Characterization and paleoenvironmental reconstruction of n-alkanes in black shales of the second member of the Dingqinghu Formation in the Lunpola Basin, Qingzang (Tibet) Plateau

The Lunpola Basin has continuously deposited a series of lacustrine dark shales since the Cenozoic era, thus providing a valuable record of climatic and environmental changes. The continuous deposition of lacustrine black shale serves as an exemplary material for studying the characteristics of climate and environmental variations within the plateau. In this study, geological investigations were conducted on the second member of the Dingqinghu Formation in the Oligocene sedimentary profile of the Jiangriacuo area. Collected samples were analyzed using chromatography to comprehensively examine longitudinal variations in CPI (carbon preference index), ACL (average chain length), Paq (proportion of aquatic macrophytes to the total plant community), and chain length. This approach facilitated the reconstruction of plant community evolution, changes in ancient lake water levels, and regional climate variations in the central Qingzang (Tibet) Plateau. The results indicate that: (1) The primary source of sedimentary organic matter in the second member of the Dingqinghu Formation transitioned from terrestrial higher plants to aquatic submerged plants. (2) During the Oligocene period in the Jiangriacuo area of the Lunpola Basin, the climatic background changed from wet to dry. This resulted in salinity stratification in the ancient lake's water body, creating a favorable sedimentary environment for preserving organic matter and forming black shales. (3) The Paq index was used to reconstruct the succession changes of aquatic plants in the ancient lake basin of the second member of the Dingqinghu Formation in the Jiangriacuo area of the Lunpola Basin. Based on the types of plants adapted to different water depths, it was determined that the ancient lake's water level showed a downward trend. As the water level declined to depth suitable for the growth of submerged plants, their prolific growth ensued, making them the main source of organic matter in the sediment.