Orbital and sub-orbital pacing of mudstones in the Dongying Depression, eastern China: Implications for middle Eocene East Asian climate evolution

Abstract

The middle Eocene was a key phase in East Asian and global climate evolution; however, the understanding of the astronomical driving mechanisms of key climate changes during this phase in eastern China is lacking. A 390-m-thick, continuous lacustrine mudstone succession in the Dongying Depression (eastern China) provides an ideal middle Eocene sedimentary record. This study established an astronomical time scale (ca. 38.97−43.55 Ma) for the Dongying mudstones by double tuning of gamma-ray logs and calcite series. Based on the tuned pollen, geochemical element and nitrogen isotope data, paleo-humidity and paleo-salinity evolution profiles were constructed, showing that Dongying mudstones experienced a phased decrease in salinity and an increase in humidity. The rapid wetting accompanied by abnormal high salinity during ca. 40.0−40.6 Ma indicates that Dongying mudstones responded to the middle Eocene Climatic Optimum (MECO) event. Multi-parameter analysis showed that the rhythmic changes of Dongying mudstones at different scales were driven by multi-scaled astronomical cycles. This study focused on the intensity changes of recorded obliquity and semi-precession signals. The signals of semi-precession weakened in the dry pre-MECO period, while those of obliquity weakened in the wetter MECO period. Considering the limitation of latitude on the influence of the two cycles on the surface sunshine, we speculate that the signals of obliquity and semi-precession may reflect the intensities of high-latitude dry airflow and low-latitude wet airflow, respectively, liking to the current East Asian monsoon (EAM) system. This study expends understanding of the role of astronomical forcing on the onset and strengthening of the EAM.