The Geochemical Characteristics and Environmental Implications of the Paleocene–Eocene in the Jiangling Depression, Southwestern Jianghan Basin

Abstract

Various isotopic and palynological indicators have shown interspersed periods of aridity and humidity for the late Paleocene to early Eocene in central China, so the paleoclimate conditions remain unclear. This research investigates the environmental characteristics of a saline lake in the Jiangling depression, southwestern Jianghan Basin, from the Paleocene to the Eocene, using bulk-rock geochemistry in a 1280 m sediment core. The ratios of FeO/MnO, Al2O3/MgO, and C-value indicate a semi-humid to semi-arid climate in the early–middle Paleocene. There was a rapid shift to a humid climate during the late Paleocene to early Eocene, following a short time of intense dryness. The Eocene climate was arid, but experienced intermittent humidity. The variation trend of the CIA, CIW and PIA was similar to that of FeO/MnO, Al2O3/MgO, and the C-value, so chemical weathering of the surrounding rocks was controlled by climate change. The lake redox conditions in the Jiangling depression from the Paleocene to the Eocene were reconstructed using the ratios of U/Th, Ni/Co, and V/Cr. During humidity and alternations of aridity and humidity, the lake water received external water input, resulting in weak stratification, so the sediments were in oxidizing conditions. During aridity, lakes become endorheic, leading to sediments forming in reduced conditions. The salinity of the lake in the Jiangling depression from the Paleocene to the Eocene was determined through analysis of sedimentary sequences and the trend of the Sr/Ba ratio. In the early–middle Paleocene, lake salinity varied greatly. From the late Paleocene to the early Eocene, lake salinity decreased. In the Eocene, lake salinity increased and halite precipitated, but lake salinity finally decreased due to a humid climate. During the late Paleocene–early Eocene, the occurrence of multiple humid climates in the Jiangling depression were not merely regional effects. The most significant humidity was caused by a global hyperthermal (PETM), which caused a huge increase in precipitation in the whole of East Asia and even in low latitudes around the world.