Astronomical forcing of lake-level changes along the low-latitude Asian continental margin and the global sea level responses during the early-middle Eocene

The early and middle Eocene was an important transition period from greenhouse to icehouse climate in the Paleogene. Exploring the response relationship between the lake and global sea levels driven by astronomical orbital cycles is of great significance for understanding the climatic characteristics and evolution of continental marginal lacustrine basins in low-latitude areas. In this study, time series analysis and sedimentary noise model are used for the cyclostratigraphic analysis of the early and middle Eocene of Wushi sag, Beibuwan Basin, South China Sea. Subsequently, the co-evolution relationship of lakes/seas in the continental margin and the enrichment model of organic matter were established. Results indicate astronomical cycle signals in the early to middle Eocene of the Beibuwan Basin. The sedimentary durations of Member 2 and 3 of the Liushagang formation (LS-2 and LS-3) are 7.9 and 8.5 Myr, respectively, with sedimentation rates ranging from 6.8 to 10.5 cm/kyr. The significant in-phase relationship between the lake and global sea level changes in the continental margin lacustrine basins suggests that the aquifer-eustasy hypothesis may not explain all the relationships between sea and lake-level changes in the greenhouse background. The 1.2 Myr long obliquity cycles influence organic matter enrichment on a million-year scale through relationships of heat and moisture supply between low and middle-high latitudes. At the minimum of the 1.2 Myr obliquity modulation, the transfer of heat and moisture from low to mid-to-high latitude areas decreases, leading to a rise in sea and lake levels in low-latitude areas, reduction in terrestrial inputs, and an increase in organic matter. This study provides evidence for the obliquity cycle control of organic matter enrichment in Asian palaeolakes under the greenhouse climate background of the early and middle Eocene at low latitudes.