High-resolution sedimentology, ichnology, and benthic marine redox conditions from Late Permian to the earliest Triassic at Shangsi, South China: Local, regional, and global signals and driving mechanisms

Anoxia has been invoked as the most plausible killing mechanism for the end-Permian mass extinction (EPME) in the marine realm. Understanding the marine redox conditions before and during this event is critical for evaluating the anoxia hypothesis. High-resolution trace fossil data (ichnofabric index (ii), maximum burrow diameter (MBD), and ichnocoenosis) were used and analyzed to reconstruct benthic marine redox conditions from the Lopingian (Late Permian) to the earliest Triassic in the Shangsi section, Sichuan, South China. The results were correlated with other redox-sensitive proxies, including local (trace fossil data, pyrite framboid size distribution, iron speciation, and trace elements) and global proxies (sulfur isotopes of carbonate-associated sulfate (delta S-34(CAS)) and evaporite (delta S-34(evaporite)) and uranium isotopes of marine carbonate (delta U-2(38)carb)), and with other sections (Xiaojiaba and Ganxi sections in South China), to evaluate the spatial impacts of benthic oxygen-level variations at Shangsi. Detailed sedimentological studies have also been conducted to investigate depositional processes, sedimentation rates, sedimentary environments, and relative sea-level changes throughout the studied section in an attempt to uncover the factors driving benthic oxygen level variations in the Shangsi section. The results are as follows: (1) High-frequency (centimeter- to decimeter-scale) and secular (meter- to tens of meters-scale) benthic oxygen-level variations were developed at Shangsi; the high-frequency variations were mostly local signals generated by transient episodic mixing and ventilation by storms and turbidity currents, and the secular variations were regional or global signals caused by regional factors as well as global climate changes. (2) Anoxic/euxinic benthic marine conditions prevailed during the middle-late Wuchiapingian at Shangsi, which persisted for 3.9 Myr (256.974-253.073 Ma). This long-term Wuchiapingian anoxia event can be correlated both regionally and globally, and was caused by paleogeographic restrictions, relative high sea levels, high marine productivities, and possible global climate changes. (3) A long-term global oxygenation event occurred during the Changhsingian, which initiated at similar to 253.513 Ma, followed by a major oxygenation pulse at 253.073 Ma, and further intensified at 252.739 Ma at Shangsi. This Changhsingian global oxygenation event was probably triggered by an early Changhsingian global cooling event, during which the average surface ocean temperature declined similar to 6 degrees C, and the cold climate persisted during most of the Changhsingian.