Pedogenic processes in loess-paleosol sediments: Clues from Li isotopes of leachate in Luochuan loess

Tracing pedogenic processes is fundamental for reconstructing climatic and environmental changes using loess deposits. Lithium isotopes can serve as a sensitive tracer to unravel these processes, owing to its active behaviors without effect by redox and biological reactions. This study investigated the Li contents and δ⁷Li values of leachate and residue ([Li]_leachate, [Li]_residue, δ⁷Li_leachate and δ⁷Li_residue) of the upper 12.5 m Luochuan loess-paleosol sediment (since the last interglacial) on the Chinese Loess Plateau, in order to better understand pedogenic processes in loess. The [Li]_leachate, varying from 0.39 to 1.97 μg/g, is mainly derived from the adsorption phase, with a significant variation in δ⁷Li_leachate, from −6.55‰ to +12.88‰. Both [Li]_leachate and δ⁷Li_leachate variations reflect weathering and adsorption processes during different periods. The [Li]_residue vary from 34.4 to 46.3 μg/g (averaging 38.4 μg/g) and their δ⁷Li_residue vary from −3.66‰ to +2.44‰ (averaging −0.22 ± 1.68‰), both in agreement with the upper continental crust values. At initial stage after loess deposited (stage I), high [Li]_leachate but low δ⁷Li_leachate are results of preferential adsorption of ⁶Li by clays and migration of ⁷Li with soil solution during weathering and eluviation processes. When overlying loess was deposited and experienced pedogenesis as the stage I, ⁷Li migrated downward and was adsorbed by clays (stage II), resulting in high [Li]_leachate and δ⁷Li_leachate in underlying loess. When loess has superimposed pedogenic and adsorption processes as the stages I and II, loess became as paleosol with highest δ⁷Li_leachate in underlying carbonate accumulation layer. A persistent increase in δ⁷Li_leachate indicates dominant control of post-depositional adsorptions on Li fractionation along solution migrations during weathering and eluviation of overlying deposits, and vice versa. These results highlight that variations in [Li]_leachate and δ⁷Li_leachate can be used to trace soil water migration processes and the magnitude of pedogenic processes in loess-paleosol sediment at different periods.