Lithological influence on Li isotope fractionation during silicate weathering

The influences of bedrock lithological composition on weathering fluxes, including dissolved lithium isotopes (δ⁷Li_diss), in rivers remain unclear. To assess this, we present new elemental, mineralogical, and lithium isotopic data from river localities in Hong Kong that drain either purely silicic volcanic or siliciclastic sedimentary bedrock that were sampled during both the wet (summer) and dry (winter) seasons. Our data show marked geochemical and mineralogical differences in relation to bedrock composition. The silicic volcanic-draining rivers exhibit δ⁷Li_diss values ranging from 8.6‰ to 20.9‰, elevated dissolved alkali metal concentrations, and relatively higher kaolinite abundances, whereas siliciclastic sedimentary-draining rivers yield δ⁷Li_diss values ranging from 3.2‰ to 8.6‰, higher dissolved alkaline-earth metal concentrations, and relatively greater illite abundances. Collectively, the volcanic-draining rivers have higher average δ⁷Li_diss of ∼12.5‰ and higher bedload kaolinite/illite average ratios of 2.4, compared to the siliciclastic sedimentary-draining bedrock rivers that yield average δ⁷Li_diss of 6.0‰ and kaolinite/illite of 0.5. Mechanistically, this could have been driven by the lower abundance of reactive minerals in the siliciclastic bedrock and/or a higher degree of fracturing in the volcanic rocks, which can both increase water–rock interaction times and secondary clay formation, driving higher δ⁷Li_diss values. Seasonal variations were also observed in the Li data, with higher Li concentrations and dissolved δ⁷Li values (up to 8.0‰ heavier) measured during the dry winter season. This is attributed to lower river discharges in the dry season, enabling longer residence times for clay formation, and higher riverine δ⁷Li values. Higher major, alkali, and alkaline-earth concentrations were also measured during the dry season, with Group 3, transition metals, and rare earth elements more concentrated in the wet season, reflecting the impact of elemental mobility. Overall, our study highlights the influence of both bedrock lithology and regional weather patterns on riverine geochemical signatures within the same tectono-climatic setting. These findings help enhance understanding of the Li cycle, wherein riverine δ⁷Li fluxes to the ocean reflect a combination of lithology, hydrology, and weathering regime, further refining the utility of lithium isotopes for assessing modern and ancient silicate weathering processes on Earth’s surface.