ASTRONOMICALLY FORCED CYCLICITY IN THE LATE ORDOVICIAN-EARLY SILURIAN, SICHUAN BASIN, CHINA

The first of the ‘big five’ mass extinctions of the Phanerozoic Eon occurred in the Early Hirnantian stage of the Ordovician period, and is associated with climatic deterioration and polar ice-sheet growth. The causes and consequences of the dramatic global climate changes that took place from the Late Ordovician to Early Silurian remain open for debate. Here we present data from three cyclic marine sections (Shuanghe, Qijiang and Wangjiawan) and four borehole well logs from South China for the Ordovician-Silurian transition interval. An approximately 12 Myr long astrochronology was constructed from interpreted 405-kyr orbital eccentricity cycles in XRF data series measured at the outcrops, and a 14 Myr long astrochronology from the borehole well logs. Based on these astrochronologies and correlation among the sites, we estimate that the duration of the Hirnantian stage was 1.46 Myr, comparable to the 1.4 Myr duration (445.2±1.4 Ma – 443.8±1.5 Ma) assigned by GTS2012. The Wufeng Formation has a duration of 3.23 Myr; and the biozone durations of the pre-pacificus and P. pacificus zones are 0.5 Myr and 1.4 Myr, comparable to the 0.6 Myr and 1.86 Myr durations assigned by GTS2012. By contrast, the 0.46 Myr and 1.0 Myr durations of the N. extraordinarius and N. persculptus zones are significantly different from the 0.73 Myr and 0.6 Myr durations assigned by GTS2012; 1.1 Myr and 1.3 Myr durations of the A. ascensus and P. acuminatus zones likewise are significantly different from the 0.43 Myr and 0.92 Myr durations assigned by GTS2012. The duration of the Hirnantian glacial maximum event that is characterized by the global d¹³C excursion (HICE) is approximately 0.46 Myr according to our reconstructed astrochronology, which corresponds to the N. extraordinarius zone and the first major extinction event of graptolites in the early Hirnantian stage. The sediment accumulation rate calculated for the Shuanghe section shows predominant ~2 Myr variations that may reflect climate and sea-level changes, primary productivity and biotic turnover driven by long-term orbital eccentricity cycles.