PALAEO-MARINE ENVIRONMENT ASSOCIATED WITH THE DISTINCT ECOSYSTEMS FOLLOWING THE END-ORDOVICIAN CRISIS IN SOUTH CHINA

The Upper Ordovician strata are considered to have recorded the first major mass extinction (85% loss in species) in Phanerzoic, episodes of high-latitude glaciation and significant oceanic turnovers. Previous studies have reported distinct oscillations in C and S cycling yet without precise calibration of oceanic redox evolution, due to the limited thickness. Here we investigated both near-shore (especially with thick Hirnantian deposits) and off-shore successions in South China, with C-S-Fe systematics and Trace-element analysis, to reconstruct secular redox variations through this interval.

Totally 50 samples collected from two sections in Upper and Lower Yangtze Platform were analyzed for Fe speciation, pyrite S isotopes and trace metal concentrations. Herein high TOC contents and elevated Fe speciation ratios indicate dominantly euxinic bottom-water conditions during the pre-glaciation and early-glaciation interval. Pyrite S isotopes also record strong microbial sulfate reduction activities, accompanied by enhanced burial of organic matter. During the late-glaciation interval, continental shelf encountered large-scale sea regression and further ocean ventilation, inducing less reducing benthic conditions. But episodic bottom water euxinia in those near-shore environments still persisted into the post-glaciation interval due to local high productivity, as evidenced by Fe and S proxies. Then oceanic anoxia recurred immediately with global sea-level rise and shift to greenhouse climate after the glaciation. Generally, shoaling of oxygen-deficiency zone (with free H₂S and abundant heavy metals), to uncertain extent, hinders the living and multiplying of benthic community. However, flourishing sponge-based biota (Anji Biota) appeared abruptly in near-shore environments in the aftermath of the biotic mass extinction, when widespread oceanic anoxia was supposed to have occupied the continental shelf. Under such scenario, the benthic community rebooted remarkably the ecosystems, featured by more than 75 species sponges across tens km². Hence this study is designed to integrate those geochemical evidences and fossil occurrences, to aim at a better understanding of potential decoupling between oceanic oxygenation and collapse/recovery of marine ecosystems during the end-Ordovician crisis.