Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China

Abstract

A high-precision time framework is critical for improving our understanding of three major events: the Late Ordovician mass extinction, the Hirnantian glaciation, and the Hirnantian isotopic carbon excursion (HICE). In the present study, we conducted detailed cyclostratigraphic analysis of a high-resolution (1–2-cm intervals) anhysteretic remanent magnetization series obtained from the Wanhe section in South China, covering the lower Katian through Hirnantian. Power spectra and derived average spectral misfit results show that the section records the ensemble of Milankovitch cycles with 405-kyr (long-eccentricity), 93–125-kyr (short-eccentricity), 33.8-kyr (main obliquity), and 17–22-kyr (precession) periods. The obliquity period of 33.8 ± 0.46 kyr indicates a precession constant k of 57.19 ± 0.53 arcsec/yr, a length of day of 22.37 ± 0.12 h, and an Earth–Moon distance of 375,330 ± 722 km (vs. 384,000 km today) during the Late Ordovician (445 Ma). The ratios of obliquity band power to total power reveal periodicities of 1.32 Myr for the s4–s3 term and 173.4 kyr for the s3–s6 term. Aided by the highly fluctuating magnetic susceptibility (MS), we recognized a series of 1.32-Myr obliquity-forced glacio-eustatic cycles with three intervals of sea-level rise and climate warming trend characterized by high MS values and two episodes of sea-level fall and climate cooling trend before the Hirnantian. The 405-kyr and subsequent 33.8-kyr calibrations produce an ~7.36-Myr floating astronomical time scale (FATS) for the studied succession, indicating that the estimated durations for the Hirnantian Stage, the worldwide Hirnantian glaciation, and the first and second phases of the Late Ordovician mass extinction were ~1225 kyr, ~830 kyr, ~440 kyr, and ~540 kyr, respectively. This work provides a reliable high-resolution time framework for further investigation of the major geological events occurred in the Late Ordovician, and improves our knowledge of Solar System behaviour and Earth–Moon history during that period.