Benthic Pleurocapsales (Cyanobacteria) Blooms Catalyzing Carbonate Precipitation and Dolomitization Following the End-Permian Mass Extinction

The widespread development of Permian/Triassic boundary microbialites is a key biosedimentary response to the mass extinction event. The microbial communities that precipitated these structures remain, however, poorly resolved. Here, we report abundant columnar microfossils from five Permian-Triassic successions in South China. Four phases of the microfossil life cycle are recorded: baeocytes, juvenile column, mature column, and baeocyte-splitting, which are interpreted as Pleurocapsales (cyanobacterial clade that reproduces by multiple fission). The great thickness (up to 13 m) of these microbial deposits and Raman spectrum analysis indicate a strong calcification capacity of the Pleurocapsales. Pronounced peaks of Mg/(Mg + Ca) ratios from microbialite and distinct Mg-enrichment in microfossils indicate that Pleurocapsales blooms may also have facilitated dolomitization of the microbialites. Common occurrence, strong biomineralization capacity, baffling framework, and the close association with carbonate minerals suggest that Pleurocapsales were key constructors of post-extinction microbialites.