HOW DO RIVER WATER LI ISOTOPE RATIOS RELATE TO CARBON TRANSFER DURING WEATHERING?

River water ⁷Li/⁶Li ratios (δ⁷Li) have been shown to trace the dissolution and precipitation of silicate minerals at the catchment scale and underlie some interpretations of carbon (C) cycle dynamics in both modern surface environments and across geologic time. However, less attention has been given to the relationship between river δ⁷Li values and other processes that transfer C between Earth’s surface and interior, such as sulfide oxidation or carbonate dissolution, and thus the conclusions that can be drawn from δ⁷Li values about the impact of weathering on the global C cycle. To address these knowledge gaps, we amass a global dataset of river δ⁷Li values and major/trace ion concentrations (n = 412), invert their dissolved chemistry for solute sources and sinks using the MEANDIR model, and compare the results with catchment geomorphic and climatic properties. These inversions quantify the proportion of carbonate weathering (R) and sulfuric acid weathering (Z), where their balance determines whether weathering acts as a net CO₂ source or sink to the atmosphere. The analyses reveal that lithology and atmospheric temperature impose the largest controls on C transfer. Broadly, we observe that sedimentary-rock dominated catchments often correlate with high Z values and high river Li/Na ratios (i.e., dissolution-dominated) regardless of their geomorphic or climatic properties. Catchments with mean annual temperatures (MAT) greater than 18°C, in contrast, consistently have low Z values and low river Li/Na ratios (i.e., precipitation-dominated) irrespective of exposed lithology. These relationships hold whether clays were included in the inversion. However, inversions with clays as solute sinks illustrate their outstanding influence on R, where the inclusion of clays almost always decreases R and the magnitude of these decreases is driven by clay chemistry. The cumulative effect of this decrease drives catchments toward being net C sinks, counteracting the influence of carbonate weathering coupled with sulfide oxidation. These findings illustrate the role of clay formation, as forced by MAT and lithology, as a dominant driver of net C transfer. However, the lack of trends between river δ⁷Li values and either R or Z underscore the limited ability of Li isotopes alone to characterize C transfer.