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 <−10‰, minimum −13.7‰). Brachiopods commonly exhibit the same δ13C compositions as micrite in a given sample, although some samples contain isotopically heavy brachiopods in a highly 13C-depleted matrix. Because the highly 13C-depleted carbonates are present in two widely separated areas between which only weakly 13C-depleted carbonates are found, there must have been multiple local diagenetic sources of 13C-depleted carbon. We infer that the highly negative δ13Ccarb values were linked to compactional influx of 13C-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 δ13Ccarb signals and to avoid potential errors in interpreting diagenetically overprinted records as primary marine carbonate events.