Data and replication code for "Lithology Governs Dissolved Lithium Isotope Ratios in Mixed-Lithology Alpine Headwater Watersheds"

Dissolved lithium isotope ratios (δ⁷Li) trace the balance between primary silicate dissolution and secondary clay formation, but mixed-lithology catchments confound this signal by superimposing source mixing on process-driven fractionation, making it difficult to isolate individual weathering drivers. Here we present δ⁷Li and major-solute data from 54 stream sites in the East River watershed (Colorado, USA), a headwater system spanning shale, sandstone, and granodioritic lithologies. Dissolved δ⁷Li spans 9.5 to 20.8‰ and shows negligible covariation with Li/Na* across the full dataset (R² = 0.002), but a strong relationship emerges in shale-dominated catchments (R² = 0.877, p = 0.0019, n = 7), demonstrating that lithologic homogeneity is a prerequisite for using conservative-normalized ratios as weathering proxies. PHREEQC speciation reveals concurrent feldspar undersaturation and clay supersaturation across all lithologic classes, consistent with ongoing incongruent weathering. Batch fractionation modeling indicates that 55–95% of released Li is sequestered into secondary phases before stream export, with an effective fractionation factor (α = 0.9835; Δ = −16.7‰) consistent with smectite and kaolinite; this factor varies systematically by lithologic class, reflecting lithologic control on secondary mineral assemblages. Beyond mineralogical controls, topographic wetness index (TWI) correlates positively with δ⁷Li (R² = 0.710; p < 0.0001), consistent with longer water–rock contact time in more convergent landscape positions promoting greater secondary mineral formation. We interpret this as a two-tier control structure: lithology sets the mineralogical potential for secondary mineral formation and shapes landscape form, while topographic wetness modulates the hydrologic opportunity for water–rock contact. Neither control alone is sufficient to explain δ⁷Li variability in mixed-lithology headwater systems. This record contains the supplementary data table (Supplementary_Table.xlsx), R analysis code (East_River_dLi7_analysis.R), and PHREEQC input file (DOE2425_SI.pqi) used to produce the results reported in the manuscript.