FRAMBOIDAL PYRITE FROM THE LOWER TRIASSIC OF THE BIANYANG SECTION (NANPANJIANG BASIN, SOUTH CHINA) AND ITS PALEOCEANOGRAPHIC IMPLICATIONS

Oceanic anoxia has been widely implicated in the end-Permian mass extinction and the delayed recovery of marine ecosystems during its aftermath. However, paleoceanographic conditions during the Early Triassic recovery interval have received much less attention than those during the Permian-Triassic boundary crisis. In this study, we have investigated the oceanic redox changes during the entire Early Triassic (~ 5 Ma) based on the framboidal pyrite petrography of Lower Triassic strata in the Bianyang section of the Nanpanjiang Basin, South China. Abundant pyrite framboids with small to medium diameters (<6 mm) were found in most beds of Induan age, as well as at the Smithian-Spathian transition and in the late Spathian. These occurrences provide evidence of an extended (~1.1-Myr-long) interval of oceanic anoxia at the beginning of the Early Triassic, followed by at least two later episodes of intense oceanic anoxia during the recovery interval with the final episode occurring nearly 5 Myr after the end-Permian crisis. Furthermore, there appears to be a significant relationship between oceanic redox conditions and d¹³C_carb variation in the Bianyang section: positive carbon isotopic excursions coincided with intensified oceanic anoxia and negative excursions with relatively better oxygenated conditions. Thus, we infer that episodic recurrence of intense marine anoxia may have been responsible for the delayed marine biotic recovery and perturbed global carbon cycle of the Early Triassic.