABSTRACT FROM AUTHOR]