Expansion of microbial-induced carbonate factory into deeper water after the Permian-Triassic mass extinction

The extinction of most heavy calcifiers, such as corals, calcareous algae and sponges, during the Permian-Triassic mass extinction (PTME) coincides with the sudden transition from a carbonate factory dominated by metazoan productivity to one consisting of microbialites and oolites. However, the nature of deeper-water carbonate (distal inner shelf to deep shelf) during the PTME is poorly understood. Here, seven deeper-water carbonate sections from South China and South Tibet were selected to examine the earliest Triassic deeper-water carbonate factory, based on petrological observations and geochemical interpretations. The results show that the earliest Triassic deeper-water carbonates formed under anoxic conditions and reveal diverse microbial structures such as extracellular polymeric substances (EPS), cyanobacteria filaments and rod-shaped grains reminiscent of bacteria in size and shape. These observations suggest that microbial proliferation expanded to relatively deeper-water settings and contributed to the precipitation of deeper-water carbonate.