Terrestrial sedimentary responses to astronomically forced climate changes during the Early Paleogene in the Bohai Bay Basin, eastern China

Abstract

High-resolution measurements of magnetic susceptibility (MS) (8029 samples) and X-ray fluorescence (XRF) elements (4367 samples) were performed on the ~403-m-thick Shahejie Formation in the Bohai Bay Basin, eastern China. The power spectral and fast Fourier transform (FFT) spectrogram analyses of the MS series reveal that the wavelength ratios of the stratigraphic cycles are ~20:5:2:1, which is consistent with the period ratios of astronomical cycles (long eccentricity, short eccentricity, obliquity, and precession). A floating-point time scale (FPTS) was developed by tuning the magnetostratigraphic susceptibility (MSS) zonation to the standard reference zonation (SRZ) established by using the same interval of semi-long eccentricity (200 kyr). The depositional durations of the lower third member (Es3l) and upper fourth member (Es4u) of the Eocene Shahejie Formation were estimated to be 2.03 ± 0.2 Myr and 1.97 ± 0.2 Myr, respectively. The sedimentary accumulation rates (SARs) during different stages were calculated based on the FPTS; the SAR of the deep to semi-deep lacustrine deposits was generally lower than 10 cm/kyr, while that of the shallow lacustrine deposits was higher than 10 cm/kyr. The significant ~1.2-Myr cycles, which were related to the planetary secular frequencies s4-s3, were coincident with the periodic climatic variation in the study area, while the ~0.84-Myr cycles may have been long-period astronomical cycles unique to the Dongying Sag. Furthermore, both of these cycles may have controlled the enrichment of the hydrocarbon source rocks in eastern China.