Shift in Orbital-Scale Climate Response Under the Coupling of Ice Sheet and Monsoon During the Late Oligocene: Sedimentary Records from the East China Sea Shelf Basin

The late Oligocene was a pivotal transitional phase within the unipolar Antarctic icehouse world. However, the interplay between ice-sheet dynamics and low-latitude hydroclimate in regulating continental margin sedimentary systems remains poorly constrained. Here, we construct a high-resolution astronomical time scale (ATS; ~27.4–23.1 Ma, resolution ~0.4 kyr) for the upper Huagang Formation in the Xihu Sag, East China Sea Shelf Basin (ECSSB), by tuning gamma-ray (GR) logs to the 405-kyr eccentricity cycle. Cyclostratigraphic analysis demonstrates that deltaic sandstone-mudstone alternations preserve full Milankovitch orbital periodicities, indicating co-modulation of sedimentary evolution by regional tectonic subsidence, intensification of the East Asian monsoon (EAM), and global sea-level change. A marked sedimentary regime shift occurred at ~25.5 Ma, transitioning from persistently sand-rich strata to high-frequency sand-mud alternations. This shift is temporally correlated with Antarctic ice-sheet retreat (Mid-Oligocene Glaciation [MOGI] to Late Oligocene Warming [LOW] transition) and intensified low-latitude hydrologic cycling, primarily driven by EAM intensification. Our findings reveal that orbital-scale signals are amplified under conditions of tectonic quiescence and high sediment supply. Moreover, during intermittent ice-sheet retreat in an icehouse climate, low-latitude monsoon processes can surpass ice-volume change as the principal control on continental margin sedimentation. This study not only provides a high-precision chronological framework for the late Oligocene strata in the ECSSB but also presents a classic example of an “ice volume-hydroclimate”-driven transition, enhancing our understanding of how orbital-scale forcing regulates shelfal sedimentary systems under global warming conditions.