Rivers play a pivotal role in regulating Earth’s long-term carbon cycle by transporting the chemical products of continental weathering to the oceans. Through this process, the breakdown of silicate minerals consumes atmospheric CO₂ and contributes to the formation of marine carbonates, making weathering a key negative feedback in Earth’s climate system. However, quantifying the intensity and spatial variability of silicate weathering across landscapes remains a major challenge, in part because traditional solute ratios can be influenced by multiple overlapping processes, including lithologic heterogeneity and hydrologic pathways. Lithium isotopes (δ⁷Li) have emerged as a powerful geochemical tracer for silicate weathering and secondary clay formation, offering potential to decouple these processes. Yet despite growing application of δ⁷Li in large river systems and tropical catchments, its behavior in alpine watersheds with complex geology and variable fluid residence times remains poorly understood. This study addresses that gap by applying Li isotopes alongside major and trace element analysis across a geochemically diverse headwater system—the East River watershed, Colorado—to evaluate how spatial variability in weathering conditions influences δ⁷Li signatures and their relationship to traditional solute indicators. 

River solute samples (n = 36) were collected from the East River watershed, Colorado in July 2024. This alpine catchment, underlain predominantly by Cretaceous Mancos Shale and intrusive igneous rocks, features steep topographic gradients, variable slope aspects, and contrasting vegetation and soil development across short distances. Such geomorphic complexity can influence fluid flow paths, water residence times, and rock–water interaction intensity, all of which contribute to spatial heterogeneity in weathering processes. These characteristics make the East River watershed a valuable natural laboratory for tracing how landscape structure mediates solute generation and isotopic signals of weathering. We applied lithium isotopes (δ⁷Li), a tracer increasingly used to infer silicate weathering and secondary mineral formation, in combination with major and trace element analyses. 

We find considerable geochemical variability among the sampled rivers. Lithium concentrations across the watershed ranged from 0.8 ppb to 13 ppb. Likewise, δ⁷Li values exhibited substantial isotopic diversity, spanning from 7‰ to 21‰. The observed range of δ⁷Li and elemental concentrations suggests pronounced spatial heterogeneity in weathering intensity and fluid–rock interactions within the watershed. In contrast to prior work, we do not observe a strong correlation between Li isotopic ratios and Li to Na ratios (R² = 0.002, p = 0.8). This poor correlation potentially indicates multiple interacting processes affecting Li dynamics, including varying fluid residence times, heterogeneous lithologic contributions, and/or multiple types of secondary clay minerals. These preliminary findings underscore that δ⁷Li signatures, when evaluated alongside traditional solute ratios, provide valuable, albeit complex, insights into geochemical weathering dynamics. Future investigations will focus on identifying the various drivers of Li fractionation in the East River watershed by applying geospatial analysis and solute mass balance models.