Negative δ13Ccarb shifts in Upper Ordovician (Hirnantian) Guanyinqiao Bed of South China linked to diagenetic carbon fluxes

The Late Ordovician Hirnantian glaciation coincided with a major perturbation of the global carbon cycle, characterized by large positive excursions of delta C-13(carb) and delta C-13(org) known as the Hirnantian carbon isotopic excursion (HICE). Previous studies from South China have reported a positive shift in delta C-13(org) but ambiguous trends in carbonate carbon isotopes. To better define delta C-13(carb) 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 delta C-13(carb) shifts near the base of the Hirnantian stage, with small shifts in two sections (to between - 2 and - 1 parts per thousand) and large shifts in three sections (mostly < - 10%o, minimum - 13.7 parts per thousand). Brachiopods commonly exhibit the same delta C-13 compositions as micrite in a given sample, although some samples contain isotopically heavy brachiopods in a highly C-13-depleted matrix. Because the highly C-13-depleted carbonates are present in two widely separated areas between which only weakly C-13-depleted carbonates are found, there must have been multiple local diagenetic sources of C-13-depleted carbon. We infer that the highly negative delta C-13(carb) values were linked to compactional influx of C-13-depleted dissolved inorganic carbon (DIC) into the Guanyinqiao Bed as a result of oxidation of organic matter through microbial sulfate reduction (MSR) or methanogenesis/anaerobic oxidation of methane (AOM) in black shales of the underlying Wufeng and overlying Longmaxi Formations. The results of the present study demonstrate the importance of careful diagenetic screening of C-isotope records in order to fully understand the origin of delta C-13(carb), signals and to avoid potential errors in interpreting diagenetically overprinted records as primary marine carbonate events.