Decoupled Responses of Lacustrine Water Level and Eustatic Sea Level to Orbital Forcing during the Early Eocene Climatic Optimum

Orbital forcing is a fundamental mechanism governing Earth’s climate system. However, under greenhouse climate conditions, its expression in low-latitude lacustrine depositional systems and its relationship to global eustatic sea-level change remain poorly constrained. This study focuses on the second member of the Liushagang Formation in the Beibu Gulf Basin, northern South China Sea, and investigates orbital-scale hydroclimate variability and lake-level response mechanisms during the Early Eocene Climatic Optimum (EECO) based on well-log and geochemical data. Spectral analysis of gamma-ray (GR) logs from Well W1 and Well W2 in the Weixinan Depression reveals distinct astronomical cyclicity. In Well W1, the identified period ratios (~19.5:5.5:2.0:1.2:1) closely match the expected Milankovitch hierarchy after normalization. The estimated sedimentation rates for the two wells are ~10.3 cm/kyr and ~7.3 cm/kyr, respectively, consistent with previous studies. Astronomical signal fitting with AstroGeoFit further supports the robustness of these orbital interpretations within the studied interval, despite the influence of stratigraphic incompleteness. A lake-level index was constructed from GR logs and XRF-derived geochemical proxies. The results indicate that lake-level variations exhibit pronounced quasi-periodic oscillations, primarily paced by precession (~20 kyr) and modulated by eccentricity. These fluctuations reflect alternating humid and arid hydroclimatic conditions, with precipitation exerting the dominant control on lacustrine water balance. Comparison with coeval global sea-level records shows that lake-level variations are not consistently in phase with eustatic sea-level changes, but instead exhibit phase offsets and nonlinear behavior at orbital timescales. This suggests that the hydrology of low-latitude lacustrine basins was governed primarily by insolation-driven regional precipitation rather than by global ice-volume-controlled eustatic sea-level change. This study provides important cyclostratigraphic evidence from a low-latitude lacustrine system and demonstrates that, under EECO greenhouse conditions, lacustrine basins can independently record orbitally driven hydroclimate variability, with important implications for understanding the global hydrological cycle under greenhouse climates.