Elemental and Li Isotopic Investigation of a Proglacial River System: Insights to Modern Chemical Weathering Processes

Abstract Dissolved and suspended sediment samples were collected from the 121 km‐long proglacial Matanuska River and five associated tributaries in Southcentral Alaska (USA), in July 2019. Li elemental and isotopic (δ 7 Li) composition of dissolved load from proglacial river water samples and XRD analyses of the accompanying suspended sediments were measured to better understand the processes controlling Li isotopic fractionation during glacial weathering. The δ 7 Li diss of the Matanuska River system ranges from +6.1 to +18.2‰ (average = +14.6‰), which is lower than that of the associated tributary samples (average = +21.0‰). A weak negative correlation between δ 7 Li diss and Li/Na* ratios is observed, indicating that fluid residence time is not the only control for the observed δ 7 Li diss variations in this study. Equilibrium‐type fractionation controls the observed δ 7 Li diss variability of the Matanuska River system with a calculated fractionation factor ( α ) of 0.988. In contrast, Rayleigh‐type fractionation regulates the δ 7 Li diss variability in tributaries, where α values range between 0.990 and 0.996. XRD analyses of suspended sediments show the presence of secondary clay mineral phases such as chlorite, illite, smectite, and Fe‐oxyhydroxides group minerals, which further supports the estimated α values. There is no correlation between δ 7 Li diss values and water temperature, pH, ionic strength of the solution, or topography, implying that these factors have limited influence on the evolution of δ 7 Li diss along the Matanuska River transect. Overall, our results suggest that isotopic fractionation via adsorption of 6 Li onto secondary mineral phases formed during glacial weathering likely explains the observed δ 7 Li diss variability in this study.