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

During the late Eocene-early Oligocene, major changes in the global climate and environment occurred, including decreases in the global temperature and the appearance of bipolar continental ice. These changes marked a move from an early Cenozoic greenhouse toward an icehouse effect. Research of this cooling event in the Atlantic and high-latitude region has increased, but few reports have studied the low-latitude region (e.g., the Northern South China Sea), and it has not been clear whether the sediment accumulation records storage signal and in response to the globally cooling event. In this study, we selected sites in the Wenchang sag, Pearl River Mouth Basin (PRMB), Northern South China Sea, and conducted analyses in cyclostratigraphy, pollen-spore assemblage, and the depositional environment. The cyclostratigraphy analysis showed that depositional cycles (including fining-upwards and coarsening-upwards sequences) were controlled mainly by astronomical forcing for long eccentricity, and the observed lithology cycles could link to the astronomical forcing for short eccentricity cycles. In addition, our results showed changes in the pollen-spore assemblage before and after the Eocene-Oligocene (E-O) shift, a gradual regressive process, and a dramatic shift from a middle-deep lake to a shallow lake and delta facies during the Wenchang to Enping formations in the Wenchang sag, PRMB. These findings suggested that the climate had deteriorated in response to global cooling. The amplification of obliquity and the coevolution between obliquity power and global pCO₂^atm during the E-O transition suggested that low-latitude sedimentary records held the signal from the high-latitude area. Furthermore, the low amplitude of the precession, obliquity, and eccentricity held the signal in accordance with the 'node' of the E-O shift where the global eustatic sea level and pCO₂^atm start declining and the carbon-oxygen isotope start a positive bias. This result may have revealed that an orbital forcing occurred during that time and probably play a key role for pCO₂^atm cross the critical threshold at the E-O shift (~33.9 Ma).