Redox conditions across the G–L boundary in South China: Evidence from pyrite morphology and sulfur isotopic compositions

Abstract

The end-Guadalupian (middle Permian) mass extinction recorded the disappearance of shelf faunas such as fusulinids, small foraminifers, brachiopods, rugose corals which flourished in the warm shallow marine habitats after the late Pennsylvanian–early Permian glaciation. The causes for this mass extinction are still unclear. Marine anoxia, often as a cause for other mass extinctions, has been paid less attention in this extinction. Here we present a comprehensive study including pyrite morphology and sulfur isotopic compositions of pyrite across the Guadalupian–Lopingian (middle–upper Permian; G–L) boundary at Tianfengping in northern Yangtze Platform and at Penglaitan in southern Yangtze Platform in South China. Our results show that the sulfur isotope ratios of pyrite mainly track the framboid size distributions in these two sections. Anoxia and intermittent euxinia occur at the G–L boundary evidenced by the coincidence between reduction of framboid size and the extremely negative sulfur isotope values of pyrite involving disproportionation, and corresponds to the main mass extinction of the end-Guadalupian, suggesting a causal link between them. Long-term dysoxic conditions during the middle Capitanian are evidenced by the relative small framboid size and lower sulfur isotope ratios of pyrite, and coincide with the first pulse of the end-Guadalupian mass extinction, also suggesting a causal link between them. The oxygen-depletion conditions in the Capitanian during the global sea level falling may be the main cause for the end-Guadalupian mass extinction.