Carbon And Sulfur Isotope Variations Through The Upper Ordovician And Lower Silurian Of South China Linked To Volcanism

Pronounced excursions of carbon and sulfur isotopes (delta C-13 and delta S-34) spanning the Ordovician-Silurian (O-S) transition have been recognized globally, yet the causes of these isotope events and their causal relationships to environmental changes, especially during Early Silurian time, remain the topics of debate. In this study, we present organic carbon and pyrite sulfur isotope (delta C-13(org) and delta S-34(pyr)) records and elemental concentrations from a drill core spanning from the Upper Ordovician (Katian) to Lower Silurian (Aeronian) succession of the Yangtze Platform, China. The newly presented delta C-13(org) profile records previously recognized positive excursions, including the Hirnantian isotopic carbon excursion (HICE), the middle Rhuddanian excursion, and the early Aeronian positive carbon isotope excursion (EACIE). In addition, two negative delta C-13(org) excursions are recognized during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian stages. The delta S-34(pyr) profile exhibits a great amount of variation (-17.8 parts per thousand to +19.6 parts per thousand) but mimics the delta C-13(org) record. Coupled positive delta C-13(org) and delta S-34(pyr) excursions during the middle Hirnantian and late Rhuddanian to Aeronian are associated with decreased total organic carbon (TOC) content and less reducing water column conditions, whereas the negative delta C-13(org) and delta S-34(pyr) excursions during the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian are associated with enhanced TOC values and the establishment of more strongly anoxic to euxinic conditions. The delta C-13 and delta S-34 excursions during the O-S transition, then, cannot be explained by changes in redox conditions or the burial of organic carbon and pyrite. The heavier and less variable middle Hirnantian and late Rhuddanian to Aeronian delta S-34(pyr) values probably reflect low marine sulfate concentration. In contrast, negative Katian to early Hirnantian and late Hirnantian to middle Rhuddanian delta S-34(pyr) excursions may reflect increased sulfate concentration. The two negative delta C-13(org) and delta S-34(pyr) excursions associated with the Katian to early Hirnantian and late Hirnantian to middle Rhuddanian correspond with episodes of volcanism. We therefore propose that volcanism was the main driver of coupled negative delta C-13(org) and delta S-34(pyr) excursions. As volcanism diminished during the middle Hirnantian and late Rhuddanian to Aeronian, both delta C-13(org) and delta S-34(pyr) returned to more positive values. Our results indicate that volcanism played a crucial role in triggering carbon and sulfur isotope perturbations and environmental changes during the Late Ordovician and Early Silurian.