Lithium isotope fractionation in Weinan loess and implications for pedogenic processes and groundwater impact

The water-rock interaction has a significant impact on reconstructing climatic and environmental changes using loess deposits. Lithium isotopes (δ⁷Li) are important tracers to track this process. This study examined the Li contents and δ⁷Li values of different phases (water, weak acid leachate and residue were recorded as [Li]_water, [Li]_leachate, [Li]_residue, δ⁷Li_water, δ⁷Li_leachate and δ⁷Li_residue) within the upper 22 m Weinan loess-paleosol sediment located on the southeastern margin of the Chinese Loess Plateau. Significant variations were observed in both the [Li] and δ⁷Li values of different fractions within the loess. The water fraction samples have the lowest [Li]_water with less than 0.05 μg/g. The leachate samples show relatively low [Li]_leachate levels ranging from 0.67 μg/g to 2.45 μg/g. In comparison, the residue samples exhibit higher [Li] values than the leachate samples, with the concentrations varying from approximately 29.45 μg/g to 41.28 μg/g. The δ⁷Li_water values are all higher than those of the corresponding leachate and residue, ranging from 14.7 ‰ to 28.2 ‰. Additionally, the δ⁷Li_water values vary from −8.4 ‰ to 8.7 ‰. The δ⁷Li of residue samples are relatively stable, ranging from −0.6 ‰ to 1.7 ‰, with an average δ⁷Li_residue = 0.5 ± 1.3 ‰. The results indicate that the [Li]_leachate in Weinan Loess-paleosol sediments has excellent potential to indicate the variations of East East Asian summer monsoons during the last glacial-interglacial cycle, compared to the values of grain size, magnetic susceptibility, and CaCO₃ content. Meanwhile, the δ⁷Li_leachate of loess not only can be utilized for tracing the pedogenic processes of weathering, eluviation, and migration, as well as for tracing groundwater impact. In the upper 15 m, the δ⁷Li_leachate can be employed to track soil water migration processes and the magnitude of pedogenic processes in loess at various time periods. Below the calcium carbonate accumulation layer (CAA, >15 m), there is groundwater, and CAA has impeded the flow of groundwater, making it difficult for the influence of groundwater to penetrate the CAA layer. Due to the prolonged exposure to capillary water, the acid-soluble phase lithium represents the lithium adsorbed by loess continuously absorbed from groundwater. δ⁷Li_leachate and δ⁷Li_water had basically reached equilibrium, with values of α ranging from 0.98 to 0.99 and an average value of α at 0.98. Our work thus shows that the δ⁷Li_leachate of loess-paleosol sediments may reflect equilibrium fractionation between sediments and groundwater, rather than pedogenic processes at the layer near groundwater.