Millennial-scale climate cycles detected in the middle Cambrian GSSP (ca. 509 Ma) of the basal Wuliuan Stage

The middle Cambrian was a critical period of Earth’s history, marked by explosive diversification of metazoans and several profound changes in Earth’s surface environments and global climate. A valid temporal framework for the middle Cambrian period and across the major bio-events is yet poorly constrained, and millennial-scale climate variability (MCV) are not fully addressed. Here, high-resolution spectral gamma-ray logs, of Potassium, Uranium, Thorium and magnetic susceptibility records are utilized to conduct a cyclostratigraphic analysis of the global stratotype section for the Wuliuan Stage in South China. A ∼1.1 Myr-long high-resolution astronomical time scale across the Wuliuan-Stage 4 is developed by astronomical tuning of gamma-ray logs to the ~100-kyr short-eccentricity cycles. We report semi-precession cycles of 11.7–7.9 kyr, which were probably associated with the twice-annual passage of the intertropical convergence zone across the intertropical zone, consistent with the paleogeographical location of South China near the equator during the middle Cambrian. Furthermore, our results suggest that MCV (with periods of 4-7 kyr) is modulated by eccentricity, obliquity, and precession cycles and can be directly generated by the harmonization between fundamental orbital cycles (e.g., obliquity and precession cycles). These results describe one of the oldest known geological candidates for solar-influenced climate change modulated by Milankovitch forcing.