High-Precision Astronomical Time Scale for the Late Ordovician-Early Silurian and Its Temporal Constraints on the End-Ordovician Mass Extinction

Key words: 
• Cyclostratigraphy /
• Astronomical chronology /
• Ordovician-Silurian transition /
• Mass extinction /
• Zircon U-Pb dating

Abstract: [Objective] The Late Ordovician to Early Silurian represents a critical transition in Earth's history, characterized by the Hirnantian glaciation and the end-Ordovician mass extinction event. However, significant uncertainties persist in the chronostratigraphic framework for this interval, limiting our understanding of the mechanisms driving biotic extinction and climatic evolution. This study aims to establish a high-resolution absolute astronomical time scale for the Late Ordovician-Early Silurian through the integration of cyclostratigraphy and isotopic geochronology, providing robust temporal constraints for major geological events. [Methods] Three sections (Shuanghe in Changning, Sichuan; Anwen in Qijiang, Chongqing; and Wangjiawan in Yichang, Hubei) were selected as study sites. High-resolution elemental data (Si, Fe, Ca, Al, Rb/Sr, etc.) were analyzed using AnalySeries and Acycle software to identify Milankovitch cycle signals. Sedimentation rates were estimated using the COCO/eCOCO and TimeOpt methods to construct floating astronomical time scales. CA-ID-TIMS zircon U-Pb dating was conducted on volcanic ash beds to obtain absolute age anchors, enabling precise conversion from the depth domain to the time domain. [Results] ① Stable sedimentary cycles corresponding to 405-kyr and ~100-kyr long and short eccentricity cycles were identified in the XRF elemental series. Floating astronomical time scales of 12.2 Myr, 10.3 Myr, and 2.75 Myr were established for the Shuanghe, Anwen, and Wangjiawan sections, respectively. ② Combined with U-Pb ages of volcanic ash beds (Shuanghe: 438.47±0.17 Ma, Anwen: 439.32±0.13 Ma), the Ordovician-Silurian boundary was recalibrated to 442.34-442.65 Ma, approximately 0.42-0.73 Myr younger than the GTS2020. ③ The duration of the Hirnantian Stage at the Wangjiawan Section was estimated at ~411 kyr, with the two pulses of the end-Ordovician mass extinction lasting 379 kyr and 89.5 kyr, respectively. ④ Paleoclimatic proxies and sea-level changes exhibited long-period fluctuations of 1.2 Myr and 2.4 Myr. [Conclusions] The high-precision astronomical time scale established in this study significantly improves the accuracy of the chronostratigraphic framework for the Late Ordovician-Early Silurian, revealing the rapid onset of the end-Ordovician mass extinction. Astronomical cycles played a significant role in driving climate-environmental evolution during this interval, providing key temporal constraints for understanding the triggering mechanisms of the end-Ordovician mass extinction.