Negative δ13Ccarb shifts in Upper Ordovician (Hirnantian) Guanyinqiao Bed of South China linked to diagenetic carbon fluxes
Abstract The Late Ordovician Hirnantian glaciation coincided with a major perturbation of the global carbon cycle, characterized by large positive excursions of δ13Ccarb and δ13Corg known as the Hirnantian carbon isotopic excursion (HICE). Previous studies from South China have reported a positive shift in δ13Corg but ambiguous trends in carbonate carbon isotopes. To better define δ13Ccarb variation through the Ordovician-Silurian (O-S) transition in South China, we investigated multiple components (brachiopods, micritic matrix, and microspar cement) in five study sections representing shallow-shelf to deep-basin facies across the Yangtze Block. All sections exhibit negative δ13Ccarb shifts near the base of the Hirnantian stage, with small shifts in two sections (to between − 2 and − 1‰) and large shifts in three sections (mostly