Extreme euxinia just prior to the Middle Triassic biotic recovery from the latest Permian mass extinction

The greatest mass extinction on Earth occurred 252 million years ago during the latest Permian. Complete biotic recovery, characterized by a return to pre-extinction diversity levels, took an extraordinarily long time (ca. 5 × 106 yr), probably because harsh conditions developed repeatedly during the Early Triassic. Here, we show the recurrence of euxinic conditions during the Early Triassic and the development of especially severe such conditions during the late Early Triassic, just prior to full biotic recovery from the mass extinction. Dibenzothiophenes and carotenoids accumulated in upper Lower Triassic sediments from South China, located on the western margin of the Paleotethys Ocean. Among these, chlorobactane, a biomarker for green-pigmented green sulfur bacteria, and okenane, a biomarker for purple sulfur photosynthetic bacteria, were identified in upper Early Triassic samples. Both compounds were detected in limestone (micrite) and we therefore infer that planktonic green and purple sulfur bacteria were the sources of these carotenoids, and that their presence indicates photic zone euxinia at the time of deposition. Concentrations of redox sensitive elements (M, V, and U), relative to that of Al, and the U/Th ratio, progressively increased during the late Early Triassic. The euxinic conditions represented by the carotenoids and the elements were likely caused by a combination of locally shallow restricted conditions and global environmental oscillations, such as those related to global warming, during the late Early Triassic. The presence of these biomarkers, together with the increase in the abundance of redox sensitive elements relative to Al and the U/Th, suggests that euxinic conditions could be one of the causes for the delayed recovery of marine communities in the Paleotethys Ocean after the latest Permian mass extinction event.