Astronomically forced climate cooling across the Eocene–Oligocene transition in the Pearl River Mouth Basin, northern South China Sea

Abstract

During the late Eocene–early Oligocene, major changes in the global climate and environment occurred, including decreases in global temperature and the appearance of Antarctic glaciation. These changes marked the transition from early Cenozoic greenhouse conditions to icehouse conditions. Research on this cooling trend in the Atlantic and high-latitude regions has increased, but few reports have studied low-latitude regions (e.g., the northern South China Sea), and it remains unclear whether sediment records in these areas preserve signals related to this global cooling event. In this study, we studied records from a lacustrine well in the Wenchang Sag, Pearl River Mouth Basin (PRMB), northern South China Sea. We conducted palynological and cyclostratigraphic analyses to reconstruct the depositional environment and assess the influence of orbital forcing. The cyclostratigraphic analysis showed that depositional cycles (including fining-upwards and coarsening-upwards sequences) were controlled mainly by long eccentricity, forcing the expansion and contraction of the lake system at ~1.2 Myr time intervals. In addition, changes in the pollen-spore assemblage before and after the Eocene–Oligocene (E-O) transition show a dramatic shift from a moderately deep lake to a shallow lake with delta facies, as recorded by the Wenchang to Enping Formations in the Wenchang Sag, PRMB. These findings suggest that the climate deteriorated in response to global cooling. The amplification of obliquity forcing and the coevolution between obliquity power and global pCO2 during the E-O transition suggest that the low-latitude sedimentary records from China contain a high-latitude signal. Furthermore, the low amplitudes of the precession, obliquity, and eccentricity cycles are coupled with the initial decreases in the global sea-level and pCO2 and a positive bias in the carbon‑oxygen isotopic compositions at ~34.3 Ma. These results suggest that orbital forcing played a role in elevating pCO2 across a critical threshold during the E-O transition (~33.9 Ma).