Carbon isotope stratigraphy of early Silurian carbonate rocks from the South China Block: Implications for dating post-Ordovician reef recovery

This study presents the first integrated carbon isotope stratigraphy and conodont biostratigraphy from the Rhuddanian to Aeronian (Llandovery, early Silurian) carbonate successions at the continuously exposed Kuaizishan and Yangdeng sections in Guizhou Province, South China Block (SCB). At the Kuaizishan section, a positive carbon isotope excursion (CIE) is recorded within the Xiangshuyuan Formation, along with the biostratigraphically diagnostic conodont Ozarkodina obesa. At the Yangdeng section, the Shiniulan Formation records both the falling limb of a positive CIE and a complete CIE, associated with Ozarkodina wangzhunia and Ozarkodina parahassi. Based on the biostratigraphic data, the CIE in the Xiangshuyuan Formation and falling limb in the Shiniulan Formation are interpreted as the early Aeronian CIE (Ozarkodina obesa Biozone), while the complete CIE in the Shiniulan Formation is attributed to the late Aeronian CIE (Ozarkodina guizhouensis Biozone). These results suggest that the Xiangshuyuan Formation spans from the middle Rhuddanian to earliest middle Aeronian, and the Shiniulan Formation from the early to late Aeronian. A composite carbon isotope curve from both sections provides a regional reference for correlating early Silurian carbonates within the SCB. The early Aeronian CIE shows comparable patterns across Laurentia, Baltica, and the SCB, indicating its global extent. In contrast, the late Aeronian CIE exhibits reduced expression towards the basin in both the Michigan Basin of Laurentia and the Upper Yangtze region of the SCB. Together, these two Aeronian CIEs highlight their global significance and stratigraphic utility. Furthermore, carbon isotope stratigraphy indicates that the widely distributed metazoan reefs in the Shiniulan Formation (the SCB) and the Minier Formation (Laurentia) are approximately coeval, providing evidence for a global-scale recovery of reef environments following the end-Ordovician mass extinction, which was completed by the late Aeronian Age.