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

Abstract

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, δ7Lileachate and δ7Liresidue) 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 δ7Lileachate, from −6.55‰ to +12.88‰. Both [Li]leachate and δ7Lileachate 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 δ7Liresidue 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 δ7Lileachate 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 δ7Lileachate in underlying loess. When loess has superimposed pedogenic and adsorption processes as the stages I and II, loess became as paleosol with highest δ7Lileachate in underlying carbonate accumulation layer. A persistent increase in δ7Lileachate 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 δ7Lileachate can be used to trace soil water migration processes and the magnitude of pedogenic processes in loess-paleosol sediment at different periods.