A Well-Constrained Middle Darriwilian Isotopic Carbon Excursion (MDICE) in South China and Its Global Implications

The Middle Darriwilian Isotopic Carbon Excursion (MDICE) is a globally recognized Ordovician carbon isotope event, but its records in the South China Block (SCB) are commonly incomplete due to regional stratigraphic hiatus. This study presents an integrated analysis of conodont biostratigraphy, carbon isotope stratigraphy, and microfacies analysis from a stratigraphically continuous succession spanning the Kuniutan to lower Pagoda formations (Darriwilian to early Katian) at the Mayang section, Upper Yangtze Platform, SCB. Microfacies data indicate a shallowing event during the deposition of the uppermost Kuniutan to lowermost Datianba formations (upper Pygodus serra Biozone, late Darriwilian), yet the absence of subaerial exposure features supports continuous sedimentation. Carbon isotope data reveal a positive excursion of ~1‰ within the Kuniutan Formation, characterized by a sustained high-value interval from the Eoplacognathus suecicus to Pygodus serra biozones and a shortened falling limb, interpreted as an apparent complete record of the MDICE. A second, larger positive excursion (2‰–3‰) near the top of the section, initiating in the upper Baltoniodus alobatus Biozone and peaking in the lower Hamarodus brevirameus Biozone, is correlated with the early Katian Guttenberg Isotopic Carbon Excursion (GICE) or its regional equivalent. Global comparison confirms the MDICE as a broadly correlative perturbation of the Ordovician carbon cycle, while pronounced regional differences in its expression highlight the importance of depositional and palaeogeographic controls.