Coupling of Graptolite Biozones and Paleoenvironmental Changes during the Ordovician-Silurian Transition: Evidence from the Lower Yangtze Platform

The Ordovician-Silurian transition (OST) was a critical interval in Earth’s history, marked by the Hirnantian glaciation and the second-largest mass extinction. The Lower Yangtze Platform preserves organic-rich graptolite-bearing siliceous Wufeng and Kaochiapien Formation shales that serve as key archives for understanding both biotic events and paleoenvironmental changes. Based on the biostratigraphic framework of Well Gudi-1 in the northwestern Lower Yangtze Platform, which identified WF2-WF3, LM2(?), and LM5 graptolite biozones with missing intervals (WF4-LM1, LM3-LM4), this study integrates sedimentological, mineralogical, and geochemical analyses to investigate the linkages between graptolite faunal dynamics and their depositional environments. The Wufeng-Kaochiapien shales are characterized by abundant biogenic silica derived from radiolarians and sponges, with microcrystalline quartz forming a rigid matrix that preserves both graptolite fossils and organic matter. Correlation of sedimentary features with specific graptolite biozones reveals distinct paleoenvironmental conditions recorded in these shales. The WF2-WF3 interval (pre-extinction, Upper Wufeng Formation) records high paleoproductivity and anoxic bottom waters, creating favorable conditions for organic matter accumulation and exceptional graptolite preservation. In contrast, the LM5 interval (Early Silurian recovery phase, Lower Kaochiapien Formation) shows enhanced biogenic silica flux, reflecting changes in ocean chemistry (silica cycle perturbation) that coincided with, and likely facilitated, the initial radiation of Silurian graptolite faunas. The missing biozones provide additional evidence of environmental perturbations: the absence of WF4-LM1 corresponds to the Hirnantian glacial lowstand and expansion of the Jiangnan Oldland, while the tectonic removal of LM3-LM4 reflects later structural deformation that also affected burial histories. This study demonstrates that the graptolite-bearing shales of the Lower Yangtze Platform are not merely potential hydrocarbon reservoirs but are high-fidelity recorders of major paleoenvironmental events that shaped Paleozoic life, particularly during the critical Ordovician-Silurian transition. The exceptional preservation of graptolites is intrinsically linked to oceanographic conditions—enhanced biogenic silica production during the early Silurian reflects changes in seawater chemistry that also promoted faunal recovery following the Hirnantian mass extinction. The integration of biostratigraphy with sedimentological and geochemical proxies provides a refined understanding of how environmental perturbations (glaciation, sea-level change, silica cycle shifts) controlled both the evolution and preservation of graptolite faunas during this critical transition.