Impacts of volcanic ash alteration on element anomalies in fine-grained mixed deposits and paleoenvironmental reconstruction: A case study of the 2nd member of the Permian Lucaogou Formation, Tiaohu-Malang Sag, Santanghu Basin

Volcanic ash input holds significant impacts on the reconstruction of sedimentary environments in saline lacustrine basins. Specifically, when abundant trace elements from volcanic ash enter lakes and are preserved in tandem with sediment, the element contents in lacustrine strata cannot accurately reflect the original sedimentary setting. Focusing on the 2nd member of the Permian Lucaogou Formation (P₂l²) in the Tiaohu-Malang Sag within the Santanghu Basin, we investigate the mechanisms by which volcanic ash alteration affects element anomalies in fine-grained mixed deposits using major, trace, and rare earth element analyses, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons, whole-rock X-ray diffraction (XRD), total organic carbon (TOC) content determination, and thin section observation. Accordingly, the saline lacustrine sedimentary environment under the action of volcanic ash is reconstructed. The results indicate that the P₂l² consists of fine-grained tuffaceous materials and carbonates, with the total content of felsic and carbonate minerals reaching up to 95 % on average, suggesting gradational mixed sedimentation at facies margins in a broad sense. Under the influence of alteration such as early dissolution in water and mid-late devitrification and organic acid corrosion, substantial volcanic ash thermodynamically unstable released various high-abundance nutrient ions (including metal ions) into pore fluids, which significantly interfered with the elements including Ni, Co, S, P, and Ga in the fine-grained mixed rocks of the P₂l². As a result, these elements cannot accurately reflect the original sedimentary environment. Biomarker compounds, together with the analysis of major and trace element data after screening and correction, reveal a hot and arid climate during the deposition of the P₂l², featuring limited water supply to the lacustrine basin and extremely high salinity and low oxygen concentration of the lake. Volcanic activity played a significant role in regulating the sedimentary environment of the P₂l². The transition from intense to intermittent volcanic eruptions corresponded to an increasingly hot and arid climate, along with elevated water evaporation, water salinity, and oxygen concentration, leading to the deposition of various types of rock assemblages.