Euxinia caused the Late Ordovician extinction: Evidence from pyrite morphology and pyritic sulfur isotopic composition in the Yangtze area, South China

The Late Ordovician mass extinction (LOME) was a severe biocrisis during the Phanerozoic and in this extinction, trilobites, brachiopods, mollusks, graptolites, conodonts and other types suffered severe mortality, and 85% of marine invertebrate species were eliminated. The potential cause of this mass extinction remains unclear, although glaciation and marine anoxia have received the greatest attention. Herein, we present findings on pyrite morphology, pyritic sulfur isotopic composition and total organic carbon content (TOC) to analyze the evolution of redox conditions across the Ordovician-Silurian boundary in the Yangtze area of South China. Our results show that long-term euxinic conditions interrupted by episodic dysoxic/oxic events occurred from the upper Katian stage to the lower Rhuddanian stage. The two extinction pulses of LOME were coincident with euxinic conditions, suggesting that marine euxinia may have been the main cause of this mass extinction event.