Late Ordovician Milankovitch Records In South China And Their Implications For Solar System Behavior

The Late Ordovician is a fascinating period during which a series of profound geological events occurred,including the Late Ordovician mass extinction,the Hirnantian glaciation and the Hirnantian isotope carbon excursion?HICE?.High-precision and high-resolution time scale is crucial for exploring the genetic mechanisms of these major geological events and their internal interconnections.Cyclostratigraphy provides an opportunity to improve the geological time scale.Based on reliable Milankovitch signals identified from the geological records,Cyclostratigraphy can be used to establishe an astronomical time scale?ATS?with a resolution of 0.02–0.4 Myr.In present international geological time scale,astronomical calibration of the Cenozoic era has been completed,and most of Mesozoic era have been calibrated to the stable 405-kyr orbital eccentricity cycles.Cyclostratigraphy of the Paleozoic Era began late due to the lack of of radioactive absolute dates,biostratigraphy,magneteostratigraphy and other temporal constraints.However,with the improvement of the technique of cyclostratigraphic analysis,especially the emergence of statistically based astronomical testing techniques such as ASM,TimeOpt and COCO,this situation is gradually improving,so that Paleozoic astrochronology has been expected to be the next frontier in cyclostratigraphy.In addition to establishing astronomical time scales,cyclostratigraphy is also recently used to reconstruct the Solar System chaotic behavior and Earth–Moon evolution parameters.Here,we present a detailed cyclostratigraphic study on the upper Pagoda Formation through the lowermost Lungmachi Formation at the Wanhe section from Yongshan County,Yunnan Province,which is a 48.49-m thick succession and spans the early Katian–late Hirnantian.A total of 2677?1–2 cm intervals?discrete samples were collected from the outcrop,and high-resolution anhysteretic remanent magnetization?ARM?and magnetic susceptibility?MS?series were obtained through magnetic measurements on all the samples.Cyclostratigraphic analysis was conducted on the ARM series,which exhibited excellent Milankovitch signals.Then,405-kyr and 33.83-kyr calibrations were performed in turn,generating a⁷.37-Myr"floating"ATS.Based on this ATS,the duration of the Hirnantian Stage of the Wanhe section was estimated to be¹226 kyr;the durations of the first and second episodes of the Late Ordovician mass extinction were⁴40 kyr and⁵40 kyr;the duration of the Hirnantian glaciation was⁸30 kyr;the duration of the HICE was⁶90 kyr.The ratios of obliquity and short-eccentricity band powers to total power reveal periodicities of¹.32 Myr for the s₄–s₃ term and 173.6 kyr for the s₃–s₆ term,the periodicity of the g₄–g₃ term decreases from 2.44 Myr to 1.41 Myr and 1.37 Myr,suggesting a disruption of the 2:1 resonance state during the late Katian,which may be related to the Solar System chaotic behavior.According to the main obliquity period of 33.83 kyr,the precession constant k of Late Ordovician?⁴45 Ma?is calculated to be 57.159193 arcsec/yr,which indicates a 22.3809-hour length of day and an Earth-Moon distance of 375,376 km.Aided by the highly fluctuant MS,we recognized a series of¹.32-Myr obliquity-forced glacio-eustasy cycles in the late Katian strata,climate cooling and sea-level falls correspond to the maximum of the¹.32-Myr cycles.Interestingly,the coupling of the long-period obliquity maximum and the long-period eccentricity node may be the inducing factor for the late Katian–Hirnantian glaciations.