关键词: 古土壤, 元素地球化学, Li同位素组成, 化学风化

Abstract: To explore the influence of plant landfall in the early Paleozoic on climate and environmental change in southwest China, we conducted elemental and lithium isotopic analyses of late Silurian-Devonian paleosols from Qujing City, Yunnan Province. Clay separation experiments were performed to eliminate the interference of unweathered primary minerals on the Li isotopic composition. The results show that the Chemical Index of Alteration (CIA) ranges from 66.8 to 82.7 with mean the degree of chemical weathering is moderate to strong. Compared with the upper Silurian and middle Devonian, the CIA average values (77.9±1.9) of the lower Devonian paleosols are higher, indicating enhanced chemical weathering. The Li contents in the clay fraction (<2 μm) range from 24.1 to 383.3×10^-6, evidently higher than in the bulk samples (7.5 to 74.5×10^-6) and the upper continental crust average (35±11×10^-6), suggesting Li enrichment in the clay minerals during the chemical weathering process. The lithium isotopic compositions of the clay fraction (δ⁷LiClay, -7.0 ‰ to +2.2 ‰ ) are lower than the UCC (~0 ‰ ), with the greatest isotopic fractionation occurring in the early Devonian (-7.0‰ ). This is primarily due to stronger chemical weathering during this period, which led to the formation of more clay minerals that preferentially adsorbed lighter ⁶Li, resulting in more negative δ⁷Li values. Meanwhile, many well-preserved plant fossils have been found in the Xujiachong Formation of the Lower Devonian. Combined with plant fossil records from the same period globally, this suggests a link between the enhanced silicate weathering may be related to the evolution of higher plants. This study provides Li isotope evidence for the hypothesis that the landing of vascular plants in the early Devonian triggered enhanced chemical weathering and reduced CO2 levels, helping to improve the understanding of the interactions between vegetation, weathering, and climate.

Key words: paleosol, element geochemistry, lithium isotope, chemical weathering