CHANGE FROM ALGAL SEA TO CYANOBACTERIAL SEA AT THE END OF THE PERMIAN AND REVERSE CHANGE AT THE END OF THE EARLY TRIASSIC

Widespread microbial communities including cyanobacteria have been commonly reported during Phanerozoic biotic crises, based on analysis of a cyanobacterial biomarker (2α-methyl hopanes) and examination of lithological textures and structures (e.g. observations of stromatolites and calcimicrobialites). The Early Triassic, following the end-Permian mass extinction, has been intensively studied for the presence of microbial communities, with a focus on stromatolites, so-called disaster forms and anachronistic facies that were opportunistic taxa during biotic crises, in addition to other microbialites. Cyanobacterial microbialites emerged within 5 m.y. of the late Permian mass extinction and persisted until the beginning of the Middle Triassic when the biota began to accelerate toward a full recovery.

Although facies studies have revealed the degree to which cyanobacteria proliferated throughout the entire Early Triassic, no previous study has conducted a biomarker analysis of cyanobacteria for this period, and the main primary producers are poorly constrained. Establishing a robust geological biomarker for cyanobacteria and primary producers would enable more reliable interpretations of the biological rock record. Here we present data on temporal changes in the abundance of marine cyanobacteria and eukaryotic algae from the Changhsingian to the Anisian, on the basis of biomarkers from three sections (Meishan, Chaohu, and Qingyan) in South China. We use monomethyl alkane ratios and the 2-a methyl hopane index as cyanobacterial biomarkers, and the n-alkyl-cyclo benzenes ratio as a biomarker of eukaryotic algae. The results show (1) a proliferation of cyanobacteria following the end-Permian mass extinction, until just before the full biotic recovery in the Middle Triassic; and (2) a sharp decrease in the abundance of eukaryotic algae following the end-Permian mass extinction, but recovery to dominate over the cyanobacteria during and after the latest Spathian. These new findings, combined with existing records of algae and stromatolites, suggest that the main primary producer in Early Triassic oceans was cyanobacteria and in the late Permian and early Middle Triassic oceans was eukaryotic algae.