Divergent Carbon Cycle Mechanisms Across the Ordovician–silurian Transition: Evidence from the Eastern Paleo-Tethys

The Tethyan realm witnessed profound environmental volatility across the Ordovician–Silurian (O–S) transition, encompassing the Hirnantian glaciation, the Late Ordovician mass extinction (LOME), and the subsequent ecological recovery. However, the precise temporal framework and the complex mechanisms driving organic carbon burial during this interval remain debated. We present an integrated study of high-resolution cyclostratigraphy, rock magnetism, and multi-proxy geochemistry from the Tiansheng section situated in the Yangtze Sea of the Eastern Tethys. By anchoring the Hirnantian Isotope Carbon Excursion (HICE) to high-precision CA-ID-TIMS U–Pb dates, we established a ~4.77 Myr astronomical time scale, providing a robust framework for investigating environmental perturbations. Our results document a sophisticated paleoenvironmental record characterized by two divergent carbon cycle mechanisms. During the Hirnantian glaciation, eccentricity-paced eustatic fall and enhanced nutrient influx promoted massive organic carbon (TOC) burial, sustaining the global HICE event. The subsequent Rhuddanian transgression caused a gradual transition of the deep-water system into a sensitized state, characterized by a paucity of both sulfur and reactive iron. Against this unbuffered background, a transient volcanic pulse triggered a distinct carbon cycle pattern: in the photic zone, volcanic Fe-fertilization stimulated intense phytoplankton blooms and a positive δ13Corg excursion through Rayleigh fractionation; concurrently, the influx of reactive volcanic Fe3+ to the seafloor supercharged benthic dissimilatory iron reduction (DIR), which consumed a significant amount of newly deposited TOC while preserving the heavy isotopic signature inherited from the surface. These findings suggest that while orbital-paced long-term climate governs the broad-scale carbon cycle, episodic localized volcanic pulses potentially perturbed the biological pump and influenced the pace of post-extinction ecological recovery.