Orbital and Millennial‐Scale Cycles Through the Hirnantian (Late Ordovician) in Southern China

Abstract

The Hirnantian period, making the end of the Ordovician with significant mass extinctions and large ice‐sheets, is a critical interval for studying paleoclimate variations. This research represents the first cyclostratigraphic study of this period, utilizing high‐resolution (1 mm sampling rate) geochemical data from the ∼7‐m thick SH‐1 drill core, capturing the latest Ordovician glaciation. Our analysis identifies the presence of Milankovitch cycles with periods of 17–21, ∼33, and ∼100‐kyr, suggesting an optimal sedimentation rate of 3.1 m/Myr. Notably, we detected signals of millennial‐scale variability, aligning with the ∼1.5‐kyr Dansgaard‐Oeschger and ∼2.4‐kyr Hallstatt heliomagnetic cycles, even in deposits lacking visible laminae alternation. This finding bolsters the hypothesis of an external origin for these millennial‐scale features. Additionally, our reconstruction of sea‐level variations, using principal component analysis and sedimentary noise modeling, reveals two intervals of sea‐level fall and one sea‐level rise, linked to an astronomical forcing cycle of ∼1.2‐Myr. The obliquity band power‐total power ratios correspond to ∼1.2‐Myr periodicities for s4‐s3 term. The synchronization of ∼1.2‐Myr cycle minimum with a sea‐level drop (low PC1 value) suggests that glacio‐eustatic variations were influenced by ∼1.2‐Myr obliquity modulation cycles. This research enhances our understanding of glacio‐eustasy during the latest Ordovician from an astronomical perspective, offering valuable insights into the interplay between orbital cycles and Earth's paleoclimate.