Lithium isotope fractionation in the mid-lower Changjiang basin: Insights from riverine geochemistry and the role of authigenic oxyhydroxides

Lithium (Li) isotopes have been extensively studied as a key proxy for silicate weathering. However, our understanding of Li isotope fractionation in lowlands and/or floodplains, particularly regarding authigenic phases, needs further refinement. In this study, we present comprehensive geochemical data, including elemental, ionic concentrations, as well as lithium and strontium (Sr) isotopic compositions, obtained from river water, suspended particulate matter (SPM), and associated oxyhydroxide fractions collected across the mid-lower Changjiang basin. The dissolved δ⁷Li values (+8.4 ‰ to +17.1 ‰) contrast with suspended particulate matter (SPM: −2.7 ‰ to −0.3 ‰) and its oxyhydroxide fraction (−15.9 ‰ to −7.0 ‰), revealing significant isotope fractionation during secondary phase formation. An inverse mixing model identifies silicate weathering (50.5 ± 4.9 %) and evaporite dissolution (35.4 ± 4.0 %) as the dominant sources of the dissolved Li, followed by urban sewage contributions of 10.4 ± 1.5 %. After correcting for non-silicate sources, dissolved δ⁷Li values conform to a steady-state batch fractionation model, reflecting the dynamic balance between primary mineral dissolution and secondary mineral (mainly clays) formation. The nearly consistent ⁸⁷Sr/⁸⁶Sr ratios of SPM oxyhydroxide fraction with those of river water strongly demonstrate their authigenic precipitation within the riverine environment. We further infer that the formation of authigenic oxyhydroxide likely played a significant role in fractionating Li isotope in the geological past, albeit secondary to clays. However, in the context of intense dam constructions, their impacts cannot be easily observed in modern river lowlands/floodplains due to reduced SPM concentrations. According to the updated global average, we hypothesize that floodplains exert little influence on elevating riverine δ⁷Li values, at least in present-day river basins surrounding the Tibetan Plateau. These findings refine our understanding of river chemistry in a highly-populated river basin, highlighting authigenic processes as a potential control on Li isotopes, warranting re-evaluation in high erosion and free-flowing river systems.