Paleoenvironmental evolution preceding the end-Permian mass extinction in the Lower Yangtze region (South China) and its controls on extreme enrichment of organic matter

The Changhsingian stage preceding the end-Permian mass extinction (EPME) was not only a significant period of global climatic-environmental change, but also an important period of widespread formation of organic-rich deposits. However, no consensus has been reached regarding the direct cause of mortality of the EPME and the main forcing mechanisms responsible for organic matter (OM) enrichment. As such, we conducted a petrological and geochemical study of the organic-rich deposits in the Talung Formation (Changqiao section), Lower Yangtze region, South China. Results show that the Lower Yangtze region was dominated by climatic fluctuations between cool-arid and warm-humid conditions during most of the Changhsingian. A paleoenvironmental reconstruction suggests that the early-middle Changhsingian was characterized by long-term anoxic to intermittent euxinic conditions, seasonal coastal upwelling, and weak basinal restriction. By contrast, the late Changhsingian was characterized by intensified euxinic conditions, weakened upwelling, and enhanced basinal restriction. The entire Changhsingian was also marked by high productivity, low detrital input, weak hydrodynamic conditions, and limited hydrothermal and volcanic activity. Quantitative analysis shows that the upwelling-driven high productivity and intensified anoxia-euxinia caused the OM enrichment, while other factors such as climate-controlled detrital inputs were less important. Two stages of climatic-oceanic dynamics were identified that reflect the relative roles and contributions of these different controlling factors. The paleoenvironmental reconstruction also indicates that euxinic conditions formed slightly earlier in deep waters, which may have been due to the combined effects of the persistent high productivity, weaker oceanic circulation, and intensified basinal restriction. Shoaling of these euxinic waters, combined with enhanced volcanism, climate warming, and ocean acidification occurred during the Permian-Triassic transition interval, could have contributed to the main stage of the EPME.