Mechanism for the synchronous changes between lake level and global sea level during the Eocene warmhouse climate: Evidence from lacustrine carbonates

Under the Eocene warmhouse climate, the lake level changes in lacustrine basins in East China have been found to synchronize with global sea level changes, although the mechanism driving these synchronized changes remains unclear. Here, we focus on the lacustrine carbonates of the upper fourth member of the Shahejie Formation (Es4s) in the Zhanhua Sag of the Bohai Bay Basin to study the controlling factors of the lake level changes in East China and to analyze the major cause of their linkage to global sea level changes. Based on observations of thin sections, cathodoluminescence, scanning electron microscopy and analyses of carbon and oxygen isotopes, the vertical changes of carbonate lithofacies and geochemistry over the Es4s have been reconstructed. The Es4s of the Bohai Bay Basin consists of a complete third-order sequence, with carbonate deposition changing from gypseous micritic carbonates during the lowstand stage, to dolomicrobialites and dolograinstones during the transgressive stage, to marlstone at the maximum lake flooding surface, and to micritic limestone during the highstand stage. Upward, oxygen isotopic compositions of carbonates show a negative excursion (from −7 ‰ to −11 ‰) in the transgression system tract, while carbon isotopic compositions instead show a positive excursion (from −1 ‰ to +4 ‰). The deposition of dolostone with lowest oxygen isotopes and highest carbon isotopes during the transgressive stage is interpreted to be related to the mixing of meteoric water with saline lake water. This study suggests that astronomically driving monsoon climate change is likely to be one of the reasons for the synchronized changes in lake level and global sea-level, and that other potential factors need to be further investigated.