Oceanic anoxia through the late Permian Changhsingian Stage in the Lower Yangtze region, South China: Evidence from sulfur isotopes and trace elements

Abstract

The late Permian oceanic anoxic event is suggested to have had a causal relationship with the Latest Permian Mass Extinction (LPME); however, the nature of spatio-temporal variations in the anoxic event prior to the LPME remains poorly known. Here we provided new constraints on this issue based on high-resolution geochemical analyses of sulfur isotopes and trace elements in the Niushan section (nearby the Meishan section) of the Lower Yangtze region, South China. Results show that the entire Upper Permian Changhsingian in the study area can be divided into five intervals according to organic carbon isotope evolutionary trend. From the bottom to top, the water was oxic (interval I) to anoxic with sporadically euxinic condition (interval II) to primarily suboxic (interval III) to anoxic with frequently euxinic condition (interval IV), to a transient euxinic event (interval VEH) under dysoxic/oxic conditions (interval V). Multiple productivity proxies (total organic carbon, Cuxs, and Znxs) suggest that the primary productivity may have experienced a stable low (interval I), then was enhanced but with strong fluctuations (interval II), followed by a transient sharp decline (interval III), and was then subsequently relatively high with small fluctuations (interval IV), and finally experienced a gradual decline with occasional peaks (interval V) through the time interval represented by the Niushan section. The relationship between redox changes and paleoproductivity supports that a redox-stratified late Permian Ocean was present in the eastern Tethys region. Combined with previous results in South China, we propose that all the deep-water sections in the northern marginal basin of the Yangtze Platform (NMBY) might display similar trends, indicating that the NMBY was part of a single paleo-ocean system during the Changhsingian, although the Niushan section records more severely euxinic benthic water. The transient euxinic event during interval extinction horizon (VEH) can be well correlated with other sections worldwide, suggesting that the end-Permian marine anoxia in the Niushan section represents a global signal. The extinction horizon accompanied by a single euxinic episode in this study provides further evidence for the LPME having been caused by widespread anoxia. The bio-environmental evolutionary history of this continuous deep-water section will help us to better evaluate the data for the nearby shallow-water Meishan section.