High-Resolution Cyclostratigraphy Uncovers Variable Orbital Forcing in the Oligocene South China Sea

The Oligocene provides an important interval for examining climatic and sedimentary variability following the Eocene–Oligocene transition, but the expression of orbital forcing in low-latitude marginal seas remains incompletely understood. We integrate natural gamma-ray (GR) logging and microfossil records from Well L9 in the Pearl River Mouth Basin, northern South China Sea, to establish a cyclostratigraphic framework spanning approximately 34.6–21.7 Ma. A floating chronology based on 405 kyr cycle counting was anchored using an independently constrained Oligocene–Miocene boundary horizon. Spectral analyses identify approximately 405 kyr, 100 kyr, and 40 kyr components, together with longer-period variability near 2.4 Myr and 1.2 Myr. Amplitude-modulation analysis reveals an approximately 1.2 Myr component in the 40 kyr envelope, supporting an obliquity-related interpretation. In contrast, the 100 kyr envelope contains approximately 1.6 Myr variability rather than a clear 2.4 Myr component, leaving the eccentricity-modulation origin of the latter unresolved. Palynological evidence indicates generally cool and humid conditions during the early–middle Oligocene, followed by late Oligocene warming. Comparison with published climate records suggests correspondence with six recognized cooling episodes, although chronological uncertainty limits precise event alignment. These results provide a regional chronological and paleoenvironmental framework and highlight the need to distinguish observed long-period sedimentary variability from its proposed orbital origin.