1060 / 2024-09-20 10:15:14

Orbitally-forced late Ordovician climate change: a record from the South China block

Cyclostratigraphy,Floating astronomical time scale,Hirnantian,HICE,South China

The Hirnantian stage (Late Ordovician) documents the second largest extinction event in the Phanerozoic Era. This catastrophic extinction was likely linked to climate cooling and accompanying environmental change. Previous study indicates that this extinction event occurred with the Hirnantian isotopic carbon excursion (HICE), a positive δ¹³C_org excursion associated with a significant rise in organic carbon burial. Previous researchers have identified the HICE event in multiple outcrops in southern China, but these sections are all condensed sections with a sediment thickness of less than one meter. This may lead to imprecise or incomplete geological information recorded in these sections. Meanwhile, there is still debate among scholars about the duration of the Hirnantian Stage, with estimates ranging from ~0.8-~2Ma. A high-precision temporal framework is essential for improving our understanding of the forcing mechanism of the extinction event. Cyclostratigraphy has been called the "third milestone of geological time interpretation" because it uses fossil evidence to establish the absolute ages and durations of various geological events and processes with precision ranging from 20ky to 400ky years. Hanggai section of Anji, South China Block subsided rapidly and has a thickness of 352.7 m, which is much thicker than many sections in South China. It can record more detailed and accurate geological events and is an ideal section for conducting Hirnantian cyclostratigraphy and carbon isotope geochemistry research. In our study, two significant positive δ¹³C_org excursion in this section (~ +4‰) were identified. The first positive δ¹³C_org excursions begins just below the base of the Hirnantian Stage and peaks in the lower part of the Normalograptus extraordinarius biozone, followed by the second peak in the middle and lower part of the Normalograptus persculptus biozone. This result can be well matched with the two biological extinction events of the Hirnantian stage and the Late Ordovician glacial event, and we can identify HICE from changes in organic carbon isotopes. After the 405-ky long eccentricity calibration, the power spectra of what results show that the section records the ensemble of Milankovitch cycles with 405-kyr (long eccentricity), 100~115-kyr (short eccentricity), 28.4~31.3-kyr (main obliquity), and 17.2~22.2-kyr (precession) periods. An integrated floating astronomical time scale (ATS) was established using the extracted stable 405-kyr eccentricity cycles and it suggests that the Hirnantian stages was 1.86 Myr. This work provides a reliable high-resolution time framework for these events, which can constrain their duration and enhance our understanding of climate change during the late Ordovician Hirnantian.