Astronomical Calibration of the Hirnantian Isotopic Carbon Excursion

The Hirnantian Isotopic Carbon Excursion (HICE) represents one of the largest-magnitude positive carbon isotope excursions in the Phanerozoic Eon. However, uncertainties regarding the timing and duration of the HICE hinders the understanding of the interaction between carbon cycle and paleoclimatic change as well as its impact on the concurrent Late Ordovician Mass Extinction (LOME). The thickness of Hirnantian Stage in the Hanggai section of the South China block is ~352.7 m, significantly greater than the < 1-m thickness observed at Wangjiawan section (Global Standard Boundary Stratotype Section and Point, GSSP). Here, we present an astrochronologically calibrated organic carbon isotopic (δ¹³Corg) data series constructed through cyclostratigraphic and chemostratigraphic analyses of the Hanggai section. Our findings reveal that the HICE encompasses two pluses of positive δ¹³Corg excursions, with a cumulative duration of ~362–480 kyr. This result is further corroborated by an analogous analysis performed on a contemporaneously deposited shallow-marine succession at the Wanhe of South China block. Our results suggest that the HICE event was synchronous with the LOME, yielding the extinction of ~52% of marine species at an exceptionally high rate of ~12 %/100 kyr. Besides, the sedimentary noise modeling analysis supports positive δ¹³C excursions parallel to glacio-eustatic sea-level regression, indicating a potential cause-and-effect relationship. Our study elucidates a detailed process of the coupling among the glaciation dynamics, climate change and carbon cycle perturbation, as well as their possible linkage to the mass extinction during the Hirnantian.